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+The Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Aeroplane Speaks
+
+Author: H. Barber
+
+Posting Date: July 21, 2008 [EBook #818]
+Release Date: February, 1997
+Last Updated: March 15, 2018
+
+Language: English
+
+Character set encoding: UTF-8
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE AEROPLANE SPEAKS ***
+
+
+
+
+Produced by Charles Keller
+
+
+
+
+
+THE AEROPLANE SPEAKS
+
+By H. Barber
+
+(Captain, Royal Flying Corps)
+
+
+
+DEDICATED TO THE SUBALTERN FLYING OFFICER
+
+
+
+
+MOTIVE
+
+The reasons impelling me to write this book, the maiden effort of
+my pen, are, firstly, a strong desire to help the ordinary man to
+understand the Aeroplane and the joys and troubles of its Pilot; and,
+secondly, to produce something of PRACTICAL assistance to the Pilot
+and his invaluable assistant the Rigger. Having had some eight years'
+experience in designing, building, and flying aeroplanes, I have hopes
+that the practical knowledge I have gained may offset the disadvantage
+of a hand more used to managing the “joy-stick” than the dreadful
+haltings, the many side-slips, the irregular speed, and, in short, the
+altogether disconcerting ways of a pen.
+
+The matter contained in the Prologue appeared in the Field of May 6th,
+13th, 20th, and 27th, 1916, and is now reprinted by the kind permission
+of the editor, Sir Theodore Cook.
+
+I have much pleasure in also acknowledging the kindness of Mr. C. G.
+Grey, editor of the Aeroplane, to whom I am indebted for the valuable
+illustrations reproduced at the end of this book.
+
+
+
+CONTENTS
+
+  PROLOGUE
+
+  PART
+  I.   THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+  II.  THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
+  III. THE GREAT TEST
+  IV.  CROSS COUNTRY
+
+
+
+  CHAPTER
+  I.   FLIGHT
+  II.  STABILITY AND CONTROL
+  III. RIGGING
+  IV.  PROPELLERS
+  V.   MAINTENANCE
+
+
+  TYPES OF AEROPLANES
+
+  GLOSSARY
+
+
+
+
+
+THE AEROPLANE SPEAKS
+
+
+
+
+PROLOGUE
+
+
+
+
+PART I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+
+The Lecture Hall at the Royal Flying Corps School for Officers was
+deserted. The pupils had dispersed, and the Officer Instructor, more
+fagged than any pupil, was out on the aerodrome watching the test of a
+new machine.
+
+Deserted, did I say? But not so. The lecture that day had been upon
+the Elementary Principles of Flight, and they lingered yet. Upon the
+Blackboard was the illustration you see in the frontispiece.
+
+“I am the side view of a Surface,” it said, mimicking the tones of the
+lecturer. “Flight is secured by driving me through the air at an angle
+inclined to the direction of motion.”
+
+“Quite right,” said the Angle. “That's me, and I'm the famous Angle of
+Incidence.”
+
+“And,” continued the Surface, “my action is to deflect the air
+downwards, and also, by fleeing from the air behind, to create a
+semi-vacuum or rarefied area over most of the top of my surface.”
+
+“This is where I come in,” a thick, gruff voice was heard, and went
+on: “I'm the Reaction. You can't have action without me. I'm a very
+considerable force, and my direction is at right-angles to you,” and
+he looked heavily at the Surface. “Like this,” said he, picking up the
+chalk with his Lift, and drifting to the Blackboard.
+
+“I act in the direction of the arrow R, that is, more or less, for the
+direction varies somewhat with the Angle of Incidence and the curvature
+of the Surface; and, strange but true, I'm stronger on the top of the
+Surface than at the bottom of it. The Wind Tunnel has proved that by
+exhaustive research--and don't forget how quickly I can grow! As the
+speed through the air increases my strength increases more rapidly than
+you might think--approximately, as the Square of the Speed; so you
+see that if the Speed of the Surface through the air is, for instance,
+doubled, then I am a good deal more than doubled. That's because I am
+the result of not only the mass of air displaced, but also the result
+of the Speed with which the Surface engages the Air. I am a product of
+those two factors, and at the speeds at which Aeroplanes fly to-day,
+and at the altitudes and consequent density of air they at present
+experience, I increase at about the Square of the Speed.
+
+“Oh, I'm a most complex and interesting personality, I assure you--in
+fact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr.
+Hyde. There's Lift, my vertical part or COMPONENT, as those who prefer
+long words would say; he always acts vertically upwards, and hates
+Gravity like poison. He's the useful and admirable part of me. Then
+there's Drift, my horizontal component, sometimes, though rather
+erroneously, called Head Resistance; he's a villain of the deepest dye,
+and must be overcome before flight can be secured.”
+
+“And I,” said the Propeller, “I screw through the air and produce the
+Thrust. I thrust the Aeroplane through the air and overcome the Drift;
+and the Lift increases with the Speed and when it equals the Gravity of
+Weight, then--there you are--Flight! And nothing mysterious about it at
+all.”
+
+“I hope you'll excuse me interrupting,” said a very beautiful young
+lady, “my name is Efficiency, and, while no doubt, all you have said is
+quite true, and that, as my young man the Designer says, `You can make a
+tea-tray fly if you slap on Power enough,' I can assure you that I'm not
+to be won quite so easily.”
+
+“Well,” eagerly replied the Lift and the Thrust, “let's be friends. Do
+tell us what we can do to help you to overcome Gravity and Drift with
+the least possible Power. That obviously seems the game to play, for
+more Power means heavier engines, and that in a way plays into the hands
+of our enemy, Gravity, besides necessitating a larger Surface or Angle
+to lift the Weight, and that increases the Drift.”
+
+“Very well,” from Efficiency, “I'll do my best, though I'm so shy, and
+I've just had such a bad time at the Factory, and I'm terribly afraid
+you'll find it awfully dry.”
+
+“Buck up, old dear!” This from several new-comers, who had just
+appeared. “We'll help you,” and one of them, so lean and long that he
+took up the whole height of the lecture room, introduced himself.
+
+“I'm the High Aspect Ratio,” he said, “and what we have got to do to
+help this young lady is to improve the proportion of Lift to Drift.
+The more Lift we can get for a certain area of Surface, the greater
+the Weight the latter can carry; and the less the Drift, then the less
+Thrust and Power required to overcome it. Now it is a fact that, if the
+Surface is shaped to have the greatest possible span, i.e., distance
+from wing-tip to wing-tip, it then engages more air and produces both a
+maximum Reaction and a better proportion of Lift to Drift.
+
+“That being so, we can then well afford to lose a little Reaction
+by reducing the Angle of Incidence to a degree giving a still better
+proportion of Lift to Drift than would otherwise be the case; for you
+must understand that the Lift-Drift Ratio depends very much upon the
+size of the Angle of Incidence, which should be as small as possible
+within certain limits. So what I say is, make the surface of Infinite
+Span with no width or chord, as they call it. That's all I require, I
+assure you, to make me quite perfect and of infinite service to Miss
+Efficiency.”
+
+“That's not practical politics,” said the Surface. “The way you talk one
+would think you were drawing L400 a year at Westminster, and working up
+a reputation as an Aeronautical Expert. I must have some depth and chord
+to take my Spars and Ribs, and again, I must have a certain chord to
+make it possible for my Camber (that's curvature) to be just right for
+the Angle of Incidence. If that's not right the air won't get a nice
+uniform compression and downward acceleration from my underside, and the
+rarefied `suction' area over the top of me will not be as even and clean
+in effect as it might be. That would spoil the Lift-Drift Ratio more
+than you can help it. Just thrust that chalk along, will you? and the
+Blackboard will show you what I mean.”
+
+“Well,” said the Aspect Ratio, “have it your own way, though I'm sorry
+to see a pretty young lady like Efficiency compromised so early in the
+game.”
+
+“Look here,” exclaimed a number of Struts, “we have got a brilliant idea
+for improving the Aspect Ratio,” and with that they hopped up on to the
+Spars. “Now,” excitedly, “place another Surface on top of us. Now do you
+see? There is double the Surface, and that being so, the proportion of
+Weight to Surface area is halved. That's less burden of work for the
+Surface, and so the Spars need not be so strong and so deep, which
+results in not so thick a Surface. That means the Chord can be
+proportionately decreased without adversely affecting the Camber.
+With the Chord decreased, the Span becomes relatively greater, and so
+produces a splendid Aspect Ratio, and an excellent proportion of Lift to
+Drift.”
+
+“I don't deny that they have rather got me there,” said the Drift, “but
+all the same, don't forget my increase due to the drift of the Struts
+and their bracing wires.”
+
+“Yes, I dare say,” replied the Surface, “but remember that my Spars are
+less deep than before, and consequently I am not so thick now, and
+shall for that reason also be able to go through the air with a less
+proportion of Drift to Lift.”
+
+“Remember me also, please,” croaked the Angle of Incidence. “Since the
+Surface has now less weight to carry for its area, I may be set at
+a still lesser and finer Angle. That means less Drift again. We are
+certainly getting on splendidly! Show us how it looks now, Blackboard.”
+ And the Blackboard obligingly showed them as follows:
+
+“Well, what do you think of that?” they all cried to the Drift.
+
+“You think you are very clever,” sneered the Drift. “But you are not
+helping Efficiency as much as you think. The suction effect on the top
+of the lower Surface will give a downward motion to the air above it and
+the result will be that the bottom of the top Surface will not secure
+as good a Reaction from the air as would otherwise be the case, and that
+means loss of Lift; and you can't help matters by increasing the gap
+between the surfaces because that means longer Struts and Wires, and
+that in itself would help me, not to speak of increasing the Weight. You
+see it's not quite so easy as you thought.”
+
+At this moment a hiccough was heard, and a rather fast and
+rakish-looking chap, named Stagger, spoke up. “How d'ye do, miss,” he
+said politely to Efficiency, with a side glance out of his wicked old
+eye. “I'm a bit of a knut, and without the slightest trouble I can
+easily minimize the disadvantage that old reprobate Drift has been
+frightening you with. I just stagger the top Surface a bit forward, and
+no longer is that suction effect dead under it. At the same time I'm
+sure the top Surface will kindly extend its Span for such distance as
+its Spars will support it without the aid of Struts. Such extension will
+be quite useful, as there will be no Surface at all underneath it to
+interfere with the Reaction above.” And the Stagger leaned forward and
+picked up the Chalk, and this is the picture he drew:
+
+Said the Blackboard, “That's not half bad! It really begins to look
+something like the real thing, eh?”
+
+“The real thing, is it?” grumbled Drift. “Just consider that contraption
+in the light of any one Principle, and I warrant you will not find
+one of them applied to perfection. The whole thing is nothing but a
+Compromise.” And he glared fixedly at poor Efficiency.
+
+“Oh, dear! Oh, dear!” she cried. “I'm always getting into trouble. What
+WILL the Designer say?”
+
+“Never mind, my dear,” said the Lift-Drift Ratio, consolingly. “You are
+improving rapidly, and quite useful enough now to think of doing a job
+of work.”
+
+“Well, that's good news,” and Efficiency wiped her eyes with her Fabric
+and became almost cheerful. “Suppose we think about finishing it now?
+There will have to be an Engine and Propeller, won't there? And a body
+to fix them in, and tanks for oil and petrol, and a tail, and,” archly,
+“one of those dashing young Pilots, what?”
+
+“Well, we are getting within sight of those interesting Factors,” said
+the Lift-Drift Ratio, “but first of all we had better decide upon the
+Area of the Surfaces, their Angle of Incidence and Camber. If we are
+to ascend as quickly as possible the Aeroplane must be SLOW in order to
+secure the best possible Lift-Drift Ratio, for the drift of the struts
+wires, body, etc., increases approximately as the square of the speed,
+but it carries with it no lift as it does in the case of the Surface.
+The less speed then, the less such drift, and the better the Aeroplane's
+proportion of lift to drift; and, being slow, we shall require a LARGE
+SURFACE in order to secure a large lift relative to the weight to be
+carried. We shall also require a LARGE ANGLE OF INCIDENCE relative to
+the horizontal, in order to secure a proper inclination of the Surface
+to the direction of motion, for you must remember that, while we shall
+fly upon an even keel and with the propeller thrust horizontal (which is
+its most efficient attitude), our flight path, which is our direction of
+motion, will be sloping upwards, and it will therefore be necessary to
+fix the Surface to the Aeroplane at a very considerable angle relative
+to the horizontal Propeller Thrust in order to secure a proper angle
+to the upwards direction of motion. Apart from that, we shall require a
+larger Angle of Incidence than in the case of a machine designed purely
+for speed, and that means a correspondingly LARGE CAMBER.
+
+“On the other hand, if we are thinking merely of Speed, then a SMALL
+SURFACE, just enough to lift the weight off the ground, will be best,
+also a SMALL ANGLE to cut the Drift down and that, of course, means a
+relatively SMALL CAMBER.
+
+“So you see the essentials for CLIMB or quick ascent and for SPEED are
+diametrically opposed. Now which is it to be?”
+
+“Nothing but perfection for me,” said Efficiency. “What I want is
+Maximum Climb and Maximum Speed for the Power the Engine produces.”
+
+And each Principle fully agreed with her beautiful sentiments, but work
+together they would not.
+
+The Aspect Ratio wanted infinite Span, and hang the Chord.
+
+The Angle of Incidence would have two Angles and two Cambers in one,
+which was manifestly absurd; the Surface insisted upon no thickness
+whatever, and would not hear of such things as Spars and Ribs; and the
+Thrust objected to anything at all likely to produce Drift, and very
+nearly wiped the whole thing off the Blackboard.
+
+There was, indeed, the makings of a very pretty quarrel when the Letter
+arrived. It was about a mile long, and began to talk at once.
+
+“I'm from the Inventor,” he said, and hope rose in the heart of each
+heated Principle. “It's really absurdly simple. All the Pilot has to do
+is to touch a button, and at his will, VARY the area of the Surface, the
+Angle of Incidence, and the Camber! And there you are--Maximum Climb or
+Maximum Speed as required! How does that suit you?”
+
+“That suits us very well,” said the Surface, “but, excuse me asking, how
+is it done without apparatus increasing the Drift and the Weight out
+of all reason? You won't mind showing us your Calculations, Working
+Drawings, Stress Diagrams, etc., will you?”
+
+Said the Letter with dignity, “I come from an Inventor so brilliantly
+clever as to be far above the unimportant matters you mention. He is no
+common working man, sir! He leaves such things to Mechanics. The point
+is, you press a button and----”
+
+“Look here,” said a Strut, rather pointedly, “where do you think you are
+going, anyway?”
+
+“Well,” from the Letter, “as a matter of fact, I'm not addressed yet,
+but, of course, there's no doubt I shall reach the very highest quarters
+and absolutely revolutionize Flight when I get there.”
+
+Said the Chalk, “I'll address you, if that's all you want; now drift
+along quickly!” And off went the Letter to The Technical Editor, “Daily
+Mauler,” London.
+
+And a League was formed, and there were Directors with Fees, and several
+out-of-service Tin Hats, and the Man-who-takes-the-credit, and a fine
+fat Guinea-pig, and all the rest of them. And the Inventor paid his
+Tailor and had a Hair-Cut, and is now a recognized Press Expert--but he
+is still waiting for those Mechanics!
+
+“I'm afraid,” said the Slide-rule, who had been busy making those
+lightning-like automatic calculations for which he is so famous, “it's
+quite impossible to fully satisfy all of you, and it is perfectly plain
+to me that we shall have to effect a Compromise and sacrifice some of
+the Lift for Speed.”
+
+Thud! What was that?
+
+Efficiency had fainted dead away! The last blow had been too much for
+her. And the Principles gathered mournfully round, but with the aid of
+the Propeller Slip[1] and a friendly lift from the Surface she was at
+length revived and regained a more normal aspect.
+
+Said the Stagger with a raffish air, “My dear young lady, I assure
+you that from the experiences of a varied career, I have learned that
+perfection is impossible, and I am sure the Designer will be quite
+satisfied if you become the Most Efficient Compromise.”
+
+“Well, that sounds so common sense,” sighed Efficiency, “I suppose it
+must be true, and if the Designer is satisfied, that's all I really care
+about. Now do let's get on with the job.”
+
+So the Chalk drew a nice long slim body to hold the Engine and the
+tanks, etc., with room for the Pilot's and Passenger's seats, and placed
+it exactly in the middle of the Biplane. And he was careful to make its
+position such that the Centre of Gravity was a little in advance of the
+Centre of Lift, so that when the Engine was not running and there was
+consequently no Thrust, the Aeroplane should be “nose-heavy” just to the
+right degree, and so take up a natural glide to Earth--and this was to
+help the Pilot and relieve him of work and worry, should he find himself
+in a fog or a cloud. And so that this tendency to glide downwards should
+not be in evidence when the Engine was running and descent not desired,
+the Thrust was placed a little below the Centre of Drift or Resistance.
+In this way it would in a measure pull the nose of the Aeroplane up and
+counterbalance the “nose-heavy” tendency.
+
+And the Engine was so mounted that when the Propeller-Thrust was
+horizontal, which is its most efficient position, the Angle of Incidence
+and the Area of the surfaces were just sufficient to give a Lift a
+little in excess of the Weight. And the Camber was such that, as far as
+it was concerned, the Lift-Drift Ratio should be the best possible for
+that Angle of Incidence. And a beautifully simple under-carriage was
+added, the outstanding features of which were simplicity, strength,
+light-weight, and minimum drift. And, last of all, there was the
+Elevator, of which you will hear more by-and-by. And this is what it
+looked like then:
+
+And Efficiency, smiling, thought that it was not such a bad compromise
+after all and that the Designer might well be satisfied.
+
+“Now,” said she, “there's just one or two points I'm a bit hazy about.
+It appears that when the Propeller shaft is horizontal and so working
+in its most efficient attitude, I shall have a Lift from the Surfaces
+slightly in excess of the Weight. That means I shall ascend slightly,
+at the same time making nearly maximum speed for the power and thrust.
+Can't I do better than that?”
+
+“Yes, indeed,” spoke up the Propeller, “though it means that I must
+assume a most undignified attitude, for helicopters[2] I never
+approved of. In order to ascend more quickly the Pilot will deflect the
+Elevator, which, by the way, you see hinged to the Tail. By that
+means he will force the whole Aeroplane to assume a greater Angle of
+Incidence. And with greater Angle, the Lift will increase, though I'm
+sorry to say the Drift will increase also. Owing to the greater Drift,
+the Speed through the air will lessen, and I'm afraid that won't be
+helpful to the Lift; but I shall now be pointing upwards, and besides
+overcoming the Drift in a forward direction I shall be doing my best
+to haul the Aeroplane skywards. At a certain angle known as the Best
+Climbing Angle, we shall have our Maximum Margin of Lift, and I'm hoping
+that may be as much as almost a thousand feet altitude a minute.”
+
+“Then, if the Pilot is green, my chance will come,” said the Maximum
+Angle of Incidence. “For if the Angle is increased over the Best
+Climbing Angle, the Drift will rush up; and the Speed, and with it the
+Lift, will, when my Angle is reached, drop to a point when the latter
+will be no more than the Weight. The Margin of Lift will have entirely
+disappeared, and there we shall be, staggering along at my tremendous
+angle, and only just maintaining horizontal flight.”
+
+“And then with luck I'll get my chance,” said the Drift. “If he is a bit
+worse than green, he'll perhaps still further increase the Angle. Then
+the Drift, largely increasing, the Speed, and consequently the Lift,
+will become still less, i.e., less than the Weight, and then--what price
+pancakes,[3] eh?”
+
+“Thank you,” from Efficiency, “that was all most informing. And now will
+you tell me, please, how the greatest Speed may be secured?”
+
+“Certainly, now it's my turn,” piped the Minimum Angle of Incidence. “By
+means of the Elevator, the Pilot places the Aeroplane at my small Angle,
+at which the Lift only just equals the Weight, and, also, at which we
+shall make greater speed with no more Drift than before. Then we get our
+greatest Speed, just maintaining horizontal flight.”
+
+“Yes; though I'm out of the horizontal and thrusting downwards,”
+ grumbled the Propeller, “and that's not efficient, though I suppose it's
+the best we can do until that Inventor fellow finds his Mechanics.”
+
+“Thank you so much,” said Efficiency. “I think I have now at any rate
+an idea of the Elementary Principles of Flight, and I don't know that I
+care to delve much deeper, for sums always give me a headache; but isn't
+there something about Stability and Control? Don't you think I ought to
+have a glimmering of them too?”
+
+“Well, I should smile,” said a spruce Spar, who had come all the way
+from America. “And that, as the Lecturer says, `will be the subject of
+our next lecture,' so be here again to-morrow, and you will be glad to
+hear that it will be distinctly more lively than the subject we have
+covered to-day.”
+
+
+
+
+PART II. THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE
+JOB
+
+Another day had passed, and the Flight Folk had again gathered together
+and were awaiting the arrival of Efficiency who, as usual, was rather
+late in making an appearance.
+
+The crowd was larger than ever, and among the newcomers some of the most
+important were the three Stabilities, named Directional, Longitudinal,
+and Lateral, with their assistants, the Rudder, Elevator, and Ailerons.
+There was Centrifugal Force, too, who would not sit still and created a
+most unfavourable impression, and Keel-Surface, the Dihedral Angle, and
+several other lesser fry.
+
+“Well,” said Centrifugal Force, “I wish this Efficiency I've heard so
+much about would get a move on. Sitting still doesn't agree with me
+at all. Motion I believe in. There's nothing like motion--the more the
+better.”
+
+“We are entirely opposed to that,” objected the three Stabilities, all
+in a breath. “Unless it's in a perfectly straight line or a perfect
+circle. Nothing but perfectly straight lines or, upon occasion, perfect
+circles satisfy us, and we are strongly suspicious of your tendencies.”
+
+“Well, we shall see what we shall see,” said the Force darkly. “But who
+in the name of blue sky is this?”
+
+And in tripped Efficiency, in a beautifully “doped” dress of the latest
+fashionable shade of khaki-coloured fabric, a perfectly stream-lined
+bonnet, and a bewitching little Morane parasol,[4] smiling as
+usual, and airily exclaiming, “I'm so sorry I'm late, but you see the
+Designer's such a funny man. He objects to skin friction,[5] and
+insisted upon me changing my fabric for one of a smoother surface, and
+that delayed me. Dear me, there are a lot more of us to-day, aren't
+there? I think I had better meet one at a time.” And turning to
+Directional Stability, she politely asked him what he preferred to do.
+
+“My purpose in life, miss,” said he, “is to keep the Aeroplane on its
+course, and to achieve that there must be, in effect, more Keel-Surface
+behind the Vertical Turning Axis than there is in front of it.”
+
+Efficiency looking a little puzzled, he added: “Just like a weathercock,
+and by Keel-Surface I mean everything you can see when you view the
+Aeroplane from the side of it--the sides of the body, struts, wires,
+etc.”
+
+“Oh, now I begin to see light,” said she: “but just exactly how does it
+work?”
+
+“I'll answer that,” said Momentum. “When perhaps by a gust of air the
+Aeroplane is blown out of its course and points in another direction, it
+doesn't immediately fly off on that new course. I'm so strong I pull it
+off the new course to a certain extent, and towards the direction of the
+old course. And so it travels, as long as my strength lasts, in a more
+or less sideways position.”
+
+“Then,” said the Keel-Surface, “I get a pressure of air all on one side,
+and as there is, in effect, most of me towards the tail, the latter
+gets pressed sideways, and the Aeroplane thus tends to assume its first
+position and course.”
+
+“I see,” said Efficiency, and, daintily holding the Chalk, she
+approached the Blackboard. “Is this what you mean?”
+
+“Yes, that's right enough,” said the Keel-Surface, “and you might
+remember, too, that I always make the Aeroplane nose into the gusts
+rather than away from them.”
+
+“If that was not the case,” broke in Lateral Stability, and affecting
+the fashionable Flying Corps stammer, “it would be a h-h-h-o-r-rible
+affair! If there were too much Keel-Surface in front, then that gust
+would blow the Aeroplane round the other way a very considerable
+distance. And the right-hand Surface being on the outside of the turn
+would have more speed, and consequently more Lift, than the Surface
+on the other side. That means a greater proportion of the Lift on that
+side, and before you could say Warp to the Ailerons over the Aeroplane
+would go--probable result a bad side-slip”
+
+“And what can the Pilot do to save such a situation as that?” said
+Efficiency.
+
+“Well,” replied Lateral Stability, “he will try to turn the Aeroplane
+sideways and back to an even keel by means of warping the Ailerons or
+little wings which are hinged on to the Wing-tips, and about which you
+will hear more later on; but if the side-slip is very bad he may not be
+able to right the Aeroplane by means of the Ailerons, and then the only
+thing for him to do is to use the Rudder and to turn the nose of the
+Aeroplane down and head-on to the direction of motion. The Aeroplane
+will then be meeting the air in the direction it is designed to do
+so, and the Surfaces and also the controls (the Rudder, Ailerons, and
+Elevator) will be working efficiently; but its attitude relative to
+the earth will probably be more or less upside-down, for the action
+of turning the Aeroplane's nose down results, as you will see by the
+illustration B, in the right wing, which is on the outside of the
+circle. travelling through the air with greater speed than the left-hand
+wing. More Speed means more Lift, so that results in overturning the
+Aeroplane still more; but now it is, at any rate, meeting the air as it
+is designed to meet it, and everything is working properly. It is then
+only necessary to warp the Elevator, as shown in illustration C, in
+order to bring the Aeroplane into a proper attitude relative to the
+earth.”
+
+“Ah!” said the Rudder, looking wise, “it's in a case like that when I
+become the Elevator and the Elevator becomes me.”
+
+“That's absurd nonsense,” said the Blackboard, “due to looseness of
+thought and expression.”
+
+“Well,” replied the Rudder, “when 'the Aeroplane is in position A and I
+am used, then I depress or ELEVATE the nose of the machine; and, if the
+Elevator is used, then it turns the Aeroplane to right or left, which is
+normally my function. Surely our roles have changed one with the other,
+and I'm then the Elevator and the Elevator is me!”
+
+Said Lateral Stability to the Rudder, “That's altogether the wrong way
+of looking at it, though I admit”--and this rather sarcastically--“that
+the way you put it sounds rather fine when you are talking of your
+experiences in the air to those 'interested in aviation' but knowing
+little about it; but it won't go down here! You are a Controlling
+Surface designed to turn the Aeroplane about its vertical axis, and the
+Elevator is a Controlling Surface designed to turn the Aeroplane about
+its lateral axis. Those are your respective jobs, and you can't possibly
+change them about. Such talk only leads to confusion, and I hope we
+shall hear no more of it.”
+
+“Thanks,” said Efficiency to Lateral Stability. “And now, please, will
+you explain your duties?”
+
+“My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.
+First of all, I sometimes arrange with the Rigger to wash-out, that is
+decrease, the Angle of Incidence on one side of the Aeroplane, and to
+effect the reverse condition, if it is not too much trouble, on the
+other side.”
+
+“But,” objected Efficiency, “the Lift varies with the Angle of
+Incidence, and surely such a condition will result in one side of the
+Aeroplane lifting more than the other side?'
+
+“That's all right,” said the Propeller, “it's meant to off-set the
+tendency of the Aeroplane to turn over sideways in the opposite
+direction to which I revolve.”
+
+“That's quite clear, though rather unexpected; but how do you counteract
+the effect of the gusts when they try to overturn the Aeroplane
+sideways?” said she, turning to Lateral Stability again.
+
+“Well,” he replied, rather miserably, “I'm not nearly so perfect as the
+Longitudinal and Directional Stabilities. The Dihedral Angle--that is,
+the upward inclination of the Surfaces towards their wing-tips--does
+what it can for me, but, in my opinion, it's a more or less futile
+effort. The Blackboard will show you the argument.” And he at once
+showed them two Surfaces, each set at a Dihedral Angle like this:
+
+“Please imagine,” said the Blackboard, “that the top V is the front
+view of a Surface flying towards you. Now if a gust blows it into the
+position of the lower V you see that the horizontal equivalent of the
+Surface on one side becomes larger, and on the other side it becomes
+smaller. That results in more Lift on the lower side and less on the
+higher side, and if the V is large enough it should produce such a
+difference in the Lift of one side to the other as to quickly turn the
+Aeroplane back to its former and normal position.”
+
+“Yes,” said the Dihedral Angle, “that's what would happen if they would
+only make me large enough; but they won't do it because it would too
+greatly decrease the horizontal equivalent, and therefore the Lift, and
+incidentally it would, as Aeroplanes are built to-day, produce an excess
+of Keel Surface above the turning axis, and that in itself would spoil
+the Lateral Stability. The Keel Surface should be equally divided above
+and below the longitudinal turning axis (upon which the Aeroplane rolls
+sideways), or the side upon which there is an excess will get blown over
+by the gusts. It strikes me that my future isn't very promising, and
+about my only chance is when the Junior Draughtsman makes a mistake, as
+he did the other day. And just think of it, they call him a Designer now
+that he's got a job at the Factory! What did he do? Why, he calculated
+the weights wrong and got the Centre of Gravity too high, and they
+didn't discover it until the machine was built. Then all they could do
+was to give me a larger Angle. That dropped the bottom of the V lower
+down, and as that's the centre of the machine, where all the Weight is,
+of course that put the Centre of Gravity in its right place. But now
+there is too much Keel Surface above, and the whole thing's a Bad
+Compromise, not at all like Our Efficiency.”
+
+And Efficiency, blushing very prettily at the compliment, then asked,
+“And how does the Centre of Gravity affect matters?”
+
+“That's easy,” said Grandfather Gravity. “I'm so heavy that if I am too
+low down I act like a pendulum and cause the Aeroplane to roll about
+sideways, and if I am too high I'm like a stick balanced on your finger,
+and then if I'm disturbed, over I go and the Aeroplane with me; and, in
+addition to that, there are the tricks I play with the Aeroplane when
+it's banked up,[6] i.e., tilted sideways for a turn, and Centrifugal
+Force sets me going the way I'm not wanted to go. No; I get on best with
+Lateral Stability when my Centre is right on the centre of Drift, or,
+at any rate, not much below it.” And with that he settled back into the
+Lecturer's Chair and went sound asleep again, for he was so very, very
+old, in fact the father of all the Principles.
+
+And the Blackboard had been busy, and now showed them a picture of
+the Aeroplane as far as they knew it, and you will see that there is
+a slight Dihedral Angle, and also, fixed to the tail, a vertical Keel
+Surface or fin, as is very often the case in order to ensure the greater
+effect of such surface being behind the vertical turning axis.
+
+But Efficiency, growing rather critical with her newly gained knowledge,
+cried out: “But where's the horizontal Tail Surface? It doesn't look
+right like that!”
+
+“This is when I have the pleasure of meeting you, my dear,” said
+Longitudinal Stability. “Here's the Tail Surface,” he said, “and in
+order to help me it must be set IN EFFECT at a much less Angle of
+Incidence than the Main Surface. To explain we must trouble the
+Blackboard again,” and this was his effort:
+
+“I have tried to make that as clear as possible,” he said. “It may
+appear a bit complicated at first, but if you will take the trouble to
+look at it for a minute you will find it quite simple. A is the normal
+and proper direction of motion of the Aeroplane, but, owing to a gust of
+air, it takes up the new nose-down position. Owing to Momentum, however,
+it does not fly straight along in that direction, but moves more or less
+in the direction B, which is the resultant of the two forces, Momentum
+and Thrust. And so you will note that the Angle of Incidence, which
+is the inclination of the Surfaces to the Direction of Motion, has
+decreased, and of course the Lift decreases with it. You will also
+see, and this is the point, that the Tail Surface has lost a higher
+proportion of its Angle, and consequently its Lift, than has the Main
+Surface. Then, such being the case, the Tail must fall and the Aeroplane
+assume its normal position again, though probably at a slightly lower
+altitude.”
+
+“I'm afraid I'm very stupid,” said Efficiency, “but please tell me why
+you lay stress upon the words 'IN EFFECT.'”
+
+“Ah! I was wondering if you would spot that,” he replied. “And there is
+a very good reason for it. You see, in some Aeroplanes the Tail Surface
+may be actually set at the same Angle on the machine as the Main
+Surface, but owing to the air being deflected downwards by the front
+Main Surface it meets the Tail Surface at a lesser angle, and indeed in
+some cases at no angle at all. The Tail is then for its surface getting
+less Lift than the Main Surface, although set at the same angle on
+the machine. It may then be said to have IN EFFECT a less Angle of
+Incidence. I'll just show you on the Blackboard.”
+
+“And now,” said Efficiency, “I have only to meet the Ailerons and the
+Rudder, haven't I?”
+
+“Here we are,” replied the Ailerons, or little wings. “Please hinge us
+on to the back of the Main Surfaces, one of us at each Wing-tip, and
+join us up to the Pilot's joystick by means of the control cables. When
+the Pilot wishes to tilt the Aeroplane sideways, he will move the stick
+and depress us upon one side, thus giving us a larger Angle of Incidence
+and so creating more Lift on that side of the Aeroplane; and, by means
+of a cable connecting us with the Ailerons on the other side of the
+Aeroplane, we shall, as we are depressed, pull them up and give them a
+reverse or negative Angle of Incidence, and that side will then get
+a reverse Lift or downward thrust, and so we are able to tilt the
+Aeroplane sideways.
+
+“And we work best when the Angle of Incidence of the Surface in front
+of us is very small, for which reason it is sometimes decreased or
+washed-out towards the Wing-tips. The reason of that is that by the
+time the air reaches us it has been deflected downwards--the greater the
+Angle of Incidence the more it is driven downwards--and in order for
+us to secure a Reaction from it, we have to take such a large Angle of
+Incidence that we produce a poor proportion of Lift to Drift; but the
+smaller the Angle of the Surface in front of us the less the air is
+deflected downwards, and consequently the less Angle is required of us,
+and the better our proportion of Lift to Drift, which, of course, makes
+us much more effective Controls.”
+
+“Yes,” said the Lateral and Directional Stabilities in one voice,
+“that's so, and the wash-out helps us also, for then the Surfaces
+towards their Wing-tips have less Drift or 'Head-Resistance,' and
+consequently the gusts will affect them and us less; but such decreased
+Angle of Incidence means decreased Lift as well as Drift, and the
+Designer does not always care to pay the price.”
+
+“Well,” said the Ailerons, “if it's not done it will mean more work for
+the Rudder, and that won't please the Pilot.”
+
+“Whatever do you mean?” asked Efficiency. “What can the Rudder have to
+do with you?”
+
+“It's like this,” they replied: “when we are deflected downwards we gain
+a larger Angle of Incidence and also enter an area of compressed air,
+and so produce more Drift than those of us on the other side of the
+Aeroplane, which are deflected upwards into an area of rarefied air due
+to the SUCTION effect (though that term is not academically correct) on
+the top of the Surface. If there is more Drift, i.e., Resistance, on
+one side of the Aeroplane than on the other side, then of course it will
+turn off its course, and if that difference in Drift is serious, as it
+will very likely be if there is no wash-out, then it will mean a good
+deal of work for the Rudder in keeping the Aeroplane on its course,
+besides creating extra Drift in doing so.”
+
+“I think, then,” said Efficiency, “I should prefer to have that
+wash-out,[7] and my friend the Designer is so clever at producing
+strength of construction for light weight, I'm pretty sure he won't
+mind paying the price in Lift. And now let me see if I can sketch the
+completed Aeroplane.”
+
+“Well, I hope that's all as it should be,” she concluded, “for to-morrow
+the Great Test in the air is due.”
+
+
+
+
+PART III. THE GREAT TEST
+
+It is five o'clock of a fine calm morning, when the Aeroplane is wheeled
+out of its shed on to the greensward of the Military Aerodrome. There
+is every promise of a good flying day, and, although the sun has not
+yet risen, it is light enough to discern the motionless layer of fleecy
+clouds some five thousand feet high, and far, far above that a few filmy
+mottled streaks of vapour. Just the kind of morning beloved of pilots.
+
+A brand new, rakish, up-to-date machine it is, of highly polished,
+beautifully finished wood, fabric as tight as a drum, polished metal,
+and every part so perfectly “streamlined” to minimize Drift, which is
+the resistance of the air to the passage of the machine, that to the
+veriest tyro the remark of the Pilot is obviously justified.
+
+“Clean looking 'bus, looks almost alive and impatient to be off. Ought
+to have a turn for speed with those lines.”
+
+“Yes,” replies the Flight-Commander, “it's the latest of its type and
+looks a beauty. Give it a good test. A special report is required on
+this machine.”
+
+The A.M.'s[8] have now placed the Aeroplane in position facing the
+gentle air that is just beginning to make itself evident; the engine
+Fitter, having made sure of a sufficiency of oil and petrol in the
+tanks, is standing by the Propeller; the Rigger, satisfied with a job
+well done, is critically “vetting” the machine by eye, four A.M.'s are
+at their posts, ready to hold the Aeroplane from jumping the blocks
+which have been placed in front of the wheels; and the Flight-Sergeant
+is awaiting the Pilot's orders.
+
+As the Pilot approaches the Aeroplane the Rigger springs to attention
+and reports, “All correct, sir,” but the Fitter does not this morning
+report the condition of the Engine, for well he knows that this Pilot
+always personally looks after the preliminary engine test. The latter,
+in leathern kit, warm flying boots and goggled, climbs into his seat,
+and now, even more than before, has the Aeroplane an almost living
+appearance, as if straining to be off and away. First he moves the
+Controls to see that everything is clear, for sometimes when the
+Aeroplane is on the ground the control lever or “joy-stick” is lashed
+fast to prevent the wind from blowing the controlling surfaces about and
+possibly damaging them.
+
+The air of this early dawn is distinctly chilly, and the A.M.'s are
+beginning to stamp their cold feet upon the dewy grass, but very careful
+and circumspect is the Pilot, as he mutters to himself, “Don't worry and
+flurry, or you'll die in a hurry.”
+
+At last he fumbles for his safety belt, but with a start remembers the
+Pilot Air Speed Indicator, and, adjusting it to zero, smiles as he hears
+the Pilot-head's gruff voice, “Well, I should think so, twenty miles an
+hour I was registering. That's likely to cause a green pilot to stall
+the Aeroplane. Pancake, they call it.” And the Pilot, who is an old hand
+and has learned a lot of things in the air that mere earth-dwellers know
+nothing about, distinctly heard the Pilot Tube, whose mouth is open to
+the air to receive its pressure, stammer. “Oh Lor! I've got an earwig
+already--hope to goodness the Rigger blows me out when I come down--and
+this morning air simply fills me with moisture; I'll never keep the
+Liquid steady in the Gauge. I'm not sure of my rubber connections
+either.”
+
+“Oh, shut up!” cry all the Wires in unison, “haven't we got our troubles
+too? We're in the most horrible state of tension. It's simply murdering
+our Factor of Safety, and how we can possibly stand it when we get the
+Lift only the Designer knows.”
+
+“That's all right,” squeak all the little Wire loops, “we're that
+accommodating, we're sure to elongate a bit and so relieve your
+tension.” For the whole Aeroplane is braced together with innumerable
+wires, many of which are at their ends bent over in the form of loops
+in order to connect with the metal fittings on the spars and
+elsewhere--cheap and easy way of making connection.
+
+“Elongate, you little devils, would you?” fairly shout the Angles of
+Incidence, Dihedral and Stagger, amid a chorus of groans from all parts
+of the Aeroplane. “What's going to happen to us then? How are we going
+to keep our adjustments upon which good flying depends?”
+
+“Butt us and screw us,”[9] wail the Wires. “Butt us and screw us, and
+death to the Loops. That's what we sang to the Designer, but he only
+looked sad and scowled at the Directors.”
+
+“And who on earth are they?” asked the Loops, trembling for their
+troublesome little lives.
+
+“Oh earth indeed,” sniffed Efficiency, who had not spoken before, having
+been rendered rather shy by being badly compromised in the Drawing
+Office. “I'd like to get some of them up between Heaven and Earth,
+I would. I'd give 'em something to think of besides their Debits and
+Credits--but all the same the Designer will get his way in the end. I'm
+his Best Girl, you know, and if we could only get rid of the Directors,
+the little Tin god, and the Man-who-takes-the-credit, we should be quite
+happy.” Then she abruptly subsides, feeling that perhaps the less said
+the better until she has made a reputation in the Air. The matter of
+that Compromise still rankled, and indeed it does seem hardly fit that
+a bold bad Tin god should flirt with Efficiency. You see there was
+a little Tin god, and he said “Boom, Boom BOOM! Nonsense! It MUST be
+done,” and things like that in a very loud voice, and the Designer
+tore his hair and was furious, but the Directors, who were thinking of
+nothing but Orders and Dividends, had the whip-hand of HIM, and so there
+you are, and so poor beautiful Miss Efficiency was compromised.
+
+All this time the Pilot is carefully buckling his belt and making
+himself perfectly easy and comfortable, as all good pilots do. As
+he straightens himself up from a careful inspection of the Deviation
+Curve[10] of the Compass and takes command of the Controls, the
+Throttle and the Ignition, the voices grow fainter and fainter until
+there is nothing but a trembling of the Lift and Drift wires to indicate
+to his understanding eye their state of tension in expectancy of the
+Great Test.
+
+“Petrol on?” shouts the Fitter to the Pilot.
+
+“Petrol on,” replies the Pilot.
+
+“Ignition off?”
+
+“Ignition off.”
+
+Round goes the Propeller, the Engine sucking in the Petrol Vapour with
+satisfied gulps. And then--
+
+“Contact?” from the Fitter.
+
+“Contact,” says the Pilot.
+
+Now one swing of the Propeller by the Fitter, and the Engine is awake
+and working. Slowly at first though, and in a weak voice demanding, “Not
+too much Throttle, please. I'm very cold and mustn't run fast until my
+Oil has thinned and is circulating freely. Three minutes slowly, as you
+love me, Pilot.”
+
+Faster and faster turn the Engine and Propeller, and the Aeroplane,
+trembling in all its parts, strains to jump the blocks and be off.
+Carefully the Pilot listens to what the Engine Revolution Indicator
+says. At last, “Steady at 1,500 revs. and I'll pick up the rest in the
+Air.” Then does he throttle down the Engine, carefully putting the lever
+back to the last notch to make sure that in such position the Throttle
+is still sufficiently open for the Engine to continue working, as
+otherwise it might lead to him “losing” his Engine in the air when
+throttling down the power for descent. Then, giving the official signal,
+he sees the blocks removed from the wheels, and the Flight-Sergeant
+saluting he knows that all is clear to ascend. One more signal, and all
+the A.M.'s run clear of the Aeroplane.
+
+Then gently, gently mind you, with none of the “crashing on” bad
+Pilots think so fine, he opens the Throttle and, the Propeller Thrust
+overcoming its enemy the Drift, the Aeroplane moves forward.
+
+“Ah!” says the Wind-screen, “that's Discipline, that is. Through
+my little window I see most things, and don't I just know that poor
+discipline always results in poor work in the air, and don't you forget
+it.”
+
+“Discipline is it?” complains the Under-carriage, as its wheels roll
+swiftly over the rather rough ground. “I'm bump getting it; and bump,
+bump, all I want, bang, bump, rattle, too!” But, as the Lift increases
+with the Speed, the complaints of the Under-carriage are stilled, and
+then, the friendly Lift becoming greater than the Weight, the Aeroplane
+swiftly and easily takes to the air.
+
+Below is left the Earth with all its bumps and troubles. Up into the
+clean clear Air moves with incredible speed and steadiness this triumph
+of the Designer, the result of how much mental effort, imagination,
+trials and errors, failures and successes, and many a life lost in high
+endeavour.
+
+Now is the mighty voice of the Engine heard as he turns the Propeller
+nine hundred times a minute. Now does the Thrust fight the Drift for all
+it's worth, and the Air Speed Indicator gasps with delight, “One hundred
+miles an hour!”
+
+And now does the burden of work fall upon the Lift and Drift Wires,
+and they scream to the Turnbuckles whose business it is to hold them in
+tension, “This is the limit! the Limit! THE LIMIT! Release us, if only
+a quarter turn.” But the Turnbuckles are locked too fast to turn their
+eyes or utter a word. Only the Locking Wires thus: “Ha! ha! the Rigger
+knew his job. He knew the trick, and there's no release here.” For
+an expert rigger will always use the locking wire in such a way as to
+oppose the slightest tendency of the turnbuckle to unscrew. The other
+kind of rigger will often use the wire in such a way as to allow the
+turnbuckle, to the “eyes” of which the wires are attached, to unscrew a
+quarter of a turn or more, with the result that the correct adjustment
+of the wires may be lost; and upon their fine adjustment much depends.
+
+And the Struts and the Spars groan in compression and pray to keep
+straight, for once “out of truth” there is, in addition to possible
+collapse, the certainty that in bending they will throw many wires out
+of adjustment.
+
+And the Fabric's quite mixed in its mind, and ejaculates, “Now, who
+would have thought I got more Lift from the top of the Surface than its
+bottom?” And then truculently to the Distance Pieces, which run from
+rib to rib, “Just keep the Ribs from rolling, will you? or you'll see me
+strip. I'm an Irishman, I am, and if my coat comes off---- Yes, Irish, I
+said. I used to come from Egypt, but I've got naturalized since the War
+began.”
+
+Then the Air Speed Indicator catches the eye of the Pilot. “Good
+enough,” he says as he gently deflects the Elevator and points the nose
+of the Aeroplane upwards in search of the elusive Best Climbing Angle.
+
+“Ha! ha!” shouts the Drift, growing stronger with the increased Angle
+of Incidence. “Ha! ha!” he laughs to the Thrust. “Now I've got you. Now
+who's Master?”
+
+And the Propeller shrieks hysterically, “Oh! look at me. I'm a
+helicopter. That's not fair. Where's Efficiency?” And she can only sadly
+reply, “Yes, indeed, but you see we're a Compromise.”
+
+And the Drift has hopes of reaching the Maximum Angle of Incidence
+and vanquishing the Thrust and the Lift. And he grows very bold as he
+strangles the Thrust; but the situation is saved by the Propeller,
+who is now bravely helicopting skywards, somewhat to the chagrin of
+Efficiency.
+
+“Much ado about nothing,” quotes the Aeroplane learnedly. “Compromise
+or not, I'm climbing a thousand feet a minute. Ask the Altimeter. He'll
+confirm it.”
+
+And so indeed it was. The vacuum box of the Altimeter was steadily
+expanding under the decreased pressure of the rarefied air, and by means
+of its little levers and its wonderful chain no larger than a hair it
+was moving the needle round the gauge and indicating the ascent at the
+rate of a thousand feet a minute.
+
+And lo! the Aeroplane has almost reached the clouds! But what's this?
+A sudden gust, and down sinks one wing and up goes the other. “Oh, my
+Horizontal Equivalent!” despairingly call the Planes: “it's eloping with
+the Lift, and what in the name of Gravity will happen? Surely there was
+enough scandal in the Factory without this, too!” For the lift varies
+with the horizontal equivalent of the planes, so that if the aeroplane
+tilts sideways beyond a certain angle, the lift becomes less than the
+weight of the machine, which must then fall. A fall in such a position
+is known as a “side-slip.”
+
+But the ever-watchful Pilot instantly depresses one aileron, elevating
+the other, with just a touch of the rudder to keep on the course, and
+the Planes welcome back their precious Lift as the Aeroplane flicks back
+to its normal position.
+
+“Bit bumpy here under these clouds,” is all the Pilot says as he heads
+for a gap between them, and the next minute the Aeroplane shoots up into
+a new world of space.
+
+“My eye!” ejaculates the Wind-screen, “talk about a view!” And indeed
+mere words will always fail to express the wonder of it. Six thousand
+feet up now, and look! The sun is rising quicker than ever mortal on
+earth witnessed its ascent. Far below is Mother Earth, wrapt in mists
+and deep blue shadows, and far above are those light, filmy, ethereal
+clouds now faintly tinged with pink And all about great mountains of
+cloud, lazily floating in space. The sun rises and they take on all
+colours, blending one with the other, from dazzling white to crimson
+and deep violet-blue. Lakes and rivers here and there in the enormous
+expanse of country below refract the level rays of the sun and, like so
+many immense diamonds, send dazzling shafts of light far upwards. The
+tops of the hills now laugh to the light of the sun, but the valleys are
+still mysterious dark blue caverns, clowned with white filmy lace-like
+streaks of vapour. And withal the increasing sense with altitude of
+vast, clean, silent solitudes of space.
+
+Lives there the man who can adequately describe this Wonder? “Never,”
+ says the Pilot, who has seen it many times, but to whom it is ever new
+and more wonderful.
+
+Up, up, up, and still up, unfalteringly speeds the Pilot and his mount.
+Sweet the drone of the Engine and steady the Thrust as the Propeller
+exultingly battles with the Drift.
+
+And look! What is that bright silver streak all along the horizon? It
+puzzled the Pilot when first he saw it, but now he knows it for the Sea,
+full fifty miles away!
+
+And on his right is the brightness of the Morn and the smiling Earth
+unveiling itself to the ardent rays of the Sun; and on his left, so
+high is he, there is yet black Night, hiding innumerable Cities, Towns,
+Villages and all those places where soon teeming multitudes of men shall
+awake, and by their unceasing toil and the spirit within them produce
+marvels of which the Aeroplane is but the harbinger.
+
+And the Pilot's soul is refreshed, and his vision, now exalted, sees
+the Earth a very garden, even as it appears at that height, with discord
+banished and a happy time come, when the Designer shall have at last
+captured Efficiency, and the Man-who-takes-the-credit is he who has
+earned it, and when kisses are the only things that go by favour.
+
+Now the Pilot anxiously scans the Barograph, which is an instrument much
+the same as the Altimeter; but in this case the expansion of the vacuum
+box causes a pen to trace a line upon a roll of paper. This paper is
+made by clockwork to pass over the point of the pen, and so a curved
+line is made which accurately registers the speed of the ascent in feet
+per minute. No longer is the ascent at the rate of a thousand feet a
+minute, and the Propeller complains to the Engine, “I'm losing my Revs.
+and the Thrust. Buck up with the Power, for the Lift is decreasing,
+though the Weight remains much the same.”
+
+Quoth the Engine: “I strangle for Air. A certain proportion, and that
+of right density, I must have to one part of Petrol, in order to give me
+full power and compression, and here at an altitude of ten thousand feet
+the Air is only two-thirds as dense as at sea-level. Oh, where is he who
+will invent a contrivance to keep me supplied with Air of right density
+and quality? It should not be impossible within certain limits.”
+
+“We fully agree,” said the dying Power and Thrust. “Only maintain Us and
+you shall be surprised at the result. For our enemy Drift decreases in
+respect of distance with the increase of altitude and rarity of air,
+and there is no limit to the speed through space if only our strength
+remains. And with oxygen for Pilot and Passengers and a steeper
+pitch[11] for the Propeller we may then circle the Earth in a day!”
+
+Ah, Reader, smile not unbelievingly, as you smiled but a few years past.
+There may be greater wonders yet. Consider that as the speed increases,
+so does the momentum or stored-up force in the mass of the aeroplane
+become terrific. And, bearing that in mind, remember that with
+altitude gravity decreases. There may yet be literally other worlds to
+conquer.[12]
+
+Now at fifteen thousand feet the conditions are chilly and rare, and the
+Pilot, with thoughts of breakfast far below, exclaims, “High enough!
+I had better get on with the Test.” And then, as he depresses the
+Elevator, the Aeroplane with relief assumes its normal horizontal
+position. Then, almost closing the Throttle, the Thrust dies away. Now,
+the nose of the Aeroplane should sink of its own volition, and the craft
+glide downward at flying speed, which is in this case a hundred miles
+an hour. That is what should happen if the Designer has carefully
+calculated the weight of every part and arranged for the centre of
+gravity to be just the right distance in front of the centre of lift.
+Thus is the Aeroplane “nose-heavy” as a glider, and just so to a degree
+ensuring a speed of glide equal to its flying speed. And the Air Speed
+Indicator is steady at one hundred miles an hour, and “That's all
+right!” exclaims the Pilot. “And very useful, too, in a fog or a cloud,”
+ he reflects, for then he can safely leave the angle of the glide to
+itself, and give all his attention, and he will need it all, to keeping
+the Aeroplane horizontal from wing-tip to wing-tip, and to keeping
+it straight on its course. The latter he will manage with the rudder,
+controlled by his feet, and the Compass will tell him whether a straight
+course is kept. The former he will control by the Ailerons, or
+little wings hinged to the tips of the planes, and the bubble in the
+Inclinometer in front of him must be kept in the middle.
+
+A Pilot, being only human, may be able to do two things at once, but
+three is a tall order, so was this Pilot relieved to find the Design not
+at fault and his craft a “natural glider.” To correct this nose-heavy
+tendency when the Engine is running, and descent not required, the
+centre of Thrust is arranged to be a little below the centre of Drift or
+Resistance, and thus acts as a counter-balance.
+
+But what is this stream of bad language from the Exhaust Pipe,
+accompanied by gouts of smoke and vapour? The Engine, now revolving at
+no more than one-tenth its normal speed, has upset the proportion of
+petrol to air, and combustion is taking place intermittently or in the
+Exhaust Pipe, where it has no business to be.
+
+“Crash, Bang, Rattle----!----!----!” and worse than that, yells the
+Exhaust, and the Aeroplane, who is a gentleman and not a box kite,[13]
+remonstrates with the severity of a Senior Officer. “See the Medical
+Officer, you young Hun. Go and see a doctor. Vocal diarrhoea, that's
+your complaint, and a very nasty one too. Bad form, bad for discipline,
+and a nuisance in the Mess. What's your Regiment? Special Reserve, you
+say? Humph! Sounds like Secondhand Bicycle Trade to me!”
+
+Now the Pilot decides to change the straight gliding descent to a spiral
+one, and, obedient to the Rudder, the Aeroplane turns to the left. But
+the Momentum (two tons at 100 miles per hour is no small affair) heavily
+resents this change of direction, and tries its level best to prevent
+it and to pull the machine sideways and outwards from its spiral
+course--that is, to make it “side-skid” outwards. But the Pilot deflects
+the Ailerons and “banks” up the planes to the correct angle, and, the
+Aeroplane skidding sideways and outwards, the lowest surfaces of
+the planes press up against the air until the pressure equals the
+centrifugal force of the Momentum, and the Aeroplane spirals steadily
+downwards.
+
+Down, down, down, and the air grows denser, and the Pilot gulps largely,
+filling his lungs with the heavier air to counteract the increasing
+pressure from without. Down through a gap in the clouds, and the
+Aerodrome springs into view, appearing no larger than a saucer, and the
+Pilot, having by now got the “feel” of the Controls, proceeds to put
+the Aeroplane through its paces. First at its Maximum Angle, staggering
+along tail-down and just maintaining horizontal flight; then a dive at
+far over flying speed, finishing with a perfect loop; then sharp turns
+with attendant vertical “banks” and then a wonderful switchback
+flight, speeding down at a hundred and fifty miles an hour with short,
+exhilarating ascents at the rate of two thousand feet a minute!
+
+All the parts are now working well together. Such wires as were before
+in undue tension have secured relief by slightly elongating their loops,
+and each one is now doing its bit, and all are sharing the burden of
+work together.
+
+The Struts and the Spars, which felt so awkward at first, have bedded
+themselves in their sockets, and are taking the compression stresses
+uncomplainingly.
+
+The Control Cables of twisted wire, a bit tight before, have slightly
+lengthened by perhaps the eighth of an inch, and, the Controls instantly
+responding to the delicate touch of the Pilot, the Aeroplane, at the
+will of its Master, darts this way and that way, dives, loops, spirals,
+and at last, in one long, magnificent glide, lands gently in front of
+its shed.
+
+“Well, what result?” calls the Flight-Commander to the Pilot.
+
+“A hundred miles an hour and a thousand feet a minute,” he briefly
+replies.
+
+“And a very good result too,” says the Aeroplane, complacently, as he is
+carefully wheeled into his shed.
+
+
+That is the way Aeroplanes speak to those who love them and understand
+them. Lots of Pilots know all about it, and can spin you wonderful
+yarns, much better than this one, if you catch them in a confidential
+mood--on leave, for instance, and after a good dinner.
+
+
+
+
+PART IV. 'CROSS COUNTRY
+
+The Aeroplane had been designed and built, and tested in the air, and
+now stood on the Aerodrome ready for its first 'cross-country flight.
+
+It had run the gauntlet of pseudo-designers, crank inventors, press
+“experts,” and politicians; of manufacturers keen on cheap work and
+large profits; of poor pilots who had funked it, and good pilots who had
+expected too much of it. Thousands of pounds had been wasted on it,
+many had gone bankrupt over it, and others it had provided with safe fat
+jobs.
+
+Somehow, and despite every conceivable obstacle, it had managed to
+muddle through, and now it was ready for its work. It was not perfect,
+for there were fifty different ways in which it might be improved, some
+of them shamefully obvious. But it was fairly sound mechanically, had a
+little inherent stability, was easily controlled, could climb a thousand
+feet a minute, and its speed was a hundred miles an hour. In short,
+quite a creditable machine, though of course the right man had not got
+the credit.
+
+It is rough, unsettled weather with a thirty mile an hour wind on the
+ground, and that means fifty more or less aloft. Lots of clouds at
+different altitudes to bother the Pilot, and the air none to clear for
+the observation of landmarks.
+
+As the Pilot and Observer approach the Aeroplane the former is clearly
+not in the best of tempers. “It's rotten luck,” he is saying, “a blank
+shame that I should have to take this blessed 'bus and join X Reserve
+Squadron, stationed a hundred and fifty miles from anywhere; and just
+as I have licked my Flight into shape. Now some slack blighter will, I
+suppose, command it and get the credit of all my work!”
+
+“Shut up, you grouser,” said the Observer. “Do you think you're the only
+one with troubles? Haven't I been through it too? Oh! I know all about
+it! You're from the Special Reserve and your C.O. doesn't like your
+style of beauty, and you won't lick his boots, and you were a bit of a
+technical knut in civil life, but now you've jolly well got to know less
+than those senior to you. Well! It's a jolly good experience for most of
+us. Perhaps conceit won't be at quite such a premium after this war. And
+what's the use of grousing? That never helped anyone. So buck up, old
+chap. Your day will come yet. Here's our machine, and I must say it
+looks a beauty!”
+
+And, as the Pilot approaches the Aeroplane, his face brightens and he
+soon forgets his troubles as he critically inspects the craft which is
+to transport him and the Observer over the hills and far away. Turning
+to the Flight-Sergeant he inquires, “Tank full of petrol and oil?”
+
+“Yes, sir,” he replies, “and everything else all correct. Propeller,
+engine, and body covers on board, sir; tool kit checked over and in the
+locker; engine and Aeroplane logbooks written up, signed, and under your
+seat; engine revs. up to mark, and all the control cables in perfect
+condition and tension.”
+
+“Very good,” said the Pilot; and then turning to the Observer, “Before
+we start you had better have a look at the course I have mapped out.
+
+“A is where we stand and we have to reach B, a hundred and fifty miles
+due North. I judge that, at the altitude we shall fly, there will be
+an East wind, for although it is not quite East on the ground it is
+probably about twenty degrees different aloft, the wind usually moving
+round clockways to about that extent. I think that it is blowing at the
+rate of about fifty miles an hour, and I therefore take a line on the
+map to C, fifty miles due West of A. The Aeroplane's speed is a hundred
+miles an hour, and so I take a line of one hundred miles from C to D.
+Our compass course will then be in the direction A--E, which is always a
+line parallel to C--D. That is, to be exact, it will be fourteen degrees
+off the C--D course, as, in this part of the globe, there is that much
+difference between the North and South lines on the map and the magnetic
+North to which the compass needle points. If the compass has an error,
+as it may have of a few degrees, that, too, must be taken into account,
+and the deviation or error curve on the dashboard will indicate it.
+
+“The Aeroplane will then always be pointing in a direction parallel to
+A--E, but, owing to the side wind, it will be actually travelling over
+the course A--B, though in a rather sideways attitude to that course.
+
+“The distance we shall travel over the A--B course in one hour is A--D.
+That is nearly eighty-seven miles, so we ought to accomplish our journey
+of a hundred and fifty miles in about one and three-quarter hours.
+
+“I hope that's quite clear to you. It's a very simple way of calculating
+the compass course, and I always do it like that.”
+
+“Yes, that's plain enough. You have drafted what engineers call 'a
+parallelogram of forces'; but suppose you have miscalculated the
+velocity of the wind, or that it should change in velocity or
+direction?”
+
+“Well, that of course will more or less alter matters,” replies the
+Pilot. “But there are any number of good landmarks such as lakes,
+rivers, towns, and railway lines. They will help to keep us on the right
+course, and the compass will, at any rate, prevent us from going far
+astray when between them.”
+
+“Well, we'd better be off, old chap. Hop aboard.” This from the Observer
+as he climbs into the front seat from which he will command a good view
+over the lower plane; and the Pilot takes his place in the rear seat,
+and, after making himself perfectly comfortable, fixing his safety belt,
+and moving the control levers to make sure that they are working freely,
+he gives the signal to the Engine Fitter to turn the propeller and so
+start the engine.
+
+Round buzzes the Propeller, and the Pilot, giving the official signal,
+the Aeroplane is released and rolls swiftly over the ground in the teeth
+of the gusty wind.
+
+In less than fifty yards it takes to the air and begins to climb rapidly
+upwards, but how different are the conditions to the calm morning of
+yesterday! If the air were visible it would be seen to be acting in the
+most extraordinary manner; crazily swirling, lifting and dropping, gusts
+viciously colliding--a mad phantasmagoria of forces!
+
+Wickedly it seizes and shakes the Aeroplane; then tries to turn it over
+sideways; then instantly changes its mind and in a second drops it into
+a hole a hundred feet deep, and if it were not for his safety belt the
+Pilot might find his seat sinking away from beneath him.
+
+Gusts strike the front of the craft like so many slaps in the face; and
+others, with the motion of mountainous waves, sometimes lift it hundreds
+of feet in a few seconds, hoping to see it plunge over the summit in a
+death-dive--and so it goes on, but the Pilot, perfectly at one with
+his mount and instantly alert to its slightest motion, is skilfully and
+naturally making perhaps fifty movements a minute of hand and feet;
+the former lightly grasping the “joy-stick” which controls the Elevator
+hinged to the tail, and also the Ailerons or little wings hinged to the
+wing-tips; and the latter moving the Rudder control-bar.
+
+A strain on the Pilot? Not a bit of it, for this is his Work which he
+loves and excels in; and given a cool head, alert eye, and a sensitive
+touch for the controls, what sport can compare with these ever-changing
+battles of the air?
+
+The Aeroplane has all this time been climbing in great wide circles,
+and is now some three thousand feet above the Aerodrome which from such
+height looks absurdly small. The buildings below now seem quite squat;
+the hills appear to have sunk away into the ground, and the whole
+country below, cut up into diminutive fields, has the appearance of
+having been lately tidied and thoroughly spring-cleaned! A doll's
+country it looks, with tiny horses and cows ornamenting the fields
+and little model motor-cars and carts stuck on the roads, the latter
+stretching away across the country like ribbons accidentally dropped.
+
+At three thousand feet altitude the Pilot is satisfied that he is now
+sufficiently high to secure, in the event of engine failure, a
+long enough glide to earth to enable him to choose and reach a good
+landing-place; and, being furthermore content with the steady running of
+the engine, he decides to climb no more but to follow the course he has
+mapped out. Consulting the compass, he places the Aeroplane on the A--E
+course and, using the Elevator, he gives his craft its minimum angle of
+incidence at which it will just maintain horizontal flight and secure
+its maximum speed.
+
+Swiftly he speeds away, and few thoughts he has now for the changing
+panorama of country, cloud, and colour. Ever present in his mind are the
+three great 'cross-country queries. “Am I on my right course? Can I see
+a good landing-ground within gliding distance?” And “How is the Engine
+running?”
+
+Keenly both he and the Observer compare their maps with the country
+below. The roads, khaki-coloured ribbons, are easily seen but are not
+of much use, for there are so many of them and they all look alike from
+such an altitude.
+
+Now where can that lake be which the map shows so plainly? He feels that
+surely he should see it by now, and has an uncomfortable feeling that
+he is flying too far West. What pilot is there indeed who has not many
+times experienced such unpleasant sensation? Few things in the air
+can create greater anxiety. Wisely, however, he sticks to his compass
+course, and the next minute he is rewarded by the sight of the lake,
+though indeed he now sees that the direction of his travel will not take
+him over it, as should be the case if he were flying over the shortest
+route to his destination. He must have slightly miscalculated the
+velocity or direction of the side-wind.
+
+“About ten degrees off,” he mutters, and, using the Rudder, corrects his
+course accordingly.
+
+Now he feels happier and that he is well on his way. The gusts, too,
+have ceased to trouble him as, at this altitude, they are not nearly so
+bad as they were near the ground the broken surface of which does much
+to produce them; and sometimes for miles he makes but a movement or two
+of the controls.
+
+The clouds just above race by with dizzy and uniform speed; the country
+below slowly unrolls, and the steady drone of the Engine is almost
+hypnotic in effect. “Sleep, sleep, sleep,” it insidiously suggests.
+“Listen to me and watch the clouds; there's nothing else to do. Dream,
+dream, dream of speeding through space for ever, and ever, and ever; and
+rest, rest, rest to the sound of my rhythmical hum. Droning on and on,
+nothing whatever matters. All things now are merged into speed through
+space and a sleepy monotonous d-d-r-r-o-o-n-n-e - - - - -.” But the
+Pilot pulls himself together with a start and peers far ahead in search
+of the next landmark. This time it is a little country town, red-roofed
+his map tells him, and roughly of cruciform shape; and, sure enough,
+there in the right direction are the broken outlines of a few red roofs
+peeping out from between the trees.
+
+Another minute and he can see this little town, a fairy town it appears,
+nestling down between the hills with its red roofs and picturesque
+shape, a glowing and lovely contrast with the dark green of the
+surrounding moors.
+
+So extraordinarily clean and tidy it looks from such a height, and
+laid out in such orderly fashion with perfectly defined squares, parks,
+avenues, and public buildings, it indeed appears hardly real, but rather
+as if it has this very day materialized from some delightful children's
+book!
+
+Every city and town you must know has its distinct individuality to the
+Pilot's eye. Some are not fairy places at all, but great dark ugly blots
+upon the fair countryside, and with tall shafts belching forth murky
+columns of smoke to defile clean space. Others, melancholy-looking
+masses of grey, slate-roofed houses, are always sad and dispirited;
+never welcoming the glad sunshine, but ever calling for leaden skies
+and a weeping Heaven. Others again, little coquettes with village green,
+white palings everywhere, bright gravel roads, and an irrepressible air
+of brightness and gaiety.
+
+Then there are the rivers, silvery streaks peacefully winding far, far
+away to the distant horizon; they and the lakes the finest landmarks
+the Pilot can have. And the forests. How can I describe them? The trees
+cannot be seen separately, but merge altogether into enormous irregular
+dark green masses sprawling over the country, and sometimes with great
+ungainly arms half encircling some town or village; and the wind passing
+over the foliage at times gives the forest an almost living appearance,
+as of some great dragon of olden times rousing itself from slumber to
+devour the peaceful villages which its arms encircle.
+
+And the Pilot and Observer fly on and on, seeing these things and many
+others which baffle my poor skill to describe--things, dear Reader, that
+you shall see, and poets sing of, and great artists paint in the days
+to come when the Designer has captured Efficiency. Then, and the time
+is near, shall you see this beautiful world as you have never seen it
+before, the garden it is, the peace it breathes, and the wonder of it.
+
+The Pilot, flying on, is now anxiously looking for the railway line
+which midway on his journey should point the course. Ah! There it is
+at last, but suddenly (and the map at fault) it plunges into the
+earth! Well the writer remembers when that happened to him on a long
+'cross-country flight in the early days of aviation. Anxiously he
+wondered “Are tunnels always straight?” and with what relief, keeping on
+a straight course, he picked up the line again some three miles farther
+on!
+
+Now at last the Pilot sees the sea, just a streak on the north-eastern
+horizon, and he knows that his flight is two-thirds over. Indeed, he
+should have seen it before, but the air is none too clear, and he is not
+yet able to discern the river which soon should cross his path. As he
+swiftly speeds on the air becomes denser and denser with what he fears
+must be the beginning of a sea-fog, perhaps drifting inland along the
+course of the river. Now does he feel real anxiety, for it is the DUTY
+of a Pilot to fear fog, his deadliest enemy. Fog not only hides the
+landmarks by which he keeps his course, but makes the control of the
+Aeroplane a matter of the greatest difficulty. He may not realize
+it, but, in keeping his machine on an even keel, he is unconsciously
+balancing it against the horizon, and with the horizon gone he is
+lost indeed. Not only that, but it also prevents him from choosing his
+landing-place, and the chances are that, landing in a fog, he will smash
+into a tree, hedge, or building, with disastrous results. The best and
+boldest pilot 'wares a fog, and so this one, finding the conditions
+becoming worse and yet worse, and being forced to descend lower and
+lower in order to keep the earth within view, wisely decides to choose a
+landing-place while there is yet time to do so.
+
+Throttling down the power of the engine he spirals downwards, keenly
+observing the country below. There are plenty of green fields to lure
+him, and his great object is to avoid one in which the grass is long,
+for that would bring his machine to a stop so suddenly as to turn it
+over; or one of rough surface likely to break the under-carriage. Now is
+perfect eyesight and a cool head indispensable. He sees and decides upon
+a field and, knowing his job, he sticks to that field with no change
+of mind to confuse him. It is none too large, and gliding just over the
+trees and head on to the wind he skilfully “stalls” his machine; that
+is, the speed having decreased sufficiently to avoid such a manoeuvre
+resulting in ascent, he, by means of the Elevator, gives the Aeroplane
+as large an angle of incidence as possible, and the undersides of the
+planes meeting the air at such a large angle act as an air-brake, and
+the Aeroplane, skimming over the ground, lessens its speed and finally
+stops just at the farther end of the field.
+
+Then, after driving the Aeroplane up to and under the lee of the hedge,
+he stops the engine, and quickly lashing the joy-stick fast in order
+to prevent the wind from blowing the controlling surfaces about and
+possibly damaging them, he hurriedly alights. Now running to the tail he
+lifts it up on to his shoulder, for the wind has become rough indeed and
+there is danger of the Aeroplane becoming unmanageable. By this action
+he decreases the angle at which the planes are inclined to the wind and
+so minimizes the latter's effect upon them. Then to the Observer, “Hurry
+up, old fellow, and try to find some rope, wire, or anything with which
+to picket the machine. The wind is rising and I shan't be able to hold
+the 'bus steady for long. Don't forget the wire-cutters. They're in the
+tool kit.” And the Observer rushes off in frantic haste, before long
+triumphantly returning with a long length of wire from a neighbouring
+fence. Blocking up the tail with some debris at hand, they soon succeed,
+with the aid of the wire, in stoutly picketing the Aeroplane to the
+roots of the high hedge in front of it; done with much care, too,
+so that the wire shall not fray the fabric or set up dangerous
+bending-stresses in the woodwork. Their work is not done yet, for the
+Observer remarking, “I don't like the look of this thick weather and
+rather fear a heavy rain-storm,” the Pilot replies, “Well, it's a
+fearful bore, but the first rule of our game is never to take an
+unnecessary risk, so out with the engine and body covers.”
+
+Working with a will they soon have the engine and the open part of the
+body which contains the seats, controls, and instruments snugly housed
+with their waterproof covers, and the Aeroplane is ready to weather the
+possible storm.
+
+Says the Observer, “I'm remarkably peckish, and methinks I spy the
+towers of one of England's stately homes showing themselves just beyond
+that wood, less than a quarter of a mile away. What ho! for a raid. What
+do you say?”
+
+“All right, you cut along and I'll stop here, for the Aeroplane must not
+be left alone. Get back as quickly as possible.”
+
+And the Observer trots off, leaving the Pilot filling his pipe and
+anxiously scrutinizing the weather conditions. Very thick it is now, but
+the day is yet young, and he has hopes of the fog lifting sufficiently
+to enable the flight to be resumed. A little impatiently he awaits the
+return of his comrade, but with never a doubt of the result, for the
+hospitality of the country house is proverbial among pilots! What old
+hand among them is there who cannot instance many a forced landing made
+pleasant by such hospitality? Never too late or too early to help with
+food, petrol, oil, tools, and assistants. Many a grateful thought has
+the writer for such kind help given in the days before the war (how long
+ago they seem!), when aeroplanes were still more imperfect than they are
+now, and involuntary descents often a part of 'cross-country flying.
+
+Ah! those early days! How fresh and inspiring they were! As one started
+off on one's first 'cross-country flight, on a machine the first of its
+design, and with everything yet to learn, and the wonders of the air yet
+to explore; then the joy of accomplishment, the dreams of Efficiency,
+the hard work and long hours better than leisure; and what a field of
+endeavour--the realms of space to conquer! And the battle still goes on
+with ever-increasing success. Who is bold enough to say what its limits
+shall be?
+
+So ruminates this Pilot-Designer, as he puffs at his pipe, until his
+reverie is abruptly disturbed by the return of the Observer.
+
+“Wake up, you AIRMAN,” the latter shouts. “Here's the very thing the
+doctor ordered! A basket of first-class grub and something to keep the
+fog out, too.”
+
+“Well, that's splendid, but don't call me newspaper names or you'll
+spoil my appetite!”
+
+Then, with hunger such as only flying can produce, they appreciatively
+discuss their lunch, and with many a grateful thought for the
+donors--and they talk shop. They can't help it, and even golf is a poor
+second to flight talk. Says the Pilot, who must have his grievance,
+“Just observe where I managed to stop the machine. Not twenty feet from
+this hedge! A little more and we should have been through it and into
+Kingdom Come! I stalled as well as one could, but the tail touched
+the ground and so I could not give the Aeroplane any larger angle of
+incidence. Could I have given it a larger angle, then the planes would
+have become a much more effective air-brake, and we should have come to
+rest in a much shorter distance. It's all the fault of the tail. There's
+hardly a type of Aeroplane in existence in which the tail could not be
+raised several feet, and that would make all the difference. High tails
+mean a large angle of incidence when the machine touches ground and,
+with enough angle, I'll guarantee to safely land the fastest machine in
+a five-acre field. You can, I am sure, imagine what a difference that
+would make where forced landings are concerned!” Then rapidly sketching
+in his notebook, he shows the Observer the following illustration:
+
+“That's very pretty,” said the Observer, “but how about Mechanical
+Difficulties, and Efficiency in respect of Flight? And, anyway, why
+hasn't such an obvious thing been done already?”
+
+“As regards the first part of your question I assure you that there's
+nothing in it, and I'll prove it to you as follows----”
+
+“Oh! That's all right, old chap. I'll take your word for it,” hurriedly
+replies the Observer, whose soul isn't tuned to a technical key.
+
+“As regards the latter part of your inquiry,” went on the Pilot, a
+little nettled at having such a poor listener, “it's very simple.
+Aeroplanes have 'just growed' like Topsy, and they consequently contain
+this and many another relic of early day design when Aeroplanes were
+more or less thrown together and anything was good enough that could get
+off the ground.”
+
+“By Jove,” interrupts the Observer, “I do believe the fog is lifting.
+Hadn't we better get the engine and body covers off, just in case it's
+really so?”
+
+“I believe you're right. I am sure those hills over there could not
+be seen a few minutes ago, and look--there's sunshine over there. We'd
+better hurry up.”
+
+Ten minutes' hard work and the covers are off, neatly folded and stowed
+aboard; the picketing wires are cast adrift, and the Pilot is once more
+in his seat. The Aeroplane has been turned to face the other end of the
+field, and, the Observer swinging round the propeller, the engine is
+awake again and slowly ticking over. Quickly the Observer climbs into
+his seat in front of the Pilot, and, the latter slightly opening the
+throttle, the Aeroplane leisurely rolls over the ground towards the
+other end of the field, from which the ascent will be made.
+
+Arriving there the Pilot turns the Aeroplane in order to face the wind
+and thus secure a quick “get-off.” Then he opens the throttle fully and
+the mighty voice of the Engine roars out “Now see me clear that hedge!”
+ and the Aeroplane races forward at its minimum angle of incidence. Tail
+up, and with ever-increasing speed, it rushes towards the hedge under
+the lee of which it has lately been at rest; and then, just as the
+Observer involuntarily pulls back an imaginary “joy-stick,” the Pilot
+moves the real one and places the machine at its best climbing angle.
+Like a living thing it responds, and instantly leaves the ground,
+clearing the hedge like a--well, like an Aeroplane with an excellent
+margin of lift. Upwards it climbs with even and powerful lift, and the
+familiar scenes below again gladden the eyes of the Pilot. Smaller and
+more and more squat grow the houses and hills; more and more doll-like
+appear the fields which are clearly outlined by the hedges; and soon the
+country below is easily identified with the map. Now they can see the
+river before them and a bay of the sea which must be crossed or skirted.
+The fog still lingers along the course of the river and between the
+hills, but is fast rolling away in grey, ghost-like masses. Out to sea
+it obscures the horizon, making it difficult to be sure where water ends
+and fog begins, and creating a strange, rather weird effect by which
+ships at a certain distance appear to be floating in space.
+
+Now the Aeroplane is almost over the river, and the next instant it
+suddenly drops into a “hole in the air.” With great suddenness it
+happens, and for some two hundred feet it drops nose-down and tilted
+over sideways; but the Pilot is prepared and has put his craft on an
+even keel in less time than it takes to tell you about it; for well he
+knows that he must expect such conditions when passing over a shore
+or, indeed, any well-defined change in the composition of the earth's
+surface. Especially is this so on a hot and sunny day, for then the warm
+surface of the earth creates columns of ascending air, the speed of the
+ascent depending upon the composition of the surface. Sandy soil, for
+instance, such as borders this river produces a quickly ascending column
+of air, whereas water and forests have not such a marked effect. Thus,
+when our Aeroplane passed over the shore of the river, it suddenly lost
+the lift due to the ascending air produced by the warm sandy soil, and
+it consequently dropped just as if it had fallen into a hole.
+
+Now the Aeroplane is over the bay and, the sea being calm, the Pilot
+looks down, down through the water, and clearly sees the bottom,
+hundreds of feet below the surface. Down through the reflection of the
+blue sky and clouds, and one might think that is all, but it isn't. Only
+those who fly know the beauties of the sea as viewed from above;
+its dappled pearly tints; its soft dark blue shadows; the beautiful
+contrasts of unusual shades of colour which are always differing and
+shifting with the changing sunshine and the ever moving position of the
+aerial observer. Ah! for some better pen than mine to describe these
+things! One with glowing words and a magic rhythm to express the wonders
+of the air and the beauty of the garden beneath--the immensity of the
+sea--the sense of space and of one's littleness there--the realization
+of the Power moving the multitudes below--the exaltation of spirit
+altitude produces--the joy of speed. A new world of sensation!
+
+Now the bay is almost crossed and the Aerodrome at B can be
+distinguished.
+
+On the Aerodrome is a little crowd waiting and watching for the arrival
+of the Aeroplane, for it is of a new and improved type and its first
+'cross-country performance is of keen interest to these men; men who
+really know something about flight.
+
+There is the Squadron Commander who has done some real flying in
+his time; several well-seasoned Flight-Commanders; a dozen or more
+Flight-Lieutenants; a knowledgeable Flight-Sergeant; a number of Air
+Mechanics, and, a little on one side and almost unnoticed, the Designer.
+
+“I hope they are all right,” said someone, “and that they haven't had
+difficulties with the fog. It rolled up very quickly, you know.”
+
+“Never fear,” remarked a Flight-Commander. “I know the Pilot well and
+he's a good 'un; far too good to carry on into a fog.”
+
+“They say the machine is really something out of the ordinary,” said
+another, “and that, for once, the Designer has been allowed full play;
+that he hasn't been forced to unduly standardize ribs, spars, struts,
+etc., and has more or less had his own way. I wonder who he is. It seems
+strange we hear so little of him.”
+
+“Ah! my boy. You do a bit more flying and you'll discover that things
+are not always as they appear from a distance!”
+
+“There she is, sir!” cries the Flight-Sergeant. “Just a speck over the
+silvery corner of that cloud.”
+
+A tiny speck it looks, some six miles distant and three thousand feet
+high; but, racing along, it rapidly appears larger and soon its outlines
+can be traced and the sunlight be seen playing upon the whirling
+propeller.
+
+Now the distant drone of the engine can be heard, but not for long, for
+suddenly it ceases and, the nose of the Aeroplane sinking, the craft
+commences gliding downwards.
+
+“Surely too far away,” says a subaltern. “It will be a wonderful machine
+if, from that distance and height, it can glide into the Aerodrome.”
+ And more than one express the opinion that it cannot be done; but
+the Designer smiles to himself, yet with a little anxiety, for his
+reputation is at stake, and Efficiency, the main reward he desires, is
+perhaps, or perhaps not, at last within his grasp!
+
+Swiftly the machine glides downwards towards them, and it can now be
+seen how surprisingly little it is affected by the rough weather and
+gusts; so much so that a little chorus of approval is heard.
+
+“Jolly good gliding angle,” says someone; and another, “Beautifully
+quick controls, what?” and from yet another, “By Jove! The Pilot must be
+sure of the machine. Look, he's stopped the engine entirely.”
+
+Then the Aeroplane with noiseless engine glides over the boundary of the
+Aerodrome, and, with just a soft soughing sound from the air it cleaves,
+lands gently not fifty yards from the onlookers.
+
+“Glad to see you,” says the Squadron Commander to the Pilot. “How do you
+like the machine?” And the Pilot replies:
+
+“I never want a better one, sir. It almost flies itself!”
+
+And the Designer turns his face homewards and towards his beloved
+drawing-office; well satisfied, but still dreaming dreams of the future
+and... looking far ahead whom should he see but Efficiency at last
+coming towards him! And to him she is all things. In her hair is the
+morning sunshine; her eyes hold the blue of the sky, and on her cheeks
+is the pearly tint of the clouds as seen from above. The passion of
+speed, the lure of space, the sense of power, and the wonder of the
+future... all these things she holds for him.
+
+“Ah!” he cries. “You'll never leave me now, when at last there is no one
+between us?”
+
+And Efficiency, smiling and blushing, but practical as ever, says:
+
+“And you will never throw those Compromises in my face?”
+
+“My dear, I love you for them! Haven't they been my life ever since I
+began striving for you ten long years ago?”
+
+And so they walked off very happily, arm-in-arm together; and if this
+hasn't bored you and you'd like some more of the same sort of thing, I'd
+just love to tell you some day of the wonderful things they accomplish
+together, and of what they dream the future holds in store.
+
+And that's the end of the Prologue.
+
+
+
+
+CHAPTER I. FLIGHT
+
+Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum.
+It therefore obeys Newton's laws[14] and resists movement. It is that
+resistance or reaction which makes flight possible.
+
+Flight is secured by driving through the air a surface[15] inclined
+upwards and towards the direction of motion.
+
+S = Side view of surface.
+
+M = Direction of motion.
+
+CHORD.--The Chord is, for practical purposes, taken to be a straight
+line from the leading edge of the surface to its trailing edge.
+
+N = A line through the surface starting from its trailing edge. The
+position of this line, which I call the Neutral Lift Line, is found by
+means of wind-tunnel research, and it varies with differences in
+the camber (curvature) of surfaces. In order to secure flight, the
+inclination of the surface must be such that the neutral lift line makes
+an angle with and ABOVE the line of motion. If it is coincident with M,
+there is no lift. If it makes an angle with M and BELOW it, then there
+is a pressure tending to force the surface down.
+
+I = Angle of Incidence. This angle is generally defined as the angle the
+chord makes with the direction of motion, but that is a bad definition,
+as it leads to misconception. The angle of incidence is best described
+as the angle the neutral lift line makes with the direction of motion
+relative to the air. You will, however, find that in nearly all rigging
+specifications the angle of incidence is taken to mean the angle the
+chord makes with a line parallel to the propeller thrust. This is
+necessary from the point of view of the practical mechanic who has to
+rig the aeroplane, for he could not find the neutral lift line, whereas
+he can easily find the chord. Again, he would certainly be in doubt as
+to “the direction of motion relative to the air,” whereas he can easily
+find a line parallel to the propeller thrust. It is a pity, however,
+that these practical considerations have resulted in a bad definition
+of the angle of incidence becoming prevalent, a consequence of which has
+been the widespread fallacy that flight may be secured with a negative
+inclination of the surface. Flight may conceivably be secured with a
+negative angle of chord, but never with a negative inclination of the
+surface. All this is only applicable to cambered surfaces. In the case
+of flat surfaces the neutral lift line coincides with the chord and the
+definition I have criticised adversely is then applicable. Flat lifting
+surfaces are, however, never used.
+
+The surface acts upon the air in the following manner:
+
+
+As the bottom of the surface meets the air, it compresses it and
+accelerates it DOWNWARDS. As a result of this definite action there is,
+of course, an equal and opposite reaction UPWARDS.
+
+The top surface, in moving forward, tends to leave the air behind
+it, thus creating a semi-vacuum or rarefied area over the top of the
+surface. Consequently the pressure of air on the top of the surface
+is decreased, thus assisting the reaction below to lift the surface
+UPWARDS.
+
+The reaction increases approximately as the square of the velocity. It
+is the result of (1) the mass of air engaged, and (2) the velocity and
+consequent force with which the surface engages the air. If the reaction
+was produced by only one of those factors it would increase in direct
+proportion to the velocity, but, since it is the product of both
+factors, it increases as V(2S).
+
+Approximately three-fifths of the reaction is due to the decrease of
+density (and consequent decrease of downward pressure) on the top of the
+surface; and only some two-fifths is due to the upward reaction secured
+by the action of the bottom surface upon the air. A practical point in
+respect of this is that, in the event of the fabric covering the surface
+getting into bad condition, it is more likely to strip off the top than
+off the bottom.
+
+The direction of the reaction is approximately at right-angles to the
+chord of the surface, as illustrated above; and it is, in considering
+flight, convenient to divide it into two component parts or values,
+thus:
+
+1. The vertical component of the reaction, i.e., Lift, which is opposed
+to Gravity, i.e., the weight of the aeroplane.
+
+2. The horizontal component, i.e., Drift (sometimes called Resistance),
+to which is opposed the thrust of the propeller.
+
+The direction of the reaction is, of course, the resultant of the forces
+Lift and Drift.
+
+The Lift is the useful part of the reaction, for it lifts the weight of
+the aeroplane.
+
+The Drift is the villain of the piece, and must be overcome by the
+Thrust in order to secure the necessary velocity to produce the
+requisite Lift for flight.
+
+DRIFT.--The drift of the whole aeroplane (we have considered only the
+lifting surface heretofore) may be conveniently divided into three
+parts, as follows:
+
+Active Drift, which is the drift produced by the lifting surfaces.
+
+Passive Drift, which is the drift produced by all the rest of the
+aeroplane--the struts, wires, fuselage, under-carriage, etc., all of
+which is known as “detrimental surface.”
+
+Skin Friction, which is the drift produced by the friction of the air
+with roughnesses of surface. The latter is practically negligible
+having regard to the smooth surface of the modern aeroplane, and its
+comparatively slow velocity compared with, for instance, the velocity of
+a propeller blade.
+
+LIFT-DRIFT RATIO.--The proportion of lift to drift is known as the
+lift-drift ratio, and is of paramount importance, for it expresses the
+efficiency of the aeroplane (as distinct from engine and propeller). A
+knowledge of the factors governing the lift-drift ratio is, as will be
+seen later, an absolute necessity to anyone responsible for the rigging
+of an aeroplane, and the maintenance of it in an efficient and safe
+condition.
+
+Those factors are as follows:
+
+1. Velocity.--The greater the velocity the greater the proportion of
+drift to lift, and consequently the less the efficiency. Considering
+the lifting surfaces alone, both the lift and the (active) drift, being
+component parts of the reaction, increase as the square of the velocity,
+and the efficiency remains the same at all speeds. But, considering the
+whole aeroplane, we must remember the passive drift. It also increases
+as the square of the velocity (with no attendant lift), and, adding
+itself to the active drift, results in increasing the proportion of
+total drift (active + passive) to lift.
+
+But for the increase in passive drift the efficiency of the aeroplane
+would not fall with increasing velocity, and it would be possible, by
+doubling the thrust, to approximately double the speed or lift--a happy
+state of affairs which can never be, but which we may, in a measure,
+approach by doing everything possible to diminish the passive drift.
+
+Every effort is then made to decrease it by “stream-lining,” i.e., by
+giving all “detrimental” parts of the aeroplane a form by which they
+will pass through the air with the least possible drift. Even the wires
+bracing the aeroplane together are, in many cases, stream-lined, and
+with a markedly good effect upon the lift-drift ratio. In the case of a
+certain well-known type of aeroplane the replacing of the ordinary wires
+by stream-lined wires added over five miles an hour to the flight speed.
+
+Head-resistance is a term often applied to passive drift, but it is apt
+to convey a wrong impression, as the drift is not nearly so much the
+result of the head or forward part of struts, wires, etc., as it is of
+the rarefied area behind.
+
+Above is illustrated the flow of air round two objects moving in the
+direction of the arrow M.
+
+In the case of A, you will note that the rarefied area DD is of very
+considerable extent; whereas in the case of B, the air flows round it
+in such a way as to meet very closely to the rear of the object, thus
+DECREASING DD.
+
+The greater the rarefied area DD. then, the less the density, and,
+consequently, the less the pressure of air upon the rear of the object.
+The less such pressure, then, the better is head-resistance D able to
+get its work in, and the more thrust will be required to overcome it.
+
+The “fineness” of the stream-line shape, i.e., the proportion of length
+to width, is determined by the velocity--the greater the velocity, the
+greater the fineness. The best degree of fineness for any given velocity
+is found by means of wind-tunnel research.
+
+The practical application of all this is, from a rigging point of view,
+the importance of adjusting all stream-line parts to be dead-on in the
+line of flight, but more of that later on.
+
+2. Angle of Incidence.--The most efficient angle of incidence varies
+with the thrust at the disposal of the designer, the weight to be
+carried, and the climb-velocity ratio desired.
+
+The best angles of incidence for these varying factors are found by
+means of wind-tunnel research and practical trial and error. Generally
+speaking, the greater the velocity the smaller should be the angle of
+incidence, in order to preserve a clean, stream-line shape of rarefied
+area and freedom from eddies. Should the angle be too great for the
+velocity, then the rarefied area becomes of irregular shape with
+attendant turbulent eddies. Such eddies possess no lift value, and since
+it has taken power to produce them, they represent drift and adversely
+affect the lift-drift ratio.
+
+From a rigging point of view, one must presume that every standard
+aeroplane has its lifting surface set at the most efficient angle, and
+the practical application of all this is in taking the greatest possible
+care to rig the surface at the correct angle and to maintain it at such
+angle. Any deviation will adversely affect the lift-drift ratio, i.e.,
+the efficiency.
+
+3. Camber.--(Refer to the second illustration in this chapter.) The
+lifting surfaces are cambered, i.e., curved, in order to decrease the
+horizontal component of the reaction, i.e., the drift.
+
+The bottom camber: If the bottom of the surface was flat, every particle
+of air meeting it would do so with a shock, and such shock would produce
+a very considerable horizontal reaction or drift. By curving it such
+shock is diminished, and the curve should be such as to produce a
+uniform (not necessarily constant) acceleration and compression of the
+air from the leading edge to the trailing edge. Any unevenness in the
+acceleration and compression of the air produces drift.
+
+The top camber: If this was flat it would produce a rarefied area of
+irregular shape. I have already explained the bad effect this has
+upon the lift-drift ratio. The top surface is then curved to produce a
+rarefied area the shape of which shall be as stream-line and free from
+attendant eddies as possible.
+
+The camber varies with the angle of incidence, the velocity, and the
+thickness of the surface. Generally speaking, the greater the velocity,
+the less the camber and angle of incidence. With infinite velocity the
+surface would be set at no angle of incidence (the neutral lift line
+coincident with the direction of motion relative to the air), and would
+be, top and bottom, of pure streamline form--i.e., of infinite fineness.
+This is, of course, carrying theory to absurdity as the surface would
+then cease to exist.
+
+The best cambers for varying velocities, angles of incidence, and
+thicknesses of surface, are found by means of wind-tunnel research.
+The practical application of all this is in taking the greatest care to
+prevent the surface from becoming distorted and thus spoiling the camber
+and consequently the lift-drift ratio.
+
+4. Aspect Ratio.--This is the proportion of span to chord. Thus, if the
+span is, for instance, 50 feet and the chord 5 feet, the surface would
+be said to have an aspect ratio of 10 to 1.
+
+For A GIVEN VELOCITY and A GIVEN AREA of surface, the greater the
+aspect ratio, the greater the reaction. It is obvious, I think, that the
+greater the span, the greater the mass of air engaged, and, as already
+explained, the reaction is partly the result of the mass of air engaged.
+
+Not only that, but, PROVIDED the chord is not decreased to an extent
+making it impossible to secure the best camber owing to the thickness
+of the surface, the greater the aspect ratio, the better the lift-drift
+ratio. The reason of this is rather obscure. It is sometimes advanced
+that it is owing to the “spill” of air from under the wing-tips. With
+a high aspect ratio the chord is less than would otherwise be the case.
+Less chord results in smaller wing-tips and consequently less “spill.”
+ This, however, appears to be a rather inadequate reason for the high
+aspect ratio producing the high lift-drift ratio. Other reasons are also
+advanced, but they are of such a contentious nature I do not think it
+well to go into them here. They are of interest to designers, but this
+is written for the practical pilot and rigger.
+
+5. Stagger.--This is the advancement of the top surface relative to the
+bottom surface, and is not, of course, applicable to a single surface,
+i.e., a monoplane. In the case of a biplane having no stagger, there
+will be “interference” and consequent loss of Efficiency unless the
+gap between the top and bottom surfaces is equal to not less than 1 1/2
+times the chord. If less than that, the air engaged by the bottom of the
+top surface will have a tendency to be drawn into the rarefied area over
+the top of the bottom surface, with the result that the surfaces will
+not secure as good a reaction as would otherwise be the case.
+
+It is not practicable to have a gap of much more than a distance equal
+to the chord, owing to the drift produced by the great length of struts
+and wires such a large gap would necessitate. By staggering the top
+surface forward, however, it is removed from the action of the lower
+surface and engages undisturbed air, with the result that the efficiency
+can in this way be increased by about 5 per cent. Theoretically the top
+plane should be staggered forward for a distance equal to about 30 per
+cent. of the chord, the exact distance depending upon the velocity
+and angle of incidence; but this is not always possible to arrange
+in designing an aeroplane, owing to difficulties of balance, desired
+position, and view of pilot, observer, etc.
+
+6. Horizontal Equivalent.--The vertical component of the reaction, i.e.,
+lift, varies as the horizontal equivalent (H.E.) of the surface, but
+the drift remains the same. Then it follows that if H.E. grows less, the
+ratio of lift to drift must do the same.
+
+A, B, and C are front views of three surfaces.
+
+A has its full H.E., and therefore, from the point of view from which
+we are at the moment considering efficiency, it has its best lift-drift
+ratio.
+
+B and C both possess the same surface as A, but one is inclined upwards
+from its centre and the other is straight but tilted. For these reasons
+their H.E.'s are, as illustrated, less than in the case of A. That means
+less vertical lift, and, the drift remaining the same (for there is
+the same amount of surface as in A to produce it), the lift-drift ratio
+falls.
+
+THE MARGIN OF POWER is the power available above that necessary to
+maintain horizontal flight.
+
+THE MARGIN OF LIFT is the height an aeroplane can gain in a given time
+and starting from a given altitude. As an example, thus: 1,000 feet the
+first minute, and starting from an altitude of 500 feet above sea-level.
+
+The margin of lift decreases with altitude, owing to the decrease in
+the density of the air, which adversely affects the engine. Provided
+the engine maintained its impulse with altitude, then, if we ignore the
+problem of the propeller, which I will go into later on, the margin of
+lift would not disappear. Moreover, greater velocity for a given power
+would be secured at a greater altitude, owing to the decreased density
+of air to be overcome. After reading that, you may like to light your
+pipe and indulge in dreams of the wonderful possibilities which may
+become realities if some brilliant genius shows us some day how to
+secure a constant power with increasing altitude. I am afraid, however,
+that will always remain impossible; but it is probable that some very
+interesting steps may be taken in that direction.
+
+THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, for
+a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, for
+a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-drift
+ratio is highest. In modern aeroplanes it is that angle of incidence
+possessed by the surface when the axis of the propeller is horizontal.
+
+THE BEST CLIMBING ANGLE is approximately half-way between the maximum
+and the optimum angles.
+
+All present-day aeroplanes are a compromise between Climb and horizontal
+Velocity. We will compare the essentials for two aeroplanes, one
+designed for maximum climb, and the other for maximum velocity.
+
+
+ESSENTIALS FOR MAXIMUM CLIMB:
+
+1. Low velocity, in order to secure the best lift-drift ratio.
+
+2. Having a low velocity, a large surface will be necessary in order to
+engage the necessary mass of air to secure the requisite lift.
+
+3. Since (1) such a climbing machine will move along an upward sloping
+path, and (2) will climb with its propeller thrust horizontal, then a
+large angle relative to the direction of the thrust will be necessary in
+order to secure the requisite angle relative to the direction of motion.
+
+The propeller thrust should be always horizontal, because the most
+efficient flying-machine (having regard to climb OR velocity) has, so
+far, been found to be an arrangement of an inclined surface driven by
+a HORIZONTAL thrust--the surface lifting the weight, and the thrust
+overcoming the drift. This is, in practice, a far more efficient
+arrangement than the helicopter, i.e., the air-screw revolving about
+a vertical axis and producing a thrust opposed to gravity. If, when
+climbing, the propeller thrust is at such an angle as to tend to haul
+the aeroplane upwards, then it is, in a measure, acting as a helicopter,
+and that means inefficiency. The reason of a helicopter being
+inefficient in practice is due to the fact that, owing to mechanical
+difficulties, it is impossible to construct within a reasonable weight
+an air-screw of the requisite dimensions. That being so, it would be
+necessary, in order to absorb the power of the engine, to revolve the
+comparatively small-surfaced air screw at an immensely greater velocity
+than that of the aeroplane's surface. As already explained, the
+lift-drift ratio falls with velocity on account of the increase in
+passive drift. This applies to a blade of a propeller or air-screw,
+which is nothing but a revolving surface set at angle of incidence, and
+which it is impossible to construct without a good deal of detrimental
+surface near the central boss.
+
+4. The velocity being low, then it follows that for that reason also the
+angle of incidence should be comparatively large.
+
+5. Camber.--Since such an aeroplane would be of low velocity, and
+therefore possess a large angle of incidence, a large camber would be
+necessary.
+
+Let us now consider the essentials for an aeroplane of maximum velocity
+for its power, and possessing merely enough lift to get off the ground,
+but no margin of lift.
+
+1. Comparatively HIGH VELOCITY.
+
+2. A comparatively SMALL SURFACE, because, being of greater velocity
+than the maximum climber, a greater mass of air will be engaged for
+a given surface and time, and therefore a smaller surface will be
+sufficient to secure the requisit lift.
+
+3. A small angle relative to the propeller thrust, since the latter
+coincides with the direction of motion.
+
+4. A comparatively small angle of incidence by reason of the high
+velocity.
+
+5. A comparatively small camber follows as a result of the small angle
+of incidence.
+
+
+SUMMARY.
+
+   Essentials for Maximum            Essentials for Maximum
+           Climb.                            Velocity
+
+   1. Low velocity.                  High velocity.
+   2. Large surface.                 Small surface.
+   3. Large angle relative to        Small angle relative to
+      propeller thrust.                 propeller thrust.
+   4. Large angle relative to        Small angle relative to direction
+      direction of motion.              of motion.
+   5. Large camber.                  Small camber.
+
+
+It is mechanically impossible to construct an aeroplane of reasonable
+weight of which it would be possible to very the above opposing
+essentials. Therefore, all aeroplanes are designed as a compromise
+between Climb and Velocity.
+
+As a rule aeroplanes are designed to have at low altitude a slight
+margin of lift when the propeller thrust is horizontal.
+
+
+ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE DESIGNED
+AS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIGHT
+MARGIN OF LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS HORIZONTAL
+
+MINIMUM ANGLE.
+
+This gives the greatest velocity during horizontal flight at a low
+altitude. Greater velocity would be secured if the surface, angle, and
+camber were smaller and designed to just maintain horizontal flight
+with a horizontal thrust. Also, in such case, the propeller would not
+be thrusting downwards, but along a horizontal line which is obviously
+a more efficient arrangement if we regard the aeroplane merely from one
+point of view, i.e., either with reference to velocity OR climb.
+
+OPTIMUM ANGLE (Thrust horizontal)
+
+The velocity is less than at the smaller minimum angle, and, as
+aeroplanes are designed to-day, the area and angle of incidence of the
+surface is such as to secure a slight ascent at a low altitude. The
+camber of the surface is designed for this angle of incidence and
+velocity. The lift-drift ratio is best at this angle.
+
+BEST CLIMBING ANGLE
+
+The velocity is now still less by reason of the increased angle
+producing increase of drift. Less velocity at A GIVEN ANGLE produces
+less lift, but the increased angle more or less offsets the loss of
+lift due to the decreased velocity, and in addition, the thrust is now
+hauling the aeroplane upwards.
+
+MAXIMUM ANGLE
+
+The greater angle has now produced so much drift as to lessen the
+velocity to a point where the combined lifts from the surface and from
+the thrust are only just able to maintain horizontal flight. Any greater
+angle will result in a still lower lift-drift ratio. The lift will then
+become less than the weight and the aeroplane will consequently fall.
+Such a fall is known as “stalling” or “pancaking.”
+
+NOTE.--The golden rule for beginners: Never exceed the Best Climbing
+Angle. Always maintain the flying speed of the aeroplane.
+
+
+By this means, when the altitude is reached where the margin of
+lift disappears (on account of loss of engine power), and which is,
+consequently, the altitude where it is just possible to maintain
+horizontal flight, the aeroplane is flying with its thrust horizontal
+and with maximum efficiency (as distinct from engine and propeller
+efficiency).
+
+The margin of lift at low altitude, and when the thrust is horizontal,
+should then be such that the higher altitude at which the margin of lift
+is lost is that altitude at which most of the aeroplane's horizontal
+flight work is done. That ensures maximum velocity when most required.
+
+Unfortunately, where aeroplanes designed for fighting are concerned, the
+altitude where most of the work is done is that at which both maximum
+velocity and maximum margin of lift for power are required.
+
+Perhaps some day a brilliant inventor will design an aeroplane of
+reasonable weight and drift of which it will be possible for the pilot
+to vary at will the above-mentioned opposing essentials. Then we shall
+get maximum velocity, or maximum margin of lift, for power as required.
+Until then the design of the aeroplane must remain a compromise between
+Velocity and Climb.
+
+
+
+
+CHAPTER II. STABILITY AND CONTROL
+
+STABILITY is a condition whereby an object disturbed has a natural
+tendency to return to its first and normal position. Example: a weight
+suspended by a cord.
+
+INSTABILITY is a condition whereby an object disturbed has a natural
+tendency to move as far as possible away from its first position, with
+no tendency to return. Example: a stick balanced vertically upon your
+finger.
+
+NEUTRAL INSTABILITY is a condition whereby an object disturbed has no
+tendency to move farther than displaced by the force of the disturbance,
+and no tendency to return to its first position.
+
+In order that an aeroplane may be reasonably controllable, it is
+necessary for it to possess some degree of stability longitudinally,
+laterally, and directionally.
+
+LONGITUDINAL STABILITY in an aeroplane is its stability about an axis
+transverse to the direction of normal horizontal flight, and without
+which it would pitch and toss.
+
+LATERAL STABILITY is its stability about its longitudinal axis, and
+without which it would roll sideways.
+
+DIRECTIONAL STABILITY is its stability about its vertical axis, and
+without which it would have no tendency to keep its course.
+
+For such directional stability to exist there must be, in effect,[16]
+more “keel-surface” behind the vertical axis than there is in front of
+it. By keel-surface I mean every-thing to be seen when looking at an
+aeroplane from the side of it--the sides of the body, undercarriage,
+struts, wires, etc. The same thing applies to a weathercock. You know
+what would happen if there was insufficient keel-surface behind the
+vertical axis upon which it is pivoted. It would turn off its proper
+course, which is opposite to the direction of the wind. It is very much
+the same in the case of an aeroplane.
+
+The above illustration represents an aeroplane (directionally stable)
+flying along the course B. A gust striking it as indicated acts upon the
+greater proportion of keel-surface behind the turning axis and throws it
+into the new course. It does not, however, travel along the new course,
+owing to its momentum in the direction B. It travels, as long as such
+momentum lasts, in a direction which is the resultant of the two forces
+Thrust and Momentum. But the centre line of the aeroplane is pointing in
+the direction of the new course. Therefore its attitude, relative to
+the direction of motion, is more or less sideways, and it consequently
+receives an air pressure in the direction C. Such pressure, acting upon
+the keel-surface, presses the tail back towards its first position in
+which the aeroplane is upon its course B.
+
+What I have described is continually going on during flight, but in
+a well-designed aeroplane such stabilizing movements are, most of the
+time, so slight as to be imperceptible to the pilot.
+
+If an aeroplane was not stabilized in this way, it would not only be
+continually trying to leave its course, but it would also possess a
+dangerous tendency to “nose away” from the direction of the side gusts.
+In such case the gust shown in the above illustration would turn the
+aeroplane round the opposite way a very considerable distance; and the
+right wing, being on the outside of the turn, would travel with greater
+velocity than the left wing. Increased velocity means increased lift;
+and so, the right wing lifting, the aeroplane would turn over sideways
+very quickly.
+
+LONGITUDINAL STABILITY.--Flat surfaces are longitudinally stable owing
+to the fact that with decreasing angles of incidence the centre line of
+pressure (C.P.) moves forward.
+
+The C.P. is a line taken across the surface, transverse to the direction
+of motion, and about which all the air forces may be said to balance, or
+through which they may be said to act.
+
+
+Imagine A to be a flat surface, attitude vertical, travelling through
+the air in the direction of motion M. Its C.P. is then obviously along
+the exact centre line of the surface as illustrated.
+
+In B, C, and D the surfaces are shown with angles of incidence
+decreasing to nothing, and you will note that the C.P. moves forward
+with the decreasing angle.
+
+Now, should some gust or eddy tend to make the surface decrease the
+angle, i.e., dive, then the C.P. moves forward and pushes the front of
+the surface up. Should the surface tend to assume too large an angle,
+then the reverse happens--the C.P. moves back and pushes the rear of the
+surface up.
+
+Flat surfaces are, then, theoretically stable longitudinally. They are
+not, however, used, on account of their poor lift-drift ratio.
+
+As already explained, cambered surfaces are used, and these are
+longitudinally unstable at those angles of incidence producing a
+reasonable lift-drift ratio, i.e., at angles below: about 12 degrees.
+
+A is a cambered surface, attitude approximately vertical, moving through
+the air in the direction M. Obviously the C. P. coincides with the
+transverse centre line of the surface.
+
+With decreasing angles, down to angles of about 30 degrees, the C.P.
+moves forward as in the case of flat surfaces (see B), but angles above
+30 degrees do not interest us, since they produce a very low ratio of
+lift to drift.
+
+Below angles of about 30 degrees (see C) the dipping front part of the
+surface assumes a negative angle of incidence resulting in the DOWNWARD
+air pressure D, and the more the angle of incidence is decreased, the
+greater such negative angle and its resultant pressure D. Since the
+C.P. is the resultant of all the air forces, its position is naturally
+affected by D, which causes it to move backwards. Now, should some gust
+or eddy tend to make the surface decrease its angle of incidence, i.e.,
+dive, then the C.P. moves backwards, and, pushing up the rear of the
+surface, causes it to dive the more. Should the surface tend to assume
+too large an angle, then the reverse happens; the pressure D decreases,
+with the result that C.P. moves forward and pushes up the front of the
+surface, thus increasing the angle still further, the final result being
+a “tail-slide.”
+
+It is therefore necessary to find a means of stabilizing the naturally
+unstable cambered surface. This is usually secured by means of a
+stabilizing surface fixed some distance in the rear of the main surface,
+and it is a necessary condition that the neutral lift lines of the two
+surfaces, when projected to meet each other, make a dihedral angle. In
+other words, the rear stabilizing surface must have a lesser angle of
+incidence than the main surface--certainly not more than one-third of
+that of the main surface. This is known as the longitudinal dihedral.
+
+I may add that the tail-plane is sometimes mounted upon the aeroplane at
+the same angle as the main surface, but, in such cases, it attacks air
+which has received a downward deflection from the main surface, thus:
+
+The angle at which the tail surface attacks the air (the angle of
+incidence) is therefore less than the angle of incidence of the main
+surface.
+
+I will now, by means of the following illustration, try to explain how
+the longitudinal dihedral secures stability:
+
+First, imagine the aeroplane travelling in the direction of motion,
+which coincides with the direction of thrust T. The weight is, of
+course, balanced about a C.P., the resultant of the C.P. of the main
+surface and the C.P. of the stabilizing surface. For the sake of
+illustration, the stabilizing surface has been given an angle of
+incidence, and therefore has a lift and C.P. In practice the stabilizer
+is often set at no angle of incidence. In such case the proposition
+remains the same, but it is, perhaps, a little easier to illustrate it
+as above.
+
+Now, we will suppose that a gust or eddy throws the machine into the
+lower position. It no longer travels in the direction of T, since the
+momentum in the old direction pulls it off that course. M is now the
+resultant of the Thrust and the Momentum, and you will note that this
+results in a decrease in the angle our old friend the neutral lift line
+makes with M, i.e., a decrease in the angle of incidence and therefore a
+decrease in lift.
+
+We will suppose that this decrease is 2 degrees. Such decrease applies
+to both main surface and stabilizer, since both are fixed rigidly to the
+aeroplane.
+
+The main surface, which had 12 degrees angle, has now only 10 degrees,
+i.e., a loss of ONE-SIXTH.
+
+The stabilizer, which had 4 degrees angle, has now only 2 degrees, i.e.,
+a loss of ONE-HALF.
+
+The latter has therefore lost a greater PROPORTION of its angle of
+incidence, and consequently its lift, than has the main surface. It must
+then fall relative to the main surface. The tail falling, the aeroplane
+then assumes its first position, though at a slightly less altitude.
+
+Should a gust throw the nose of the aeroplane up, then the reverse
+happens. Both main surface and stabilizer increase their angles of
+incidence in the same amount, but the angle, and therefore the lift, of
+the stabilizer increases in greater proportion than does the lift of the
+main surface, with the result that it lifts the tail. The aeroplane then
+assumes its first position, though at a slightly greater altitude.
+
+Do not fall into the widespread error that the angle of incidence varies
+as the angle of the aeroplane to the horizontal. It varies with such
+angle, but not as anything approaching it. Remember that the stabilizing
+effect of the longitudinal dihedral lasts only as long as there is
+momentum in the direction of the first course.
+
+These stabilizing movements are taking place all the time, even though
+imperceptible to the pilot.
+
+Aeroplanes have, in the past, been built with a stabilizing surface in
+front of the main surface instead of at the rear of it. In such
+design the main surface (which is then the tail surface as well as the
+principal lifting surface) must be set at a less angle than the forward
+stabilizing surface, in order to secure a longitudinal dihedral. The
+defect of such design lies in the fact that the main surface must have
+a certain angle to lift the weight--say 5 degrees. Then, in order to
+secure a sufficiency of longitudinal stability, it is necessary to
+set the forward stabilizer at about 15 degrees. Such a large angle of
+incidence results in a very poor lift-drift ratio (and consequently
+great loss of efficiency), except at very low velocities compared with
+the speed of modern aeroplanes. At the time such aeroplanes were built
+velocities were comparatively low, and this defect was; for that reason,
+not sufficiently appreciated. In the end it killed the “canard” or
+“tail-first” design.
+
+Aeroplanes of the Dunne and similar types possess no stabilizing surface
+distinct from the main surface, but they have a longitudinal dihedral
+which renders them stable.
+
+The main surface towards the wing-tips is given a decreasing angle
+of incidence and corresponding camber. The wing-tips then act as
+longitudinal stabilizers.
+
+This design of aeroplane, while very interesting, has not proved very
+practicable, owing to the following disadvantages: (1) The plan design
+is not, from a mechanical point of view, so sound as that of the
+ordinary aeroplane surface, which is, in plan, a parallelogram. It is,
+then, necessary to make the strength of construction greater than would
+otherwise be the case. That means extra weight. (2) The plan of the
+surface area is such that the aspect ratio is not so high as if the
+surface was arranged with its leading edges at right angles to the
+direction of motion. The lower the aspect ratio, then, the less the
+lift. This design, then, produces less lift for weight of surface than
+would the same surface if arranged as a parallelogram. (3) In order to
+secure the longitudinal dihedral, the angle of incidence has to be very
+much decreased towards the wing-tips. Then, in order that the lift-drift
+ratio may be preserved, there must be a corresponding decrease in the
+camber. That calls for surface ribs of varying cambers, and results in
+an expensive and lengthy job for the builder. (4) In order to secure
+directional stability, the surface is, in the centre, arranged to dip
+down in the form of a V, pointing towards the direction of motion.
+Should the aeroplane turn off its course, then its momentum in the
+direction of its first course causes it to move in a direction the
+resultant of the thrust and the momentum. It then moves in a more or
+less sideways attitude, which results in an air pressure upon one
+side of the V, and which tends to turn the aeroplane back to its first
+course. This arrangement of the surface results in a bad drift. Vertical
+surfaces at the wing-tips may also be set at an angle producing the same
+stabilizing effect, but they also increase the drift.
+
+The gyroscopic action of a rotary engine will affect the longitudinal
+stability when an aeroplane is turned to right or left. In the case of
+a Gnome engine, such gyroscopic action will tend to depress the nose of
+the aeroplane when it is turned to the left, and to elevate it when
+it is turned to the right. In modern aeroplanes this tendency is not
+sufficiently important to bother about. In the old days of crudely
+designed and under-powered aeroplanes this gyroscopic action was very
+marked, and led the majority of pilots to dislike turning an aeroplane
+to the right, since, in doing so, there was some danger of “stalling.”
+
+
+LATERAL STABILITY is far more difficult for the designer to secure
+than is longitudinal or directional stability. Some degree of lateral
+stability may be secured by means of the “lateral dihedral,” i.e., the
+upward inclination of the surface towards its wing-tips thus:
+
+Imagine the top V, illustrated opposite, to be the front view of a
+surface flying towards you. The horizontal equivalent (H.E.) of the left
+wing is the same as that of the right wing. Therefore, the lift of one
+wing is equal to the lift of the other, and the weight, being situated
+always in the centre, is balanced.
+
+If some movement of the air causes the surface to tilt sideways, as in
+the lower illustration, then you will note that the H.E. of the left
+wing increases, and the H.E. of the right wing decreases. The left wing
+then, having the greatest lift, rises; and the surface assumes its first
+and normal position.
+
+Unfortunately however, the righting effect is not proportional to the
+difference between the right and left H.E.'s.
+
+
+
+In the case of A, the resultant direction of the reaction of both wings
+is opposed to the direction of gravity or weight. The two forces R R
+and gravity are then evenly balanced, and the surface is in a state of
+equilibrium.
+
+In the case of B, you will note that the R R is not directly opposed
+to gravity. This results in the appearance of M, and so the resultant
+direction of motion of the aeroplane is no longer directly forward, but
+is along a line the resultant of the thrust and M. In other words,
+it is, while flying forward, at the same time moving sideways in the
+direction M.
+
+In moving sideways, the keel-surface receives, of course, a pressure
+from the air equal and opposite to M. Since such surface is greatest in
+effect towards the tail, then the latter must be pushed sideways. That
+causes the aeroplane to turn; and, the highest wing being on the
+outside of the turn, it has a greater velocity than the lower wing. That
+produces greater lift, and tends to tilt the aeroplane over still more.
+Such tilting tendency is, however, opposed by the difference in the
+H.E.'s of the two wings.
+
+It then follows that, for the lateral dihedral angle to be effective,
+such angle must be large enough to produce, when the aeroplane tilts,
+a difference in the H.E.'s of the two wings, which difference must be
+sufficient to not only oppose the tilting tendency due to the aeroplane
+turning, but sufficient to also force the aeroplane back to its original
+position of equilibrium.
+
+It is now, I hope, clear to the reader that the lateral dihedral is
+not quite so effective as would appear at first sight. Some designers,
+indeed, prefer not to use it, since its effect is not very great, and
+since it must be paid for in loss of H.E. and consequently loss of lift,
+thus decreasing the lift-drift ratio, i.e., the efficiency. Also, it is
+sometimes advanced that the lateral dihedral increases the “spill” of
+air from the wing-tips and that this adversely affects the lift-drift
+ratio.
+
+The disposition of the keel-surface affects the lateral stability. It
+should be, in effect, equally divided by the longitudinal turning axis
+of the aeroplane. If there is an excess of keel-surface above or below
+such axis, then a side gust striking it will tend to turn the aeroplane
+over sideways.
+
+The position of the centre of gravity affects lateral stability. If
+too low, it produces a pendulum effect and causes the aeroplane to roll
+sideways.
+
+If too high, it acts as a stick balanced vertically would act. If
+disturbed, it tends to travel to a position as far as possible from its
+original position. It would then tend, when moved, to turn the aeroplane
+over sideways and into an upside-down position.
+
+From the point of view of lateral stability, the best position for the
+centre of gravity is one a little below the centre of drift.
+
+Propeller torque affects lateral stability. An aeroplane tends to turn
+over sideways in the opposite direction to which the propeller revolves.
+
+This tendency is offset by increasing the angle of incidence (and
+consequently the lift) of the side tending to fall; and it is always
+advisable, if practical considerations allow it, to also decrease the
+angle upon the other side. In that way it is not necessary to depart so
+far from the normal angle of incidence at which the lift-drift ratio is
+highest.
+
+Wash-in is the term applied to the increased angle.
+
+Wash-out is the term applied to the decreased angle.
+
+Both lateral and directional stability may be improved by washing out
+the angle of incidence on both sides of the surface, thus:
+
+The decreased angle decreases the drift and therefore the effect of
+gusts upon the wing-tips which is just where they have the most effect
+upon the aeroplane, owing to the distance from the turning axis.
+
+The wash-out also renders the ailerons (lateral controlling services)
+more effective, as, in order to operate them, it is not then necessary
+to give them such a large angle of incidence as would otherwise be
+required.
+
+
+
+The less the angle of incidence of the ailerons, the better their
+lift-drift ratio, i.e., their efficiency. You will note that, while the
+aileron attached to the surface with washed-out angle is operated to the
+same extent as the aileron illustrated above it, its angle of incidence
+is considerably less. Its efficiency is therefore greater.
+
+The advantages of the wash-in must, of course be paid for in some loss
+of lift, as the lift decreases with the decreased angle.
+
+In order to secure all the above described advantages, a combination is
+sometimes effected, thus:
+
+BANKING.--An aeroplane turned off its course to right or left does not
+at once proceed along its new course. Its momentum in the direction of
+its first course causes it to travel along a line the resultant of such
+momentum and the thrust. In other words, it more or less skids sideways
+and away from the centre of the turn. Its lifting surfaces do not then
+meet the air in their correct attitude, and the lift may fall to such
+an extent as to become less than the weight, in which case the aeroplane
+must fall. This bad effect is minimized by “banking,” i.e., tilting the
+aeroplane sideways. The bottom of the lifting surface is in that way
+opposed to the air through which it is moving in the direction of the
+momentum and receives an opposite air pressure. The rarefied area over
+the top of the surface is rendered still more rare, and this, of course,
+assists the air pressure in opposing the momentum.
+
+The velocity of the “skid,” or sideways movement, is then only such
+as is necessary to secure an air pressure equal and opposite to the
+centrifugal force of the turn.
+
+The sharper the turn, the greater the effect of the centrifugal force,
+and therefore the steeper should be the “bank.” Experentia docet.
+
+The position of the centre of gravity affects banking. A low C.G. will
+tend to swing outward from the centre of the turn, and will cause the
+aeroplane to bank--perhaps too much, in which case the pilot must remedy
+matters by operating the ailerons.
+
+A high C.G. also tends to swing outward from the centre of the turn. It
+will tend to make the aeroplane bank the wrong way, and such effect must
+be remedied by means of the ailerons.
+
+The pleasantest machine from a banking point of view is one in which
+the C.G. is a little below the centre of drift. It tends to bank the
+aeroplane the right way for the turn, and the pilot can, if necessary,
+perfect the bank by means of the ailerons.
+
+The disposition of the keel-surface affects banking. It should be,
+in effect, evenly divided by the longitudinal axis. An excess of
+keel-surface above the longitudinal axis will, when banking, receive an
+air pressure causing the aeroplane to bank, perhaps too much. An excess
+of keel-surface below the axis has the reverse effect.
+
+
+SIDE-SLIPPING.--This usually occurs as a result of over-banking. It is
+always the result of the aeroplane tilting sideways and thus decreasing
+the horizontal equivalent, and therefore the lift, of the surface. An
+excessive “bank,” or sideways tilt, results in the H.E., and therefore
+the lift, becoming less than the weight, when, of course, the aeroplane
+must fall, i.e., side-slip.
+
+
+When making a very sharp turn it is necessary to bank very steeply
+indeed. If, at the same time, the longitudinal axis of the aeroplane
+remains approximately horizontal, then there must be a fall, and the
+direction of motion will be the resultant of the thrust and the fall as
+illustrated above in sketch A. The lifting surfaces and the controlling
+surfaces are not then meeting the air in the correct attitude, with the
+result that, in addition to falling, the aeroplane will probably become
+quite unmanageable.
+
+The Pilot, however, prevents such a state of affairs from happening by
+“nosing-down,” i.e., by operating the rudder to turn the nose of the
+aeroplane downward and towards the direction of motion as illustrated
+in sketch B. This results in the higher wing, which is on the outside
+of the turn, travelling with greater velocity, and therefore securing a
+greater reaction than the lower wing, thus tending to tilt the aeroplane
+over still more. The aeroplane is now almost upside-down, but its
+attitude relative to the direction of motion is correct and the
+controlling surfaces are all of them working efficiently. The recovery
+of a normal attitude relative to the Earth is then made as illustrated
+in sketch C.
+
+The Pilot must then learn to know just the angle of bank at which the
+margin of lift is lost, and, if a sharp turn necessitates banking beyond
+that angle, he must “nose-down.”
+
+In this matter of banking and nosing-down, and, indeed, regarding
+stability and control generally, the golden rule for all but very
+experienced pilots should be: Keep the aeroplane in such an attitude
+that the air pressure is always directly in the pilot's face. The
+aeroplane is then always engaging the air as designed to do so, and
+both lifting and controlling surfaces are acting efficiently. The only
+exception to this rule is a vertical dive, and I think that is obviously
+not an attitude for any but very experienced pilots to hanker after.
+
+SPINNING.--This is the worst of all predicaments the pilot can find
+himself in. Fortunately it rarely happens.
+
+It is due to the combination of (1) a very steep spiral descent of small
+radius, and (2) insufficiency of keel-surface behind the vertical axis,
+or the jamming of the rudder end or elevator into a position by which
+the aeroplane is forced into an increasingly steep and small spiral.
+
+Owing to the small radius of such a spiral, the mass of the aeroplane
+may gain a rotary momentum greater, in effect, than the air pressure of
+the keel-surface or controlling surfaces opposed to it; and, when once
+such a condition occurs, it is difficult to see what can be done by the
+pilot to remedy it. The sensible pilot will not go beyond reasonable
+limits of steepness and radius when executing spiral descents.
+
+GLIDING DESCENT WITHOUT PROPELLER THRUST.--All aeroplanes are, or should
+be, designed to assume their gliding angle when the power and thrust is
+cut off. This relieves the pilot of work, worry, and danger should he
+find himself in a fog or cloud. The Pilot, although he may not realize
+it, maintains the correct attitude of the aeroplane by observing its
+position relative to the horizon. Flying into a fog or cloud the horizon
+is lost to view, and he must then rely upon his instruments--(1) the
+compass for direction; (2) an inclinometer (arched spirit-level) mounted
+transversely to the longitudinal axis, for lateral stability; and (3) an
+inclinometer mounted parallel to the longitudinal axis, or the airspeed
+indicator, which will indicate a nose-down position by increase in air
+speed, and a tail-down position by decrease in air speed.
+
+The pilot is then under the necessity of watching three instruments
+and manipulating his three controls to keep the instruments indicating
+longitudinal, lateral, and directional stability. That is a feat beyond
+the capacity of the ordinary man. If, however, by the simple movement
+of throttling down the power and thrust, he can be relieved of looking
+after the longitudinal stability, he then has only two instruments to
+watch. That is no small job in itself, but it is, at any rate, fairly
+practicable.
+
+Aeroplanes are, then, designed, or should be, so that the centre of
+gravity is slightly forward of centre of lift. The aeroplane is then, as
+a glider, nose-heavy--and the distance the C.G. is placed in advance
+of the C.L. should be such as to ensure a gliding angle producing a
+velocity the same as the normal flying speed (for which the strength of
+construction has been designed).
+
+In order that this nose-heavy tendency should not exist when the thrust
+is working and descent not required, the centre of thrust is placed a
+little below the centre of drift or resistance, and thus tends to pull
+up the nose of the aeroplane.
+
+The distance the centre of thrust is placed below the centre of drift
+should be such as to produce a force equal and opposite to that due to
+the C.G. being forward of the C.L.
+
+LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired, it is best to
+throttle the engine down at point A. The C.G. being forward of the C.P.,
+then causes the aeroplane to nose-down, and assists the pilot in making
+a reasonably small loop along the course C and in securing a quick
+recovery. If the engine is not throttled down, then the aeroplane may
+be expected to follow the course D, which results in a longer nose dive
+than in the case of the course C.
+
+A steady, gentle movement of the elevator is necessary. A jerky movement
+may change the direction of motion so suddenly as to produce dangerous
+air stresses upon the surfaces, in which case there is a possibility of
+collapse.
+
+If an upside-down flight is desired, the engine may, or may not, be
+throttled down at point A. If not throttled down, then the elevator must
+be operated to secure a course approximately in the direction B. If it
+is throttled down, then the course must be one of a steeper angle than
+B, or there will be danger of stalling.
+
+
+Diagram p. 88.--This is not set at quite the correct angle. Path B
+should slope slightly downwards from Position A.
+
+
+
+
+CHAPTER III. RIGGING
+
+In order to rig an aeroplane intelligently, and to maintain it in an
+efficient and safe condition, it is necessary to possess a knowledge
+of the stresses it is called upon to endure, and the strains likely to
+appear.
+
+
+STRESS is the load or burden a body is called upon to bear. It is
+usually expressed by the result found by dividing the load by the number
+of superficial square inches contained in the cross-sectional area of
+the body.
+
+Thus, if, for instance, the object illustrated above contains 4 square
+inches of cross-sectional area, and the total load it is called upon to
+endure is 10 tons, the stress would be expressed as 2 1/2 tons.
+
+
+STRAIN is the deformation produced by stress.
+
+
+THE FACTOR OF SAFETY is usually expressed by the result found by
+dividing the stress at which it is known the body will collapse, by
+the maximum stress it will be called upon to endure. For instance, if a
+control wire be called upon to endure a maximum stress of 2 cwts., and
+the known stress at which it will collapse is 10 cwts., the factor of
+safety is then 5.
+
+[cwts. = centerweights = 100 pound units as in cent & century.
+Interestingly enough, this word only exists today in abbreviation form,
+probably of centreweights, but the dictionary entries, even from a
+hundred years ago do not list this as a word, but do list c. or C. as
+the previous popular abbreviation as in Roman Numerals] The word listed
+is “hundredweight. Michael S. Hart, 1997]
+
+
+COMPRESSION.--The simple stress of compression tends to produce a
+crushing strain. Example: the interplane and fuselage struts.
+
+
+TENSION.--The simple stress of tension tends to produce the strain of
+elongation. Example: all the wires.
+
+
+BENDING.--The compound stress of bending is a combination of compression
+and tension.
+
+The above sketch illustrates a straight piece of wood of which the top,
+centre, and bottom lines are of equal length. We will now imagine it
+bent to form a circle, thus:
+
+The centre line is still the same length as before being bent; but the
+top line, being farther from the centre of the circle, is now longer
+than the centre line. That can be due only to the strain of elongation
+produced by the stress of tension. The wood between the centre line and
+the top line is then in tension; and the farther from the centre, the
+greater the strain, and consequently the greater the tension.
+
+The bottom line, being nearest to the centre of the circle, is now
+shorter than the centre line. That can be due only to the strain of
+crushing produced by the stress of compression. The wood between the
+centre and bottom lines is then in compression; and the nearer the
+centre of the circle, the greater the strain, and consequently the
+greater the compression.
+
+It then follows that there is neither tension nor compression, i.e., no
+stress, at the centre line, and that the wood immediately surrounding it
+is under considerably less stress than the wood farther away. This being
+so, the wood in the centre may be hollowed out without unduly weakening
+struts and spars. In this way 25 to 33 per cent. is saved in the weight
+of wood in an aeroplane.
+
+The strength of wood is in its fibres, which should, as far as possible,
+run without break from one end of a strut or spar to the other end. A
+point to remember is that the outside fibres, being farthest removed
+from the centre line, are doing by far the greatest work.
+
+
+SHEAR STRESS IS such that, when material collapses under it, one part
+slides over the other. Example: all the locking pins.
+
+Some of the bolts are also in shear or “sideways” stress, owing to lugs
+under their heads and from which wires are taken. Such a wire, exerting
+a sideways pull upon a bolt, tries to break it in such a way as to make
+one piece of the bolt slide over the other piece.
+
+TORSION.--This is a twisting stress compounded of compression, tension,
+and shear stresses. Example: the propeller shaft.
+
+
+NATURE OF WOOD UNDER STRESS.--Wood, for its weight, takes the stress
+of compression far better than any other stress. For instance: a
+walking-stick of less than 1 lb. in weight will, if kept perfectly
+straight, probably stand up to a compression stress of a ton or more
+before crushing; whereas, if the same stick is put under a bending
+stress, it will probably collapse to a stress of not more than about
+50 lb. That is a very great difference, and, since weight is of the
+greatest importance, the design of an aeroplane is always such as to,
+as far as possible, keep the various wooden parts of its construction in
+direct compression. Weight being of such vital importance, and designers
+all trying to outdo each other in saving weight, it follows that the
+factor of safety is rather low in an aeroplane. The parts in direct
+compression will, however, take the stresses safely provided the
+following conditions are carefully observed.
+
+
+CONDITIONS TO BE OBSERVED:
+
+
+1. All the spars and struts must be perfectly straight.
+
+The above sketch illustrates a section through an interplane strut. If
+the strut is to be kept straight, i.e., prevented from bending, then
+the stress of compression must be equally disposed about the centre of
+strength. If it is not straight, then there will be more compression
+on one side of the centre of strength than on the other side. That is
+a step towards getting compression on one side and tension on the other
+side, in which case it may be forced to take a bending stress for which
+it is not designed. Even if it does not collapse it will, in effect,
+become shorter, and thus throw out of adjustment the gap and all the
+wires attached to the top and bottom of the strut, with the result that
+the flight efficiency of the aeroplane will be spoiled.
+
+The only exception to the above condition is what is known as the
+Arch. For instance, in the case of the Maurice Farman, the spars of the
+centre-section plane, which have to take the weight of the nacelle, are
+arched upwards. If this was not done, it is possible that rough
+landings might result in the weight causing the spars to become slightly
+distorted downwards. That would produce a dangerous bending stress,
+but, as long as the wood is arched, or, at any rate, kept from bending
+downwards, it will remain in direct compression and no danger can
+result.
+
+
+2. Struts and spars must be symmetrical. By that I mean that the
+cross-sectional dimensions must be correct, as otherwise there will be
+bulging places on the outside, with the result that the stress will not
+be evenly disposed about the centre of strength, and a bending stress
+may be produced.
+
+
+3. Struts, spars, etc., must be undamaged. Remember that, from what I
+have already explained about bending stresses, the outside fibres of
+the wood are doing by far the most work. If these get bruised or scored,
+then the strut or spar suffers in strength much more than one might
+think at first sight; and, if it ever gets a tendency to bend, it is
+likely to collapse at that point.
+
+
+4. The wood must have a good, clear grain with no cross-grain, knots,
+or shakes. Such blemishes produce weak places and, if a tendency to bend
+appears, then it may collapse at such a point.
+
+5. The struts, spars, etc., must be properly bedded into their sockets
+or fittings. To begin with, they must be of good pushing or gentle
+tapping fit. They must never be driven in with a heavy hammer. Then
+again, a strut must bed well down all over its cross-sectional area
+as illustrated above; otherwise the stress of compression will not be
+evenly disposed about the centre of strength, and that may produce a
+bending stress. The bottom of the strut or spar should be covered
+with some sort of paint, bedded into the socket or fitting, and then
+withdrawn to see if the paint has stuck all over the bed.
+
+
+6. The atmosphere is sometimes much damper than at other times, and this
+causes wood to expand and contract appreciably. This would not matter
+but for the fact that it does not expand and contract uniformly, but
+becomes unsymmetrical, i.e., distorted. I have already explained
+the danger of that in condition 2. This should be minimized by WELL
+VARNISHING THE WOOD to keep the moisture out of it.
+
+
+FUNCTION OF INTERPLANE STRUTS.--These struts have to keep the lifting
+surfaces or “planes” apart, but this is only part of their work. They
+must keep the planes apart, so that the latter are in their correct
+attitude. That is only so when the spars of the bottom plane are
+parallel with those of the top plane. Also, the chord of the top plane
+must be parallel with the chord of the bottom plane. If that is not so,
+then one plane will not have the same angle of incidence as the other
+one. At first sight one might think that all that is necessary is to cut
+all the struts to be the same length, but that is not the case.
+
+Sometimes, as illustrated above, the rear spar is not so thick as the
+main spar, and it is then necessary to make up for that difference by
+making the rear struts correspondingly longer. If that is not done, then
+the top and bottom chords will not be parallel, and the top and bottom
+planes will have different angles of incidence. Also, the sockets or
+fittings, or even the spars upon which they are placed, sometimes
+vary in thickness owing to faulty manufacture. This must be offset by
+altering the length of the struts. The best way to proceed is to measure
+the distance between the top and bottom spars by the side of each strut,
+and if that distance, or “gap” as it is called, is not as stated in the
+aeroplane's specifications, then make it correct by changing the length
+of the strut. This applies to both front and rear interplane struts.
+When measuring the gap, always be careful to measure from the centre
+of the spar, as it may be set at an angle, and the rear of it may be
+considerably lower than its front.
+
+
+BORING HOLES IN WOOD.--It should be a strict rule that no spar be used
+which has an unnecessary hole in it. Before boring a hole, its position
+should be confirmed by whoever is in charge of the workshop. A bolt-hole
+should be of a size to enable the bolt to be pushed in, or, at any rate,
+not more than gently tapped in. Bolts should not be hammered in, as that
+may split the spar. On the other hand, a bolt should not be slack in its
+hole, as, in such a case, it may work sideways and split the spar, not
+to speak of throwing out of adjustment the wires leading from the lug or
+socket under the bolt-head.
+
+
+WASHERS.--Under the bolt-head, and also under the nut, a washer must be
+placed--a very large washer compared with the size which would be used
+in all-metal construction. This is to disperse the stress over a large
+area; otherwise the washer may be pulled into the wood and weaken it,
+besides possibly throwing out of adjustment the wires attached to the
+bolt or the fitting it is holding to the spar.
+
+
+LOCKING.--Now as regards locking the bolts. If split pins are used,
+be sure to see that they are used in such a way that the nut cannot
+possibly unscrew at all. The split pin should be passed through the bolt
+as near as possible to the nut. It should not be passed through both nut
+and bolt.
+
+If it is locked by burring over the edge of the bolt, do not use a heavy
+hammer and try to spread the whole head of the bolt. That might damage
+the woodwork inside the fabric-covered surface. Use a small, light
+hammer, and gently tap round the edge of the bolt until it is burred
+over.
+
+
+TURNBUCKLES.--A turnbuckle is composed of a central barrel into each end
+of which is screwed an eye-bolt. Wires are taken from the eyes of the
+eye-bolt, and so, by turning the barrel, they can be adjusted to their
+proper tension. Eye-bolts must be a good fit in the barrel; that is to
+say, not slack and not very tight. Theoretically it is not necessary
+to screw the eye-bolt into the barrel for a distance greater than the
+diameter of the bolt, but, in practice, it is better to screw it in
+for a considerably greater distance than that if a reasonable degree of
+safety is to be secured.
+
+Now about turning the barrel to secure the right adjustment. The barrel
+looks solid, but, as a matter of fact, it is hollow and much more frail
+than it appears. For that reason it should not be turned by seizing it
+with pliers, as that may distort it and spoil the bore within it. The
+best method is to pass a piece of wire through the hole in its centre,
+and to use that as a lever. When the correct adjustment has been
+secured, the turnbuckle must be locked to prevent it from unscrewing. It
+is quite possible to lock it in such a way as to allow it to unscrew a
+quarter or a half turn, and that would throw the wires out of the very
+fine adjustment necessary. The proper way is to use the locking wire so
+that its direction is such as to oppose the tendency of the barrel to
+unscrew, thus:
+
+
+WIRES.--The following points should be carefully observed where wire is
+concerned:
+
+1. Quality.--It must not be too hard or too soft. An easy practical way
+of learning to know the approximate quality of wire is as follows:
+
+Take three pieces, all of the same gauge, and each about a foot in
+length. One piece should be too soft, another too hard, and the third
+piece of the right quality. Fix them in a vice, about an inch apart and
+in a vertical position, and with the light from a window shining upon
+them. Burnish them if necessary, and you will see a band of light
+reflected from each wire.
+
+Now bend the wires over as far as possible and away from the light.
+Where the soft wire is concerned, it will squash out at the bend, and
+this will be indicated by the band of light, which will broaden at that
+point. In the case of the wire which is too hard, the band of light will
+broaden very little at the turn, but, if you look carefully, you will
+see some little roughnesses of surface. In the case of the wire of the
+right quality, the band of light may broaden a very little at the turn,
+but there will be no roughnesses of surface.
+
+By making this experiment two or three times one can soon learn to know
+really bad wire from good, and also learn to know the strength of hand
+necessary to bend the right quality.
+
+2. It must not be damaged. That is to say, it must be unkinked,
+rustless, and unscored.
+
+3. Now as regards keeping wire in good condition. Where outside wires
+are concerned, they should be kept WELL GREASED OR OILED, especially
+where bent over at the ends. Internal bracing wires cannot be reached
+for the purpose of regreasing them, as they are inside fabric-covered
+surfaces. They should be prevented from rusting by being painted with
+an anti-rust mixture. Great care should be taken to see that the wire
+is perfectly clean and dry before being painted. A greasy finger-mark is
+sufficient to stop the paint from sticking to the wire. In such a case
+there will be a little space between the paint and the wire. Air may
+enter there and cause the wire to rust.
+
+4. Tension of Wires.--The tension to which the wires are adjusted is
+of the greatest importance. All the wires should be of the same tension
+when the aeroplane is supported in such a way as to throw no stress upon
+them. If some wires are in greater tension than others, the aeroplane
+will quickly become distorted and lose its efficiency.
+
+In order to secure the same tension of all wires, the aeroplane, when
+being rigged, should be supported by packing underneath the lower
+surfaces as well as by packing underneath the fuselage or nacelle. In
+this way the anti-lift wires are relieved of the weight, and there is no
+stress upon any of the wires.
+
+As a general rule the wires of an aeroplane are tensioned too much. The
+tension should be sufficient to keep the framework rigid. Anything
+more than that lowers the factor of safety, throws various parts of the
+framework into undue compression, pulls the fittings into the wood, and
+will, in the end, distort the whole framework of the aeroplane.
+
+Only experience will teach the rigger what tension to employ. Much may
+be done by learning the construction of the various types of aeroplanes,
+the work the various parts do, and in cultivating a touch for tensioning
+wires by constantly handling them.
+
+5. Wires with no Opposition Wires.--In some few cases wires will be
+found which have no opposition wires pulling in the opposite direction.
+For instance, an auxiliary lift wire may run from the bottom of a strut
+to a spar in the top plane at a point between struts. In such a case
+great care should be taken not to tighten the wire beyond barely taking
+up the slack.
+
+Such a wire must be a little slack, or, as illustrated above, it will
+distort the framework. That, in the example given, will spoil the camber
+(curvature) of the surface, and result in changing both the lift and
+the drift at that part of the surface. Such a condition will cause the
+aeroplane to lose its directional stability and also to fly one wing
+down.
+
+I cannot impress this matter of tension upon the reader too strongly.
+It is of the utmost importance. When this, and also accuracy in securing
+the various adjustments, has been learned, one is on the way to becoming
+a good rigger.
+
+6. Wire Loops.--Wire is often bent over at its end in the form of a
+loop, in order to connect with a turnbuckle or fitting. These loops,
+even when made as perfectly as possible, have a tendency to elongate,
+thus spoiling the adjustment of the wires Great care should be taken
+to minimize this as far as possible. The rules to be observed are as
+follows:
+
+(a) The size of the loop should be as small as possible within reason.
+By that I mean it should not be so small as to create the possibility of
+the wire breaking.
+
+
+(b) The shape of the loop should be symmetrical.
+
+
+(c) It should have well-defined shoulders in order to prevent the
+ferrule from slipping up. At the same time, a shoulder should not have
+an angular place.
+
+
+(d) When the loop is finished it should be undamaged, and it should not
+be, as is often the case, badly scored.
+
+
+7. Stranded Wire Cable.--No splice should be served with twine until it
+has been inspected by whoever is in charge of the workshop. The serving
+may cover bad work.
+
+Should a strand become broken, then the cable should be replaced at once
+by another one.
+
+Control cables have a way of wearing out and fraying wherever they pass
+round pulleys. Every time an aeroplane comes down from flight the rigger
+should carefully examine the cables, especially where they pass round
+pulleys. If he finds a strand broken, he should replace the cable.
+
+The ailerons' balance cable on the top of the top plane is often
+forgotten, since it is necessary to fetch a high pair of steps in
+order to examine it. Don't slack this, or some gusty day the pilot may
+unexpectedly find himself minus the aileron control.
+
+CONTROLLING SURFACES.--The greatest care should be exercised in rigging
+the aileron, rudder, and elevator properly, for the pilot entirely
+depends upon them in managing the aeroplane.
+
+The ailerons and elevator should be rigged so that, when the aeroplane
+is in flight, they are in a fair true line with the surface in front and
+to which they are hinged.
+
+If the surface to which they are hinged is not a lifting surface, then
+they should be rigged to be in a fair true line with it as illustrated
+above.
+
+If the controlling surface is, as illustrated, hinged to the back of a
+lifting surface, then it should be rigged a little below the position it
+would occupy if in a fair true line with the surface in front. This is
+because, in such a case, it is set at an angle of incidence. This angle
+will, during flight, cause it to lift a little above the position in
+which it has been rigged. It is able to lift owing to a certain amount
+of slack in the control wire holding it--and one cannot adjust the
+control wire to have no slack, because that would cause it to bind
+against the pulleys and make the operation of it too hard for the pilot.
+It is therefore necessary to rig it a little below the position it would
+occupy if it was rigged in a fair true line with the surface in front.
+Remember that this only applies when it is hinged to a lifting surface.
+The greater the angle of incidence (and therefore the lift) of the
+surface in front, then the more the controlling surface will have to be
+rigged down.
+
+As a general rule it is safe to rig it down so that its trailing
+
+edge is 1/2 to 3/4 inch below the position it would occupy if in a fair
+line with the surface in front; or about 1/2 inch down for every 18
+inches of chord of the controlling surface.
+
+When making these adjustments the pilot's control levers should be in
+their neutral positions. It is not sufficient to lash them. They should
+be rigidly blocked into position with wood packing.
+
+The surfaces must not be distorted in any way. If they are held true by
+bracing wires, then such wires must be carefully adjusted. If they are
+distorted and there are no bracing wires with which to true them, then
+some of the internal framework will probably have to be replaced.
+
+The controlling surfaces should never be adjusted with a view to
+altering the stability of the aeroplane. Nothing can be accomplished in
+that way. The only result will be to spoil the control of the aeroplane.
+
+
+FABRIC-COVERED SURFACES.--First of all make sure that there is no
+distortion of spars or ribs, and that they are perfectly sound. Then
+adjust the internal bracing wires so that the ribs are parallel to the
+direction of flight. The ribs usually cause the fabric to make a ridge
+where they occur, and, if such ridge is not parallel to the direction of
+flight, it will produce excessive drift. As a rule the ribs are at right
+angles to both main and rear spars.
+
+The tension of the internal bracing wires should be just sufficient to
+give rigidity to the framework. They should not be tensioned above that
+unless the wires are, at their ends, bent to form loops. In that case a
+little extra tension may be given to offset the probable elongation of
+the loops.
+
+The turnbuckles must now be generously greased, and served round with
+adhesive tape. The wires must be rendered perfectly dry and clean,
+and then painted with an anti-rust mixture. The woodwork must be well
+varnished.
+
+If it is necessary to bore holes in the spars for the purpose of
+receiving, for instance, socket bolts, then their places should be
+marked before being bored and their positions confirmed by whoever is in
+charge of the workshop. All is now ready for the sail-maker to cover the
+surface with fabric.
+
+
+ADJUSTMENT OF CONTROL CABLES.--The adjustment of the control cables is
+quite an art, and upon it will depend to a large degree the quick and
+easy control of the aeroplane by the pilot.
+
+The method is as follows:
+
+After having rigged the controlling surfaces, and as far as possible
+secured the correct adjustment of the control cables, then remove the
+packing which has kept the control levers rigid. Then, sitting in the
+pilot's seat, move the control levers SMARTLY. Tension the control
+cables so that when the levers are smartly moved there is no perceptible
+snatch or lag. Be careful not to tension the cables more than necessary
+to take out the snatch. If tensioned too much they will (1) bind round
+the pulleys and result in hard work for the pilot; (2) throw dangerous
+stresses upon the controlling surfaces, which are of rather flimsy
+construction; and (3) cause the cables to fray round the pulleys quicker
+than would otherwise be the case.
+
+Now, after having tensioned the cables sufficiently to take out the
+snatch, place the levers in their neutral positions, and move them to
+and fro about 1/8 inch either side of such positions. If the adjustment
+is correct, it should be possible to see the controlling surfaces move.
+If they do not move, then the control cables are too slack.
+
+
+FLYING POSITION.--Before rigging an aeroplane or making any adjustments
+it is necessary to place it in what is known as its “flying position.” I
+may add that it would be better termed its “rigging position.”
+
+In the case of an aeroplane fitted with a stationary engine this is
+secured by packing up the machine so that the engine foundations are
+perfectly horizontal both longitudinally and laterally. This position
+is found by placing a straight-edge and a spirit-level across the engine
+foundations (both longitudinally and laterally), and great care should
+be taken to see that the bubble is exactly in the centre of the level.
+The slightest error will assume magnitude towards the extremities of the
+aeroplane. Great care should be taken to block up the aeroplane rigidly.
+In case it gets accidentally disturbed while the work is going on, it
+is well to constantly verify the flying position by running the
+straight-edge and spirit-level over the engine foundations. The
+straight-edge should be carefully tested before being used, as, being
+generally made of wood, it will not remain true long. Place it lightly
+in a vice, and in such a position that a spirit-level on top shows
+the bubble exactly in the centre. Now slowly move the level along the
+straight-edge, and the bubble should remain exactly in the centre. If
+it does not do so, then the straight-edge is not true and must be
+corrected. THIS SHOULD NEVER BE OMITTED.
+
+In the case of aeroplanes fitted with engines of the rotary type, the
+“flying position” is some special attitude laid down in the aeroplane's
+specifications, and great care should be taken to secure accuracy.
+
+
+ANGLE OF INCIDENCE.--One method of finding the angle of incidence is as
+follows:
+
+First place the aeroplane in its flying position. The corner of the
+straight-edge must be placed underneath and against the CENTRE of the
+rear spar, and held in a horizontal position parallel to the ribs. This
+is secured by using a spirit-level. The set measurement will then be
+from the top of the straight-edge to the centre of the bottom surface
+of the main spar, or it may be from the top of the straight-edge to the
+lowest part of the leading edge. Care should be taken to measure from
+the centre of the spar and to see that the bubble is exactly in the
+centre of the level. Remember that all this will be useless if the
+aeroplane has not been placed accurately in its flying position.
+
+This method of finding the angle of incidence must be used under every
+part of the lower surface where struts occur. It should not be used
+between the struts, because, in such places, the spars may have taken a
+slight permanent set up or down; not, perhaps, sufficiently bad to make
+any material difference to the flying of the machine, but quite bad
+enough to throw out the angle of incidence, which cannot be corrected at
+such a place.
+
+If the angle is wrong, it should then be corrected as follows:
+
+If it is too great, then the rear spar must be warped up until it is
+right, and this is done by slackening ALL the wires going to the top of
+the strut, and then tightening ALL the wires going to the bottom of the
+strut.
+
+If the angle is too small, then slacken ALL the wires going to the
+bottom of the strut, and tighten ALL the wires going to the top of the
+strut, until the correct adjustment is secured.
+
+Never attempt to adjust the angle by warping the main spar.
+
+The set measurement, which is of course stated in the aeroplane's
+specifications, should be accurate to 1/16 inch.
+
+
+LATERAL DIHEDRAL ANGLE.--One method of securing this is as follows,
+and this method will, at the same time, secure the correct angle of
+incidence:
+
+The strings, drawn very tight, must be taken over both the main and rear
+spars of the top surface. They must run between points on the spars just
+inside the outer struts. The set measurement (which should be accurate
+to 1/16 inch or less) is then from the strings down to four points on
+the main and rear spars of the centre-section surface. These points
+should be just inside the four centre-section struts; that is to say,
+as far as possible away from the centre of the centre-section. Do
+not attempt to take the set measurement near the centre of the
+centre-section.
+
+The strings should be as tight as possible, and, if it can be arranged,
+the best way to accomplish that is as shown in the above illustration,
+i.e., by weighting the strings down to the spars by means of weights
+and tying each end of the strings to a strut. This will give a tight and
+motionless string.
+
+However carefully the above adjustment is made, there is sure to be some
+slight error. This is of no great importance, provided it is divided
+equally between the left- and right-hand wings. In order to make sure of
+this, certain check measurements should be taken as follows:
+
+Each bay must be diagonally measured, and such measurements must be the
+same to within 1/16 inch on each side of the aeroplane. As a rule such
+diagonal measurements are taken from the bottom socket of one strut to
+the top socket of another strut, but this is bad practice, because of
+possible inaccuracies due to faulty manufacture.
+
+The points between which the diagonal measurements are taken should be
+at fixed distances from the butts of the spars, such distances being the
+same on each side of the aeroplane, thus:
+
+
+It would be better to use the centre line of the aeroplane rather than
+the butts of the spars. It is not practicable to do so, however, as the
+centre line probably runs through the petrol tanks, etc.
+
+
+THE DIHEDRAL BOARD.--Another method of securing the dihedral angle, and
+also the angle of incidence, is by means of the dihedral board. It is
+a light handy thing to use, but leads to many errors, and should not be
+used unless necessary. The reasons are as follows:
+
+The dihedral board is probably not true. If it must be used, then it
+should be very carefully tested for truth before-hand. Another reason
+against its use is that it has to be placed on the spars in a position
+between the struts, and that is just where the spars may have a little
+permanent set up or down, or some inaccuracy of surface which will, of
+course, throw out the accuracy of the adjustment. The method of using it
+is as follows:
+
+The board is cut to the same angle as that specified for the upward
+inclination of the surface towards its wing-tips. It is placed on the
+spar as indicated above, and it is provided with two short legs to raise
+it above the flanges of the ribs (which cross over the spars), as they
+may vary in depth. A spirit-level is then placed on the board, and the
+wires must be adjusted to give the surface such an inclination as to
+result in the bubble being in the centre of the level. This operation
+must be performed in respect of each bay both front and rear. The bays
+must then be diagonally measured as already explained.
+
+
+YET ANOTHER METHOD of finding the dihedral angle, and at the same time
+the angle of incidence, is as follows:
+
+A horizontal line is taken from underneath the butt of each spar, and
+the set measurement is either the angle it makes with the spar, or
+a fixed measurement from the line to the spar taken at a specified
+distance from the butt. This operation must be performed in respect of
+both main and rear spars, and all the bays must be measured diagonally
+afterwards.
+
+
+
+Whichever method is used, be sure that after the job is done the spars
+are perfectly straight.
+
+
+STAGGER.--The stagger is the distance the top surface is in advance of
+the bottom surface when the aeroplane is in flying position. The set
+measurement is obtained as follows:
+
+Plumb-lines must be dropped over the leading edge of the top surface
+wherever struts occur, and also near the fuselage. The set measurement
+is taken from the front of the lower leading edge to the plumb-lines. It
+makes a difference whether the measurement is taken along a horizontal
+line (which can be found by using a straight-edge and a spirit-level)
+or along a projection of the chord. The line along which the measurement
+should be taken is laid down in the aeroplane's specifications.
+
+If a mistake is made and the measurement taken along the wrong line, it
+may result in a difference of perhaps 1/4 will, in flight, be nose-heavy
+or tail-heavy.
+
+After the adjustments of the angles of incidence, dihedral, and stagger
+have been secured, it is as well to confirm all of them, as, in making
+the last adjustment, the first one may have been spoiled.
+
+
+OVER-ALL ADJUSTMENTS.--The following over-all check measurements should
+now be taken.
+
+The straight lines AC and BC should be equal to within 1/8 inch. The
+point C is the centre of the propeller, or, in the case of a “pusher”
+ aeroplane, the centre of the nacelle. The points A and B are marked on
+the main spar, and must in each case be the same distance from the butt
+of the spar. The rigger should not attempt to make A and B merely the
+sockets of the outer struts, as they may not have been placed quite
+accurately by the manufacturer. The lines AC and BC must be taken
+from both top and bottom spars--two measurements on each side of the
+aeroplane.
+
+The two measurements FD and FE should be equal to within 1/8 inch. F is
+the centre of the fuselage or rudder-post. D and E are points marked on
+both top and bottom rear spars, and each must be the same fixed
+distance from the butt of the spar. Two measurements on each side of the
+aeroplane.
+
+If these over-all measurements are not correct, then it is probably due
+to some of the drift or anti-drift wires being too tight or too slack.
+It may possibly be due to the fuselage being out of truth, but of course
+the rigger should have made quite sure that the fuselage was true before
+rigging the rest of the machine. Again, it may be due to the internal
+bracing wires within the lifting surfaces not being accurately adjusted,
+but of course this should have been seen to before covering the surfaces
+with fabric.
+
+
+FUSELAGE.--The method of truing the fuselage is laid down in the
+aeroplane's specifications. After it has been adjusted according to the
+specified directions, it should then be arranged on trestles in such
+a way as to make about three-quarters of it towards the tail stick out
+unsupported. In this way it will assume a condition as near as
+possible to flying conditions, and when it is in this position the set
+measurements should be confirmed. If this is not done it may be out of
+truth, but perhaps appear all right when supported by trestles at both
+ends, as, in such case, its weight may keep it true as long as it is
+resting upon the trestles.
+
+
+THE TAIL-PLANE (EMPENNAGE).--The exact angle of incidence of the
+tail-plane is laid down in the aeroplane's specifications. It is
+necessary to make sure that the spars are horizontal when the aeroplane
+is in flying position and the tail unsupported as explained above under
+the heading of Fuselage. If the spars are tapered, then make sure that
+their centre lines are horizontal.
+
+
+UNDERCARRIAGE.--The undercarriage must be very carefully aligned as laid
+down in the specifications.
+
+1. The aeroplane must be placed in its flying position and sufficiently
+high to ensure the wheels being off the ground when rigged. When in this
+position the axle must be horizontal and the bracing wires adjusted to
+secure the various set measurements stated in the specifications.
+
+2. Make sure that the struts bed well down into their sockets.
+
+3. Make sure that the shock absorbers are of equal tension. In the case
+of rubber shock absorbers, both the number of turns and the lengths must
+be equal.
+
+
+HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.
+
+
+DIRECTIONAL STABILITY will be badly affected if there is more drift
+(i.e., resistance) on one side of the aeroplane than there is on the
+other side. The aeroplane will tend to turn towards the side having the
+most drift. This may be caused as follows:
+
+1. The angle of incidence of the main surface or the tail surface may
+be wrong. The greater the angle of incidence, the greater the drift. The
+less the angle, the less the drift.
+
+2. If the alignment of the fuselage, fin in front of the rudder, the
+struts or stream-line wires, or, in the case of the Maurice Farman, the
+front outriggers, are not absolutely correct--that is to say, if they
+are turned a little to the left or to the right instead of being in line
+with the direction of flight--then they will act as a rudder and cause
+the aeroplane to turn off its course.
+
+3. If any part of the surface is distorted, it will cause the aeroplane
+to turn off its course. The surface is cambered, i.e., curved, to pass
+through the air with the least possible drift. If, owing perhaps to the
+leading edge, spars, or trailing edge becoming bent, the curvature is
+spoiled, that will result in changing the amount of drift on one side of
+the aeroplane, which will then have a tendency to turn off its course.
+
+
+LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible reason
+for such a condition is a difference in the lifts of right and left
+wings. That may be caused as follows:
+
+1. The angle of incidence may be wrong. If it is too great, it will
+produce more lift than on the other side of the aeroplane; and if too
+small, it will produce less lift than on the other side--the result
+being that, in either case, the aeroplane will try to fly one wing down.
+
+2. Distorted Surfaces.--If some part of the surface is distorted, then
+its camber is spoiled, and the lift will not be the same on both sides
+of the aeroplane, and that, of course, will cause it to fly one wing
+down.
+
+
+LONGITUDINAL INSTABILITY may be due to the following reasons:
+
+1. The stagger may be wrong. The top surface may have drifted back a
+little owing to some of the wires, probably the incidence wires, having
+elongated their loops or having pulled the fittings into the wood. If
+the top surface is not staggered forward to the correct degree, then
+consequently the whole of its lift is too far back, and it will then
+have a tendency to lift up the tail of the machine too much. The
+aeroplane would then be said to be “nose-heavy.”
+
+A 1/4-inch area in the stagger will make a very considerable difference
+to the longitudinal stability.
+
+2. If the angle of incidence of the main surface is not right, it will
+have a bad effect, especially in the case of an aeroplane with a lifting
+tail-plane.
+
+If the angle is too great, it will produce an excess of lift, and that
+may lift up the nose of the aeroplane and result in a tendency to fly
+“tail-down.” If the angle is too small, it will produce a decreased
+lift, and the aeroplane may have a tendency to fly “nose-down.”
+
+3. The fuselage may have become warped upward or downward, thus giving
+the tail-plane an incorrect angle of incidence. If it has too much
+angle, it will lift too much, and the aeroplane will be “nose-heavy.” If
+it has too little angle, then it will not lift enough, and the aeroplane
+will be “tail-heavy.”
+
+4. (The least likely reason.) The tail-plane may be mounted upon
+the fuselage at a wrong angle of incidence, in which case it must
+be corrected. If nose-heavy, it should be given a smaller angle of
+incidence. If tail-heavy, it should be given a larger angle; but
+care should be taken not to give it too great an angle, because the
+longitudinal stability entirely depends upon the tail-plane being set at
+a much smaller angle of incidence than is the main surface, and if
+that difference is decreased too much, the aeroplane will become
+uncontrollable longitudinally. Sometimes the tail-plane is mounted on
+the aeroplane at the same angle as the main surface, but it actually
+engages the air at a lesser angle, owing to the air being deflected
+downwards by the main surface. There is then, in effect, a longitudinal
+dihedral as explained and illustrated in Chapter I.
+
+
+CLIMBS BADLY.--Such a condition is, apart from engine or propeller
+trouble, probably due to (1) distorted surfaces, or (2) too small an
+angle of incidence.
+
+
+FLIGHT SPEED POOR.--Such a condition is, apart from engine or propeller
+trouble, probably due to (1) distorted surfaces, (2) too great an
+angle of incidence, or (3) dirt or mud, and consequently excessive
+skin-friction.
+
+
+INEFFICIENT CONTROL is probably due to (1) wrong setting of control
+surfaces, (2) distortion of control surfaces, or (3) control cables
+being badly tensioned.
+
+
+WILL NOT TAXI STRAIGHT.--If the aeroplane is uncontrollable on the
+ground, it is probably due to (1) alignment of undercarriage being
+wrong, or (2) unequal tension of shock absorbers.
+
+
+
+
+CHAPTER IV. THE PROPELLER, OR “AIR-SCREW”
+
+The sole object of the propeller is to translate the power of the engine
+into thrust.
+
+The propeller screws through the air, and its blades, being set at an
+angle inclined to the direction of motion, secure a reaction, as in the
+case of the aeroplane's lifting surface.
+
+This reaction may be conveniently divided into two component parts or
+values, namely, Thrust and Drift.
+
+The Thrust is opposed to the Drift of the aeroplane, and must be equal
+and opposite to it at flying speed. If it falls off in power, then the
+flying speed must decrease to a velocity, at which the aeroplane drift
+equals the decreased thrust.
+
+The Drift of the propeller may be conveniently divided into the
+following component values:
+
+
+Active Drift, produced by the useful thrusting part of the propeller.
+
+
+Passive Drift, produced by all the rest of the propeller, i.e., by its
+detrimental surface.
+
+
+Skin Friction, produced by the friction of the air with roughnesses of
+surface.
+
+
+Eddies attending the movement of the air caused by the action of the
+propeller.
+
+
+Cavitation (very marked at excessive speed of revolution). A tendency of
+the propeller to produce a cavity or semi-vacuum in which it revolves,
+the thrust decreasing with increase of speed and cavitation.
+
+
+THRUST-DRIFT RATIO.--The proportion of thrust to drift is of paramount
+importance, for it expresses the efficiency of the propeller. It is
+affected by the following factors: Speed of Revolution.--The greater the
+speed, the greater the proportion of drift to thrust. This is due to
+the increase with speed of the passive drift, which carries with it no
+increase in thrust. For this reason propellers are often geared down to
+revolve at a lower speed than that of the engine.
+
+
+Angle of Incidence.--The same reasons as in the case of the aeroplane
+surface.
+
+Surface Area.--Ditto.
+
+Aspect Ratio.--Ditto.
+
+Camber.--Ditto.
+
+In addition to the above factors there are, when it comes to actually
+designing a propeller, mechanical difficulties to consider. For
+instance, the blades must be of a certain strength and consequent
+thickness. That, in itself, limits the aspect ratio, for it will
+necessitate a chord long enough in proportion to the thickness to make
+a good camber possible. Again, the diameter of the propeller must be
+limited, having regard to the fact that greater diameters than those
+used to-day would not only result in excessive weight of construction,
+but would also necessitate a very high undercarriage to keep the
+propeller off the ground, and such undercarriage would not only produce
+excessive drift, but would also tend to make the aeroplane stand on
+its nose when alighting. The latter difficulty cannot be overcome by
+mounting the propeller higher, as the centre of its thrust must be
+approximately coincident with the centre of aeroplane drift.
+
+
+MAINTENANCE OF EFFICIENCY.
+
+
+The following conditions must be observed:
+
+
+1. PITCH ANGLE.--The angle, at any given point on the propeller, at
+which the blade is set is known as the pitch angle, and it must be
+correct to half a degree if reasonable efficiency is to be maintained.
+
+This angle secures the “pitch,” which is the distance the propeller
+advances during one revolution, supposing the air to be solid. The air,
+as a matter of fact, gives back to the thrust of the blades just as the
+pebbles slip back as one ascends a shingle beach. Such “give-back” is
+known as Slip. If a propeller has a pitch of, say, 10 feet, but actually
+advances, say, only 8 feet owing to slip, then it will be said to
+possess 20 per cent. slip.
+
+Thus, the pitch must equal the flying speed of the aeroplane plus
+the slip of the propeller. For example, let us find the pitch of a
+propeller, given the following conditions:
+
+          Flying speed.............. 70 miles per hour.
+          Propeller revolutions..... 1,200 per minute.
+          Slip...................... 15 per cent.
+
+First find the distance in feet the aeroplane will travel forward in one
+minute. That is--
+
+   369,600 feet (70 miles)
+   ------------------------ = 6,160 feet per minute.
+      60     “ (minutes)
+
+
+Now divide the feet per minute by the propeller revolutions per minute,
+add 15 per cent. for the slip, and the result will be the propeller
+pitch:
+
+     6,160
+     ----- + 15 per cent. = 5 feet 1 3/5 inches.
+     1,200
+
+
+In order to secure a constant pitch from root to tip of blade, the pitch
+angle decreases towards the tip. This is necessary, since the end of the
+blade travels faster than its root, and yet must advance forward at the
+same speed as the rest of the propeller. For example, two men ascending
+a hill. One prefers to walk fast and the other slowly, but they wish to
+arrive at the top of the hill simultaneously. Then the fast walker
+must travel a farther distance than the slow one, and his angle of path
+(pitch angle) must be smaller than the angle of path taken by the slow
+walker. Their pitch angles are different, but their pitch (in this case
+altitude reached in a given time) is the same.
+
+In order to test the pitch angle, the propeller must be mounted upon
+a shaft at right angles to a beam the face of which must be perfectly
+level, thus:
+
+First select a point on the blade at some distance (say about 2 feet)
+from the centre of the propeller. At that point find, by means of a
+protractor, the angle a projection of the chord makes with the face of
+the beam. That angle is the pitch angle of the blade at that point.
+
+Now lay out the angle on paper, thus:
+
+The line above and parallel to the circumference line must be placed
+in a position making the distance between the two lines equal to the
+specified pitch, which is, or should be, marked upon the boss of the
+propeller.
+
+Now find the circumference of the propeller where the pitch angle is
+being tested. For example, if that place is 2 feet radius from the
+centre, then the circumference will be 2 feet X 2 = 4 feet diameter,
+which, if multiplied by 3.1416 = 15.56 feet circumference.
+
+Now mark off the circumference distance, which is represented above by
+A-B, and reduce it in scale for convenience.
+
+The distance a vertical line makes between B and the chord dine is
+the pitch at the point where the angle is being tested, and it should
+coincide with the specified pitch. You will note, from the above
+illustration, that the actual pitch line should meet the junction of the
+chord line and top line.
+
+The propeller should be tested at several points, about a foot apart, on
+each blade; and the diagram, provided the propeller is not faulty, will
+then look like this:
+
+At each point tested the actual pitch coincides with the specified
+pitch: a satisfactory condition.
+
+A faulty propeller will produce a diagram something like this:
+
+
+At every point tested the pitch angle is wrong, for nowhere does the
+actual pitch coincide with the specified pitch. Angles A, C, and D, are
+too large, and B is too small. The angle should be correct to half a
+degree if reasonable efficiency is to be maintained.
+
+A fault in the pitch angle may be due to (1) faulty manufacture,
+(2) distortion, or (3) the shaft hole through the boss being out of
+position.
+
+
+2. STRAIGHTNESS.--To test for straightness the propeller must be mounted
+upon a shaft. Now bring the tip of one blade round to graze some fixed
+object. Mark the point it grazes. Now bring the other tip round, and it
+should come within 1/8 inch of the mark. If it does not do so, it is due
+to (1) faulty manufacture, (2) distortion, or (3) to the hole through
+the boss being out of position.
+
+
+3. LENGTH.--The blades should be of equal length to inch.
+
+
+4. BALANCE.--The usual method of testing a propeller for balance is as
+follows: Mount it upon a shaft, which must be on ball-bearings. Place
+the propeller in a horizontal position, and it should remain in that
+position. If a weight of a trifle over an ounce placed in a bolt-hole on
+one side of the boss fails to disturb the balance, then the propeller is
+usually regarded as unfit for use.
+
+The above method is rather futile, as it does not test for the balance
+of centrifugal force, which comes into play as soon as the propeller
+revolves. It can be tested as follows:
+
+
+The propeller must be in a horizontal position, and then weighed at
+fixed points, such as A, B, C, D, E, and F, and the weights noted. The
+points A, B, and C must, of course, be at the same fixed distances from
+the centre of the propeller as the points D, E, and F. Now reverse the
+propeller and weigh at each point again. Note the results. The first
+series of weights should correspond to the second series, thus:
+
+      Weight A should equal weight F.
+         “  B   “     “     “  E.
+         “  C   “     “     “  D.
+
+
+There is no standard practice as to the degree of error permissible, but
+if there are any appreciable differences the propeller is unfit for use.
+
+
+5. SURFACE AREA.--The surface area of the blades should be equal. Test
+with callipers thus:
+
+The points between which the distances are taken must, of course, be at
+the same distance from the centre in the case of each blade.
+
+There is no standard practice as to the degree of error permissible.
+If, however, there is an error of over 1/8 inch, the propeller is really
+unfit for use.
+
+
+6. CAMBER.--The camber (curvature) of the blades should be (1) equal,
+(2) decrease evenly towards the tips of the blades, and (3) the greatest
+depth of the curve should, at any point of the blade, be approximately
+at the same percentage of the chord from the leading edge as at other
+points.
+
+It is difficult to test the top camber without a set of templates, but
+a fairly accurate idea of the concave camber can be secured by slowly
+passing a straight-edge along the blade, thus:
+
+The camber can now be easily seen, and as the straight-edge is passed
+along the blade, the observer should look for any irregularities of the
+curvature, which should gradually and evenly decrease towards the tip of
+the blade.
+
+
+7. THE JOINTS.--The usual method for testing the glued joints is by
+revolving the propeller at greater speed than it will be called upon to
+make during flight, and then carefully examining the joints to see if
+they have opened. It is not likely, however, that the reader will have
+the opportunity of making this test. He should, however, examine all the
+joints very carefully, trying by hand to see if they are quite sound.
+Suspect a propeller of which the joints appear to hold any thickness of
+glue. Sometimes the joints in the boss open a little, but this is not
+dangerous unless they extend to the blades, as the bolts will hold the
+laminations together.
+
+
+8. CONDITION OF SURFACE.--The surface should be very smooth, especially
+towards the tips of the blades. Some propeller tips have a speed of
+over 30,000 feet a minute, and any roughness will produce a bad drift or
+resistance and lower the efficiency.
+
+
+9. MOUNTING.--Great care should be taken to see that the propeller
+is mounted quite straight on its shaft. Test in the same way as for
+straightness. If it is not straight, it is possibly due to some of the
+propeller bolts being too slack or to others having been pulled up too
+tightly.
+
+
+FLUTTER.--Propeller “flutter,” or vibration, may be due to faulty pitch
+angle, balance, camber, or surface area. It causes a condition sometimes
+mistaken for engine trouble, and one which may easily lead to the
+collapse of the propeller.
+
+
+CARE OF PROPELLERS.--The care of propellers is of the greatest
+importance, as they become distorted very easily.
+
+
+1. Do not store them in a very damp or a very dry place.
+
+
+2. Do not store them where the sun will shine upon them.
+
+
+3. Never leave them long in a horizontal position or leaning up against
+a wall.
+
+
+4. They should be hung on horizontal pegs, and the position of the
+propellers should be vertical.
+
+
+If the points I have impressed upon you in these notes are not attended
+to, you may be sure of the following results:
+
+
+1. Lack of efficiency, resulting in less aeroplane speed and climb than
+would otherwise be the case.
+
+
+2. Propeller “flutter” and possible collapse.
+
+
+3. A bad stress upon the propeller shaft and its bearings.
+
+
+TRACTOR.--A propeller mounted in front of the main surface.
+
+
+PUSHER.--A propeller mounted behind the main surface.
+
+
+FOUR-BLADED PROPELLERS.--Four-bladed propellers are suitable only when
+the pitch is comparatively large.
+
+For a given pitch, and having regard to “interference,” they are not so
+efficient as two-bladed propellers.
+
+The smaller the pitch, the less the “gap,” i.e., the distance, measured
+in the direction of the thrust, between the spiral courses of the
+blades.
+
+If the gap is too small, then the following blade will engage air
+which the preceding blade has put into motion, with the result that the
+following blade will not secure as good a reaction as would otherwise be
+the case. It is very much the same as in the case of the aeroplane gap.
+
+For a given pitch, the gap of a four-bladed propeller is only half
+that of a two-bladed one. Therefore the four-bladed propeller is only
+suitable for large pitch, as such pitch produces spirals with a large
+gap, thus offsetting the decrease in gap caused by the numerous blades.
+
+The greater the speed of rotation, the less the pitch for a given
+aeroplane speed. Then, in order to secure a large pitch and consequently
+a good gap, the four-bladed propeller is usually geared to rotate at a
+lower speed than would be the case if directly attached to the engine
+crank-shaft.
+
+
+
+
+CHAPTER V. MAINTENANCE
+
+CLEANLINESS.--The fabric must be kept clean and free from oil, as that
+will rot it. To take out dirt or oily patches, try acetone. If that will
+not remedy matters, then try petrol, but use it sparingly, as otherwise
+it will take off an unnecessary amount of dope. If that will not remove
+the dirt, then hot water and soap will do so, but, in that case, be
+sure to use soap having no alkali in it, as otherwise it may injure the
+fabric. Use the water sparingly, or it may get inside the planes and
+rust the internal bracing wires, or cause some of the wooden framework
+to swell.
+
+The wheels of the undercarriage have a way of throwing up mud on to
+the lower surface. This should, if possible, be taken off while wet. It
+should never be scraped off when dry, as that may injure the fabric. If
+dry, then it should be moistened before being removed.
+
+Measures should be taken to prevent dirt from collecting upon any
+part of the aeroplane, as, otherwise, excessive skin-friction will be
+produced with resultant loss of flight speed. The wires, being greasy,
+collect dirt very easily.
+
+
+CONTROL CABLES.--After every flight the rigger should pass his hand over
+the control cables and carefully examine them near pulleys. Removal of
+grease may be necessary to make a close inspection possible. If only one
+strand is broken the wire should be replaced. Do not forget the aileron
+balance wire on the top surface.
+
+Once a day try the tension of the control cables by smartly moving the
+control levers about as explained elsewhere.
+
+
+WIRES.--All the wires should be kept well greased or oiled, and in the
+correct tension. When examining the wires, it is necessary to place the
+aeroplane on level ground, as otherwise it may be twisted, thus throwing
+some wires into undue tension and slackening others. The best way, if
+there is time, is to pack the machine up into its “flying position.”
+
+If you see a slack wire, do not jump to the conclusion that it must
+be tensioned. Perhaps its opposition wire is too tight, in which case
+slacken it, and possibly you will find that will tighten the slack wire.
+
+Carefully examine all wires and their connections near the propeller,
+and be sure that they are snaked round with safety wire, so that the
+latter may keep them out of the way of the propeller if they come
+adrift.
+
+The wires inside the fuselage should be cleaned and regreased about once
+a fortnight.
+
+
+STRUTS AND SOCKETS.--These should be carefully examined to see if any
+splitting has occurred.
+
+
+DISTORTION.--Carefully examine all surfaces, including the controlling
+surfaces, to see whether any distortion has occurred. If distortion can
+be corrected by the adjustment of wires, well and good; but if not, then
+some of the internal framework probably requires replacement.
+
+
+ADJUSTMENTS.--Verify the angles of incidence; dihedral, and stagger, and
+the rigging position of the controlling-surfaces, as often as possible.
+
+
+UNDERCARRIAGE.--Constantly examine the alignment and fittings of the
+undercarriage, and the condition of tyres and shock absorbers. The
+latter, when made of rubber, wear quickest underneath. Inspect axles and
+skids to see if there are any signs of them becoming bent. The wheels
+should be taken off occasionally and greased.
+
+
+LOCKING ARRANGEMENTS.--Constantly inspect the locking arrangements of
+turnbuckles, bolts, etc. Pay particular attention to the control cable
+connections, and to all moving parts in respect of the controls.
+
+
+LUBRICATION.--Keep all moving parts, such as pulleys, control levers,
+and hinges of controlling surfaces, well greased.
+
+
+SPECIAL INSPECTION.--Apart from constantly examining the aeroplane with
+reference to the above points I have made, I think that, in the case of
+an aeroplane in constant use it is an excellent thing to make a special
+inspection of every part, say once a week. This will take from two to
+three hours, according to the type of aeroplane. In order to carry it
+out methodically, the rigger should have a list of every part down to
+the smallest split-pin. He can then check the parts as he examines them,
+and nothing will be passed over. This, I know from experience, greatly
+increases the confidence of the pilot, and tends to produce good work in
+the air.
+
+
+WINDY WEATHER.--The aeroplane, when on the ground, should face the
+wind; and it is advisable to lash the control lever fast, so that the
+controlling surfaces may not be blown about and possibly damaged.
+
+
+“VETTING” BY EYE.--This should be practiced at every opportunity, and,
+if persevered in, it is possible to become quite expert in diagnosing by
+eye faults in flight efficiency, stability and control.
+
+The aeroplane should be standing upon level ground, or, better than
+that, packed up into its “flying position.”
+
+Now stand in front of it and line up the leading edge with the main
+spar, rear spar, and trailing edge. Their shadows can usually be seen
+through the fabric. Allowance must, of course, be made for wash-in and
+wash-out; otherwise, the parts I have specified should be parallel with
+each other.
+
+Now line up the centre part of the main-plane with the tail-plane. The
+latter should be horizontal.
+
+Next, sight each interplane front strut with its rear strut. They should
+be parallel.
+
+Then, standing on one side of the aeroplane, sight all the front struts.
+The one nearest to you should cover all the others. This applies to the
+rear struts also.
+
+Look for distortion of leading edges, main and rear spars, trailing
+edges, tail-plane and controlling surfaces.
+
+This sort of thing, if practiced constantly, will not only develop an
+expert eye for diagnosis of faults, but will also greatly assist in
+impressing upon the memory the characteristics and possible troubles of
+the various types of aeroplanes.
+
+
+MISHANDLING OF THE GROUND.--This is the cause of a lot of unnecessary
+damage. The golden rule to observe is: PRODUCE NO BENDING STRESSES.
+
+Nearly all the wood in an aeroplane is designed to take merely the
+stress of direct compression, and it cannot be bent safely. Therefore,
+in packing an aeroplane up from the ground, or in pulling or pushing it
+about, be careful to stress it in such a way as to produce, as far
+as possible, only direct compression stresses. For instance, if it is
+necessary to support the lifting surface, then the packing should be
+arranged to come directly under the struts so that they may take the
+stress in the form of compression for which they are designed. Such
+supports should be covered with soft packing in order to prevent the
+fabric from becoming damaged.
+
+When pulling an aeroplane along, if possible, pull from the top of the
+undercarriage struts. If necessary to pull from elsewhere, then do so by
+grasping the interplane struts as low down as possible.
+
+Never lay fabric-covered parts upon a concrete floor. Any slight
+movement will cause the fabric to scrape over the floor with resultant
+damage.
+
+Struts, spars, etc., should never be left about the floor, as in such
+position they are likely to become scored. I have already explained the
+importance of protecting the outside fibres of the wood. Remember
+also that wood becomes distorted easily. This particularly applies to
+interplane struts. If there are no proper racks to stand them in, then
+the best plan is to lean them up against the wall in as near a vertical
+position as possible.
+
+
+TIME.--Learn to know the time necessary to complete any of the various
+rigging jobs. This is really important. Ignorance of this will lead
+to bitter disappointments in civil life; and, where Service flying
+is concerned, it will, to say the least of it, earn unpopularity with
+senior officers, and fail to develop respect and good work where men are
+concerned.
+
+
+THE AEROPLANE SHED.--This should be kept as clean and orderly as
+possible. A clean, smart shed produces briskness, energy, and pride of
+work. A dirty, disorderly shed nearly always produces slackness and poor
+quality of work, lost tools and mislaid material.
+
+
+
+
+GLOSSARY
+
+Aeronautics--The science of aerial navigation.
+
+Aerofoil--A rigid structure, of large superficial area relative to its
+thickness, designed to obtain, when driven through the air at an
+angle inclined to the direction of motion, a reaction from the air
+approximately at right angles to its surface. Always cambered when
+intended to secure a reaction in one direction only. As the term
+“aerofoil” is hardly ever used in practical aeronautics, I have,
+throughout this book, used the term SURFACE, which, while academically
+incorrect, since it does not indicate thickness, is a term usually
+used to describe the cambered lifting surfaces, i.e., the “planes” or
+“wings,” and the stabilizers and the controlling aerofoils.
+
+Aerodrome--The name usually applied to a ground used for the practice
+of aviation. It really means “flying machine,” but is never used in that
+sense nowadays.
+
+Aeroplane--A power-driven aerofoil with stabilizing and controlling
+surfaces.
+
+Acceleration--The rate of change of velocity.
+
+Angle of Incidence--The angle at which the “neutral lift line” of a
+surface attacks the air.
+
+Angle of Incidence, Rigger's--The angle the chord of a surface makes
+with a line parallel to the axis of the propeller.
+
+Angle of Incidence, Maximum--The greatest angle of incidence at which,
+for a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+Angle of Incidence, Minimum--The smallest angle of incidence at which,
+for a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+Angle of Incidence, Best Climbing--That angle of incidence at which an
+aeroplane ascends quickest. An angle approximately halfway between the
+maximum and optimum angles.
+
+Angle of Incidence, Optimum--The angle of incidence at which the
+lift-drift ratio is the highest.
+
+
+Angle, Gliding--The angle between the horizontal and the path along
+which an aeroplane at normal flying speed, but not under engine power,
+descends in still air.
+
+Angle, Dihedral--The angle between two planes.
+
+Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
+be at a lateral dihedral angle when it is inclined upward towards its
+wing-tips.
+
+Angle, Longitudinal Dihedral--The main surface and tail surface are said
+to be at a longitudinal dihedral angle when the projections of their
+neutral lift lines meet and produce an angle above them.
+
+Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting “chords”
+ for “neutral life lines.”
+
+Angle, Pitch--The angle at any given point of a propeller, at which
+the blade is inclined to the direction of motion when the propeller is
+revolving but the aeroplane stationary.
+
+Altimeter--An instrument used for measuring height.
+
+Air-Speed Indicator--An instrument used for measuring air pressures or
+velocities. It consequently indicates whether the surface is securing
+the requisite reaction for flight. Usually calibrated in miles per hour,
+in which case it indicates the correct number of miles per hour at only
+one altitude. This is owing to the density of the air decreasing with
+increase of altitude and necessitating a greater speed through space
+to secure the same air pressure as would be secured by less speed at a
+lower altitude. It would be more correct to calibrate it in units of air
+pressure.
+
+Air Pocket--A local movement or condition of the air causing an
+aeroplane to drop or lose its correct attitude.
+
+Aspect-Ratio--The proportion of span to chord of a surface.
+
+Air-Screw (Propeller)--A surface so shaped that its rotation about an
+axis produces a force (thrust) in the direction of its axis.
+
+Aileron--A controlling surface, usually situated at the wing-tip, the
+operation of which turns an aeroplane about its longitudinal axis;
+causes an aeroplane to tilt sideways.
+
+
+Aviation--The art of driving an aeroplane.
+
+Aviator--The driver of an aeroplane.
+
+Barograph--A recording barometer, the charts of which can be calibrated
+for showing air density or height.
+
+Barometer--An instrument used for indicating the density of air.
+
+Bank, to--To turn an aeroplane about its longitudinal axis (to tilt
+sideways) when turning to left or right.
+
+Biplane--An aeroplane of which the main lifting surface consists of a
+surface or pair of wings mounted above another surface or pair of wings.
+
+Bay--The space enclosed by two struts and whatever they are fixed to.
+
+Boom--A term usually applied to the long spars joining the tail of a
+“pusher” aeroplane to its main lifting surface.
+
+Bracing--A system of struts and tie wires to transfer a force from one
+point to another.
+
+Canard--Literally “duck.” The name which was given to a type of
+aeroplane of which the longitudinal stabilizing surface (empennage)
+was mounted in front of the main lifting surface. Sometimes termed
+“tail-first” aeroplanes, but such term is erroneous, as in such a design
+the main lifting surface acts as, and is, the empennage.
+
+Cabre--To fly or glide at an excessive angle of incidence; tail down.
+
+Camber--Curvature.
+
+Chord--Usually taken to be a straight line between the trailing and
+leading edges of a surface.
+
+Cell--The whole of the lower surface, that part of the upper surface
+directly over it, together with the struts and wires holding them
+together.
+
+Centre (Line) of Pressure--A line running from wing-tip to wing-tip, and
+through which all the air forces acting upon the surface may be said to
+act, or about which they may be said to balance.
+
+Centre (Line) of Pressure, Resultant--A line transverse to the
+longitudinal axis, and the position of which is the resultant of the
+centres of pressure of two or more surfaces.
+
+Centre of Gravity--The centre of weight.
+
+Cabane--A combination of two pylons, situated over the fuselage, and
+from which anti-lift wires are suspended.
+
+Cloche--Literally “bell.” Is applied to the bell-shaped construction
+which forms the lower part of the pilot's control lever in a Bleriot
+monoplane, and to which the control cables are attached.
+
+Centrifugal Force--Every body which moves in a curved path is urged
+outwards from the centre of the curve by a force termed “centrifugal.”
+
+Control Lever--A lever by means of which the controlling surfaces
+are operated. It usually operates the ailerons and elevator. The
+“joy-stick”.
+
+Cavitation, Propeller--The tendency to produce a cavity in the air.
+
+Distance Piece--A long, thin piece of wood (sometimes tape) passing
+through and attached to all the ribs in order to prevent them from
+rolling over sideways.
+
+Displacement--Change of position.
+
+Drift (of an aeroplane as distinct from the propeller)--The horizontal
+component of the reaction produced by the action of driving through the
+air a surface inclined upwards and towards its direction of motion PLUS
+the horizontal component of the reaction produced by the “detrimental”
+ surface PLUS resistance due to “skin-friction.” Sometimes termed
+“head-resistance.”
+
+Drift, Active--Drift produced by the lifting surface.
+
+Drift, Passive--Drift produced by the detrimental surface.
+
+Drift (of a propeller)--Analogous to the drift of an aeroplane. It is
+convenient to include “cavitation” within this term.
+
+Drift, to--To be carried by a current of air; to make leeway.
+
+Dive, to--To descend so steeply as to produce a speed greater than the
+normal flying speed.
+
+Dope, to--To paint a fabric with a special fluid for the purpose of
+tightening and protecting it.
+
+Density--Mass of unit volume, for instance, pounds per cubic foot.
+
+Efficiency--Output            Input
+
+Efficiency (of an aeroplane as distinct from engine and propeller)--
+
+       Lift and Velocity
+    Thrust (= aeroplane drift)
+
+Efficiency, Engine--Brake horse-power
+
+       Indicated horse-power
+
+Efficiency, Propeller--
+
+                              Thrust horse-power
+                       Horse-power received from engine
+                               (= propeller drift)
+
+NOTE.--The above terms can, of course, be expressed in foot-pounds. It
+is then only necessary to divide the upper term by the lower one to find
+the measure of efficiency.
+
+Elevator--A controlling surface, usually hinged to the rear of the
+tail-plane, the operation of which turns an aeroplane about an axis
+which is transverse to the direction of normal horizontal flight.
+
+Empennage--See “Tail-plane.”
+
+Energy--Stored work. For instance, a given weight of coal or petroleum
+stores a given quantity of energy which may be expressed in foot-pounds.
+
+Extension--That part of the upper surface extending beyond the span of
+the lower surface.
+
+Edge, Leading--The front edge of a surface relative to its normal
+direction of motion.
+
+Edge, Trailing--The rear edge of a surface relative to its normal
+direction of motion.
+
+Factor of Safety--Usually taken to mean the result found by dividing the
+stress at which a body will collapse by the maximum stress it will be
+called upon to bear.
+
+Fineness (of stream-line)--The proportion of length to maximum width.
+
+Flying Position--A special position in which an aeroplane must be placed
+when rigging it or making adjustments. It varies with different types of
+aeroplanes. Would be more correctly described as “rigging position.”
+
+Fuselage--That part of an aeroplane containing the pilot, and to which
+is fixed the tail-plane.
+
+Fin--Additional keel-surface, usually mounted at the rear of an
+aeroplane.
+
+Flange (of a rib)--That horizontal part of a rib which prevents it from
+bending sideways.
+
+Flight--The sustenance of a body heavier than air by means of its action
+upon the air.
+
+Foot-pound--A measure of work representing the weight of 1 lb. raised 1
+foot.
+
+Fairing--Usually made of thin sheet aluminum, wood, or a light
+construction of wood and fabric; and bent round detrimental surface in
+order to give it a “fair” or “stream-like” shape.
+
+Gravity--Is the force of the Earth's attraction upon a body. It
+decreases with increase of distance from the Earth. See “Weight.”
+
+Gravity, Specific--Density of substance                   Density of water.
+Thus, if the density of water is 10 lb. per unit volume, the same unit
+volume of petrol, if weighing 7 lb., would be said to have a specific
+gravity of 7/10, i.e., 0.7.
+
+Gap (of an aeroplane)--The distance between the upper and lower surfaces
+of a biplane. In a triplane or multiplane, the distance between a
+surface and the one first above it.
+
+Gap, Propeller--The distance, measured in the direction of the thrust,
+between the spiral courses of the blades.
+
+Girder--A structure designed to resist bending, and to combine lightness
+and strength.
+
+Gyroscope--A heavy circular wheel revolving at high speed, the effect of
+which is a tendency to maintain its plane of rotation against disturbing
+forces.
+
+Hangar--An aeroplane shed.
+
+Head-Resistance--Drift. The resistance of the air to the passage of a
+body.
+
+Helicopter--An air-screw revolving about a vertical axis, the direction
+of its thrust being opposed to gravity.
+
+Horizontal Equivalent--The plan view of a body whatever its attitude may
+be.
+
+Impulse--A force causing a body to gain or lose momentum.
+
+Inclinometer--A curved form of spirit-level used for indicating the
+attitude of a body relative to the horizontal.
+
+Instability--An inherent tendency of a body, which, if the body is
+disturbed, causes it to move into a position as far as possible away
+from its first position.
+
+Instability, Neutral--An inherent tendency of a body to remain in the
+position given it by the force of a disturbance, with no tendency to
+move farther or to return to its first position.
+
+Inertia--The inherent resistance to displacement of a body as distinct
+from resistance the result of an external force.
+
+Joy-Stick--See “Control Lever.”
+
+Keel-Surface--Everything to be seen when viewing an aeroplane from the
+side of it.
+
+King-Post--A bracing strut; in an aeroplane, usually passing through a
+surface and attached to the main spar, and from the end or ends of which
+wires are taken to spar, surface, or other part of the construction in
+order to prevent distortion. When used in connection with a controlling
+surface, it usually performs the additional function of a lever, control
+cables connecting its ends with the pilot's control lever.
+
+Lift--The vertical component of the reaction produced by the action
+of driving through the air a surface inclined upwards and towards its
+direction of motion.
+
+Lift, Margin of--The height an aeroplane can gain in a given time and
+starting from a given altitude.
+
+Lift-Drift Ratio--The proportion of lift to drift.
+
+Loading--The weight carried by an aerofoil. Usually expressed in pounds
+per square foot of superficial area.
+
+Longeron--The term usually applied to any long spar running length-ways
+of a fuselage.
+
+Mass--The mass of a body is a measure of the quantity of material in it.
+
+Momentum--The product of the mass and velocity of a body is known as
+“momentum.”
+
+Monoplane--An aeroplane of which the main lifting surface consists of
+one surface or one pair of wings.
+
+Multiplane--An aeroplane of which the main lifting surface consists of
+numerous surfaces or pairs of wings mounted one above the other.
+
+Montant--Fuselage strut.
+
+Nacelle--That part of an aeroplane containing the engine and pilot and
+passenger, and to which the tail plane is not fixed.
+
+Neutral Lift Line--A line taken through a surface in a forward direction
+relative to its direction of motion, and starting from its trailing
+edge. If the attitude of the surface is such as to make the said line
+coincident with the direction of motion, it results in no lift, the
+reaction then consisting solely of drift. The position of the neutral
+lift line, i.e., the angle it makes with the chord, varies with
+differences of camber, and it is found by means of wind-tunnel research.
+
+Newton's Laws of Motion--1. If a body be at rest, it will remain at
+rest; or, if in motion, it will move uniformly in a straight line until
+acted upon by some force.
+
+2. The rate of change of the quantity of motion (momentum) is
+proportional to the force which causes it, and takes place in the
+direction of the straight line in which the force acts. If a body be
+acted upon by several forces, it will obey each as though the others did
+not exist, and this whether the body be at rest or in motion.
+
+3. To every action there is opposed an equal and opposite reaction.
+
+Ornithopter (or Orthopter)--A flapping wing design of aircraft intended
+to imitate the flight of a bird.
+
+Outrigger--This term is usually applied to the framework connecting the
+main surface with an elevator placed in advance of it. Sometimes applied
+to the “tail-boom” framework connecting the tail-plane with the main
+lifting surface.
+
+Pancake, to--To “stall ”
+
+Plane--This term is often applied to a lifting surface. Such application
+is not quite correct, since “plane” indicates a flat surface, and the
+lifting surfaces are always cambered.
+
+Propeller--See “Air-Screw.”
+
+Propeller, Tractor--An air-screw mounted in front of the main lifting
+surface.
+
+Propeller, Pusher--An air-screw mounted behind the main lifting surface.
+
+Pusher--An aeroplane of which the propeller is mounted behind the main
+lifting surface.
+
+Pylon--Any V-shaped construction from the point of which wires are
+taken.
+
+Power--Rate of working.
+
+Power, Horse--One horse-power represents a force sufficient to raise
+33,000 lbs. 1 foot in a minute.
+
+Power, Indicated Horse--The I.H.P. of an engine is a measure of the rate
+at which work is done by the pressure upon the piston or pistons, as
+distinct from the rate at which the engine does work. The latter is
+usually termed “brake horse-power,” since it may be measured by an
+absorption brake.
+
+Power, Margin of--The available quantity of power above that necessary
+to maintain horizontal flight at the optimum angle.
+
+Pitot Tube--A form of air-speed indicator consisting of a tube with open
+end facing the wind, which, combined with a static pressure or suction
+tube, is used in conjunction with a gauge for measuring air pressures or
+velocities. (No. 1 in diagram.)
+
+Pitch, Propeller--The distance a propeller advances during one
+revolution supposing the air to be solid.
+
+Pitch, to--To plunge nose-down.
+
+Reaction--A force, equal and opposite to the force of the action
+producing it.
+
+Rudder--A controlling surface, usually hinged to the tail, the operation
+of which turns an aeroplane about an axis which is vertical in normal
+horizontal flight; causes an aeroplane to turn to left or right of the
+pilot.
+
+Roll, to--To turn about the longitudinal axis.
+
+Rib, Ordinary--A light curved wooden part mounted in a fore and aft
+direction within a surface. The ordinary ribs give the surface its
+camber, carry the fabric, and transfer the lift from the fabric to the
+spars.
+
+Rib, Compression--Acts as an ordinary rib, besides bearing the stress of
+compression produced by the tension of the internal bracing wires.
+
+Rib, False--A subsidiary rib, usually used to improve the camber of the
+front part of the surface.
+
+Right and Left Hand--Always used relative to the position of the pilot.
+When observing an aeroplane from the front of it, the right hand side of
+it is then on the left hand of the observer.
+
+Remou--A local movement or condition of the air which may cause
+displacement of an aeroplane.
+
+Rudder-Bar--A control lever moved by the pilot's feet, and operating the
+rudder.
+
+Surface--See “Aerofoil.”
+
+Surface, Detrimental--All exterior parts of an aeroplane including
+the propeller, but excluding the (aeroplane) lifting and (propeller)
+thrusting surfaces.
+
+Surface, Controlling--A surface the operation of which turns an
+aeroplane about one of its axes.
+
+Skin-Friction--The friction of the air with roughness of surface. A form
+of drift.
+
+Span---The distance from wing-tip to wing-tip.
+
+Stagger--The distance the upper surface is forward of the lower surface
+when the axis of the propeller is horizontal.
+
+Stability--The inherent tendency of a body, when disturbed, to return to
+its normal position.
+
+Stability, Directional--The stability about an axis which is vertical
+during normal horizontal flight, and without which an aeroplane has no
+natural tendency to remain upon its course.
+
+Stability, Longitudinal--The stability of an aeroplane about an axis
+transverse to the direction of normal horizontal flight, and without
+which it has no tendency to oppose pitching and tossing.
+
+Stability, Lateral--The stability of an aeroplane about its longitudinal
+axis, and without which it has no tendency to oppose sideways rolling.
+
+Stabilizer--A surface, such as fin or tail-plane, designed to give an
+aeroplane inherent stability.
+
+Stall, to--To give or allow an aeroplane an angle of incidence greater
+than the “maximum” angle, the result being a fall in the lift-drift
+ratio, the lift consequently becoming less than the weight of the
+aeroplane, which must then fall, i.e., “stall” or “pancake.”
+
+Stress--Burden or load.
+
+Strain--Deformation produced by stress.
+
+Side-Slip, to--To fall as a result of an excessive “bank” or “roll.”
+
+Skid, to--To be carried sideways by centrifugal force when turning to
+left or right.
+
+Skid, Undercarriage--A spar, mounted in a fore and aft direction, and to
+which the wheels of the undercarriage are sometimes attached. Should
+a wheel give way the skid is then supposed to act like the runner of a
+sleigh and to support the aeroplane.
+
+Skid, Tail--A piece of wood or other material, orientable, and fitted
+with shock absorbers, situated under the tail of an aeroplane in order
+to support it upon the ground and to absorb the shock of alighting.
+
+
+Section--Any separate part of the top surface, that part of the bottom
+surface immediately underneath it, with their struts and wires.
+
+Spar--Any long piece of wood or other material.
+
+Spar, Main--A spar within a surface and to which all the ribs are
+attached, such spar being the one situated nearest to the centre of
+pressure. It transfers more than half the lift from the ribs to the
+bracing.
+
+Spar, Rear--A spar within a surface, and to which all the ribs are
+attached, such spar being situated at the rear of the centre of pressure
+and at a greater distance from it than is the main spar. It transfers
+less than half of the lift from the ribs to the bracing.
+
+Strut--Any wooden member intended to take merely the stress of direct
+compression.
+
+Strut, Interplane--A strut holding the top and bottom surfaces apart.
+
+Strut, Fuselage--A strut holding the fuselage longerons apart. It should
+be stated whether top, bottom, or side. If side, then it should be
+stated whether right or left hand. Montant.
+
+Strut, Extension--A strut supporting an “extension” when not in flight.
+It may also prevent the extension from collapsing upwards during flight.
+
+Strut, Undercarriage--
+
+Strut, Dope--A strut within a surface, so placed as to prevent the
+tension of the doped fabric from distorting the framework.
+
+Serving--To bind round with wire, cord, or similar material. Usually
+used in connection with wood joints and wire cable splices.
+
+Slip, Propeller--The pitch less the distance the propeller advances
+during one revolution.
+
+Stream-Line--A form or shape of detrimental surface designed to produce
+minimum drift.
+
+Toss, to--To plunge tail-down.
+
+Torque, Propeller--The tendency of a propeller to turn an aeroplane
+about its longitudinal axis in a direction opposite to that in which the
+propeller revolves.
+
+Tail-Slide--A fall whereby the tail of an aeroplane leads.
+
+Tractor--An aeroplane of which the propeller is mounted in front of the
+main lifting surface.
+
+Triplane--An aeroplane of which the main lifting surface consists of
+three surfaces or pairs of wings mounted one above the other.
+
+Tail-Plane--A horizontal stabilizing surface mounted at some distance
+behind the main lifting surface. Empennage.
+
+Turnbuckle--A form of wire-tightener, consisting of a barrel into each
+end of which is screwed an eyebolt. Wires are attached to the eyebolts
+and the required degree of tension is secured by means of rotating the
+barrel.
+
+Thrust, Propeller--See “Air-Screw.”
+
+Undercarriage--That part of an aeroplane beneath the fuselage or
+nacelle, and intended to support the aeroplane when at rest, and to
+absorb the shock of alighting.
+
+Velocity--Rate of displacement; speed.
+
+Volplane--A gliding descent.
+
+Weight--Is a measure of the force of the Earth's attraction (gravity)
+upon a body. The standard unit of weight in this country is 1 lb., and
+is the force of the Earth's attraction on a piece of platinum called
+the standard pound, deposited with the Board of Trade in London. At the
+centre of the Earth a body will be attracted with equal force in
+every direction. It will therefore have no weight, though its mass
+is unchanged. Gravity, of which weight is a measure, decreases with
+increase of altitude.
+
+Web (of a rib)--That vertical part of a rib which prevents it from
+bending upwards.
+
+Warp, to--To distort a surface in order to vary its angle of incidence.
+To vary the angle of incidence of a controlling surface.
+
+Wash--The disturbance of air produced by the flight of an aeroplane.
+
+Wash-in--An increasing angle of incidence of a surface towards its
+wing-tip.
+
+Wash-out--A decreasing angle of incidence of a surface towards its
+wing-tip.
+
+Wing-tip--The right- or left-hand extremity of a surface.
+
+Wire--A wire is, in Aeronautics, always known by the name of its
+function.
+
+Wire, Lift or Flying--A wire opposed to the direction of lift, and used
+to prevent a surface from collapsing upward during flight.
+
+Wire, Anti-lift or Landing--A wire opposed to the direction of gravity,
+and used to sustain a surface when it is at rest.
+
+Wire, Drift--A wire opposed to the direction of drift, and used to
+prevent a surface from collapsing backwards during flight.
+
+Wire, Anti-drift--A wire opposed to the tension of a drift wire, and
+used to prevent such tension from distorting the framework.
+
+Wire, Incidence--A wire running from the top of an interplane strut
+to the bottom of the interplane strut in front of or behind it.
+It maintains the “stagger” and assists in maintaining the angle of
+incidence. Sometimes termed “stagger wire.”
+
+Wire, Bracing--Any wire holding together the framework of any part of
+an aeroplane. It is not, however, usually applied to the wires described
+above unless the function performed includes a function additional to
+those described above. Thus, a lift wire, while strictly speaking a
+bracing wire, is not usually described as one unless it performs the
+additional function of bracing some well-defined part such as the
+undercarriage. It will then be said to be an “undercarriage bracing lift
+wire.” It might, perhaps, be acting as a drift wire also, in which
+case it will then be de-scribed as an “undercarriage bracing lift-drift
+wire.” It should always be stated whether a bracing wire is (1) top, (2)
+bottom, (3) cross, or (4) side. If a “side bracing wire,” then it should
+be stated whether right- or left-hand.
+
+Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)
+within a surface.
+
+Wire, Top Bracing--A bracing wire, approximately horizontal and situated
+between the top longerons of fuselate, between top tail booms, or at the
+top of similar construction.
+
+Wire, Bottom Bracing--Ditto, substituting “bottom” for “top.”
+
+Wire, Side Bracing--A bracing wire crossing diagonally a side bay of
+fuselage, tail boom bay, undercarriage side bay or centre-section side
+bay. This term is not usually used with reference to incidence wires,
+although they cross diagonally the side bays of the cell. It should be
+stated whether right- or left-hand.
+
+Wire, Cross Bracing--A bracing wire, the position of which is diagonal
+from right to left when viewing it from the front of an aeroplane.
+
+Wire, Control Bracing--A wire preventing distortion of a controlling
+surface.
+
+Wire, Control--A wire connecting a controlling surface with the pilot's
+control lever, wheel, or rudder-bar.
+
+Wire, Aileron Gap--A wire connecting top and bottom ailerons.
+
+Wire, Aileron Balance--A wire connecting the right- and left-hand top
+ailerons. Sometimes termed the “aileron compensating wire.”
+
+Wire, Snaking--A wire, usually of soft metal, wound spirally or tied
+round another wire, and attached at each end to the framework. Used to
+prevent the wire round which it is “snaked” from becoming, in the event
+of its displacement, entangled with the propeller.
+
+Wire, Locking--A wire used to prevent a turnbuckle barrel or other
+fitting from losing its adjustment.
+
+Wing--Strictly speaking, a wing is one of the surfaces of an
+ornithopter. The term is, however, often applied to the lifting surface
+of an aeroplane when such surface is divided into two parts, one being
+the left-hand “wing,” and the other the right-hand “wing.”
+
+Wind-Tunnel--A large tube used for experimenting with surfaces and
+models, and through which a current of air is made to flow by artificial
+means.
+
+Work--Force X displacement.
+
+Wind-Screen--A small transparent screen mounted in front of the pilot to
+protect his face from the air pressure.
+
+
+
+
+
+FOOTNOTES:
+
+
+[1] Propeller Slip: As the propeller screws through the air, the
+latter to a certain extent gives back to the thrust of the propellor
+blades, just as the shingle on the beach slips back as you ascend it.
+Such “give-back” is known as “slip,” and anyone behind the propellor
+will feel the slip as a strong draught of air.
+
+[2] Helicopter. An air-screw revolving upon a vertical axis. If driven
+with sufficient power, it will lift vertically, but having regard to the
+mechanical difficulties of such construction, it is a most inefficient
+way of securing lift compared with the arrangement of an inclined
+surface driven by a propeller revolving about a horizontal axis.
+
+[3] Pancakes: Pilot's slang for stalling an aeroplane and dropping
+like a pancake.
+
+[4] Morane parasol: A type of Morane monoplane in which the lifting
+surfaces are raised above the pilot in order to afford him a good view
+of the earth.
+
+[5] Skin friction is that part of the drift due to the friction of the
+air with roughnesses upon the surface of the aeroplane.
+
+[6] Banking: When an aeroplane is turned to the left or the right
+the centrifugal force of its momentum causes it to skid sideways and
+outwards away from the centre of the turn. To minimize such action the
+pilot banks, i.e., tilts, the aeroplane sideways in order to oppose the
+underside of the planes to the air. The aeroplane will not then skid
+outwards beyond the slight skid necessary to secure a sufficient
+pressure of air to balance the centrifugal force.
+
+[7] An explanation of the way in which the wash-out is combined with a
+wash-in to offset propellor torque will be found on p. 82.
+
+[8] A.M.'s: Air Mechanics.
+
+[9] Butt means to thicken at the end. Screw means to machine a
+thread on the butt-end of the wire, and in this way the wire can make
+connection with the desired place by being screwed into a metal fitting,
+thus eliminating the disadvantage of the unsatisfactory loop.
+
+[10] Deviation curve: A curved line indicating any errors in the
+compass.
+
+[11] A propeller screws through the air, and the distance it advances
+during one revolution, supposing the air to be solid, is known as the
+pitch. The pitch, which depends upon the angle of the propeller blades,
+must be equal to the speed of the aeroplane, plus the slip, and if, on
+account of the rarity of the air the speed of the aeroplane increases,
+then the angle and pitch should be correspondingly increased. Propellers
+with a pitch capable of being varied by the pilot are the dream of
+propeller designers. For explanation of “slip” see Chapter IV. on
+propellers.
+
+[12] Getting out of my depth? Invading the realms of fancy? Well,
+perhaps so, but at any rate it is possible that extraordinary speed
+through space may be secured if means are found to maintain the impulse
+of the engine and the thrust-drift efficiency of the propeller at great
+altitude.
+
+[13] Box-kite. The first crude form of biplane.
+
+[14] See Newton's laws in the Glossary at the end of the book.
+
+[15] See “Aerofoil” in the Glossary.
+
+[16] “In effect” because, although there may be actually the greatest
+proportion of keel-surface In front of the vertical axis, such surface
+may be much nearer to the axis than is the keel-surface towards the
+tail. The latter may then be actually less than the surface in front,
+but, being farther from the axis, it has a greater leverage, and
+consequently is greater in effect than the surface in front.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
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+
+<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
+  <head>
+    <title>
+      The Aeroplane Speaks, by H. Barber
+    </title>
+    <style type="text/css" xml:space="preserve">
+
+    body { margin:5%; background:#faebd0; text-align:justify}
+    P { text-indent: 1em; margin-top: .25em; margin-bottom: .25em; }
+    H1,H2,H3,H4,H5,H6 { text-align: center; margin-left: 15%; margin-right: 15%; }
+    hr  { width: 50%; text-align: center;}
+    .foot { margin-left: 20%; margin-right: 20%; text-align: justify; text-indent: -3em; font-size: 90%; }
+    blockquote {font-size: 97%; font-style: italic; margin-left: 10%; margin-right: 10%;}
+    .mynote    {background-color: #DDE; color: #000; padding: .5em; margin-left: 10%; margin-right: 10%; font-family: sans-serif; font-size: 95%;}
+    .toc       { margin-left: 10%; margin-bottom: .75em;}
+    .toc2      { margin-left: 20%;}
+    div.fig    { display:block; margin:0 auto; text-align:center; }
+    div.middle { margin-left: 20%; margin-right: 20%; text-align: justify; }
+    .figleft   {float: left; margin-left: 0%; margin-right: 1%;}
+    .figright  {float: right; margin-right: 0%; margin-left: 1%;}
+    .pagenum   {display:inline; font-size: 70%; font-style:normal;
+               margin: 0; padding: 0; position: absolute; right: 1%;
+               text-align: right;}
+    pre        { font-style: italic; font-size: 90%; margin-left: 10%;}
+
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+  <body>
+<pre xml:space="preserve">
+
+The Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Aeroplane Speaks
+
+Author: H. Barber
+
+Release Date: July 21, 2008 [EBook #818]
+Last Updated: March 15, 2018
+
+Language: English
+
+Character set encoding: UTF-8
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE AEROPLANE SPEAKS ***
+
+
+
+
+Produced by Charles Keller, and David Widger
+
+
+
+
+
+</pre>
+    <p>
+      <br /><br />
+    </p>
+    <h1>
+      THE AEROPLANE SPEAKS
+    </h1>
+    <p>
+      <br />
+    </p>
+    <h2>
+      By H. Barber
+    </h2>
+    <p>
+      <br />
+    </p>
+    <h3>
+      (Captain, Royal Flying Corps)
+    </h3>
+    <p>
+      <br /> <br />
+    </p>
+    <h3>
+      DEDICATED TO THE SUBALTERN FLYING OFFICER
+    </h3>
+    <p>
+      <br /> <br />
+    </p>
+    <hr />
+    <p>
+      <br /> <br /> <a name="link2H_4_0001" id="link2H_4_0001">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      MOTIVE
+    </h2>
+    <p>
+      The reasons impelling me to write this book, the maiden effort of my pen,
+      are, firstly, a strong desire to help the ordinary man to understand the
+      Aeroplane and the joys and troubles of its Pilot; and, secondly, to
+      produce something of PRACTICAL assistance to the Pilot and his invaluable
+      assistant the Rigger. Having had some eight years' experience in
+      designing, building, and flying aeroplanes, I have hopes that the
+      practical knowledge I have gained may offset the disadvantage of a hand
+      more used to managing the &ldquo;joy-stick&rdquo; than the dreadful haltings, the many
+      side-slips, the irregular speed, and, in short, the altogether
+      disconcerting ways of a pen.
+    </p>
+    <p>
+      The matter contained in the Prologue appeared in the Field of May 6th,
+      13th, 20th, and 27th, 1916, and is now reprinted by the kind permission of
+      the editor, Sir Theodore Cook.
+    </p>
+    <p>
+      I have much pleasure in also acknowledging the kindness of Mr. C. G. Grey,
+      editor of the Aeroplane, to whom I am indebted for the valuable
+      illustrations reproduced at the end of this book.
+    </p>
+    <p>
+      <br /> <br />
+    </p>
+    <hr />
+    <p>
+      <br /> <br />
+    </p>
+    <blockquote>
+      <p class="toc">
+        <big><b>CONTENTS</b></big>
+      </p>
+      <p>
+        <br />
+      </p>
+      <p class="toc">
+        <a href="#link2H_4_0001"> MOTIVE </a>
+      </p>
+      <p class="toc">
+        <a href="#link2H_4_0002"> <b>THE AEROPLANE SPEAKS</b> </a>
+      </p>
+      <p>
+        <br />
+      </p>
+      <p class="toc">
+        <a href="#link2H_PROL"> PROLOGUE </a>
+      </p>
+      <p>
+        <br />
+      </p>
+      <p class="toc">
+        <a href="#link2H_PART1"> PART I. THE ELEMENTARY PRINCIPLES AIR THEIR
+        GRIEVANCES </a>
+      </p>
+      <p class="toc">
+        <a href="#link2H_PART2"> PART II. THE PRINCIPLES, HAVING SETTLED THEIR
+        DIFFERENCES, FINISH THE </a>
+      </p>
+      <p class="toc">
+        <a href="#link2H_PART3"> PART III. THE GREAT TEST </a>
+      </p>
+      <p class="toc">
+        <a href="#link2H_PART4"> PART IV. 'CROSS COUNTRY </a>
+      </p>
+      <p>
+        <br />
+      </p>
+      <p class="toc">
+        <a href="#link2HCH0001"> CHAPTER I. FLIGHT </a>
+      </p>
+      <p class="toc">
+        <a href="#link2HCH0002"> CHAPTER II. STABILITY AND CONTROL </a>
+      </p>
+      <p class="toc">
+        <a href="#link2HCH0003"> CHAPTER III. RIGGING </a>
+      </p>
+      <p class="toc">
+        <a href="#link2HCH0004"> CHAPTER IV. THE PROPELLER, OR &ldquo;AIR-SCREW&rdquo; </a>
+      </p>
+      <p class="toc">
+        <a href="#link2HCH0005"> CHAPTER V. MAINTENANCE </a>
+      </p>
+      <p>
+        <br />
+      </p>
+      <p class="toc">
+        <a href="#link2H_GLOS"> GLOSSARY </a>
+      </p>
+      <p class="toc">
+        <a href="#link2H_FOOT"> FOOTNOTES </a>
+      </p>
+    </blockquote>
+    <p>
+      <br /> <br />
+    </p>
+    <hr />
+    <p>
+      <br /> <br /> <a name="link2H_4_0002" id="link2H_4_0002">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h1>
+      THE AEROPLANE SPEAKS
+    </h1>
+    <p>
+      <a name="link2H_PROL" id="link2H_PROL">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      PROLOGUE
+    </h2>
+    <p>
+      <a name="link2H_PART1" id="link2H_PART1">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      PART I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+    </h2>
+    <p>
+      The Lecture Hall at the Royal Flying Corps School for Officers was
+      deserted. The pupils had dispersed, and the Officer Instructor, more
+      fagged than any pupil, was out on the aerodrome watching the test of a new
+      machine.
+    </p>
+    <p>
+      Deserted, did I say? But not so. The lecture that day had been upon the
+      Elementary Principles of Flight, and they lingered yet. Upon the
+      Blackboard was the illustration you see in the frontispiece.
+    </p>
+    <p>
+      &ldquo;I am the side view of a Surface,&rdquo; it said, mimicking the tones of the
+      lecturer. &ldquo;Flight is secured by driving me through the air at an angle
+      inclined to the direction of motion.&rdquo;
+     </p>
+    <p>
+      &ldquo;Quite right,&rdquo; said the Angle. &ldquo;That's me, and I'm the famous Angle of
+      Incidence.&rdquo;
+     </p>
+    <p>
+      &ldquo;And,&rdquo; continued the Surface, &ldquo;my action is to deflect the air downwards,
+      and also, by fleeing from the air behind, to create a semi-vacuum or
+      rarefied area over most of the top of my surface.&rdquo;
+     </p>
+    <p>
+      &ldquo;This is where I come in,&rdquo; a thick, gruff voice was heard, and went on:
+      &ldquo;I'm the Reaction. You can't have action without me. I'm a very
+      considerable force, and my direction is at right-angles to you,&rdquo; and he
+      looked heavily at the Surface. &ldquo;Like this,&rdquo; said he, picking up the chalk
+      with his Lift, and drifting to the Blackboard.
+    </p>
+    <p>
+      &ldquo;I act in the direction of the arrow R, that is, more or less, for the
+      direction varies somewhat with the Angle of Incidence and the curvature of
+      the Surface; and, strange but true, I'm stronger on the top of the Surface
+      than at the bottom of it. The Wind Tunnel has proved that by exhaustive
+      research&mdash;and don't forget how quickly I can grow! As the speed
+      through the air increases my strength increases more rapidly than you
+      might think&mdash;approximately, as the Square of the Speed; so you see
+      that if the Speed of the Surface through the air is, for instance,
+      doubled, then I am a good deal more than doubled. That's because I am the
+      result of not only the mass of air displaced, but also the result of the
+      Speed with which the Surface engages the Air. I am a product of those two
+      factors, and at the speeds at which Aeroplanes fly to-day, and at the
+      altitudes and consequent density of air they at present experience, I
+      increase at about the Square of the Speed.
+    </p>
+    <p>
+      &ldquo;Oh, I'm a most complex and interesting personality, I assure you&mdash;in
+      fact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr. Hyde.
+      There's Lift, my vertical part or COMPONENT, as those who prefer long
+      words would say; he always acts vertically upwards, and hates Gravity like
+      poison. He's the useful and admirable part of me. Then there's Drift, my
+      horizontal component, sometimes, though rather erroneously, called Head
+      Resistance; he's a villain of the deepest dye, and must be overcome before
+      flight can be secured.&rdquo;
+     </p>
+    <p>
+      &ldquo;And I,&rdquo; said the Propeller, &ldquo;I screw through the air and produce the
+      Thrust. I thrust the Aeroplane through the air and overcome the Drift; and
+      the Lift increases with the Speed and when it equals the Gravity of
+      Weight, then&mdash;there you are&mdash;Flight! And nothing mysterious
+      about it at all.&rdquo;
+     </p>
+    <p>
+      &ldquo;I hope you'll excuse me interrupting,&rdquo; said a very beautiful young lady,
+      &ldquo;my name is Efficiency, and, while no doubt, all you have said is quite
+      true, and that, as my young man the Designer says, `You can make a
+      tea-tray fly if you slap on Power enough,' I can assure you that I'm not
+      to be won quite so easily.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well,&rdquo; eagerly replied the Lift and the Thrust, &ldquo;let's be friends. Do
+      tell us what we can do to help you to overcome Gravity and Drift with the
+      least possible Power. That obviously seems the game to play, for more
+      Power means heavier engines, and that in a way plays into the hands of our
+      enemy, Gravity, besides necessitating a larger Surface or Angle to lift
+      the Weight, and that increases the Drift.&rdquo;
+     </p>
+    <p>
+      &ldquo;Very well,&rdquo; from Efficiency, &ldquo;I'll do my best, though I'm so shy, and
+      I've just had such a bad time at the Factory, and I'm terribly afraid
+      you'll find it awfully dry.&rdquo;
+     </p>
+    <p>
+      &ldquo;Buck up, old dear!&rdquo; This from several new-comers, who had just appeared.
+      &ldquo;We'll help you,&rdquo; and one of them, so lean and long that he took up the
+      whole height of the lecture room, introduced himself.
+    </p>
+    <p>
+      &ldquo;I'm the High Aspect Ratio,&rdquo; he said, &ldquo;and what we have got to do to help
+      this young lady is to improve the proportion of Lift to Drift. The more
+      Lift we can get for a certain area of Surface, the greater the Weight the
+      latter can carry; and the less the Drift, then the less Thrust and Power
+      required to overcome it. Now it is a fact that, if the Surface is shaped
+      to have the greatest possible span, i.e., distance from wing-tip to
+      wing-tip, it then engages more air and produces both a maximum Reaction
+      and a better proportion of Lift to Drift.
+    </p>
+    <p>
+      &ldquo;That being so, we can then well afford to lose a little Reaction by
+      reducing the Angle of Incidence to a degree giving a still better
+      proportion of Lift to Drift than would otherwise be the case; for you must
+      understand that the Lift-Drift Ratio depends very much upon the size of
+      the Angle of Incidence, which should be as small as possible within
+      certain limits. So what I say is, make the surface of Infinite Span with
+      no width or chord, as they call it. That's all I require, I assure you, to
+      make me quite perfect and of infinite service to Miss Efficiency.&rdquo;
+     </p>
+    <p>
+      &ldquo;That's not practical politics,&rdquo; said the Surface. &ldquo;The way you talk one
+      would think you were drawing L400 a year at Westminster, and working up a
+      reputation as an Aeronautical Expert. I must have some depth and chord to
+      take my Spars and Ribs, and again, I must have a certain chord to make it
+      possible for my Camber (that's curvature) to be just right for the Angle
+      of Incidence. If that's not right the air won't get a nice uniform
+      compression and downward acceleration from my underside, and the rarefied
+      `suction' area over the top of me will not be as even and clean in effect
+      as it might be. That would spoil the Lift-Drift Ratio more than you can
+      help it. Just thrust that chalk along, will you? and the Blackboard will
+      show you what I mean.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well,&rdquo; said the Aspect Ratio, &ldquo;have it your own way, though I'm sorry to
+      see a pretty young lady like Efficiency compromised so early in the game.&rdquo;
+     </p>
+    <p>
+      &ldquo;Look here,&rdquo; exclaimed a number of Struts, &ldquo;we have got a brilliant idea
+      for improving the Aspect Ratio,&rdquo; and with that they hopped up on to the
+      Spars. &ldquo;Now,&rdquo; excitedly, &ldquo;place another Surface on top of us. Now do you
+      see? There is double the Surface, and that being so, the proportion of
+      Weight to Surface area is halved. That's less burden of work for the
+      Surface, and so the Spars need not be so strong and so deep, which results
+      in not so thick a Surface. That means the Chord can be proportionately
+      decreased without adversely affecting the Camber. With the Chord
+      decreased, the Span becomes relatively greater, and so produces a splendid
+      Aspect Ratio, and an excellent proportion of Lift to Drift.&rdquo;
+     </p>
+    <p>
+      &ldquo;I don't deny that they have rather got me there,&rdquo; said the Drift, &ldquo;but
+      all the same, don't forget my increase due to the drift of the Struts and
+      their bracing wires.&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes, I dare say,&rdquo; replied the Surface, &ldquo;but remember that my Spars are
+      less deep than before, and consequently I am not so thick now, and shall
+      for that reason also be able to go through the air with a less proportion
+      of Drift to Lift.&rdquo;
+     </p>
+    <p>
+      &ldquo;Remember me also, please,&rdquo; croaked the Angle of Incidence. &ldquo;Since the
+      Surface has now less weight to carry for its area, I may be set at a still
+      lesser and finer Angle. That means less Drift again. We are certainly
+      getting on splendidly! Show us how it looks now, Blackboard.&rdquo; And the
+      Blackboard obligingly showed them as follows:
+    </p>
+    <p>
+      &ldquo;Well, what do you think of that?&rdquo; they all cried to the Drift.
+    </p>
+    <p>
+      &ldquo;You think you are very clever,&rdquo; sneered the Drift. &ldquo;But you are not
+      helping Efficiency as much as you think. The suction effect on the top of
+      the lower Surface will give a downward motion to the air above it and the
+      result will be that the bottom of the top Surface will not secure as good
+      a Reaction from the air as would otherwise be the case, and that means
+      loss of Lift; and you can't help matters by increasing the gap between the
+      surfaces because that means longer Struts and Wires, and that in itself
+      would help me, not to speak of increasing the Weight. You see it's not
+      quite so easy as you thought.&rdquo;
+     </p>
+    <p>
+      At this moment a hiccough was heard, and a rather fast and rakish-looking
+      chap, named Stagger, spoke up. &ldquo;How d'ye do, miss,&rdquo; he said politely to
+      Efficiency, with a side glance out of his wicked old eye. &ldquo;I'm a bit of a
+      knut, and without the slightest trouble I can easily minimize the
+      disadvantage that old reprobate Drift has been frightening you with. I
+      just stagger the top Surface a bit forward, and no longer is that suction
+      effect dead under it. At the same time I'm sure the top Surface will
+      kindly extend its Span for such distance as its Spars will support it
+      without the aid of Struts. Such extension will be quite useful, as there
+      will be no Surface at all underneath it to interfere with the Reaction
+      above.&rdquo; And the Stagger leaned forward and picked up the Chalk, and this
+      is the picture he drew:
+    </p>
+    <p>
+      Said the Blackboard, &ldquo;That's not half bad! It really begins to look
+      something like the real thing, eh?&rdquo;
+     </p>
+    <p>
+      &ldquo;The real thing, is it?&rdquo; grumbled Drift. &ldquo;Just consider that contraption
+      in the light of any one Principle, and I warrant you will not find one of
+      them applied to perfection. The whole thing is nothing but a Compromise.&rdquo;
+       And he glared fixedly at poor Efficiency.
+    </p>
+    <p>
+      &ldquo;Oh, dear! Oh, dear!&rdquo; she cried. &ldquo;I'm always getting into trouble. What
+      WILL the Designer say?&rdquo;
+     </p>
+    <p>
+      &ldquo;Never mind, my dear,&rdquo; said the Lift-Drift Ratio, consolingly. &ldquo;You are
+      improving rapidly, and quite useful enough now to think of doing a job of
+      work.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, that's good news,&rdquo; and Efficiency wiped her eyes with her Fabric
+      and became almost cheerful. &ldquo;Suppose we think about finishing it now?
+      There will have to be an Engine and Propeller, won't there? And a body to
+      fix them in, and tanks for oil and petrol, and a tail, and,&rdquo; archly, &ldquo;one
+      of those dashing young Pilots, what?&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, we are getting within sight of those interesting Factors,&rdquo; said the
+      Lift-Drift Ratio, &ldquo;but first of all we had better decide upon the Area of
+      the Surfaces, their Angle of Incidence and Camber. If we are to ascend as
+      quickly as possible the Aeroplane must be SLOW in order to secure the best
+      possible Lift-Drift Ratio, for the drift of the struts wires, body, etc.,
+      increases approximately as the square of the speed, but it carries with it
+      no lift as it does in the case of the Surface. The less speed then, the
+      less such drift, and the better the Aeroplane's proportion of lift to
+      drift; and, being slow, we shall require a LARGE SURFACE in order to
+      secure a large lift relative to the weight to be carried. We shall also
+      require a LARGE ANGLE OF INCIDENCE relative to the horizontal, in order to
+      secure a proper inclination of the Surface to the direction of motion, for
+      you must remember that, while we shall fly upon an even keel and with the
+      propeller thrust horizontal (which is its most efficient attitude), our
+      flight path, which is our direction of motion, will be sloping upwards,
+      and it will therefore be necessary to fix the Surface to the Aeroplane at
+      a very considerable angle relative to the horizontal Propeller Thrust in
+      order to secure a proper angle to the upwards direction of motion. Apart
+      from that, we shall require a larger Angle of Incidence than in the case
+      of a machine designed purely for speed, and that means a correspondingly
+      LARGE CAMBER.
+    </p>
+    <p>
+      &ldquo;On the other hand, if we are thinking merely of Speed, then a SMALL
+      SURFACE, just enough to lift the weight off the ground, will be best, also
+      a SMALL ANGLE to cut the Drift down and that, of course, means a
+      relatively SMALL CAMBER.
+    </p>
+    <p>
+      &ldquo;So you see the essentials for CLIMB or quick ascent and for SPEED are
+      diametrically opposed. Now which is it to be?&rdquo;
+     </p>
+    <p>
+      &ldquo;Nothing but perfection for me,&rdquo; said Efficiency. &ldquo;What I want is Maximum
+      Climb and Maximum Speed for the Power the Engine produces.&rdquo;
+     </p>
+    <p>
+      And each Principle fully agreed with her beautiful sentiments, but work
+      together they would not.
+    </p>
+    <p>
+      The Aspect Ratio wanted infinite Span, and hang the Chord.
+    </p>
+    <p>
+      The Angle of Incidence would have two Angles and two Cambers in one, which
+      was manifestly absurd; the Surface insisted upon no thickness whatever,
+      and would not hear of such things as Spars and Ribs; and the Thrust
+      objected to anything at all likely to produce Drift, and very nearly wiped
+      the whole thing off the Blackboard.
+    </p>
+    <p>
+      There was, indeed, the makings of a very pretty quarrel when the Letter
+      arrived. It was about a mile long, and began to talk at once.
+    </p>
+    <p>
+      &ldquo;I'm from the Inventor,&rdquo; he said, and hope rose in the heart of each
+      heated Principle. &ldquo;It's really absurdly simple. All the Pilot has to do is
+      to touch a button, and at his will, VARY the area of the Surface, the
+      Angle of Incidence, and the Camber! And there you are&mdash;Maximum Climb
+      or Maximum Speed as required! How does that suit you?&rdquo;
+     </p>
+    <p>
+      &ldquo;That suits us very well,&rdquo; said the Surface, &ldquo;but, excuse me asking, how
+      is it done without apparatus increasing the Drift and the Weight out of
+      all reason? You won't mind showing us your Calculations, Working Drawings,
+      Stress Diagrams, etc., will you?&rdquo;
+     </p>
+    <p>
+      Said the Letter with dignity, &ldquo;I come from an Inventor so brilliantly
+      clever as to be far above the unimportant matters you mention. He is no
+      common working man, sir! He leaves such things to Mechanics. The point is,
+      you press a button and&mdash;&mdash;&rdquo;
+     </p>
+    <p>
+      &ldquo;Look here,&rdquo; said a Strut, rather pointedly, &ldquo;where do you think you are
+      going, anyway?&rdquo;
+     </p>
+    <p>
+      &ldquo;Well,&rdquo; from the Letter, &ldquo;as a matter of fact, I'm not addressed yet, but,
+      of course, there's no doubt I shall reach the very highest quarters and
+      absolutely revolutionize Flight when I get there.&rdquo;
+     </p>
+    <p>
+      Said the Chalk, &ldquo;I'll address you, if that's all you want; now drift along
+      quickly!&rdquo; And off went the Letter to The Technical Editor, &ldquo;Daily Mauler,&rdquo;
+       London.
+    </p>
+    <p>
+      And a League was formed, and there were Directors with Fees, and several
+      out-of-service Tin Hats, and the Man-who-takes-the-credit, and a fine fat
+      Guinea-pig, and all the rest of them. And the Inventor paid his Tailor and
+      had a Hair-Cut, and is now a recognized Press Expert&mdash;but he is still
+      waiting for those Mechanics!
+    </p>
+    <p>
+      &ldquo;I'm afraid,&rdquo; said the Slide-rule, who had been busy making those
+      lightning-like automatic calculations for which he is so famous, &ldquo;it's
+      quite impossible to fully satisfy all of you, and it is perfectly plain to
+      me that we shall have to effect a Compromise and sacrifice some of the
+      Lift for Speed.&rdquo;
+     </p>
+    <p>
+      Thud! What was that?
+    </p>
+    <p>
+      Efficiency had fainted dead away! The last blow had been too much for her.
+      And the Principles gathered mournfully round, but with the aid of the
+      Propeller Slip<a href="#linknote-1" name="linknoteref-1" id="linknoteref-1"><small>1</small></a>
+      and a friendly lift from the Surface she was at length revived and
+      regained a more normal aspect.
+    </p>
+    <p>
+      Said the Stagger with a raffish air, &ldquo;My dear young lady, I assure you
+      that from the experiences of a varied career, I have learned that
+      perfection is impossible, and I am sure the Designer will be quite
+      satisfied if you become the Most Efficient Compromise.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, that sounds so common sense,&rdquo; sighed Efficiency, &ldquo;I suppose it must
+      be true, and if the Designer is satisfied, that's all I really care about.
+      Now do let's get on with the job.&rdquo;
+     </p>
+    <p>
+      So the Chalk drew a nice long slim body to hold the Engine and the tanks,
+      etc., with room for the Pilot's and Passenger's seats, and placed it
+      exactly in the middle of the Biplane. And he was careful to make its
+      position such that the Centre of Gravity was a little in advance of the
+      Centre of Lift, so that when the Engine was not running and there was
+      consequently no Thrust, the Aeroplane should be &ldquo;nose-heavy&rdquo; just to the
+      right degree, and so take up a natural glide to Earth&mdash;and this was
+      to help the Pilot and relieve him of work and worry, should he find
+      himself in a fog or a cloud. And so that this tendency to glide downwards
+      should not be in evidence when the Engine was running and descent not
+      desired, the Thrust was placed a little below the Centre of Drift or
+      Resistance. In this way it would in a measure pull the nose of the
+      Aeroplane up and counterbalance the &ldquo;nose-heavy&rdquo; tendency.
+    </p>
+    <p>
+      And the Engine was so mounted that when the Propeller-Thrust was
+      horizontal, which is its most efficient position, the Angle of Incidence
+      and the Area of the surfaces were just sufficient to give a Lift a little
+      in excess of the Weight. And the Camber was such that, as far as it was
+      concerned, the Lift-Drift Ratio should be the best possible for that Angle
+      of Incidence. And a beautifully simple under-carriage was added, the
+      outstanding features of which were simplicity, strength, light-weight, and
+      minimum drift. And, last of all, there was the Elevator, of which you will
+      hear more by-and-by. And this is what it looked like then:
+    </p>
+    <p>
+      And Efficiency, smiling, thought that it was not such a bad compromise
+      after all and that the Designer might well be satisfied.
+    </p>
+    <p>
+      &ldquo;Now,&rdquo; said she, &ldquo;there's just one or two points I'm a bit hazy about. It
+      appears that when the Propeller shaft is horizontal and so working in its
+      most efficient attitude, I shall have a Lift from the Surfaces slightly in
+      excess of the Weight. That means I shall ascend slightly, at the same time
+      making nearly maximum speed for the power and thrust. Can't I do better
+      than that?&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes, indeed,&rdquo; spoke up the Propeller, &ldquo;though it means that I must assume
+      a most undignified attitude, for helicopters<a href="#linknote-2"
+       name="linknoteref-2" id="linknoteref-2"><small>2</small></a> I never
+      approved of. In order to ascend more quickly the Pilot will deflect the
+      Elevator, which, by the way, you see hinged to the Tail. By that means he
+      will force the whole Aeroplane to assume a greater Angle of Incidence. And
+      with greater Angle, the Lift will increase, though I'm sorry to say the
+      Drift will increase also. Owing to the greater Drift, the Speed through
+      the air will lessen, and I'm afraid that won't be helpful to the Lift; but
+      I shall now be pointing upwards, and besides overcoming the Drift in a
+      forward direction I shall be doing my best to haul the Aeroplane skywards.
+      At a certain angle known as the Best Climbing Angle, we shall have our
+      Maximum Margin of Lift, and I'm hoping that may be as much as almost a
+      thousand feet altitude a minute.&rdquo;
+     </p>
+    <p>
+      &ldquo;Then, if the Pilot is green, my chance will come,&rdquo; said the Maximum Angle
+      of Incidence. &ldquo;For if the Angle is increased over the Best Climbing Angle,
+      the Drift will rush up; and the Speed, and with it the Lift, will, when my
+      Angle is reached, drop to a point when the latter will be no more than the
+      Weight. The Margin of Lift will have entirely disappeared, and there we
+      shall be, staggering along at my tremendous angle, and only just
+      maintaining horizontal flight.&rdquo;
+     </p>
+    <p>
+      &ldquo;And then with luck I'll get my chance,&rdquo; said the Drift. &ldquo;If he is a bit
+      worse than green, he'll perhaps still further increase the Angle. Then the
+      Drift, largely increasing, the Speed, and consequently the Lift, will
+      become still less, i.e., less than the Weight, and then&mdash;what price
+      pancakes,<a href="#linknote-3" name="linknoteref-3" id="linknoteref-3"><small>3</small></a>
+      eh?&rdquo;
+     </p>
+    <p>
+      &ldquo;Thank you,&rdquo; from Efficiency, &ldquo;that was all most informing. And now will
+      you tell me, please, how the greatest Speed may be secured?&rdquo;
+     </p>
+    <p>
+      &ldquo;Certainly, now it's my turn,&rdquo; piped the Minimum Angle of Incidence. &ldquo;By
+      means of the Elevator, the Pilot places the Aeroplane at my small Angle,
+      at which the Lift only just equals the Weight, and, also, at which we
+      shall make greater speed with no more Drift than before. Then we get our
+      greatest Speed, just maintaining horizontal flight.&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes; though I'm out of the horizontal and thrusting downwards,&rdquo; grumbled
+      the Propeller, &ldquo;and that's not efficient, though I suppose it's the best
+      we can do until that Inventor fellow finds his Mechanics.&rdquo;
+     </p>
+    <p>
+      &ldquo;Thank you so much,&rdquo; said Efficiency. &ldquo;I think I have now at any rate an
+      idea of the Elementary Principles of Flight, and I don't know that I care
+      to delve much deeper, for sums always give me a headache; but isn't there
+      something about Stability and Control? Don't you think I ought to have a
+      glimmering of them too?&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, I should smile,&rdquo; said a spruce Spar, who had come all the way from
+      America. &ldquo;And that, as the Lecturer says, `will be the subject of our next
+      lecture,' so be here again to-morrow, and you will be glad to hear that it
+      will be distinctly more lively than the subject we have covered to-day.&rdquo;
+     </p>
+    <p>
+      <a name="link2H_PART2" id="link2H_PART2">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      PART II. THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE
+    </h2>
+    <p>
+      JOB
+    </p>
+    <p>
+      Another day had passed, and the Flight Folk had again gathered together
+      and were awaiting the arrival of Efficiency who, as usual, was rather late
+      in making an appearance.
+    </p>
+    <p>
+      The crowd was larger than ever, and among the newcomers some of the most
+      important were the three Stabilities, named Directional, Longitudinal, and
+      Lateral, with their assistants, the Rudder, Elevator, and Ailerons. There
+      was Centrifugal Force, too, who would not sit still and created a most
+      unfavourable impression, and Keel-Surface, the Dihedral Angle, and several
+      other lesser fry.
+    </p>
+    <p>
+      &ldquo;Well,&rdquo; said Centrifugal Force, &ldquo;I wish this Efficiency I've heard so much
+      about would get a move on. Sitting still doesn't agree with me at all.
+      Motion I believe in. There's nothing like motion&mdash;the more the
+      better.&rdquo;
+     </p>
+    <p>
+      &ldquo;We are entirely opposed to that,&rdquo; objected the three Stabilities, all in
+      a breath. &ldquo;Unless it's in a perfectly straight line or a perfect circle.
+      Nothing but perfectly straight lines or, upon occasion, perfect circles
+      satisfy us, and we are strongly suspicious of your tendencies.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, we shall see what we shall see,&rdquo; said the Force darkly. &ldquo;But who in
+      the name of blue sky is this?&rdquo;
+     </p>
+    <p>
+      And in tripped Efficiency, in a beautifully &ldquo;doped&rdquo; dress of the latest
+      fashionable shade of khaki-coloured fabric, a perfectly stream-lined
+      bonnet, and a bewitching little Morane parasol,<a href="#linknote-4"
+       name="linknoteref-4" id="linknoteref-4"><small>4</small></a> smiling as
+      usual, and airily exclaiming, &ldquo;I'm so sorry I'm late, but you see the
+      Designer's such a funny man. He objects to skin friction,<a
+      href="#linknote-5" name="linknoteref-5" id="linknoteref-5"><small>5</small></a>
+      and insisted upon me changing my fabric for one of a smoother surface, and
+      that delayed me. Dear me, there are a lot more of us to-day, aren't there?
+      I think I had better meet one at a time.&rdquo; And turning to Directional
+      Stability, she politely asked him what he preferred to do.
+    </p>
+    <p>
+      &ldquo;My purpose in life, miss,&rdquo; said he, &ldquo;is to keep the Aeroplane on its
+      course, and to achieve that there must be, in effect, more Keel-Surface
+      behind the Vertical Turning Axis than there is in front of it.&rdquo;
+     </p>
+    <p>
+      Efficiency looking a little puzzled, he added: &ldquo;Just like a weathercock,
+      and by Keel-Surface I mean everything you can see when you view the
+      Aeroplane from the side of it&mdash;the sides of the body, struts, wires,
+      etc.&rdquo;
+     </p>
+    <p>
+      &ldquo;Oh, now I begin to see light,&rdquo; said she: &ldquo;but just exactly how does it
+      work?&rdquo;
+     </p>
+    <p>
+      &ldquo;I'll answer that,&rdquo; said Momentum. &ldquo;When perhaps by a gust of air the
+      Aeroplane is blown out of its course and points in another direction, it
+      doesn't immediately fly off on that new course. I'm so strong I pull it
+      off the new course to a certain extent, and towards the direction of the
+      old course. And so it travels, as long as my strength lasts, in a more or
+      less sideways position.&rdquo;
+     </p>
+    <p>
+      &ldquo;Then,&rdquo; said the Keel-Surface, &ldquo;I get a pressure of air all on one side,
+      and as there is, in effect, most of me towards the tail, the latter gets
+      pressed sideways, and the Aeroplane thus tends to assume its first
+      position and course.&rdquo;
+     </p>
+    <p>
+      &ldquo;I see,&rdquo; said Efficiency, and, daintily holding the Chalk, she approached
+      the Blackboard. &ldquo;Is this what you mean?&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes, that's right enough,&rdquo; said the Keel-Surface, &ldquo;and you might
+      remember, too, that I always make the Aeroplane nose into the gusts rather
+      than away from them.&rdquo;
+     </p>
+    <p>
+      &ldquo;If that was not the case,&rdquo; broke in Lateral Stability, and affecting the
+      fashionable Flying Corps stammer, &ldquo;it would be a h-h-h-o-r-rible affair!
+      If there were too much Keel-Surface in front, then that gust would blow
+      the Aeroplane round the other way a very considerable distance. And the
+      right-hand Surface being on the outside of the turn would have more speed,
+      and consequently more Lift, than the Surface on the other side. That means
+      a greater proportion of the Lift on that side, and before you could say
+      Warp to the Ailerons over the Aeroplane would go&mdash;probable result a
+      bad side-slip&rdquo;
+     </p>
+    <p>
+      &ldquo;And what can the Pilot do to save such a situation as that?&rdquo; said
+      Efficiency.
+    </p>
+    <p>
+      &ldquo;Well,&rdquo; replied Lateral Stability, &ldquo;he will try to turn the Aeroplane
+      sideways and back to an even keel by means of warping the Ailerons or
+      little wings which are hinged on to the Wing-tips, and about which you
+      will hear more later on; but if the side-slip is very bad he may not be
+      able to right the Aeroplane by means of the Ailerons, and then the only
+      thing for him to do is to use the Rudder and to turn the nose of the
+      Aeroplane down and head-on to the direction of motion. The Aeroplane will
+      then be meeting the air in the direction it is designed to do so, and the
+      Surfaces and also the controls (the Rudder, Ailerons, and Elevator) will
+      be working efficiently; but its attitude relative to the earth will
+      probably be more or less upside-down, for the action of turning the
+      Aeroplane's nose down results, as you will see by the illustration B, in
+      the right wing, which is on the outside of the circle. travelling through
+      the air with greater speed than the left-hand wing. More Speed means more
+      Lift, so that results in overturning the Aeroplane still more; but now it
+      is, at any rate, meeting the air as it is designed to meet it, and
+      everything is working properly. It is then only necessary to warp the
+      Elevator, as shown in illustration C, in order to bring the Aeroplane into
+      a proper attitude relative to the earth.&rdquo;
+     </p>
+    <p>
+      &ldquo;Ah!&rdquo; said the Rudder, looking wise, &ldquo;it's in a case like that when I
+      become the Elevator and the Elevator becomes me.&rdquo;
+     </p>
+    <p>
+      &ldquo;That's absurd nonsense,&rdquo; said the Blackboard, &ldquo;due to looseness of
+      thought and expression.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well,&rdquo; replied the Rudder, &ldquo;when 'the Aeroplane is in position A and I am
+      used, then I depress or ELEVATE the nose of the machine; and, if the
+      Elevator is used, then it turns the Aeroplane to right or left, which is
+      normally my function. Surely our roles have changed one with the other,
+      and I'm then the Elevator and the Elevator is me!&rdquo;
+     </p>
+    <p>
+      Said Lateral Stability to the Rudder, &ldquo;That's altogether the wrong way of
+      looking at it, though I admit&rdquo;&mdash;and this rather sarcastically&mdash;&ldquo;that
+      the way you put it sounds rather fine when you are talking of your
+      experiences in the air to those 'interested in aviation' but knowing
+      little about it; but it won't go down here! You are a Controlling Surface
+      designed to turn the Aeroplane about its vertical axis, and the Elevator
+      is a Controlling Surface designed to turn the Aeroplane about its lateral
+      axis. Those are your respective jobs, and you can't possibly change them
+      about. Such talk only leads to confusion, and I hope we shall hear no more
+      of it.&rdquo;
+     </p>
+    <p>
+      &ldquo;Thanks,&rdquo; said Efficiency to Lateral Stability. &ldquo;And now, please, will you
+      explain your duties?&rdquo;
+     </p>
+    <p>
+      &ldquo;My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.
+      First of all, I sometimes arrange with the Rigger to wash-out, that is
+      decrease, the Angle of Incidence on one side of the Aeroplane, and to
+      effect the reverse condition, if it is not too much trouble, on the other
+      side.&rdquo;
+     </p>
+    <p>
+      &ldquo;But,&rdquo; objected Efficiency, &ldquo;the Lift varies with the Angle of Incidence,
+      and surely such a condition will result in one side of the Aeroplane
+      lifting more than the other side?'
+    </p>
+    <p>
+      &ldquo;That's all right,&rdquo; said the Propeller, &ldquo;it's meant to off-set the
+      tendency of the Aeroplane to turn over sideways in the opposite direction
+      to which I revolve.&rdquo;
+     </p>
+    <p>
+      &ldquo;That's quite clear, though rather unexpected; but how do you counteract
+      the effect of the gusts when they try to overturn the Aeroplane sideways?&rdquo;
+       said she, turning to Lateral Stability again.
+    </p>
+    <p>
+      &ldquo;Well,&rdquo; he replied, rather miserably, &ldquo;I'm not nearly so perfect as the
+      Longitudinal and Directional Stabilities. The Dihedral Angle&mdash;that
+      is, the upward inclination of the Surfaces towards their wing-tips&mdash;does
+      what it can for me, but, in my opinion, it's a more or less futile effort.
+      The Blackboard will show you the argument.&rdquo; And he at once showed them two
+      Surfaces, each set at a Dihedral Angle like this:
+    </p>
+    <p>
+      &ldquo;Please imagine,&rdquo; said the Blackboard, &ldquo;that the top V is the front view
+      of a Surface flying towards you. Now if a gust blows it into the position
+      of the lower V you see that the horizontal equivalent of the Surface on
+      one side becomes larger, and on the other side it becomes smaller. That
+      results in more Lift on the lower side and less on the higher side, and if
+      the V is large enough it should produce such a difference in the Lift of
+      one side to the other as to quickly turn the Aeroplane back to its former
+      and normal position.&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes,&rdquo; said the Dihedral Angle, &ldquo;that's what would happen if they would
+      only make me large enough; but they won't do it because it would too
+      greatly decrease the horizontal equivalent, and therefore the Lift, and
+      incidentally it would, as Aeroplanes are built to-day, produce an excess
+      of Keel Surface above the turning axis, and that in itself would spoil the
+      Lateral Stability. The Keel Surface should be equally divided above and
+      below the longitudinal turning axis (upon which the Aeroplane rolls
+      sideways), or the side upon which there is an excess will get blown over
+      by the gusts. It strikes me that my future isn't very promising, and about
+      my only chance is when the Junior Draughtsman makes a mistake, as he did
+      the other day. And just think of it, they call him a Designer now that
+      he's got a job at the Factory! What did he do? Why, he calculated the
+      weights wrong and got the Centre of Gravity too high, and they didn't
+      discover it until the machine was built. Then all they could do was to
+      give me a larger Angle. That dropped the bottom of the V lower down, and
+      as that's the centre of the machine, where all the Weight is, of course
+      that put the Centre of Gravity in its right place. But now there is too
+      much Keel Surface above, and the whole thing's a Bad Compromise, not at
+      all like Our Efficiency.&rdquo;
+     </p>
+    <p>
+      And Efficiency, blushing very prettily at the compliment, then asked, &ldquo;And
+      how does the Centre of Gravity affect matters?&rdquo;
+     </p>
+    <p>
+      &ldquo;That's easy,&rdquo; said Grandfather Gravity. &ldquo;I'm so heavy that if I am too
+      low down I act like a pendulum and cause the Aeroplane to roll about
+      sideways, and if I am too high I'm like a stick balanced on your finger,
+      and then if I'm disturbed, over I go and the Aeroplane with me; and, in
+      addition to that, there are the tricks I play with the Aeroplane when it's
+      banked up,<a href="#linknote-6" name="linknoteref-6" id="linknoteref-6"><small>6</small></a>
+      i.e., tilted sideways for a turn, and Centrifugal Force sets me going the
+      way I'm not wanted to go. No; I get on best with Lateral Stability when my
+      Centre is right on the centre of Drift, or, at any rate, not much below
+      it.&rdquo; And with that he settled back into the Lecturer's Chair and went
+      sound asleep again, for he was so very, very old, in fact the father of
+      all the Principles.
+    </p>
+    <p>
+      And the Blackboard had been busy, and now showed them a picture of the
+      Aeroplane as far as they knew it, and you will see that there is a slight
+      Dihedral Angle, and also, fixed to the tail, a vertical Keel Surface or
+      fin, as is very often the case in order to ensure the greater effect of
+      such surface being behind the vertical turning axis.
+    </p>
+    <p>
+      But Efficiency, growing rather critical with her newly gained knowledge,
+      cried out: &ldquo;But where's the horizontal Tail Surface? It doesn't look right
+      like that!&rdquo;
+     </p>
+    <p>
+      &ldquo;This is when I have the pleasure of meeting you, my dear,&rdquo; said
+      Longitudinal Stability. &ldquo;Here's the Tail Surface,&rdquo; he said, &ldquo;and in order
+      to help me it must be set IN EFFECT at a much less Angle of Incidence than
+      the Main Surface. To explain we must trouble the Blackboard again,&rdquo; and
+      this was his effort:
+    </p>
+    <p>
+      &ldquo;I have tried to make that as clear as possible,&rdquo; he said. &ldquo;It may appear
+      a bit complicated at first, but if you will take the trouble to look at it
+      for a minute you will find it quite simple. A is the normal and proper
+      direction of motion of the Aeroplane, but, owing to a gust of air, it
+      takes up the new nose-down position. Owing to Momentum, however, it does
+      not fly straight along in that direction, but moves more or less in the
+      direction B, which is the resultant of the two forces, Momentum and
+      Thrust. And so you will note that the Angle of Incidence, which is the
+      inclination of the Surfaces to the Direction of Motion, has decreased, and
+      of course the Lift decreases with it. You will also see, and this is the
+      point, that the Tail Surface has lost a higher proportion of its Angle,
+      and consequently its Lift, than has the Main Surface. Then, such being the
+      case, the Tail must fall and the Aeroplane assume its normal position
+      again, though probably at a slightly lower altitude.&rdquo;
+     </p>
+    <p>
+      &ldquo;I'm afraid I'm very stupid,&rdquo; said Efficiency, &ldquo;but please tell me why you
+      lay stress upon the words 'IN EFFECT.'&rdquo;
+     </p>
+    <p>
+      &ldquo;Ah! I was wondering if you would spot that,&rdquo; he replied. &ldquo;And there is a
+      very good reason for it. You see, in some Aeroplanes the Tail Surface may
+      be actually set at the same Angle on the machine as the Main Surface, but
+      owing to the air being deflected downwards by the front Main Surface it
+      meets the Tail Surface at a lesser angle, and indeed in some cases at no
+      angle at all. The Tail is then for its surface getting less Lift than the
+      Main Surface, although set at the same angle on the machine. It may then
+      be said to have IN EFFECT a less Angle of Incidence. I'll just show you on
+      the Blackboard.&rdquo;
+     </p>
+    <p>
+      &ldquo;And now,&rdquo; said Efficiency, &ldquo;I have only to meet the Ailerons and the
+      Rudder, haven't I?&rdquo;
+     </p>
+    <p>
+      &ldquo;Here we are,&rdquo; replied the Ailerons, or little wings. &ldquo;Please hinge us on
+      to the back of the Main Surfaces, one of us at each Wing-tip, and join us
+      up to the Pilot's joystick by means of the control cables. When the Pilot
+      wishes to tilt the Aeroplane sideways, he will move the stick and depress
+      us upon one side, thus giving us a larger Angle of Incidence and so
+      creating more Lift on that side of the Aeroplane; and, by means of a cable
+      connecting us with the Ailerons on the other side of the Aeroplane, we
+      shall, as we are depressed, pull them up and give them a reverse or
+      negative Angle of Incidence, and that side will then get a reverse Lift or
+      downward thrust, and so we are able to tilt the Aeroplane sideways.
+    </p>
+    <p>
+      &ldquo;And we work best when the Angle of Incidence of the Surface in front of
+      us is very small, for which reason it is sometimes decreased or washed-out
+      towards the Wing-tips. The reason of that is that by the time the air
+      reaches us it has been deflected downwards&mdash;the greater the Angle of
+      Incidence the more it is driven downwards&mdash;and in order for us to
+      secure a Reaction from it, we have to take such a large Angle of Incidence
+      that we produce a poor proportion of Lift to Drift; but the smaller the
+      Angle of the Surface in front of us the less the air is deflected
+      downwards, and consequently the less Angle is required of us, and the
+      better our proportion of Lift to Drift, which, of course, makes us much
+      more effective Controls.&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes,&rdquo; said the Lateral and Directional Stabilities in one voice, &ldquo;that's
+      so, and the wash-out helps us also, for then the Surfaces towards their
+      Wing-tips have less Drift or 'Head-Resistance,' and consequently the gusts
+      will affect them and us less; but such decreased Angle of Incidence means
+      decreased Lift as well as Drift, and the Designer does not always care to
+      pay the price.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well,&rdquo; said the Ailerons, &ldquo;if it's not done it will mean more work for
+      the Rudder, and that won't please the Pilot.&rdquo;
+     </p>
+    <p>
+      &ldquo;Whatever do you mean?&rdquo; asked Efficiency. &ldquo;What can the Rudder have to do
+      with you?&rdquo;
+     </p>
+    <p>
+      &ldquo;It's like this,&rdquo; they replied: &ldquo;when we are deflected downwards we gain a
+      larger Angle of Incidence and also enter an area of compressed air, and so
+      produce more Drift than those of us on the other side of the Aeroplane,
+      which are deflected upwards into an area of rarefied air due to the
+      SUCTION effect (though that term is not academically correct) on the top
+      of the Surface. If there is more Drift, i.e., Resistance, on one side of
+      the Aeroplane than on the other side, then of course it will turn off its
+      course, and if that difference in Drift is serious, as it will very likely
+      be if there is no wash-out, then it will mean a good deal of work for the
+      Rudder in keeping the Aeroplane on its course, besides creating extra
+      Drift in doing so.&rdquo;
+     </p>
+    <p>
+      &ldquo;I think, then,&rdquo; said Efficiency, &ldquo;I should prefer to have that wash-out,<a
+      href="#linknote-7" name="linknoteref-7" id="linknoteref-7"><small>7</small></a>
+      and my friend the Designer is so clever at producing strength of
+      construction for light weight, I'm pretty sure he won't mind paying the
+      price in Lift. And now let me see if I can sketch the completed
+      Aeroplane.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, I hope that's all as it should be,&rdquo; she concluded, &ldquo;for to-morrow
+      the Great Test in the air is due.&rdquo;
+     </p>
+    <p>
+      <a name="link2H_PART3" id="link2H_PART3">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      PART III. THE GREAT TEST
+    </h2>
+    <p>
+      It is five o'clock of a fine calm morning, when the Aeroplane is wheeled
+      out of its shed on to the greensward of the Military Aerodrome. There is
+      every promise of a good flying day, and, although the sun has not yet
+      risen, it is light enough to discern the motionless layer of fleecy clouds
+      some five thousand feet high, and far, far above that a few filmy mottled
+      streaks of vapour. Just the kind of morning beloved of pilots.
+    </p>
+    <p>
+      A brand new, rakish, up-to-date machine it is, of highly polished,
+      beautifully finished wood, fabric as tight as a drum, polished metal, and
+      every part so perfectly &ldquo;streamlined&rdquo; to minimize Drift, which is the
+      resistance of the air to the passage of the machine, that to the veriest
+      tyro the remark of the Pilot is obviously justified.
+    </p>
+    <p>
+      &ldquo;Clean looking 'bus, looks almost alive and impatient to be off. Ought to
+      have a turn for speed with those lines.&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes,&rdquo; replies the Flight-Commander, &ldquo;it's the latest of its type and
+      looks a beauty. Give it a good test. A special report is required on this
+      machine.&rdquo;
+     </p>
+    <p>
+      The A.M.'s<a href="#linknote-8" name="linknoteref-8" id="linknoteref-8"><small>8</small></a>
+      have now placed the Aeroplane in position facing the gentle air that is
+      just beginning to make itself evident; the engine Fitter, having made sure
+      of a sufficiency of oil and petrol in the tanks, is standing by the
+      Propeller; the Rigger, satisfied with a job well done, is critically
+      &ldquo;vetting&rdquo; the machine by eye, four A.M.'s are at their posts, ready to
+      hold the Aeroplane from jumping the blocks which have been placed in front
+      of the wheels; and the Flight-Sergeant is awaiting the Pilot's orders.
+    </p>
+    <p>
+      As the Pilot approaches the Aeroplane the Rigger springs to attention and
+      reports, &ldquo;All correct, sir,&rdquo; but the Fitter does not this morning report
+      the condition of the Engine, for well he knows that this Pilot always
+      personally looks after the preliminary engine test. The latter, in
+      leathern kit, warm flying boots and goggled, climbs into his seat, and
+      now, even more than before, has the Aeroplane an almost living appearance,
+      as if straining to be off and away. First he moves the Controls to see
+      that everything is clear, for sometimes when the Aeroplane is on the
+      ground the control lever or &ldquo;joy-stick&rdquo; is lashed fast to prevent the wind
+      from blowing the controlling surfaces about and possibly damaging them.
+    </p>
+    <p>
+      The air of this early dawn is distinctly chilly, and the A.M.'s are
+      beginning to stamp their cold feet upon the dewy grass, but very careful
+      and circumspect is the Pilot, as he mutters to himself, &ldquo;Don't worry and
+      flurry, or you'll die in a hurry.&rdquo;
+     </p>
+    <p>
+      At last he fumbles for his safety belt, but with a start remembers the
+      Pilot Air Speed Indicator, and, adjusting it to zero, smiles as he hears
+      the Pilot-head's gruff voice, &ldquo;Well, I should think so, twenty miles an
+      hour I was registering. That's likely to cause a green pilot to stall the
+      Aeroplane. Pancake, they call it.&rdquo; And the Pilot, who is an old hand and
+      has learned a lot of things in the air that mere earth-dwellers know
+      nothing about, distinctly heard the Pilot Tube, whose mouth is open to the
+      air to receive its pressure, stammer. &ldquo;Oh Lor! I've got an earwig already&mdash;hope
+      to goodness the Rigger blows me out when I come down&mdash;and this
+      morning air simply fills me with moisture; I'll never keep the Liquid
+      steady in the Gauge. I'm not sure of my rubber connections either.&rdquo;
+     </p>
+    <p>
+      &ldquo;Oh, shut up!&rdquo; cry all the Wires in unison, &ldquo;haven't we got our troubles
+      too? We're in the most horrible state of tension. It's simply murdering
+      our Factor of Safety, and how we can possibly stand it when we get the
+      Lift only the Designer knows.&rdquo;
+     </p>
+    <p>
+      &ldquo;That's all right,&rdquo; squeak all the little Wire loops, &ldquo;we're that
+      accommodating, we're sure to elongate a bit and so relieve your tension.&rdquo;
+       For the whole Aeroplane is braced together with innumerable wires, many of
+      which are at their ends bent over in the form of loops in order to connect
+      with the metal fittings on the spars and elsewhere&mdash;cheap and easy
+      way of making connection.
+    </p>
+    <p>
+      &ldquo;Elongate, you little devils, would you?&rdquo; fairly shout the Angles of
+      Incidence, Dihedral and Stagger, amid a chorus of groans from all parts of
+      the Aeroplane. &ldquo;What's going to happen to us then? How are we going to
+      keep our adjustments upon which good flying depends?&rdquo;
+     </p>
+    <p>
+      &ldquo;Butt us and screw us,"<a href="#linknote-9" name="linknoteref-9"
+       id="linknoteref-9"><small>9</small></a> wail the Wires. &ldquo;Butt us and screw
+      us, and death to the Loops. That's what we sang to the Designer, but he
+      only looked sad and scowled at the Directors.&rdquo;
+     </p>
+    <p>
+      &ldquo;And who on earth are they?&rdquo; asked the Loops, trembling for their
+      troublesome little lives.
+    </p>
+    <p>
+      &ldquo;Oh earth indeed,&rdquo; sniffed Efficiency, who had not spoken before, having
+      been rendered rather shy by being badly compromised in the Drawing Office.
+      &ldquo;I'd like to get some of them up between Heaven and Earth, I would. I'd
+      give 'em something to think of besides their Debits and Credits&mdash;but
+      all the same the Designer will get his way in the end. I'm his Best Girl,
+      you know, and if we could only get rid of the Directors, the little Tin
+      god, and the Man-who-takes-the-credit, we should be quite happy.&rdquo; Then she
+      abruptly subsides, feeling that perhaps the less said the better until she
+      has made a reputation in the Air. The matter of that Compromise still
+      rankled, and indeed it does seem hardly fit that a bold bad Tin god should
+      flirt with Efficiency. You see there was a little Tin god, and he said
+      &ldquo;Boom, Boom BOOM! Nonsense! It MUST be done,&rdquo; and things like that in a
+      very loud voice, and the Designer tore his hair and was furious, but the
+      Directors, who were thinking of nothing but Orders and Dividends, had the
+      whip-hand of HIM, and so there you are, and so poor beautiful Miss
+      Efficiency was compromised.
+    </p>
+    <p>
+      All this time the Pilot is carefully buckling his belt and making himself
+      perfectly easy and comfortable, as all good pilots do. As he straightens
+      himself up from a careful inspection of the Deviation Curve<a
+      href="#linknote-10" name="linknoteref-10" id="linknoteref-10"><small>10</small></a>
+      of the Compass and takes command of the Controls, the Throttle and the
+      Ignition, the voices grow fainter and fainter until there is nothing but a
+      trembling of the Lift and Drift wires to indicate to his understanding eye
+      their state of tension in expectancy of the Great Test.
+    </p>
+    <p>
+      &ldquo;Petrol on?&rdquo; shouts the Fitter to the Pilot.
+    </p>
+    <p>
+      &ldquo;Petrol on,&rdquo; replies the Pilot.
+    </p>
+    <p>
+      &ldquo;Ignition off?&rdquo;
+     </p>
+    <p>
+      &ldquo;Ignition off.&rdquo;
+     </p>
+    <p>
+      Round goes the Propeller, the Engine sucking in the Petrol Vapour with
+      satisfied gulps. And then&mdash;
+    </p>
+    <p>
+      &ldquo;Contact?&rdquo; from the Fitter.
+    </p>
+    <p>
+      &ldquo;Contact,&rdquo; says the Pilot.
+    </p>
+    <p>
+      Now one swing of the Propeller by the Fitter, and the Engine is awake and
+      working. Slowly at first though, and in a weak voice demanding, &ldquo;Not too
+      much Throttle, please. I'm very cold and mustn't run fast until my Oil has
+      thinned and is circulating freely. Three minutes slowly, as you love me,
+      Pilot.&rdquo;
+     </p>
+    <p>
+      Faster and faster turn the Engine and Propeller, and the Aeroplane,
+      trembling in all its parts, strains to jump the blocks and be off.
+      Carefully the Pilot listens to what the Engine Revolution Indicator says.
+      At last, &ldquo;Steady at 1,500 revs. and I'll pick up the rest in the Air.&rdquo;
+       Then does he throttle down the Engine, carefully putting the lever back to
+      the last notch to make sure that in such position the Throttle is still
+      sufficiently open for the Engine to continue working, as otherwise it
+      might lead to him &ldquo;losing&rdquo; his Engine in the air when throttling down the
+      power for descent. Then, giving the official signal, he sees the blocks
+      removed from the wheels, and the Flight-Sergeant saluting he knows that
+      all is clear to ascend. One more signal, and all the A.M.'s run clear of
+      the Aeroplane.
+    </p>
+    <p>
+      Then gently, gently mind you, with none of the &ldquo;crashing on&rdquo; bad Pilots
+      think so fine, he opens the Throttle and, the Propeller Thrust overcoming
+      its enemy the Drift, the Aeroplane moves forward.
+    </p>
+    <p>
+      &ldquo;Ah!&rdquo; says the Wind-screen, &ldquo;that's Discipline, that is. Through my little
+      window I see most things, and don't I just know that poor discipline
+      always results in poor work in the air, and don't you forget it.&rdquo;
+     </p>
+    <p>
+      &ldquo;Discipline is it?&rdquo; complains the Under-carriage, as its wheels roll
+      swiftly over the rather rough ground. &ldquo;I'm bump getting it; and bump,
+      bump, all I want, bang, bump, rattle, too!&rdquo; But, as the Lift increases
+      with the Speed, the complaints of the Under-carriage are stilled, and
+      then, the friendly Lift becoming greater than the Weight, the Aeroplane
+      swiftly and easily takes to the air.
+    </p>
+    <p>
+      Below is left the Earth with all its bumps and troubles. Up into the clean
+      clear Air moves with incredible speed and steadiness this triumph of the
+      Designer, the result of how much mental effort, imagination, trials and
+      errors, failures and successes, and many a life lost in high endeavour.
+    </p>
+    <p>
+      Now is the mighty voice of the Engine heard as he turns the Propeller nine
+      hundred times a minute. Now does the Thrust fight the Drift for all it's
+      worth, and the Air Speed Indicator gasps with delight, &ldquo;One hundred miles
+      an hour!&rdquo;
+     </p>
+    <p>
+      And now does the burden of work fall upon the Lift and Drift Wires, and
+      they scream to the Turnbuckles whose business it is to hold them in
+      tension, &ldquo;This is the limit! the Limit! THE LIMIT! Release us, if only a
+      quarter turn.&rdquo; But the Turnbuckles are locked too fast to turn their eyes
+      or utter a word. Only the Locking Wires thus: &ldquo;Ha! ha! the Rigger knew his
+      job. He knew the trick, and there's no release here.&rdquo; For an expert rigger
+      will always use the locking wire in such a way as to oppose the slightest
+      tendency of the turnbuckle to unscrew. The other kind of rigger will often
+      use the wire in such a way as to allow the turnbuckle, to the &ldquo;eyes&rdquo; of
+      which the wires are attached, to unscrew a quarter of a turn or more, with
+      the result that the correct adjustment of the wires may be lost; and upon
+      their fine adjustment much depends.
+    </p>
+    <p>
+      And the Struts and the Spars groan in compression and pray to keep
+      straight, for once &ldquo;out of truth&rdquo; there is, in addition to possible
+      collapse, the certainty that in bending they will throw many wires out of
+      adjustment.
+    </p>
+    <p>
+      And the Fabric's quite mixed in its mind, and ejaculates, &ldquo;Now, who would
+      have thought I got more Lift from the top of the Surface than its bottom?&rdquo;
+       And then truculently to the Distance Pieces, which run from rib to rib,
+      &ldquo;Just keep the Ribs from rolling, will you? or you'll see me strip. I'm an
+      Irishman, I am, and if my coat comes off&mdash;&mdash; Yes, Irish, I said.
+      I used to come from Egypt, but I've got naturalized since the War began.&rdquo;
+     </p>
+    <p>
+      Then the Air Speed Indicator catches the eye of the Pilot. &ldquo;Good enough,&rdquo;
+       he says as he gently deflects the Elevator and points the nose of the
+      Aeroplane upwards in search of the elusive Best Climbing Angle.
+    </p>
+    <p>
+      &ldquo;Ha! ha!&rdquo; shouts the Drift, growing stronger with the increased Angle of
+      Incidence. &ldquo;Ha! ha!&rdquo; he laughs to the Thrust. &ldquo;Now I've got you. Now who's
+      Master?&rdquo;
+     </p>
+    <p>
+      And the Propeller shrieks hysterically, &ldquo;Oh! look at me. I'm a helicopter.
+      That's not fair. Where's Efficiency?&rdquo; And she can only sadly reply, &ldquo;Yes,
+      indeed, but you see we're a Compromise.&rdquo;
+     </p>
+    <p>
+      And the Drift has hopes of reaching the Maximum Angle of Incidence and
+      vanquishing the Thrust and the Lift. And he grows very bold as he
+      strangles the Thrust; but the situation is saved by the Propeller, who is
+      now bravely helicopting skywards, somewhat to the chagrin of Efficiency.
+    </p>
+    <p>
+      &ldquo;Much ado about nothing,&rdquo; quotes the Aeroplane learnedly. &ldquo;Compromise or
+      not, I'm climbing a thousand feet a minute. Ask the Altimeter. He'll
+      confirm it.&rdquo;
+     </p>
+    <p>
+      And so indeed it was. The vacuum box of the Altimeter was steadily
+      expanding under the decreased pressure of the rarefied air, and by means
+      of its little levers and its wonderful chain no larger than a hair it was
+      moving the needle round the gauge and indicating the ascent at the rate of
+      a thousand feet a minute.
+    </p>
+    <p>
+      And lo! the Aeroplane has almost reached the clouds! But what's this? A
+      sudden gust, and down sinks one wing and up goes the other. &ldquo;Oh, my
+      Horizontal Equivalent!&rdquo; despairingly call the Planes: &ldquo;it's eloping with
+      the Lift, and what in the name of Gravity will happen? Surely there was
+      enough scandal in the Factory without this, too!&rdquo; For the lift varies with
+      the horizontal equivalent of the planes, so that if the aeroplane tilts
+      sideways beyond a certain angle, the lift becomes less than the weight of
+      the machine, which must then fall. A fall in such a position is known as a
+      &ldquo;side-slip.&rdquo;
+     </p>
+    <p>
+      But the ever-watchful Pilot instantly depresses one aileron, elevating the
+      other, with just a touch of the rudder to keep on the course, and the
+      Planes welcome back their precious Lift as the Aeroplane flicks back to
+      its normal position.
+    </p>
+    <p>
+      &ldquo;Bit bumpy here under these clouds,&rdquo; is all the Pilot says as he heads for
+      a gap between them, and the next minute the Aeroplane shoots up into a new
+      world of space.
+    </p>
+    <p>
+      &ldquo;My eye!&rdquo; ejaculates the Wind-screen, &ldquo;talk about a view!&rdquo; And indeed mere
+      words will always fail to express the wonder of it. Six thousand feet up
+      now, and look! The sun is rising quicker than ever mortal on earth
+      witnessed its ascent. Far below is Mother Earth, wrapt in mists and deep
+      blue shadows, and far above are those light, filmy, ethereal clouds now
+      faintly tinged with pink And all about great mountains of cloud, lazily
+      floating in space. The sun rises and they take on all colours, blending
+      one with the other, from dazzling white to crimson and deep violet-blue.
+      Lakes and rivers here and there in the enormous expanse of country below
+      refract the level rays of the sun and, like so many immense diamonds, send
+      dazzling shafts of light far upwards. The tops of the hills now laugh to
+      the light of the sun, but the valleys are still mysterious dark blue
+      caverns, clowned with white filmy lace-like streaks of vapour. And withal
+      the increasing sense with altitude of vast, clean, silent solitudes of
+      space.
+    </p>
+    <p>
+      Lives there the man who can adequately describe this Wonder? &ldquo;Never,&rdquo; says
+      the Pilot, who has seen it many times, but to whom it is ever new and more
+      wonderful.
+    </p>
+    <p>
+      Up, up, up, and still up, unfalteringly speeds the Pilot and his mount.
+      Sweet the drone of the Engine and steady the Thrust as the Propeller
+      exultingly battles with the Drift.
+    </p>
+    <p>
+      And look! What is that bright silver streak all along the horizon? It
+      puzzled the Pilot when first he saw it, but now he knows it for the Sea,
+      full fifty miles away!
+    </p>
+    <p>
+      And on his right is the brightness of the Morn and the smiling Earth
+      unveiling itself to the ardent rays of the Sun; and on his left, so high
+      is he, there is yet black Night, hiding innumerable Cities, Towns,
+      Villages and all those places where soon teeming multitudes of men shall
+      awake, and by their unceasing toil and the spirit within them produce
+      marvels of which the Aeroplane is but the harbinger.
+    </p>
+    <p>
+      And the Pilot's soul is refreshed, and his vision, now exalted, sees the
+      Earth a very garden, even as it appears at that height, with discord
+      banished and a happy time come, when the Designer shall have at last
+      captured Efficiency, and the Man-who-takes-the-credit is he who has earned
+      it, and when kisses are the only things that go by favour.
+    </p>
+    <p>
+      Now the Pilot anxiously scans the Barograph, which is an instrument much
+      the same as the Altimeter; but in this case the expansion of the vacuum
+      box causes a pen to trace a line upon a roll of paper. This paper is made
+      by clockwork to pass over the point of the pen, and so a curved line is
+      made which accurately registers the speed of the ascent in feet per
+      minute. No longer is the ascent at the rate of a thousand feet a minute,
+      and the Propeller complains to the Engine, &ldquo;I'm losing my Revs. and the
+      Thrust. Buck up with the Power, for the Lift is decreasing, though the
+      Weight remains much the same.&rdquo;
+     </p>
+    <p>
+      Quoth the Engine: &ldquo;I strangle for Air. A certain proportion, and that of
+      right density, I must have to one part of Petrol, in order to give me full
+      power and compression, and here at an altitude of ten thousand feet the
+      Air is only two-thirds as dense as at sea-level. Oh, where is he who will
+      invent a contrivance to keep me supplied with Air of right density and
+      quality? It should not be impossible within certain limits.&rdquo;
+     </p>
+    <p>
+      &ldquo;We fully agree,&rdquo; said the dying Power and Thrust. &ldquo;Only maintain Us and
+      you shall be surprised at the result. For our enemy Drift decreases in
+      respect of distance with the increase of altitude and rarity of air, and
+      there is no limit to the speed through space if only our strength remains.
+      And with oxygen for Pilot and Passengers and a steeper pitch<a
+      href="#linknote-11" name="linknoteref-11" id="linknoteref-11"><small>11</small></a>
+      for the Propeller we may then circle the Earth in a day!&rdquo;
+     </p>
+    <p>
+      Ah, Reader, smile not unbelievingly, as you smiled but a few years past.
+      There may be greater wonders yet. Consider that as the speed increases, so
+      does the momentum or stored-up force in the mass of the aeroplane become
+      terrific. And, bearing that in mind, remember that with altitude gravity
+      decreases. There may yet be literally other worlds to conquer.<a
+      href="#linknote-12" name="linknoteref-12" id="linknoteref-12"><small>12</small></a>
+    </p>
+    <p>
+      Now at fifteen thousand feet the conditions are chilly and rare, and the
+      Pilot, with thoughts of breakfast far below, exclaims, &ldquo;High enough! I had
+      better get on with the Test.&rdquo; And then, as he depresses the Elevator, the
+      Aeroplane with relief assumes its normal horizontal position. Then, almost
+      closing the Throttle, the Thrust dies away. Now, the nose of the Aeroplane
+      should sink of its own volition, and the craft glide downward at flying
+      speed, which is in this case a hundred miles an hour. That is what should
+      happen if the Designer has carefully calculated the weight of every part
+      and arranged for the centre of gravity to be just the right distance in
+      front of the centre of lift. Thus is the Aeroplane &ldquo;nose-heavy&rdquo; as a
+      glider, and just so to a degree ensuring a speed of glide equal to its
+      flying speed. And the Air Speed Indicator is steady at one hundred miles
+      an hour, and &ldquo;That's all right!&rdquo; exclaims the Pilot. &ldquo;And very useful,
+      too, in a fog or a cloud,&rdquo; he reflects, for then he can safely leave the
+      angle of the glide to itself, and give all his attention, and he will need
+      it all, to keeping the Aeroplane horizontal from wing-tip to wing-tip, and
+      to keeping it straight on its course. The latter he will manage with the
+      rudder, controlled by his feet, and the Compass will tell him whether a
+      straight course is kept. The former he will control by the Ailerons, or
+      little wings hinged to the tips of the planes, and the bubble in the
+      Inclinometer in front of him must be kept in the middle.
+    </p>
+    <p>
+      A Pilot, being only human, may be able to do two things at once, but three
+      is a tall order, so was this Pilot relieved to find the Design not at
+      fault and his craft a &ldquo;natural glider.&rdquo; To correct this nose-heavy
+      tendency when the Engine is running, and descent not required, the centre
+      of Thrust is arranged to be a little below the centre of Drift or
+      Resistance, and thus acts as a counter-balance.
+    </p>
+    <p>
+      But what is this stream of bad language from the Exhaust Pipe, accompanied
+      by gouts of smoke and vapour? The Engine, now revolving at no more than
+      one-tenth its normal speed, has upset the proportion of petrol to air, and
+      combustion is taking place intermittently or in the Exhaust Pipe, where it
+      has no business to be.
+    </p>
+    <p>
+      &ldquo;Crash, Bang, Rattle&mdash;&mdash;!&mdash;&mdash;!&mdash;&mdash;!&rdquo; and
+      worse than that, yells the Exhaust, and the Aeroplane, who is a gentleman
+      and not a box kite,<a href="#linknote-13" name="linknoteref-13"
+       id="linknoteref-13"><small>13</small></a> remonstrates with the severity
+      of a Senior Officer. &ldquo;See the Medical Officer, you young Hun. Go and see a
+      doctor. Vocal diarrhoea, that's your complaint, and a very nasty one too.
+      Bad form, bad for discipline, and a nuisance in the Mess. What's your
+      Regiment? Special Reserve, you say? Humph! Sounds like Secondhand Bicycle
+      Trade to me!&rdquo;
+     </p>
+    <p>
+      Now the Pilot decides to change the straight gliding descent to a spiral
+      one, and, obedient to the Rudder, the Aeroplane turns to the left. But the
+      Momentum (two tons at 100 miles per hour is no small affair) heavily
+      resents this change of direction, and tries its level best to prevent it
+      and to pull the machine sideways and outwards from its spiral course&mdash;that
+      is, to make it &ldquo;side-skid&rdquo; outwards. But the Pilot deflects the Ailerons
+      and &ldquo;banks&rdquo; up the planes to the correct angle, and, the Aeroplane
+      skidding sideways and outwards, the lowest surfaces of the planes press up
+      against the air until the pressure equals the centrifugal force of the
+      Momentum, and the Aeroplane spirals steadily downwards.
+    </p>
+    <p>
+      Down, down, down, and the air grows denser, and the Pilot gulps largely,
+      filling his lungs with the heavier air to counteract the increasing
+      pressure from without. Down through a gap in the clouds, and the Aerodrome
+      springs into view, appearing no larger than a saucer, and the Pilot,
+      having by now got the &ldquo;feel&rdquo; of the Controls, proceeds to put the
+      Aeroplane through its paces. First at its Maximum Angle, staggering along
+      tail-down and just maintaining horizontal flight; then a dive at far over
+      flying speed, finishing with a perfect loop; then sharp turns with
+      attendant vertical &ldquo;banks&rdquo; and then a wonderful switchback flight,
+      speeding down at a hundred and fifty miles an hour with short,
+      exhilarating ascents at the rate of two thousand feet a minute!
+    </p>
+    <p>
+      All the parts are now working well together. Such wires as were before in
+      undue tension have secured relief by slightly elongating their loops, and
+      each one is now doing its bit, and all are sharing the burden of work
+      together.
+    </p>
+    <p>
+      The Struts and the Spars, which felt so awkward at first, have bedded
+      themselves in their sockets, and are taking the compression stresses
+      uncomplainingly.
+    </p>
+    <p>
+      The Control Cables of twisted wire, a bit tight before, have slightly
+      lengthened by perhaps the eighth of an inch, and, the Controls instantly
+      responding to the delicate touch of the Pilot, the Aeroplane, at the will
+      of its Master, darts this way and that way, dives, loops, spirals, and at
+      last, in one long, magnificent glide, lands gently in front of its shed.
+    </p>
+    <p>
+      &ldquo;Well, what result?&rdquo; calls the Flight-Commander to the Pilot.
+    </p>
+    <p>
+      &ldquo;A hundred miles an hour and a thousand feet a minute,&rdquo; he briefly
+      replies.
+    </p>
+    <p>
+      &ldquo;And a very good result too,&rdquo; says the Aeroplane, complacently, as he is
+      carefully wheeled into his shed.
+    </p>
+    <p>
+      That is the way Aeroplanes speak to those who love them and understand
+      them. Lots of Pilots know all about it, and can spin you wonderful yarns,
+      much better than this one, if you catch them in a confidential mood&mdash;on
+      leave, for instance, and after a good dinner.
+    </p>
+    <p>
+      <a name="link2H_PART4" id="link2H_PART4">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      PART IV. 'CROSS COUNTRY
+    </h2>
+    <p>
+      The Aeroplane had been designed and built, and tested in the air, and now
+      stood on the Aerodrome ready for its first 'cross-country flight.
+    </p>
+    <p>
+      It had run the gauntlet of pseudo-designers, crank inventors, press
+      &ldquo;experts,&rdquo; and politicians; of manufacturers keen on cheap work and large
+      profits; of poor pilots who had funked it, and good pilots who had
+      expected too much of it. Thousands of pounds had been wasted on it, many
+      had gone bankrupt over it, and others it had provided with safe fat jobs.
+    </p>
+    <p>
+      Somehow, and despite every conceivable obstacle, it had managed to muddle
+      through, and now it was ready for its work. It was not perfect, for there
+      were fifty different ways in which it might be improved, some of them
+      shamefully obvious. But it was fairly sound mechanically, had a little
+      inherent stability, was easily controlled, could climb a thousand feet a
+      minute, and its speed was a hundred miles an hour. In short, quite a
+      creditable machine, though of course the right man had not got the credit.
+    </p>
+    <p>
+      It is rough, unsettled weather with a thirty mile an hour wind on the
+      ground, and that means fifty more or less aloft. Lots of clouds at
+      different altitudes to bother the Pilot, and the air none to clear for the
+      observation of landmarks.
+    </p>
+    <p>
+      As the Pilot and Observer approach the Aeroplane the former is clearly not
+      in the best of tempers. &ldquo;It's rotten luck,&rdquo; he is saying, &ldquo;a blank shame
+      that I should have to take this blessed 'bus and join X Reserve Squadron,
+      stationed a hundred and fifty miles from anywhere; and just as I have
+      licked my Flight into shape. Now some slack blighter will, I suppose,
+      command it and get the credit of all my work!&rdquo;
+     </p>
+    <p>
+      &ldquo;Shut up, you grouser,&rdquo; said the Observer. &ldquo;Do you think you're the only
+      one with troubles? Haven't I been through it too? Oh! I know all about it!
+      You're from the Special Reserve and your C.O. doesn't like your style of
+      beauty, and you won't lick his boots, and you were a bit of a technical
+      knut in civil life, but now you've jolly well got to know less than those
+      senior to you. Well! It's a jolly good experience for most of us. Perhaps
+      conceit won't be at quite such a premium after this war. And what's the
+      use of grousing? That never helped anyone. So buck up, old chap. Your day
+      will come yet. Here's our machine, and I must say it looks a beauty!&rdquo;
+     </p>
+    <p>
+      And, as the Pilot approaches the Aeroplane, his face brightens and he soon
+      forgets his troubles as he critically inspects the craft which is to
+      transport him and the Observer over the hills and far away. Turning to the
+      Flight-Sergeant he inquires, &ldquo;Tank full of petrol and oil?&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes, sir,&rdquo; he replies, &ldquo;and everything else all correct. Propeller,
+      engine, and body covers on board, sir; tool kit checked over and in the
+      locker; engine and Aeroplane logbooks written up, signed, and under your
+      seat; engine revs. up to mark, and all the control cables in perfect
+      condition and tension.&rdquo;
+     </p>
+    <p>
+      &ldquo;Very good,&rdquo; said the Pilot; and then turning to the Observer, &ldquo;Before we
+      start you had better have a look at the course I have mapped out.
+    </p>
+    <p>
+      &ldquo;A is where we stand and we have to reach B, a hundred and fifty miles due
+      North. I judge that, at the altitude we shall fly, there will be an East
+      wind, for although it is not quite East on the ground it is probably about
+      twenty degrees different aloft, the wind usually moving round clockways to
+      about that extent. I think that it is blowing at the rate of about fifty
+      miles an hour, and I therefore take a line on the map to C, fifty miles
+      due West of A. The Aeroplane's speed is a hundred miles an hour, and so I
+      take a line of one hundred miles from C to D. Our compass course will then
+      be in the direction A&mdash;E, which is always a line parallel to C&mdash;D.
+      That is, to be exact, it will be fourteen degrees off the C&mdash;D
+      course, as, in this part of the globe, there is that much difference
+      between the North and South lines on the map and the magnetic North to
+      which the compass needle points. If the compass has an error, as it may
+      have of a few degrees, that, too, must be taken into account, and the
+      deviation or error curve on the dashboard will indicate it.
+    </p>
+    <p>
+      &ldquo;The Aeroplane will then always be pointing in a direction parallel to A&mdash;E,
+      but, owing to the side wind, it will be actually travelling over the
+      course A&mdash;B, though in a rather sideways attitude to that course.
+    </p>
+    <p>
+      &ldquo;The distance we shall travel over the A&mdash;B course in one hour is A&mdash;D.
+      That is nearly eighty-seven miles, so we ought to accomplish our journey
+      of a hundred and fifty miles in about one and three-quarter hours.
+    </p>
+    <p>
+      &ldquo;I hope that's quite clear to you. It's a very simple way of calculating
+      the compass course, and I always do it like that.&rdquo;
+     </p>
+    <p>
+      &ldquo;Yes, that's plain enough. You have drafted what engineers call 'a
+      parallelogram of forces'; but suppose you have miscalculated the velocity
+      of the wind, or that it should change in velocity or direction?&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, that of course will more or less alter matters,&rdquo; replies the Pilot.
+      &ldquo;But there are any number of good landmarks such as lakes, rivers, towns,
+      and railway lines. They will help to keep us on the right course, and the
+      compass will, at any rate, prevent us from going far astray when between
+      them.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, we'd better be off, old chap. Hop aboard.&rdquo; This from the Observer
+      as he climbs into the front seat from which he will command a good view
+      over the lower plane; and the Pilot takes his place in the rear seat, and,
+      after making himself perfectly comfortable, fixing his safety belt, and
+      moving the control levers to make sure that they are working freely, he
+      gives the signal to the Engine Fitter to turn the propeller and so start
+      the engine.
+    </p>
+    <p>
+      Round buzzes the Propeller, and the Pilot, giving the official signal, the
+      Aeroplane is released and rolls swiftly over the ground in the teeth of
+      the gusty wind.
+    </p>
+    <p>
+      In less than fifty yards it takes to the air and begins to climb rapidly
+      upwards, but how different are the conditions to the calm morning of
+      yesterday! If the air were visible it would be seen to be acting in the
+      most extraordinary manner; crazily swirling, lifting and dropping, gusts
+      viciously colliding&mdash;a mad phantasmagoria of forces!
+    </p>
+    <p>
+      Wickedly it seizes and shakes the Aeroplane; then tries to turn it over
+      sideways; then instantly changes its mind and in a second drops it into a
+      hole a hundred feet deep, and if it were not for his safety belt the Pilot
+      might find his seat sinking away from beneath him.
+    </p>
+    <p>
+      Gusts strike the front of the craft like so many slaps in the face; and
+      others, with the motion of mountainous waves, sometimes lift it hundreds
+      of feet in a few seconds, hoping to see it plunge over the summit in a
+      death-dive&mdash;and so it goes on, but the Pilot, perfectly at one with
+      his mount and instantly alert to its slightest motion, is skilfully and
+      naturally making perhaps fifty movements a minute of hand and feet; the
+      former lightly grasping the &ldquo;joy-stick&rdquo; which controls the Elevator hinged
+      to the tail, and also the Ailerons or little wings hinged to the
+      wing-tips; and the latter moving the Rudder control-bar.
+    </p>
+    <p>
+      A strain on the Pilot? Not a bit of it, for this is his Work which he
+      loves and excels in; and given a cool head, alert eye, and a sensitive
+      touch for the controls, what sport can compare with these ever-changing
+      battles of the air?
+    </p>
+    <p>
+      The Aeroplane has all this time been climbing in great wide circles, and
+      is now some three thousand feet above the Aerodrome which from such height
+      looks absurdly small. The buildings below now seem quite squat; the hills
+      appear to have sunk away into the ground, and the whole country below, cut
+      up into diminutive fields, has the appearance of having been lately tidied
+      and thoroughly spring-cleaned! A doll's country it looks, with tiny horses
+      and cows ornamenting the fields and little model motor-cars and carts
+      stuck on the roads, the latter stretching away across the country like
+      ribbons accidentally dropped.
+    </p>
+    <p>
+      At three thousand feet altitude the Pilot is satisfied that he is now
+      sufficiently high to secure, in the event of engine failure, a long enough
+      glide to earth to enable him to choose and reach a good landing-place;
+      and, being furthermore content with the steady running of the engine, he
+      decides to climb no more but to follow the course he has mapped out.
+      Consulting the compass, he places the Aeroplane on the A&mdash;E course
+      and, using the Elevator, he gives his craft its minimum angle of incidence
+      at which it will just maintain horizontal flight and secure its maximum
+      speed.
+    </p>
+    <p>
+      Swiftly he speeds away, and few thoughts he has now for the changing
+      panorama of country, cloud, and colour. Ever present in his mind are the
+      three great 'cross-country queries. &ldquo;Am I on my right course? Can I see a
+      good landing-ground within gliding distance?&rdquo; And &ldquo;How is the Engine
+      running?&rdquo;
+     </p>
+    <p>
+      Keenly both he and the Observer compare their maps with the country below.
+      The roads, khaki-coloured ribbons, are easily seen but are not of much
+      use, for there are so many of them and they all look alike from such an
+      altitude.
+    </p>
+    <p>
+      Now where can that lake be which the map shows so plainly? He feels that
+      surely he should see it by now, and has an uncomfortable feeling that he
+      is flying too far West. What pilot is there indeed who has not many times
+      experienced such unpleasant sensation? Few things in the air can create
+      greater anxiety. Wisely, however, he sticks to his compass course, and the
+      next minute he is rewarded by the sight of the lake, though indeed he now
+      sees that the direction of his travel will not take him over it, as should
+      be the case if he were flying over the shortest route to his destination.
+      He must have slightly miscalculated the velocity or direction of the
+      side-wind.
+    </p>
+    <p>
+      &ldquo;About ten degrees off,&rdquo; he mutters, and, using the Rudder, corrects his
+      course accordingly.
+    </p>
+    <p>
+      Now he feels happier and that he is well on his way. The gusts, too, have
+      ceased to trouble him as, at this altitude, they are not nearly so bad as
+      they were near the ground the broken surface of which does much to produce
+      them; and sometimes for miles he makes but a movement or two of the
+      controls.
+    </p>
+    <p>
+      The clouds just above race by with dizzy and uniform speed; the country
+      below slowly unrolls, and the steady drone of the Engine is almost
+      hypnotic in effect. &ldquo;Sleep, sleep, sleep,&rdquo; it insidiously suggests.
+      &ldquo;Listen to me and watch the clouds; there's nothing else to do. Dream,
+      dream, dream of speeding through space for ever, and ever, and ever; and
+      rest, rest, rest to the sound of my rhythmical hum. Droning on and on,
+      nothing whatever matters. All things now are merged into speed through
+      space and a sleepy monotonous d-d-r-r-o-o-n-n-e - - - - -.&rdquo; But the Pilot
+      pulls himself together with a start and peers far ahead in search of the
+      next landmark. This time it is a little country town, red-roofed his map
+      tells him, and roughly of cruciform shape; and, sure enough, there in the
+      right direction are the broken outlines of a few red roofs peeping out
+      from between the trees.
+    </p>
+    <p>
+      Another minute and he can see this little town, a fairy town it appears,
+      nestling down between the hills with its red roofs and picturesque shape,
+      a glowing and lovely contrast with the dark green of the surrounding
+      moors.
+    </p>
+    <p>
+      So extraordinarily clean and tidy it looks from such a height, and laid
+      out in such orderly fashion with perfectly defined squares, parks,
+      avenues, and public buildings, it indeed appears hardly real, but rather
+      as if it has this very day materialized from some delightful children's
+      book!
+    </p>
+    <p>
+      Every city and town you must know has its distinct individuality to the
+      Pilot's eye. Some are not fairy places at all, but great dark ugly blots
+      upon the fair countryside, and with tall shafts belching forth murky
+      columns of smoke to defile clean space. Others, melancholy-looking masses
+      of grey, slate-roofed houses, are always sad and dispirited; never
+      welcoming the glad sunshine, but ever calling for leaden skies and a
+      weeping Heaven. Others again, little coquettes with village green, white
+      palings everywhere, bright gravel roads, and an irrepressible air of
+      brightness and gaiety.
+    </p>
+    <p>
+      Then there are the rivers, silvery streaks peacefully winding far, far
+      away to the distant horizon; they and the lakes the finest landmarks the
+      Pilot can have. And the forests. How can I describe them? The trees cannot
+      be seen separately, but merge altogether into enormous irregular dark
+      green masses sprawling over the country, and sometimes with great ungainly
+      arms half encircling some town or village; and the wind passing over the
+      foliage at times gives the forest an almost living appearance, as of some
+      great dragon of olden times rousing itself from slumber to devour the
+      peaceful villages which its arms encircle.
+    </p>
+    <p>
+      And the Pilot and Observer fly on and on, seeing these things and many
+      others which baffle my poor skill to describe&mdash;things, dear Reader,
+      that you shall see, and poets sing of, and great artists paint in the days
+      to come when the Designer has captured Efficiency. Then, and the time is
+      near, shall you see this beautiful world as you have never seen it before,
+      the garden it is, the peace it breathes, and the wonder of it.
+    </p>
+    <p>
+      The Pilot, flying on, is now anxiously looking for the railway line which
+      midway on his journey should point the course. Ah! There it is at last,
+      but suddenly (and the map at fault) it plunges into the earth! Well the
+      writer remembers when that happened to him on a long 'cross-country flight
+      in the early days of aviation. Anxiously he wondered &ldquo;Are tunnels always
+      straight?&rdquo; and with what relief, keeping on a straight course, he picked
+      up the line again some three miles farther on!
+    </p>
+    <p>
+      Now at last the Pilot sees the sea, just a streak on the north-eastern
+      horizon, and he knows that his flight is two-thirds over. Indeed, he
+      should have seen it before, but the air is none too clear, and he is not
+      yet able to discern the river which soon should cross his path. As he
+      swiftly speeds on the air becomes denser and denser with what he fears
+      must be the beginning of a sea-fog, perhaps drifting inland along the
+      course of the river. Now does he feel real anxiety, for it is the DUTY of
+      a Pilot to fear fog, his deadliest enemy. Fog not only hides the landmarks
+      by which he keeps his course, but makes the control of the Aeroplane a
+      matter of the greatest difficulty. He may not realize it, but, in keeping
+      his machine on an even keel, he is unconsciously balancing it against the
+      horizon, and with the horizon gone he is lost indeed. Not only that, but
+      it also prevents him from choosing his landing-place, and the chances are
+      that, landing in a fog, he will smash into a tree, hedge, or building,
+      with disastrous results. The best and boldest pilot 'wares a fog, and so
+      this one, finding the conditions becoming worse and yet worse, and being
+      forced to descend lower and lower in order to keep the earth within view,
+      wisely decides to choose a landing-place while there is yet time to do so.
+    </p>
+    <p>
+      Throttling down the power of the engine he spirals downwards, keenly
+      observing the country below. There are plenty of green fields to lure him,
+      and his great object is to avoid one in which the grass is long, for that
+      would bring his machine to a stop so suddenly as to turn it over; or one
+      of rough surface likely to break the under-carriage. Now is perfect
+      eyesight and a cool head indispensable. He sees and decides upon a field
+      and, knowing his job, he sticks to that field with no change of mind to
+      confuse him. It is none too large, and gliding just over the trees and
+      head on to the wind he skilfully &ldquo;stalls&rdquo; his machine; that is, the speed
+      having decreased sufficiently to avoid such a manoeuvre resulting in
+      ascent, he, by means of the Elevator, gives the Aeroplane as large an
+      angle of incidence as possible, and the undersides of the planes meeting
+      the air at such a large angle act as an air-brake, and the Aeroplane,
+      skimming over the ground, lessens its speed and finally stops just at the
+      farther end of the field.
+    </p>
+    <p>
+      Then, after driving the Aeroplane up to and under the lee of the hedge, he
+      stops the engine, and quickly lashing the joy-stick fast in order to
+      prevent the wind from blowing the controlling surfaces about and possibly
+      damaging them, he hurriedly alights. Now running to the tail he lifts it
+      up on to his shoulder, for the wind has become rough indeed and there is
+      danger of the Aeroplane becoming unmanageable. By this action he decreases
+      the angle at which the planes are inclined to the wind and so minimizes
+      the latter's effect upon them. Then to the Observer, &ldquo;Hurry up, old
+      fellow, and try to find some rope, wire, or anything with which to picket
+      the machine. The wind is rising and I shan't be able to hold the 'bus
+      steady for long. Don't forget the wire-cutters. They're in the tool kit.&rdquo;
+       And the Observer rushes off in frantic haste, before long triumphantly
+      returning with a long length of wire from a neighbouring fence. Blocking
+      up the tail with some debris at hand, they soon succeed, with the aid of
+      the wire, in stoutly picketing the Aeroplane to the roots of the high
+      hedge in front of it; done with much care, too, so that the wire shall not
+      fray the fabric or set up dangerous bending-stresses in the woodwork.
+      Their work is not done yet, for the Observer remarking, &ldquo;I don't like the
+      look of this thick weather and rather fear a heavy rain-storm,&rdquo; the Pilot
+      replies, &ldquo;Well, it's a fearful bore, but the first rule of our game is
+      never to take an unnecessary risk, so out with the engine and body
+      covers.&rdquo;
+     </p>
+    <p>
+      Working with a will they soon have the engine and the open part of the
+      body which contains the seats, controls, and instruments snugly housed
+      with their waterproof covers, and the Aeroplane is ready to weather the
+      possible storm.
+    </p>
+    <p>
+      Says the Observer, &ldquo;I'm remarkably peckish, and methinks I spy the towers
+      of one of England's stately homes showing themselves just beyond that
+      wood, less than a quarter of a mile away. What ho! for a raid. What do you
+      say?&rdquo;
+     </p>
+    <p>
+      &ldquo;All right, you cut along and I'll stop here, for the Aeroplane must not
+      be left alone. Get back as quickly as possible.&rdquo;
+     </p>
+    <p>
+      And the Observer trots off, leaving the Pilot filling his pipe and
+      anxiously scrutinizing the weather conditions. Very thick it is now, but
+      the day is yet young, and he has hopes of the fog lifting sufficiently to
+      enable the flight to be resumed. A little impatiently he awaits the return
+      of his comrade, but with never a doubt of the result, for the hospitality
+      of the country house is proverbial among pilots! What old hand among them
+      is there who cannot instance many a forced landing made pleasant by such
+      hospitality? Never too late or too early to help with food, petrol, oil,
+      tools, and assistants. Many a grateful thought has the writer for such
+      kind help given in the days before the war (how long ago they seem!), when
+      aeroplanes were still more imperfect than they are now, and involuntary
+      descents often a part of 'cross-country flying.
+    </p>
+    <p>
+      Ah! those early days! How fresh and inspiring they were! As one started
+      off on one's first 'cross-country flight, on a machine the first of its
+      design, and with everything yet to learn, and the wonders of the air yet
+      to explore; then the joy of accomplishment, the dreams of Efficiency, the
+      hard work and long hours better than leisure; and what a field of
+      endeavour&mdash;the realms of space to conquer! And the battle still goes
+      on with ever-increasing success. Who is bold enough to say what its limits
+      shall be?
+    </p>
+    <p>
+      So ruminates this Pilot-Designer, as he puffs at his pipe, until his
+      reverie is abruptly disturbed by the return of the Observer.
+    </p>
+    <p>
+      &ldquo;Wake up, you AIRMAN,&rdquo; the latter shouts. &ldquo;Here's the very thing the
+      doctor ordered! A basket of first-class grub and something to keep the fog
+      out, too.&rdquo;
+     </p>
+    <p>
+      &ldquo;Well, that's splendid, but don't call me newspaper names or you'll spoil
+      my appetite!&rdquo;
+     </p>
+    <p>
+      Then, with hunger such as only flying can produce, they appreciatively
+      discuss their lunch, and with many a grateful thought for the donors&mdash;and
+      they talk shop. They can't help it, and even golf is a poor second to
+      flight talk. Says the Pilot, who must have his grievance, &ldquo;Just observe
+      where I managed to stop the machine. Not twenty feet from this hedge! A
+      little more and we should have been through it and into Kingdom Come! I
+      stalled as well as one could, but the tail touched the ground and so I
+      could not give the Aeroplane any larger angle of incidence. Could I have
+      given it a larger angle, then the planes would have become a much more
+      effective air-brake, and we should have come to rest in a much shorter
+      distance. It's all the fault of the tail. There's hardly a type of
+      Aeroplane in existence in which the tail could not be raised several feet,
+      and that would make all the difference. High tails mean a large angle of
+      incidence when the machine touches ground and, with enough angle, I'll
+      guarantee to safely land the fastest machine in a five-acre field. You
+      can, I am sure, imagine what a difference that would make where forced
+      landings are concerned!&rdquo; Then rapidly sketching in his notebook, he shows
+      the Observer the following illustration:
+    </p>
+    <p>
+      &ldquo;That's very pretty,&rdquo; said the Observer, &ldquo;but how about Mechanical
+      Difficulties, and Efficiency in respect of Flight? And, anyway, why hasn't
+      such an obvious thing been done already?&rdquo;
+     </p>
+    <p>
+      &ldquo;As regards the first part of your question I assure you that there's
+      nothing in it, and I'll prove it to you as follows&mdash;&mdash;&rdquo;
+     </p>
+    <p>
+      &ldquo;Oh! That's all right, old chap. I'll take your word for it,&rdquo; hurriedly
+      replies the Observer, whose soul isn't tuned to a technical key.
+    </p>
+    <p>
+      &ldquo;As regards the latter part of your inquiry,&rdquo; went on the Pilot, a little
+      nettled at having such a poor listener, &ldquo;it's very simple. Aeroplanes have
+      'just growed' like Topsy, and they consequently contain this and many
+      another relic of early day design when Aeroplanes were more or less thrown
+      together and anything was good enough that could get off the ground.&rdquo;
+     </p>
+    <p>
+      &ldquo;By Jove,&rdquo; interrupts the Observer, &ldquo;I do believe the fog is lifting.
+      Hadn't we better get the engine and body covers off, just in case it's
+      really so?&rdquo;
+     </p>
+    <p>
+      &ldquo;I believe you're right. I am sure those hills over there could not be
+      seen a few minutes ago, and look&mdash;there's sunshine over there. We'd
+      better hurry up.&rdquo;
+     </p>
+    <p>
+      Ten minutes' hard work and the covers are off, neatly folded and stowed
+      aboard; the picketing wires are cast adrift, and the Pilot is once more in
+      his seat. The Aeroplane has been turned to face the other end of the
+      field, and, the Observer swinging round the propeller, the engine is awake
+      again and slowly ticking over. Quickly the Observer climbs into his seat
+      in front of the Pilot, and, the latter slightly opening the throttle, the
+      Aeroplane leisurely rolls over the ground towards the other end of the
+      field, from which the ascent will be made.
+    </p>
+    <p>
+      Arriving there the Pilot turns the Aeroplane in order to face the wind and
+      thus secure a quick &ldquo;get-off.&rdquo; Then he opens the throttle fully and the
+      mighty voice of the Engine roars out &ldquo;Now see me clear that hedge!&rdquo; and
+      the Aeroplane races forward at its minimum angle of incidence. Tail up,
+      and with ever-increasing speed, it rushes towards the hedge under the lee
+      of which it has lately been at rest; and then, just as the Observer
+      involuntarily pulls back an imaginary &ldquo;joy-stick,&rdquo; the Pilot moves the
+      real one and places the machine at its best climbing angle. Like a living
+      thing it responds, and instantly leaves the ground, clearing the hedge
+      like a&mdash;well, like an Aeroplane with an excellent margin of lift.
+      Upwards it climbs with even and powerful lift, and the familiar scenes
+      below again gladden the eyes of the Pilot. Smaller and more and more squat
+      grow the houses and hills; more and more doll-like appear the fields which
+      are clearly outlined by the hedges; and soon the country below is easily
+      identified with the map. Now they can see the river before them and a bay
+      of the sea which must be crossed or skirted. The fog still lingers along
+      the course of the river and between the hills, but is fast rolling away in
+      grey, ghost-like masses. Out to sea it obscures the horizon, making it
+      difficult to be sure where water ends and fog begins, and creating a
+      strange, rather weird effect by which ships at a certain distance appear
+      to be floating in space.
+    </p>
+    <p>
+      Now the Aeroplane is almost over the river, and the next instant it
+      suddenly drops into a &ldquo;hole in the air.&rdquo; With great suddenness it happens,
+      and for some two hundred feet it drops nose-down and tilted over sideways;
+      but the Pilot is prepared and has put his craft on an even keel in less
+      time than it takes to tell you about it; for well he knows that he must
+      expect such conditions when passing over a shore or, indeed, any
+      well-defined change in the composition of the earth's surface. Especially
+      is this so on a hot and sunny day, for then the warm surface of the earth
+      creates columns of ascending air, the speed of the ascent depending upon
+      the composition of the surface. Sandy soil, for instance, such as borders
+      this river produces a quickly ascending column of air, whereas water and
+      forests have not such a marked effect. Thus, when our Aeroplane passed
+      over the shore of the river, it suddenly lost the lift due to the
+      ascending air produced by the warm sandy soil, and it consequently dropped
+      just as if it had fallen into a hole.
+    </p>
+    <p>
+      Now the Aeroplane is over the bay and, the sea being calm, the Pilot looks
+      down, down through the water, and clearly sees the bottom, hundreds of
+      feet below the surface. Down through the reflection of the blue sky and
+      clouds, and one might think that is all, but it isn't. Only those who fly
+      know the beauties of the sea as viewed from above; its dappled pearly
+      tints; its soft dark blue shadows; the beautiful contrasts of unusual
+      shades of colour which are always differing and shifting with the changing
+      sunshine and the ever moving position of the aerial observer. Ah! for some
+      better pen than mine to describe these things! One with glowing words and
+      a magic rhythm to express the wonders of the air and the beauty of the
+      garden beneath&mdash;the immensity of the sea&mdash;the sense of space and
+      of one's littleness there&mdash;the realization of the Power moving the
+      multitudes below&mdash;the exaltation of spirit altitude produces&mdash;the
+      joy of speed. A new world of sensation!
+    </p>
+    <p>
+      Now the bay is almost crossed and the Aerodrome at B can be distinguished.
+    </p>
+    <p>
+      On the Aerodrome is a little crowd waiting and watching for the arrival of
+      the Aeroplane, for it is of a new and improved type and its first
+      'cross-country performance is of keen interest to these men; men who
+      really know something about flight.
+    </p>
+    <p>
+      There is the Squadron Commander who has done some real flying in his time;
+      several well-seasoned Flight-Commanders; a dozen or more
+      Flight-Lieutenants; a knowledgeable Flight-Sergeant; a number of Air
+      Mechanics, and, a little on one side and almost unnoticed, the Designer.
+    </p>
+    <p>
+      &ldquo;I hope they are all right,&rdquo; said someone, &ldquo;and that they haven't had
+      difficulties with the fog. It rolled up very quickly, you know.&rdquo;
+     </p>
+    <p>
+      &ldquo;Never fear,&rdquo; remarked a Flight-Commander. &ldquo;I know the Pilot well and he's
+      a good 'un; far too good to carry on into a fog.&rdquo;
+     </p>
+    <p>
+      &ldquo;They say the machine is really something out of the ordinary,&rdquo; said
+      another, &ldquo;and that, for once, the Designer has been allowed full play;
+      that he hasn't been forced to unduly standardize ribs, spars, struts,
+      etc., and has more or less had his own way. I wonder who he is. It seems
+      strange we hear so little of him.&rdquo;
+     </p>
+    <p>
+      &ldquo;Ah! my boy. You do a bit more flying and you'll discover that things are
+      not always as they appear from a distance!&rdquo;
+     </p>
+    <p>
+      &ldquo;There she is, sir!&rdquo; cries the Flight-Sergeant. &ldquo;Just a speck over the
+      silvery corner of that cloud.&rdquo;
+     </p>
+    <p>
+      A tiny speck it looks, some six miles distant and three thousand feet
+      high; but, racing along, it rapidly appears larger and soon its outlines
+      can be traced and the sunlight be seen playing upon the whirling
+      propeller.
+    </p>
+    <p>
+      Now the distant drone of the engine can be heard, but not for long, for
+      suddenly it ceases and, the nose of the Aeroplane sinking, the craft
+      commences gliding downwards.
+    </p>
+    <p>
+      &ldquo;Surely too far away,&rdquo; says a subaltern. &ldquo;It will be a wonderful machine
+      if, from that distance and height, it can glide into the Aerodrome.&rdquo; And
+      more than one express the opinion that it cannot be done; but the Designer
+      smiles to himself, yet with a little anxiety, for his reputation is at
+      stake, and Efficiency, the main reward he desires, is perhaps, or perhaps
+      not, at last within his grasp!
+    </p>
+    <p>
+      Swiftly the machine glides downwards towards them, and it can now be seen
+      how surprisingly little it is affected by the rough weather and gusts; so
+      much so that a little chorus of approval is heard.
+    </p>
+    <p>
+      &ldquo;Jolly good gliding angle,&rdquo; says someone; and another, &ldquo;Beautifully quick
+      controls, what?&rdquo; and from yet another, &ldquo;By Jove! The Pilot must be sure of
+      the machine. Look, he's stopped the engine entirely.&rdquo;
+     </p>
+    <p>
+      Then the Aeroplane with noiseless engine glides over the boundary of the
+      Aerodrome, and, with just a soft soughing sound from the air it cleaves,
+      lands gently not fifty yards from the onlookers.
+    </p>
+    <p>
+      &ldquo;Glad to see you,&rdquo; says the Squadron Commander to the Pilot. &ldquo;How do you
+      like the machine?&rdquo; And the Pilot replies:
+    </p>
+    <p>
+      &ldquo;I never want a better one, sir. It almost flies itself!&rdquo;
+     </p>
+    <p>
+      And the Designer turns his face homewards and towards his beloved
+      drawing-office; well satisfied, but still dreaming dreams of the future
+      and... looking far ahead whom should he see but Efficiency at last coming
+      towards him! And to him she is all things. In her hair is the morning
+      sunshine; her eyes hold the blue of the sky, and on her cheeks is the
+      pearly tint of the clouds as seen from above. The passion of speed, the
+      lure of space, the sense of power, and the wonder of the future... all
+      these things she holds for him.
+    </p>
+    <p>
+      &ldquo;Ah!&rdquo; he cries. &ldquo;You'll never leave me now, when at last there is no one
+      between us?&rdquo;
+     </p>
+    <p>
+      And Efficiency, smiling and blushing, but practical as ever, says:
+    </p>
+    <p>
+      &ldquo;And you will never throw those Compromises in my face?&rdquo;
+     </p>
+    <p>
+      &ldquo;My dear, I love you for them! Haven't they been my life ever since I
+      began striving for you ten long years ago?&rdquo;
+     </p>
+    <p>
+      And so they walked off very happily, arm-in-arm together; and if this
+      hasn't bored you and you'd like some more of the same sort of thing, I'd
+      just love to tell you some day of the wonderful things they accomplish
+      together, and of what they dream the future holds in store.
+    </p>
+    <p>
+      And that's the end of the Prologue.
+    </p>
+    <p>
+      <a name="link2HCH0001" id="link2HCH0001">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      CHAPTER I. FLIGHT
+    </h2>
+    <p>
+      Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum. It
+      therefore obeys Newton's laws<a href="#linknote-14" name="linknoteref-14"
+       id="linknoteref-14"><small>14</small></a> and resists movement. It is that
+      resistance or reaction which makes flight possible.
+    </p>
+    <p>
+      Flight is secured by driving through the air a surface<a
+      href="#linknote-15" name="linknoteref-15" id="linknoteref-15"><small>15</small></a>
+      inclined upwards and towards the direction of motion.
+    </p>
+    <p>
+      S = Side view of surface.
+    </p>
+    <p>
+      M = Direction of motion.
+    </p>
+    <p>
+      CHORD.&mdash;The Chord is, for practical purposes, taken to be a straight
+      line from the leading edge of the surface to its trailing edge.
+    </p>
+    <p>
+      N = A line through the surface starting from its trailing edge. The
+      position of this line, which I call the Neutral Lift Line, is found by
+      means of wind-tunnel research, and it varies with differences in the
+      camber (curvature) of surfaces. In order to secure flight, the inclination
+      of the surface must be such that the neutral lift line makes an angle with
+      and ABOVE the line of motion. If it is coincident with M, there is no
+      lift. If it makes an angle with M and BELOW it, then there is a pressure
+      tending to force the surface down.
+    </p>
+    <p>
+      I = Angle of Incidence. This angle is generally defined as the angle the
+      chord makes with the direction of motion, but that is a bad definition, as
+      it leads to misconception. The angle of incidence is best described as the
+      angle the neutral lift line makes with the direction of motion relative to
+      the air. You will, however, find that in nearly all rigging specifications
+      the angle of incidence is taken to mean the angle the chord makes with a
+      line parallel to the propeller thrust. This is necessary from the point of
+      view of the practical mechanic who has to rig the aeroplane, for he could
+      not find the neutral lift line, whereas he can easily find the chord.
+      Again, he would certainly be in doubt as to &ldquo;the direction of motion
+      relative to the air,&rdquo; whereas he can easily find a line parallel to the
+      propeller thrust. It is a pity, however, that these practical
+      considerations have resulted in a bad definition of the angle of incidence
+      becoming prevalent, a consequence of which has been the widespread fallacy
+      that flight may be secured with a negative inclination of the surface.
+      Flight may conceivably be secured with a negative angle of chord, but
+      never with a negative inclination of the surface. All this is only
+      applicable to cambered surfaces. In the case of flat surfaces the neutral
+      lift line coincides with the chord and the definition I have criticised
+      adversely is then applicable. Flat lifting surfaces are, however, never
+      used.
+    </p>
+    <p>
+      The surface acts upon the air in the following manner:
+    </p>
+    <p>
+      As the bottom of the surface meets the air, it compresses it and
+      accelerates it DOWNWARDS. As a result of this definite action there is, of
+      course, an equal and opposite reaction UPWARDS.
+    </p>
+    <p>
+      The top surface, in moving forward, tends to leave the air behind it, thus
+      creating a semi-vacuum or rarefied area over the top of the surface.
+      Consequently the pressure of air on the top of the surface is decreased,
+      thus assisting the reaction below to lift the surface UPWARDS.
+    </p>
+    <p>
+      The reaction increases approximately as the square of the velocity. It is
+      the result of (1) the mass of air engaged, and (2) the velocity and
+      consequent force with which the surface engages the air. If the reaction
+      was produced by only one of those factors it would increase in direct
+      proportion to the velocity, but, since it is the product of both factors,
+      it increases as V(2S).
+    </p>
+    <p>
+      Approximately three-fifths of the reaction is due to the decrease of
+      density (and consequent decrease of downward pressure) on the top of the
+      surface; and only some two-fifths is due to the upward reaction secured by
+      the action of the bottom surface upon the air. A practical point in
+      respect of this is that, in the event of the fabric covering the surface
+      getting into bad condition, it is more likely to strip off the top than
+      off the bottom.
+    </p>
+    <p>
+      The direction of the reaction is approximately at right-angles to the
+      chord of the surface, as illustrated above; and it is, in considering
+      flight, convenient to divide it into two component parts or values, thus:
+    </p>
+    <p>
+      1. The vertical component of the reaction, i.e., Lift, which is opposed to
+      Gravity, i.e., the weight of the aeroplane.
+    </p>
+    <p>
+      2. The horizontal component, i.e., Drift (sometimes called Resistance), to
+      which is opposed the thrust of the propeller.
+    </p>
+    <p>
+      The direction of the reaction is, of course, the resultant of the forces
+      Lift and Drift.
+    </p>
+    <p>
+      The Lift is the useful part of the reaction, for it lifts the weight of
+      the aeroplane.
+    </p>
+    <p>
+      The Drift is the villain of the piece, and must be overcome by the Thrust
+      in order to secure the necessary velocity to produce the requisite Lift
+      for flight.
+    </p>
+    <p>
+      DRIFT.&mdash;The drift of the whole aeroplane (we have considered only the
+      lifting surface heretofore) may be conveniently divided into three parts,
+      as follows:
+    </p>
+    <p>
+      Active Drift, which is the drift produced by the lifting surfaces.
+    </p>
+    <p>
+      Passive Drift, which is the drift produced by all the rest of the
+      aeroplane&mdash;the struts, wires, fuselage, under-carriage, etc., all of
+      which is known as &ldquo;detrimental surface.&rdquo;
+     </p>
+    <p>
+      Skin Friction, which is the drift produced by the friction of the air with
+      roughnesses of surface. The latter is practically negligible having regard
+      to the smooth surface of the modern aeroplane, and its comparatively slow
+      velocity compared with, for instance, the velocity of a propeller blade.
+    </p>
+    <p>
+      LIFT-DRIFT RATIO.&mdash;The proportion of lift to drift is known as the
+      lift-drift ratio, and is of paramount importance, for it expresses the
+      efficiency of the aeroplane (as distinct from engine and propeller). A
+      knowledge of the factors governing the lift-drift ratio is, as will be
+      seen later, an absolute necessity to anyone responsible for the rigging of
+      an aeroplane, and the maintenance of it in an efficient and safe
+      condition.
+    </p>
+    <p>
+      Those factors are as follows:
+    </p>
+    <p>
+      1. Velocity.&mdash;The greater the velocity the greater the proportion of
+      drift to lift, and consequently the less the efficiency. Considering the
+      lifting surfaces alone, both the lift and the (active) drift, being
+      component parts of the reaction, increase as the square of the velocity,
+      and the efficiency remains the same at all speeds. But, considering the
+      whole aeroplane, we must remember the passive drift. It also increases as
+      the square of the velocity (with no attendant lift), and, adding itself to
+      the active drift, results in increasing the proportion of total drift
+      (active + passive) to lift.
+    </p>
+    <p>
+      But for the increase in passive drift the efficiency of the aeroplane
+      would not fall with increasing velocity, and it would be possible, by
+      doubling the thrust, to approximately double the speed or lift&mdash;a
+      happy state of affairs which can never be, but which we may, in a measure,
+      approach by doing everything possible to diminish the passive drift.
+    </p>
+    <p>
+      Every effort is then made to decrease it by &ldquo;stream-lining,&rdquo; i.e., by
+      giving all &ldquo;detrimental&rdquo; parts of the aeroplane a form by which they will
+      pass through the air with the least possible drift. Even the wires bracing
+      the aeroplane together are, in many cases, stream-lined, and with a
+      markedly good effect upon the lift-drift ratio. In the case of a certain
+      well-known type of aeroplane the replacing of the ordinary wires by
+      stream-lined wires added over five miles an hour to the flight speed.
+    </p>
+    <p>
+      Head-resistance is a term often applied to passive drift, but it is apt to
+      convey a wrong impression, as the drift is not nearly so much the result
+      of the head or forward part of struts, wires, etc., as it is of the
+      rarefied area behind.
+    </p>
+    <p>
+      Above is illustrated the flow of air round two objects moving in the
+      direction of the arrow M.
+    </p>
+    <p>
+      In the case of A, you will note that the rarefied area DD is of very
+      considerable extent; whereas in the case of B, the air flows round it in
+      such a way as to meet very closely to the rear of the object, thus
+      DECREASING DD.
+    </p>
+    <p>
+      The greater the rarefied area DD. then, the less the density, and,
+      consequently, the less the pressure of air upon the rear of the object.
+      The less such pressure, then, the better is head-resistance D able to get
+      its work in, and the more thrust will be required to overcome it.
+    </p>
+    <p>
+      The &ldquo;fineness&rdquo; of the stream-line shape, i.e., the proportion of length to
+      width, is determined by the velocity&mdash;the greater the velocity, the
+      greater the fineness. The best degree of fineness for any given velocity
+      is found by means of wind-tunnel research.
+    </p>
+    <p>
+      The practical application of all this is, from a rigging point of view,
+      the importance of adjusting all stream-line parts to be dead-on in the
+      line of flight, but more of that later on.
+    </p>
+    <p>
+      2. Angle of Incidence.&mdash;The most efficient angle of incidence varies
+      with the thrust at the disposal of the designer, the weight to be carried,
+      and the climb-velocity ratio desired.
+    </p>
+    <p>
+      The best angles of incidence for these varying factors are found by means
+      of wind-tunnel research and practical trial and error. Generally speaking,
+      the greater the velocity the smaller should be the angle of incidence, in
+      order to preserve a clean, stream-line shape of rarefied area and freedom
+      from eddies. Should the angle be too great for the velocity, then the
+      rarefied area becomes of irregular shape with attendant turbulent eddies.
+      Such eddies possess no lift value, and since it has taken power to produce
+      them, they represent drift and adversely affect the lift-drift ratio.
+    </p>
+    <p>
+      From a rigging point of view, one must presume that every standard
+      aeroplane has its lifting surface set at the most efficient angle, and the
+      practical application of all this is in taking the greatest possible care
+      to rig the surface at the correct angle and to maintain it at such angle.
+      Any deviation will adversely affect the lift-drift ratio, i.e., the
+      efficiency.
+    </p>
+    <p>
+      3. Camber.&mdash;(Refer to the second illustration in this chapter.) The
+      lifting surfaces are cambered, i.e., curved, in order to decrease the
+      horizontal component of the reaction, i.e., the drift.
+    </p>
+    <p>
+      The bottom camber: If the bottom of the surface was flat, every particle
+      of air meeting it would do so with a shock, and such shock would produce a
+      very considerable horizontal reaction or drift. By curving it such shock
+      is diminished, and the curve should be such as to produce a uniform (not
+      necessarily constant) acceleration and compression of the air from the
+      leading edge to the trailing edge. Any unevenness in the acceleration and
+      compression of the air produces drift.
+    </p>
+    <p>
+      The top camber: If this was flat it would produce a rarefied area of
+      irregular shape. I have already explained the bad effect this has upon the
+      lift-drift ratio. The top surface is then curved to produce a rarefied
+      area the shape of which shall be as stream-line and free from attendant
+      eddies as possible.
+    </p>
+    <p>
+      The camber varies with the angle of incidence, the velocity, and the
+      thickness of the surface. Generally speaking, the greater the velocity,
+      the less the camber and angle of incidence. With infinite velocity the
+      surface would be set at no angle of incidence (the neutral lift line
+      coincident with the direction of motion relative to the air), and would
+      be, top and bottom, of pure streamline form&mdash;i.e., of infinite
+      fineness. This is, of course, carrying theory to absurdity as the surface
+      would then cease to exist.
+    </p>
+    <p>
+      The best cambers for varying velocities, angles of incidence, and
+      thicknesses of surface, are found by means of wind-tunnel research. The
+      practical application of all this is in taking the greatest care to
+      prevent the surface from becoming distorted and thus spoiling the camber
+      and consequently the lift-drift ratio.
+    </p>
+    <p>
+      4. Aspect Ratio.&mdash;This is the proportion of span to chord. Thus, if
+      the span is, for instance, 50 feet and the chord 5 feet, the surface would
+      be said to have an aspect ratio of 10 to 1.
+    </p>
+    <p>
+      For A GIVEN VELOCITY and A GIVEN AREA of surface, the greater the aspect
+      ratio, the greater the reaction. It is obvious, I think, that the greater
+      the span, the greater the mass of air engaged, and, as already explained,
+      the reaction is partly the result of the mass of air engaged.
+    </p>
+    <p>
+      Not only that, but, PROVIDED the chord is not decreased to an extent
+      making it impossible to secure the best camber owing to the thickness of
+      the surface, the greater the aspect ratio, the better the lift-drift
+      ratio. The reason of this is rather obscure. It is sometimes advanced that
+      it is owing to the &ldquo;spill&rdquo; of air from under the wing-tips. With a high
+      aspect ratio the chord is less than would otherwise be the case. Less
+      chord results in smaller wing-tips and consequently less &ldquo;spill.&rdquo; This,
+      however, appears to be a rather inadequate reason for the high aspect
+      ratio producing the high lift-drift ratio. Other reasons are also
+      advanced, but they are of such a contentious nature I do not think it well
+      to go into them here. They are of interest to designers, but this is
+      written for the practical pilot and rigger.
+    </p>
+    <p>
+      5. Stagger.&mdash;This is the advancement of the top surface relative to
+      the bottom surface, and is not, of course, applicable to a single surface,
+      i.e., a monoplane. In the case of a biplane having no stagger, there will
+      be &ldquo;interference&rdquo; and consequent loss of Efficiency unless the gap between
+      the top and bottom surfaces is equal to not less than 1 1/2 times the
+      chord. If less than that, the air engaged by the bottom of the top surface
+      will have a tendency to be drawn into the rarefied area over the top of
+      the bottom surface, with the result that the surfaces will not secure as
+      good a reaction as would otherwise be the case.
+    </p>
+    <p>
+      It is not practicable to have a gap of much more than a distance equal to
+      the chord, owing to the drift produced by the great length of struts and
+      wires such a large gap would necessitate. By staggering the top surface
+      forward, however, it is removed from the action of the lower surface and
+      engages undisturbed air, with the result that the efficiency can in this
+      way be increased by about 5 per cent. Theoretically the top plane should
+      be staggered forward for a distance equal to about 30 per cent. of the
+      chord, the exact distance depending upon the velocity and angle of
+      incidence; but this is not always possible to arrange in designing an
+      aeroplane, owing to difficulties of balance, desired position, and view of
+      pilot, observer, etc.
+    </p>
+    <p>
+      6. Horizontal Equivalent.&mdash;The vertical component of the reaction,
+      i.e., lift, varies as the horizontal equivalent (H.E.) of the surface, but
+      the drift remains the same. Then it follows that if H.E. grows less, the
+      ratio of lift to drift must do the same.
+    </p>
+    <p>
+      A, B, and C are front views of three surfaces.
+    </p>
+    <p>
+      A has its full H.E., and therefore, from the point of view from which we
+      are at the moment considering efficiency, it has its best lift-drift
+      ratio.
+    </p>
+    <p>
+      B and C both possess the same surface as A, but one is inclined upwards
+      from its centre and the other is straight but tilted. For these reasons
+      their H.E.'s are, as illustrated, less than in the case of A. That means
+      less vertical lift, and, the drift remaining the same (for there is the
+      same amount of surface as in A to produce it), the lift-drift ratio falls.
+    </p>
+    <p>
+      THE MARGIN OF POWER is the power available above that necessary to
+      maintain horizontal flight.
+    </p>
+    <p>
+      THE MARGIN OF LIFT is the height an aeroplane can gain in a given time and
+      starting from a given altitude. As an example, thus: 1,000 feet the first
+      minute, and starting from an altitude of 500 feet above sea-level.
+    </p>
+    <p>
+      The margin of lift decreases with altitude, owing to the decrease in the
+      density of the air, which adversely affects the engine. Provided the
+      engine maintained its impulse with altitude, then, if we ignore the
+      problem of the propeller, which I will go into later on, the margin of
+      lift would not disappear. Moreover, greater velocity for a given power
+      would be secured at a greater altitude, owing to the decreased density of
+      air to be overcome. After reading that, you may like to light your pipe
+      and indulge in dreams of the wonderful possibilities which may become
+      realities if some brilliant genius shows us some day how to secure a
+      constant power with increasing altitude. I am afraid, however, that will
+      always remain impossible; but it is probable that some very interesting
+      steps may be taken in that direction.
+    </p>
+    <p>
+      THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, for a given
+      power, surface (including detrimental surface), and weight, horizontal
+      flight can be maintained.
+    </p>
+    <p>
+      THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, for a given
+      power, surface (including detrimental surface), and weight, horizontal
+      flight can be maintained.
+    </p>
+    <p>
+      THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-drift ratio
+      is highest. In modern aeroplanes it is that angle of incidence possessed
+      by the surface when the axis of the propeller is horizontal.
+    </p>
+    <p>
+      THE BEST CLIMBING ANGLE is approximately half-way between the maximum and
+      the optimum angles.
+    </p>
+    <p>
+      All present-day aeroplanes are a compromise between Climb and horizontal
+      Velocity. We will compare the essentials for two aeroplanes, one designed
+      for maximum climb, and the other for maximum velocity.
+    </p>
+    <p>
+      ESSENTIALS FOR MAXIMUM CLIMB:
+    </p>
+    <p>
+      1. Low velocity, in order to secure the best lift-drift ratio.
+    </p>
+    <p>
+      2. Having a low velocity, a large surface will be necessary in order to
+      engage the necessary mass of air to secure the requisite lift.
+    </p>
+    <p>
+      3. Since (1) such a climbing machine will move along an upward sloping
+      path, and (2) will climb with its propeller thrust horizontal, then a
+      large angle relative to the direction of the thrust will be necessary in
+      order to secure the requisite angle relative to the direction of motion.
+    </p>
+    <p>
+      The propeller thrust should be always horizontal, because the most
+      efficient flying-machine (having regard to climb OR velocity) has, so far,
+      been found to be an arrangement of an inclined surface driven by a
+      HORIZONTAL thrust&mdash;the surface lifting the weight, and the thrust
+      overcoming the drift. This is, in practice, a far more efficient
+      arrangement than the helicopter, i.e., the air-screw revolving about a
+      vertical axis and producing a thrust opposed to gravity. If, when
+      climbing, the propeller thrust is at such an angle as to tend to haul the
+      aeroplane upwards, then it is, in a measure, acting as a helicopter, and
+      that means inefficiency. The reason of a helicopter being inefficient in
+      practice is due to the fact that, owing to mechanical difficulties, it is
+      impossible to construct within a reasonable weight an air-screw of the
+      requisite dimensions. That being so, it would be necessary, in order to
+      absorb the power of the engine, to revolve the comparatively
+      small-surfaced air screw at an immensely greater velocity than that of the
+      aeroplane's surface. As already explained, the lift-drift ratio falls with
+      velocity on account of the increase in passive drift. This applies to a
+      blade of a propeller or air-screw, which is nothing but a revolving
+      surface set at angle of incidence, and which it is impossible to construct
+      without a good deal of detrimental surface near the central boss.
+    </p>
+    <p>
+      4. The velocity being low, then it follows that for that reason also the
+      angle of incidence should be comparatively large.
+    </p>
+    <p>
+      5. Camber.&mdash;Since such an aeroplane would be of low velocity, and
+      therefore possess a large angle of incidence, a large camber would be
+      necessary.
+    </p>
+    <p>
+      Let us now consider the essentials for an aeroplane of maximum velocity
+      for its power, and possessing merely enough lift to get off the ground,
+      but no margin of lift.
+    </p>
+    <p>
+      1. Comparatively HIGH VELOCITY.
+    </p>
+    <p>
+      2. A comparatively SMALL SURFACE, because, being of greater velocity than
+      the maximum climber, a greater mass of air will be engaged for a given
+      surface and time, and therefore a smaller surface will be sufficient to
+      secure the requisit lift.
+    </p>
+    <p>
+      3. A small angle relative to the propeller thrust, since the latter
+      coincides with the direction of motion.
+    </p>
+    <p>
+      4. A comparatively small angle of incidence by reason of the high
+      velocity.
+    </p>
+    <p>
+      5. A comparatively small camber follows as a result of the small angle of
+      incidence.
+    </p>
+    <p>
+      <br />
+    </p>
+    <h3>
+      SUMMARY.
+    </h3>
+<pre xml:space="preserve">
+   Essentials for Maximum            Essentials for Maximum
+           Climb.                            Velocity
+
+   1. Low velocity.                  High velocity.
+   2. Large surface.                 Small surface.
+   3. Large angle relative to        Small angle relative to
+      propeller thrust.                 propeller thrust.
+   4. Large angle relative to        Small angle relative to direction
+      direction of motion.              of motion.
+   5. Large camber.                  Small camber.
+</pre>
+    <p>
+      It is mechanically impossible to construct an aeroplane of reasonable
+      weight of which it would be possible to very the above opposing
+      essentials. Therefore, all aeroplanes are designed as a compromise between
+      Climb and Velocity.
+    </p>
+    <p>
+      As a rule aeroplanes are designed to have at low altitude a slight margin
+      of lift when the propeller thrust is horizontal.
+    </p>
+    <p>
+      ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE DESIGNED AS
+      A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIGHT MARGIN OF
+      LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS HORIZONTAL
+    </p>
+    <p>
+      MINIMUM ANGLE.
+    </p>
+    <p>
+      This gives the greatest velocity during horizontal flight at a low
+      altitude. Greater velocity would be secured if the surface, angle, and
+      camber were smaller and designed to just maintain horizontal flight with a
+      horizontal thrust. Also, in such case, the propeller would not be
+      thrusting downwards, but along a horizontal line which is obviously a more
+      efficient arrangement if we regard the aeroplane merely from one point of
+      view, i.e., either with reference to velocity OR climb.
+    </p>
+    <p>
+      OPTIMUM ANGLE (Thrust horizontal)
+    </p>
+    <p>
+      The velocity is less than at the smaller minimum angle, and, as aeroplanes
+      are designed to-day, the area and angle of incidence of the surface is
+      such as to secure a slight ascent at a low altitude. The camber of the
+      surface is designed for this angle of incidence and velocity. The
+      lift-drift ratio is best at this angle.
+    </p>
+    <p>
+      BEST CLIMBING ANGLE
+    </p>
+    <p>
+      The velocity is now still less by reason of the increased angle producing
+      increase of drift. Less velocity at A GIVEN ANGLE produces less lift, but
+      the increased angle more or less offsets the loss of lift due to the
+      decreased velocity, and in addition, the thrust is now hauling the
+      aeroplane upwards.
+    </p>
+    <p>
+      MAXIMUM ANGLE
+    </p>
+    <p>
+      The greater angle has now produced so much drift as to lessen the velocity
+      to a point where the combined lifts from the surface and from the thrust
+      are only just able to maintain horizontal flight. Any greater angle will
+      result in a still lower lift-drift ratio. The lift will then become less
+      than the weight and the aeroplane will consequently fall. Such a fall is
+      known as &ldquo;stalling&rdquo; or &ldquo;pancaking.&rdquo;
+     </p>
+    <p>
+      NOTE.&mdash;The golden rule for beginners: Never exceed the Best Climbing
+      Angle. Always maintain the flying speed of the aeroplane.
+    </p>
+    <p>
+      By this means, when the altitude is reached where the margin of lift
+      disappears (on account of loss of engine power), and which is,
+      consequently, the altitude where it is just possible to maintain
+      horizontal flight, the aeroplane is flying with its thrust horizontal and
+      with maximum efficiency (as distinct from engine and propeller
+      efficiency).
+    </p>
+    <p>
+      The margin of lift at low altitude, and when the thrust is horizontal,
+      should then be such that the higher altitude at which the margin of lift
+      is lost is that altitude at which most of the aeroplane's horizontal
+      flight work is done. That ensures maximum velocity when most required.
+    </p>
+    <p>
+      Unfortunately, where aeroplanes designed for fighting are concerned, the
+      altitude where most of the work is done is that at which both maximum
+      velocity and maximum margin of lift for power are required.
+    </p>
+    <p>
+      Perhaps some day a brilliant inventor will design an aeroplane of
+      reasonable weight and drift of which it will be possible for the pilot to
+      vary at will the above-mentioned opposing essentials. Then we shall get
+      maximum velocity, or maximum margin of lift, for power as required. Until
+      then the design of the aeroplane must remain a compromise between Velocity
+      and Climb.
+    </p>
+    <p>
+      <a name="link2HCH0002" id="link2HCH0002">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      CHAPTER II. STABILITY AND CONTROL
+    </h2>
+    <p>
+      STABILITY is a condition whereby an object disturbed has a natural
+      tendency to return to its first and normal position. Example: a weight
+      suspended by a cord.
+    </p>
+    <p>
+      INSTABILITY is a condition whereby an object disturbed has a natural
+      tendency to move as far as possible away from its first position, with no
+      tendency to return. Example: a stick balanced vertically upon your finger.
+    </p>
+    <p>
+      NEUTRAL INSTABILITY is a condition whereby an object disturbed has no
+      tendency to move farther than displaced by the force of the disturbance,
+      and no tendency to return to its first position.
+    </p>
+    <p>
+      In order that an aeroplane may be reasonably controllable, it is necessary
+      for it to possess some degree of stability longitudinally, laterally, and
+      directionally.
+    </p>
+    <p>
+      LONGITUDINAL STABILITY in an aeroplane is its stability about an axis
+      transverse to the direction of normal horizontal flight, and without which
+      it would pitch and toss.
+    </p>
+    <p>
+      LATERAL STABILITY is its stability about its longitudinal axis, and
+      without which it would roll sideways.
+    </p>
+    <p>
+      DIRECTIONAL STABILITY is its stability about its vertical axis, and
+      without which it would have no tendency to keep its course.
+    </p>
+    <p>
+      For such directional stability to exist there must be, in effect,<a
+      href="#linknote-16" name="linknoteref-16" id="linknoteref-16"><small>16</small></a>
+      more &ldquo;keel-surface&rdquo; behind the vertical axis than there is in front of it.
+      By keel-surface I mean every-thing to be seen when looking at an aeroplane
+      from the side of it&mdash;the sides of the body, undercarriage, struts,
+      wires, etc. The same thing applies to a weathercock. You know what would
+      happen if there was insufficient keel-surface behind the vertical axis
+      upon which it is pivoted. It would turn off its proper course, which is
+      opposite to the direction of the wind. It is very much the same in the
+      case of an aeroplane.
+    </p>
+    <p>
+      The above illustration represents an aeroplane (directionally stable)
+      flying along the course B. A gust striking it as indicated acts upon the
+      greater proportion of keel-surface behind the turning axis and throws it
+      into the new course. It does not, however, travel along the new course,
+      owing to its momentum in the direction B. It travels, as long as such
+      momentum lasts, in a direction which is the resultant of the two forces
+      Thrust and Momentum. But the centre line of the aeroplane is pointing in
+      the direction of the new course. Therefore its attitude, relative to the
+      direction of motion, is more or less sideways, and it consequently
+      receives an air pressure in the direction C. Such pressure, acting upon
+      the keel-surface, presses the tail back towards its first position in
+      which the aeroplane is upon its course B.
+    </p>
+    <p>
+      What I have described is continually going on during flight, but in a
+      well-designed aeroplane such stabilizing movements are, most of the time,
+      so slight as to be imperceptible to the pilot.
+    </p>
+    <p>
+      If an aeroplane was not stabilized in this way, it would not only be
+      continually trying to leave its course, but it would also possess a
+      dangerous tendency to &ldquo;nose away&rdquo; from the direction of the side gusts. In
+      such case the gust shown in the above illustration would turn the
+      aeroplane round the opposite way a very considerable distance; and the
+      right wing, being on the outside of the turn, would travel with greater
+      velocity than the left wing. Increased velocity means increased lift; and
+      so, the right wing lifting, the aeroplane would turn over sideways very
+      quickly.
+    </p>
+    <p>
+      LONGITUDINAL STABILITY.&mdash;Flat surfaces are longitudinally stable
+      owing to the fact that with decreasing angles of incidence the centre line
+      of pressure (C.P.) moves forward.
+    </p>
+    <p>
+      The C.P. is a line taken across the surface, transverse to the direction
+      of motion, and about which all the air forces may be said to balance, or
+      through which they may be said to act.
+    </p>
+    <p>
+      Imagine A to be a flat surface, attitude vertical, travelling through the
+      air in the direction of motion M. Its C.P. is then obviously along the
+      exact centre line of the surface as illustrated.
+    </p>
+    <p>
+      In B, C, and D the surfaces are shown with angles of incidence decreasing
+      to nothing, and you will note that the C.P. moves forward with the
+      decreasing angle.
+    </p>
+    <p>
+      Now, should some gust or eddy tend to make the surface decrease the angle,
+      i.e., dive, then the C.P. moves forward and pushes the front of the
+      surface up. Should the surface tend to assume too large an angle, then the
+      reverse happens&mdash;the C.P. moves back and pushes the rear of the
+      surface up.
+    </p>
+    <p>
+      Flat surfaces are, then, theoretically stable longitudinally. They are
+      not, however, used, on account of their poor lift-drift ratio.
+    </p>
+    <p>
+      As already explained, cambered surfaces are used, and these are
+      longitudinally unstable at those angles of incidence producing a
+      reasonable lift-drift ratio, i.e., at angles below: about 12 degrees.
+    </p>
+    <p>
+      A is a cambered surface, attitude approximately vertical, moving through
+      the air in the direction M. Obviously the C. P. coincides with the
+      transverse centre line of the surface.
+    </p>
+    <p>
+      With decreasing angles, down to angles of about 30 degrees, the C.P. moves
+      forward as in the case of flat surfaces (see B), but angles above 30
+      degrees do not interest us, since they produce a very low ratio of lift to
+      drift.
+    </p>
+    <p>
+      Below angles of about 30 degrees (see C) the dipping front part of the
+      surface assumes a negative angle of incidence resulting in the DOWNWARD
+      air pressure D, and the more the angle of incidence is decreased, the
+      greater such negative angle and its resultant pressure D. Since the C.P.
+      is the resultant of all the air forces, its position is naturally affected
+      by D, which causes it to move backwards. Now, should some gust or eddy
+      tend to make the surface decrease its angle of incidence, i.e., dive, then
+      the C.P. moves backwards, and, pushing up the rear of the surface, causes
+      it to dive the more. Should the surface tend to assume too large an angle,
+      then the reverse happens; the pressure D decreases, with the result that
+      C.P. moves forward and pushes up the front of the surface, thus increasing
+      the angle still further, the final result being a &ldquo;tail-slide.&rdquo;
+     </p>
+    <p>
+      It is therefore necessary to find a means of stabilizing the naturally
+      unstable cambered surface. This is usually secured by means of a
+      stabilizing surface fixed some distance in the rear of the main surface,
+      and it is a necessary condition that the neutral lift lines of the two
+      surfaces, when projected to meet each other, make a dihedral angle. In
+      other words, the rear stabilizing surface must have a lesser angle of
+      incidence than the main surface&mdash;certainly not more than one-third of
+      that of the main surface. This is known as the longitudinal dihedral.
+    </p>
+    <p>
+      I may add that the tail-plane is sometimes mounted upon the aeroplane at
+      the same angle as the main surface, but, in such cases, it attacks air
+      which has received a downward deflection from the main surface, thus:
+    </p>
+    <p>
+      The angle at which the tail surface attacks the air (the angle of
+      incidence) is therefore less than the angle of incidence of the main
+      surface.
+    </p>
+    <p>
+      I will now, by means of the following illustration, try to explain how the
+      longitudinal dihedral secures stability:
+    </p>
+    <p>
+      First, imagine the aeroplane travelling in the direction of motion, which
+      coincides with the direction of thrust T. The weight is, of course,
+      balanced about a C.P., the resultant of the C.P. of the main surface and
+      the C.P. of the stabilizing surface. For the sake of illustration, the
+      stabilizing surface has been given an angle of incidence, and therefore
+      has a lift and C.P. In practice the stabilizer is often set at no angle of
+      incidence. In such case the proposition remains the same, but it is,
+      perhaps, a little easier to illustrate it as above.
+    </p>
+    <p>
+      Now, we will suppose that a gust or eddy throws the machine into the lower
+      position. It no longer travels in the direction of T, since the momentum
+      in the old direction pulls it off that course. M is now the resultant of
+      the Thrust and the Momentum, and you will note that this results in a
+      decrease in the angle our old friend the neutral lift line makes with M,
+      i.e., a decrease in the angle of incidence and therefore a decrease in
+      lift.
+    </p>
+    <p>
+      We will suppose that this decrease is 2 degrees. Such decrease applies to
+      both main surface and stabilizer, since both are fixed rigidly to the
+      aeroplane.
+    </p>
+    <p>
+      The main surface, which had 12 degrees angle, has now only 10 degrees,
+      i.e., a loss of ONE-SIXTH.
+    </p>
+    <p>
+      The stabilizer, which had 4 degrees angle, has now only 2 degrees, i.e., a
+      loss of ONE-HALF.
+    </p>
+    <p>
+      The latter has therefore lost a greater PROPORTION of its angle of
+      incidence, and consequently its lift, than has the main surface. It must
+      then fall relative to the main surface. The tail falling, the aeroplane
+      then assumes its first position, though at a slightly less altitude.
+    </p>
+    <p>
+      Should a gust throw the nose of the aeroplane up, then the reverse
+      happens. Both main surface and stabilizer increase their angles of
+      incidence in the same amount, but the angle, and therefore the lift, of
+      the stabilizer increases in greater proportion than does the lift of the
+      main surface, with the result that it lifts the tail. The aeroplane then
+      assumes its first position, though at a slightly greater altitude.
+    </p>
+    <p>
+      Do not fall into the widespread error that the angle of incidence varies
+      as the angle of the aeroplane to the horizontal. It varies with such
+      angle, but not as anything approaching it. Remember that the stabilizing
+      effect of the longitudinal dihedral lasts only as long as there is
+      momentum in the direction of the first course.
+    </p>
+    <p>
+      These stabilizing movements are taking place all the time, even though
+      imperceptible to the pilot.
+    </p>
+    <p>
+      Aeroplanes have, in the past, been built with a stabilizing surface in
+      front of the main surface instead of at the rear of it. In such design the
+      main surface (which is then the tail surface as well as the principal
+      lifting surface) must be set at a less angle than the forward stabilizing
+      surface, in order to secure a longitudinal dihedral. The defect of such
+      design lies in the fact that the main surface must have a certain angle to
+      lift the weight&mdash;say 5 degrees. Then, in order to secure a
+      sufficiency of longitudinal stability, it is necessary to set the forward
+      stabilizer at about 15 degrees. Such a large angle of incidence results in
+      a very poor lift-drift ratio (and consequently great loss of efficiency),
+      except at very low velocities compared with the speed of modern
+      aeroplanes. At the time such aeroplanes were built velocities were
+      comparatively low, and this defect was; for that reason, not sufficiently
+      appreciated. In the end it killed the &ldquo;canard&rdquo; or &ldquo;tail-first&rdquo; design.
+    </p>
+    <p>
+      Aeroplanes of the Dunne and similar types possess no stabilizing surface
+      distinct from the main surface, but they have a longitudinal dihedral
+      which renders them stable.
+    </p>
+    <p>
+      The main surface towards the wing-tips is given a decreasing angle of
+      incidence and corresponding camber. The wing-tips then act as longitudinal
+      stabilizers.
+    </p>
+    <p>
+      This design of aeroplane, while very interesting, has not proved very
+      practicable, owing to the following disadvantages: (1) The plan design is
+      not, from a mechanical point of view, so sound as that of the ordinary
+      aeroplane surface, which is, in plan, a parallelogram. It is, then,
+      necessary to make the strength of construction greater than would
+      otherwise be the case. That means extra weight. (2) The plan of the
+      surface area is such that the aspect ratio is not so high as if the
+      surface was arranged with its leading edges at right angles to the
+      direction of motion. The lower the aspect ratio, then, the less the lift.
+      This design, then, produces less lift for weight of surface than would the
+      same surface if arranged as a parallelogram. (3) In order to secure the
+      longitudinal dihedral, the angle of incidence has to be very much
+      decreased towards the wing-tips. Then, in order that the lift-drift ratio
+      may be preserved, there must be a corresponding decrease in the camber.
+      That calls for surface ribs of varying cambers, and results in an
+      expensive and lengthy job for the builder. (4) In order to secure
+      directional stability, the surface is, in the centre, arranged to dip down
+      in the form of a V, pointing towards the direction of motion. Should the
+      aeroplane turn off its course, then its momentum in the direction of its
+      first course causes it to move in a direction the resultant of the thrust
+      and the momentum. It then moves in a more or less sideways attitude, which
+      results in an air pressure upon one side of the V, and which tends to turn
+      the aeroplane back to its first course. This arrangement of the surface
+      results in a bad drift. Vertical surfaces at the wing-tips may also be set
+      at an angle producing the same stabilizing effect, but they also increase
+      the drift.
+    </p>
+    <p>
+      The gyroscopic action of a rotary engine will affect the longitudinal
+      stability when an aeroplane is turned to right or left. In the case of a
+      Gnome engine, such gyroscopic action will tend to depress the nose of the
+      aeroplane when it is turned to the left, and to elevate it when it is
+      turned to the right. In modern aeroplanes this tendency is not
+      sufficiently important to bother about. In the old days of crudely
+      designed and under-powered aeroplanes this gyroscopic action was very
+      marked, and led the majority of pilots to dislike turning an aeroplane to
+      the right, since, in doing so, there was some danger of &ldquo;stalling.&rdquo;
+     </p>
+    <p>
+      LATERAL STABILITY is far more difficult for the designer to secure than is
+      longitudinal or directional stability. Some degree of lateral stability
+      may be secured by means of the &ldquo;lateral dihedral,&rdquo; i.e., the upward
+      inclination of the surface towards its wing-tips thus:
+    </p>
+    <p>
+      Imagine the top V, illustrated opposite, to be the front view of a surface
+      flying towards you. The horizontal equivalent (H.E.) of the left wing is
+      the same as that of the right wing. Therefore, the lift of one wing is
+      equal to the lift of the other, and the weight, being situated always in
+      the centre, is balanced.
+    </p>
+    <p>
+      If some movement of the air causes the surface to tilt sideways, as in the
+      lower illustration, then you will note that the H.E. of the left wing
+      increases, and the H.E. of the right wing decreases. The left wing then,
+      having the greatest lift, rises; and the surface assumes its first and
+      normal position.
+    </p>
+    <p>
+      Unfortunately however, the righting effect is not proportional to the
+      difference between the right and left H.E.'s.
+    </p>
+    <p>
+      In the case of A, the resultant direction of the reaction of both wings is
+      opposed to the direction of gravity or weight. The two forces R R and
+      gravity are then evenly balanced, and the surface is in a state of
+      equilibrium.
+    </p>
+    <p>
+      In the case of B, you will note that the R R is not directly opposed to
+      gravity. This results in the appearance of M, and so the resultant
+      direction of motion of the aeroplane is no longer directly forward, but is
+      along a line the resultant of the thrust and M. In other words, it is,
+      while flying forward, at the same time moving sideways in the direction M.
+    </p>
+    <p>
+      In moving sideways, the keel-surface receives, of course, a pressure from
+      the air equal and opposite to M. Since such surface is greatest in effect
+      towards the tail, then the latter must be pushed sideways. That causes the
+      aeroplane to turn; and, the highest wing being on the outside of the turn,
+      it has a greater velocity than the lower wing. That produces greater lift,
+      and tends to tilt the aeroplane over still more. Such tilting tendency is,
+      however, opposed by the difference in the H.E.'s of the two wings.
+    </p>
+    <p>
+      It then follows that, for the lateral dihedral angle to be effective, such
+      angle must be large enough to produce, when the aeroplane tilts, a
+      difference in the H.E.'s of the two wings, which difference must be
+      sufficient to not only oppose the tilting tendency due to the aeroplane
+      turning, but sufficient to also force the aeroplane back to its original
+      position of equilibrium.
+    </p>
+    <p>
+      It is now, I hope, clear to the reader that the lateral dihedral is not
+      quite so effective as would appear at first sight. Some designers, indeed,
+      prefer not to use it, since its effect is not very great, and since it
+      must be paid for in loss of H.E. and consequently loss of lift, thus
+      decreasing the lift-drift ratio, i.e., the efficiency. Also, it is
+      sometimes advanced that the lateral dihedral increases the &ldquo;spill&rdquo; of air
+      from the wing-tips and that this adversely affects the lift-drift ratio.
+    </p>
+    <p>
+      The disposition of the keel-surface affects the lateral stability. It
+      should be, in effect, equally divided by the longitudinal turning axis of
+      the aeroplane. If there is an excess of keel-surface above or below such
+      axis, then a side gust striking it will tend to turn the aeroplane over
+      sideways.
+    </p>
+    <p>
+      The position of the centre of gravity affects lateral stability. If too
+      low, it produces a pendulum effect and causes the aeroplane to roll
+      sideways.
+    </p>
+    <p>
+      If too high, it acts as a stick balanced vertically would act. If
+      disturbed, it tends to travel to a position as far as possible from its
+      original position. It would then tend, when moved, to turn the aeroplane
+      over sideways and into an upside-down position.
+    </p>
+    <p>
+      From the point of view of lateral stability, the best position for the
+      centre of gravity is one a little below the centre of drift.
+    </p>
+    <p>
+      Propeller torque affects lateral stability. An aeroplane tends to turn
+      over sideways in the opposite direction to which the propeller revolves.
+    </p>
+    <p>
+      This tendency is offset by increasing the angle of incidence (and
+      consequently the lift) of the side tending to fall; and it is always
+      advisable, if practical considerations allow it, to also decrease the
+      angle upon the other side. In that way it is not necessary to depart so
+      far from the normal angle of incidence at which the lift-drift ratio is
+      highest.
+    </p>
+    <p>
+      Wash-in is the term applied to the increased angle.
+    </p>
+    <p>
+      Wash-out is the term applied to the decreased angle.
+    </p>
+    <p>
+      Both lateral and directional stability may be improved by washing out the
+      angle of incidence on both sides of the surface, thus:
+    </p>
+    <p>
+      The decreased angle decreases the drift and therefore the effect of gusts
+      upon the wing-tips which is just where they have the most effect upon the
+      aeroplane, owing to the distance from the turning axis.
+    </p>
+    <p>
+      The wash-out also renders the ailerons (lateral controlling services) more
+      effective, as, in order to operate them, it is not then necessary to give
+      them such a large angle of incidence as would otherwise be required.
+    </p>
+    <p>
+      The less the angle of incidence of the ailerons, the better their
+      lift-drift ratio, i.e., their efficiency. You will note that, while the
+      aileron attached to the surface with washed-out angle is operated to the
+      same extent as the aileron illustrated above it, its angle of incidence is
+      considerably less. Its efficiency is therefore greater.
+    </p>
+    <p>
+      The advantages of the wash-in must, of course be paid for in some loss of
+      lift, as the lift decreases with the decreased angle.
+    </p>
+    <p>
+      In order to secure all the above described advantages, a combination is
+      sometimes effected, thus:
+    </p>
+    <p>
+      BANKING.&mdash;An aeroplane turned off its course to right or left does
+      not at once proceed along its new course. Its momentum in the direction of
+      its first course causes it to travel along a line the resultant of such
+      momentum and the thrust. In other words, it more or less skids sideways
+      and away from the centre of the turn. Its lifting surfaces do not then
+      meet the air in their correct attitude, and the lift may fall to such an
+      extent as to become less than the weight, in which case the aeroplane must
+      fall. This bad effect is minimized by &ldquo;banking,&rdquo; i.e., tilting the
+      aeroplane sideways. The bottom of the lifting surface is in that way
+      opposed to the air through which it is moving in the direction of the
+      momentum and receives an opposite air pressure. The rarefied area over the
+      top of the surface is rendered still more rare, and this, of course,
+      assists the air pressure in opposing the momentum.
+    </p>
+    <p>
+      The velocity of the &ldquo;skid,&rdquo; or sideways movement, is then only such as is
+      necessary to secure an air pressure equal and opposite to the centrifugal
+      force of the turn.
+    </p>
+    <p>
+      The sharper the turn, the greater the effect of the centrifugal force, and
+      therefore the steeper should be the &ldquo;bank.&rdquo; Experentia docet.
+    </p>
+    <p>
+      The position of the centre of gravity affects banking. A low C.G. will
+      tend to swing outward from the centre of the turn, and will cause the
+      aeroplane to bank&mdash;perhaps too much, in which case the pilot must
+      remedy matters by operating the ailerons.
+    </p>
+    <p>
+      A high C.G. also tends to swing outward from the centre of the turn. It
+      will tend to make the aeroplane bank the wrong way, and such effect must
+      be remedied by means of the ailerons.
+    </p>
+    <p>
+      The pleasantest machine from a banking point of view is one in which the
+      C.G. is a little below the centre of drift. It tends to bank the aeroplane
+      the right way for the turn, and the pilot can, if necessary, perfect the
+      bank by means of the ailerons.
+    </p>
+    <p>
+      The disposition of the keel-surface affects banking. It should be, in
+      effect, evenly divided by the longitudinal axis. An excess of keel-surface
+      above the longitudinal axis will, when banking, receive an air pressure
+      causing the aeroplane to bank, perhaps too much. An excess of keel-surface
+      below the axis has the reverse effect.
+    </p>
+    <p>
+      SIDE-SLIPPING.&mdash;This usually occurs as a result of over-banking. It
+      is always the result of the aeroplane tilting sideways and thus decreasing
+      the horizontal equivalent, and therefore the lift, of the surface. An
+      excessive &ldquo;bank,&rdquo; or sideways tilt, results in the H.E., and therefore the
+      lift, becoming less than the weight, when, of course, the aeroplane must
+      fall, i.e., side-slip.
+    </p>
+    <p>
+      When making a very sharp turn it is necessary to bank very steeply indeed.
+      If, at the same time, the longitudinal axis of the aeroplane remains
+      approximately horizontal, then there must be a fall, and the direction of
+      motion will be the resultant of the thrust and the fall as illustrated
+      above in sketch A. The lifting surfaces and the controlling surfaces are
+      not then meeting the air in the correct attitude, with the result that, in
+      addition to falling, the aeroplane will probably become quite
+      unmanageable.
+    </p>
+    <p>
+      The Pilot, however, prevents such a state of affairs from happening by
+      &ldquo;nosing-down,&rdquo; i.e., by operating the rudder to turn the nose of the
+      aeroplane downward and towards the direction of motion as illustrated in
+      sketch B. This results in the higher wing, which is on the outside of the
+      turn, travelling with greater velocity, and therefore securing a greater
+      reaction than the lower wing, thus tending to tilt the aeroplane over
+      still more. The aeroplane is now almost upside-down, but its attitude
+      relative to the direction of motion is correct and the controlling
+      surfaces are all of them working efficiently. The recovery of a normal
+      attitude relative to the Earth is then made as illustrated in sketch C.
+    </p>
+    <p>
+      The Pilot must then learn to know just the angle of bank at which the
+      margin of lift is lost, and, if a sharp turn necessitates banking beyond
+      that angle, he must &ldquo;nose-down.&rdquo;
+     </p>
+    <p>
+      In this matter of banking and nosing-down, and, indeed, regarding
+      stability and control generally, the golden rule for all but very
+      experienced pilots should be: Keep the aeroplane in such an attitude that
+      the air pressure is always directly in the pilot's face. The aeroplane is
+      then always engaging the air as designed to do so, and both lifting and
+      controlling surfaces are acting efficiently. The only exception to this
+      rule is a vertical dive, and I think that is obviously not an attitude for
+      any but very experienced pilots to hanker after.
+    </p>
+    <p>
+      SPINNING.&mdash;This is the worst of all predicaments the pilot can find
+      himself in. Fortunately it rarely happens.
+    </p>
+    <p>
+      It is due to the combination of (1) a very steep spiral descent of small
+      radius, and (2) insufficiency of keel-surface behind the vertical axis, or
+      the jamming of the rudder end or elevator into a position by which the
+      aeroplane is forced into an increasingly steep and small spiral.
+    </p>
+    <p>
+      Owing to the small radius of such a spiral, the mass of the aeroplane may
+      gain a rotary momentum greater, in effect, than the air pressure of the
+      keel-surface or controlling surfaces opposed to it; and, when once such a
+      condition occurs, it is difficult to see what can be done by the pilot to
+      remedy it. The sensible pilot will not go beyond reasonable limits of
+      steepness and radius when executing spiral descents.
+    </p>
+    <p>
+      GLIDING DESCENT WITHOUT PROPELLER THRUST.&mdash;All aeroplanes are, or
+      should be, designed to assume their gliding angle when the power and
+      thrust is cut off. This relieves the pilot of work, worry, and danger
+      should he find himself in a fog or cloud. The Pilot, although he may not
+      realize it, maintains the correct attitude of the aeroplane by observing
+      its position relative to the horizon. Flying into a fog or cloud the
+      horizon is lost to view, and he must then rely upon his instruments&mdash;(1)
+      the compass for direction; (2) an inclinometer (arched spirit-level)
+      mounted transversely to the longitudinal axis, for lateral stability; and
+      (3) an inclinometer mounted parallel to the longitudinal axis, or the
+      airspeed indicator, which will indicate a nose-down position by increase
+      in air speed, and a tail-down position by decrease in air speed.
+    </p>
+    <p>
+      The pilot is then under the necessity of watching three instruments and
+      manipulating his three controls to keep the instruments indicating
+      longitudinal, lateral, and directional stability. That is a feat beyond
+      the capacity of the ordinary man. If, however, by the simple movement of
+      throttling down the power and thrust, he can be relieved of looking after
+      the longitudinal stability, he then has only two instruments to watch.
+      That is no small job in itself, but it is, at any rate, fairly
+      practicable.
+    </p>
+    <p>
+      Aeroplanes are, then, designed, or should be, so that the centre of
+      gravity is slightly forward of centre of lift. The aeroplane is then, as a
+      glider, nose-heavy&mdash;and the distance the C.G. is placed in advance of
+      the C.L. should be such as to ensure a gliding angle producing a velocity
+      the same as the normal flying speed (for which the strength of
+      construction has been designed).
+    </p>
+    <p>
+      In order that this nose-heavy tendency should not exist when the thrust is
+      working and descent not required, the centre of thrust is placed a little
+      below the centre of drift or resistance, and thus tends to pull up the
+      nose of the aeroplane.
+    </p>
+    <p>
+      The distance the centre of thrust is placed below the centre of drift
+      should be such as to produce a force equal and opposite to that due to the
+      C.G. being forward of the C.L.
+    </p>
+    <p>
+      LOOPING AND UPSIDE DOWN FLYING.&mdash;If a loop is desired, it is best to
+      throttle the engine down at point A. The C.G. being forward of the C.P.,
+      then causes the aeroplane to nose-down, and assists the pilot in making a
+      reasonably small loop along the course C and in securing a quick recovery.
+      If the engine is not throttled down, then the aeroplane may be expected to
+      follow the course D, which results in a longer nose dive than in the case
+      of the course C.
+    </p>
+    <p>
+      A steady, gentle movement of the elevator is necessary. A jerky movement
+      may change the direction of motion so suddenly as to produce dangerous air
+      stresses upon the surfaces, in which case there is a possibility of
+      collapse.
+    </p>
+    <p>
+      If an upside-down flight is desired, the engine may, or may not, be
+      throttled down at point A. If not throttled down, then the elevator must
+      be operated to secure a course approximately in the direction B. If it is
+      throttled down, then the course must be one of a steeper angle than B, or
+      there will be danger of stalling.
+    </p>
+    <p>
+      Diagram p. 88.&mdash;This is not set at quite the correct angle. Path B
+      should slope slightly downwards from Position A.
+    </p>
+    <p>
+      <a name="link2HCH0003" id="link2HCH0003">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      CHAPTER III. RIGGING
+    </h2>
+    <p>
+      In order to rig an aeroplane intelligently, and to maintain it in an
+      efficient and safe condition, it is necessary to possess a knowledge of
+      the stresses it is called upon to endure, and the strains likely to
+      appear.
+    </p>
+    <p>
+      STRESS is the load or burden a body is called upon to bear. It is usually
+      expressed by the result found by dividing the load by the number of
+      superficial square inches contained in the cross-sectional area of the
+      body.
+    </p>
+    <p>
+      Thus, if, for instance, the object illustrated above contains 4 square
+      inches of cross-sectional area, and the total load it is called upon to
+      endure is 10 tons, the stress would be expressed as 2 1/2 tons.
+    </p>
+    <p>
+      STRAIN is the deformation produced by stress.
+    </p>
+    <p>
+      THE FACTOR OF SAFETY is usually expressed by the result found by dividing
+      the stress at which it is known the body will collapse, by the maximum
+      stress it will be called upon to endure. For instance, if a control wire
+      be called upon to endure a maximum stress of 2 cwts., and the known stress
+      at which it will collapse is 10 cwts., the factor of safety is then 5.
+    </p>
+    <p>
+      [cwts. = centerweights = 100 pound units as in cent &amp; century.
+      Interestingly enough, this word only exists today in abbreviation form,
+      probably of centreweights, but the dictionary entries, even from a hundred
+      years ago do not list this as a word, but do list c. or C. as the previous
+      popular abbreviation as in Roman Numerals] The word listed is
+      &ldquo;hundredweight. Michael S. Hart, 1997]
+    </p>
+    <p>
+      COMPRESSION.&mdash;The simple stress of compression tends to produce a
+      crushing strain. Example: the interplane and fuselage struts.
+    </p>
+    <p>
+      TENSION.&mdash;The simple stress of tension tends to produce the strain of
+      elongation. Example: all the wires.
+    </p>
+    <p>
+      BENDING.&mdash;The compound stress of bending is a combination of
+      compression and tension.
+    </p>
+    <p>
+      The above sketch illustrates a straight piece of wood of which the top,
+      centre, and bottom lines are of equal length. We will now imagine it bent
+      to form a circle, thus:
+    </p>
+    <p>
+      The centre line is still the same length as before being bent; but the top
+      line, being farther from the centre of the circle, is now longer than the
+      centre line. That can be due only to the strain of elongation produced by
+      the stress of tension. The wood between the centre line and the top line
+      is then in tension; and the farther from the centre, the greater the
+      strain, and consequently the greater the tension.
+    </p>
+    <p>
+      The bottom line, being nearest to the centre of the circle, is now shorter
+      than the centre line. That can be due only to the strain of crushing
+      produced by the stress of compression. The wood between the centre and
+      bottom lines is then in compression; and the nearer the centre of the
+      circle, the greater the strain, and consequently the greater the
+      compression.
+    </p>
+    <p>
+      It then follows that there is neither tension nor compression, i.e., no
+      stress, at the centre line, and that the wood immediately surrounding it
+      is under considerably less stress than the wood farther away. This being
+      so, the wood in the centre may be hollowed out without unduly weakening
+      struts and spars. In this way 25 to 33 per cent. is saved in the weight of
+      wood in an aeroplane.
+    </p>
+    <p>
+      The strength of wood is in its fibres, which should, as far as possible,
+      run without break from one end of a strut or spar to the other end. A
+      point to remember is that the outside fibres, being farthest removed from
+      the centre line, are doing by far the greatest work.
+    </p>
+    <p>
+      SHEAR STRESS IS such that, when material collapses under it, one part
+      slides over the other. Example: all the locking pins.
+    </p>
+    <p>
+      Some of the bolts are also in shear or &ldquo;sideways&rdquo; stress, owing to lugs
+      under their heads and from which wires are taken. Such a wire, exerting a
+      sideways pull upon a bolt, tries to break it in such a way as to make one
+      piece of the bolt slide over the other piece.
+    </p>
+    <p>
+      TORSION.&mdash;This is a twisting stress compounded of compression,
+      tension, and shear stresses. Example: the propeller shaft.
+    </p>
+    <p>
+      NATURE OF WOOD UNDER STRESS.&mdash;Wood, for its weight, takes the stress
+      of compression far better than any other stress. For instance: a
+      walking-stick of less than 1 lb. in weight will, if kept perfectly
+      straight, probably stand up to a compression stress of a ton or more
+      before crushing; whereas, if the same stick is put under a bending stress,
+      it will probably collapse to a stress of not more than about 50 lb. That
+      is a very great difference, and, since weight is of the greatest
+      importance, the design of an aeroplane is always such as to, as far as
+      possible, keep the various wooden parts of its construction in direct
+      compression. Weight being of such vital importance, and designers all
+      trying to outdo each other in saving weight, it follows that the factor of
+      safety is rather low in an aeroplane. The parts in direct compression
+      will, however, take the stresses safely provided the following conditions
+      are carefully observed.
+    </p>
+    <p>
+      CONDITIONS TO BE OBSERVED:
+    </p>
+    <p>
+      1. All the spars and struts must be perfectly straight.
+    </p>
+    <p>
+      The above sketch illustrates a section through an interplane strut. If the
+      strut is to be kept straight, i.e., prevented from bending, then the
+      stress of compression must be equally disposed about the centre of
+      strength. If it is not straight, then there will be more compression on
+      one side of the centre of strength than on the other side. That is a step
+      towards getting compression on one side and tension on the other side, in
+      which case it may be forced to take a bending stress for which it is not
+      designed. Even if it does not collapse it will, in effect, become shorter,
+      and thus throw out of adjustment the gap and all the wires attached to the
+      top and bottom of the strut, with the result that the flight efficiency of
+      the aeroplane will be spoiled.
+    </p>
+    <p>
+      The only exception to the above condition is what is known as the Arch.
+      For instance, in the case of the Maurice Farman, the spars of the
+      centre-section plane, which have to take the weight of the nacelle, are
+      arched upwards. If this was not done, it is possible that rough landings
+      might result in the weight causing the spars to become slightly distorted
+      downwards. That would produce a dangerous bending stress, but, as long as
+      the wood is arched, or, at any rate, kept from bending downwards, it will
+      remain in direct compression and no danger can result.
+    </p>
+    <p>
+      2. Struts and spars must be symmetrical. By that I mean that the
+      cross-sectional dimensions must be correct, as otherwise there will be
+      bulging places on the outside, with the result that the stress will not be
+      evenly disposed about the centre of strength, and a bending stress may be
+      produced.
+    </p>
+    <p>
+      3. Struts, spars, etc., must be undamaged. Remember that, from what I have
+      already explained about bending stresses, the outside fibres of the wood
+      are doing by far the most work. If these get bruised or scored, then the
+      strut or spar suffers in strength much more than one might think at first
+      sight; and, if it ever gets a tendency to bend, it is likely to collapse
+      at that point.
+    </p>
+    <p>
+      4. The wood must have a good, clear grain with no cross-grain, knots, or
+      shakes. Such blemishes produce weak places and, if a tendency to bend
+      appears, then it may collapse at such a point.
+    </p>
+    <p>
+      5. The struts, spars, etc., must be properly bedded into their sockets or
+      fittings. To begin with, they must be of good pushing or gentle tapping
+      fit. They must never be driven in with a heavy hammer. Then again, a strut
+      must bed well down all over its cross-sectional area as illustrated above;
+      otherwise the stress of compression will not be evenly disposed about the
+      centre of strength, and that may produce a bending stress. The bottom of
+      the strut or spar should be covered with some sort of paint, bedded into
+      the socket or fitting, and then withdrawn to see if the paint has stuck
+      all over the bed.
+    </p>
+    <p>
+      6. The atmosphere is sometimes much damper than at other times, and this
+      causes wood to expand and contract appreciably. This would not matter but
+      for the fact that it does not expand and contract uniformly, but becomes
+      unsymmetrical, i.e., distorted. I have already explained the danger of
+      that in condition 2. This should be minimized by WELL VARNISHING THE WOOD
+      to keep the moisture out of it.
+    </p>
+    <p>
+      FUNCTION OF INTERPLANE STRUTS.&mdash;These struts have to keep the lifting
+      surfaces or &ldquo;planes&rdquo; apart, but this is only part of their work. They must
+      keep the planes apart, so that the latter are in their correct attitude.
+      That is only so when the spars of the bottom plane are parallel with those
+      of the top plane. Also, the chord of the top plane must be parallel with
+      the chord of the bottom plane. If that is not so, then one plane will not
+      have the same angle of incidence as the other one. At first sight one
+      might think that all that is necessary is to cut all the struts to be the
+      same length, but that is not the case.
+    </p>
+    <p>
+      Sometimes, as illustrated above, the rear spar is not so thick as the main
+      spar, and it is then necessary to make up for that difference by making
+      the rear struts correspondingly longer. If that is not done, then the top
+      and bottom chords will not be parallel, and the top and bottom planes will
+      have different angles of incidence. Also, the sockets or fittings, or even
+      the spars upon which they are placed, sometimes vary in thickness owing to
+      faulty manufacture. This must be offset by altering the length of the
+      struts. The best way to proceed is to measure the distance between the top
+      and bottom spars by the side of each strut, and if that distance, or &ldquo;gap&rdquo;
+       as it is called, is not as stated in the aeroplane's specifications, then
+      make it correct by changing the length of the strut. This applies to both
+      front and rear interplane struts. When measuring the gap, always be
+      careful to measure from the centre of the spar, as it may be set at an
+      angle, and the rear of it may be considerably lower than its front.
+    </p>
+    <p>
+      BORING HOLES IN WOOD.&mdash;It should be a strict rule that no spar be
+      used which has an unnecessary hole in it. Before boring a hole, its
+      position should be confirmed by whoever is in charge of the workshop. A
+      bolt-hole should be of a size to enable the bolt to be pushed in, or, at
+      any rate, not more than gently tapped in. Bolts should not be hammered in,
+      as that may split the spar. On the other hand, a bolt should not be slack
+      in its hole, as, in such a case, it may work sideways and split the spar,
+      not to speak of throwing out of adjustment the wires leading from the lug
+      or socket under the bolt-head.
+    </p>
+    <p>
+      WASHERS.&mdash;Under the bolt-head, and also under the nut, a washer must
+      be placed&mdash;a very large washer compared with the size which would be
+      used in all-metal construction. This is to disperse the stress over a
+      large area; otherwise the washer may be pulled into the wood and weaken
+      it, besides possibly throwing out of adjustment the wires attached to the
+      bolt or the fitting it is holding to the spar.
+    </p>
+    <p>
+      LOCKING.&mdash;Now as regards locking the bolts. If split pins are used,
+      be sure to see that they are used in such a way that the nut cannot
+      possibly unscrew at all. The split pin should be passed through the bolt
+      as near as possible to the nut. It should not be passed through both nut
+      and bolt.
+    </p>
+    <p>
+      If it is locked by burring over the edge of the bolt, do not use a heavy
+      hammer and try to spread the whole head of the bolt. That might damage the
+      woodwork inside the fabric-covered surface. Use a small, light hammer, and
+      gently tap round the edge of the bolt until it is burred over.
+    </p>
+    <p>
+      TURNBUCKLES.&mdash;A turnbuckle is composed of a central barrel into each
+      end of which is screwed an eye-bolt. Wires are taken from the eyes of the
+      eye-bolt, and so, by turning the barrel, they can be adjusted to their
+      proper tension. Eye-bolts must be a good fit in the barrel; that is to
+      say, not slack and not very tight. Theoretically it is not necessary to
+      screw the eye-bolt into the barrel for a distance greater than the
+      diameter of the bolt, but, in practice, it is better to screw it in for a
+      considerably greater distance than that if a reasonable degree of safety
+      is to be secured.
+    </p>
+    <p>
+      Now about turning the barrel to secure the right adjustment. The barrel
+      looks solid, but, as a matter of fact, it is hollow and much more frail
+      than it appears. For that reason it should not be turned by seizing it
+      with pliers, as that may distort it and spoil the bore within it. The best
+      method is to pass a piece of wire through the hole in its centre, and to
+      use that as a lever. When the correct adjustment has been secured, the
+      turnbuckle must be locked to prevent it from unscrewing. It is quite
+      possible to lock it in such a way as to allow it to unscrew a quarter or a
+      half turn, and that would throw the wires out of the very fine adjustment
+      necessary. The proper way is to use the locking wire so that its direction
+      is such as to oppose the tendency of the barrel to unscrew, thus:
+    </p>
+    <p>
+      WIRES.&mdash;The following points should be carefully observed where wire
+      is concerned:
+    </p>
+    <p>
+      1. Quality.&mdash;It must not be too hard or too soft. An easy practical
+      way of learning to know the approximate quality of wire is as follows:
+    </p>
+    <p>
+      Take three pieces, all of the same gauge, and each about a foot in length.
+      One piece should be too soft, another too hard, and the third piece of the
+      right quality. Fix them in a vice, about an inch apart and in a vertical
+      position, and with the light from a window shining upon them. Burnish them
+      if necessary, and you will see a band of light reflected from each wire.
+    </p>
+    <p>
+      Now bend the wires over as far as possible and away from the light. Where
+      the soft wire is concerned, it will squash out at the bend, and this will
+      be indicated by the band of light, which will broaden at that point. In
+      the case of the wire which is too hard, the band of light will broaden
+      very little at the turn, but, if you look carefully, you will see some
+      little roughnesses of surface. In the case of the wire of the right
+      quality, the band of light may broaden a very little at the turn, but
+      there will be no roughnesses of surface.
+    </p>
+    <p>
+      By making this experiment two or three times one can soon learn to know
+      really bad wire from good, and also learn to know the strength of hand
+      necessary to bend the right quality.
+    </p>
+    <p>
+      2. It must not be damaged. That is to say, it must be unkinked, rustless,
+      and unscored.
+    </p>
+    <p>
+      3. Now as regards keeping wire in good condition. Where outside wires are
+      concerned, they should be kept WELL GREASED OR OILED, especially where
+      bent over at the ends. Internal bracing wires cannot be reached for the
+      purpose of regreasing them, as they are inside fabric-covered surfaces.
+      They should be prevented from rusting by being painted with an anti-rust
+      mixture. Great care should be taken to see that the wire is perfectly
+      clean and dry before being painted. A greasy finger-mark is sufficient to
+      stop the paint from sticking to the wire. In such a case there will be a
+      little space between the paint and the wire. Air may enter there and cause
+      the wire to rust.
+    </p>
+    <p>
+      4. Tension of Wires.&mdash;The tension to which the wires are adjusted is
+      of the greatest importance. All the wires should be of the same tension
+      when the aeroplane is supported in such a way as to throw no stress upon
+      them. If some wires are in greater tension than others, the aeroplane will
+      quickly become distorted and lose its efficiency.
+    </p>
+    <p>
+      In order to secure the same tension of all wires, the aeroplane, when
+      being rigged, should be supported by packing underneath the lower surfaces
+      as well as by packing underneath the fuselage or nacelle. In this way the
+      anti-lift wires are relieved of the weight, and there is no stress upon
+      any of the wires.
+    </p>
+    <p>
+      As a general rule the wires of an aeroplane are tensioned too much. The
+      tension should be sufficient to keep the framework rigid. Anything more
+      than that lowers the factor of safety, throws various parts of the
+      framework into undue compression, pulls the fittings into the wood, and
+      will, in the end, distort the whole framework of the aeroplane.
+    </p>
+    <p>
+      Only experience will teach the rigger what tension to employ. Much may be
+      done by learning the construction of the various types of aeroplanes, the
+      work the various parts do, and in cultivating a touch for tensioning wires
+      by constantly handling them.
+    </p>
+    <p>
+      5. Wires with no Opposition Wires.&mdash;In some few cases wires will be
+      found which have no opposition wires pulling in the opposite direction.
+      For instance, an auxiliary lift wire may run from the bottom of a strut to
+      a spar in the top plane at a point between struts. In such a case great
+      care should be taken not to tighten the wire beyond barely taking up the
+      slack.
+    </p>
+    <p>
+      Such a wire must be a little slack, or, as illustrated above, it will
+      distort the framework. That, in the example given, will spoil the camber
+      (curvature) of the surface, and result in changing both the lift and the
+      drift at that part of the surface. Such a condition will cause the
+      aeroplane to lose its directional stability and also to fly one wing down.
+    </p>
+    <p>
+      I cannot impress this matter of tension upon the reader too strongly. It
+      is of the utmost importance. When this, and also accuracy in securing the
+      various adjustments, has been learned, one is on the way to becoming a
+      good rigger.
+    </p>
+    <p>
+      6. Wire Loops.&mdash;Wire is often bent over at its end in the form of a
+      loop, in order to connect with a turnbuckle or fitting. These loops, even
+      when made as perfectly as possible, have a tendency to elongate, thus
+      spoiling the adjustment of the wires Great care should be taken to
+      minimize this as far as possible. The rules to be observed are as follows:
+    </p>
+    <p>
+      (a) The size of the loop should be as small as possible within reason. By
+      that I mean it should not be so small as to create the possibility of the
+      wire breaking.
+    </p>
+    <p>
+      (b) The shape of the loop should be symmetrical.
+    </p>
+    <p>
+      (c) It should have well-defined shoulders in order to prevent the ferrule
+      from slipping up. At the same time, a shoulder should not have an angular
+      place.
+    </p>
+    <p>
+      (d) When the loop is finished it should be undamaged, and it should not
+      be, as is often the case, badly scored.
+    </p>
+    <p>
+      7. Stranded Wire Cable.&mdash;No splice should be served with twine until
+      it has been inspected by whoever is in charge of the workshop. The serving
+      may cover bad work.
+    </p>
+    <p>
+      Should a strand become broken, then the cable should be replaced at once
+      by another one.
+    </p>
+    <p>
+      Control cables have a way of wearing out and fraying wherever they pass
+      round pulleys. Every time an aeroplane comes down from flight the rigger
+      should carefully examine the cables, especially where they pass round
+      pulleys. If he finds a strand broken, he should replace the cable.
+    </p>
+    <p>
+      The ailerons' balance cable on the top of the top plane is often
+      forgotten, since it is necessary to fetch a high pair of steps in order to
+      examine it. Don't slack this, or some gusty day the pilot may unexpectedly
+      find himself minus the aileron control.
+    </p>
+    <p>
+      CONTROLLING SURFACES.&mdash;The greatest care should be exercised in
+      rigging the aileron, rudder, and elevator properly, for the pilot entirely
+      depends upon them in managing the aeroplane.
+    </p>
+    <p>
+      The ailerons and elevator should be rigged so that, when the aeroplane is
+      in flight, they are in a fair true line with the surface in front and to
+      which they are hinged.
+    </p>
+    <p>
+      If the surface to which they are hinged is not a lifting surface, then
+      they should be rigged to be in a fair true line with it as illustrated
+      above.
+    </p>
+    <p>
+      If the controlling surface is, as illustrated, hinged to the back of a
+      lifting surface, then it should be rigged a little below the position it
+      would occupy if in a fair true line with the surface in front. This is
+      because, in such a case, it is set at an angle of incidence. This angle
+      will, during flight, cause it to lift a little above the position in which
+      it has been rigged. It is able to lift owing to a certain amount of slack
+      in the control wire holding it&mdash;and one cannot adjust the control
+      wire to have no slack, because that would cause it to bind against the
+      pulleys and make the operation of it too hard for the pilot. It is
+      therefore necessary to rig it a little below the position it would occupy
+      if it was rigged in a fair true line with the surface in front. Remember
+      that this only applies when it is hinged to a lifting surface. The greater
+      the angle of incidence (and therefore the lift) of the surface in front,
+      then the more the controlling surface will have to be rigged down.
+    </p>
+    <p>
+      As a general rule it is safe to rig it down so that its trailing
+    </p>
+    <p>
+      edge is 1/2 to 3/4 inch below the position it would occupy if in a fair
+      line with the surface in front; or about 1/2 inch down for every 18 inches
+      of chord of the controlling surface.
+    </p>
+    <p>
+      When making these adjustments the pilot's control levers should be in
+      their neutral positions. It is not sufficient to lash them. They should be
+      rigidly blocked into position with wood packing.
+    </p>
+    <p>
+      The surfaces must not be distorted in any way. If they are held true by
+      bracing wires, then such wires must be carefully adjusted. If they are
+      distorted and there are no bracing wires with which to true them, then
+      some of the internal framework will probably have to be replaced.
+    </p>
+    <p>
+      The controlling surfaces should never be adjusted with a view to altering
+      the stability of the aeroplane. Nothing can be accomplished in that way.
+      The only result will be to spoil the control of the aeroplane.
+    </p>
+    <p>
+      FABRIC-COVERED SURFACES.&mdash;First of all make sure that there is no
+      distortion of spars or ribs, and that they are perfectly sound. Then
+      adjust the internal bracing wires so that the ribs are parallel to the
+      direction of flight. The ribs usually cause the fabric to make a ridge
+      where they occur, and, if such ridge is not parallel to the direction of
+      flight, it will produce excessive drift. As a rule the ribs are at right
+      angles to both main and rear spars.
+    </p>
+    <p>
+      The tension of the internal bracing wires should be just sufficient to
+      give rigidity to the framework. They should not be tensioned above that
+      unless the wires are, at their ends, bent to form loops. In that case a
+      little extra tension may be given to offset the probable elongation of the
+      loops.
+    </p>
+    <p>
+      The turnbuckles must now be generously greased, and served round with
+      adhesive tape. The wires must be rendered perfectly dry and clean, and
+      then painted with an anti-rust mixture. The woodwork must be well
+      varnished.
+    </p>
+    <p>
+      If it is necessary to bore holes in the spars for the purpose of
+      receiving, for instance, socket bolts, then their places should be marked
+      before being bored and their positions confirmed by whoever is in charge
+      of the workshop. All is now ready for the sail-maker to cover the surface
+      with fabric.
+    </p>
+    <p>
+      ADJUSTMENT OF CONTROL CABLES.&mdash;The adjustment of the control cables
+      is quite an art, and upon it will depend to a large degree the quick and
+      easy control of the aeroplane by the pilot.
+    </p>
+    <p>
+      The method is as follows:
+    </p>
+    <p>
+      After having rigged the controlling surfaces, and as far as possible
+      secured the correct adjustment of the control cables, then remove the
+      packing which has kept the control levers rigid. Then, sitting in the
+      pilot's seat, move the control levers SMARTLY. Tension the control cables
+      so that when the levers are smartly moved there is no perceptible snatch
+      or lag. Be careful not to tension the cables more than necessary to take
+      out the snatch. If tensioned too much they will (1) bind round the pulleys
+      and result in hard work for the pilot; (2) throw dangerous stresses upon
+      the controlling surfaces, which are of rather flimsy construction; and (3)
+      cause the cables to fray round the pulleys quicker than would otherwise be
+      the case.
+    </p>
+    <p>
+      Now, after having tensioned the cables sufficiently to take out the
+      snatch, place the levers in their neutral positions, and move them to and
+      fro about 1/8 inch either side of such positions. If the adjustment is
+      correct, it should be possible to see the controlling surfaces move. If
+      they do not move, then the control cables are too slack.
+    </p>
+    <p>
+      FLYING POSITION.&mdash;Before rigging an aeroplane or making any
+      adjustments it is necessary to place it in what is known as its &ldquo;flying
+      position.&rdquo; I may add that it would be better termed its &ldquo;rigging
+      position.&rdquo;
+     </p>
+    <p>
+      In the case of an aeroplane fitted with a stationary engine this is
+      secured by packing up the machine so that the engine foundations are
+      perfectly horizontal both longitudinally and laterally. This position is
+      found by placing a straight-edge and a spirit-level across the engine
+      foundations (both longitudinally and laterally), and great care should be
+      taken to see that the bubble is exactly in the centre of the level. The
+      slightest error will assume magnitude towards the extremities of the
+      aeroplane. Great care should be taken to block up the aeroplane rigidly.
+      In case it gets accidentally disturbed while the work is going on, it is
+      well to constantly verify the flying position by running the straight-edge
+      and spirit-level over the engine foundations. The straight-edge should be
+      carefully tested before being used, as, being generally made of wood, it
+      will not remain true long. Place it lightly in a vice, and in such a
+      position that a spirit-level on top shows the bubble exactly in the
+      centre. Now slowly move the level along the straight-edge, and the bubble
+      should remain exactly in the centre. If it does not do so, then the
+      straight-edge is not true and must be corrected. THIS SHOULD NEVER BE
+      OMITTED.
+    </p>
+    <p>
+      In the case of aeroplanes fitted with engines of the rotary type, the
+      &ldquo;flying position&rdquo; is some special attitude laid down in the aeroplane's
+      specifications, and great care should be taken to secure accuracy.
+    </p>
+    <p>
+      ANGLE OF INCIDENCE.&mdash;One method of finding the angle of incidence is
+      as follows:
+    </p>
+    <p>
+      First place the aeroplane in its flying position. The corner of the
+      straight-edge must be placed underneath and against the CENTRE of the rear
+      spar, and held in a horizontal position parallel to the ribs. This is
+      secured by using a spirit-level. The set measurement will then be from the
+      top of the straight-edge to the centre of the bottom surface of the main
+      spar, or it may be from the top of the straight-edge to the lowest part of
+      the leading edge. Care should be taken to measure from the centre of the
+      spar and to see that the bubble is exactly in the centre of the level.
+      Remember that all this will be useless if the aeroplane has not been
+      placed accurately in its flying position.
+    </p>
+    <p>
+      This method of finding the angle of incidence must be used under every
+      part of the lower surface where struts occur. It should not be used
+      between the struts, because, in such places, the spars may have taken a
+      slight permanent set up or down; not, perhaps, sufficiently bad to make
+      any material difference to the flying of the machine, but quite bad enough
+      to throw out the angle of incidence, which cannot be corrected at such a
+      place.
+    </p>
+    <p>
+      If the angle is wrong, it should then be corrected as follows:
+    </p>
+    <p>
+      If it is too great, then the rear spar must be warped up until it is
+      right, and this is done by slackening ALL the wires going to the top of
+      the strut, and then tightening ALL the wires going to the bottom of the
+      strut.
+    </p>
+    <p>
+      If the angle is too small, then slacken ALL the wires going to the bottom
+      of the strut, and tighten ALL the wires going to the top of the strut,
+      until the correct adjustment is secured.
+    </p>
+    <p>
+      Never attempt to adjust the angle by warping the main spar.
+    </p>
+    <p>
+      The set measurement, which is of course stated in the aeroplane's
+      specifications, should be accurate to 1/16 inch.
+    </p>
+    <p>
+      LATERAL DIHEDRAL ANGLE.&mdash;One method of securing this is as follows,
+      and this method will, at the same time, secure the correct angle of
+      incidence:
+    </p>
+    <p>
+      The strings, drawn very tight, must be taken over both the main and rear
+      spars of the top surface. They must run between points on the spars just
+      inside the outer struts. The set measurement (which should be accurate to
+      1/16 inch or less) is then from the strings down to four points on the
+      main and rear spars of the centre-section surface. These points should be
+      just inside the four centre-section struts; that is to say, as far as
+      possible away from the centre of the centre-section. Do not attempt to
+      take the set measurement near the centre of the centre-section.
+    </p>
+    <p>
+      The strings should be as tight as possible, and, if it can be arranged,
+      the best way to accomplish that is as shown in the above illustration,
+      i.e., by weighting the strings down to the spars by means of weights and
+      tying each end of the strings to a strut. This will give a tight and
+      motionless string.
+    </p>
+    <p>
+      However carefully the above adjustment is made, there is sure to be some
+      slight error. This is of no great importance, provided it is divided
+      equally between the left- and right-hand wings. In order to make sure of
+      this, certain check measurements should be taken as follows:
+    </p>
+    <p>
+      Each bay must be diagonally measured, and such measurements must be the
+      same to within 1/16 inch on each side of the aeroplane. As a rule such
+      diagonal measurements are taken from the bottom socket of one strut to the
+      top socket of another strut, but this is bad practice, because of possible
+      inaccuracies due to faulty manufacture.
+    </p>
+    <p>
+      The points between which the diagonal measurements are taken should be at
+      fixed distances from the butts of the spars, such distances being the same
+      on each side of the aeroplane, thus:
+    </p>
+    <p>
+      It would be better to use the centre line of the aeroplane rather than the
+      butts of the spars. It is not practicable to do so, however, as the centre
+      line probably runs through the petrol tanks, etc.
+    </p>
+    <p>
+      THE DIHEDRAL BOARD.&mdash;Another method of securing the dihedral angle,
+      and also the angle of incidence, is by means of the dihedral board. It is
+      a light handy thing to use, but leads to many errors, and should not be
+      used unless necessary. The reasons are as follows:
+    </p>
+    <p>
+      The dihedral board is probably not true. If it must be used, then it
+      should be very carefully tested for truth before-hand. Another reason
+      against its use is that it has to be placed on the spars in a position
+      between the struts, and that is just where the spars may have a little
+      permanent set up or down, or some inaccuracy of surface which will, of
+      course, throw out the accuracy of the adjustment. The method of using it
+      is as follows:
+    </p>
+    <p>
+      The board is cut to the same angle as that specified for the upward
+      inclination of the surface towards its wing-tips. It is placed on the spar
+      as indicated above, and it is provided with two short legs to raise it
+      above the flanges of the ribs (which cross over the spars), as they may
+      vary in depth. A spirit-level is then placed on the board, and the wires
+      must be adjusted to give the surface such an inclination as to result in
+      the bubble being in the centre of the level. This operation must be
+      performed in respect of each bay both front and rear. The bays must then
+      be diagonally measured as already explained.
+    </p>
+    <p>
+      YET ANOTHER METHOD of finding the dihedral angle, and at the same time the
+      angle of incidence, is as follows:
+    </p>
+    <p>
+      A horizontal line is taken from underneath the butt of each spar, and the
+      set measurement is either the angle it makes with the spar, or a fixed
+      measurement from the line to the spar taken at a specified distance from
+      the butt. This operation must be performed in respect of both main and
+      rear spars, and all the bays must be measured diagonally afterwards.
+    </p>
+    <p>
+      Whichever method is used, be sure that after the job is done the spars are
+      perfectly straight.
+    </p>
+    <p>
+      STAGGER.&mdash;The stagger is the distance the top surface is in advance
+      of the bottom surface when the aeroplane is in flying position. The set
+      measurement is obtained as follows:
+    </p>
+    <p>
+      Plumb-lines must be dropped over the leading edge of the top surface
+      wherever struts occur, and also near the fuselage. The set measurement is
+      taken from the front of the lower leading edge to the plumb-lines. It
+      makes a difference whether the measurement is taken along a horizontal
+      line (which can be found by using a straight-edge and a spirit-level) or
+      along a projection of the chord. The line along which the measurement
+      should be taken is laid down in the aeroplane's specifications.
+    </p>
+    <p>
+      If a mistake is made and the measurement taken along the wrong line, it
+      may result in a difference of perhaps 1/4 will, in flight, be nose-heavy
+      or tail-heavy.
+    </p>
+    <p>
+      After the adjustments of the angles of incidence, dihedral, and stagger
+      have been secured, it is as well to confirm all of them, as, in making the
+      last adjustment, the first one may have been spoiled.
+    </p>
+    <p>
+      OVER-ALL ADJUSTMENTS.&mdash;The following over-all check measurements
+      should now be taken.
+    </p>
+    <p>
+      The straight lines AC and BC should be equal to within 1/8 inch. The point
+      C is the centre of the propeller, or, in the case of a &ldquo;pusher&rdquo; aeroplane,
+      the centre of the nacelle. The points A and B are marked on the main spar,
+      and must in each case be the same distance from the butt of the spar. The
+      rigger should not attempt to make A and B merely the sockets of the outer
+      struts, as they may not have been placed quite accurately by the
+      manufacturer. The lines AC and BC must be taken from both top and bottom
+      spars&mdash;two measurements on each side of the aeroplane.
+    </p>
+    <p>
+      The two measurements FD and FE should be equal to within 1/8 inch. F is
+      the centre of the fuselage or rudder-post. D and E are points marked on
+      both top and bottom rear spars, and each must be the same fixed distance
+      from the butt of the spar. Two measurements on each side of the aeroplane.
+    </p>
+    <p>
+      If these over-all measurements are not correct, then it is probably due to
+      some of the drift or anti-drift wires being too tight or too slack. It may
+      possibly be due to the fuselage being out of truth, but of course the
+      rigger should have made quite sure that the fuselage was true before
+      rigging the rest of the machine. Again, it may be due to the internal
+      bracing wires within the lifting surfaces not being accurately adjusted,
+      but of course this should have been seen to before covering the surfaces
+      with fabric.
+    </p>
+    <p>
+      FUSELAGE.&mdash;The method of truing the fuselage is laid down in the
+      aeroplane's specifications. After it has been adjusted according to the
+      specified directions, it should then be arranged on trestles in such a way
+      as to make about three-quarters of it towards the tail stick out
+      unsupported. In this way it will assume a condition as near as possible to
+      flying conditions, and when it is in this position the set measurements
+      should be confirmed. If this is not done it may be out of truth, but
+      perhaps appear all right when supported by trestles at both ends, as, in
+      such case, its weight may keep it true as long as it is resting upon the
+      trestles.
+    </p>
+    <p>
+      THE TAIL-PLANE (EMPENNAGE).&mdash;The exact angle of incidence of the
+      tail-plane is laid down in the aeroplane's specifications. It is necessary
+      to make sure that the spars are horizontal when the aeroplane is in flying
+      position and the tail unsupported as explained above under the heading of
+      Fuselage. If the spars are tapered, then make sure that their centre lines
+      are horizontal.
+    </p>
+    <p>
+      UNDERCARRIAGE.&mdash;The undercarriage must be very carefully aligned as
+      laid down in the specifications.
+    </p>
+    <p>
+      1. The aeroplane must be placed in its flying position and sufficiently
+      high to ensure the wheels being off the ground when rigged. When in this
+      position the axle must be horizontal and the bracing wires adjusted to
+      secure the various set measurements stated in the specifications.
+    </p>
+    <p>
+      2. Make sure that the struts bed well down into their sockets.
+    </p>
+    <p>
+      3. Make sure that the shock absorbers are of equal tension. In the case of
+      rubber shock absorbers, both the number of turns and the lengths must be
+      equal.
+    </p>
+    <p>
+      HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.
+    </p>
+    <p>
+      DIRECTIONAL STABILITY will be badly affected if there is more drift (i.e.,
+      resistance) on one side of the aeroplane than there is on the other side.
+      The aeroplane will tend to turn towards the side having the most drift.
+      This may be caused as follows:
+    </p>
+    <p>
+      1. The angle of incidence of the main surface or the tail surface may be
+      wrong. The greater the angle of incidence, the greater the drift. The less
+      the angle, the less the drift.
+    </p>
+    <p>
+      2. If the alignment of the fuselage, fin in front of the rudder, the
+      struts or stream-line wires, or, in the case of the Maurice Farman, the
+      front outriggers, are not absolutely correct&mdash;that is to say, if they
+      are turned a little to the left or to the right instead of being in line
+      with the direction of flight&mdash;then they will act as a rudder and
+      cause the aeroplane to turn off its course.
+    </p>
+    <p>
+      3. If any part of the surface is distorted, it will cause the aeroplane to
+      turn off its course. The surface is cambered, i.e., curved, to pass
+      through the air with the least possible drift. If, owing perhaps to the
+      leading edge, spars, or trailing edge becoming bent, the curvature is
+      spoiled, that will result in changing the amount of drift on one side of
+      the aeroplane, which will then have a tendency to turn off its course.
+    </p>
+    <p>
+      LATERAL INSTABILITY (FLYING ONE WING DOWN).&mdash;The only possible reason
+      for such a condition is a difference in the lifts of right and left wings.
+      That may be caused as follows:
+    </p>
+    <p>
+      1. The angle of incidence may be wrong. If it is too great, it will
+      produce more lift than on the other side of the aeroplane; and if too
+      small, it will produce less lift than on the other side&mdash;the result
+      being that, in either case, the aeroplane will try to fly one wing down.
+    </p>
+    <p>
+      2. Distorted Surfaces.&mdash;If some part of the surface is distorted,
+      then its camber is spoiled, and the lift will not be the same on both
+      sides of the aeroplane, and that, of course, will cause it to fly one wing
+      down.
+    </p>
+    <p>
+      LONGITUDINAL INSTABILITY may be due to the following reasons:
+    </p>
+    <p>
+      1. The stagger may be wrong. The top surface may have drifted back a
+      little owing to some of the wires, probably the incidence wires, having
+      elongated their loops or having pulled the fittings into the wood. If the
+      top surface is not staggered forward to the correct degree, then
+      consequently the whole of its lift is too far back, and it will then have
+      a tendency to lift up the tail of the machine too much. The aeroplane
+      would then be said to be &ldquo;nose-heavy.&rdquo;
+     </p>
+    <p>
+      A 1/4-inch area in the stagger will make a very considerable difference to
+      the longitudinal stability.
+    </p>
+    <p>
+      2. If the angle of incidence of the main surface is not right, it will
+      have a bad effect, especially in the case of an aeroplane with a lifting
+      tail-plane.
+    </p>
+    <p>
+      If the angle is too great, it will produce an excess of lift, and that may
+      lift up the nose of the aeroplane and result in a tendency to fly
+      &ldquo;tail-down.&rdquo; If the angle is too small, it will produce a decreased lift,
+      and the aeroplane may have a tendency to fly &ldquo;nose-down.&rdquo;
+     </p>
+    <p>
+      3. The fuselage may have become warped upward or downward, thus giving the
+      tail-plane an incorrect angle of incidence. If it has too much angle, it
+      will lift too much, and the aeroplane will be &ldquo;nose-heavy.&rdquo; If it has too
+      little angle, then it will not lift enough, and the aeroplane will be
+      &ldquo;tail-heavy.&rdquo;
+     </p>
+    <p>
+      4. (The least likely reason.) The tail-plane may be mounted upon the
+      fuselage at a wrong angle of incidence, in which case it must be
+      corrected. If nose-heavy, it should be given a smaller angle of incidence.
+      If tail-heavy, it should be given a larger angle; but care should be taken
+      not to give it too great an angle, because the longitudinal stability
+      entirely depends upon the tail-plane being set at a much smaller angle of
+      incidence than is the main surface, and if that difference is decreased
+      too much, the aeroplane will become uncontrollable longitudinally.
+      Sometimes the tail-plane is mounted on the aeroplane at the same angle as
+      the main surface, but it actually engages the air at a lesser angle, owing
+      to the air being deflected downwards by the main surface. There is then,
+      in effect, a longitudinal dihedral as explained and illustrated in Chapter
+      I.
+    </p>
+    <p>
+      CLIMBS BADLY.&mdash;Such a condition is, apart from engine or propeller
+      trouble, probably due to (1) distorted surfaces, or (2) too small an angle
+      of incidence.
+    </p>
+    <p>
+      FLIGHT SPEED POOR.&mdash;Such a condition is, apart from engine or
+      propeller trouble, probably due to (1) distorted surfaces, (2) too great
+      an angle of incidence, or (3) dirt or mud, and consequently excessive
+      skin-friction.
+    </p>
+    <p>
+      INEFFICIENT CONTROL is probably due to (1) wrong setting of control
+      surfaces, (2) distortion of control surfaces, or (3) control cables being
+      badly tensioned.
+    </p>
+    <p>
+      WILL NOT TAXI STRAIGHT.&mdash;If the aeroplane is uncontrollable on the
+      ground, it is probably due to (1) alignment of undercarriage being wrong,
+      or (2) unequal tension of shock absorbers.
+    </p>
+    <p>
+      <a name="link2HCH0004" id="link2HCH0004">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      CHAPTER IV. THE PROPELLER, OR &ldquo;AIR-SCREW&rdquo;
+     </h2>
+    <p>
+      The sole object of the propeller is to translate the power of the engine
+      into thrust.
+    </p>
+    <p>
+      The propeller screws through the air, and its blades, being set at an
+      angle inclined to the direction of motion, secure a reaction, as in the
+      case of the aeroplane's lifting surface.
+    </p>
+    <p>
+      This reaction may be conveniently divided into two component parts or
+      values, namely, Thrust and Drift.
+    </p>
+    <p>
+      The Thrust is opposed to the Drift of the aeroplane, and must be equal and
+      opposite to it at flying speed. If it falls off in power, then the flying
+      speed must decrease to a velocity, at which the aeroplane drift equals the
+      decreased thrust.
+    </p>
+    <p>
+      The Drift of the propeller may be conveniently divided into the following
+      component values:
+    </p>
+    <p>
+      Active Drift, produced by the useful thrusting part of the propeller.
+    </p>
+    <p>
+      Passive Drift, produced by all the rest of the propeller, i.e., by its
+      detrimental surface.
+    </p>
+    <p>
+      Skin Friction, produced by the friction of the air with roughnesses of
+      surface.
+    </p>
+    <p>
+      Eddies attending the movement of the air caused by the action of the
+      propeller.
+    </p>
+    <p>
+      Cavitation (very marked at excessive speed of revolution). A tendency of
+      the propeller to produce a cavity or semi-vacuum in which it revolves, the
+      thrust decreasing with increase of speed and cavitation.
+    </p>
+    <p>
+      THRUST-DRIFT RATIO.&mdash;The proportion of thrust to drift is of
+      paramount importance, for it expresses the efficiency of the propeller. It
+      is affected by the following factors: Speed of Revolution.&mdash;The
+      greater the speed, the greater the proportion of drift to thrust. This is
+      due to the increase with speed of the passive drift, which carries with it
+      no increase in thrust. For this reason propellers are often geared down to
+      revolve at a lower speed than that of the engine.
+    </p>
+    <p>
+      Angle of Incidence.&mdash;The same reasons as in the case of the aeroplane
+      surface.
+    </p>
+    <p>
+      Surface Area.&mdash;Ditto.
+    </p>
+    <p>
+      Aspect Ratio.&mdash;Ditto.
+    </p>
+    <p>
+      Camber.&mdash;Ditto.
+    </p>
+    <p>
+      In addition to the above factors there are, when it comes to actually
+      designing a propeller, mechanical difficulties to consider. For instance,
+      the blades must be of a certain strength and consequent thickness. That,
+      in itself, limits the aspect ratio, for it will necessitate a chord long
+      enough in proportion to the thickness to make a good camber possible.
+      Again, the diameter of the propeller must be limited, having regard to the
+      fact that greater diameters than those used to-day would not only result
+      in excessive weight of construction, but would also necessitate a very
+      high undercarriage to keep the propeller off the ground, and such
+      undercarriage would not only produce excessive drift, but would also tend
+      to make the aeroplane stand on its nose when alighting. The latter
+      difficulty cannot be overcome by mounting the propeller higher, as the
+      centre of its thrust must be approximately coincident with the centre of
+      aeroplane drift.
+    </p>
+    <p>
+      MAINTENANCE OF EFFICIENCY.
+    </p>
+    <p>
+      The following conditions must be observed:
+    </p>
+    <p>
+      1. PITCH ANGLE.&mdash;The angle, at any given point on the propeller, at
+      which the blade is set is known as the pitch angle, and it must be correct
+      to half a degree if reasonable efficiency is to be maintained.
+    </p>
+    <p>
+      This angle secures the &ldquo;pitch,&rdquo; which is the distance the propeller
+      advances during one revolution, supposing the air to be solid. The air, as
+      a matter of fact, gives back to the thrust of the blades just as the
+      pebbles slip back as one ascends a shingle beach. Such &ldquo;give-back&rdquo; is
+      known as Slip. If a propeller has a pitch of, say, 10 feet, but actually
+      advances, say, only 8 feet owing to slip, then it will be said to possess
+      20 per cent. slip.
+    </p>
+    <p>
+      Thus, the pitch must equal the flying speed of the aeroplane plus the slip
+      of the propeller. For example, let us find the pitch of a propeller, given
+      the following conditions:
+    </p>
+<pre xml:space="preserve">
+          Flying speed.............. 70 miles per hour.
+          Propeller revolutions..... 1,200 per minute.
+          Slip...................... 15 per cent.
+</pre>
+    <p>
+      First find the distance in feet the aeroplane will travel forward in one
+      minute. That is&mdash;
+    </p>
+<pre xml:space="preserve">
+   369,600 feet (70 miles)
+   &mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash; = 6,160 feet per minute.
+      60     &ldquo; (minutes)
+</pre>
+    <p>
+      Now divide the feet per minute by the propeller revolutions per minute,
+      add 15 per cent. for the slip, and the result will be the propeller pitch:
+    </p>
+<pre xml:space="preserve">
+     6,160
+     &mdash;&mdash;- + 15 per cent. = 5 feet 1 3/5 inches.
+     1,200
+</pre>
+    <p>
+      In order to secure a constant pitch from root to tip of blade, the pitch
+      angle decreases towards the tip. This is necessary, since the end of the
+      blade travels faster than its root, and yet must advance forward at the
+      same speed as the rest of the propeller. For example, two men ascending a
+      hill. One prefers to walk fast and the other slowly, but they wish to
+      arrive at the top of the hill simultaneously. Then the fast walker must
+      travel a farther distance than the slow one, and his angle of path (pitch
+      angle) must be smaller than the angle of path taken by the slow walker.
+      Their pitch angles are different, but their pitch (in this case altitude
+      reached in a given time) is the same.
+    </p>
+    <p>
+      In order to test the pitch angle, the propeller must be mounted upon a
+      shaft at right angles to a beam the face of which must be perfectly level,
+      thus:
+    </p>
+    <p>
+      First select a point on the blade at some distance (say about 2 feet) from
+      the centre of the propeller. At that point find, by means of a protractor,
+      the angle a projection of the chord makes with the face of the beam. That
+      angle is the pitch angle of the blade at that point.
+    </p>
+    <p>
+      Now lay out the angle on paper, thus:
+    </p>
+    <p>
+      The line above and parallel to the circumference line must be placed in a
+      position making the distance between the two lines equal to the specified
+      pitch, which is, or should be, marked upon the boss of the propeller.
+    </p>
+    <p>
+      Now find the circumference of the propeller where the pitch angle is being
+      tested. For example, if that place is 2 feet radius from the centre, then
+      the circumference will be 2 feet X 2 = 4 feet diameter, which, if
+      multiplied by 3.1416 = 15.56 feet circumference.
+    </p>
+    <p>
+      Now mark off the circumference distance, which is represented above by
+      A-B, and reduce it in scale for convenience.
+    </p>
+    <p>
+      The distance a vertical line makes between B and the chord dine is the
+      pitch at the point where the angle is being tested, and it should coincide
+      with the specified pitch. You will note, from the above illustration, that
+      the actual pitch line should meet the junction of the chord line and top
+      line.
+    </p>
+    <p>
+      The propeller should be tested at several points, about a foot apart, on
+      each blade; and the diagram, provided the propeller is not faulty, will
+      then look like this:
+    </p>
+    <p>
+      At each point tested the actual pitch coincides with the specified pitch:
+      a satisfactory condition.
+    </p>
+    <p>
+      A faulty propeller will produce a diagram something like this:
+    </p>
+    <p>
+      At every point tested the pitch angle is wrong, for nowhere does the
+      actual pitch coincide with the specified pitch. Angles A, C, and D, are
+      too large, and B is too small. The angle should be correct to half a
+      degree if reasonable efficiency is to be maintained.
+    </p>
+    <p>
+      A fault in the pitch angle may be due to (1) faulty manufacture, (2)
+      distortion, or (3) the shaft hole through the boss being out of position.
+    </p>
+    <p>
+      2. STRAIGHTNESS.&mdash;To test for straightness the propeller must be
+      mounted upon a shaft. Now bring the tip of one blade round to graze some
+      fixed object. Mark the point it grazes. Now bring the other tip round, and
+      it should come within 1/8 inch of the mark. If it does not do so, it is
+      due to (1) faulty manufacture, (2) distortion, or (3) to the hole through
+      the boss being out of position.
+    </p>
+    <p>
+      3. LENGTH.&mdash;The blades should be of equal length to inch.
+    </p>
+    <p>
+      4. BALANCE.&mdash;The usual method of testing a propeller for balance is
+      as follows: Mount it upon a shaft, which must be on ball-bearings. Place
+      the propeller in a horizontal position, and it should remain in that
+      position. If a weight of a trifle over an ounce placed in a bolt-hole on
+      one side of the boss fails to disturb the balance, then the propeller is
+      usually regarded as unfit for use.
+    </p>
+    <p>
+      The above method is rather futile, as it does not test for the balance of
+      centrifugal force, which comes into play as soon as the propeller
+      revolves. It can be tested as follows:
+    </p>
+    <p>
+      The propeller must be in a horizontal position, and then weighed at fixed
+      points, such as A, B, C, D, E, and F, and the weights noted. The points A,
+      B, and C must, of course, be at the same fixed distances from the centre
+      of the propeller as the points D, E, and F. Now reverse the propeller and
+      weigh at each point again. Note the results. The first series of weights
+      should correspond to the second series, thus:
+    </p>
+<pre xml:space="preserve">
+      Weight A should equal weight F.
+         &ldquo;  B   &ldquo;     &ldquo;     &ldquo;  E.
+         &ldquo;  C   &ldquo;     &ldquo;     &ldquo;  D.
+</pre>
+    <p>
+      There is no standard practice as to the degree of error permissible, but
+      if there are any appreciable differences the propeller is unfit for use.
+    </p>
+    <p>
+      5. SURFACE AREA.&mdash;The surface area of the blades should be equal.
+      Test with callipers thus:
+    </p>
+    <p>
+      The points between which the distances are taken must, of course, be at
+      the same distance from the centre in the case of each blade.
+    </p>
+    <p>
+      There is no standard practice as to the degree of error permissible. If,
+      however, there is an error of over 1/8 inch, the propeller is really unfit
+      for use.
+    </p>
+    <p>
+      6. CAMBER.&mdash;The camber (curvature) of the blades should be (1) equal,
+      (2) decrease evenly towards the tips of the blades, and (3) the greatest
+      depth of the curve should, at any point of the blade, be approximately at
+      the same percentage of the chord from the leading edge as at other points.
+    </p>
+    <p>
+      It is difficult to test the top camber without a set of templates, but a
+      fairly accurate idea of the concave camber can be secured by slowly
+      passing a straight-edge along the blade, thus:
+    </p>
+    <p>
+      The camber can now be easily seen, and as the straight-edge is passed
+      along the blade, the observer should look for any irregularities of the
+      curvature, which should gradually and evenly decrease towards the tip of
+      the blade.
+    </p>
+    <p>
+      7. THE JOINTS.&mdash;The usual method for testing the glued joints is by
+      revolving the propeller at greater speed than it will be called upon to
+      make during flight, and then carefully examining the joints to see if they
+      have opened. It is not likely, however, that the reader will have the
+      opportunity of making this test. He should, however, examine all the
+      joints very carefully, trying by hand to see if they are quite sound.
+      Suspect a propeller of which the joints appear to hold any thickness of
+      glue. Sometimes the joints in the boss open a little, but this is not
+      dangerous unless they extend to the blades, as the bolts will hold the
+      laminations together.
+    </p>
+    <p>
+      8. CONDITION OF SURFACE.&mdash;The surface should be very smooth,
+      especially towards the tips of the blades. Some propeller tips have a
+      speed of over 30,000 feet a minute, and any roughness will produce a bad
+      drift or resistance and lower the efficiency.
+    </p>
+    <p>
+      9. MOUNTING.&mdash;Great care should be taken to see that the propeller is
+      mounted quite straight on its shaft. Test in the same way as for
+      straightness. If it is not straight, it is possibly due to some of the
+      propeller bolts being too slack or to others having been pulled up too
+      tightly.
+    </p>
+    <p>
+      FLUTTER.&mdash;Propeller &ldquo;flutter,&rdquo; or vibration, may be due to faulty
+      pitch angle, balance, camber, or surface area. It causes a condition
+      sometimes mistaken for engine trouble, and one which may easily lead to
+      the collapse of the propeller.
+    </p>
+    <p>
+      CARE OF PROPELLERS.&mdash;The care of propellers is of the greatest
+      importance, as they become distorted very easily.
+    </p>
+    <p>
+      1. Do not store them in a very damp or a very dry place.
+    </p>
+    <p>
+      2. Do not store them where the sun will shine upon them.
+    </p>
+    <p>
+      3. Never leave them long in a horizontal position or leaning up against a
+      wall.
+    </p>
+    <p>
+      4. They should be hung on horizontal pegs, and the position of the
+      propellers should be vertical.
+    </p>
+    <p>
+      If the points I have impressed upon you in these notes are not attended
+      to, you may be sure of the following results:
+    </p>
+    <p>
+      1. Lack of efficiency, resulting in less aeroplane speed and climb than
+      would otherwise be the case.
+    </p>
+    <p>
+      2. Propeller &ldquo;flutter&rdquo; and possible collapse.
+    </p>
+    <p>
+      3. A bad stress upon the propeller shaft and its bearings.
+    </p>
+    <p>
+      TRACTOR.&mdash;A propeller mounted in front of the main surface.
+    </p>
+    <p>
+      PUSHER.&mdash;A propeller mounted behind the main surface.
+    </p>
+    <p>
+      FOUR-BLADED PROPELLERS.&mdash;Four-bladed propellers are suitable only
+      when the pitch is comparatively large.
+    </p>
+    <p>
+      For a given pitch, and having regard to &ldquo;interference,&rdquo; they are not so
+      efficient as two-bladed propellers.
+    </p>
+    <p>
+      The smaller the pitch, the less the &ldquo;gap,&rdquo; i.e., the distance, measured in
+      the direction of the thrust, between the spiral courses of the blades.
+    </p>
+    <p>
+      If the gap is too small, then the following blade will engage air which
+      the preceding blade has put into motion, with the result that the
+      following blade will not secure as good a reaction as would otherwise be
+      the case. It is very much the same as in the case of the aeroplane gap.
+    </p>
+    <p>
+      For a given pitch, the gap of a four-bladed propeller is only half that of
+      a two-bladed one. Therefore the four-bladed propeller is only suitable for
+      large pitch, as such pitch produces spirals with a large gap, thus
+      offsetting the decrease in gap caused by the numerous blades.
+    </p>
+    <p>
+      The greater the speed of rotation, the less the pitch for a given
+      aeroplane speed. Then, in order to secure a large pitch and consequently a
+      good gap, the four-bladed propeller is usually geared to rotate at a lower
+      speed than would be the case if directly attached to the engine
+      crank-shaft.
+    </p>
+    <p>
+      <a name="link2HCH0005" id="link2HCH0005">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      CHAPTER V. MAINTENANCE
+    </h2>
+    <p>
+      CLEANLINESS.&mdash;The fabric must be kept clean and free from oil, as
+      that will rot it. To take out dirt or oily patches, try acetone. If that
+      will not remedy matters, then try petrol, but use it sparingly, as
+      otherwise it will take off an unnecessary amount of dope. If that will not
+      remove the dirt, then hot water and soap will do so, but, in that case, be
+      sure to use soap having no alkali in it, as otherwise it may injure the
+      fabric. Use the water sparingly, or it may get inside the planes and rust
+      the internal bracing wires, or cause some of the wooden framework to
+      swell.
+    </p>
+    <p>
+      The wheels of the undercarriage have a way of throwing up mud on to the
+      lower surface. This should, if possible, be taken off while wet. It should
+      never be scraped off when dry, as that may injure the fabric. If dry, then
+      it should be moistened before being removed.
+    </p>
+    <p>
+      Measures should be taken to prevent dirt from collecting upon any part of
+      the aeroplane, as, otherwise, excessive skin-friction will be produced
+      with resultant loss of flight speed. The wires, being greasy, collect dirt
+      very easily.
+    </p>
+    <p>
+      CONTROL CABLES.&mdash;After every flight the rigger should pass his hand
+      over the control cables and carefully examine them near pulleys. Removal
+      of grease may be necessary to make a close inspection possible. If only
+      one strand is broken the wire should be replaced. Do not forget the
+      aileron balance wire on the top surface.
+    </p>
+    <p>
+      Once a day try the tension of the control cables by smartly moving the
+      control levers about as explained elsewhere.
+    </p>
+    <p>
+      WIRES.&mdash;All the wires should be kept well greased or oiled, and in
+      the correct tension. When examining the wires, it is necessary to place
+      the aeroplane on level ground, as otherwise it may be twisted, thus
+      throwing some wires into undue tension and slackening others. The best
+      way, if there is time, is to pack the machine up into its &ldquo;flying
+      position.&rdquo;
+     </p>
+    <p>
+      If you see a slack wire, do not jump to the conclusion that it must be
+      tensioned. Perhaps its opposition wire is too tight, in which case slacken
+      it, and possibly you will find that will tighten the slack wire.
+    </p>
+    <p>
+      Carefully examine all wires and their connections near the propeller, and
+      be sure that they are snaked round with safety wire, so that the latter
+      may keep them out of the way of the propeller if they come adrift.
+    </p>
+    <p>
+      The wires inside the fuselage should be cleaned and regreased about once a
+      fortnight.
+    </p>
+    <p>
+      STRUTS AND SOCKETS.&mdash;These should be carefully examined to see if any
+      splitting has occurred.
+    </p>
+    <p>
+      DISTORTION.&mdash;Carefully examine all surfaces, including the
+      controlling surfaces, to see whether any distortion has occurred. If
+      distortion can be corrected by the adjustment of wires, well and good; but
+      if not, then some of the internal framework probably requires replacement.
+    </p>
+    <p>
+      ADJUSTMENTS.&mdash;Verify the angles of incidence; dihedral, and stagger,
+      and the rigging position of the controlling-surfaces, as often as
+      possible.
+    </p>
+    <p>
+      UNDERCARRIAGE.&mdash;Constantly examine the alignment and fittings of the
+      undercarriage, and the condition of tyres and shock absorbers. The latter,
+      when made of rubber, wear quickest underneath. Inspect axles and skids to
+      see if there are any signs of them becoming bent. The wheels should be
+      taken off occasionally and greased.
+    </p>
+    <p>
+      LOCKING ARRANGEMENTS.&mdash;Constantly inspect the locking arrangements of
+      turnbuckles, bolts, etc. Pay particular attention to the control cable
+      connections, and to all moving parts in respect of the controls.
+    </p>
+    <p>
+      LUBRICATION.&mdash;Keep all moving parts, such as pulleys, control levers,
+      and hinges of controlling surfaces, well greased.
+    </p>
+    <p>
+      SPECIAL INSPECTION.&mdash;Apart from constantly examining the aeroplane
+      with reference to the above points I have made, I think that, in the case
+      of an aeroplane in constant use it is an excellent thing to make a special
+      inspection of every part, say once a week. This will take from two to
+      three hours, according to the type of aeroplane. In order to carry it out
+      methodically, the rigger should have a list of every part down to the
+      smallest split-pin. He can then check the parts as he examines them, and
+      nothing will be passed over. This, I know from experience, greatly
+      increases the confidence of the pilot, and tends to produce good work in
+      the air.
+    </p>
+    <p>
+      WINDY WEATHER.&mdash;The aeroplane, when on the ground, should face the
+      wind; and it is advisable to lash the control lever fast, so that the
+      controlling surfaces may not be blown about and possibly damaged.
+    </p>
+    <p>
+      &ldquo;VETTING&rdquo; BY EYE.&mdash;This should be practiced at every opportunity,
+      and, if persevered in, it is possible to become quite expert in diagnosing
+      by eye faults in flight efficiency, stability and control.
+    </p>
+    <p>
+      The aeroplane should be standing upon level ground, or, better than that,
+      packed up into its &ldquo;flying position.&rdquo;
+     </p>
+    <p>
+      Now stand in front of it and line up the leading edge with the main spar,
+      rear spar, and trailing edge. Their shadows can usually be seen through
+      the fabric. Allowance must, of course, be made for wash-in and wash-out;
+      otherwise, the parts I have specified should be parallel with each other.
+    </p>
+    <p>
+      Now line up the centre part of the main-plane with the tail-plane. The
+      latter should be horizontal.
+    </p>
+    <p>
+      Next, sight each interplane front strut with its rear strut. They should
+      be parallel.
+    </p>
+    <p>
+      Then, standing on one side of the aeroplane, sight all the front struts.
+      The one nearest to you should cover all the others. This applies to the
+      rear struts also.
+    </p>
+    <p>
+      Look for distortion of leading edges, main and rear spars, trailing edges,
+      tail-plane and controlling surfaces.
+    </p>
+    <p>
+      This sort of thing, if practiced constantly, will not only develop an
+      expert eye for diagnosis of faults, but will also greatly assist in
+      impressing upon the memory the characteristics and possible troubles of
+      the various types of aeroplanes.
+    </p>
+    <p>
+      MISHANDLING OF THE GROUND.&mdash;This is the cause of a lot of unnecessary
+      damage. The golden rule to observe is: PRODUCE NO BENDING STRESSES.
+    </p>
+    <p>
+      Nearly all the wood in an aeroplane is designed to take merely the stress
+      of direct compression, and it cannot be bent safely. Therefore, in packing
+      an aeroplane up from the ground, or in pulling or pushing it about, be
+      careful to stress it in such a way as to produce, as far as possible, only
+      direct compression stresses. For instance, if it is necessary to support
+      the lifting surface, then the packing should be arranged to come directly
+      under the struts so that they may take the stress in the form of
+      compression for which they are designed. Such supports should be covered
+      with soft packing in order to prevent the fabric from becoming damaged.
+    </p>
+    <p>
+      When pulling an aeroplane along, if possible, pull from the top of the
+      undercarriage struts. If necessary to pull from elsewhere, then do so by
+      grasping the interplane struts as low down as possible.
+    </p>
+    <p>
+      Never lay fabric-covered parts upon a concrete floor. Any slight movement
+      will cause the fabric to scrape over the floor with resultant damage.
+    </p>
+    <p>
+      Struts, spars, etc., should never be left about the floor, as in such
+      position they are likely to become scored. I have already explained the
+      importance of protecting the outside fibres of the wood. Remember also
+      that wood becomes distorted easily. This particularly applies to
+      interplane struts. If there are no proper racks to stand them in, then the
+      best plan is to lean them up against the wall in as near a vertical
+      position as possible.
+    </p>
+    <p>
+      TIME.&mdash;Learn to know the time necessary to complete any of the
+      various rigging jobs. This is really important. Ignorance of this will
+      lead to bitter disappointments in civil life; and, where Service flying is
+      concerned, it will, to say the least of it, earn unpopularity with senior
+      officers, and fail to develop respect and good work where men are
+      concerned.
+    </p>
+    <p>
+      THE AEROPLANE SHED.&mdash;This should be kept as clean and orderly as
+      possible. A clean, smart shed produces briskness, energy, and pride of
+      work. A dirty, disorderly shed nearly always produces slackness and poor
+      quality of work, lost tools and mislaid material.
+    </p>
+    <p>
+      <a name="link2H_GLOS" id="link2H_GLOS">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      GLOSSARY
+    </h2>
+    <h3>
+      Aeronautics&mdash;The science of aerial navigation.
+    </h3>
+    <p>
+      Aerofoil&mdash;A rigid structure, of large superficial area relative to
+      its thickness, designed to obtain, when driven through the air at an angle
+      inclined to the direction of motion, a reaction from the air approximately
+      at right angles to its surface. Always cambered when intended to secure a
+      reaction in one direction only. As the term &ldquo;aerofoil&rdquo; is hardly ever used
+      in practical aeronautics, I have, throughout this book, used the term
+      SURFACE, which, while academically incorrect, since it does not indicate
+      thickness, is a term usually used to describe the cambered lifting
+      surfaces, i.e., the &ldquo;planes&rdquo; or &ldquo;wings,&rdquo; and the stabilizers and the
+      controlling aerofoils.
+    </p>
+    <p>
+      Aerodrome&mdash;The name usually applied to a ground used for the practice
+      of aviation. It really means &ldquo;flying machine,&rdquo; but is never used in that
+      sense nowadays.
+    </p>
+    <p>
+      Aeroplane&mdash;A power-driven aerofoil with stabilizing and controlling
+      surfaces.
+    </p>
+    <p>
+      Acceleration&mdash;The rate of change of velocity.
+    </p>
+    <p>
+      Angle of Incidence&mdash;The angle at which the &ldquo;neutral lift line&rdquo; of a
+      surface attacks the air.
+    </p>
+    <p>
+      Angle of Incidence, Rigger's&mdash;The angle the chord of a surface makes
+      with a line parallel to the axis of the propeller.
+    </p>
+    <p>
+      Angle of Incidence, Maximum&mdash;The greatest angle of incidence at
+      which, for a given power, surface (including detrimental surface), and
+      weight, horizontal flight can be maintained.
+    </p>
+    <p>
+      Angle of Incidence, Minimum&mdash;The smallest angle of incidence at
+      which, for a given power, surface (including detrimental surface), and
+      weight, horizontal flight can be maintained.
+    </p>
+    <p>
+      Angle of Incidence, Best Climbing&mdash;That angle of incidence at which
+      an aeroplane ascends quickest. An angle approximately halfway between the
+      maximum and optimum angles.
+    </p>
+    <p>
+      Angle of Incidence, Optimum&mdash;The angle of incidence at which the
+      lift-drift ratio is the highest.
+    </p>
+    <p>
+      Angle, Gliding&mdash;The angle between the horizontal and the path along
+      which an aeroplane at normal flying speed, but not under engine power,
+      descends in still air.
+    </p>
+    <p>
+      Angle, Dihedral&mdash;The angle between two planes.
+    </p>
+    <p>
+      Angle, Lateral Dihedral&mdash;The lifting surface of an aeroplane is said
+      to be at a lateral dihedral angle when it is inclined upward towards its
+      wing-tips.
+    </p>
+    <p>
+      Angle, Longitudinal Dihedral&mdash;The main surface and tail surface are
+      said to be at a longitudinal dihedral angle when the projections of their
+      neutral lift lines meet and produce an angle above them.
+    </p>
+    <p>
+      Angle, Rigger's Longitudinal Dihedral&mdash;Ditto, but substituting
+      &ldquo;chords&rdquo; for &ldquo;neutral life lines.&rdquo;
+     </p>
+    <p>
+      Angle, Pitch&mdash;The angle at any given point of a propeller, at which
+      the blade is inclined to the direction of motion when the propeller is
+      revolving but the aeroplane stationary.
+    </p>
+    <p>
+      Altimeter&mdash;An instrument used for measuring height.
+    </p>
+    <p>
+      Air-Speed Indicator&mdash;An instrument used for measuring air pressures
+      or velocities. It consequently indicates whether the surface is securing
+      the requisite reaction for flight. Usually calibrated in miles per hour,
+      in which case it indicates the correct number of miles per hour at only
+      one altitude. This is owing to the density of the air decreasing with
+      increase of altitude and necessitating a greater speed through space to
+      secure the same air pressure as would be secured by less speed at a lower
+      altitude. It would be more correct to calibrate it in units of air
+      pressure.
+    </p>
+    <p>
+      Air Pocket&mdash;A local movement or condition of the air causing an
+      aeroplane to drop or lose its correct attitude.
+    </p>
+    <p>
+      Aspect-Ratio&mdash;The proportion of span to chord of a surface.
+    </p>
+    <p>
+      Air-Screw (Propeller)&mdash;A surface so shaped that its rotation about an
+      axis produces a force (thrust) in the direction of its axis.
+    </p>
+    <p>
+      Aileron&mdash;A controlling surface, usually situated at the wing-tip, the
+      operation of which turns an aeroplane about its longitudinal axis; causes
+      an aeroplane to tilt sideways.
+    </p>
+    <p>
+      Aviation&mdash;The art of driving an aeroplane.
+    </p>
+    <p>
+      Aviator&mdash;The driver of an aeroplane.
+    </p>
+    <p>
+      Barograph&mdash;A recording barometer, the charts of which can be
+      calibrated for showing air density or height.
+    </p>
+    <p>
+      Barometer&mdash;An instrument used for indicating the density of air.
+    </p>
+    <p>
+      Bank, to&mdash;To turn an aeroplane about its longitudinal axis (to tilt
+      sideways) when turning to left or right.
+    </p>
+    <p>
+      Biplane&mdash;An aeroplane of which the main lifting surface consists of a
+      surface or pair of wings mounted above another surface or pair of wings.
+    </p>
+    <p>
+      Bay&mdash;The space enclosed by two struts and whatever they are fixed to.
+    </p>
+    <p>
+      Boom&mdash;A term usually applied to the long spars joining the tail of a
+      &ldquo;pusher&rdquo; aeroplane to its main lifting surface.
+    </p>
+    <p>
+      Bracing&mdash;A system of struts and tie wires to transfer a force from
+      one point to another.
+    </p>
+    <p>
+      Canard&mdash;Literally &ldquo;duck.&rdquo; The name which was given to a type of
+      aeroplane of which the longitudinal stabilizing surface (empennage) was
+      mounted in front of the main lifting surface. Sometimes termed
+      &ldquo;tail-first&rdquo; aeroplanes, but such term is erroneous, as in such a design
+      the main lifting surface acts as, and is, the empennage.
+    </p>
+    <p>
+      Cabre&mdash;To fly or glide at an excessive angle of incidence; tail down.
+    </p>
+    <p>
+      Camber&mdash;Curvature.
+    </p>
+    <p>
+      Chord&mdash;Usually taken to be a straight line between the trailing and
+      leading edges of a surface.
+    </p>
+    <p>
+      Cell&mdash;The whole of the lower surface, that part of the upper surface
+      directly over it, together with the struts and wires holding them
+      together.
+    </p>
+    <p>
+      Centre (Line) of Pressure&mdash;A line running from wing-tip to wing-tip,
+      and through which all the air forces acting upon the surface may be said
+      to act, or about which they may be said to balance.
+    </p>
+    <p>
+      Centre (Line) of Pressure, Resultant&mdash;A line transverse to the
+      longitudinal axis, and the position of which is the resultant of the
+      centres of pressure of two or more surfaces.
+    </p>
+    <p>
+      Centre of Gravity&mdash;The centre of weight.
+    </p>
+    <p>
+      Cabane&mdash;A combination of two pylons, situated over the fuselage, and
+      from which anti-lift wires are suspended.
+    </p>
+    <p>
+      Cloche&mdash;Literally &ldquo;bell.&rdquo; Is applied to the bell-shaped construction
+      which forms the lower part of the pilot's control lever in a Bleriot
+      monoplane, and to which the control cables are attached.
+    </p>
+    <p>
+      Centrifugal Force&mdash;Every body which moves in a curved path is urged
+      outwards from the centre of the curve by a force termed &ldquo;centrifugal.&rdquo;
+     </p>
+    <p>
+      Control Lever&mdash;A lever by means of which the controlling surfaces are
+      operated. It usually operates the ailerons and elevator. The &ldquo;joy-stick&rdquo;.
+    </p>
+    <p>
+      Cavitation, Propeller&mdash;The tendency to produce a cavity in the air.
+    </p>
+    <p>
+      Distance Piece&mdash;A long, thin piece of wood (sometimes tape) passing
+      through and attached to all the ribs in order to prevent them from rolling
+      over sideways.
+    </p>
+    <p>
+      Displacement&mdash;Change of position.
+    </p>
+    <p>
+      Drift (of an aeroplane as distinct from the propeller)&mdash;The
+      horizontal component of the reaction produced by the action of driving
+      through the air a surface inclined upwards and towards its direction of
+      motion PLUS the horizontal component of the reaction produced by the
+      &ldquo;detrimental&rdquo; surface PLUS resistance due to &ldquo;skin-friction.&rdquo; Sometimes
+      termed &ldquo;head-resistance.&rdquo;
+     </p>
+    <p>
+      Drift, Active&mdash;Drift produced by the lifting surface.
+    </p>
+    <p>
+      Drift, Passive&mdash;Drift produced by the detrimental surface.
+    </p>
+    <p>
+      Drift (of a propeller)&mdash;Analogous to the drift of an aeroplane. It is
+      convenient to include &ldquo;cavitation&rdquo; within this term.
+    </p>
+    <p>
+      Drift, to&mdash;To be carried by a current of air; to make leeway.
+    </p>
+    <p>
+      Dive, to&mdash;To descend so steeply as to produce a speed greater than
+      the normal flying speed.
+    </p>
+    <p>
+      Dope, to&mdash;To paint a fabric with a special fluid for the purpose of
+      tightening and protecting it.
+    </p>
+    <p>
+      Density&mdash;Mass of unit volume, for instance, pounds per cubic foot.
+    </p>
+    <p>
+      Efficiency&mdash;Output Input
+    </p>
+    <p>
+      Efficiency (of an aeroplane as distinct from engine and propeller)&mdash;
+    </p>
+<pre xml:space="preserve">
+       Lift and Velocity
+    Thrust (= aeroplane drift)
+</pre>
+    <p>
+      Efficiency, Engine&mdash;Brake horse-power
+    </p>
+<pre xml:space="preserve">
+       Indicated horse-power
+</pre>
+    <p>
+      Efficiency, Propeller&mdash;
+    </p>
+<pre xml:space="preserve">
+                              Thrust horse-power
+                       Horse-power received from engine
+                               (= propeller drift)
+</pre>
+    <p>
+      NOTE.&mdash;The above terms can, of course, be expressed in foot-pounds.
+      It is then only necessary to divide the upper term by the lower one to
+      find the measure of efficiency.
+    </p>
+    <p>
+      Elevator&mdash;A controlling surface, usually hinged to the rear of the
+      tail-plane, the operation of which turns an aeroplane about an axis which
+      is transverse to the direction of normal horizontal flight.
+    </p>
+    <p>
+      Empennage&mdash;See &ldquo;Tail-plane.&rdquo;
+     </p>
+    <p>
+      Energy&mdash;Stored work. For instance, a given weight of coal or
+      petroleum stores a given quantity of energy which may be expressed in
+      foot-pounds.
+    </p>
+    <p>
+      Extension&mdash;That part of the upper surface extending beyond the span
+      of the lower surface.
+    </p>
+    <p>
+      Edge, Leading&mdash;The front edge of a surface relative to its normal
+      direction of motion.
+    </p>
+    <p>
+      Edge, Trailing&mdash;The rear edge of a surface relative to its normal
+      direction of motion.
+    </p>
+    <p>
+      Factor of Safety&mdash;Usually taken to mean the result found by dividing
+      the stress at which a body will collapse by the maximum stress it will be
+      called upon to bear.
+    </p>
+    <p>
+      Fineness (of stream-line)&mdash;The proportion of length to maximum width.
+    </p>
+    <p>
+      Flying Position&mdash;A special position in which an aeroplane must be
+      placed when rigging it or making adjustments. It varies with different
+      types of aeroplanes. Would be more correctly described as &ldquo;rigging
+      position.&rdquo;
+     </p>
+    <p>
+      Fuselage&mdash;That part of an aeroplane containing the pilot, and to
+      which is fixed the tail-plane.
+    </p>
+    <p>
+      Fin&mdash;Additional keel-surface, usually mounted at the rear of an
+      aeroplane.
+    </p>
+    <p>
+      Flange (of a rib)&mdash;That horizontal part of a rib which prevents it
+      from bending sideways.
+    </p>
+    <p>
+      Flight&mdash;The sustenance of a body heavier than air by means of its
+      action upon the air.
+    </p>
+    <p>
+      Foot-pound&mdash;A measure of work representing the weight of 1 lb. raised
+      1 foot.
+    </p>
+    <p>
+      Fairing&mdash;Usually made of thin sheet aluminum, wood, or a light
+      construction of wood and fabric; and bent round detrimental surface in
+      order to give it a &ldquo;fair&rdquo; or &ldquo;stream-like&rdquo; shape.
+    </p>
+    <p>
+      Gravity&mdash;Is the force of the Earth's attraction upon a body. It
+      decreases with increase of distance from the Earth. See &ldquo;Weight.&rdquo;
+     </p>
+    <p>
+      Gravity, Specific&mdash;Density of substance Density of water. Thus, if
+      the density of water is 10 lb. per unit volume, the same unit volume of
+      petrol, if weighing 7 lb., would be said to have a specific gravity of
+      7/10, i.e., 0.7.
+    </p>
+    <p>
+      Gap (of an aeroplane)&mdash;The distance between the upper and lower
+      surfaces of a biplane. In a triplane or multiplane, the distance between a
+      surface and the one first above it.
+    </p>
+    <p>
+      Gap, Propeller&mdash;The distance, measured in the direction of the
+      thrust, between the spiral courses of the blades.
+    </p>
+    <p>
+      Girder&mdash;A structure designed to resist bending, and to combine
+      lightness and strength.
+    </p>
+    <p>
+      Gyroscope&mdash;A heavy circular wheel revolving at high speed, the effect
+      of which is a tendency to maintain its plane of rotation against
+      disturbing forces.
+    </p>
+    <p>
+      Hangar&mdash;An aeroplane shed.
+    </p>
+    <p>
+      Head-Resistance&mdash;Drift. The resistance of the air to the passage of a
+      body.
+    </p>
+    <p>
+      Helicopter&mdash;An air-screw revolving about a vertical axis, the
+      direction of its thrust being opposed to gravity.
+    </p>
+    <p>
+      Horizontal Equivalent&mdash;The plan view of a body whatever its attitude
+      may be.
+    </p>
+    <p>
+      Impulse&mdash;A force causing a body to gain or lose momentum.
+    </p>
+    <p>
+      Inclinometer&mdash;A curved form of spirit-level used for indicating the
+      attitude of a body relative to the horizontal.
+    </p>
+    <p>
+      Instability&mdash;An inherent tendency of a body, which, if the body is
+      disturbed, causes it to move into a position as far as possible away from
+      its first position.
+    </p>
+    <p>
+      Instability, Neutral&mdash;An inherent tendency of a body to remain in the
+      position given it by the force of a disturbance, with no tendency to move
+      farther or to return to its first position.
+    </p>
+    <p>
+      Inertia&mdash;The inherent resistance to displacement of a body as
+      distinct from resistance the result of an external force.
+    </p>
+    <p>
+      Joy-Stick&mdash;See &ldquo;Control Lever.&rdquo;
+     </p>
+    <p>
+      Keel-Surface&mdash;Everything to be seen when viewing an aeroplane from
+      the side of it.
+    </p>
+    <p>
+      King-Post&mdash;A bracing strut; in an aeroplane, usually passing through
+      a surface and attached to the main spar, and from the end or ends of which
+      wires are taken to spar, surface, or other part of the construction in
+      order to prevent distortion. When used in connection with a controlling
+      surface, it usually performs the additional function of a lever, control
+      cables connecting its ends with the pilot's control lever.
+    </p>
+    <p>
+      Lift&mdash;The vertical component of the reaction produced by the action
+      of driving through the air a surface inclined upwards and towards its
+      direction of motion.
+    </p>
+    <p>
+      Lift, Margin of&mdash;The height an aeroplane can gain in a given time and
+      starting from a given altitude.
+    </p>
+    <p>
+      Lift-Drift Ratio&mdash;The proportion of lift to drift.
+    </p>
+    <p>
+      Loading&mdash;The weight carried by an aerofoil. Usually expressed in
+      pounds per square foot of superficial area.
+    </p>
+    <p>
+      Longeron&mdash;The term usually applied to any long spar running
+      length-ways of a fuselage.
+    </p>
+    <p>
+      Mass&mdash;The mass of a body is a measure of the quantity of material in
+      it.
+    </p>
+    <p>
+      Momentum&mdash;The product of the mass and velocity of a body is known as
+      &ldquo;momentum.&rdquo;
+     </p>
+    <p>
+      Monoplane&mdash;An aeroplane of which the main lifting surface consists of
+      one surface or one pair of wings.
+    </p>
+    <p>
+      Multiplane&mdash;An aeroplane of which the main lifting surface consists
+      of numerous surfaces or pairs of wings mounted one above the other.
+    </p>
+    <p>
+      Montant&mdash;Fuselage strut.
+    </p>
+    <p>
+      Nacelle&mdash;That part of an aeroplane containing the engine and pilot
+      and passenger, and to which the tail plane is not fixed.
+    </p>
+    <p>
+      Neutral Lift Line&mdash;A line taken through a surface in a forward
+      direction relative to its direction of motion, and starting from its
+      trailing edge. If the attitude of the surface is such as to make the said
+      line coincident with the direction of motion, it results in no lift, the
+      reaction then consisting solely of drift. The position of the neutral lift
+      line, i.e., the angle it makes with the chord, varies with differences of
+      camber, and it is found by means of wind-tunnel research.
+    </p>
+    <p>
+      Newton's Laws of Motion&mdash;1. If a body be at rest, it will remain at
+      rest; or, if in motion, it will move uniformly in a straight line until
+      acted upon by some force.
+    </p>
+    <p>
+      2. The rate of change of the quantity of motion (momentum) is proportional
+      to the force which causes it, and takes place in the direction of the
+      straight line in which the force acts. If a body be acted upon by several
+      forces, it will obey each as though the others did not exist, and this
+      whether the body be at rest or in motion.
+    </p>
+    <p>
+      3. To every action there is opposed an equal and opposite reaction.
+    </p>
+    <p>
+      Ornithopter (or Orthopter)&mdash;A flapping wing design of aircraft
+      intended to imitate the flight of a bird.
+    </p>
+    <p>
+      Outrigger&mdash;This term is usually applied to the framework connecting
+      the main surface with an elevator placed in advance of it. Sometimes
+      applied to the &ldquo;tail-boom&rdquo; framework connecting the tail-plane with the
+      main lifting surface.
+    </p>
+    <p>
+      Pancake, to&mdash;To &ldquo;stall &rdquo;
+     </p>
+    <p>
+      Plane&mdash;This term is often applied to a lifting surface. Such
+      application is not quite correct, since &ldquo;plane&rdquo; indicates a flat surface,
+      and the lifting surfaces are always cambered.
+    </p>
+    <p>
+      Propeller&mdash;See &ldquo;Air-Screw.&rdquo;
+     </p>
+    <p>
+      Propeller, Tractor&mdash;An air-screw mounted in front of the main lifting
+      surface.
+    </p>
+    <p>
+      Propeller, Pusher&mdash;An air-screw mounted behind the main lifting
+      surface.
+    </p>
+    <p>
+      Pusher&mdash;An aeroplane of which the propeller is mounted behind the
+      main lifting surface.
+    </p>
+    <p>
+      Pylon&mdash;Any V-shaped construction from the point of which wires are
+      taken.
+    </p>
+    <p>
+      Power&mdash;Rate of working.
+    </p>
+    <p>
+      Power, Horse&mdash;One horse-power represents a force sufficient to raise
+      33,000 lbs. 1 foot in a minute.
+    </p>
+    <p>
+      Power, Indicated Horse&mdash;The I.H.P. of an engine is a measure of the
+      rate at which work is done by the pressure upon the piston or pistons, as
+      distinct from the rate at which the engine does work. The latter is
+      usually termed &ldquo;brake horse-power,&rdquo; since it may be measured by an
+      absorption brake.
+    </p>
+    <p>
+      Power, Margin of&mdash;The available quantity of power above that
+      necessary to maintain horizontal flight at the optimum angle.
+    </p>
+    <p>
+      Pitot Tube&mdash;A form of air-speed indicator consisting of a tube with
+      open end facing the wind, which, combined with a static pressure or
+      suction tube, is used in conjunction with a gauge for measuring air
+      pressures or velocities. (No. 1 in diagram.)
+    </p>
+    <p>
+      Pitch, Propeller&mdash;The distance a propeller advances during one
+      revolution supposing the air to be solid.
+    </p>
+    <p>
+      Pitch, to&mdash;To plunge nose-down.
+    </p>
+    <p>
+      Reaction&mdash;A force, equal and opposite to the force of the action
+      producing it.
+    </p>
+    <p>
+      Rudder&mdash;A controlling surface, usually hinged to the tail, the
+      operation of which turns an aeroplane about an axis which is vertical in
+      normal horizontal flight; causes an aeroplane to turn to left or right of
+      the pilot.
+    </p>
+    <p>
+      Roll, to&mdash;To turn about the longitudinal axis.
+    </p>
+    <p>
+      Rib, Ordinary&mdash;A light curved wooden part mounted in a fore and aft
+      direction within a surface. The ordinary ribs give the surface its camber,
+      carry the fabric, and transfer the lift from the fabric to the spars.
+    </p>
+    <p>
+      Rib, Compression&mdash;Acts as an ordinary rib, besides bearing the stress
+      of compression produced by the tension of the internal bracing wires.
+    </p>
+    <p>
+      Rib, False&mdash;A subsidiary rib, usually used to improve the camber of
+      the front part of the surface.
+    </p>
+    <p>
+      Right and Left Hand&mdash;Always used relative to the position of the
+      pilot. When observing an aeroplane from the front of it, the right hand
+      side of it is then on the left hand of the observer.
+    </p>
+    <p>
+      Remou&mdash;A local movement or condition of the air which may cause
+      displacement of an aeroplane.
+    </p>
+    <p>
+      Rudder-Bar&mdash;A control lever moved by the pilot's feet, and operating
+      the rudder.
+    </p>
+    <p>
+      Surface&mdash;See &ldquo;Aerofoil.&rdquo;
+     </p>
+    <p>
+      Surface, Detrimental&mdash;All exterior parts of an aeroplane including
+      the propeller, but excluding the (aeroplane) lifting and (propeller)
+      thrusting surfaces.
+    </p>
+    <p>
+      Surface, Controlling&mdash;A surface the operation of which turns an
+      aeroplane about one of its axes.
+    </p>
+    <p>
+      Skin-Friction&mdash;The friction of the air with roughness of surface. A
+      form of drift.
+    </p>
+    <p>
+      Span&mdash;-The distance from wing-tip to wing-tip.
+    </p>
+    <p>
+      Stagger&mdash;The distance the upper surface is forward of the lower
+      surface when the axis of the propeller is horizontal.
+    </p>
+    <p>
+      Stability&mdash;The inherent tendency of a body, when disturbed, to return
+      to its normal position.
+    </p>
+    <p>
+      Stability, Directional&mdash;The stability about an axis which is vertical
+      during normal horizontal flight, and without which an aeroplane has no
+      natural tendency to remain upon its course.
+    </p>
+    <p>
+      Stability, Longitudinal&mdash;The stability of an aeroplane about an axis
+      transverse to the direction of normal horizontal flight, and without which
+      it has no tendency to oppose pitching and tossing.
+    </p>
+    <p>
+      Stability, Lateral&mdash;The stability of an aeroplane about its
+      longitudinal axis, and without which it has no tendency to oppose sideways
+      rolling.
+    </p>
+    <p>
+      Stabilizer&mdash;A surface, such as fin or tail-plane, designed to give an
+      aeroplane inherent stability.
+    </p>
+    <p>
+      Stall, to&mdash;To give or allow an aeroplane an angle of incidence
+      greater than the &ldquo;maximum&rdquo; angle, the result being a fall in the
+      lift-drift ratio, the lift consequently becoming less than the weight of
+      the aeroplane, which must then fall, i.e., &ldquo;stall&rdquo; or &ldquo;pancake.&rdquo;
+     </p>
+    <p>
+      Stress&mdash;Burden or load.
+    </p>
+    <p>
+      Strain&mdash;Deformation produced by stress.
+    </p>
+    <p>
+      Side-Slip, to&mdash;To fall as a result of an excessive &ldquo;bank&rdquo; or &ldquo;roll.&rdquo;
+     </p>
+    <p>
+      Skid, to&mdash;To be carried sideways by centrifugal force when turning to
+      left or right.
+    </p>
+    <p>
+      Skid, Undercarriage&mdash;A spar, mounted in a fore and aft direction, and
+      to which the wheels of the undercarriage are sometimes attached. Should a
+      wheel give way the skid is then supposed to act like the runner of a
+      sleigh and to support the aeroplane.
+    </p>
+    <p>
+      Skid, Tail&mdash;A piece of wood or other material, orientable, and fitted
+      with shock absorbers, situated under the tail of an aeroplane in order to
+      support it upon the ground and to absorb the shock of alighting.
+    </p>
+    <p>
+      Section&mdash;Any separate part of the top surface, that part of the
+      bottom surface immediately underneath it, with their struts and wires.
+    </p>
+    <p>
+      Spar&mdash;Any long piece of wood or other material.
+    </p>
+    <p>
+      Spar, Main&mdash;A spar within a surface and to which all the ribs are
+      attached, such spar being the one situated nearest to the centre of
+      pressure. It transfers more than half the lift from the ribs to the
+      bracing.
+    </p>
+    <p>
+      Spar, Rear&mdash;A spar within a surface, and to which all the ribs are
+      attached, such spar being situated at the rear of the centre of pressure
+      and at a greater distance from it than is the main spar. It transfers less
+      than half of the lift from the ribs to the bracing.
+    </p>
+    <p>
+      Strut&mdash;Any wooden member intended to take merely the stress of direct
+      compression.
+    </p>
+    <p>
+      Strut, Interplane&mdash;A strut holding the top and bottom surfaces apart.
+    </p>
+    <p>
+      Strut, Fuselage&mdash;A strut holding the fuselage longerons apart. It
+      should be stated whether top, bottom, or side. If side, then it should be
+      stated whether right or left hand. Montant.
+    </p>
+    <p>
+      Strut, Extension&mdash;A strut supporting an &ldquo;extension&rdquo; when not in
+      flight. It may also prevent the extension from collapsing upwards during
+      flight.
+    </p>
+    <p>
+      Strut, Undercarriage&mdash;
+    </p>
+    <p>
+      Strut, Dope&mdash;A strut within a surface, so placed as to prevent the
+      tension of the doped fabric from distorting the framework.
+    </p>
+    <p>
+      Serving&mdash;To bind round with wire, cord, or similar material. Usually
+      used in connection with wood joints and wire cable splices.
+    </p>
+    <p>
+      Slip, Propeller&mdash;The pitch less the distance the propeller advances
+      during one revolution.
+    </p>
+    <p>
+      Stream-Line&mdash;A form or shape of detrimental surface designed to
+      produce minimum drift.
+    </p>
+    <p>
+      Toss, to&mdash;To plunge tail-down.
+    </p>
+    <p>
+      Torque, Propeller&mdash;The tendency of a propeller to turn an aeroplane
+      about its longitudinal axis in a direction opposite to that in which the
+      propeller revolves.
+    </p>
+    <p>
+      Tail-Slide&mdash;A fall whereby the tail of an aeroplane leads.
+    </p>
+    <p>
+      Tractor&mdash;An aeroplane of which the propeller is mounted in front of
+      the main lifting surface.
+    </p>
+    <p>
+      Triplane&mdash;An aeroplane of which the main lifting surface consists of
+      three surfaces or pairs of wings mounted one above the other.
+    </p>
+    <p>
+      Tail-Plane&mdash;A horizontal stabilizing surface mounted at some distance
+      behind the main lifting surface. Empennage.
+    </p>
+    <p>
+      Turnbuckle&mdash;A form of wire-tightener, consisting of a barrel into
+      each end of which is screwed an eyebolt. Wires are attached to the
+      eyebolts and the required degree of tension is secured by means of
+      rotating the barrel.
+    </p>
+    <p>
+      Thrust, Propeller&mdash;See &ldquo;Air-Screw.&rdquo;
+     </p>
+    <p>
+      Undercarriage&mdash;That part of an aeroplane beneath the fuselage or
+      nacelle, and intended to support the aeroplane when at rest, and to absorb
+      the shock of alighting.
+    </p>
+    <p>
+      Velocity&mdash;Rate of displacement; speed.
+    </p>
+    <p>
+      Volplane&mdash;A gliding descent.
+    </p>
+    <p>
+      Weight&mdash;Is a measure of the force of the Earth's attraction (gravity)
+      upon a body. The standard unit of weight in this country is 1 lb., and is
+      the force of the Earth's attraction on a piece of platinum called the
+      standard pound, deposited with the Board of Trade in London. At the centre
+      of the Earth a body will be attracted with equal force in every direction.
+      It will therefore have no weight, though its mass is unchanged. Gravity,
+      of which weight is a measure, decreases with increase of altitude.
+    </p>
+    <p>
+      Web (of a rib)&mdash;That vertical part of a rib which prevents it from
+      bending upwards.
+    </p>
+    <p>
+      Warp, to&mdash;To distort a surface in order to vary its angle of
+      incidence. To vary the angle of incidence of a controlling surface.
+    </p>
+    <p>
+      Wash&mdash;The disturbance of air produced by the flight of an aeroplane.
+    </p>
+    <p>
+      Wash-in&mdash;An increasing angle of incidence of a surface towards its
+      wing-tip.
+    </p>
+    <p>
+      Wash-out&mdash;A decreasing angle of incidence of a surface towards its
+      wing-tip.
+    </p>
+    <p>
+      Wing-tip&mdash;The right- or left-hand extremity of a surface.
+    </p>
+    <p>
+      Wire&mdash;A wire is, in Aeronautics, always known by the name of its
+      function.
+    </p>
+    <p>
+      Wire, Lift or Flying&mdash;A wire opposed to the direction of lift, and
+      used to prevent a surface from collapsing upward during flight.
+    </p>
+    <p>
+      Wire, Anti-lift or Landing&mdash;A wire opposed to the direction of
+      gravity, and used to sustain a surface when it is at rest.
+    </p>
+    <p>
+      Wire, Drift&mdash;A wire opposed to the direction of drift, and used to
+      prevent a surface from collapsing backwards during flight.
+    </p>
+    <p>
+      Wire, Anti-drift&mdash;A wire opposed to the tension of a drift wire, and
+      used to prevent such tension from distorting the framework.
+    </p>
+    <p>
+      Wire, Incidence&mdash;A wire running from the top of an interplane strut
+      to the bottom of the interplane strut in front of or behind it. It
+      maintains the &ldquo;stagger&rdquo; and assists in maintaining the angle of incidence.
+      Sometimes termed &ldquo;stagger wire.&rdquo;
+     </p>
+    <p>
+      Wire, Bracing&mdash;Any wire holding together the framework of any part of
+      an aeroplane. It is not, however, usually applied to the wires described
+      above unless the function performed includes a function additional to
+      those described above. Thus, a lift wire, while strictly speaking a
+      bracing wire, is not usually described as one unless it performs the
+      additional function of bracing some well-defined part such as the
+      undercarriage. It will then be said to be an &ldquo;undercarriage bracing lift
+      wire.&rdquo; It might, perhaps, be acting as a drift wire also, in which case it
+      will then be de-scribed as an &ldquo;undercarriage bracing lift-drift wire.&rdquo; It
+      should always be stated whether a bracing wire is (1) top, (2) bottom, (3)
+      cross, or (4) side. If a &ldquo;side bracing wire,&rdquo; then it should be stated
+      whether right- or left-hand.
+    </p>
+    <p>
+      Wire, Internal Bracing&mdash;A bracing wire (usually drift or anti-drift)
+      within a surface.
+    </p>
+    <p>
+      Wire, Top Bracing&mdash;A bracing wire, approximately horizontal and
+      situated between the top longerons of fuselate, between top tail booms, or
+      at the top of similar construction.
+    </p>
+    <p>
+      Wire, Bottom Bracing&mdash;Ditto, substituting &ldquo;bottom&rdquo; for &ldquo;top.&rdquo;
+     </p>
+    <p>
+      Wire, Side Bracing&mdash;A bracing wire crossing diagonally a side bay of
+      fuselage, tail boom bay, undercarriage side bay or centre-section side
+      bay. This term is not usually used with reference to incidence wires,
+      although they cross diagonally the side bays of the cell. It should be
+      stated whether right- or left-hand.
+    </p>
+    <p>
+      Wire, Cross Bracing&mdash;A bracing wire, the position of which is
+      diagonal from right to left when viewing it from the front of an
+      aeroplane.
+    </p>
+    <p>
+      Wire, Control Bracing&mdash;A wire preventing distortion of a controlling
+      surface.
+    </p>
+    <p>
+      Wire, Control&mdash;A wire connecting a controlling surface with the
+      pilot's control lever, wheel, or rudder-bar.
+    </p>
+    <p>
+      Wire, Aileron Gap&mdash;A wire connecting top and bottom ailerons.
+    </p>
+    <p>
+      Wire, Aileron Balance&mdash;A wire connecting the right- and left-hand top
+      ailerons. Sometimes termed the &ldquo;aileron compensating wire.&rdquo;
+     </p>
+    <p>
+      Wire, Snaking&mdash;A wire, usually of soft metal, wound spirally or tied
+      round another wire, and attached at each end to the framework. Used to
+      prevent the wire round which it is &ldquo;snaked&rdquo; from becoming, in the event of
+      its displacement, entangled with the propeller.
+    </p>
+    <p>
+      Wire, Locking&mdash;A wire used to prevent a turnbuckle barrel or other
+      fitting from losing its adjustment.
+    </p>
+    <p>
+      Wing&mdash;Strictly speaking, a wing is one of the surfaces of an
+      ornithopter. The term is, however, often applied to the lifting surface of
+      an aeroplane when such surface is divided into two parts, one being the
+      left-hand &ldquo;wing,&rdquo; and the other the right-hand &ldquo;wing.&rdquo;
+     </p>
+    <p>
+      Wind-Tunnel&mdash;A large tube used for experimenting with surfaces and
+      models, and through which a current of air is made to flow by artificial
+      means.
+    </p>
+    <p>
+      Work&mdash;Force X displacement.
+    </p>
+    <p>
+      Wind-Screen&mdash;A small transparent screen mounted in front of the pilot
+      to protect his face from the air pressure.
+    </p>
+    <p>
+      <a name="link2H_FOOT" id="link2H_FOOT">
+      <!--  H2 anchor --> </a>
+    </p>
+    <div style="height: 4em;">
+      <br /><br /><br /><br />
+    </div>
+    <h2>
+      FOOTNOTES:]
+    </h2>
+    <p>
+      <a name="linknote-1" id="linknote-1">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      1 (<a href="#linknoteref-1">return</a>)<br /> [ Propeller Slip: As the
+      propeller screws through the air, the latter to a certain extent gives
+      back to the thrust of the propellor blades, just as the shingle on the
+      beach slips back as you ascend it. Such &ldquo;give-back&rdquo; is known as &ldquo;slip,&rdquo;
+       and anyone behind the propellor will feel the slip as a strong draught of
+      air.]
+    </p>
+    <p>
+      <a name="linknote-2" id="linknote-2">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      2 (<a href="#linknoteref-2">return</a>)<br /> [ Helicopter. An air-screw
+      revolving upon a vertical axis. If driven with sufficient power, it will
+      lift vertically, but having regard to the mechanical difficulties of such
+      construction, it is a most inefficient way of securing lift compared with
+      the arrangement of an inclined surface driven by a propeller revolving
+      about a horizontal axis.]
+    </p>
+    <p>
+      <a name="linknote-3" id="linknote-3">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      3 (<a href="#linknoteref-3">return</a>)<br /> [ Pancakes: Pilot's slang for
+      stalling an aeroplane and dropping like a pancake.]
+    </p>
+    <p>
+      <a name="linknote-4" id="linknote-4">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      4 (<a href="#linknoteref-4">return</a>)<br /> [ Morane parasol: A type of
+      Morane monoplane in which the lifting surfaces are raised above the pilot
+      in order to afford him a good view of the earth.]
+    </p>
+    <p>
+      <a name="linknote-5" id="linknote-5">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      5 (<a href="#linknoteref-5">return</a>)<br /> [ Skin friction is that part
+      of the drift due to the friction of the air with roughnesses upon the
+      surface of the aeroplane.]
+    </p>
+    <p>
+      <a name="linknote-6" id="linknote-6">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      6 (<a href="#linknoteref-6">return</a>)<br /> [ Banking: When an aeroplane
+      is turned to the left or the right the centrifugal force of its momentum
+      causes it to skid sideways and outwards away from the centre of the turn.
+      To minimize such action the pilot banks, i.e., tilts, the aeroplane
+      sideways in order to oppose the underside of the planes to the air. The
+      aeroplane will not then skid outwards beyond the slight skid necessary to
+      secure a sufficient pressure of air to balance the centrifugal force.]
+    </p>
+    <p>
+      <a name="linknote-7" id="linknote-7">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      7 (<a href="#linknoteref-7">return</a>)<br /> [ An explanation of the way
+      in which the wash-out is combined with a wash-in to offset propellor
+      torque will be found on p. 82.]
+    </p>
+    <p>
+      <a name="linknote-8" id="linknote-8">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      8 (<a href="#linknoteref-8">return</a>)<br /> [ A.M.'s: Air Mechanics.]
+    </p>
+    <p>
+      <a name="linknote-9" id="linknote-9">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      9 (<a href="#linknoteref-9">return</a>)<br /> [ Butt means to thicken at
+      the end. Screw means to machine a thread on the butt-end of the wire, and
+      in this way the wire can make connection with the desired place by being
+      screwed into a metal fitting, thus eliminating the disadvantage of the
+      unsatisfactory loop.]
+    </p>
+    <p>
+      <a name="linknote-10" id="linknote-10">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      10 (<a href="#linknoteref-10">return</a>)<br /> [ Deviation curve: A curved
+      line indicating any errors in the compass.]
+    </p>
+    <p>
+      <a name="linknote-11" id="linknote-11">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      11 (<a href="#linknoteref-11">return</a>)<br /> [ A propeller screws
+      through the air, and the distance it advances during one revolution,
+      supposing the air to be solid, is known as the pitch. The pitch, which
+      depends upon the angle of the propeller blades, must be equal to the speed
+      of the aeroplane, plus the slip, and if, on account of the rarity of the
+      air the speed of the aeroplane increases, then the angle and pitch should
+      be correspondingly increased. Propellers with a pitch capable of being
+      varied by the pilot are the dream of propeller designers. For explanation
+      of &ldquo;slip&rdquo; see Chapter IV. on propellers.]
+    </p>
+    <p>
+      <a name="linknote-12" id="linknote-12">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      12 (<a href="#linknoteref-12">return</a>)<br /> [ Getting out of my depth?
+      Invading the realms of fancy? Well, perhaps so, but at any rate it is
+      possible that extraordinary speed through space may be secured if means
+      are found to maintain the impulse of the engine and the thrust-drift
+      efficiency of the propeller at great altitude.]
+    </p>
+    <p>
+      <a name="linknote-13" id="linknote-13">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      13 (<a href="#linknoteref-13">return</a>)<br /> [ Box-kite. The first crude
+      form of biplane.]
+    </p>
+    <p>
+      <a name="linknote-14" id="linknote-14">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      14 (<a href="#linknoteref-14">return</a>)<br /> [ See Newton's laws in the
+      Glossary at the end of the book.]
+    </p>
+    <p>
+      <a name="linknote-15" id="linknote-15">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      15 (<a href="#linknoteref-15">return</a>)<br /> [ See &ldquo;Aerofoil&rdquo; in the
+      Glossary.]
+    </p>
+    <p>
+      <a name="linknote-16" id="linknote-16">
+      <!-- Note --></a>
+    </p>
+    <p class="foot">
+      16 (<a href="#linknoteref-16">return</a>)<br /> [ &ldquo;In effect&rdquo; because,
+      although there may be actually the greatest proportion of keel-surface In
+      front of the vertical axis, such surface may be much nearer to the axis
+      than is the keel-surface towards the tail. The latter may then be actually
+      less than the surface in front, but, being farther from the axis, it has a
+      greater leverage, and consequently is greater in effect than the surface
+      in front.]
+    </p>
+    <p>
+      <br /><br />
+    </p>
+<pre xml:space="preserve">
+
+
+
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+  </body>
+</html>
diff --git a/818.txt b/818.txt
new file mode 100644
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+++ b/818.txt
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+The Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Aeroplane Speaks
+
+Author: H. Barber
+
+Posting Date: July 21, 2008 [EBook #818]
+Release Date: February, 1997
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE AEROPLANE SPEAKS ***
+
+
+
+
+Produced by Charles Keller
+
+
+
+
+
+THE AEROPLANE SPEAKS
+
+By H. Barber
+
+(Captain, Royal Flying Corps)
+
+
+
+DEDICATED TO THE SUBALTERN FLYING OFFICER
+
+
+
+
+MOTIVE
+
+The reasons impelling me to write this book, the maiden effort of
+my pen, are, firstly, a strong desire to help the ordinary man to
+understand the Aeroplane and the joys and troubles of its Pilot; and,
+secondly, to produce something of PRACTICAL assistance to the Pilot
+and his invaluable assistant the Rigger. Having had some eight years'
+experience in designing, building, and flying aeroplanes, I have hopes
+that the practical knowledge I have gained may offset the disadvantage
+of a hand more used to managing the "joy-stick" than the dreadful
+haltings, the many side-slips, the irregular speed, and, in short, the
+altogether disconcerting ways of a pen.
+
+The matter contained in the Prologue appeared in the Field of May 6th,
+13th, 20th, and 27th, 1916, and is now reprinted by the kind permission
+of the editor, Sir Theodore Cook.
+
+I have much pleasure in also acknowledging the kindness of Mr. C. G.
+Grey, editor of the Aeroplane, to whom I am indebted for the valuable
+illustrations reproduced at the end of this book.
+
+
+
+CONTENTS
+
+  PROLOGUE
+
+  PART
+  I.   THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+  II.  THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
+  III. THE GREAT TEST
+  IV.  CROSS COUNTRY
+
+
+
+  CHAPTER
+  I.   FLIGHT
+  II.  STABILITY AND CONTROL
+  III. RIGGING
+  IV.  PROPELLERS
+  V.   MAINTENANCE
+
+
+  TYPES OF AEROPLANES
+
+  GLOSSARY
+
+
+
+
+
+THE AEROPLANE SPEAKS
+
+
+
+
+PROLOGUE
+
+
+
+
+PART I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+
+The Lecture Hall at the Royal Flying Corps School for Officers was
+deserted. The pupils had dispersed, and the Officer Instructor, more
+fagged than any pupil, was out on the aerodrome watching the test of a
+new machine.
+
+Deserted, did I say? But not so. The lecture that day had been upon
+the Elementary Principles of Flight, and they lingered yet. Upon the
+Blackboard was the illustration you see in the frontispiece.
+
+"I am the side view of a Surface," it said, mimicking the tones of the
+lecturer. "Flight is secured by driving me through the air at an angle
+inclined to the direction of motion."
+
+"Quite right," said the Angle. "That's me, and I'm the famous Angle of
+Incidence."
+
+"And," continued the Surface, "my action is to deflect the air
+downwards, and also, by fleeing from the air behind, to create a
+semi-vacuum or rarefied area over most of the top of my surface."
+
+"This is where I come in," a thick, gruff voice was heard, and went
+on: "I'm the Reaction. You can't have action without me. I'm a very
+considerable force, and my direction is at right-angles to you," and
+he looked heavily at the Surface. "Like this," said he, picking up the
+chalk with his Lift, and drifting to the Blackboard.
+
+"I act in the direction of the arrow R, that is, more or less, for the
+direction varies somewhat with the Angle of Incidence and the curvature
+of the Surface; and, strange but true, I'm stronger on the top of the
+Surface than at the bottom of it. The Wind Tunnel has proved that by
+exhaustive research--and don't forget how quickly I can grow! As the
+speed through the air increases my strength increases more rapidly than
+you might think--approximately, as the Square of the Speed; so you
+see that if the Speed of the Surface through the air is, for instance,
+doubled, then I am a good deal more than doubled. That's because I am
+the result of not only the mass of air displaced, but also the result
+of the Speed with which the Surface engages the Air. I am a product of
+those two factors, and at the speeds at which Aeroplanes fly to-day,
+and at the altitudes and consequent density of air they at present
+experience, I increase at about the Square of the Speed.
+
+"Oh, I'm a most complex and interesting personality, I assure you--in
+fact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr.
+Hyde. There's Lift, my vertical part or COMPONENT, as those who prefer
+long words would say; he always acts vertically upwards, and hates
+Gravity like poison. He's the useful and admirable part of me. Then
+there's Drift, my horizontal component, sometimes, though rather
+erroneously, called Head Resistance; he's a villain of the deepest dye,
+and must be overcome before flight can be secured."
+
+"And I," said the Propeller, "I screw through the air and produce the
+Thrust. I thrust the Aeroplane through the air and overcome the Drift;
+and the Lift increases with the Speed and when it equals the Gravity of
+Weight, then--there you are--Flight! And nothing mysterious about it at
+all."
+
+"I hope you'll excuse me interrupting," said a very beautiful young
+lady, "my name is Efficiency, and, while no doubt, all you have said is
+quite true, and that, as my young man the Designer says, `You can make a
+tea-tray fly if you slap on Power enough,' I can assure you that I'm not
+to be won quite so easily."
+
+"Well," eagerly replied the Lift and the Thrust, "let's be friends. Do
+tell us what we can do to help you to overcome Gravity and Drift with
+the least possible Power. That obviously seems the game to play, for
+more Power means heavier engines, and that in a way plays into the hands
+of our enemy, Gravity, besides necessitating a larger Surface or Angle
+to lift the Weight, and that increases the Drift."
+
+"Very well," from Efficiency, "I'll do my best, though I'm so shy, and
+I've just had such a bad time at the Factory, and I'm terribly afraid
+you'll find it awfully dry."
+
+"Buck up, old dear!" This from several new-comers, who had just
+appeared. "We'll help you," and one of them, so lean and long that he
+took up the whole height of the lecture room, introduced himself.
+
+"I'm the High Aspect Ratio," he said, "and what we have got to do to
+help this young lady is to improve the proportion of Lift to Drift.
+The more Lift we can get for a certain area of Surface, the greater
+the Weight the latter can carry; and the less the Drift, then the less
+Thrust and Power required to overcome it. Now it is a fact that, if the
+Surface is shaped to have the greatest possible span, i.e., distance
+from wing-tip to wing-tip, it then engages more air and produces both a
+maximum Reaction and a better proportion of Lift to Drift.
+
+"That being so, we can then well afford to lose a little Reaction
+by reducing the Angle of Incidence to a degree giving a still better
+proportion of Lift to Drift than would otherwise be the case; for you
+must understand that the Lift-Drift Ratio depends very much upon the
+size of the Angle of Incidence, which should be as small as possible
+within certain limits. So what I say is, make the surface of Infinite
+Span with no width or chord, as they call it. That's all I require, I
+assure you, to make me quite perfect and of infinite service to Miss
+Efficiency."
+
+"That's not practical politics," said the Surface. "The way you talk one
+would think you were drawing L400 a year at Westminster, and working up
+a reputation as an Aeronautical Expert. I must have some depth and chord
+to take my Spars and Ribs, and again, I must have a certain chord to
+make it possible for my Camber (that's curvature) to be just right for
+the Angle of Incidence. If that's not right the air won't get a nice
+uniform compression and downward acceleration from my underside, and the
+rarefied `suction' area over the top of me will not be as even and clean
+in effect as it might be. That would spoil the Lift-Drift Ratio more
+than you can help it. Just thrust that chalk along, will you? and the
+Blackboard will show you what I mean."
+
+"Well," said the Aspect Ratio, "have it your own way, though I'm sorry
+to see a pretty young lady like Efficiency compromised so early in the
+game."
+
+"Look here," exclaimed a number of Struts, "we have got a brilliant idea
+for improving the Aspect Ratio," and with that they hopped up on to the
+Spars. "Now," excitedly, "place another Surface on top of us. Now do you
+see? There is double the Surface, and that being so, the proportion of
+Weight to Surface area is halved. That's less burden of work for the
+Surface, and so the Spars need not be so strong and so deep, which
+results in not so thick a Surface. That means the Chord can be
+proportionately decreased without adversely affecting the Camber.
+With the Chord decreased, the Span becomes relatively greater, and so
+produces a splendid Aspect Ratio, and an excellent proportion of Lift to
+Drift."
+
+"I don't deny that they have rather got me there," said the Drift, "but
+all the same, don't forget my increase due to the drift of the Struts
+and their bracing wires."
+
+"Yes, I dare say," replied the Surface, "but remember that my Spars are
+less deep than before, and consequently I am not so thick now, and
+shall for that reason also be able to go through the air with a less
+proportion of Drift to Lift."
+
+"Remember me also, please," croaked the Angle of Incidence. "Since the
+Surface has now less weight to carry for its area, I may be set at
+a still lesser and finer Angle. That means less Drift again. We are
+certainly getting on splendidly! Show us how it looks now, Blackboard."
+And the Blackboard obligingly showed them as follows:
+
+"Well, what do you think of that?" they all cried to the Drift.
+
+"You think you are very clever," sneered the Drift. "But you are not
+helping Efficiency as much as you think. The suction effect on the top
+of the lower Surface will give a downward motion to the air above it and
+the result will be that the bottom of the top Surface will not secure
+as good a Reaction from the air as would otherwise be the case, and that
+means loss of Lift; and you can't help matters by increasing the gap
+between the surfaces because that means longer Struts and Wires, and
+that in itself would help me, not to speak of increasing the Weight. You
+see it's not quite so easy as you thought."
+
+At this moment a hiccough was heard, and a rather fast and
+rakish-looking chap, named Stagger, spoke up. "How d'ye do, miss," he
+said politely to Efficiency, with a side glance out of his wicked old
+eye. "I'm a bit of a knut, and without the slightest trouble I can
+easily minimize the disadvantage that old reprobate Drift has been
+frightening you with. I just stagger the top Surface a bit forward, and
+no longer is that suction effect dead under it. At the same time I'm
+sure the top Surface will kindly extend its Span for such distance as
+its Spars will support it without the aid of Struts. Such extension will
+be quite useful, as there will be no Surface at all underneath it to
+interfere with the Reaction above." And the Stagger leaned forward and
+picked up the Chalk, and this is the picture he drew:
+
+Said the Blackboard, "That's not half bad! It really begins to look
+something like the real thing, eh?"
+
+"The real thing, is it?" grumbled Drift. "Just consider that contraption
+in the light of any one Principle, and I warrant you will not find
+one of them applied to perfection. The whole thing is nothing but a
+Compromise." And he glared fixedly at poor Efficiency.
+
+"Oh, dear! Oh, dear!" she cried. "I'm always getting into trouble. What
+WILL the Designer say?"
+
+"Never mind, my dear," said the Lift-Drift Ratio, consolingly. "You are
+improving rapidly, and quite useful enough now to think of doing a job
+of work."
+
+"Well, that's good news," and Efficiency wiped her eyes with her Fabric
+and became almost cheerful. "Suppose we think about finishing it now?
+There will have to be an Engine and Propeller, won't there? And a body
+to fix them in, and tanks for oil and petrol, and a tail, and," archly,
+"one of those dashing young Pilots, what?"
+
+"Well, we are getting within sight of those interesting Factors," said
+the Lift-Drift Ratio, "but first of all we had better decide upon the
+Area of the Surfaces, their Angle of Incidence and Camber. If we are
+to ascend as quickly as possible the Aeroplane must be SLOW in order to
+secure the best possible Lift-Drift Ratio, for the drift of the struts
+wires, body, etc., increases approximately as the square of the speed,
+but it carries with it no lift as it does in the case of the Surface.
+The less speed then, the less such drift, and the better the Aeroplane's
+proportion of lift to drift; and, being slow, we shall require a LARGE
+SURFACE in order to secure a large lift relative to the weight to be
+carried. We shall also require a LARGE ANGLE OF INCIDENCE relative to
+the horizontal, in order to secure a proper inclination of the Surface
+to the direction of motion, for you must remember that, while we shall
+fly upon an even keel and with the propeller thrust horizontal (which is
+its most efficient attitude), our flight path, which is our direction of
+motion, will be sloping upwards, and it will therefore be necessary to
+fix the Surface to the Aeroplane at a very considerable angle relative
+to the horizontal Propeller Thrust in order to secure a proper angle
+to the upwards direction of motion. Apart from that, we shall require a
+larger Angle of Incidence than in the case of a machine designed purely
+for speed, and that means a correspondingly LARGE CAMBER.
+
+"On the other hand, if we are thinking merely of Speed, then a SMALL
+SURFACE, just enough to lift the weight off the ground, will be best,
+also a SMALL ANGLE to cut the Drift down and that, of course, means a
+relatively SMALL CAMBER.
+
+"So you see the essentials for CLIMB or quick ascent and for SPEED are
+diametrically opposed. Now which is it to be?"
+
+"Nothing but perfection for me," said Efficiency. "What I want is
+Maximum Climb and Maximum Speed for the Power the Engine produces."
+
+And each Principle fully agreed with her beautiful sentiments, but work
+together they would not.
+
+The Aspect Ratio wanted infinite Span, and hang the Chord.
+
+The Angle of Incidence would have two Angles and two Cambers in one,
+which was manifestly absurd; the Surface insisted upon no thickness
+whatever, and would not hear of such things as Spars and Ribs; and the
+Thrust objected to anything at all likely to produce Drift, and very
+nearly wiped the whole thing off the Blackboard.
+
+There was, indeed, the makings of a very pretty quarrel when the Letter
+arrived. It was about a mile long, and began to talk at once.
+
+"I'm from the Inventor," he said, and hope rose in the heart of each
+heated Principle. "It's really absurdly simple. All the Pilot has to do
+is to touch a button, and at his will, VARY the area of the Surface, the
+Angle of Incidence, and the Camber! And there you are--Maximum Climb or
+Maximum Speed as required! How does that suit you?"
+
+"That suits us very well," said the Surface, "but, excuse me asking, how
+is it done without apparatus increasing the Drift and the Weight out
+of all reason? You won't mind showing us your Calculations, Working
+Drawings, Stress Diagrams, etc., will you?"
+
+Said the Letter with dignity, "I come from an Inventor so brilliantly
+clever as to be far above the unimportant matters you mention. He is no
+common working man, sir! He leaves such things to Mechanics. The point
+is, you press a button and----"
+
+"Look here," said a Strut, rather pointedly, "where do you think you are
+going, anyway?"
+
+"Well," from the Letter, "as a matter of fact, I'm not addressed yet,
+but, of course, there's no doubt I shall reach the very highest quarters
+and absolutely revolutionize Flight when I get there."
+
+Said the Chalk, "I'll address you, if that's all you want; now drift
+along quickly!" And off went the Letter to The Technical Editor, "Daily
+Mauler," London.
+
+And a League was formed, and there were Directors with Fees, and several
+out-of-service Tin Hats, and the Man-who-takes-the-credit, and a fine
+fat Guinea-pig, and all the rest of them. And the Inventor paid his
+Tailor and had a Hair-Cut, and is now a recognized Press Expert--but he
+is still waiting for those Mechanics!
+
+"I'm afraid," said the Slide-rule, who had been busy making those
+lightning-like automatic calculations for which he is so famous, "it's
+quite impossible to fully satisfy all of you, and it is perfectly plain
+to me that we shall have to effect a Compromise and sacrifice some of
+the Lift for Speed."
+
+Thud! What was that?
+
+Efficiency had fainted dead away! The last blow had been too much for
+her. And the Principles gathered mournfully round, but with the aid of
+the Propeller Slip[1] and a friendly lift from the Surface she was at
+length revived and regained a more normal aspect.
+
+Said the Stagger with a raffish air, "My dear young lady, I assure
+you that from the experiences of a varied career, I have learned that
+perfection is impossible, and I am sure the Designer will be quite
+satisfied if you become the Most Efficient Compromise."
+
+"Well, that sounds so common sense," sighed Efficiency, "I suppose it
+must be true, and if the Designer is satisfied, that's all I really care
+about. Now do let's get on with the job."
+
+So the Chalk drew a nice long slim body to hold the Engine and the
+tanks, etc., with room for the Pilot's and Passenger's seats, and placed
+it exactly in the middle of the Biplane. And he was careful to make its
+position such that the Centre of Gravity was a little in advance of the
+Centre of Lift, so that when the Engine was not running and there was
+consequently no Thrust, the Aeroplane should be "nose-heavy" just to the
+right degree, and so take up a natural glide to Earth--and this was to
+help the Pilot and relieve him of work and worry, should he find himself
+in a fog or a cloud. And so that this tendency to glide downwards should
+not be in evidence when the Engine was running and descent not desired,
+the Thrust was placed a little below the Centre of Drift or Resistance.
+In this way it would in a measure pull the nose of the Aeroplane up and
+counterbalance the "nose-heavy" tendency.
+
+And the Engine was so mounted that when the Propeller-Thrust was
+horizontal, which is its most efficient position, the Angle of Incidence
+and the Area of the surfaces were just sufficient to give a Lift a
+little in excess of the Weight. And the Camber was such that, as far as
+it was concerned, the Lift-Drift Ratio should be the best possible for
+that Angle of Incidence. And a beautifully simple under-carriage was
+added, the outstanding features of which were simplicity, strength,
+light-weight, and minimum drift. And, last of all, there was the
+Elevator, of which you will hear more by-and-by. And this is what it
+looked like then:
+
+And Efficiency, smiling, thought that it was not such a bad compromise
+after all and that the Designer might well be satisfied.
+
+"Now," said she, "there's just one or two points I'm a bit hazy about.
+It appears that when the Propeller shaft is horizontal and so working
+in its most efficient attitude, I shall have a Lift from the Surfaces
+slightly in excess of the Weight. That means I shall ascend slightly,
+at the same time making nearly maximum speed for the power and thrust.
+Can't I do better than that?"
+
+"Yes, indeed," spoke up the Propeller, "though it means that I must
+assume a most undignified attitude, for helicopters[2] I never
+approved of. In order to ascend more quickly the Pilot will deflect the
+Elevator, which, by the way, you see hinged to the Tail. By that
+means he will force the whole Aeroplane to assume a greater Angle of
+Incidence. And with greater Angle, the Lift will increase, though I'm
+sorry to say the Drift will increase also. Owing to the greater Drift,
+the Speed through the air will lessen, and I'm afraid that won't be
+helpful to the Lift; but I shall now be pointing upwards, and besides
+overcoming the Drift in a forward direction I shall be doing my best
+to haul the Aeroplane skywards. At a certain angle known as the Best
+Climbing Angle, we shall have our Maximum Margin of Lift, and I'm hoping
+that may be as much as almost a thousand feet altitude a minute."
+
+"Then, if the Pilot is green, my chance will come," said the Maximum
+Angle of Incidence. "For if the Angle is increased over the Best
+Climbing Angle, the Drift will rush up; and the Speed, and with it the
+Lift, will, when my Angle is reached, drop to a point when the latter
+will be no more than the Weight. The Margin of Lift will have entirely
+disappeared, and there we shall be, staggering along at my tremendous
+angle, and only just maintaining horizontal flight."
+
+"And then with luck I'll get my chance," said the Drift. "If he is a bit
+worse than green, he'll perhaps still further increase the Angle. Then
+the Drift, largely increasing, the Speed, and consequently the Lift,
+will become still less, i.e., less than the Weight, and then--what price
+pancakes,[3] eh?"
+
+"Thank you," from Efficiency, "that was all most informing. And now will
+you tell me, please, how the greatest Speed may be secured?"
+
+"Certainly, now it's my turn," piped the Minimum Angle of Incidence. "By
+means of the Elevator, the Pilot places the Aeroplane at my small Angle,
+at which the Lift only just equals the Weight, and, also, at which we
+shall make greater speed with no more Drift than before. Then we get our
+greatest Speed, just maintaining horizontal flight."
+
+"Yes; though I'm out of the horizontal and thrusting downwards,"
+grumbled the Propeller, "and that's not efficient, though I suppose it's
+the best we can do until that Inventor fellow finds his Mechanics."
+
+"Thank you so much," said Efficiency. "I think I have now at any rate
+an idea of the Elementary Principles of Flight, and I don't know that I
+care to delve much deeper, for sums always give me a headache; but isn't
+there something about Stability and Control? Don't you think I ought to
+have a glimmering of them too?"
+
+"Well, I should smile," said a spruce Spar, who had come all the way
+from America. "And that, as the Lecturer says, `will be the subject of
+our next lecture,' so be here again to-morrow, and you will be glad to
+hear that it will be distinctly more lively than the subject we have
+covered to-day."
+
+
+
+
+PART II. THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE
+JOB
+
+Another day had passed, and the Flight Folk had again gathered together
+and were awaiting the arrival of Efficiency who, as usual, was rather
+late in making an appearance.
+
+The crowd was larger than ever, and among the newcomers some of the most
+important were the three Stabilities, named Directional, Longitudinal,
+and Lateral, with their assistants, the Rudder, Elevator, and Ailerons.
+There was Centrifugal Force, too, who would not sit still and created a
+most unfavourable impression, and Keel-Surface, the Dihedral Angle, and
+several other lesser fry.
+
+"Well," said Centrifugal Force, "I wish this Efficiency I've heard so
+much about would get a move on. Sitting still doesn't agree with me
+at all. Motion I believe in. There's nothing like motion--the more the
+better."
+
+"We are entirely opposed to that," objected the three Stabilities, all
+in a breath. "Unless it's in a perfectly straight line or a perfect
+circle. Nothing but perfectly straight lines or, upon occasion, perfect
+circles satisfy us, and we are strongly suspicious of your tendencies."
+
+"Well, we shall see what we shall see," said the Force darkly. "But who
+in the name of blue sky is this?"
+
+And in tripped Efficiency, in a beautifully "doped" dress of the latest
+fashionable shade of khaki-coloured fabric, a perfectly stream-lined
+bonnet, and a bewitching little Morane parasol,[4] smiling as
+usual, and airily exclaiming, "I'm so sorry I'm late, but you see the
+Designer's such a funny man. He objects to skin friction,[5] and
+insisted upon me changing my fabric for one of a smoother surface, and
+that delayed me. Dear me, there are a lot more of us to-day, aren't
+there? I think I had better meet one at a time." And turning to
+Directional Stability, she politely asked him what he preferred to do.
+
+"My purpose in life, miss," said he, "is to keep the Aeroplane on its
+course, and to achieve that there must be, in effect, more Keel-Surface
+behind the Vertical Turning Axis than there is in front of it."
+
+Efficiency looking a little puzzled, he added: "Just like a weathercock,
+and by Keel-Surface I mean everything you can see when you view the
+Aeroplane from the side of it--the sides of the body, struts, wires,
+etc."
+
+"Oh, now I begin to see light," said she: "but just exactly how does it
+work?"
+
+"I'll answer that," said Momentum. "When perhaps by a gust of air the
+Aeroplane is blown out of its course and points in another direction, it
+doesn't immediately fly off on that new course. I'm so strong I pull it
+off the new course to a certain extent, and towards the direction of the
+old course. And so it travels, as long as my strength lasts, in a more
+or less sideways position."
+
+"Then," said the Keel-Surface, "I get a pressure of air all on one side,
+and as there is, in effect, most of me towards the tail, the latter
+gets pressed sideways, and the Aeroplane thus tends to assume its first
+position and course."
+
+"I see," said Efficiency, and, daintily holding the Chalk, she
+approached the Blackboard. "Is this what you mean?"
+
+"Yes, that's right enough," said the Keel-Surface, "and you might
+remember, too, that I always make the Aeroplane nose into the gusts
+rather than away from them."
+
+"If that was not the case," broke in Lateral Stability, and affecting
+the fashionable Flying Corps stammer, "it would be a h-h-h-o-r-rible
+affair! If there were too much Keel-Surface in front, then that gust
+would blow the Aeroplane round the other way a very considerable
+distance. And the right-hand Surface being on the outside of the turn
+would have more speed, and consequently more Lift, than the Surface
+on the other side. That means a greater proportion of the Lift on that
+side, and before you could say Warp to the Ailerons over the Aeroplane
+would go--probable result a bad side-slip"
+
+"And what can the Pilot do to save such a situation as that?" said
+Efficiency.
+
+"Well," replied Lateral Stability, "he will try to turn the Aeroplane
+sideways and back to an even keel by means of warping the Ailerons or
+little wings which are hinged on to the Wing-tips, and about which you
+will hear more later on; but if the side-slip is very bad he may not be
+able to right the Aeroplane by means of the Ailerons, and then the only
+thing for him to do is to use the Rudder and to turn the nose of the
+Aeroplane down and head-on to the direction of motion. The Aeroplane
+will then be meeting the air in the direction it is designed to do
+so, and the Surfaces and also the controls (the Rudder, Ailerons, and
+Elevator) will be working efficiently; but its attitude relative to
+the earth will probably be more or less upside-down, for the action
+of turning the Aeroplane's nose down results, as you will see by the
+illustration B, in the right wing, which is on the outside of the
+circle. travelling through the air with greater speed than the left-hand
+wing. More Speed means more Lift, so that results in overturning the
+Aeroplane still more; but now it is, at any rate, meeting the air as it
+is designed to meet it, and everything is working properly. It is then
+only necessary to warp the Elevator, as shown in illustration C, in
+order to bring the Aeroplane into a proper attitude relative to the
+earth."
+
+"Ah!" said the Rudder, looking wise, "it's in a case like that when I
+become the Elevator and the Elevator becomes me."
+
+"That's absurd nonsense," said the Blackboard, "due to looseness of
+thought and expression."
+
+"Well," replied the Rudder, "when 'the Aeroplane is in position A and I
+am used, then I depress or ELEVATE the nose of the machine; and, if the
+Elevator is used, then it turns the Aeroplane to right or left, which is
+normally my function. Surely our roles have changed one with the other,
+and I'm then the Elevator and the Elevator is me!"
+
+Said Lateral Stability to the Rudder, "That's altogether the wrong way
+of looking at it, though I admit"--and this rather sarcastically--"that
+the way you put it sounds rather fine when you are talking of your
+experiences in the air to those 'interested in aviation' but knowing
+little about it; but it won't go down here! You are a Controlling
+Surface designed to turn the Aeroplane about its vertical axis, and the
+Elevator is a Controlling Surface designed to turn the Aeroplane about
+its lateral axis. Those are your respective jobs, and you can't possibly
+change them about. Such talk only leads to confusion, and I hope we
+shall hear no more of it."
+
+"Thanks," said Efficiency to Lateral Stability. "And now, please, will
+you explain your duties?"
+
+"My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.
+First of all, I sometimes arrange with the Rigger to wash-out, that is
+decrease, the Angle of Incidence on one side of the Aeroplane, and to
+effect the reverse condition, if it is not too much trouble, on the
+other side."
+
+"But," objected Efficiency, "the Lift varies with the Angle of
+Incidence, and surely such a condition will result in one side of the
+Aeroplane lifting more than the other side?'
+
+"That's all right," said the Propeller, "it's meant to off-set the
+tendency of the Aeroplane to turn over sideways in the opposite
+direction to which I revolve."
+
+"That's quite clear, though rather unexpected; but how do you counteract
+the effect of the gusts when they try to overturn the Aeroplane
+sideways?" said she, turning to Lateral Stability again.
+
+"Well," he replied, rather miserably, "I'm not nearly so perfect as the
+Longitudinal and Directional Stabilities. The Dihedral Angle--that is,
+the upward inclination of the Surfaces towards their wing-tips--does
+what it can for me, but, in my opinion, it's a more or less futile
+effort. The Blackboard will show you the argument." And he at once
+showed them two Surfaces, each set at a Dihedral Angle like this:
+
+"Please imagine," said the Blackboard, "that the top V is the front
+view of a Surface flying towards you. Now if a gust blows it into the
+position of the lower V you see that the horizontal equivalent of the
+Surface on one side becomes larger, and on the other side it becomes
+smaller. That results in more Lift on the lower side and less on the
+higher side, and if the V is large enough it should produce such a
+difference in the Lift of one side to the other as to quickly turn the
+Aeroplane back to its former and normal position."
+
+"Yes," said the Dihedral Angle, "that's what would happen if they would
+only make me large enough; but they won't do it because it would too
+greatly decrease the horizontal equivalent, and therefore the Lift, and
+incidentally it would, as Aeroplanes are built to-day, produce an excess
+of Keel Surface above the turning axis, and that in itself would spoil
+the Lateral Stability. The Keel Surface should be equally divided above
+and below the longitudinal turning axis (upon which the Aeroplane rolls
+sideways), or the side upon which there is an excess will get blown over
+by the gusts. It strikes me that my future isn't very promising, and
+about my only chance is when the Junior Draughtsman makes a mistake, as
+he did the other day. And just think of it, they call him a Designer now
+that he's got a job at the Factory! What did he do? Why, he calculated
+the weights wrong and got the Centre of Gravity too high, and they
+didn't discover it until the machine was built. Then all they could do
+was to give me a larger Angle. That dropped the bottom of the V lower
+down, and as that's the centre of the machine, where all the Weight is,
+of course that put the Centre of Gravity in its right place. But now
+there is too much Keel Surface above, and the whole thing's a Bad
+Compromise, not at all like Our Efficiency."
+
+And Efficiency, blushing very prettily at the compliment, then asked,
+"And how does the Centre of Gravity affect matters?"
+
+"That's easy," said Grandfather Gravity. "I'm so heavy that if I am too
+low down I act like a pendulum and cause the Aeroplane to roll about
+sideways, and if I am too high I'm like a stick balanced on your finger,
+and then if I'm disturbed, over I go and the Aeroplane with me; and, in
+addition to that, there are the tricks I play with the Aeroplane when
+it's banked up,[6] i.e., tilted sideways for a turn, and Centrifugal
+Force sets me going the way I'm not wanted to go. No; I get on best with
+Lateral Stability when my Centre is right on the centre of Drift, or,
+at any rate, not much below it." And with that he settled back into the
+Lecturer's Chair and went sound asleep again, for he was so very, very
+old, in fact the father of all the Principles.
+
+And the Blackboard had been busy, and now showed them a picture of
+the Aeroplane as far as they knew it, and you will see that there is
+a slight Dihedral Angle, and also, fixed to the tail, a vertical Keel
+Surface or fin, as is very often the case in order to ensure the greater
+effect of such surface being behind the vertical turning axis.
+
+But Efficiency, growing rather critical with her newly gained knowledge,
+cried out: "But where's the horizontal Tail Surface? It doesn't look
+right like that!"
+
+"This is when I have the pleasure of meeting you, my dear," said
+Longitudinal Stability. "Here's the Tail Surface," he said, "and in
+order to help me it must be set IN EFFECT at a much less Angle of
+Incidence than the Main Surface. To explain we must trouble the
+Blackboard again," and this was his effort:
+
+"I have tried to make that as clear as possible," he said. "It may
+appear a bit complicated at first, but if you will take the trouble to
+look at it for a minute you will find it quite simple. A is the normal
+and proper direction of motion of the Aeroplane, but, owing to a gust of
+air, it takes up the new nose-down position. Owing to Momentum, however,
+it does not fly straight along in that direction, but moves more or less
+in the direction B, which is the resultant of the two forces, Momentum
+and Thrust. And so you will note that the Angle of Incidence, which
+is the inclination of the Surfaces to the Direction of Motion, has
+decreased, and of course the Lift decreases with it. You will also
+see, and this is the point, that the Tail Surface has lost a higher
+proportion of its Angle, and consequently its Lift, than has the Main
+Surface. Then, such being the case, the Tail must fall and the Aeroplane
+assume its normal position again, though probably at a slightly lower
+altitude."
+
+"I'm afraid I'm very stupid," said Efficiency, "but please tell me why
+you lay stress upon the words 'IN EFFECT.'"
+
+"Ah! I was wondering if you would spot that," he replied. "And there is
+a very good reason for it. You see, in some Aeroplanes the Tail Surface
+may be actually set at the same Angle on the machine as the Main
+Surface, but owing to the air being deflected downwards by the front
+Main Surface it meets the Tail Surface at a lesser angle, and indeed in
+some cases at no angle at all. The Tail is then for its surface getting
+less Lift than the Main Surface, although set at the same angle on
+the machine. It may then be said to have IN EFFECT a less Angle of
+Incidence. I'll just show you on the Blackboard."
+
+"And now," said Efficiency, "I have only to meet the Ailerons and the
+Rudder, haven't I?"
+
+"Here we are," replied the Ailerons, or little wings. "Please hinge us
+on to the back of the Main Surfaces, one of us at each Wing-tip, and
+join us up to the Pilot's joystick by means of the control cables. When
+the Pilot wishes to tilt the Aeroplane sideways, he will move the stick
+and depress us upon one side, thus giving us a larger Angle of Incidence
+and so creating more Lift on that side of the Aeroplane; and, by means
+of a cable connecting us with the Ailerons on the other side of the
+Aeroplane, we shall, as we are depressed, pull them up and give them a
+reverse or negative Angle of Incidence, and that side will then get
+a reverse Lift or downward thrust, and so we are able to tilt the
+Aeroplane sideways.
+
+"And we work best when the Angle of Incidence of the Surface in front
+of us is very small, for which reason it is sometimes decreased or
+washed-out towards the Wing-tips. The reason of that is that by the
+time the air reaches us it has been deflected downwards--the greater the
+Angle of Incidence the more it is driven downwards--and in order for
+us to secure a Reaction from it, we have to take such a large Angle of
+Incidence that we produce a poor proportion of Lift to Drift; but the
+smaller the Angle of the Surface in front of us the less the air is
+deflected downwards, and consequently the less Angle is required of us,
+and the better our proportion of Lift to Drift, which, of course, makes
+us much more effective Controls."
+
+"Yes," said the Lateral and Directional Stabilities in one voice,
+"that's so, and the wash-out helps us also, for then the Surfaces
+towards their Wing-tips have less Drift or 'Head-Resistance,' and
+consequently the gusts will affect them and us less; but such decreased
+Angle of Incidence means decreased Lift as well as Drift, and the
+Designer does not always care to pay the price."
+
+"Well," said the Ailerons, "if it's not done it will mean more work for
+the Rudder, and that won't please the Pilot."
+
+"Whatever do you mean?" asked Efficiency. "What can the Rudder have to
+do with you?"
+
+"It's like this," they replied: "when we are deflected downwards we gain
+a larger Angle of Incidence and also enter an area of compressed air,
+and so produce more Drift than those of us on the other side of the
+Aeroplane, which are deflected upwards into an area of rarefied air due
+to the SUCTION effect (though that term is not academically correct) on
+the top of the Surface. If there is more Drift, i.e., Resistance, on
+one side of the Aeroplane than on the other side, then of course it will
+turn off its course, and if that difference in Drift is serious, as it
+will very likely be if there is no wash-out, then it will mean a good
+deal of work for the Rudder in keeping the Aeroplane on its course,
+besides creating extra Drift in doing so."
+
+"I think, then," said Efficiency, "I should prefer to have that
+wash-out,[7] and my friend the Designer is so clever at producing
+strength of construction for light weight, I'm pretty sure he won't
+mind paying the price in Lift. And now let me see if I can sketch the
+completed Aeroplane."
+
+"Well, I hope that's all as it should be," she concluded, "for to-morrow
+the Great Test in the air is due."
+
+
+
+
+PART III. THE GREAT TEST
+
+It is five o'clock of a fine calm morning, when the Aeroplane is wheeled
+out of its shed on to the greensward of the Military Aerodrome. There
+is every promise of a good flying day, and, although the sun has not
+yet risen, it is light enough to discern the motionless layer of fleecy
+clouds some five thousand feet high, and far, far above that a few filmy
+mottled streaks of vapour. Just the kind of morning beloved of pilots.
+
+A brand new, rakish, up-to-date machine it is, of highly polished,
+beautifully finished wood, fabric as tight as a drum, polished metal,
+and every part so perfectly "streamlined" to minimize Drift, which is
+the resistance of the air to the passage of the machine, that to the
+veriest tyro the remark of the Pilot is obviously justified.
+
+"Clean looking 'bus, looks almost alive and impatient to be off. Ought
+to have a turn for speed with those lines."
+
+"Yes," replies the Flight-Commander, "it's the latest of its type and
+looks a beauty. Give it a good test. A special report is required on
+this machine."
+
+The A.M.'s[8] have now placed the Aeroplane in position facing the
+gentle air that is just beginning to make itself evident; the engine
+Fitter, having made sure of a sufficiency of oil and petrol in the
+tanks, is standing by the Propeller; the Rigger, satisfied with a job
+well done, is critically "vetting" the machine by eye, four A.M.'s are
+at their posts, ready to hold the Aeroplane from jumping the blocks
+which have been placed in front of the wheels; and the Flight-Sergeant
+is awaiting the Pilot's orders.
+
+As the Pilot approaches the Aeroplane the Rigger springs to attention
+and reports, "All correct, sir," but the Fitter does not this morning
+report the condition of the Engine, for well he knows that this Pilot
+always personally looks after the preliminary engine test. The latter,
+in leathern kit, warm flying boots and goggled, climbs into his seat,
+and now, even more than before, has the Aeroplane an almost living
+appearance, as if straining to be off and away. First he moves the
+Controls to see that everything is clear, for sometimes when the
+Aeroplane is on the ground the control lever or "joy-stick" is lashed
+fast to prevent the wind from blowing the controlling surfaces about and
+possibly damaging them.
+
+The air of this early dawn is distinctly chilly, and the A.M.'s are
+beginning to stamp their cold feet upon the dewy grass, but very careful
+and circumspect is the Pilot, as he mutters to himself, "Don't worry and
+flurry, or you'll die in a hurry."
+
+At last he fumbles for his safety belt, but with a start remembers the
+Pilot Air Speed Indicator, and, adjusting it to zero, smiles as he hears
+the Pilot-head's gruff voice, "Well, I should think so, twenty miles an
+hour I was registering. That's likely to cause a green pilot to stall
+the Aeroplane. Pancake, they call it." And the Pilot, who is an old hand
+and has learned a lot of things in the air that mere earth-dwellers know
+nothing about, distinctly heard the Pilot Tube, whose mouth is open to
+the air to receive its pressure, stammer. "Oh Lor! I've got an earwig
+already--hope to goodness the Rigger blows me out when I come down--and
+this morning air simply fills me with moisture; I'll never keep the
+Liquid steady in the Gauge. I'm not sure of my rubber connections
+either."
+
+"Oh, shut up!" cry all the Wires in unison, "haven't we got our troubles
+too? We're in the most horrible state of tension. It's simply murdering
+our Factor of Safety, and how we can possibly stand it when we get the
+Lift only the Designer knows."
+
+"That's all right," squeak all the little Wire loops, "we're that
+accommodating, we're sure to elongate a bit and so relieve your
+tension." For the whole Aeroplane is braced together with innumerable
+wires, many of which are at their ends bent over in the form of loops
+in order to connect with the metal fittings on the spars and
+elsewhere--cheap and easy way of making connection.
+
+"Elongate, you little devils, would you?" fairly shout the Angles of
+Incidence, Dihedral and Stagger, amid a chorus of groans from all parts
+of the Aeroplane. "What's going to happen to us then? How are we going
+to keep our adjustments upon which good flying depends?"
+
+"Butt us and screw us,"[9] wail the Wires. "Butt us and screw us, and
+death to the Loops. That's what we sang to the Designer, but he only
+looked sad and scowled at the Directors."
+
+"And who on earth are they?" asked the Loops, trembling for their
+troublesome little lives.
+
+"Oh earth indeed," sniffed Efficiency, who had not spoken before, having
+been rendered rather shy by being badly compromised in the Drawing
+Office. "I'd like to get some of them up between Heaven and Earth,
+I would. I'd give 'em something to think of besides their Debits and
+Credits--but all the same the Designer will get his way in the end. I'm
+his Best Girl, you know, and if we could only get rid of the Directors,
+the little Tin god, and the Man-who-takes-the-credit, we should be quite
+happy." Then she abruptly subsides, feeling that perhaps the less said
+the better until she has made a reputation in the Air. The matter of
+that Compromise still rankled, and indeed it does seem hardly fit that
+a bold bad Tin god should flirt with Efficiency. You see there was
+a little Tin god, and he said "Boom, Boom BOOM! Nonsense! It MUST be
+done," and things like that in a very loud voice, and the Designer
+tore his hair and was furious, but the Directors, who were thinking of
+nothing but Orders and Dividends, had the whip-hand of HIM, and so there
+you are, and so poor beautiful Miss Efficiency was compromised.
+
+All this time the Pilot is carefully buckling his belt and making
+himself perfectly easy and comfortable, as all good pilots do. As
+he straightens himself up from a careful inspection of the Deviation
+Curve[10] of the Compass and takes command of the Controls, the
+Throttle and the Ignition, the voices grow fainter and fainter until
+there is nothing but a trembling of the Lift and Drift wires to indicate
+to his understanding eye their state of tension in expectancy of the
+Great Test.
+
+"Petrol on?" shouts the Fitter to the Pilot.
+
+"Petrol on," replies the Pilot.
+
+"Ignition off?"
+
+"Ignition off."
+
+Round goes the Propeller, the Engine sucking in the Petrol Vapour with
+satisfied gulps. And then--
+
+"Contact?" from the Fitter.
+
+"Contact," says the Pilot.
+
+Now one swing of the Propeller by the Fitter, and the Engine is awake
+and working. Slowly at first though, and in a weak voice demanding, "Not
+too much Throttle, please. I'm very cold and mustn't run fast until my
+Oil has thinned and is circulating freely. Three minutes slowly, as you
+love me, Pilot."
+
+Faster and faster turn the Engine and Propeller, and the Aeroplane,
+trembling in all its parts, strains to jump the blocks and be off.
+Carefully the Pilot listens to what the Engine Revolution Indicator
+says. At last, "Steady at 1,500 revs. and I'll pick up the rest in the
+Air." Then does he throttle down the Engine, carefully putting the lever
+back to the last notch to make sure that in such position the Throttle
+is still sufficiently open for the Engine to continue working, as
+otherwise it might lead to him "losing" his Engine in the air when
+throttling down the power for descent. Then, giving the official signal,
+he sees the blocks removed from the wheels, and the Flight-Sergeant
+saluting he knows that all is clear to ascend. One more signal, and all
+the A.M.'s run clear of the Aeroplane.
+
+Then gently, gently mind you, with none of the "crashing on" bad
+Pilots think so fine, he opens the Throttle and, the Propeller Thrust
+overcoming its enemy the Drift, the Aeroplane moves forward.
+
+"Ah!" says the Wind-screen, "that's Discipline, that is. Through
+my little window I see most things, and don't I just know that poor
+discipline always results in poor work in the air, and don't you forget
+it."
+
+"Discipline is it?" complains the Under-carriage, as its wheels roll
+swiftly over the rather rough ground. "I'm bump getting it; and bump,
+bump, all I want, bang, bump, rattle, too!" But, as the Lift increases
+with the Speed, the complaints of the Under-carriage are stilled, and
+then, the friendly Lift becoming greater than the Weight, the Aeroplane
+swiftly and easily takes to the air.
+
+Below is left the Earth with all its bumps and troubles. Up into the
+clean clear Air moves with incredible speed and steadiness this triumph
+of the Designer, the result of how much mental effort, imagination,
+trials and errors, failures and successes, and many a life lost in high
+endeavour.
+
+Now is the mighty voice of the Engine heard as he turns the Propeller
+nine hundred times a minute. Now does the Thrust fight the Drift for all
+it's worth, and the Air Speed Indicator gasps with delight, "One hundred
+miles an hour!"
+
+And now does the burden of work fall upon the Lift and Drift Wires,
+and they scream to the Turnbuckles whose business it is to hold them in
+tension, "This is the limit! the Limit! THE LIMIT! Release us, if only
+a quarter turn." But the Turnbuckles are locked too fast to turn their
+eyes or utter a word. Only the Locking Wires thus: "Ha! ha! the Rigger
+knew his job. He knew the trick, and there's no release here." For
+an expert rigger will always use the locking wire in such a way as to
+oppose the slightest tendency of the turnbuckle to unscrew. The other
+kind of rigger will often use the wire in such a way as to allow the
+turnbuckle, to the "eyes" of which the wires are attached, to unscrew a
+quarter of a turn or more, with the result that the correct adjustment
+of the wires may be lost; and upon their fine adjustment much depends.
+
+And the Struts and the Spars groan in compression and pray to keep
+straight, for once "out of truth" there is, in addition to possible
+collapse, the certainty that in bending they will throw many wires out
+of adjustment.
+
+And the Fabric's quite mixed in its mind, and ejaculates, "Now, who
+would have thought I got more Lift from the top of the Surface than its
+bottom?" And then truculently to the Distance Pieces, which run from
+rib to rib, "Just keep the Ribs from rolling, will you? or you'll see me
+strip. I'm an Irishman, I am, and if my coat comes off---- Yes, Irish, I
+said. I used to come from Egypt, but I've got naturalized since the War
+began."
+
+Then the Air Speed Indicator catches the eye of the Pilot. "Good
+enough," he says as he gently deflects the Elevator and points the nose
+of the Aeroplane upwards in search of the elusive Best Climbing Angle.
+
+"Ha! ha!" shouts the Drift, growing stronger with the increased Angle
+of Incidence. "Ha! ha!" he laughs to the Thrust. "Now I've got you. Now
+who's Master?"
+
+And the Propeller shrieks hysterically, "Oh! look at me. I'm a
+helicopter. That's not fair. Where's Efficiency?" And she can only sadly
+reply, "Yes, indeed, but you see we're a Compromise."
+
+And the Drift has hopes of reaching the Maximum Angle of Incidence
+and vanquishing the Thrust and the Lift. And he grows very bold as he
+strangles the Thrust; but the situation is saved by the Propeller,
+who is now bravely helicopting skywards, somewhat to the chagrin of
+Efficiency.
+
+"Much ado about nothing," quotes the Aeroplane learnedly. "Compromise
+or not, I'm climbing a thousand feet a minute. Ask the Altimeter. He'll
+confirm it."
+
+And so indeed it was. The vacuum box of the Altimeter was steadily
+expanding under the decreased pressure of the rarefied air, and by means
+of its little levers and its wonderful chain no larger than a hair it
+was moving the needle round the gauge and indicating the ascent at the
+rate of a thousand feet a minute.
+
+And lo! the Aeroplane has almost reached the clouds! But what's this?
+A sudden gust, and down sinks one wing and up goes the other. "Oh, my
+Horizontal Equivalent!" despairingly call the Planes: "it's eloping with
+the Lift, and what in the name of Gravity will happen? Surely there was
+enough scandal in the Factory without this, too!" For the lift varies
+with the horizontal equivalent of the planes, so that if the aeroplane
+tilts sideways beyond a certain angle, the lift becomes less than the
+weight of the machine, which must then fall. A fall in such a position
+is known as a "side-slip."
+
+But the ever-watchful Pilot instantly depresses one aileron, elevating
+the other, with just a touch of the rudder to keep on the course, and
+the Planes welcome back their precious Lift as the Aeroplane flicks back
+to its normal position.
+
+"Bit bumpy here under these clouds," is all the Pilot says as he heads
+for a gap between them, and the next minute the Aeroplane shoots up into
+a new world of space.
+
+"My eye!" ejaculates the Wind-screen, "talk about a view!" And indeed
+mere words will always fail to express the wonder of it. Six thousand
+feet up now, and look! The sun is rising quicker than ever mortal on
+earth witnessed its ascent. Far below is Mother Earth, wrapt in mists
+and deep blue shadows, and far above are those light, filmy, ethereal
+clouds now faintly tinged with pink And all about great mountains of
+cloud, lazily floating in space. The sun rises and they take on all
+colours, blending one with the other, from dazzling white to crimson
+and deep violet-blue. Lakes and rivers here and there in the enormous
+expanse of country below refract the level rays of the sun and, like so
+many immense diamonds, send dazzling shafts of light far upwards. The
+tops of the hills now laugh to the light of the sun, but the valleys are
+still mysterious dark blue caverns, clowned with white filmy lace-like
+streaks of vapour. And withal the increasing sense with altitude of
+vast, clean, silent solitudes of space.
+
+Lives there the man who can adequately describe this Wonder? "Never,"
+says the Pilot, who has seen it many times, but to whom it is ever new
+and more wonderful.
+
+Up, up, up, and still up, unfalteringly speeds the Pilot and his mount.
+Sweet the drone of the Engine and steady the Thrust as the Propeller
+exultingly battles with the Drift.
+
+And look! What is that bright silver streak all along the horizon? It
+puzzled the Pilot when first he saw it, but now he knows it for the Sea,
+full fifty miles away!
+
+And on his right is the brightness of the Morn and the smiling Earth
+unveiling itself to the ardent rays of the Sun; and on his left, so
+high is he, there is yet black Night, hiding innumerable Cities, Towns,
+Villages and all those places where soon teeming multitudes of men shall
+awake, and by their unceasing toil and the spirit within them produce
+marvels of which the Aeroplane is but the harbinger.
+
+And the Pilot's soul is refreshed, and his vision, now exalted, sees
+the Earth a very garden, even as it appears at that height, with discord
+banished and a happy time come, when the Designer shall have at last
+captured Efficiency, and the Man-who-takes-the-credit is he who has
+earned it, and when kisses are the only things that go by favour.
+
+Now the Pilot anxiously scans the Barograph, which is an instrument much
+the same as the Altimeter; but in this case the expansion of the vacuum
+box causes a pen to trace a line upon a roll of paper. This paper is
+made by clockwork to pass over the point of the pen, and so a curved
+line is made which accurately registers the speed of the ascent in feet
+per minute. No longer is the ascent at the rate of a thousand feet a
+minute, and the Propeller complains to the Engine, "I'm losing my Revs.
+and the Thrust. Buck up with the Power, for the Lift is decreasing,
+though the Weight remains much the same."
+
+Quoth the Engine: "I strangle for Air. A certain proportion, and that
+of right density, I must have to one part of Petrol, in order to give me
+full power and compression, and here at an altitude of ten thousand feet
+the Air is only two-thirds as dense as at sea-level. Oh, where is he who
+will invent a contrivance to keep me supplied with Air of right density
+and quality? It should not be impossible within certain limits."
+
+"We fully agree," said the dying Power and Thrust. "Only maintain Us and
+you shall be surprised at the result. For our enemy Drift decreases in
+respect of distance with the increase of altitude and rarity of air,
+and there is no limit to the speed through space if only our strength
+remains. And with oxygen for Pilot and Passengers and a steeper
+pitch[11] for the Propeller we may then circle the Earth in a day!"
+
+Ah, Reader, smile not unbelievingly, as you smiled but a few years past.
+There may be greater wonders yet. Consider that as the speed increases,
+so does the momentum or stored-up force in the mass of the aeroplane
+become terrific. And, bearing that in mind, remember that with
+altitude gravity decreases. There may yet be literally other worlds to
+conquer.[12]
+
+Now at fifteen thousand feet the conditions are chilly and rare, and the
+Pilot, with thoughts of breakfast far below, exclaims, "High enough!
+I had better get on with the Test." And then, as he depresses the
+Elevator, the Aeroplane with relief assumes its normal horizontal
+position. Then, almost closing the Throttle, the Thrust dies away. Now,
+the nose of the Aeroplane should sink of its own volition, and the craft
+glide downward at flying speed, which is in this case a hundred miles
+an hour. That is what should happen if the Designer has carefully
+calculated the weight of every part and arranged for the centre of
+gravity to be just the right distance in front of the centre of lift.
+Thus is the Aeroplane "nose-heavy" as a glider, and just so to a degree
+ensuring a speed of glide equal to its flying speed. And the Air Speed
+Indicator is steady at one hundred miles an hour, and "That's all
+right!" exclaims the Pilot. "And very useful, too, in a fog or a cloud,"
+he reflects, for then he can safely leave the angle of the glide to
+itself, and give all his attention, and he will need it all, to keeping
+the Aeroplane horizontal from wing-tip to wing-tip, and to keeping
+it straight on its course. The latter he will manage with the rudder,
+controlled by his feet, and the Compass will tell him whether a straight
+course is kept. The former he will control by the Ailerons, or
+little wings hinged to the tips of the planes, and the bubble in the
+Inclinometer in front of him must be kept in the middle.
+
+A Pilot, being only human, may be able to do two things at once, but
+three is a tall order, so was this Pilot relieved to find the Design not
+at fault and his craft a "natural glider." To correct this nose-heavy
+tendency when the Engine is running, and descent not required, the
+centre of Thrust is arranged to be a little below the centre of Drift or
+Resistance, and thus acts as a counter-balance.
+
+But what is this stream of bad language from the Exhaust Pipe,
+accompanied by gouts of smoke and vapour? The Engine, now revolving at
+no more than one-tenth its normal speed, has upset the proportion of
+petrol to air, and combustion is taking place intermittently or in the
+Exhaust Pipe, where it has no business to be.
+
+"Crash, Bang, Rattle----!----!----!" and worse than that, yells the
+Exhaust, and the Aeroplane, who is a gentleman and not a box kite,[13]
+remonstrates with the severity of a Senior Officer. "See the Medical
+Officer, you young Hun. Go and see a doctor. Vocal diarrhoea, that's
+your complaint, and a very nasty one too. Bad form, bad for discipline,
+and a nuisance in the Mess. What's your Regiment? Special Reserve, you
+say? Humph! Sounds like Secondhand Bicycle Trade to me!"
+
+Now the Pilot decides to change the straight gliding descent to a spiral
+one, and, obedient to the Rudder, the Aeroplane turns to the left. But
+the Momentum (two tons at 100 miles per hour is no small affair) heavily
+resents this change of direction, and tries its level best to prevent
+it and to pull the machine sideways and outwards from its spiral
+course--that is, to make it "side-skid" outwards. But the Pilot deflects
+the Ailerons and "banks" up the planes to the correct angle, and, the
+Aeroplane skidding sideways and outwards, the lowest surfaces of
+the planes press up against the air until the pressure equals the
+centrifugal force of the Momentum, and the Aeroplane spirals steadily
+downwards.
+
+Down, down, down, and the air grows denser, and the Pilot gulps largely,
+filling his lungs with the heavier air to counteract the increasing
+pressure from without. Down through a gap in the clouds, and the
+Aerodrome springs into view, appearing no larger than a saucer, and the
+Pilot, having by now got the "feel" of the Controls, proceeds to put
+the Aeroplane through its paces. First at its Maximum Angle, staggering
+along tail-down and just maintaining horizontal flight; then a dive at
+far over flying speed, finishing with a perfect loop; then sharp turns
+with attendant vertical "banks" and then a wonderful switchback
+flight, speeding down at a hundred and fifty miles an hour with short,
+exhilarating ascents at the rate of two thousand feet a minute!
+
+All the parts are now working well together. Such wires as were before
+in undue tension have secured relief by slightly elongating their loops,
+and each one is now doing its bit, and all are sharing the burden of
+work together.
+
+The Struts and the Spars, which felt so awkward at first, have bedded
+themselves in their sockets, and are taking the compression stresses
+uncomplainingly.
+
+The Control Cables of twisted wire, a bit tight before, have slightly
+lengthened by perhaps the eighth of an inch, and, the Controls instantly
+responding to the delicate touch of the Pilot, the Aeroplane, at the
+will of its Master, darts this way and that way, dives, loops, spirals,
+and at last, in one long, magnificent glide, lands gently in front of
+its shed.
+
+"Well, what result?" calls the Flight-Commander to the Pilot.
+
+"A hundred miles an hour and a thousand feet a minute," he briefly
+replies.
+
+"And a very good result too," says the Aeroplane, complacently, as he is
+carefully wheeled into his shed.
+
+
+That is the way Aeroplanes speak to those who love them and understand
+them. Lots of Pilots know all about it, and can spin you wonderful
+yarns, much better than this one, if you catch them in a confidential
+mood--on leave, for instance, and after a good dinner.
+
+
+
+
+PART IV. 'CROSS COUNTRY
+
+The Aeroplane had been designed and built, and tested in the air, and
+now stood on the Aerodrome ready for its first 'cross-country flight.
+
+It had run the gauntlet of pseudo-designers, crank inventors, press
+"experts," and politicians; of manufacturers keen on cheap work and
+large profits; of poor pilots who had funked it, and good pilots who had
+expected too much of it. Thousands of pounds had been wasted on it,
+many had gone bankrupt over it, and others it had provided with safe fat
+jobs.
+
+Somehow, and despite every conceivable obstacle, it had managed to
+muddle through, and now it was ready for its work. It was not perfect,
+for there were fifty different ways in which it might be improved, some
+of them shamefully obvious. But it was fairly sound mechanically, had a
+little inherent stability, was easily controlled, could climb a thousand
+feet a minute, and its speed was a hundred miles an hour. In short,
+quite a creditable machine, though of course the right man had not got
+the credit.
+
+It is rough, unsettled weather with a thirty mile an hour wind on the
+ground, and that means fifty more or less aloft. Lots of clouds at
+different altitudes to bother the Pilot, and the air none to clear for
+the observation of landmarks.
+
+As the Pilot and Observer approach the Aeroplane the former is clearly
+not in the best of tempers. "It's rotten luck," he is saying, "a blank
+shame that I should have to take this blessed 'bus and join X Reserve
+Squadron, stationed a hundred and fifty miles from anywhere; and just
+as I have licked my Flight into shape. Now some slack blighter will, I
+suppose, command it and get the credit of all my work!"
+
+"Shut up, you grouser," said the Observer. "Do you think you're the only
+one with troubles? Haven't I been through it too? Oh! I know all about
+it! You're from the Special Reserve and your C.O. doesn't like your
+style of beauty, and you won't lick his boots, and you were a bit of a
+technical knut in civil life, but now you've jolly well got to know less
+than those senior to you. Well! It's a jolly good experience for most of
+us. Perhaps conceit won't be at quite such a premium after this war. And
+what's the use of grousing? That never helped anyone. So buck up, old
+chap. Your day will come yet. Here's our machine, and I must say it
+looks a beauty!"
+
+And, as the Pilot approaches the Aeroplane, his face brightens and he
+soon forgets his troubles as he critically inspects the craft which is
+to transport him and the Observer over the hills and far away. Turning
+to the Flight-Sergeant he inquires, "Tank full of petrol and oil?"
+
+"Yes, sir," he replies, "and everything else all correct. Propeller,
+engine, and body covers on board, sir; tool kit checked over and in the
+locker; engine and Aeroplane logbooks written up, signed, and under your
+seat; engine revs. up to mark, and all the control cables in perfect
+condition and tension."
+
+"Very good," said the Pilot; and then turning to the Observer, "Before
+we start you had better have a look at the course I have mapped out.
+
+"A is where we stand and we have to reach B, a hundred and fifty miles
+due North. I judge that, at the altitude we shall fly, there will be
+an East wind, for although it is not quite East on the ground it is
+probably about twenty degrees different aloft, the wind usually moving
+round clockways to about that extent. I think that it is blowing at the
+rate of about fifty miles an hour, and I therefore take a line on the
+map to C, fifty miles due West of A. The Aeroplane's speed is a hundred
+miles an hour, and so I take a line of one hundred miles from C to D.
+Our compass course will then be in the direction A--E, which is always a
+line parallel to C--D. That is, to be exact, it will be fourteen degrees
+off the C--D course, as, in this part of the globe, there is that much
+difference between the North and South lines on the map and the magnetic
+North to which the compass needle points. If the compass has an error,
+as it may have of a few degrees, that, too, must be taken into account,
+and the deviation or error curve on the dashboard will indicate it.
+
+"The Aeroplane will then always be pointing in a direction parallel to
+A--E, but, owing to the side wind, it will be actually travelling over
+the course A--B, though in a rather sideways attitude to that course.
+
+"The distance we shall travel over the A--B course in one hour is A--D.
+That is nearly eighty-seven miles, so we ought to accomplish our journey
+of a hundred and fifty miles in about one and three-quarter hours.
+
+"I hope that's quite clear to you. It's a very simple way of calculating
+the compass course, and I always do it like that."
+
+"Yes, that's plain enough. You have drafted what engineers call 'a
+parallelogram of forces'; but suppose you have miscalculated the
+velocity of the wind, or that it should change in velocity or
+direction?"
+
+"Well, that of course will more or less alter matters," replies the
+Pilot. "But there are any number of good landmarks such as lakes,
+rivers, towns, and railway lines. They will help to keep us on the right
+course, and the compass will, at any rate, prevent us from going far
+astray when between them."
+
+"Well, we'd better be off, old chap. Hop aboard." This from the Observer
+as he climbs into the front seat from which he will command a good view
+over the lower plane; and the Pilot takes his place in the rear seat,
+and, after making himself perfectly comfortable, fixing his safety belt,
+and moving the control levers to make sure that they are working freely,
+he gives the signal to the Engine Fitter to turn the propeller and so
+start the engine.
+
+Round buzzes the Propeller, and the Pilot, giving the official signal,
+the Aeroplane is released and rolls swiftly over the ground in the teeth
+of the gusty wind.
+
+In less than fifty yards it takes to the air and begins to climb rapidly
+upwards, but how different are the conditions to the calm morning of
+yesterday! If the air were visible it would be seen to be acting in the
+most extraordinary manner; crazily swirling, lifting and dropping, gusts
+viciously colliding--a mad phantasmagoria of forces!
+
+Wickedly it seizes and shakes the Aeroplane; then tries to turn it over
+sideways; then instantly changes its mind and in a second drops it into
+a hole a hundred feet deep, and if it were not for his safety belt the
+Pilot might find his seat sinking away from beneath him.
+
+Gusts strike the front of the craft like so many slaps in the face; and
+others, with the motion of mountainous waves, sometimes lift it hundreds
+of feet in a few seconds, hoping to see it plunge over the summit in a
+death-dive--and so it goes on, but the Pilot, perfectly at one with
+his mount and instantly alert to its slightest motion, is skilfully and
+naturally making perhaps fifty movements a minute of hand and feet;
+the former lightly grasping the "joy-stick" which controls the Elevator
+hinged to the tail, and also the Ailerons or little wings hinged to the
+wing-tips; and the latter moving the Rudder control-bar.
+
+A strain on the Pilot? Not a bit of it, for this is his Work which he
+loves and excels in; and given a cool head, alert eye, and a sensitive
+touch for the controls, what sport can compare with these ever-changing
+battles of the air?
+
+The Aeroplane has all this time been climbing in great wide circles,
+and is now some three thousand feet above the Aerodrome which from such
+height looks absurdly small. The buildings below now seem quite squat;
+the hills appear to have sunk away into the ground, and the whole
+country below, cut up into diminutive fields, has the appearance of
+having been lately tidied and thoroughly spring-cleaned! A doll's
+country it looks, with tiny horses and cows ornamenting the fields
+and little model motor-cars and carts stuck on the roads, the latter
+stretching away across the country like ribbons accidentally dropped.
+
+At three thousand feet altitude the Pilot is satisfied that he is now
+sufficiently high to secure, in the event of engine failure, a
+long enough glide to earth to enable him to choose and reach a good
+landing-place; and, being furthermore content with the steady running of
+the engine, he decides to climb no more but to follow the course he has
+mapped out. Consulting the compass, he places the Aeroplane on the A--E
+course and, using the Elevator, he gives his craft its minimum angle of
+incidence at which it will just maintain horizontal flight and secure
+its maximum speed.
+
+Swiftly he speeds away, and few thoughts he has now for the changing
+panorama of country, cloud, and colour. Ever present in his mind are the
+three great 'cross-country queries. "Am I on my right course? Can I see
+a good landing-ground within gliding distance?" And "How is the Engine
+running?"
+
+Keenly both he and the Observer compare their maps with the country
+below. The roads, khaki-coloured ribbons, are easily seen but are not
+of much use, for there are so many of them and they all look alike from
+such an altitude.
+
+Now where can that lake be which the map shows so plainly? He feels that
+surely he should see it by now, and has an uncomfortable feeling that
+he is flying too far West. What pilot is there indeed who has not many
+times experienced such unpleasant sensation? Few things in the air
+can create greater anxiety. Wisely, however, he sticks to his compass
+course, and the next minute he is rewarded by the sight of the lake,
+though indeed he now sees that the direction of his travel will not take
+him over it, as should be the case if he were flying over the shortest
+route to his destination. He must have slightly miscalculated the
+velocity or direction of the side-wind.
+
+"About ten degrees off," he mutters, and, using the Rudder, corrects his
+course accordingly.
+
+Now he feels happier and that he is well on his way. The gusts, too,
+have ceased to trouble him as, at this altitude, they are not nearly so
+bad as they were near the ground the broken surface of which does much
+to produce them; and sometimes for miles he makes but a movement or two
+of the controls.
+
+The clouds just above race by with dizzy and uniform speed; the country
+below slowly unrolls, and the steady drone of the Engine is almost
+hypnotic in effect. "Sleep, sleep, sleep," it insidiously suggests.
+"Listen to me and watch the clouds; there's nothing else to do. Dream,
+dream, dream of speeding through space for ever, and ever, and ever; and
+rest, rest, rest to the sound of my rhythmical hum. Droning on and on,
+nothing whatever matters. All things now are merged into speed through
+space and a sleepy monotonous d-d-r-r-o-o-n-n-e - - - - -." But the
+Pilot pulls himself together with a start and peers far ahead in search
+of the next landmark. This time it is a little country town, red-roofed
+his map tells him, and roughly of cruciform shape; and, sure enough,
+there in the right direction are the broken outlines of a few red roofs
+peeping out from between the trees.
+
+Another minute and he can see this little town, a fairy town it appears,
+nestling down between the hills with its red roofs and picturesque
+shape, a glowing and lovely contrast with the dark green of the
+surrounding moors.
+
+So extraordinarily clean and tidy it looks from such a height, and
+laid out in such orderly fashion with perfectly defined squares, parks,
+avenues, and public buildings, it indeed appears hardly real, but rather
+as if it has this very day materialized from some delightful children's
+book!
+
+Every city and town you must know has its distinct individuality to the
+Pilot's eye. Some are not fairy places at all, but great dark ugly blots
+upon the fair countryside, and with tall shafts belching forth murky
+columns of smoke to defile clean space. Others, melancholy-looking
+masses of grey, slate-roofed houses, are always sad and dispirited;
+never welcoming the glad sunshine, but ever calling for leaden skies
+and a weeping Heaven. Others again, little coquettes with village green,
+white palings everywhere, bright gravel roads, and an irrepressible air
+of brightness and gaiety.
+
+Then there are the rivers, silvery streaks peacefully winding far, far
+away to the distant horizon; they and the lakes the finest landmarks
+the Pilot can have. And the forests. How can I describe them? The trees
+cannot be seen separately, but merge altogether into enormous irregular
+dark green masses sprawling over the country, and sometimes with great
+ungainly arms half encircling some town or village; and the wind passing
+over the foliage at times gives the forest an almost living appearance,
+as of some great dragon of olden times rousing itself from slumber to
+devour the peaceful villages which its arms encircle.
+
+And the Pilot and Observer fly on and on, seeing these things and many
+others which baffle my poor skill to describe--things, dear Reader, that
+you shall see, and poets sing of, and great artists paint in the days
+to come when the Designer has captured Efficiency. Then, and the time
+is near, shall you see this beautiful world as you have never seen it
+before, the garden it is, the peace it breathes, and the wonder of it.
+
+The Pilot, flying on, is now anxiously looking for the railway line
+which midway on his journey should point the course. Ah! There it is
+at last, but suddenly (and the map at fault) it plunges into the
+earth! Well the writer remembers when that happened to him on a long
+'cross-country flight in the early days of aviation. Anxiously he
+wondered "Are tunnels always straight?" and with what relief, keeping on
+a straight course, he picked up the line again some three miles farther
+on!
+
+Now at last the Pilot sees the sea, just a streak on the north-eastern
+horizon, and he knows that his flight is two-thirds over. Indeed, he
+should have seen it before, but the air is none too clear, and he is not
+yet able to discern the river which soon should cross his path. As he
+swiftly speeds on the air becomes denser and denser with what he fears
+must be the beginning of a sea-fog, perhaps drifting inland along the
+course of the river. Now does he feel real anxiety, for it is the DUTY
+of a Pilot to fear fog, his deadliest enemy. Fog not only hides the
+landmarks by which he keeps his course, but makes the control of the
+Aeroplane a matter of the greatest difficulty. He may not realize
+it, but, in keeping his machine on an even keel, he is unconsciously
+balancing it against the horizon, and with the horizon gone he is
+lost indeed. Not only that, but it also prevents him from choosing his
+landing-place, and the chances are that, landing in a fog, he will smash
+into a tree, hedge, or building, with disastrous results. The best and
+boldest pilot 'wares a fog, and so this one, finding the conditions
+becoming worse and yet worse, and being forced to descend lower and
+lower in order to keep the earth within view, wisely decides to choose a
+landing-place while there is yet time to do so.
+
+Throttling down the power of the engine he spirals downwards, keenly
+observing the country below. There are plenty of green fields to lure
+him, and his great object is to avoid one in which the grass is long,
+for that would bring his machine to a stop so suddenly as to turn it
+over; or one of rough surface likely to break the under-carriage. Now is
+perfect eyesight and a cool head indispensable. He sees and decides upon
+a field and, knowing his job, he sticks to that field with no change
+of mind to confuse him. It is none too large, and gliding just over the
+trees and head on to the wind he skilfully "stalls" his machine; that
+is, the speed having decreased sufficiently to avoid such a manoeuvre
+resulting in ascent, he, by means of the Elevator, gives the Aeroplane
+as large an angle of incidence as possible, and the undersides of the
+planes meeting the air at such a large angle act as an air-brake, and
+the Aeroplane, skimming over the ground, lessens its speed and finally
+stops just at the farther end of the field.
+
+Then, after driving the Aeroplane up to and under the lee of the hedge,
+he stops the engine, and quickly lashing the joy-stick fast in order
+to prevent the wind from blowing the controlling surfaces about and
+possibly damaging them, he hurriedly alights. Now running to the tail he
+lifts it up on to his shoulder, for the wind has become rough indeed and
+there is danger of the Aeroplane becoming unmanageable. By this action
+he decreases the angle at which the planes are inclined to the wind and
+so minimizes the latter's effect upon them. Then to the Observer, "Hurry
+up, old fellow, and try to find some rope, wire, or anything with which
+to picket the machine. The wind is rising and I shan't be able to hold
+the 'bus steady for long. Don't forget the wire-cutters. They're in the
+tool kit." And the Observer rushes off in frantic haste, before long
+triumphantly returning with a long length of wire from a neighbouring
+fence. Blocking up the tail with some debris at hand, they soon succeed,
+with the aid of the wire, in stoutly picketing the Aeroplane to the
+roots of the high hedge in front of it; done with much care, too,
+so that the wire shall not fray the fabric or set up dangerous
+bending-stresses in the woodwork. Their work is not done yet, for the
+Observer remarking, "I don't like the look of this thick weather and
+rather fear a heavy rain-storm," the Pilot replies, "Well, it's a
+fearful bore, but the first rule of our game is never to take an
+unnecessary risk, so out with the engine and body covers."
+
+Working with a will they soon have the engine and the open part of the
+body which contains the seats, controls, and instruments snugly housed
+with their waterproof covers, and the Aeroplane is ready to weather the
+possible storm.
+
+Says the Observer, "I'm remarkably peckish, and methinks I spy the
+towers of one of England's stately homes showing themselves just beyond
+that wood, less than a quarter of a mile away. What ho! for a raid. What
+do you say?"
+
+"All right, you cut along and I'll stop here, for the Aeroplane must not
+be left alone. Get back as quickly as possible."
+
+And the Observer trots off, leaving the Pilot filling his pipe and
+anxiously scrutinizing the weather conditions. Very thick it is now, but
+the day is yet young, and he has hopes of the fog lifting sufficiently
+to enable the flight to be resumed. A little impatiently he awaits the
+return of his comrade, but with never a doubt of the result, for the
+hospitality of the country house is proverbial among pilots! What old
+hand among them is there who cannot instance many a forced landing made
+pleasant by such hospitality? Never too late or too early to help with
+food, petrol, oil, tools, and assistants. Many a grateful thought has
+the writer for such kind help given in the days before the war (how long
+ago they seem!), when aeroplanes were still more imperfect than they are
+now, and involuntary descents often a part of 'cross-country flying.
+
+Ah! those early days! How fresh and inspiring they were! As one started
+off on one's first 'cross-country flight, on a machine the first of its
+design, and with everything yet to learn, and the wonders of the air yet
+to explore; then the joy of accomplishment, the dreams of Efficiency,
+the hard work and long hours better than leisure; and what a field of
+endeavour--the realms of space to conquer! And the battle still goes on
+with ever-increasing success. Who is bold enough to say what its limits
+shall be?
+
+So ruminates this Pilot-Designer, as he puffs at his pipe, until his
+reverie is abruptly disturbed by the return of the Observer.
+
+"Wake up, you AIRMAN," the latter shouts. "Here's the very thing the
+doctor ordered! A basket of first-class grub and something to keep the
+fog out, too."
+
+"Well, that's splendid, but don't call me newspaper names or you'll
+spoil my appetite!"
+
+Then, with hunger such as only flying can produce, they appreciatively
+discuss their lunch, and with many a grateful thought for the
+donors--and they talk shop. They can't help it, and even golf is a poor
+second to flight talk. Says the Pilot, who must have his grievance,
+"Just observe where I managed to stop the machine. Not twenty feet from
+this hedge! A little more and we should have been through it and into
+Kingdom Come! I stalled as well as one could, but the tail touched
+the ground and so I could not give the Aeroplane any larger angle of
+incidence. Could I have given it a larger angle, then the planes would
+have become a much more effective air-brake, and we should have come to
+rest in a much shorter distance. It's all the fault of the tail. There's
+hardly a type of Aeroplane in existence in which the tail could not be
+raised several feet, and that would make all the difference. High tails
+mean a large angle of incidence when the machine touches ground and,
+with enough angle, I'll guarantee to safely land the fastest machine in
+a five-acre field. You can, I am sure, imagine what a difference that
+would make where forced landings are concerned!" Then rapidly sketching
+in his notebook, he shows the Observer the following illustration:
+
+"That's very pretty," said the Observer, "but how about Mechanical
+Difficulties, and Efficiency in respect of Flight? And, anyway, why
+hasn't such an obvious thing been done already?"
+
+"As regards the first part of your question I assure you that there's
+nothing in it, and I'll prove it to you as follows----"
+
+"Oh! That's all right, old chap. I'll take your word for it," hurriedly
+replies the Observer, whose soul isn't tuned to a technical key.
+
+"As regards the latter part of your inquiry," went on the Pilot, a
+little nettled at having such a poor listener, "it's very simple.
+Aeroplanes have 'just growed' like Topsy, and they consequently contain
+this and many another relic of early day design when Aeroplanes were
+more or less thrown together and anything was good enough that could get
+off the ground."
+
+"By Jove," interrupts the Observer, "I do believe the fog is lifting.
+Hadn't we better get the engine and body covers off, just in case it's
+really so?"
+
+"I believe you're right. I am sure those hills over there could not
+be seen a few minutes ago, and look--there's sunshine over there. We'd
+better hurry up."
+
+Ten minutes' hard work and the covers are off, neatly folded and stowed
+aboard; the picketing wires are cast adrift, and the Pilot is once more
+in his seat. The Aeroplane has been turned to face the other end of the
+field, and, the Observer swinging round the propeller, the engine is
+awake again and slowly ticking over. Quickly the Observer climbs into
+his seat in front of the Pilot, and, the latter slightly opening the
+throttle, the Aeroplane leisurely rolls over the ground towards the
+other end of the field, from which the ascent will be made.
+
+Arriving there the Pilot turns the Aeroplane in order to face the wind
+and thus secure a quick "get-off." Then he opens the throttle fully and
+the mighty voice of the Engine roars out "Now see me clear that hedge!"
+and the Aeroplane races forward at its minimum angle of incidence. Tail
+up, and with ever-increasing speed, it rushes towards the hedge under
+the lee of which it has lately been at rest; and then, just as the
+Observer involuntarily pulls back an imaginary "joy-stick," the Pilot
+moves the real one and places the machine at its best climbing angle.
+Like a living thing it responds, and instantly leaves the ground,
+clearing the hedge like a--well, like an Aeroplane with an excellent
+margin of lift. Upwards it climbs with even and powerful lift, and the
+familiar scenes below again gladden the eyes of the Pilot. Smaller and
+more and more squat grow the houses and hills; more and more doll-like
+appear the fields which are clearly outlined by the hedges; and soon the
+country below is easily identified with the map. Now they can see the
+river before them and a bay of the sea which must be crossed or skirted.
+The fog still lingers along the course of the river and between the
+hills, but is fast rolling away in grey, ghost-like masses. Out to sea
+it obscures the horizon, making it difficult to be sure where water ends
+and fog begins, and creating a strange, rather weird effect by which
+ships at a certain distance appear to be floating in space.
+
+Now the Aeroplane is almost over the river, and the next instant it
+suddenly drops into a "hole in the air." With great suddenness it
+happens, and for some two hundred feet it drops nose-down and tilted
+over sideways; but the Pilot is prepared and has put his craft on an
+even keel in less time than it takes to tell you about it; for well he
+knows that he must expect such conditions when passing over a shore
+or, indeed, any well-defined change in the composition of the earth's
+surface. Especially is this so on a hot and sunny day, for then the warm
+surface of the earth creates columns of ascending air, the speed of the
+ascent depending upon the composition of the surface. Sandy soil, for
+instance, such as borders this river produces a quickly ascending column
+of air, whereas water and forests have not such a marked effect. Thus,
+when our Aeroplane passed over the shore of the river, it suddenly lost
+the lift due to the ascending air produced by the warm sandy soil, and
+it consequently dropped just as if it had fallen into a hole.
+
+Now the Aeroplane is over the bay and, the sea being calm, the Pilot
+looks down, down through the water, and clearly sees the bottom,
+hundreds of feet below the surface. Down through the reflection of the
+blue sky and clouds, and one might think that is all, but it isn't. Only
+those who fly know the beauties of the sea as viewed from above;
+its dappled pearly tints; its soft dark blue shadows; the beautiful
+contrasts of unusual shades of colour which are always differing and
+shifting with the changing sunshine and the ever moving position of the
+aerial observer. Ah! for some better pen than mine to describe these
+things! One with glowing words and a magic rhythm to express the wonders
+of the air and the beauty of the garden beneath--the immensity of the
+sea--the sense of space and of one's littleness there--the realization
+of the Power moving the multitudes below--the exaltation of spirit
+altitude produces--the joy of speed. A new world of sensation!
+
+Now the bay is almost crossed and the Aerodrome at B can be
+distinguished.
+
+On the Aerodrome is a little crowd waiting and watching for the arrival
+of the Aeroplane, for it is of a new and improved type and its first
+'cross-country performance is of keen interest to these men; men who
+really know something about flight.
+
+There is the Squadron Commander who has done some real flying in
+his time; several well-seasoned Flight-Commanders; a dozen or more
+Flight-Lieutenants; a knowledgeable Flight-Sergeant; a number of Air
+Mechanics, and, a little on one side and almost unnoticed, the Designer.
+
+"I hope they are all right," said someone, "and that they haven't had
+difficulties with the fog. It rolled up very quickly, you know."
+
+"Never fear," remarked a Flight-Commander. "I know the Pilot well and
+he's a good 'un; far too good to carry on into a fog."
+
+"They say the machine is really something out of the ordinary," said
+another, "and that, for once, the Designer has been allowed full play;
+that he hasn't been forced to unduly standardize ribs, spars, struts,
+etc., and has more or less had his own way. I wonder who he is. It seems
+strange we hear so little of him."
+
+"Ah! my boy. You do a bit more flying and you'll discover that things
+are not always as they appear from a distance!"
+
+"There she is, sir!" cries the Flight-Sergeant. "Just a speck over the
+silvery corner of that cloud."
+
+A tiny speck it looks, some six miles distant and three thousand feet
+high; but, racing along, it rapidly appears larger and soon its outlines
+can be traced and the sunlight be seen playing upon the whirling
+propeller.
+
+Now the distant drone of the engine can be heard, but not for long, for
+suddenly it ceases and, the nose of the Aeroplane sinking, the craft
+commences gliding downwards.
+
+"Surely too far away," says a subaltern. "It will be a wonderful machine
+if, from that distance and height, it can glide into the Aerodrome."
+And more than one express the opinion that it cannot be done; but
+the Designer smiles to himself, yet with a little anxiety, for his
+reputation is at stake, and Efficiency, the main reward he desires, is
+perhaps, or perhaps not, at last within his grasp!
+
+Swiftly the machine glides downwards towards them, and it can now be
+seen how surprisingly little it is affected by the rough weather and
+gusts; so much so that a little chorus of approval is heard.
+
+"Jolly good gliding angle," says someone; and another, "Beautifully
+quick controls, what?" and from yet another, "By Jove! The Pilot must be
+sure of the machine. Look, he's stopped the engine entirely."
+
+Then the Aeroplane with noiseless engine glides over the boundary of the
+Aerodrome, and, with just a soft soughing sound from the air it cleaves,
+lands gently not fifty yards from the onlookers.
+
+"Glad to see you," says the Squadron Commander to the Pilot. "How do you
+like the machine?" And the Pilot replies:
+
+"I never want a better one, sir. It almost flies itself!"
+
+And the Designer turns his face homewards and towards his beloved
+drawing-office; well satisfied, but still dreaming dreams of the future
+and... looking far ahead whom should he see but Efficiency at last
+coming towards him! And to him she is all things. In her hair is the
+morning sunshine; her eyes hold the blue of the sky, and on her cheeks
+is the pearly tint of the clouds as seen from above. The passion of
+speed, the lure of space, the sense of power, and the wonder of the
+future... all these things she holds for him.
+
+"Ah!" he cries. "You'll never leave me now, when at last there is no one
+between us?"
+
+And Efficiency, smiling and blushing, but practical as ever, says:
+
+"And you will never throw those Compromises in my face?"
+
+"My dear, I love you for them! Haven't they been my life ever since I
+began striving for you ten long years ago?"
+
+And so they walked off very happily, arm-in-arm together; and if this
+hasn't bored you and you'd like some more of the same sort of thing, I'd
+just love to tell you some day of the wonderful things they accomplish
+together, and of what they dream the future holds in store.
+
+And that's the end of the Prologue.
+
+
+
+
+CHAPTER I. FLIGHT
+
+Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum.
+It therefore obeys Newton's laws[14] and resists movement. It is that
+resistance or reaction which makes flight possible.
+
+Flight is secured by driving through the air a surface[15] inclined
+upwards and towards the direction of motion.
+
+S = Side view of surface.
+
+M = Direction of motion.
+
+CHORD.--The Chord is, for practical purposes, taken to be a straight
+line from the leading edge of the surface to its trailing edge.
+
+N = A line through the surface starting from its trailing edge. The
+position of this line, which I call the Neutral Lift Line, is found by
+means of wind-tunnel research, and it varies with differences in
+the camber (curvature) of surfaces. In order to secure flight, the
+inclination of the surface must be such that the neutral lift line makes
+an angle with and ABOVE the line of motion. If it is coincident with M,
+there is no lift. If it makes an angle with M and BELOW it, then there
+is a pressure tending to force the surface down.
+
+I = Angle of Incidence. This angle is generally defined as the angle the
+chord makes with the direction of motion, but that is a bad definition,
+as it leads to misconception. The angle of incidence is best described
+as the angle the neutral lift line makes with the direction of motion
+relative to the air. You will, however, find that in nearly all rigging
+specifications the angle of incidence is taken to mean the angle the
+chord makes with a line parallel to the propeller thrust. This is
+necessary from the point of view of the practical mechanic who has to
+rig the aeroplane, for he could not find the neutral lift line, whereas
+he can easily find the chord. Again, he would certainly be in doubt as
+to "the direction of motion relative to the air," whereas he can easily
+find a line parallel to the propeller thrust. It is a pity, however,
+that these practical considerations have resulted in a bad definition
+of the angle of incidence becoming prevalent, a consequence of which has
+been the widespread fallacy that flight may be secured with a negative
+inclination of the surface. Flight may conceivably be secured with a
+negative angle of chord, but never with a negative inclination of the
+surface. All this is only applicable to cambered surfaces. In the case
+of flat surfaces the neutral lift line coincides with the chord and the
+definition I have criticised adversely is then applicable. Flat lifting
+surfaces are, however, never used.
+
+The surface acts upon the air in the following manner:
+
+
+As the bottom of the surface meets the air, it compresses it and
+accelerates it DOWNWARDS. As a result of this definite action there is,
+of course, an equal and opposite reaction UPWARDS.
+
+The top surface, in moving forward, tends to leave the air behind
+it, thus creating a semi-vacuum or rarefied area over the top of the
+surface. Consequently the pressure of air on the top of the surface
+is decreased, thus assisting the reaction below to lift the surface
+UPWARDS.
+
+The reaction increases approximately as the square of the velocity. It
+is the result of (1) the mass of air engaged, and (2) the velocity and
+consequent force with which the surface engages the air. If the reaction
+was produced by only one of those factors it would increase in direct
+proportion to the velocity, but, since it is the product of both
+factors, it increases as V(2S).
+
+Approximately three-fifths of the reaction is due to the decrease of
+density (and consequent decrease of downward pressure) on the top of the
+surface; and only some two-fifths is due to the upward reaction secured
+by the action of the bottom surface upon the air. A practical point in
+respect of this is that, in the event of the fabric covering the surface
+getting into bad condition, it is more likely to strip off the top than
+off the bottom.
+
+The direction of the reaction is approximately at right-angles to the
+chord of the surface, as illustrated above; and it is, in considering
+flight, convenient to divide it into two component parts or values,
+thus:
+
+1. The vertical component of the reaction, i.e., Lift, which is opposed
+to Gravity, i.e., the weight of the aeroplane.
+
+2. The horizontal component, i.e., Drift (sometimes called Resistance),
+to which is opposed the thrust of the propeller.
+
+The direction of the reaction is, of course, the resultant of the forces
+Lift and Drift.
+
+The Lift is the useful part of the reaction, for it lifts the weight of
+the aeroplane.
+
+The Drift is the villain of the piece, and must be overcome by the
+Thrust in order to secure the necessary velocity to produce the
+requisite Lift for flight.
+
+DRIFT.--The drift of the whole aeroplane (we have considered only the
+lifting surface heretofore) may be conveniently divided into three
+parts, as follows:
+
+Active Drift, which is the drift produced by the lifting surfaces.
+
+Passive Drift, which is the drift produced by all the rest of the
+aeroplane--the struts, wires, fuselage, under-carriage, etc., all of
+which is known as "detrimental surface."
+
+Skin Friction, which is the drift produced by the friction of the air
+with roughnesses of surface. The latter is practically negligible
+having regard to the smooth surface of the modern aeroplane, and its
+comparatively slow velocity compared with, for instance, the velocity of
+a propeller blade.
+
+LIFT-DRIFT RATIO.--The proportion of lift to drift is known as the
+lift-drift ratio, and is of paramount importance, for it expresses the
+efficiency of the aeroplane (as distinct from engine and propeller). A
+knowledge of the factors governing the lift-drift ratio is, as will be
+seen later, an absolute necessity to anyone responsible for the rigging
+of an aeroplane, and the maintenance of it in an efficient and safe
+condition.
+
+Those factors are as follows:
+
+1. Velocity.--The greater the velocity the greater the proportion of
+drift to lift, and consequently the less the efficiency. Considering
+the lifting surfaces alone, both the lift and the (active) drift, being
+component parts of the reaction, increase as the square of the velocity,
+and the efficiency remains the same at all speeds. But, considering the
+whole aeroplane, we must remember the passive drift. It also increases
+as the square of the velocity (with no attendant lift), and, adding
+itself to the active drift, results in increasing the proportion of
+total drift (active + passive) to lift.
+
+But for the increase in passive drift the efficiency of the aeroplane
+would not fall with increasing velocity, and it would be possible, by
+doubling the thrust, to approximately double the speed or lift--a happy
+state of affairs which can never be, but which we may, in a measure,
+approach by doing everything possible to diminish the passive drift.
+
+Every effort is then made to decrease it by "stream-lining," i.e., by
+giving all "detrimental" parts of the aeroplane a form by which they
+will pass through the air with the least possible drift. Even the wires
+bracing the aeroplane together are, in many cases, stream-lined, and
+with a markedly good effect upon the lift-drift ratio. In the case of a
+certain well-known type of aeroplane the replacing of the ordinary wires
+by stream-lined wires added over five miles an hour to the flight speed.
+
+Head-resistance is a term often applied to passive drift, but it is apt
+to convey a wrong impression, as the drift is not nearly so much the
+result of the head or forward part of struts, wires, etc., as it is of
+the rarefied area behind.
+
+Above is illustrated the flow of air round two objects moving in the
+direction of the arrow M.
+
+In the case of A, you will note that the rarefied area DD is of very
+considerable extent; whereas in the case of B, the air flows round it
+in such a way as to meet very closely to the rear of the object, thus
+DECREASING DD.
+
+The greater the rarefied area DD. then, the less the density, and,
+consequently, the less the pressure of air upon the rear of the object.
+The less such pressure, then, the better is head-resistance D able to
+get its work in, and the more thrust will be required to overcome it.
+
+The "fineness" of the stream-line shape, i.e., the proportion of length
+to width, is determined by the velocity--the greater the velocity, the
+greater the fineness. The best degree of fineness for any given velocity
+is found by means of wind-tunnel research.
+
+The practical application of all this is, from a rigging point of view,
+the importance of adjusting all stream-line parts to be dead-on in the
+line of flight, but more of that later on.
+
+2. Angle of Incidence.--The most efficient angle of incidence varies
+with the thrust at the disposal of the designer, the weight to be
+carried, and the climb-velocity ratio desired.
+
+The best angles of incidence for these varying factors are found by
+means of wind-tunnel research and practical trial and error. Generally
+speaking, the greater the velocity the smaller should be the angle of
+incidence, in order to preserve a clean, stream-line shape of rarefied
+area and freedom from eddies. Should the angle be too great for the
+velocity, then the rarefied area becomes of irregular shape with
+attendant turbulent eddies. Such eddies possess no lift value, and since
+it has taken power to produce them, they represent drift and adversely
+affect the lift-drift ratio.
+
+From a rigging point of view, one must presume that every standard
+aeroplane has its lifting surface set at the most efficient angle, and
+the practical application of all this is in taking the greatest possible
+care to rig the surface at the correct angle and to maintain it at such
+angle. Any deviation will adversely affect the lift-drift ratio, i.e.,
+the efficiency.
+
+3. Camber.--(Refer to the second illustration in this chapter.) The
+lifting surfaces are cambered, i.e., curved, in order to decrease the
+horizontal component of the reaction, i.e., the drift.
+
+The bottom camber: If the bottom of the surface was flat, every particle
+of air meeting it would do so with a shock, and such shock would produce
+a very considerable horizontal reaction or drift. By curving it such
+shock is diminished, and the curve should be such as to produce a
+uniform (not necessarily constant) acceleration and compression of the
+air from the leading edge to the trailing edge. Any unevenness in the
+acceleration and compression of the air produces drift.
+
+The top camber: If this was flat it would produce a rarefied area of
+irregular shape. I have already explained the bad effect this has
+upon the lift-drift ratio. The top surface is then curved to produce a
+rarefied area the shape of which shall be as stream-line and free from
+attendant eddies as possible.
+
+The camber varies with the angle of incidence, the velocity, and the
+thickness of the surface. Generally speaking, the greater the velocity,
+the less the camber and angle of incidence. With infinite velocity the
+surface would be set at no angle of incidence (the neutral lift line
+coincident with the direction of motion relative to the air), and would
+be, top and bottom, of pure streamline form--i.e., of infinite fineness.
+This is, of course, carrying theory to absurdity as the surface would
+then cease to exist.
+
+The best cambers for varying velocities, angles of incidence, and
+thicknesses of surface, are found by means of wind-tunnel research.
+The practical application of all this is in taking the greatest care to
+prevent the surface from becoming distorted and thus spoiling the camber
+and consequently the lift-drift ratio.
+
+4. Aspect Ratio.--This is the proportion of span to chord. Thus, if the
+span is, for instance, 50 feet and the chord 5 feet, the surface would
+be said to have an aspect ratio of 10 to 1.
+
+For A GIVEN VELOCITY and A GIVEN AREA of surface, the greater the
+aspect ratio, the greater the reaction. It is obvious, I think, that the
+greater the span, the greater the mass of air engaged, and, as already
+explained, the reaction is partly the result of the mass of air engaged.
+
+Not only that, but, PROVIDED the chord is not decreased to an extent
+making it impossible to secure the best camber owing to the thickness
+of the surface, the greater the aspect ratio, the better the lift-drift
+ratio. The reason of this is rather obscure. It is sometimes advanced
+that it is owing to the "spill" of air from under the wing-tips. With
+a high aspect ratio the chord is less than would otherwise be the case.
+Less chord results in smaller wing-tips and consequently less "spill."
+This, however, appears to be a rather inadequate reason for the high
+aspect ratio producing the high lift-drift ratio. Other reasons are also
+advanced, but they are of such a contentious nature I do not think it
+well to go into them here. They are of interest to designers, but this
+is written for the practical pilot and rigger.
+
+5. Stagger.--This is the advancement of the top surface relative to the
+bottom surface, and is not, of course, applicable to a single surface,
+i.e., a monoplane. In the case of a biplane having no stagger, there
+will be "interference" and consequent loss of Efficiency unless the
+gap between the top and bottom surfaces is equal to not less than 1 1/2
+times the chord. If less than that, the air engaged by the bottom of the
+top surface will have a tendency to be drawn into the rarefied area over
+the top of the bottom surface, with the result that the surfaces will
+not secure as good a reaction as would otherwise be the case.
+
+It is not practicable to have a gap of much more than a distance equal
+to the chord, owing to the drift produced by the great length of struts
+and wires such a large gap would necessitate. By staggering the top
+surface forward, however, it is removed from the action of the lower
+surface and engages undisturbed air, with the result that the efficiency
+can in this way be increased by about 5 per cent. Theoretically the top
+plane should be staggered forward for a distance equal to about 30 per
+cent. of the chord, the exact distance depending upon the velocity
+and angle of incidence; but this is not always possible to arrange
+in designing an aeroplane, owing to difficulties of balance, desired
+position, and view of pilot, observer, etc.
+
+6. Horizontal Equivalent.--The vertical component of the reaction, i.e.,
+lift, varies as the horizontal equivalent (H.E.) of the surface, but
+the drift remains the same. Then it follows that if H.E. grows less, the
+ratio of lift to drift must do the same.
+
+A, B, and C are front views of three surfaces.
+
+A has its full H.E., and therefore, from the point of view from which
+we are at the moment considering efficiency, it has its best lift-drift
+ratio.
+
+B and C both possess the same surface as A, but one is inclined upwards
+from its centre and the other is straight but tilted. For these reasons
+their H.E.'s are, as illustrated, less than in the case of A. That means
+less vertical lift, and, the drift remaining the same (for there is
+the same amount of surface as in A to produce it), the lift-drift ratio
+falls.
+
+THE MARGIN OF POWER is the power available above that necessary to
+maintain horizontal flight.
+
+THE MARGIN OF LIFT is the height an aeroplane can gain in a given time
+and starting from a given altitude. As an example, thus: 1,000 feet the
+first minute, and starting from an altitude of 500 feet above sea-level.
+
+The margin of lift decreases with altitude, owing to the decrease in
+the density of the air, which adversely affects the engine. Provided
+the engine maintained its impulse with altitude, then, if we ignore the
+problem of the propeller, which I will go into later on, the margin of
+lift would not disappear. Moreover, greater velocity for a given power
+would be secured at a greater altitude, owing to the decreased density
+of air to be overcome. After reading that, you may like to light your
+pipe and indulge in dreams of the wonderful possibilities which may
+become realities if some brilliant genius shows us some day how to
+secure a constant power with increasing altitude. I am afraid, however,
+that will always remain impossible; but it is probable that some very
+interesting steps may be taken in that direction.
+
+THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, for
+a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, for
+a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-drift
+ratio is highest. In modern aeroplanes it is that angle of incidence
+possessed by the surface when the axis of the propeller is horizontal.
+
+THE BEST CLIMBING ANGLE is approximately half-way between the maximum
+and the optimum angles.
+
+All present-day aeroplanes are a compromise between Climb and horizontal
+Velocity. We will compare the essentials for two aeroplanes, one
+designed for maximum climb, and the other for maximum velocity.
+
+
+ESSENTIALS FOR MAXIMUM CLIMB:
+
+1. Low velocity, in order to secure the best lift-drift ratio.
+
+2. Having a low velocity, a large surface will be necessary in order to
+engage the necessary mass of air to secure the requisite lift.
+
+3. Since (1) such a climbing machine will move along an upward sloping
+path, and (2) will climb with its propeller thrust horizontal, then a
+large angle relative to the direction of the thrust will be necessary in
+order to secure the requisite angle relative to the direction of motion.
+
+The propeller thrust should be always horizontal, because the most
+efficient flying-machine (having regard to climb OR velocity) has, so
+far, been found to be an arrangement of an inclined surface driven by
+a HORIZONTAL thrust--the surface lifting the weight, and the thrust
+overcoming the drift. This is, in practice, a far more efficient
+arrangement than the helicopter, i.e., the air-screw revolving about
+a vertical axis and producing a thrust opposed to gravity. If, when
+climbing, the propeller thrust is at such an angle as to tend to haul
+the aeroplane upwards, then it is, in a measure, acting as a helicopter,
+and that means inefficiency. The reason of a helicopter being
+inefficient in practice is due to the fact that, owing to mechanical
+difficulties, it is impossible to construct within a reasonable weight
+an air-screw of the requisite dimensions. That being so, it would be
+necessary, in order to absorb the power of the engine, to revolve the
+comparatively small-surfaced air screw at an immensely greater velocity
+than that of the aeroplane's surface. As already explained, the
+lift-drift ratio falls with velocity on account of the increase in
+passive drift. This applies to a blade of a propeller or air-screw,
+which is nothing but a revolving surface set at angle of incidence, and
+which it is impossible to construct without a good deal of detrimental
+surface near the central boss.
+
+4. The velocity being low, then it follows that for that reason also the
+angle of incidence should be comparatively large.
+
+5. Camber.--Since such an aeroplane would be of low velocity, and
+therefore possess a large angle of incidence, a large camber would be
+necessary.
+
+Let us now consider the essentials for an aeroplane of maximum velocity
+for its power, and possessing merely enough lift to get off the ground,
+but no margin of lift.
+
+1. Comparatively HIGH VELOCITY.
+
+2. A comparatively SMALL SURFACE, because, being of greater velocity
+than the maximum climber, a greater mass of air will be engaged for
+a given surface and time, and therefore a smaller surface will be
+sufficient to secure the requisit lift.
+
+3. A small angle relative to the propeller thrust, since the latter
+coincides with the direction of motion.
+
+4. A comparatively small angle of incidence by reason of the high
+velocity.
+
+5. A comparatively small camber follows as a result of the small angle
+of incidence.
+
+
+SUMMARY.
+
+   Essentials for Maximum            Essentials for Maximum
+           Climb.                            Velocity
+
+   1. Low velocity.                  High velocity.
+   2. Large surface.                 Small surface.
+   3. Large angle relative to        Small angle relative to
+      propeller thrust.                 propeller thrust.
+   4. Large angle relative to        Small angle relative to direction
+      direction of motion.              of motion.
+   5. Large camber.                  Small camber.
+
+
+It is mechanically impossible to construct an aeroplane of reasonable
+weight of which it would be possible to very the above opposing
+essentials. Therefore, all aeroplanes are designed as a compromise
+between Climb and Velocity.
+
+As a rule aeroplanes are designed to have at low altitude a slight
+margin of lift when the propeller thrust is horizontal.
+
+
+ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE DESIGNED
+AS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIGHT
+MARGIN OF LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS HORIZONTAL
+
+MINIMUM ANGLE.
+
+This gives the greatest velocity during horizontal flight at a low
+altitude. Greater velocity would be secured if the surface, angle, and
+camber were smaller and designed to just maintain horizontal flight
+with a horizontal thrust. Also, in such case, the propeller would not
+be thrusting downwards, but along a horizontal line which is obviously
+a more efficient arrangement if we regard the aeroplane merely from one
+point of view, i.e., either with reference to velocity OR climb.
+
+OPTIMUM ANGLE (Thrust horizontal)
+
+The velocity is less than at the smaller minimum angle, and, as
+aeroplanes are designed to-day, the area and angle of incidence of the
+surface is such as to secure a slight ascent at a low altitude. The
+camber of the surface is designed for this angle of incidence and
+velocity. The lift-drift ratio is best at this angle.
+
+BEST CLIMBING ANGLE
+
+The velocity is now still less by reason of the increased angle
+producing increase of drift. Less velocity at A GIVEN ANGLE produces
+less lift, but the increased angle more or less offsets the loss of
+lift due to the decreased velocity, and in addition, the thrust is now
+hauling the aeroplane upwards.
+
+MAXIMUM ANGLE
+
+The greater angle has now produced so much drift as to lessen the
+velocity to a point where the combined lifts from the surface and from
+the thrust are only just able to maintain horizontal flight. Any greater
+angle will result in a still lower lift-drift ratio. The lift will then
+become less than the weight and the aeroplane will consequently fall.
+Such a fall is known as "stalling" or "pancaking."
+
+NOTE.--The golden rule for beginners: Never exceed the Best Climbing
+Angle. Always maintain the flying speed of the aeroplane.
+
+
+By this means, when the altitude is reached where the margin of
+lift disappears (on account of loss of engine power), and which is,
+consequently, the altitude where it is just possible to maintain
+horizontal flight, the aeroplane is flying with its thrust horizontal
+and with maximum efficiency (as distinct from engine and propeller
+efficiency).
+
+The margin of lift at low altitude, and when the thrust is horizontal,
+should then be such that the higher altitude at which the margin of lift
+is lost is that altitude at which most of the aeroplane's horizontal
+flight work is done. That ensures maximum velocity when most required.
+
+Unfortunately, where aeroplanes designed for fighting are concerned, the
+altitude where most of the work is done is that at which both maximum
+velocity and maximum margin of lift for power are required.
+
+Perhaps some day a brilliant inventor will design an aeroplane of
+reasonable weight and drift of which it will be possible for the pilot
+to vary at will the above-mentioned opposing essentials. Then we shall
+get maximum velocity, or maximum margin of lift, for power as required.
+Until then the design of the aeroplane must remain a compromise between
+Velocity and Climb.
+
+
+
+
+CHAPTER II. STABILITY AND CONTROL
+
+STABILITY is a condition whereby an object disturbed has a natural
+tendency to return to its first and normal position. Example: a weight
+suspended by a cord.
+
+INSTABILITY is a condition whereby an object disturbed has a natural
+tendency to move as far as possible away from its first position, with
+no tendency to return. Example: a stick balanced vertically upon your
+finger.
+
+NEUTRAL INSTABILITY is a condition whereby an object disturbed has no
+tendency to move farther than displaced by the force of the disturbance,
+and no tendency to return to its first position.
+
+In order that an aeroplane may be reasonably controllable, it is
+necessary for it to possess some degree of stability longitudinally,
+laterally, and directionally.
+
+LONGITUDINAL STABILITY in an aeroplane is its stability about an axis
+transverse to the direction of normal horizontal flight, and without
+which it would pitch and toss.
+
+LATERAL STABILITY is its stability about its longitudinal axis, and
+without which it would roll sideways.
+
+DIRECTIONAL STABILITY is its stability about its vertical axis, and
+without which it would have no tendency to keep its course.
+
+For such directional stability to exist there must be, in effect,[16]
+more "keel-surface" behind the vertical axis than there is in front of
+it. By keel-surface I mean every-thing to be seen when looking at an
+aeroplane from the side of it--the sides of the body, undercarriage,
+struts, wires, etc. The same thing applies to a weathercock. You know
+what would happen if there was insufficient keel-surface behind the
+vertical axis upon which it is pivoted. It would turn off its proper
+course, which is opposite to the direction of the wind. It is very much
+the same in the case of an aeroplane.
+
+The above illustration represents an aeroplane (directionally stable)
+flying along the course B. A gust striking it as indicated acts upon the
+greater proportion of keel-surface behind the turning axis and throws it
+into the new course. It does not, however, travel along the new course,
+owing to its momentum in the direction B. It travels, as long as such
+momentum lasts, in a direction which is the resultant of the two forces
+Thrust and Momentum. But the centre line of the aeroplane is pointing in
+the direction of the new course. Therefore its attitude, relative to
+the direction of motion, is more or less sideways, and it consequently
+receives an air pressure in the direction C. Such pressure, acting upon
+the keel-surface, presses the tail back towards its first position in
+which the aeroplane is upon its course B.
+
+What I have described is continually going on during flight, but in
+a well-designed aeroplane such stabilizing movements are, most of the
+time, so slight as to be imperceptible to the pilot.
+
+If an aeroplane was not stabilized in this way, it would not only be
+continually trying to leave its course, but it would also possess a
+dangerous tendency to "nose away" from the direction of the side gusts.
+In such case the gust shown in the above illustration would turn the
+aeroplane round the opposite way a very considerable distance; and the
+right wing, being on the outside of the turn, would travel with greater
+velocity than the left wing. Increased velocity means increased lift;
+and so, the right wing lifting, the aeroplane would turn over sideways
+very quickly.
+
+LONGITUDINAL STABILITY.--Flat surfaces are longitudinally stable owing
+to the fact that with decreasing angles of incidence the centre line of
+pressure (C.P.) moves forward.
+
+The C.P. is a line taken across the surface, transverse to the direction
+of motion, and about which all the air forces may be said to balance, or
+through which they may be said to act.
+
+
+Imagine A to be a flat surface, attitude vertical, travelling through
+the air in the direction of motion M. Its C.P. is then obviously along
+the exact centre line of the surface as illustrated.
+
+In B, C, and D the surfaces are shown with angles of incidence
+decreasing to nothing, and you will note that the C.P. moves forward
+with the decreasing angle.
+
+Now, should some gust or eddy tend to make the surface decrease the
+angle, i.e., dive, then the C.P. moves forward and pushes the front of
+the surface up. Should the surface tend to assume too large an angle,
+then the reverse happens--the C.P. moves back and pushes the rear of the
+surface up.
+
+Flat surfaces are, then, theoretically stable longitudinally. They are
+not, however, used, on account of their poor lift-drift ratio.
+
+As already explained, cambered surfaces are used, and these are
+longitudinally unstable at those angles of incidence producing a
+reasonable lift-drift ratio, i.e., at angles below: about 12 degrees.
+
+A is a cambered surface, attitude approximately vertical, moving through
+the air in the direction M. Obviously the C. P. coincides with the
+transverse centre line of the surface.
+
+With decreasing angles, down to angles of about 30 degrees, the C.P.
+moves forward as in the case of flat surfaces (see B), but angles above
+30 degrees do not interest us, since they produce a very low ratio of
+lift to drift.
+
+Below angles of about 30 degrees (see C) the dipping front part of the
+surface assumes a negative angle of incidence resulting in the DOWNWARD
+air pressure D, and the more the angle of incidence is decreased, the
+greater such negative angle and its resultant pressure D. Since the
+C.P. is the resultant of all the air forces, its position is naturally
+affected by D, which causes it to move backwards. Now, should some gust
+or eddy tend to make the surface decrease its angle of incidence, i.e.,
+dive, then the C.P. moves backwards, and, pushing up the rear of the
+surface, causes it to dive the more. Should the surface tend to assume
+too large an angle, then the reverse happens; the pressure D decreases,
+with the result that C.P. moves forward and pushes up the front of the
+surface, thus increasing the angle still further, the final result being
+a "tail-slide."
+
+It is therefore necessary to find a means of stabilizing the naturally
+unstable cambered surface. This is usually secured by means of a
+stabilizing surface fixed some distance in the rear of the main surface,
+and it is a necessary condition that the neutral lift lines of the two
+surfaces, when projected to meet each other, make a dihedral angle. In
+other words, the rear stabilizing surface must have a lesser angle of
+incidence than the main surface--certainly not more than one-third of
+that of the main surface. This is known as the longitudinal dihedral.
+
+I may add that the tail-plane is sometimes mounted upon the aeroplane at
+the same angle as the main surface, but, in such cases, it attacks air
+which has received a downward deflection from the main surface, thus:
+
+The angle at which the tail surface attacks the air (the angle of
+incidence) is therefore less than the angle of incidence of the main
+surface.
+
+I will now, by means of the following illustration, try to explain how
+the longitudinal dihedral secures stability:
+
+First, imagine the aeroplane travelling in the direction of motion,
+which coincides with the direction of thrust T. The weight is, of
+course, balanced about a C.P., the resultant of the C.P. of the main
+surface and the C.P. of the stabilizing surface. For the sake of
+illustration, the stabilizing surface has been given an angle of
+incidence, and therefore has a lift and C.P. In practice the stabilizer
+is often set at no angle of incidence. In such case the proposition
+remains the same, but it is, perhaps, a little easier to illustrate it
+as above.
+
+Now, we will suppose that a gust or eddy throws the machine into the
+lower position. It no longer travels in the direction of T, since the
+momentum in the old direction pulls it off that course. M is now the
+resultant of the Thrust and the Momentum, and you will note that this
+results in a decrease in the angle our old friend the neutral lift line
+makes with M, i.e., a decrease in the angle of incidence and therefore a
+decrease in lift.
+
+We will suppose that this decrease is 2 degrees. Such decrease applies
+to both main surface and stabilizer, since both are fixed rigidly to the
+aeroplane.
+
+The main surface, which had 12 degrees angle, has now only 10 degrees,
+i.e., a loss of ONE-SIXTH.
+
+The stabilizer, which had 4 degrees angle, has now only 2 degrees, i.e.,
+a loss of ONE-HALF.
+
+The latter has therefore lost a greater PROPORTION of its angle of
+incidence, and consequently its lift, than has the main surface. It must
+then fall relative to the main surface. The tail falling, the aeroplane
+then assumes its first position, though at a slightly less altitude.
+
+Should a gust throw the nose of the aeroplane up, then the reverse
+happens. Both main surface and stabilizer increase their angles of
+incidence in the same amount, but the angle, and therefore the lift, of
+the stabilizer increases in greater proportion than does the lift of the
+main surface, with the result that it lifts the tail. The aeroplane then
+assumes its first position, though at a slightly greater altitude.
+
+Do not fall into the widespread error that the angle of incidence varies
+as the angle of the aeroplane to the horizontal. It varies with such
+angle, but not as anything approaching it. Remember that the stabilizing
+effect of the longitudinal dihedral lasts only as long as there is
+momentum in the direction of the first course.
+
+These stabilizing movements are taking place all the time, even though
+imperceptible to the pilot.
+
+Aeroplanes have, in the past, been built with a stabilizing surface in
+front of the main surface instead of at the rear of it. In such
+design the main surface (which is then the tail surface as well as the
+principal lifting surface) must be set at a less angle than the forward
+stabilizing surface, in order to secure a longitudinal dihedral. The
+defect of such design lies in the fact that the main surface must have
+a certain angle to lift the weight--say 5 degrees. Then, in order to
+secure a sufficiency of longitudinal stability, it is necessary to
+set the forward stabilizer at about 15 degrees. Such a large angle of
+incidence results in a very poor lift-drift ratio (and consequently
+great loss of efficiency), except at very low velocities compared with
+the speed of modern aeroplanes. At the time such aeroplanes were built
+velocities were comparatively low, and this defect was; for that reason,
+not sufficiently appreciated. In the end it killed the "canard" or
+"tail-first" design.
+
+Aeroplanes of the Dunne and similar types possess no stabilizing surface
+distinct from the main surface, but they have a longitudinal dihedral
+which renders them stable.
+
+The main surface towards the wing-tips is given a decreasing angle
+of incidence and corresponding camber. The wing-tips then act as
+longitudinal stabilizers.
+
+This design of aeroplane, while very interesting, has not proved very
+practicable, owing to the following disadvantages: (1) The plan design
+is not, from a mechanical point of view, so sound as that of the
+ordinary aeroplane surface, which is, in plan, a parallelogram. It is,
+then, necessary to make the strength of construction greater than would
+otherwise be the case. That means extra weight. (2) The plan of the
+surface area is such that the aspect ratio is not so high as if the
+surface was arranged with its leading edges at right angles to the
+direction of motion. The lower the aspect ratio, then, the less the
+lift. This design, then, produces less lift for weight of surface than
+would the same surface if arranged as a parallelogram. (3) In order to
+secure the longitudinal dihedral, the angle of incidence has to be very
+much decreased towards the wing-tips. Then, in order that the lift-drift
+ratio may be preserved, there must be a corresponding decrease in the
+camber. That calls for surface ribs of varying cambers, and results in
+an expensive and lengthy job for the builder. (4) In order to secure
+directional stability, the surface is, in the centre, arranged to dip
+down in the form of a V, pointing towards the direction of motion.
+Should the aeroplane turn off its course, then its momentum in the
+direction of its first course causes it to move in a direction the
+resultant of the thrust and the momentum. It then moves in a more or
+less sideways attitude, which results in an air pressure upon one
+side of the V, and which tends to turn the aeroplane back to its first
+course. This arrangement of the surface results in a bad drift. Vertical
+surfaces at the wing-tips may also be set at an angle producing the same
+stabilizing effect, but they also increase the drift.
+
+The gyroscopic action of a rotary engine will affect the longitudinal
+stability when an aeroplane is turned to right or left. In the case of
+a Gnome engine, such gyroscopic action will tend to depress the nose of
+the aeroplane when it is turned to the left, and to elevate it when
+it is turned to the right. In modern aeroplanes this tendency is not
+sufficiently important to bother about. In the old days of crudely
+designed and under-powered aeroplanes this gyroscopic action was very
+marked, and led the majority of pilots to dislike turning an aeroplane
+to the right, since, in doing so, there was some danger of "stalling."
+
+
+LATERAL STABILITY is far more difficult for the designer to secure
+than is longitudinal or directional stability. Some degree of lateral
+stability may be secured by means of the "lateral dihedral," i.e., the
+upward inclination of the surface towards its wing-tips thus:
+
+Imagine the top V, illustrated opposite, to be the front view of a
+surface flying towards you. The horizontal equivalent (H.E.) of the left
+wing is the same as that of the right wing. Therefore, the lift of one
+wing is equal to the lift of the other, and the weight, being situated
+always in the centre, is balanced.
+
+If some movement of the air causes the surface to tilt sideways, as in
+the lower illustration, then you will note that the H.E. of the left
+wing increases, and the H.E. of the right wing decreases. The left wing
+then, having the greatest lift, rises; and the surface assumes its first
+and normal position.
+
+Unfortunately however, the righting effect is not proportional to the
+difference between the right and left H.E.'s.
+
+
+
+In the case of A, the resultant direction of the reaction of both wings
+is opposed to the direction of gravity or weight. The two forces R R
+and gravity are then evenly balanced, and the surface is in a state of
+equilibrium.
+
+In the case of B, you will note that the R R is not directly opposed
+to gravity. This results in the appearance of M, and so the resultant
+direction of motion of the aeroplane is no longer directly forward, but
+is along a line the resultant of the thrust and M. In other words,
+it is, while flying forward, at the same time moving sideways in the
+direction M.
+
+In moving sideways, the keel-surface receives, of course, a pressure
+from the air equal and opposite to M. Since such surface is greatest in
+effect towards the tail, then the latter must be pushed sideways. That
+causes the aeroplane to turn; and, the highest wing being on the
+outside of the turn, it has a greater velocity than the lower wing. That
+produces greater lift, and tends to tilt the aeroplane over still more.
+Such tilting tendency is, however, opposed by the difference in the
+H.E.'s of the two wings.
+
+It then follows that, for the lateral dihedral angle to be effective,
+such angle must be large enough to produce, when the aeroplane tilts,
+a difference in the H.E.'s of the two wings, which difference must be
+sufficient to not only oppose the tilting tendency due to the aeroplane
+turning, but sufficient to also force the aeroplane back to its original
+position of equilibrium.
+
+It is now, I hope, clear to the reader that the lateral dihedral is
+not quite so effective as would appear at first sight. Some designers,
+indeed, prefer not to use it, since its effect is not very great, and
+since it must be paid for in loss of H.E. and consequently loss of lift,
+thus decreasing the lift-drift ratio, i.e., the efficiency. Also, it is
+sometimes advanced that the lateral dihedral increases the "spill" of
+air from the wing-tips and that this adversely affects the lift-drift
+ratio.
+
+The disposition of the keel-surface affects the lateral stability. It
+should be, in effect, equally divided by the longitudinal turning axis
+of the aeroplane. If there is an excess of keel-surface above or below
+such axis, then a side gust striking it will tend to turn the aeroplane
+over sideways.
+
+The position of the centre of gravity affects lateral stability. If
+too low, it produces a pendulum effect and causes the aeroplane to roll
+sideways.
+
+If too high, it acts as a stick balanced vertically would act. If
+disturbed, it tends to travel to a position as far as possible from its
+original position. It would then tend, when moved, to turn the aeroplane
+over sideways and into an upside-down position.
+
+From the point of view of lateral stability, the best position for the
+centre of gravity is one a little below the centre of drift.
+
+Propeller torque affects lateral stability. An aeroplane tends to turn
+over sideways in the opposite direction to which the propeller revolves.
+
+This tendency is offset by increasing the angle of incidence (and
+consequently the lift) of the side tending to fall; and it is always
+advisable, if practical considerations allow it, to also decrease the
+angle upon the other side. In that way it is not necessary to depart so
+far from the normal angle of incidence at which the lift-drift ratio is
+highest.
+
+Wash-in is the term applied to the increased angle.
+
+Wash-out is the term applied to the decreased angle.
+
+Both lateral and directional stability may be improved by washing out
+the angle of incidence on both sides of the surface, thus:
+
+The decreased angle decreases the drift and therefore the effect of
+gusts upon the wing-tips which is just where they have the most effect
+upon the aeroplane, owing to the distance from the turning axis.
+
+The wash-out also renders the ailerons (lateral controlling services)
+more effective, as, in order to operate them, it is not then necessary
+to give them such a large angle of incidence as would otherwise be
+required.
+
+
+
+The less the angle of incidence of the ailerons, the better their
+lift-drift ratio, i.e., their efficiency. You will note that, while the
+aileron attached to the surface with washed-out angle is operated to the
+same extent as the aileron illustrated above it, its angle of incidence
+is considerably less. Its efficiency is therefore greater.
+
+The advantages of the wash-in must, of course be paid for in some loss
+of lift, as the lift decreases with the decreased angle.
+
+In order to secure all the above described advantages, a combination is
+sometimes effected, thus:
+
+BANKING.--An aeroplane turned off its course to right or left does not
+at once proceed along its new course. Its momentum in the direction of
+its first course causes it to travel along a line the resultant of such
+momentum and the thrust. In other words, it more or less skids sideways
+and away from the centre of the turn. Its lifting surfaces do not then
+meet the air in their correct attitude, and the lift may fall to such
+an extent as to become less than the weight, in which case the aeroplane
+must fall. This bad effect is minimized by "banking," i.e., tilting the
+aeroplane sideways. The bottom of the lifting surface is in that way
+opposed to the air through which it is moving in the direction of the
+momentum and receives an opposite air pressure. The rarefied area over
+the top of the surface is rendered still more rare, and this, of course,
+assists the air pressure in opposing the momentum.
+
+The velocity of the "skid," or sideways movement, is then only such
+as is necessary to secure an air pressure equal and opposite to the
+centrifugal force of the turn.
+
+The sharper the turn, the greater the effect of the centrifugal force,
+and therefore the steeper should be the "bank." Experentia docet.
+
+The position of the centre of gravity affects banking. A low C.G. will
+tend to swing outward from the centre of the turn, and will cause the
+aeroplane to bank--perhaps too much, in which case the pilot must remedy
+matters by operating the ailerons.
+
+A high C.G. also tends to swing outward from the centre of the turn. It
+will tend to make the aeroplane bank the wrong way, and such effect must
+be remedied by means of the ailerons.
+
+The pleasantest machine from a banking point of view is one in which
+the C.G. is a little below the centre of drift. It tends to bank the
+aeroplane the right way for the turn, and the pilot can, if necessary,
+perfect the bank by means of the ailerons.
+
+The disposition of the keel-surface affects banking. It should be,
+in effect, evenly divided by the longitudinal axis. An excess of
+keel-surface above the longitudinal axis will, when banking, receive an
+air pressure causing the aeroplane to bank, perhaps too much. An excess
+of keel-surface below the axis has the reverse effect.
+
+
+SIDE-SLIPPING.--This usually occurs as a result of over-banking. It is
+always the result of the aeroplane tilting sideways and thus decreasing
+the horizontal equivalent, and therefore the lift, of the surface. An
+excessive "bank," or sideways tilt, results in the H.E., and therefore
+the lift, becoming less than the weight, when, of course, the aeroplane
+must fall, i.e., side-slip.
+
+
+When making a very sharp turn it is necessary to bank very steeply
+indeed. If, at the same time, the longitudinal axis of the aeroplane
+remains approximately horizontal, then there must be a fall, and the
+direction of motion will be the resultant of the thrust and the fall as
+illustrated above in sketch A. The lifting surfaces and the controlling
+surfaces are not then meeting the air in the correct attitude, with the
+result that, in addition to falling, the aeroplane will probably become
+quite unmanageable.
+
+The Pilot, however, prevents such a state of affairs from happening by
+"nosing-down," i.e., by operating the rudder to turn the nose of the
+aeroplane downward and towards the direction of motion as illustrated
+in sketch B. This results in the higher wing, which is on the outside
+of the turn, travelling with greater velocity, and therefore securing a
+greater reaction than the lower wing, thus tending to tilt the aeroplane
+over still more. The aeroplane is now almost upside-down, but its
+attitude relative to the direction of motion is correct and the
+controlling surfaces are all of them working efficiently. The recovery
+of a normal attitude relative to the Earth is then made as illustrated
+in sketch C.
+
+The Pilot must then learn to know just the angle of bank at which the
+margin of lift is lost, and, if a sharp turn necessitates banking beyond
+that angle, he must "nose-down."
+
+In this matter of banking and nosing-down, and, indeed, regarding
+stability and control generally, the golden rule for all but very
+experienced pilots should be: Keep the aeroplane in such an attitude
+that the air pressure is always directly in the pilot's face. The
+aeroplane is then always engaging the air as designed to do so, and
+both lifting and controlling surfaces are acting efficiently. The only
+exception to this rule is a vertical dive, and I think that is obviously
+not an attitude for any but very experienced pilots to hanker after.
+
+SPINNING.--This is the worst of all predicaments the pilot can find
+himself in. Fortunately it rarely happens.
+
+It is due to the combination of (1) a very steep spiral descent of small
+radius, and (2) insufficiency of keel-surface behind the vertical axis,
+or the jamming of the rudder end or elevator into a position by which
+the aeroplane is forced into an increasingly steep and small spiral.
+
+Owing to the small radius of such a spiral, the mass of the aeroplane
+may gain a rotary momentum greater, in effect, than the air pressure of
+the keel-surface or controlling surfaces opposed to it; and, when once
+such a condition occurs, it is difficult to see what can be done by the
+pilot to remedy it. The sensible pilot will not go beyond reasonable
+limits of steepness and radius when executing spiral descents.
+
+GLIDING DESCENT WITHOUT PROPELLER THRUST.--All aeroplanes are, or should
+be, designed to assume their gliding angle when the power and thrust is
+cut off. This relieves the pilot of work, worry, and danger should he
+find himself in a fog or cloud. The Pilot, although he may not realize
+it, maintains the correct attitude of the aeroplane by observing its
+position relative to the horizon. Flying into a fog or cloud the horizon
+is lost to view, and he must then rely upon his instruments--(1) the
+compass for direction; (2) an inclinometer (arched spirit-level) mounted
+transversely to the longitudinal axis, for lateral stability; and (3) an
+inclinometer mounted parallel to the longitudinal axis, or the airspeed
+indicator, which will indicate a nose-down position by increase in air
+speed, and a tail-down position by decrease in air speed.
+
+The pilot is then under the necessity of watching three instruments
+and manipulating his three controls to keep the instruments indicating
+longitudinal, lateral, and directional stability. That is a feat beyond
+the capacity of the ordinary man. If, however, by the simple movement
+of throttling down the power and thrust, he can be relieved of looking
+after the longitudinal stability, he then has only two instruments to
+watch. That is no small job in itself, but it is, at any rate, fairly
+practicable.
+
+Aeroplanes are, then, designed, or should be, so that the centre of
+gravity is slightly forward of centre of lift. The aeroplane is then, as
+a glider, nose-heavy--and the distance the C.G. is placed in advance
+of the C.L. should be such as to ensure a gliding angle producing a
+velocity the same as the normal flying speed (for which the strength of
+construction has been designed).
+
+In order that this nose-heavy tendency should not exist when the thrust
+is working and descent not required, the centre of thrust is placed a
+little below the centre of drift or resistance, and thus tends to pull
+up the nose of the aeroplane.
+
+The distance the centre of thrust is placed below the centre of drift
+should be such as to produce a force equal and opposite to that due to
+the C.G. being forward of the C.L.
+
+LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired, it is best to
+throttle the engine down at point A. The C.G. being forward of the C.P.,
+then causes the aeroplane to nose-down, and assists the pilot in making
+a reasonably small loop along the course C and in securing a quick
+recovery. If the engine is not throttled down, then the aeroplane may
+be expected to follow the course D, which results in a longer nose dive
+than in the case of the course C.
+
+A steady, gentle movement of the elevator is necessary. A jerky movement
+may change the direction of motion so suddenly as to produce dangerous
+air stresses upon the surfaces, in which case there is a possibility of
+collapse.
+
+If an upside-down flight is desired, the engine may, or may not, be
+throttled down at point A. If not throttled down, then the elevator must
+be operated to secure a course approximately in the direction B. If it
+is throttled down, then the course must be one of a steeper angle than
+B, or there will be danger of stalling.
+
+
+Diagram p. 88.--This is not set at quite the correct angle. Path B
+should slope slightly downwards from Position A.
+
+
+
+
+CHAPTER III. RIGGING
+
+In order to rig an aeroplane intelligently, and to maintain it in an
+efficient and safe condition, it is necessary to possess a knowledge
+of the stresses it is called upon to endure, and the strains likely to
+appear.
+
+
+STRESS is the load or burden a body is called upon to bear. It is
+usually expressed by the result found by dividing the load by the number
+of superficial square inches contained in the cross-sectional area of
+the body.
+
+Thus, if, for instance, the object illustrated above contains 4 square
+inches of cross-sectional area, and the total load it is called upon to
+endure is 10 tons, the stress would be expressed as 2 1/2 tons.
+
+
+STRAIN is the deformation produced by stress.
+
+
+THE FACTOR OF SAFETY is usually expressed by the result found by
+dividing the stress at which it is known the body will collapse, by
+the maximum stress it will be called upon to endure. For instance, if a
+control wire be called upon to endure a maximum stress of 2 cwts., and
+the known stress at which it will collapse is 10 cwts., the factor of
+safety is then 5.
+
+[cwts. = centerweights = 100 pound units as in cent & century.
+Interestingly enough, this word only exists today in abbreviation form,
+probably of centreweights, but the dictionary entries, even from a
+hundred years ago do not list this as a word, but do list c. or C. as
+the previous popular abbreviation as in Roman Numerals] The word listed
+is "hundredweight. Michael S. Hart, 1997]
+
+
+COMPRESSION.--The simple stress of compression tends to produce a
+crushing strain. Example: the interplane and fuselage struts.
+
+
+TENSION.--The simple stress of tension tends to produce the strain of
+elongation. Example: all the wires.
+
+
+BENDING.--The compound stress of bending is a combination of compression
+and tension.
+
+The above sketch illustrates a straight piece of wood of which the top,
+centre, and bottom lines are of equal length. We will now imagine it
+bent to form a circle, thus:
+
+The centre line is still the same length as before being bent; but the
+top line, being farther from the centre of the circle, is now longer
+than the centre line. That can be due only to the strain of elongation
+produced by the stress of tension. The wood between the centre line and
+the top line is then in tension; and the farther from the centre, the
+greater the strain, and consequently the greater the tension.
+
+The bottom line, being nearest to the centre of the circle, is now
+shorter than the centre line. That can be due only to the strain of
+crushing produced by the stress of compression. The wood between the
+centre and bottom lines is then in compression; and the nearer the
+centre of the circle, the greater the strain, and consequently the
+greater the compression.
+
+It then follows that there is neither tension nor compression, i.e., no
+stress, at the centre line, and that the wood immediately surrounding it
+is under considerably less stress than the wood farther away. This being
+so, the wood in the centre may be hollowed out without unduly weakening
+struts and spars. In this way 25 to 33 per cent. is saved in the weight
+of wood in an aeroplane.
+
+The strength of wood is in its fibres, which should, as far as possible,
+run without break from one end of a strut or spar to the other end. A
+point to remember is that the outside fibres, being farthest removed
+from the centre line, are doing by far the greatest work.
+
+
+SHEAR STRESS IS such that, when material collapses under it, one part
+slides over the other. Example: all the locking pins.
+
+Some of the bolts are also in shear or "sideways" stress, owing to lugs
+under their heads and from which wires are taken. Such a wire, exerting
+a sideways pull upon a bolt, tries to break it in such a way as to make
+one piece of the bolt slide over the other piece.
+
+TORSION.--This is a twisting stress compounded of compression, tension,
+and shear stresses. Example: the propeller shaft.
+
+
+NATURE OF WOOD UNDER STRESS.--Wood, for its weight, takes the stress
+of compression far better than any other stress. For instance: a
+walking-stick of less than 1 lb. in weight will, if kept perfectly
+straight, probably stand up to a compression stress of a ton or more
+before crushing; whereas, if the same stick is put under a bending
+stress, it will probably collapse to a stress of not more than about
+50 lb. That is a very great difference, and, since weight is of the
+greatest importance, the design of an aeroplane is always such as to,
+as far as possible, keep the various wooden parts of its construction in
+direct compression. Weight being of such vital importance, and designers
+all trying to outdo each other in saving weight, it follows that the
+factor of safety is rather low in an aeroplane. The parts in direct
+compression will, however, take the stresses safely provided the
+following conditions are carefully observed.
+
+
+CONDITIONS TO BE OBSERVED:
+
+
+1. All the spars and struts must be perfectly straight.
+
+The above sketch illustrates a section through an interplane strut. If
+the strut is to be kept straight, i.e., prevented from bending, then
+the stress of compression must be equally disposed about the centre of
+strength. If it is not straight, then there will be more compression
+on one side of the centre of strength than on the other side. That is
+a step towards getting compression on one side and tension on the other
+side, in which case it may be forced to take a bending stress for which
+it is not designed. Even if it does not collapse it will, in effect,
+become shorter, and thus throw out of adjustment the gap and all the
+wires attached to the top and bottom of the strut, with the result that
+the flight efficiency of the aeroplane will be spoiled.
+
+The only exception to the above condition is what is known as the
+Arch. For instance, in the case of the Maurice Farman, the spars of the
+centre-section plane, which have to take the weight of the nacelle, are
+arched upwards. If this was not done, it is possible that rough
+landings might result in the weight causing the spars to become slightly
+distorted downwards. That would produce a dangerous bending stress,
+but, as long as the wood is arched, or, at any rate, kept from bending
+downwards, it will remain in direct compression and no danger can
+result.
+
+
+2. Struts and spars must be symmetrical. By that I mean that the
+cross-sectional dimensions must be correct, as otherwise there will be
+bulging places on the outside, with the result that the stress will not
+be evenly disposed about the centre of strength, and a bending stress
+may be produced.
+
+
+3. Struts, spars, etc., must be undamaged. Remember that, from what I
+have already explained about bending stresses, the outside fibres of
+the wood are doing by far the most work. If these get bruised or scored,
+then the strut or spar suffers in strength much more than one might
+think at first sight; and, if it ever gets a tendency to bend, it is
+likely to collapse at that point.
+
+
+4. The wood must have a good, clear grain with no cross-grain, knots,
+or shakes. Such blemishes produce weak places and, if a tendency to bend
+appears, then it may collapse at such a point.
+
+5. The struts, spars, etc., must be properly bedded into their sockets
+or fittings. To begin with, they must be of good pushing or gentle
+tapping fit. They must never be driven in with a heavy hammer. Then
+again, a strut must bed well down all over its cross-sectional area
+as illustrated above; otherwise the stress of compression will not be
+evenly disposed about the centre of strength, and that may produce a
+bending stress. The bottom of the strut or spar should be covered
+with some sort of paint, bedded into the socket or fitting, and then
+withdrawn to see if the paint has stuck all over the bed.
+
+
+6. The atmosphere is sometimes much damper than at other times, and this
+causes wood to expand and contract appreciably. This would not matter
+but for the fact that it does not expand and contract uniformly, but
+becomes unsymmetrical, i.e., distorted. I have already explained
+the danger of that in condition 2. This should be minimized by WELL
+VARNISHING THE WOOD to keep the moisture out of it.
+
+
+FUNCTION OF INTERPLANE STRUTS.--These struts have to keep the lifting
+surfaces or "planes" apart, but this is only part of their work. They
+must keep the planes apart, so that the latter are in their correct
+attitude. That is only so when the spars of the bottom plane are
+parallel with those of the top plane. Also, the chord of the top plane
+must be parallel with the chord of the bottom plane. If that is not so,
+then one plane will not have the same angle of incidence as the other
+one. At first sight one might think that all that is necessary is to cut
+all the struts to be the same length, but that is not the case.
+
+Sometimes, as illustrated above, the rear spar is not so thick as the
+main spar, and it is then necessary to make up for that difference by
+making the rear struts correspondingly longer. If that is not done, then
+the top and bottom chords will not be parallel, and the top and bottom
+planes will have different angles of incidence. Also, the sockets or
+fittings, or even the spars upon which they are placed, sometimes
+vary in thickness owing to faulty manufacture. This must be offset by
+altering the length of the struts. The best way to proceed is to measure
+the distance between the top and bottom spars by the side of each strut,
+and if that distance, or "gap" as it is called, is not as stated in the
+aeroplane's specifications, then make it correct by changing the length
+of the strut. This applies to both front and rear interplane struts.
+When measuring the gap, always be careful to measure from the centre
+of the spar, as it may be set at an angle, and the rear of it may be
+considerably lower than its front.
+
+
+BORING HOLES IN WOOD.--It should be a strict rule that no spar be used
+which has an unnecessary hole in it. Before boring a hole, its position
+should be confirmed by whoever is in charge of the workshop. A bolt-hole
+should be of a size to enable the bolt to be pushed in, or, at any rate,
+not more than gently tapped in. Bolts should not be hammered in, as that
+may split the spar. On the other hand, a bolt should not be slack in its
+hole, as, in such a case, it may work sideways and split the spar, not
+to speak of throwing out of adjustment the wires leading from the lug or
+socket under the bolt-head.
+
+
+WASHERS.--Under the bolt-head, and also under the nut, a washer must be
+placed--a very large washer compared with the size which would be used
+in all-metal construction. This is to disperse the stress over a large
+area; otherwise the washer may be pulled into the wood and weaken it,
+besides possibly throwing out of adjustment the wires attached to the
+bolt or the fitting it is holding to the spar.
+
+
+LOCKING.--Now as regards locking the bolts. If split pins are used,
+be sure to see that they are used in such a way that the nut cannot
+possibly unscrew at all. The split pin should be passed through the bolt
+as near as possible to the nut. It should not be passed through both nut
+and bolt.
+
+If it is locked by burring over the edge of the bolt, do not use a heavy
+hammer and try to spread the whole head of the bolt. That might damage
+the woodwork inside the fabric-covered surface. Use a small, light
+hammer, and gently tap round the edge of the bolt until it is burred
+over.
+
+
+TURNBUCKLES.--A turnbuckle is composed of a central barrel into each end
+of which is screwed an eye-bolt. Wires are taken from the eyes of the
+eye-bolt, and so, by turning the barrel, they can be adjusted to their
+proper tension. Eye-bolts must be a good fit in the barrel; that is to
+say, not slack and not very tight. Theoretically it is not necessary
+to screw the eye-bolt into the barrel for a distance greater than the
+diameter of the bolt, but, in practice, it is better to screw it in
+for a considerably greater distance than that if a reasonable degree of
+safety is to be secured.
+
+Now about turning the barrel to secure the right adjustment. The barrel
+looks solid, but, as a matter of fact, it is hollow and much more frail
+than it appears. For that reason it should not be turned by seizing it
+with pliers, as that may distort it and spoil the bore within it. The
+best method is to pass a piece of wire through the hole in its centre,
+and to use that as a lever. When the correct adjustment has been
+secured, the turnbuckle must be locked to prevent it from unscrewing. It
+is quite possible to lock it in such a way as to allow it to unscrew a
+quarter or a half turn, and that would throw the wires out of the very
+fine adjustment necessary. The proper way is to use the locking wire so
+that its direction is such as to oppose the tendency of the barrel to
+unscrew, thus:
+
+
+WIRES.--The following points should be carefully observed where wire is
+concerned:
+
+1. Quality.--It must not be too hard or too soft. An easy practical way
+of learning to know the approximate quality of wire is as follows:
+
+Take three pieces, all of the same gauge, and each about a foot in
+length. One piece should be too soft, another too hard, and the third
+piece of the right quality. Fix them in a vice, about an inch apart and
+in a vertical position, and with the light from a window shining upon
+them. Burnish them if necessary, and you will see a band of light
+reflected from each wire.
+
+Now bend the wires over as far as possible and away from the light.
+Where the soft wire is concerned, it will squash out at the bend, and
+this will be indicated by the band of light, which will broaden at that
+point. In the case of the wire which is too hard, the band of light will
+broaden very little at the turn, but, if you look carefully, you will
+see some little roughnesses of surface. In the case of the wire of the
+right quality, the band of light may broaden a very little at the turn,
+but there will be no roughnesses of surface.
+
+By making this experiment two or three times one can soon learn to know
+really bad wire from good, and also learn to know the strength of hand
+necessary to bend the right quality.
+
+2. It must not be damaged. That is to say, it must be unkinked,
+rustless, and unscored.
+
+3. Now as regards keeping wire in good condition. Where outside wires
+are concerned, they should be kept WELL GREASED OR OILED, especially
+where bent over at the ends. Internal bracing wires cannot be reached
+for the purpose of regreasing them, as they are inside fabric-covered
+surfaces. They should be prevented from rusting by being painted with
+an anti-rust mixture. Great care should be taken to see that the wire
+is perfectly clean and dry before being painted. A greasy finger-mark is
+sufficient to stop the paint from sticking to the wire. In such a case
+there will be a little space between the paint and the wire. Air may
+enter there and cause the wire to rust.
+
+4. Tension of Wires.--The tension to which the wires are adjusted is
+of the greatest importance. All the wires should be of the same tension
+when the aeroplane is supported in such a way as to throw no stress upon
+them. If some wires are in greater tension than others, the aeroplane
+will quickly become distorted and lose its efficiency.
+
+In order to secure the same tension of all wires, the aeroplane, when
+being rigged, should be supported by packing underneath the lower
+surfaces as well as by packing underneath the fuselage or nacelle. In
+this way the anti-lift wires are relieved of the weight, and there is no
+stress upon any of the wires.
+
+As a general rule the wires of an aeroplane are tensioned too much. The
+tension should be sufficient to keep the framework rigid. Anything
+more than that lowers the factor of safety, throws various parts of the
+framework into undue compression, pulls the fittings into the wood, and
+will, in the end, distort the whole framework of the aeroplane.
+
+Only experience will teach the rigger what tension to employ. Much may
+be done by learning the construction of the various types of aeroplanes,
+the work the various parts do, and in cultivating a touch for tensioning
+wires by constantly handling them.
+
+5. Wires with no Opposition Wires.--In some few cases wires will be
+found which have no opposition wires pulling in the opposite direction.
+For instance, an auxiliary lift wire may run from the bottom of a strut
+to a spar in the top plane at a point between struts. In such a case
+great care should be taken not to tighten the wire beyond barely taking
+up the slack.
+
+Such a wire must be a little slack, or, as illustrated above, it will
+distort the framework. That, in the example given, will spoil the camber
+(curvature) of the surface, and result in changing both the lift and
+the drift at that part of the surface. Such a condition will cause the
+aeroplane to lose its directional stability and also to fly one wing
+down.
+
+I cannot impress this matter of tension upon the reader too strongly.
+It is of the utmost importance. When this, and also accuracy in securing
+the various adjustments, has been learned, one is on the way to becoming
+a good rigger.
+
+6. Wire Loops.--Wire is often bent over at its end in the form of a
+loop, in order to connect with a turnbuckle or fitting. These loops,
+even when made as perfectly as possible, have a tendency to elongate,
+thus spoiling the adjustment of the wires Great care should be taken
+to minimize this as far as possible. The rules to be observed are as
+follows:
+
+(a) The size of the loop should be as small as possible within reason.
+By that I mean it should not be so small as to create the possibility of
+the wire breaking.
+
+
+(b) The shape of the loop should be symmetrical.
+
+
+(c) It should have well-defined shoulders in order to prevent the
+ferrule from slipping up. At the same time, a shoulder should not have
+an angular place.
+
+
+(d) When the loop is finished it should be undamaged, and it should not
+be, as is often the case, badly scored.
+
+
+7. Stranded Wire Cable.--No splice should be served with twine until it
+has been inspected by whoever is in charge of the workshop. The serving
+may cover bad work.
+
+Should a strand become broken, then the cable should be replaced at once
+by another one.
+
+Control cables have a way of wearing out and fraying wherever they pass
+round pulleys. Every time an aeroplane comes down from flight the rigger
+should carefully examine the cables, especially where they pass round
+pulleys. If he finds a strand broken, he should replace the cable.
+
+The ailerons' balance cable on the top of the top plane is often
+forgotten, since it is necessary to fetch a high pair of steps in
+order to examine it. Don't slack this, or some gusty day the pilot may
+unexpectedly find himself minus the aileron control.
+
+CONTROLLING SURFACES.--The greatest care should be exercised in rigging
+the aileron, rudder, and elevator properly, for the pilot entirely
+depends upon them in managing the aeroplane.
+
+The ailerons and elevator should be rigged so that, when the aeroplane
+is in flight, they are in a fair true line with the surface in front and
+to which they are hinged.
+
+If the surface to which they are hinged is not a lifting surface, then
+they should be rigged to be in a fair true line with it as illustrated
+above.
+
+If the controlling surface is, as illustrated, hinged to the back of a
+lifting surface, then it should be rigged a little below the position it
+would occupy if in a fair true line with the surface in front. This is
+because, in such a case, it is set at an angle of incidence. This angle
+will, during flight, cause it to lift a little above the position in
+which it has been rigged. It is able to lift owing to a certain amount
+of slack in the control wire holding it--and one cannot adjust the
+control wire to have no slack, because that would cause it to bind
+against the pulleys and make the operation of it too hard for the pilot.
+It is therefore necessary to rig it a little below the position it would
+occupy if it was rigged in a fair true line with the surface in front.
+Remember that this only applies when it is hinged to a lifting surface.
+The greater the angle of incidence (and therefore the lift) of the
+surface in front, then the more the controlling surface will have to be
+rigged down.
+
+As a general rule it is safe to rig it down so that its trailing
+
+edge is 1/2 to 3/4 inch below the position it would occupy if in a fair
+line with the surface in front; or about 1/2 inch down for every 18
+inches of chord of the controlling surface.
+
+When making these adjustments the pilot's control levers should be in
+their neutral positions. It is not sufficient to lash them. They should
+be rigidly blocked into position with wood packing.
+
+The surfaces must not be distorted in any way. If they are held true by
+bracing wires, then such wires must be carefully adjusted. If they are
+distorted and there are no bracing wires with which to true them, then
+some of the internal framework will probably have to be replaced.
+
+The controlling surfaces should never be adjusted with a view to
+altering the stability of the aeroplane. Nothing can be accomplished in
+that way. The only result will be to spoil the control of the aeroplane.
+
+
+FABRIC-COVERED SURFACES.--First of all make sure that there is no
+distortion of spars or ribs, and that they are perfectly sound. Then
+adjust the internal bracing wires so that the ribs are parallel to the
+direction of flight. The ribs usually cause the fabric to make a ridge
+where they occur, and, if such ridge is not parallel to the direction of
+flight, it will produce excessive drift. As a rule the ribs are at right
+angles to both main and rear spars.
+
+The tension of the internal bracing wires should be just sufficient to
+give rigidity to the framework. They should not be tensioned above that
+unless the wires are, at their ends, bent to form loops. In that case a
+little extra tension may be given to offset the probable elongation of
+the loops.
+
+The turnbuckles must now be generously greased, and served round with
+adhesive tape. The wires must be rendered perfectly dry and clean,
+and then painted with an anti-rust mixture. The woodwork must be well
+varnished.
+
+If it is necessary to bore holes in the spars for the purpose of
+receiving, for instance, socket bolts, then their places should be
+marked before being bored and their positions confirmed by whoever is in
+charge of the workshop. All is now ready for the sail-maker to cover the
+surface with fabric.
+
+
+ADJUSTMENT OF CONTROL CABLES.--The adjustment of the control cables is
+quite an art, and upon it will depend to a large degree the quick and
+easy control of the aeroplane by the pilot.
+
+The method is as follows:
+
+After having rigged the controlling surfaces, and as far as possible
+secured the correct adjustment of the control cables, then remove the
+packing which has kept the control levers rigid. Then, sitting in the
+pilot's seat, move the control levers SMARTLY. Tension the control
+cables so that when the levers are smartly moved there is no perceptible
+snatch or lag. Be careful not to tension the cables more than necessary
+to take out the snatch. If tensioned too much they will (1) bind round
+the pulleys and result in hard work for the pilot; (2) throw dangerous
+stresses upon the controlling surfaces, which are of rather flimsy
+construction; and (3) cause the cables to fray round the pulleys quicker
+than would otherwise be the case.
+
+Now, after having tensioned the cables sufficiently to take out the
+snatch, place the levers in their neutral positions, and move them to
+and fro about 1/8 inch either side of such positions. If the adjustment
+is correct, it should be possible to see the controlling surfaces move.
+If they do not move, then the control cables are too slack.
+
+
+FLYING POSITION.--Before rigging an aeroplane or making any adjustments
+it is necessary to place it in what is known as its "flying position." I
+may add that it would be better termed its "rigging position."
+
+In the case of an aeroplane fitted with a stationary engine this is
+secured by packing up the machine so that the engine foundations are
+perfectly horizontal both longitudinally and laterally. This position
+is found by placing a straight-edge and a spirit-level across the engine
+foundations (both longitudinally and laterally), and great care should
+be taken to see that the bubble is exactly in the centre of the level.
+The slightest error will assume magnitude towards the extremities of the
+aeroplane. Great care should be taken to block up the aeroplane rigidly.
+In case it gets accidentally disturbed while the work is going on, it
+is well to constantly verify the flying position by running the
+straight-edge and spirit-level over the engine foundations. The
+straight-edge should be carefully tested before being used, as, being
+generally made of wood, it will not remain true long. Place it lightly
+in a vice, and in such a position that a spirit-level on top shows
+the bubble exactly in the centre. Now slowly move the level along the
+straight-edge, and the bubble should remain exactly in the centre. If
+it does not do so, then the straight-edge is not true and must be
+corrected. THIS SHOULD NEVER BE OMITTED.
+
+In the case of aeroplanes fitted with engines of the rotary type, the
+"flying position" is some special attitude laid down in the aeroplane's
+specifications, and great care should be taken to secure accuracy.
+
+
+ANGLE OF INCIDENCE.--One method of finding the angle of incidence is as
+follows:
+
+First place the aeroplane in its flying position. The corner of the
+straight-edge must be placed underneath and against the CENTRE of the
+rear spar, and held in a horizontal position parallel to the ribs. This
+is secured by using a spirit-level. The set measurement will then be
+from the top of the straight-edge to the centre of the bottom surface
+of the main spar, or it may be from the top of the straight-edge to the
+lowest part of the leading edge. Care should be taken to measure from
+the centre of the spar and to see that the bubble is exactly in the
+centre of the level. Remember that all this will be useless if the
+aeroplane has not been placed accurately in its flying position.
+
+This method of finding the angle of incidence must be used under every
+part of the lower surface where struts occur. It should not be used
+between the struts, because, in such places, the spars may have taken a
+slight permanent set up or down; not, perhaps, sufficiently bad to make
+any material difference to the flying of the machine, but quite bad
+enough to throw out the angle of incidence, which cannot be corrected at
+such a place.
+
+If the angle is wrong, it should then be corrected as follows:
+
+If it is too great, then the rear spar must be warped up until it is
+right, and this is done by slackening ALL the wires going to the top of
+the strut, and then tightening ALL the wires going to the bottom of the
+strut.
+
+If the angle is too small, then slacken ALL the wires going to the
+bottom of the strut, and tighten ALL the wires going to the top of the
+strut, until the correct adjustment is secured.
+
+Never attempt to adjust the angle by warping the main spar.
+
+The set measurement, which is of course stated in the aeroplane's
+specifications, should be accurate to 1/16 inch.
+
+
+LATERAL DIHEDRAL ANGLE.--One method of securing this is as follows,
+and this method will, at the same time, secure the correct angle of
+incidence:
+
+The strings, drawn very tight, must be taken over both the main and rear
+spars of the top surface. They must run between points on the spars just
+inside the outer struts. The set measurement (which should be accurate
+to 1/16 inch or less) is then from the strings down to four points on
+the main and rear spars of the centre-section surface. These points
+should be just inside the four centre-section struts; that is to say,
+as far as possible away from the centre of the centre-section. Do
+not attempt to take the set measurement near the centre of the
+centre-section.
+
+The strings should be as tight as possible, and, if it can be arranged,
+the best way to accomplish that is as shown in the above illustration,
+i.e., by weighting the strings down to the spars by means of weights
+and tying each end of the strings to a strut. This will give a tight and
+motionless string.
+
+However carefully the above adjustment is made, there is sure to be some
+slight error. This is of no great importance, provided it is divided
+equally between the left- and right-hand wings. In order to make sure of
+this, certain check measurements should be taken as follows:
+
+Each bay must be diagonally measured, and such measurements must be the
+same to within 1/16 inch on each side of the aeroplane. As a rule such
+diagonal measurements are taken from the bottom socket of one strut to
+the top socket of another strut, but this is bad practice, because of
+possible inaccuracies due to faulty manufacture.
+
+The points between which the diagonal measurements are taken should be
+at fixed distances from the butts of the spars, such distances being the
+same on each side of the aeroplane, thus:
+
+
+It would be better to use the centre line of the aeroplane rather than
+the butts of the spars. It is not practicable to do so, however, as the
+centre line probably runs through the petrol tanks, etc.
+
+
+THE DIHEDRAL BOARD.--Another method of securing the dihedral angle, and
+also the angle of incidence, is by means of the dihedral board. It is
+a light handy thing to use, but leads to many errors, and should not be
+used unless necessary. The reasons are as follows:
+
+The dihedral board is probably not true. If it must be used, then it
+should be very carefully tested for truth before-hand. Another reason
+against its use is that it has to be placed on the spars in a position
+between the struts, and that is just where the spars may have a little
+permanent set up or down, or some inaccuracy of surface which will, of
+course, throw out the accuracy of the adjustment. The method of using it
+is as follows:
+
+The board is cut to the same angle as that specified for the upward
+inclination of the surface towards its wing-tips. It is placed on the
+spar as indicated above, and it is provided with two short legs to raise
+it above the flanges of the ribs (which cross over the spars), as they
+may vary in depth. A spirit-level is then placed on the board, and the
+wires must be adjusted to give the surface such an inclination as to
+result in the bubble being in the centre of the level. This operation
+must be performed in respect of each bay both front and rear. The bays
+must then be diagonally measured as already explained.
+
+
+YET ANOTHER METHOD of finding the dihedral angle, and at the same time
+the angle of incidence, is as follows:
+
+A horizontal line is taken from underneath the butt of each spar, and
+the set measurement is either the angle it makes with the spar, or
+a fixed measurement from the line to the spar taken at a specified
+distance from the butt. This operation must be performed in respect of
+both main and rear spars, and all the bays must be measured diagonally
+afterwards.
+
+
+
+Whichever method is used, be sure that after the job is done the spars
+are perfectly straight.
+
+
+STAGGER.--The stagger is the distance the top surface is in advance of
+the bottom surface when the aeroplane is in flying position. The set
+measurement is obtained as follows:
+
+Plumb-lines must be dropped over the leading edge of the top surface
+wherever struts occur, and also near the fuselage. The set measurement
+is taken from the front of the lower leading edge to the plumb-lines. It
+makes a difference whether the measurement is taken along a horizontal
+line (which can be found by using a straight-edge and a spirit-level)
+or along a projection of the chord. The line along which the measurement
+should be taken is laid down in the aeroplane's specifications.
+
+If a mistake is made and the measurement taken along the wrong line, it
+may result in a difference of perhaps 1/4 will, in flight, be nose-heavy
+or tail-heavy.
+
+After the adjustments of the angles of incidence, dihedral, and stagger
+have been secured, it is as well to confirm all of them, as, in making
+the last adjustment, the first one may have been spoiled.
+
+
+OVER-ALL ADJUSTMENTS.--The following over-all check measurements should
+now be taken.
+
+The straight lines AC and BC should be equal to within 1/8 inch. The
+point C is the centre of the propeller, or, in the case of a "pusher"
+aeroplane, the centre of the nacelle. The points A and B are marked on
+the main spar, and must in each case be the same distance from the butt
+of the spar. The rigger should not attempt to make A and B merely the
+sockets of the outer struts, as they may not have been placed quite
+accurately by the manufacturer. The lines AC and BC must be taken
+from both top and bottom spars--two measurements on each side of the
+aeroplane.
+
+The two measurements FD and FE should be equal to within 1/8 inch. F is
+the centre of the fuselage or rudder-post. D and E are points marked on
+both top and bottom rear spars, and each must be the same fixed
+distance from the butt of the spar. Two measurements on each side of the
+aeroplane.
+
+If these over-all measurements are not correct, then it is probably due
+to some of the drift or anti-drift wires being too tight or too slack.
+It may possibly be due to the fuselage being out of truth, but of course
+the rigger should have made quite sure that the fuselage was true before
+rigging the rest of the machine. Again, it may be due to the internal
+bracing wires within the lifting surfaces not being accurately adjusted,
+but of course this should have been seen to before covering the surfaces
+with fabric.
+
+
+FUSELAGE.--The method of truing the fuselage is laid down in the
+aeroplane's specifications. After it has been adjusted according to the
+specified directions, it should then be arranged on trestles in such
+a way as to make about three-quarters of it towards the tail stick out
+unsupported. In this way it will assume a condition as near as
+possible to flying conditions, and when it is in this position the set
+measurements should be confirmed. If this is not done it may be out of
+truth, but perhaps appear all right when supported by trestles at both
+ends, as, in such case, its weight may keep it true as long as it is
+resting upon the trestles.
+
+
+THE TAIL-PLANE (EMPENNAGE).--The exact angle of incidence of the
+tail-plane is laid down in the aeroplane's specifications. It is
+necessary to make sure that the spars are horizontal when the aeroplane
+is in flying position and the tail unsupported as explained above under
+the heading of Fuselage. If the spars are tapered, then make sure that
+their centre lines are horizontal.
+
+
+UNDERCARRIAGE.--The undercarriage must be very carefully aligned as laid
+down in the specifications.
+
+1. The aeroplane must be placed in its flying position and sufficiently
+high to ensure the wheels being off the ground when rigged. When in this
+position the axle must be horizontal and the bracing wires adjusted to
+secure the various set measurements stated in the specifications.
+
+2. Make sure that the struts bed well down into their sockets.
+
+3. Make sure that the shock absorbers are of equal tension. In the case
+of rubber shock absorbers, both the number of turns and the lengths must
+be equal.
+
+
+HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.
+
+
+DIRECTIONAL STABILITY will be badly affected if there is more drift
+(i.e., resistance) on one side of the aeroplane than there is on the
+other side. The aeroplane will tend to turn towards the side having the
+most drift. This may be caused as follows:
+
+1. The angle of incidence of the main surface or the tail surface may
+be wrong. The greater the angle of incidence, the greater the drift. The
+less the angle, the less the drift.
+
+2. If the alignment of the fuselage, fin in front of the rudder, the
+struts or stream-line wires, or, in the case of the Maurice Farman, the
+front outriggers, are not absolutely correct--that is to say, if they
+are turned a little to the left or to the right instead of being in line
+with the direction of flight--then they will act as a rudder and cause
+the aeroplane to turn off its course.
+
+3. If any part of the surface is distorted, it will cause the aeroplane
+to turn off its course. The surface is cambered, i.e., curved, to pass
+through the air with the least possible drift. If, owing perhaps to the
+leading edge, spars, or trailing edge becoming bent, the curvature is
+spoiled, that will result in changing the amount of drift on one side of
+the aeroplane, which will then have a tendency to turn off its course.
+
+
+LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible reason
+for such a condition is a difference in the lifts of right and left
+wings. That may be caused as follows:
+
+1. The angle of incidence may be wrong. If it is too great, it will
+produce more lift than on the other side of the aeroplane; and if too
+small, it will produce less lift than on the other side--the result
+being that, in either case, the aeroplane will try to fly one wing down.
+
+2. Distorted Surfaces.--If some part of the surface is distorted, then
+its camber is spoiled, and the lift will not be the same on both sides
+of the aeroplane, and that, of course, will cause it to fly one wing
+down.
+
+
+LONGITUDINAL INSTABILITY may be due to the following reasons:
+
+1. The stagger may be wrong. The top surface may have drifted back a
+little owing to some of the wires, probably the incidence wires, having
+elongated their loops or having pulled the fittings into the wood. If
+the top surface is not staggered forward to the correct degree, then
+consequently the whole of its lift is too far back, and it will then
+have a tendency to lift up the tail of the machine too much. The
+aeroplane would then be said to be "nose-heavy."
+
+A 1/4-inch area in the stagger will make a very considerable difference
+to the longitudinal stability.
+
+2. If the angle of incidence of the main surface is not right, it will
+have a bad effect, especially in the case of an aeroplane with a lifting
+tail-plane.
+
+If the angle is too great, it will produce an excess of lift, and that
+may lift up the nose of the aeroplane and result in a tendency to fly
+"tail-down." If the angle is too small, it will produce a decreased
+lift, and the aeroplane may have a tendency to fly "nose-down."
+
+3. The fuselage may have become warped upward or downward, thus giving
+the tail-plane an incorrect angle of incidence. If it has too much
+angle, it will lift too much, and the aeroplane will be "nose-heavy." If
+it has too little angle, then it will not lift enough, and the aeroplane
+will be "tail-heavy."
+
+4. (The least likely reason.) The tail-plane may be mounted upon
+the fuselage at a wrong angle of incidence, in which case it must
+be corrected. If nose-heavy, it should be given a smaller angle of
+incidence. If tail-heavy, it should be given a larger angle; but
+care should be taken not to give it too great an angle, because the
+longitudinal stability entirely depends upon the tail-plane being set at
+a much smaller angle of incidence than is the main surface, and if
+that difference is decreased too much, the aeroplane will become
+uncontrollable longitudinally. Sometimes the tail-plane is mounted on
+the aeroplane at the same angle as the main surface, but it actually
+engages the air at a lesser angle, owing to the air being deflected
+downwards by the main surface. There is then, in effect, a longitudinal
+dihedral as explained and illustrated in Chapter I.
+
+
+CLIMBS BADLY.--Such a condition is, apart from engine or propeller
+trouble, probably due to (1) distorted surfaces, or (2) too small an
+angle of incidence.
+
+
+FLIGHT SPEED POOR.--Such a condition is, apart from engine or propeller
+trouble, probably due to (1) distorted surfaces, (2) too great an
+angle of incidence, or (3) dirt or mud, and consequently excessive
+skin-friction.
+
+
+INEFFICIENT CONTROL is probably due to (1) wrong setting of control
+surfaces, (2) distortion of control surfaces, or (3) control cables
+being badly tensioned.
+
+
+WILL NOT TAXI STRAIGHT.--If the aeroplane is uncontrollable on the
+ground, it is probably due to (1) alignment of undercarriage being
+wrong, or (2) unequal tension of shock absorbers.
+
+
+
+
+CHAPTER IV. THE PROPELLER, OR "AIR-SCREW"
+
+The sole object of the propeller is to translate the power of the engine
+into thrust.
+
+The propeller screws through the air, and its blades, being set at an
+angle inclined to the direction of motion, secure a reaction, as in the
+case of the aeroplane's lifting surface.
+
+This reaction may be conveniently divided into two component parts or
+values, namely, Thrust and Drift.
+
+The Thrust is opposed to the Drift of the aeroplane, and must be equal
+and opposite to it at flying speed. If it falls off in power, then the
+flying speed must decrease to a velocity, at which the aeroplane drift
+equals the decreased thrust.
+
+The Drift of the propeller may be conveniently divided into the
+following component values:
+
+
+Active Drift, produced by the useful thrusting part of the propeller.
+
+
+Passive Drift, produced by all the rest of the propeller, i.e., by its
+detrimental surface.
+
+
+Skin Friction, produced by the friction of the air with roughnesses of
+surface.
+
+
+Eddies attending the movement of the air caused by the action of the
+propeller.
+
+
+Cavitation (very marked at excessive speed of revolution). A tendency of
+the propeller to produce a cavity or semi-vacuum in which it revolves,
+the thrust decreasing with increase of speed and cavitation.
+
+
+THRUST-DRIFT RATIO.--The proportion of thrust to drift is of paramount
+importance, for it expresses the efficiency of the propeller. It is
+affected by the following factors: Speed of Revolution.--The greater the
+speed, the greater the proportion of drift to thrust. This is due to
+the increase with speed of the passive drift, which carries with it no
+increase in thrust. For this reason propellers are often geared down to
+revolve at a lower speed than that of the engine.
+
+
+Angle of Incidence.--The same reasons as in the case of the aeroplane
+surface.
+
+Surface Area.--Ditto.
+
+Aspect Ratio.--Ditto.
+
+Camber.--Ditto.
+
+In addition to the above factors there are, when it comes to actually
+designing a propeller, mechanical difficulties to consider. For
+instance, the blades must be of a certain strength and consequent
+thickness. That, in itself, limits the aspect ratio, for it will
+necessitate a chord long enough in proportion to the thickness to make
+a good camber possible. Again, the diameter of the propeller must be
+limited, having regard to the fact that greater diameters than those
+used to-day would not only result in excessive weight of construction,
+but would also necessitate a very high undercarriage to keep the
+propeller off the ground, and such undercarriage would not only produce
+excessive drift, but would also tend to make the aeroplane stand on
+its nose when alighting. The latter difficulty cannot be overcome by
+mounting the propeller higher, as the centre of its thrust must be
+approximately coincident with the centre of aeroplane drift.
+
+
+MAINTENANCE OF EFFICIENCY.
+
+
+The following conditions must be observed:
+
+
+1. PITCH ANGLE.--The angle, at any given point on the propeller, at
+which the blade is set is known as the pitch angle, and it must be
+correct to half a degree if reasonable efficiency is to be maintained.
+
+This angle secures the "pitch," which is the distance the propeller
+advances during one revolution, supposing the air to be solid. The air,
+as a matter of fact, gives back to the thrust of the blades just as the
+pebbles slip back as one ascends a shingle beach. Such "give-back" is
+known as Slip. If a propeller has a pitch of, say, 10 feet, but actually
+advances, say, only 8 feet owing to slip, then it will be said to
+possess 20 per cent. slip.
+
+Thus, the pitch must equal the flying speed of the aeroplane plus
+the slip of the propeller. For example, let us find the pitch of a
+propeller, given the following conditions:
+
+          Flying speed.............. 70 miles per hour.
+          Propeller revolutions..... 1,200 per minute.
+          Slip...................... 15 per cent.
+
+First find the distance in feet the aeroplane will travel forward in one
+minute. That is--
+
+   369,600 feet (70 miles)
+   ------------------------ = 6,160 feet per minute.
+      60     " (minutes)
+
+
+Now divide the feet per minute by the propeller revolutions per minute,
+add 15 per cent. for the slip, and the result will be the propeller
+pitch:
+
+     6,160
+     ----- + 15 per cent. = 5 feet 1 3/5 inches.
+     1,200
+
+
+In order to secure a constant pitch from root to tip of blade, the pitch
+angle decreases towards the tip. This is necessary, since the end of the
+blade travels faster than its root, and yet must advance forward at the
+same speed as the rest of the propeller. For example, two men ascending
+a hill. One prefers to walk fast and the other slowly, but they wish to
+arrive at the top of the hill simultaneously. Then the fast walker
+must travel a farther distance than the slow one, and his angle of path
+(pitch angle) must be smaller than the angle of path taken by the slow
+walker. Their pitch angles are different, but their pitch (in this case
+altitude reached in a given time) is the same.
+
+In order to test the pitch angle, the propeller must be mounted upon
+a shaft at right angles to a beam the face of which must be perfectly
+level, thus:
+
+First select a point on the blade at some distance (say about 2 feet)
+from the centre of the propeller. At that point find, by means of a
+protractor, the angle a projection of the chord makes with the face of
+the beam. That angle is the pitch angle of the blade at that point.
+
+Now lay out the angle on paper, thus:
+
+The line above and parallel to the circumference line must be placed
+in a position making the distance between the two lines equal to the
+specified pitch, which is, or should be, marked upon the boss of the
+propeller.
+
+Now find the circumference of the propeller where the pitch angle is
+being tested. For example, if that place is 2 feet radius from the
+centre, then the circumference will be 2 feet X 2 = 4 feet diameter,
+which, if multiplied by 3.1416 = 15.56 feet circumference.
+
+Now mark off the circumference distance, which is represented above by
+A-B, and reduce it in scale for convenience.
+
+The distance a vertical line makes between B and the chord dine is
+the pitch at the point where the angle is being tested, and it should
+coincide with the specified pitch. You will note, from the above
+illustration, that the actual pitch line should meet the junction of the
+chord line and top line.
+
+The propeller should be tested at several points, about a foot apart, on
+each blade; and the diagram, provided the propeller is not faulty, will
+then look like this:
+
+At each point tested the actual pitch coincides with the specified
+pitch: a satisfactory condition.
+
+A faulty propeller will produce a diagram something like this:
+
+
+At every point tested the pitch angle is wrong, for nowhere does the
+actual pitch coincide with the specified pitch. Angles A, C, and D, are
+too large, and B is too small. The angle should be correct to half a
+degree if reasonable efficiency is to be maintained.
+
+A fault in the pitch angle may be due to (1) faulty manufacture,
+(2) distortion, or (3) the shaft hole through the boss being out of
+position.
+
+
+2. STRAIGHTNESS.--To test for straightness the propeller must be mounted
+upon a shaft. Now bring the tip of one blade round to graze some fixed
+object. Mark the point it grazes. Now bring the other tip round, and it
+should come within 1/8 inch of the mark. If it does not do so, it is due
+to (1) faulty manufacture, (2) distortion, or (3) to the hole through
+the boss being out of position.
+
+
+3. LENGTH.--The blades should be of equal length to inch.
+
+
+4. BALANCE.--The usual method of testing a propeller for balance is as
+follows: Mount it upon a shaft, which must be on ball-bearings. Place
+the propeller in a horizontal position, and it should remain in that
+position. If a weight of a trifle over an ounce placed in a bolt-hole on
+one side of the boss fails to disturb the balance, then the propeller is
+usually regarded as unfit for use.
+
+The above method is rather futile, as it does not test for the balance
+of centrifugal force, which comes into play as soon as the propeller
+revolves. It can be tested as follows:
+
+
+The propeller must be in a horizontal position, and then weighed at
+fixed points, such as A, B, C, D, E, and F, and the weights noted. The
+points A, B, and C must, of course, be at the same fixed distances from
+the centre of the propeller as the points D, E, and F. Now reverse the
+propeller and weigh at each point again. Note the results. The first
+series of weights should correspond to the second series, thus:
+
+      Weight A should equal weight F.
+         "  B   "     "     "  E.
+         "  C   "     "     "  D.
+
+
+There is no standard practice as to the degree of error permissible, but
+if there are any appreciable differences the propeller is unfit for use.
+
+
+5. SURFACE AREA.--The surface area of the blades should be equal. Test
+with callipers thus:
+
+The points between which the distances are taken must, of course, be at
+the same distance from the centre in the case of each blade.
+
+There is no standard practice as to the degree of error permissible.
+If, however, there is an error of over 1/8 inch, the propeller is really
+unfit for use.
+
+
+6. CAMBER.--The camber (curvature) of the blades should be (1) equal,
+(2) decrease evenly towards the tips of the blades, and (3) the greatest
+depth of the curve should, at any point of the blade, be approximately
+at the same percentage of the chord from the leading edge as at other
+points.
+
+It is difficult to test the top camber without a set of templates, but
+a fairly accurate idea of the concave camber can be secured by slowly
+passing a straight-edge along the blade, thus:
+
+The camber can now be easily seen, and as the straight-edge is passed
+along the blade, the observer should look for any irregularities of the
+curvature, which should gradually and evenly decrease towards the tip of
+the blade.
+
+
+7. THE JOINTS.--The usual method for testing the glued joints is by
+revolving the propeller at greater speed than it will be called upon to
+make during flight, and then carefully examining the joints to see if
+they have opened. It is not likely, however, that the reader will have
+the opportunity of making this test. He should, however, examine all the
+joints very carefully, trying by hand to see if they are quite sound.
+Suspect a propeller of which the joints appear to hold any thickness of
+glue. Sometimes the joints in the boss open a little, but this is not
+dangerous unless they extend to the blades, as the bolts will hold the
+laminations together.
+
+
+8. CONDITION OF SURFACE.--The surface should be very smooth, especially
+towards the tips of the blades. Some propeller tips have a speed of
+over 30,000 feet a minute, and any roughness will produce a bad drift or
+resistance and lower the efficiency.
+
+
+9. MOUNTING.--Great care should be taken to see that the propeller
+is mounted quite straight on its shaft. Test in the same way as for
+straightness. If it is not straight, it is possibly due to some of the
+propeller bolts being too slack or to others having been pulled up too
+tightly.
+
+
+FLUTTER.--Propeller "flutter," or vibration, may be due to faulty pitch
+angle, balance, camber, or surface area. It causes a condition sometimes
+mistaken for engine trouble, and one which may easily lead to the
+collapse of the propeller.
+
+
+CARE OF PROPELLERS.--The care of propellers is of the greatest
+importance, as they become distorted very easily.
+
+
+1. Do not store them in a very damp or a very dry place.
+
+
+2. Do not store them where the sun will shine upon them.
+
+
+3. Never leave them long in a horizontal position or leaning up against
+a wall.
+
+
+4. They should be hung on horizontal pegs, and the position of the
+propellers should be vertical.
+
+
+If the points I have impressed upon you in these notes are not attended
+to, you may be sure of the following results:
+
+
+1. Lack of efficiency, resulting in less aeroplane speed and climb than
+would otherwise be the case.
+
+
+2. Propeller "flutter" and possible collapse.
+
+
+3. A bad stress upon the propeller shaft and its bearings.
+
+
+TRACTOR.--A propeller mounted in front of the main surface.
+
+
+PUSHER.--A propeller mounted behind the main surface.
+
+
+FOUR-BLADED PROPELLERS.--Four-bladed propellers are suitable only when
+the pitch is comparatively large.
+
+For a given pitch, and having regard to "interference," they are not so
+efficient as two-bladed propellers.
+
+The smaller the pitch, the less the "gap," i.e., the distance, measured
+in the direction of the thrust, between the spiral courses of the
+blades.
+
+If the gap is too small, then the following blade will engage air
+which the preceding blade has put into motion, with the result that the
+following blade will not secure as good a reaction as would otherwise be
+the case. It is very much the same as in the case of the aeroplane gap.
+
+For a given pitch, the gap of a four-bladed propeller is only half
+that of a two-bladed one. Therefore the four-bladed propeller is only
+suitable for large pitch, as such pitch produces spirals with a large
+gap, thus offsetting the decrease in gap caused by the numerous blades.
+
+The greater the speed of rotation, the less the pitch for a given
+aeroplane speed. Then, in order to secure a large pitch and consequently
+a good gap, the four-bladed propeller is usually geared to rotate at a
+lower speed than would be the case if directly attached to the engine
+crank-shaft.
+
+
+
+
+CHAPTER V. MAINTENANCE
+
+CLEANLINESS.--The fabric must be kept clean and free from oil, as that
+will rot it. To take out dirt or oily patches, try acetone. If that will
+not remedy matters, then try petrol, but use it sparingly, as otherwise
+it will take off an unnecessary amount of dope. If that will not remove
+the dirt, then hot water and soap will do so, but, in that case, be
+sure to use soap having no alkali in it, as otherwise it may injure the
+fabric. Use the water sparingly, or it may get inside the planes and
+rust the internal bracing wires, or cause some of the wooden framework
+to swell.
+
+The wheels of the undercarriage have a way of throwing up mud on to
+the lower surface. This should, if possible, be taken off while wet. It
+should never be scraped off when dry, as that may injure the fabric. If
+dry, then it should be moistened before being removed.
+
+Measures should be taken to prevent dirt from collecting upon any
+part of the aeroplane, as, otherwise, excessive skin-friction will be
+produced with resultant loss of flight speed. The wires, being greasy,
+collect dirt very easily.
+
+
+CONTROL CABLES.--After every flight the rigger should pass his hand over
+the control cables and carefully examine them near pulleys. Removal of
+grease may be necessary to make a close inspection possible. If only one
+strand is broken the wire should be replaced. Do not forget the aileron
+balance wire on the top surface.
+
+Once a day try the tension of the control cables by smartly moving the
+control levers about as explained elsewhere.
+
+
+WIRES.--All the wires should be kept well greased or oiled, and in the
+correct tension. When examining the wires, it is necessary to place the
+aeroplane on level ground, as otherwise it may be twisted, thus throwing
+some wires into undue tension and slackening others. The best way, if
+there is time, is to pack the machine up into its "flying position."
+
+If you see a slack wire, do not jump to the conclusion that it must
+be tensioned. Perhaps its opposition wire is too tight, in which case
+slacken it, and possibly you will find that will tighten the slack wire.
+
+Carefully examine all wires and their connections near the propeller,
+and be sure that they are snaked round with safety wire, so that the
+latter may keep them out of the way of the propeller if they come
+adrift.
+
+The wires inside the fuselage should be cleaned and regreased about once
+a fortnight.
+
+
+STRUTS AND SOCKETS.--These should be carefully examined to see if any
+splitting has occurred.
+
+
+DISTORTION.--Carefully examine all surfaces, including the controlling
+surfaces, to see whether any distortion has occurred. If distortion can
+be corrected by the adjustment of wires, well and good; but if not, then
+some of the internal framework probably requires replacement.
+
+
+ADJUSTMENTS.--Verify the angles of incidence; dihedral, and stagger, and
+the rigging position of the controlling-surfaces, as often as possible.
+
+
+UNDERCARRIAGE.--Constantly examine the alignment and fittings of the
+undercarriage, and the condition of tyres and shock absorbers. The
+latter, when made of rubber, wear quickest underneath. Inspect axles and
+skids to see if there are any signs of them becoming bent. The wheels
+should be taken off occasionally and greased.
+
+
+LOCKING ARRANGEMENTS.--Constantly inspect the locking arrangements of
+turnbuckles, bolts, etc. Pay particular attention to the control cable
+connections, and to all moving parts in respect of the controls.
+
+
+LUBRICATION.--Keep all moving parts, such as pulleys, control levers,
+and hinges of controlling surfaces, well greased.
+
+
+SPECIAL INSPECTION.--Apart from constantly examining the aeroplane with
+reference to the above points I have made, I think that, in the case of
+an aeroplane in constant use it is an excellent thing to make a special
+inspection of every part, say once a week. This will take from two to
+three hours, according to the type of aeroplane. In order to carry it
+out methodically, the rigger should have a list of every part down to
+the smallest split-pin. He can then check the parts as he examines them,
+and nothing will be passed over. This, I know from experience, greatly
+increases the confidence of the pilot, and tends to produce good work in
+the air.
+
+
+WINDY WEATHER.--The aeroplane, when on the ground, should face the
+wind; and it is advisable to lash the control lever fast, so that the
+controlling surfaces may not be blown about and possibly damaged.
+
+
+"VETTING" BY EYE.--This should be practiced at every opportunity, and,
+if persevered in, it is possible to become quite expert in diagnosing by
+eye faults in flight efficiency, stability and control.
+
+The aeroplane should be standing upon level ground, or, better than
+that, packed up into its "flying position."
+
+Now stand in front of it and line up the leading edge with the main
+spar, rear spar, and trailing edge. Their shadows can usually be seen
+through the fabric. Allowance must, of course, be made for wash-in and
+wash-out; otherwise, the parts I have specified should be parallel with
+each other.
+
+Now line up the centre part of the main-plane with the tail-plane. The
+latter should be horizontal.
+
+Next, sight each interplane front strut with its rear strut. They should
+be parallel.
+
+Then, standing on one side of the aeroplane, sight all the front struts.
+The one nearest to you should cover all the others. This applies to the
+rear struts also.
+
+Look for distortion of leading edges, main and rear spars, trailing
+edges, tail-plane and controlling surfaces.
+
+This sort of thing, if practiced constantly, will not only develop an
+expert eye for diagnosis of faults, but will also greatly assist in
+impressing upon the memory the characteristics and possible troubles of
+the various types of aeroplanes.
+
+
+MISHANDLING OF THE GROUND.--This is the cause of a lot of unnecessary
+damage. The golden rule to observe is: PRODUCE NO BENDING STRESSES.
+
+Nearly all the wood in an aeroplane is designed to take merely the
+stress of direct compression, and it cannot be bent safely. Therefore,
+in packing an aeroplane up from the ground, or in pulling or pushing it
+about, be careful to stress it in such a way as to produce, as far
+as possible, only direct compression stresses. For instance, if it is
+necessary to support the lifting surface, then the packing should be
+arranged to come directly under the struts so that they may take the
+stress in the form of compression for which they are designed. Such
+supports should be covered with soft packing in order to prevent the
+fabric from becoming damaged.
+
+When pulling an aeroplane along, if possible, pull from the top of the
+undercarriage struts. If necessary to pull from elsewhere, then do so by
+grasping the interplane struts as low down as possible.
+
+Never lay fabric-covered parts upon a concrete floor. Any slight
+movement will cause the fabric to scrape over the floor with resultant
+damage.
+
+Struts, spars, etc., should never be left about the floor, as in such
+position they are likely to become scored. I have already explained the
+importance of protecting the outside fibres of the wood. Remember
+also that wood becomes distorted easily. This particularly applies to
+interplane struts. If there are no proper racks to stand them in, then
+the best plan is to lean them up against the wall in as near a vertical
+position as possible.
+
+
+TIME.--Learn to know the time necessary to complete any of the various
+rigging jobs. This is really important. Ignorance of this will lead
+to bitter disappointments in civil life; and, where Service flying
+is concerned, it will, to say the least of it, earn unpopularity with
+senior officers, and fail to develop respect and good work where men are
+concerned.
+
+
+THE AEROPLANE SHED.--This should be kept as clean and orderly as
+possible. A clean, smart shed produces briskness, energy, and pride of
+work. A dirty, disorderly shed nearly always produces slackness and poor
+quality of work, lost tools and mislaid material.
+
+
+
+
+GLOSSARY
+
+Aeronautics--The science of aerial navigation.
+
+Aerofoil--A rigid structure, of large superficial area relative to its
+thickness, designed to obtain, when driven through the air at an
+angle inclined to the direction of motion, a reaction from the air
+approximately at right angles to its surface. Always cambered when
+intended to secure a reaction in one direction only. As the term
+"aerofoil" is hardly ever used in practical aeronautics, I have,
+throughout this book, used the term SURFACE, which, while academically
+incorrect, since it does not indicate thickness, is a term usually
+used to describe the cambered lifting surfaces, i.e., the "planes" or
+"wings," and the stabilizers and the controlling aerofoils.
+
+Aerodrome--The name usually applied to a ground used for the practice
+of aviation. It really means "flying machine," but is never used in that
+sense nowadays.
+
+Aeroplane--A power-driven aerofoil with stabilizing and controlling
+surfaces.
+
+Acceleration--The rate of change of velocity.
+
+Angle of Incidence--The angle at which the "neutral lift line" of a
+surface attacks the air.
+
+Angle of Incidence, Rigger's--The angle the chord of a surface makes
+with a line parallel to the axis of the propeller.
+
+Angle of Incidence, Maximum--The greatest angle of incidence at which,
+for a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+Angle of Incidence, Minimum--The smallest angle of incidence at which,
+for a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+
+Angle of Incidence, Best Climbing--That angle of incidence at which an
+aeroplane ascends quickest. An angle approximately halfway between the
+maximum and optimum angles.
+
+Angle of Incidence, Optimum--The angle of incidence at which the
+lift-drift ratio is the highest.
+
+
+Angle, Gliding--The angle between the horizontal and the path along
+which an aeroplane at normal flying speed, but not under engine power,
+descends in still air.
+
+Angle, Dihedral--The angle between two planes.
+
+Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
+be at a lateral dihedral angle when it is inclined upward towards its
+wing-tips.
+
+Angle, Longitudinal Dihedral--The main surface and tail surface are said
+to be at a longitudinal dihedral angle when the projections of their
+neutral lift lines meet and produce an angle above them.
+
+Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting "chords"
+for "neutral life lines."
+
+Angle, Pitch--The angle at any given point of a propeller, at which
+the blade is inclined to the direction of motion when the propeller is
+revolving but the aeroplane stationary.
+
+Altimeter--An instrument used for measuring height.
+
+Air-Speed Indicator--An instrument used for measuring air pressures or
+velocities. It consequently indicates whether the surface is securing
+the requisite reaction for flight. Usually calibrated in miles per hour,
+in which case it indicates the correct number of miles per hour at only
+one altitude. This is owing to the density of the air decreasing with
+increase of altitude and necessitating a greater speed through space
+to secure the same air pressure as would be secured by less speed at a
+lower altitude. It would be more correct to calibrate it in units of air
+pressure.
+
+Air Pocket--A local movement or condition of the air causing an
+aeroplane to drop or lose its correct attitude.
+
+Aspect-Ratio--The proportion of span to chord of a surface.
+
+Air-Screw (Propeller)--A surface so shaped that its rotation about an
+axis produces a force (thrust) in the direction of its axis.
+
+Aileron--A controlling surface, usually situated at the wing-tip, the
+operation of which turns an aeroplane about its longitudinal axis;
+causes an aeroplane to tilt sideways.
+
+
+Aviation--The art of driving an aeroplane.
+
+Aviator--The driver of an aeroplane.
+
+Barograph--A recording barometer, the charts of which can be calibrated
+for showing air density or height.
+
+Barometer--An instrument used for indicating the density of air.
+
+Bank, to--To turn an aeroplane about its longitudinal axis (to tilt
+sideways) when turning to left or right.
+
+Biplane--An aeroplane of which the main lifting surface consists of a
+surface or pair of wings mounted above another surface or pair of wings.
+
+Bay--The space enclosed by two struts and whatever they are fixed to.
+
+Boom--A term usually applied to the long spars joining the tail of a
+"pusher" aeroplane to its main lifting surface.
+
+Bracing--A system of struts and tie wires to transfer a force from one
+point to another.
+
+Canard--Literally "duck." The name which was given to a type of
+aeroplane of which the longitudinal stabilizing surface (empennage)
+was mounted in front of the main lifting surface. Sometimes termed
+"tail-first" aeroplanes, but such term is erroneous, as in such a design
+the main lifting surface acts as, and is, the empennage.
+
+Cabre--To fly or glide at an excessive angle of incidence; tail down.
+
+Camber--Curvature.
+
+Chord--Usually taken to be a straight line between the trailing and
+leading edges of a surface.
+
+Cell--The whole of the lower surface, that part of the upper surface
+directly over it, together with the struts and wires holding them
+together.
+
+Centre (Line) of Pressure--A line running from wing-tip to wing-tip, and
+through which all the air forces acting upon the surface may be said to
+act, or about which they may be said to balance.
+
+Centre (Line) of Pressure, Resultant--A line transverse to the
+longitudinal axis, and the position of which is the resultant of the
+centres of pressure of two or more surfaces.
+
+Centre of Gravity--The centre of weight.
+
+Cabane--A combination of two pylons, situated over the fuselage, and
+from which anti-lift wires are suspended.
+
+Cloche--Literally "bell." Is applied to the bell-shaped construction
+which forms the lower part of the pilot's control lever in a Bleriot
+monoplane, and to which the control cables are attached.
+
+Centrifugal Force--Every body which moves in a curved path is urged
+outwards from the centre of the curve by a force termed "centrifugal."
+
+Control Lever--A lever by means of which the controlling surfaces
+are operated. It usually operates the ailerons and elevator. The
+"joy-stick".
+
+Cavitation, Propeller--The tendency to produce a cavity in the air.
+
+Distance Piece--A long, thin piece of wood (sometimes tape) passing
+through and attached to all the ribs in order to prevent them from
+rolling over sideways.
+
+Displacement--Change of position.
+
+Drift (of an aeroplane as distinct from the propeller)--The horizontal
+component of the reaction produced by the action of driving through the
+air a surface inclined upwards and towards its direction of motion PLUS
+the horizontal component of the reaction produced by the "detrimental"
+surface PLUS resistance due to "skin-friction." Sometimes termed
+"head-resistance."
+
+Drift, Active--Drift produced by the lifting surface.
+
+Drift, Passive--Drift produced by the detrimental surface.
+
+Drift (of a propeller)--Analogous to the drift of an aeroplane. It is
+convenient to include "cavitation" within this term.
+
+Drift, to--To be carried by a current of air; to make leeway.
+
+Dive, to--To descend so steeply as to produce a speed greater than the
+normal flying speed.
+
+Dope, to--To paint a fabric with a special fluid for the purpose of
+tightening and protecting it.
+
+Density--Mass of unit volume, for instance, pounds per cubic foot.
+
+Efficiency--Output            Input
+
+Efficiency (of an aeroplane as distinct from engine and propeller)--
+
+       Lift and Velocity
+    Thrust (= aeroplane drift)
+
+Efficiency, Engine--Brake horse-power
+
+       Indicated horse-power
+
+Efficiency, Propeller--
+
+                              Thrust horse-power
+                       Horse-power received from engine
+                               (= propeller drift)
+
+NOTE.--The above terms can, of course, be expressed in foot-pounds. It
+is then only necessary to divide the upper term by the lower one to find
+the measure of efficiency.
+
+Elevator--A controlling surface, usually hinged to the rear of the
+tail-plane, the operation of which turns an aeroplane about an axis
+which is transverse to the direction of normal horizontal flight.
+
+Empennage--See "Tail-plane."
+
+Energy--Stored work. For instance, a given weight of coal or petroleum
+stores a given quantity of energy which may be expressed in foot-pounds.
+
+Extension--That part of the upper surface extending beyond the span of
+the lower surface.
+
+Edge, Leading--The front edge of a surface relative to its normal
+direction of motion.
+
+Edge, Trailing--The rear edge of a surface relative to its normal
+direction of motion.
+
+Factor of Safety--Usually taken to mean the result found by dividing the
+stress at which a body will collapse by the maximum stress it will be
+called upon to bear.
+
+Fineness (of stream-line)--The proportion of length to maximum width.
+
+Flying Position--A special position in which an aeroplane must be placed
+when rigging it or making adjustments. It varies with different types of
+aeroplanes. Would be more correctly described as "rigging position."
+
+Fuselage--That part of an aeroplane containing the pilot, and to which
+is fixed the tail-plane.
+
+Fin--Additional keel-surface, usually mounted at the rear of an
+aeroplane.
+
+Flange (of a rib)--That horizontal part of a rib which prevents it from
+bending sideways.
+
+Flight--The sustenance of a body heavier than air by means of its action
+upon the air.
+
+Foot-pound--A measure of work representing the weight of 1 lb. raised 1
+foot.
+
+Fairing--Usually made of thin sheet aluminum, wood, or a light
+construction of wood and fabric; and bent round detrimental surface in
+order to give it a "fair" or "stream-like" shape.
+
+Gravity--Is the force of the Earth's attraction upon a body. It
+decreases with increase of distance from the Earth. See "Weight."
+
+Gravity, Specific--Density of substance                   Density of water.
+Thus, if the density of water is 10 lb. per unit volume, the same unit
+volume of petrol, if weighing 7 lb., would be said to have a specific
+gravity of 7/10, i.e., 0.7.
+
+Gap (of an aeroplane)--The distance between the upper and lower surfaces
+of a biplane. In a triplane or multiplane, the distance between a
+surface and the one first above it.
+
+Gap, Propeller--The distance, measured in the direction of the thrust,
+between the spiral courses of the blades.
+
+Girder--A structure designed to resist bending, and to combine lightness
+and strength.
+
+Gyroscope--A heavy circular wheel revolving at high speed, the effect of
+which is a tendency to maintain its plane of rotation against disturbing
+forces.
+
+Hangar--An aeroplane shed.
+
+Head-Resistance--Drift. The resistance of the air to the passage of a
+body.
+
+Helicopter--An air-screw revolving about a vertical axis, the direction
+of its thrust being opposed to gravity.
+
+Horizontal Equivalent--The plan view of a body whatever its attitude may
+be.
+
+Impulse--A force causing a body to gain or lose momentum.
+
+Inclinometer--A curved form of spirit-level used for indicating the
+attitude of a body relative to the horizontal.
+
+Instability--An inherent tendency of a body, which, if the body is
+disturbed, causes it to move into a position as far as possible away
+from its first position.
+
+Instability, Neutral--An inherent tendency of a body to remain in the
+position given it by the force of a disturbance, with no tendency to
+move farther or to return to its first position.
+
+Inertia--The inherent resistance to displacement of a body as distinct
+from resistance the result of an external force.
+
+Joy-Stick--See "Control Lever."
+
+Keel-Surface--Everything to be seen when viewing an aeroplane from the
+side of it.
+
+King-Post--A bracing strut; in an aeroplane, usually passing through a
+surface and attached to the main spar, and from the end or ends of which
+wires are taken to spar, surface, or other part of the construction in
+order to prevent distortion. When used in connection with a controlling
+surface, it usually performs the additional function of a lever, control
+cables connecting its ends with the pilot's control lever.
+
+Lift--The vertical component of the reaction produced by the action
+of driving through the air a surface inclined upwards and towards its
+direction of motion.
+
+Lift, Margin of--The height an aeroplane can gain in a given time and
+starting from a given altitude.
+
+Lift-Drift Ratio--The proportion of lift to drift.
+
+Loading--The weight carried by an aerofoil. Usually expressed in pounds
+per square foot of superficial area.
+
+Longeron--The term usually applied to any long spar running length-ways
+of a fuselage.
+
+Mass--The mass of a body is a measure of the quantity of material in it.
+
+Momentum--The product of the mass and velocity of a body is known as
+"momentum."
+
+Monoplane--An aeroplane of which the main lifting surface consists of
+one surface or one pair of wings.
+
+Multiplane--An aeroplane of which the main lifting surface consists of
+numerous surfaces or pairs of wings mounted one above the other.
+
+Montant--Fuselage strut.
+
+Nacelle--That part of an aeroplane containing the engine and pilot and
+passenger, and to which the tail plane is not fixed.
+
+Neutral Lift Line--A line taken through a surface in a forward direction
+relative to its direction of motion, and starting from its trailing
+edge. If the attitude of the surface is such as to make the said line
+coincident with the direction of motion, it results in no lift, the
+reaction then consisting solely of drift. The position of the neutral
+lift line, i.e., the angle it makes with the chord, varies with
+differences of camber, and it is found by means of wind-tunnel research.
+
+Newton's Laws of Motion--1. If a body be at rest, it will remain at
+rest; or, if in motion, it will move uniformly in a straight line until
+acted upon by some force.
+
+2. The rate of change of the quantity of motion (momentum) is
+proportional to the force which causes it, and takes place in the
+direction of the straight line in which the force acts. If a body be
+acted upon by several forces, it will obey each as though the others did
+not exist, and this whether the body be at rest or in motion.
+
+3. To every action there is opposed an equal and opposite reaction.
+
+Ornithopter (or Orthopter)--A flapping wing design of aircraft intended
+to imitate the flight of a bird.
+
+Outrigger--This term is usually applied to the framework connecting the
+main surface with an elevator placed in advance of it. Sometimes applied
+to the "tail-boom" framework connecting the tail-plane with the main
+lifting surface.
+
+Pancake, to--To "stall "
+
+Plane--This term is often applied to a lifting surface. Such application
+is not quite correct, since "plane" indicates a flat surface, and the
+lifting surfaces are always cambered.
+
+Propeller--See "Air-Screw."
+
+Propeller, Tractor--An air-screw mounted in front of the main lifting
+surface.
+
+Propeller, Pusher--An air-screw mounted behind the main lifting surface.
+
+Pusher--An aeroplane of which the propeller is mounted behind the main
+lifting surface.
+
+Pylon--Any V-shaped construction from the point of which wires are
+taken.
+
+Power--Rate of working.
+
+Power, Horse--One horse-power represents a force sufficient to raise
+33,000 lbs. 1 foot in a minute.
+
+Power, Indicated Horse--The I.H.P. of an engine is a measure of the rate
+at which work is done by the pressure upon the piston or pistons, as
+distinct from the rate at which the engine does work. The latter is
+usually termed "brake horse-power," since it may be measured by an
+absorption brake.
+
+Power, Margin of--The available quantity of power above that necessary
+to maintain horizontal flight at the optimum angle.
+
+Pitot Tube--A form of air-speed indicator consisting of a tube with open
+end facing the wind, which, combined with a static pressure or suction
+tube, is used in conjunction with a gauge for measuring air pressures or
+velocities. (No. 1 in diagram.)
+
+Pitch, Propeller--The distance a propeller advances during one
+revolution supposing the air to be solid.
+
+Pitch, to--To plunge nose-down.
+
+Reaction--A force, equal and opposite to the force of the action
+producing it.
+
+Rudder--A controlling surface, usually hinged to the tail, the operation
+of which turns an aeroplane about an axis which is vertical in normal
+horizontal flight; causes an aeroplane to turn to left or right of the
+pilot.
+
+Roll, to--To turn about the longitudinal axis.
+
+Rib, Ordinary--A light curved wooden part mounted in a fore and aft
+direction within a surface. The ordinary ribs give the surface its
+camber, carry the fabric, and transfer the lift from the fabric to the
+spars.
+
+Rib, Compression--Acts as an ordinary rib, besides bearing the stress of
+compression produced by the tension of the internal bracing wires.
+
+Rib, False--A subsidiary rib, usually used to improve the camber of the
+front part of the surface.
+
+Right and Left Hand--Always used relative to the position of the pilot.
+When observing an aeroplane from the front of it, the right hand side of
+it is then on the left hand of the observer.
+
+Remou--A local movement or condition of the air which may cause
+displacement of an aeroplane.
+
+Rudder-Bar--A control lever moved by the pilot's feet, and operating the
+rudder.
+
+Surface--See "Aerofoil."
+
+Surface, Detrimental--All exterior parts of an aeroplane including
+the propeller, but excluding the (aeroplane) lifting and (propeller)
+thrusting surfaces.
+
+Surface, Controlling--A surface the operation of which turns an
+aeroplane about one of its axes.
+
+Skin-Friction--The friction of the air with roughness of surface. A form
+of drift.
+
+Span---The distance from wing-tip to wing-tip.
+
+Stagger--The distance the upper surface is forward of the lower surface
+when the axis of the propeller is horizontal.
+
+Stability--The inherent tendency of a body, when disturbed, to return to
+its normal position.
+
+Stability, Directional--The stability about an axis which is vertical
+during normal horizontal flight, and without which an aeroplane has no
+natural tendency to remain upon its course.
+
+Stability, Longitudinal--The stability of an aeroplane about an axis
+transverse to the direction of normal horizontal flight, and without
+which it has no tendency to oppose pitching and tossing.
+
+Stability, Lateral--The stability of an aeroplane about its longitudinal
+axis, and without which it has no tendency to oppose sideways rolling.
+
+Stabilizer--A surface, such as fin or tail-plane, designed to give an
+aeroplane inherent stability.
+
+Stall, to--To give or allow an aeroplane an angle of incidence greater
+than the "maximum" angle, the result being a fall in the lift-drift
+ratio, the lift consequently becoming less than the weight of the
+aeroplane, which must then fall, i.e., "stall" or "pancake."
+
+Stress--Burden or load.
+
+Strain--Deformation produced by stress.
+
+Side-Slip, to--To fall as a result of an excessive "bank" or "roll."
+
+Skid, to--To be carried sideways by centrifugal force when turning to
+left or right.
+
+Skid, Undercarriage--A spar, mounted in a fore and aft direction, and to
+which the wheels of the undercarriage are sometimes attached. Should
+a wheel give way the skid is then supposed to act like the runner of a
+sleigh and to support the aeroplane.
+
+Skid, Tail--A piece of wood or other material, orientable, and fitted
+with shock absorbers, situated under the tail of an aeroplane in order
+to support it upon the ground and to absorb the shock of alighting.
+
+
+Section--Any separate part of the top surface, that part of the bottom
+surface immediately underneath it, with their struts and wires.
+
+Spar--Any long piece of wood or other material.
+
+Spar, Main--A spar within a surface and to which all the ribs are
+attached, such spar being the one situated nearest to the centre of
+pressure. It transfers more than half the lift from the ribs to the
+bracing.
+
+Spar, Rear--A spar within a surface, and to which all the ribs are
+attached, such spar being situated at the rear of the centre of pressure
+and at a greater distance from it than is the main spar. It transfers
+less than half of the lift from the ribs to the bracing.
+
+Strut--Any wooden member intended to take merely the stress of direct
+compression.
+
+Strut, Interplane--A strut holding the top and bottom surfaces apart.
+
+Strut, Fuselage--A strut holding the fuselage longerons apart. It should
+be stated whether top, bottom, or side. If side, then it should be
+stated whether right or left hand. Montant.
+
+Strut, Extension--A strut supporting an "extension" when not in flight.
+It may also prevent the extension from collapsing upwards during flight.
+
+Strut, Undercarriage--
+
+Strut, Dope--A strut within a surface, so placed as to prevent the
+tension of the doped fabric from distorting the framework.
+
+Serving--To bind round with wire, cord, or similar material. Usually
+used in connection with wood joints and wire cable splices.
+
+Slip, Propeller--The pitch less the distance the propeller advances
+during one revolution.
+
+Stream-Line--A form or shape of detrimental surface designed to produce
+minimum drift.
+
+Toss, to--To plunge tail-down.
+
+Torque, Propeller--The tendency of a propeller to turn an aeroplane
+about its longitudinal axis in a direction opposite to that in which the
+propeller revolves.
+
+Tail-Slide--A fall whereby the tail of an aeroplane leads.
+
+Tractor--An aeroplane of which the propeller is mounted in front of the
+main lifting surface.
+
+Triplane--An aeroplane of which the main lifting surface consists of
+three surfaces or pairs of wings mounted one above the other.
+
+Tail-Plane--A horizontal stabilizing surface mounted at some distance
+behind the main lifting surface. Empennage.
+
+Turnbuckle--A form of wire-tightener, consisting of a barrel into each
+end of which is screwed an eyebolt. Wires are attached to the eyebolts
+and the required degree of tension is secured by means of rotating the
+barrel.
+
+Thrust, Propeller--See "Air-Screw."
+
+Undercarriage--That part of an aeroplane beneath the fuselage or
+nacelle, and intended to support the aeroplane when at rest, and to
+absorb the shock of alighting.
+
+Velocity--Rate of displacement; speed.
+
+Volplane--A gliding descent.
+
+Weight--Is a measure of the force of the Earth's attraction (gravity)
+upon a body. The standard unit of weight in this country is 1 lb., and
+is the force of the Earth's attraction on a piece of platinum called
+the standard pound, deposited with the Board of Trade in London. At the
+centre of the Earth a body will be attracted with equal force in
+every direction. It will therefore have no weight, though its mass
+is unchanged. Gravity, of which weight is a measure, decreases with
+increase of altitude.
+
+Web (of a rib)--That vertical part of a rib which prevents it from
+bending upwards.
+
+Warp, to--To distort a surface in order to vary its angle of incidence.
+To vary the angle of incidence of a controlling surface.
+
+Wash--The disturbance of air produced by the flight of an aeroplane.
+
+Wash-in--An increasing angle of incidence of a surface towards its
+wing-tip.
+
+Wash-out--A decreasing angle of incidence of a surface towards its
+wing-tip.
+
+Wing-tip--The right- or left-hand extremity of a surface.
+
+Wire--A wire is, in Aeronautics, always known by the name of its
+function.
+
+Wire, Lift or Flying--A wire opposed to the direction of lift, and used
+to prevent a surface from collapsing upward during flight.
+
+Wire, Anti-lift or Landing--A wire opposed to the direction of gravity,
+and used to sustain a surface when it is at rest.
+
+Wire, Drift--A wire opposed to the direction of drift, and used to
+prevent a surface from collapsing backwards during flight.
+
+Wire, Anti-drift--A wire opposed to the tension of a drift wire, and
+used to prevent such tension from distorting the framework.
+
+Wire, Incidence--A wire running from the top of an interplane strut
+to the bottom of the interplane strut in front of or behind it.
+It maintains the "stagger" and assists in maintaining the angle of
+incidence. Sometimes termed "stagger wire."
+
+Wire, Bracing--Any wire holding together the framework of any part of
+an aeroplane. It is not, however, usually applied to the wires described
+above unless the function performed includes a function additional to
+those described above. Thus, a lift wire, while strictly speaking a
+bracing wire, is not usually described as one unless it performs the
+additional function of bracing some well-defined part such as the
+undercarriage. It will then be said to be an "undercarriage bracing lift
+wire." It might, perhaps, be acting as a drift wire also, in which
+case it will then be de-scribed as an "undercarriage bracing lift-drift
+wire." It should always be stated whether a bracing wire is (1) top, (2)
+bottom, (3) cross, or (4) side. If a "side bracing wire," then it should
+be stated whether right- or left-hand.
+
+Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)
+within a surface.
+
+Wire, Top Bracing--A bracing wire, approximately horizontal and situated
+between the top longerons of fuselate, between top tail booms, or at the
+top of similar construction.
+
+Wire, Bottom Bracing--Ditto, substituting "bottom" for "top."
+
+Wire, Side Bracing--A bracing wire crossing diagonally a side bay of
+fuselage, tail boom bay, undercarriage side bay or centre-section side
+bay. This term is not usually used with reference to incidence wires,
+although they cross diagonally the side bays of the cell. It should be
+stated whether right- or left-hand.
+
+Wire, Cross Bracing--A bracing wire, the position of which is diagonal
+from right to left when viewing it from the front of an aeroplane.
+
+Wire, Control Bracing--A wire preventing distortion of a controlling
+surface.
+
+Wire, Control--A wire connecting a controlling surface with the pilot's
+control lever, wheel, or rudder-bar.
+
+Wire, Aileron Gap--A wire connecting top and bottom ailerons.
+
+Wire, Aileron Balance--A wire connecting the right- and left-hand top
+ailerons. Sometimes termed the "aileron compensating wire."
+
+Wire, Snaking--A wire, usually of soft metal, wound spirally or tied
+round another wire, and attached at each end to the framework. Used to
+prevent the wire round which it is "snaked" from becoming, in the event
+of its displacement, entangled with the propeller.
+
+Wire, Locking--A wire used to prevent a turnbuckle barrel or other
+fitting from losing its adjustment.
+
+Wing--Strictly speaking, a wing is one of the surfaces of an
+ornithopter. The term is, however, often applied to the lifting surface
+of an aeroplane when such surface is divided into two parts, one being
+the left-hand "wing," and the other the right-hand "wing."
+
+Wind-Tunnel--A large tube used for experimenting with surfaces and
+models, and through which a current of air is made to flow by artificial
+means.
+
+Work--Force X displacement.
+
+Wind-Screen--A small transparent screen mounted in front of the pilot to
+protect his face from the air pressure.
+
+
+
+
+
+FOOTNOTES:
+
+
+[1] Propeller Slip: As the propeller screws through the air, the
+latter to a certain extent gives back to the thrust of the propellor
+blades, just as the shingle on the beach slips back as you ascend it.
+Such "give-back" is known as "slip," and anyone behind the propellor
+will feel the slip as a strong draught of air.
+
+[2] Helicopter. An air-screw revolving upon a vertical axis. If driven
+with sufficient power, it will lift vertically, but having regard to the
+mechanical difficulties of such construction, it is a most inefficient
+way of securing lift compared with the arrangement of an inclined
+surface driven by a propeller revolving about a horizontal axis.
+
+[3] Pancakes: Pilot's slang for stalling an aeroplane and dropping
+like a pancake.
+
+[4] Morane parasol: A type of Morane monoplane in which the lifting
+surfaces are raised above the pilot in order to afford him a good view
+of the earth.
+
+[5] Skin friction is that part of the drift due to the friction of the
+air with roughnesses upon the surface of the aeroplane.
+
+[6] Banking: When an aeroplane is turned to the left or the right
+the centrifugal force of its momentum causes it to skid sideways and
+outwards away from the centre of the turn. To minimize such action the
+pilot banks, i.e., tilts, the aeroplane sideways in order to oppose the
+underside of the planes to the air. The aeroplane will not then skid
+outwards beyond the slight skid necessary to secure a sufficient
+pressure of air to balance the centrifugal force.
+
+[7] An explanation of the way in which the wash-out is combined with a
+wash-in to offset propellor torque will be found on p. 82.
+
+[8] A.M.'s: Air Mechanics.
+
+[9] Butt means to thicken at the end. Screw means to machine a
+thread on the butt-end of the wire, and in this way the wire can make
+connection with the desired place by being screwed into a metal fitting,
+thus eliminating the disadvantage of the unsatisfactory loop.
+
+[10] Deviation curve: A curved line indicating any errors in the
+compass.
+
+[11] A propeller screws through the air, and the distance it advances
+during one revolution, supposing the air to be solid, is known as the
+pitch. The pitch, which depends upon the angle of the propeller blades,
+must be equal to the speed of the aeroplane, plus the slip, and if, on
+account of the rarity of the air the speed of the aeroplane increases,
+then the angle and pitch should be correspondingly increased. Propellers
+with a pitch capable of being varied by the pilot are the dream of
+propeller designers. For explanation of "slip" see Chapter IV. on
+propellers.
+
+[12] Getting out of my depth? Invading the realms of fancy? Well,
+perhaps so, but at any rate it is possible that extraordinary speed
+through space may be secured if means are found to maintain the impulse
+of the engine and the thrust-drift efficiency of the propeller at great
+altitude.
+
+[13] Box-kite. The first crude form of biplane.
+
+[14] See Newton's laws in the Glossary at the end of the book.
+
+[15] See "Aerofoil" in the Glossary.
+
+[16] "In effect" because, although there may be actually the greatest
+proportion of keel-surface In front of the vertical axis, such surface
+may be much nearer to the axis than is the keel-surface towards the
+tail. The latter may then be actually less than the surface in front,
+but, being farther from the axis, it has a greater leverage, and
+consequently is greater in effect than the surface in front.
+
+
+
+
+
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+*The Project Gutenberg Etext of The Aeroplane Speaks, by Barber*
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+The Aeroplane Speaks
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+by H. Barber
+Captain, Royal Flying Corps
+
+February, 1997  [Etext #818]
+
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+*The Project Gutenberg Etext of The Aeroplane Speaks, by Barber*
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+Scanned by Charles Keller with OmniPage Professional OCR software
+
+
+
+
+
+THE AEROPLANE SPEAKS
+
+BY H. BARBER
+(CAPTAIN, ROYAL FLYING CORPS)
+
+
+
+DEDICATED TO THE SUBALTERN FLYING OFFICER
+
+
+
+
+MOTIVE
+
+The reasons impelling me to write this book, the maiden
+effort of my pen, are, firstly, a strong desire to help the
+ordinary man to understand the Aeroplane and the joys
+and troubles of its Pilot; and, secondly, to produce something
+of PRACTICAL assistance to the Pilot and his invaluable assistant
+the Rigger. Having had some eight years' experience in
+designing, building, and flying aeroplanes, I have hopes
+that the practical knowledge I have gained may offset the
+disadvantage of a hand more used to managing the ``joy-
+stick'' than the dreadful haltings, the many side-slips, the
+irregular speed, and, in short, the altogether disconcerting
+ways of a pen.
+
+The matter contained in the Prologue appeared in the
+Field of May 6th, 13th, 20th, and 27th, 1916, and is now
+reprinted by the kind permission of the editor, Sir Theodore
+Cook.
+
+I have much pleasure in also acknowledging the kindness
+of Mr. C. G. Grey, editor of the Aeroplane, to whom I am
+indebted for the valuable illustrations reproduced at the
+end of this book.
+
+
+
+CONTENTS
+
+PROLOGUE
+
+PART
+I.   THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+II.  THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
+III. THE GREAT TEST
+IV.  CROSS COUNTRY 
+
+
+
+CHAPTER
+I.   FLIGHT
+II.  STABILITY AND CONTROL
+III. RIGGING
+IV.  PROPELLERS 
+V.   MAINTENANCE
+
+
+
+TYPES OF AEROPLANES
+
+GLOSSARY
+
+
+
+
+THE AEROPLANE SPEAKS
+
+PROLOGUE
+
+PART I
+
+THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+
+The Lecture Hall at the Royal Flying Corps School for
+Officers was deserted. The pupils had dispersed, and the
+Officer Instructor, more fagged than any pupil, was out on
+the aerodrome watching the test of a new machine.
+
+Deserted, did I say? But not so. The lecture that day
+had been upon the Elementary Principles of Flight, and
+they lingered yet. Upon the Blackboard was the illustration
+you see in the frontispiece.
+
+``I am the side view of a Surface,'' it said, mimicking
+the tones of the lecturer. ``Flight is secured by driving me
+through the air at an angle inclined to the direction of
+motion.''
+
+``Quite right,'' said the Angle. ``That's me, and I'm
+the famous Angle of Incidence.''
+
+``And,'' continued the Surface, ``my action is to deflect
+the air downwards, and also, by fleeing from the air behind,
+to create a semi-vacuum or rarefied area over most of the
+top of my surface.''
+
+``This is where I come in,'' a thick, gruff voice was
+heard, and went on: ``I'm the Reaction. You can't have
+action without me. I'm a very considerable force, and my
+direction is at right-angles to you,'' and he looked heavily
+at the Surface. ``Like this,'' said he, picking up the chalk
+with his Lift, and drifting to the Blackboard.
+
+``I act in the direction of the arrow R, that is, more or
+less, for the direction varies somewhat with the Angle of
+Incidence and the curvature of the Surface; and, strange
+but true, I'm stronger on the top of the Surface than at
+the bottom of it. The Wind Tunnel has proved that by
+exhaustive research--and don't forget how quickly I can
+grow! As the speed through the air increases my strength
+increases more rapidly than you might think--approximately,
+as the Square of the Speed; so you see that if the Speed of
+the Surface through the air is, for instance, doubled, then
+I am a good deal more than doubled. That's because I
+am the result of not only the mass of air displaced, but also
+the result of the Speed with which the Surface engages
+the Air. I am a product of those two factors, and at the
+speeds at which Aeroplanes fly to-day, and at the altitudes
+and consequent density of air they at present experience,
+I increase at about the Square of the Speed.
+
+``Oh, I'm a most complex and interesting personality, I
+assure you--in fact, a dual personality, a sort of aeronautical
+Dr. Jekyll and Mr. Hyde. There's Lift, my vertical part or
+COMPONENT, as those who prefer long words would say; he
+always acts vertically upwards, and hates Gravity like poison.
+He's the useful and admirable part of me. Then there's Drift,
+my horizontal component, sometimes, though rather erroneously,
+called Head Resistance; he's a villain of the deepest
+dye, and must be overcome before flight can be secured.''
+
+``And I,'' said the Propeller, ``I screw through the air and
+produce the Thrust. I thrust the Aeroplane through the air
+and overcome the Drift; and the Lift increases with the Speed
+and when it equals the Gravity of Weight, then--there you
+are--Flight! And nothing mysterious about it at all.''
+
+``I hope you'll excuse me interrupting,'' said a very
+beautiful young lady, ``my name is Efficiency, and, while
+no doubt, all you have said is quite true, and that, as my
+young man the Designer says, `You can make a tea-tray
+fly if you slap on Power enough,' I can assure you that I'm
+not to be won quite so easily.''
+
+``Well,'' eagerly replied the Lift and the Thrust, ``let's
+be friends. Do tell us what we can do to help you to overcome
+Gravity and Drift with the least possible Power. That
+obviously seems the game to play, for more Power means
+heavier engines, and that in a way plays into the hands of
+our enemy, Gravity, besides necessitating a larger Surface
+or Angle to lift the Weight, and that increases the Drift.''
+
+``Very well,'' from Efficiency, ``I'll do my best, though
+I'm so shy, and I've just had such a bad time at the Factory,
+and I'm terribly afraid you'll find it awefully dry.''
+
+``Buck up, old dear!'' This from several new-comers,
+who had just appeared. ``We'll help you,'' and one of
+them, so lean and long that he took up the whole height of
+the lecture room, introduced himself.
+
+``I'm the High Aspect Ratio,'' he said, ``and what we
+have got to do to help this young lady is to improve the
+proportion of Lift to Drift. The more Lift we can get for a
+certain area of Surface, the greater the Weight the latter
+can carry; and the less the Drift, then the less Thrust and
+Power required to overcome it. Now it is a fact that, if
+the Surface is shaped to have the greatest possible span,
+i.e., distance from wing-tip to wing-tip, it then engages more
+air and produces both a maximum Reaction and a better
+proportion of Lift to Drift.
+
+``That being so, we can then well afford to lose a little
+Reaction by reducing the Angle of Incidence to a degree
+giving a still better proportion of Lift to Drift than would
+otherwise be the case; for you must understand that the
+Lift-Drift Ratio depends very much upon the size of the
+Angle of Incidence, which should be as small as possible
+within certain limits. So what I say is, make the surface of
+Infinite Span with no width or chord, as they call it. That's
+all I require, I assure you, to make me quite perfect and of
+infinite service to Miss Efficiency.''
+
+``That's not practical politics,'' said the Surface. ``The
+way you talk one would think you were drawing L400 a
+year at Westminster, and working up a reputation as an
+Aeronautical Expert. I must have some depth and chord
+to take my Spars and Ribs, and again, I must have a certain
+chord to make it possible for my Camber (that's curvature)
+to be just right for the Angle of Incidence. If that's not
+right the air won't get a nice uniform compression and
+downward acceleration from my underside, and the rarefied
+`suction' area over the top of me will not be as even and clean
+in effect as it might be. That would spoil the Lift-Drift Ratio
+more than you can help it. Just thrust that chalk along, will
+you? and the Blackboard will show you what I mean.''
+
+``Well,'' said the Aspect Ratio, ``have it your own way,
+though I'm sorry to see a pretty young lady like Efficiency
+compromised so early in the game.''
+
+``Look here,'' exclaimed a number of Struts, ``we have
+got a brilliant idea for improving the Aspect Ratio,'' and
+with that they hopped up on to the Spars. ``Now,'' excitedly,
+``place another Surface on top of us. Now do you
+see? There is double the Surface, and that being so, the
+proportion of Weight to Surface area is halved. That's
+less burden of work for the Surface, and so the Spars need
+not be so strong and so deep, which results in not so thick
+a Surface. That means the Chord can be proportionately
+decreased without adversely affecting the Camber. With
+the Chord decreased, the Span becomes relatively greater,
+and so produces a splendid Aspect Ratio, and an excellent
+proportion of Lift to Drift.''
+
+``I don't deny that they have rather got me there,''
+said the Drift, ``but all the same, don't forget my increase
+due to the drift of the Struts and their bracing wires.''
+
+``Yes, I dare say,'' replied the Surface, ``but remember
+that my Spars are less deep than before, and consequently I
+am not so thick now, and shall for that reason also be able
+to go through the air with a less proportion of Drift to Lift.''
+
+``Remember me also, please,'' croaked the Angle of
+Incidence. ``Since the Surface has now less weight to carry
+for its area, I may be set at a still lesser and finer Angle.
+That means less Drift again. We are certainly getting on
+splendidly! Show us how it looks now, Blackboard.'' And
+the Blackboard obligingly showed them as follows:
+
+``Well, what do you think of that?'' they all cried to the
+Drift.
+
+``You think you are very clever,'' sneered the Drift.
+``But you are not helping Efficiency as much as you think.
+The suction effect on the top of the lower Surface will give
+a downward motion to the air above it and the result will
+be that the bottom of the top Surface will not secure as good
+a Reaction from the air as would otherwise be the case,
+and that means loss of Lift; and you can't help matters
+by increasing the gap between the surfaces because that
+means longer Struts and Wires, and that in itself would
+help me, not to speak of increasing the Weight. You see
+it's not quite so easy as you thought.''
+
+At this moment a hiccough was heard, and a rather fast
+and rakish-looking chap, named Stagger, spoke up. ``How
+d'ye do, miss,'' he said politely to Efficiency, with a side
+glance out of his wicked old eye. ``I'm a bit of a knut,
+and without the slightest trouble I can easily minimize
+the disadvantage that old reprobate Drift has been frightening
+you with. I just stagger the top Surface a bit forward,
+and no longer is that suction effect dead under it. At the
+same time I'm sure the top Surface will kindly extend its
+Span for such distance as its Spars will support it without
+the aid of Struts. Such extension will be quite useful, as
+there will be no Surface at all underneath it to interfere
+with the Reaction above.'' And the Stagger leaned
+forward and picked up the Chalk, and this is the picture
+he drew:
+
+Said the Blackboard, ``That's not half bad! It really
+begins to look something like the real thing, eh?''
+
+``The real thing, is it?'' grumbled Drift. ``Just consider
+that contraption in the light of any one Principle, and I
+warrant you will not find one of them applied to perfection.
+The whole thing is nothing but a Compromise.'' And he
+glared fixedly at poor Efficiency.
+
+``Oh, dear! Oh, dear!'' she cried. ``I'm always getting
+into trouble. What WILL the Designer say?''
+
+``Never mind, my dear,'' said the Lift-Drift Ratio,
+consolingly. ``You are improving rapidly, and quite useful
+enough now to think of doing a job of work.''
+
+``Well, that's good news,'' and Efficiency wiped her eyes
+with her Fabric and became almost cheerful. ``Suppose
+we think about finishing it now? There will have to be an
+Engine and Propeller, won't there? And a body to fix
+them in, and tanks for oil and petrol, and a tail, and,'' archly,
+``one of those dashing young Pilots, what?''
+
+``Well, we are getting within sight of those interesting
+Factors,'' said the Lift-Drift Ratio, ``but first of all we
+had better decide upon the Area of the Surfaces, their Angle
+of Incidence and Camber. If we are to ascend as quickly
+as possible the Aeroplane must be SLOW in order to secure
+the best possible Lift-Drift Ratio, for the drift of the struts
+wires, body, etc., increases approximately as the square
+of the speed, but it carries with it no lift as it does in the
+case of the Surface. The less speed then, the less such
+drift, and the better the Aeroplane's proportion of lift to
+drift; and, being slow, we shall require a LARGE SURFACE in
+order to secure a large lift relative to the weight to be carried.
+We shall also require a LARGE ANGLE OF INCIDENCE relative to
+the horizontal, in order to secure a proper inclination of
+the Surface to the direction of motion, for you must remember
+that, while we shall fly upon an even keel and with
+the propeller thrust horizontal (which is its most efficient
+attitude), our flight path, which is our direction of motion,
+will be sloping upwards, and it will therefore be necessary
+to fix the Surface to the Aeroplane at a very considerable
+angle relative to the horizontal Propeller Thrust in order to
+secure a proper angle to the upwards direction of motion.
+Apart from that, we shall require a larger Angle of Incidence
+than in the case of a machine designed purely for speed,
+and that means a correspondingly LARGE CAMBER.
+
+``On the other hand, if we are thinking merely of Speed,
+then a SMALL SURFACE, just enough to lift the weight off the
+ground, will be best, also a SMALL ANGLE to cut the Drift down
+and that, of course, means a relatively SMALL CAMBER.
+
+``So you see the essentials for CLIMB or quick ascent and
+for SPEED are diametrically opposed. Now which is it to be?''
+
+``Nothing but perfection for me,'' said Efficiency. ``What
+I want is Maximum Climb and Maximum Speed for the
+Power the Engine produces.''
+
+And each Principle fully agreed with her beautiful
+sentiments, but work together they would not.
+
+The Aspect Ratio wanted infinite Span, and hang the
+Chord.
+
+The Angle of Incidence would have two Angles and two
+Cambers in one, which was manifestly absurd; the Surface
+insisted upon no thickness whatever, and would not hear
+of such things as Spars and Ribs; and the Thrust objected
+to anything at all likely to produce Drift, and very nearly
+wiped the whole thing off the Blackboard.
+
+There was, indeed, the makings of a very pretty quarrel
+when the Letter arrived. It was about a mile long, and
+began to talk at once.
+
+``I'm from the Inventor,'' he said, and hope rose in the
+heart of each heated Principle. ``It's really absurdly simple.
+All the Pilot has to do is to touch a button, and at his will,
+VARY the area of the Surface, the Angle of Incidence,
+and the Camber! And there you are--Maximum Climb or
+Maximum Speed as required! How does that suit you?''
+
+``That suits us very well,'' said the Surface, ``but, excuse
+me asking, how is it done without apparatus increasing the
+Drift and the Weight out of all reason? You won't mind
+showing us your Calculations, Working Drawings, Stress
+Diagrams, etc., will you?''
+
+Said the Letter with dignity, ``I come from an Inventor
+so brilliantly clever as to be far above the unimportant
+matters you mention. He is no common working man,
+sir! He leaves such things to Mechanics. The point is, you
+press a button and----''
+
+``Look here,'' said a Strut, rather pointedly, ``where do
+you think you are going, anyway?''
+
+``Well,'' from the Letter, ``as a matter of fact, I'm not
+addressed yet, but, of course, there's no doubt I shall reach
+the very highest quarters and absolutely revolutionize Flight
+when I get there.''
+
+Said the Chalk, ``I'll address you, if that's all you want;
+now drift along quickly!'' And off went the Letter to The
+Technical Editor, ``Daily Mauler,'' London.
+
+And a League was formed, and there were Directors with
+Fees, and several out-of-service Tin Hats, and the Man-who-
+takes-the-credit, and a fine fat Guinea-pig, and all the rest
+of them. And the Inventor paid his Tailor and had a Hair-
+Cut, and is now a recognized Press Expert--but he is still
+waiting for those Mechanics!
+
+``I'm afraid,'' said the Slide-rule, who had been busy
+making those lightning-like automatic calculations for which
+he is so famous, ``it's quite impossible to fully satisfy all of
+you, and it is perfectly plain to me that we shall have to effect
+a Compromise and sacrifice some of the Lift for Speed.''
+
+Thud! What was that?
+
+Efficiency had fainted dead away! The last blow had
+been too much for her. And the Principles gathered mournfully
+round, but with the aid of the Propeller Slip[[1]] and a
+friendly lift from the Surface she was at length revived and
+regained a more normal aspect.
+
+
+[[1]] Propeller Slip: As the propeller screws through the air,
+the latter to a certain extent gives back to the thrust of the
+propellor blades, just as the shingle on the beach slips back
+as you ascend it. Such ``give-back'' is known as ``slip,''
+and anyone behind the propellor will feel the slip as a
+strong draught of air.
+
+
+Said the Stagger with a raffish air, ``My dear young lady,
+I assure you that from the experiences of a varied career,
+I have learned that perfection is impossible, and I am sure
+the Designer will be quite satisfied if you become the Most
+Efficient Compromise.''
+
+``Well, that sounds so common sense,'' sighed Efficiency,
+``I suppose it must be true, and if the Designer is satisfied,
+that's all I really care about. Now do let's get on with the job.''
+
+So the Chalk drew a nice long slim body to hold the
+Engine and the tanks, etc., with room for the Pilot's and
+Passenger's seats, and placed it exactly in the middle of the
+Biplane. And he was careful to make its position such that
+the Centre of Gravity was a little in advance of the Centre
+of Lift, so that when the Engine was not running and there
+was consequently no Thrust, the Aeroplane should be ``nose-
+heavy'' just to the right degree, and so take up a natural
+glide to Earth--and this was to help the Pilot and relieve
+him of work and worry, should he find himself in a fog or
+a cloud. And so that this tendency to glide downwards
+should not be in evidence when the Engine was running and
+descent not desired, the Thrust was placed a little below
+the Centre of Drift or Resistance. In this way it would in
+a measure pull the nose of the Aeroplane up and counterbalance
+the ``nose-heavy'' tendency.
+
+And the Engine was so mounted that when the Propeller-
+Thrust was horizontal, which is its most efficient position,
+the Angle of Incidence and the Area of the surfaces were
+just sufficient to give a Lift a little in excess of the Weight.
+And the Camber was such that, as far as it was concerned,
+the Lift-Drift Ratio should be the best possible for that Angle
+of Incidence. And a beautifully simple under-carriage was
+added, the outstanding features of which were simplicity,
+strength, light-weight, and minimum drift. And, last of
+all, there was the Elevator, of which you will hear more
+by-and-by. And this is what it looked like then:
+
+And Efficiency, smiling, thought that it was not such a
+bad compromise after all and that the Designer might well
+be satisfied.
+
+``Now,'' said she, ``there's just one or two points I'm
+a bit hazy about. It appears that when the Propeller shaft
+is horizontal and so working in its most efficient attitude,
+I shall have a Lift from the Surfaces slightly in excess of the
+Weight. That means I shall ascend slightly, at the same time
+making nearly maximum speed for the power and thrust.
+Can't I do better than that?''
+
+``Yes, indeed,'' spoke up the Propeller, ``though it means
+that I must assume a most undignified attitude, for helicopters[[2]]
+I never approved of. In order to ascend more
+quickly the Pilot will deflect the Elevator, which, by the
+way, you see hinged to the Tail. By that means he will
+force the whole Aeroplane to assume a greater Angle of
+Incidence. And with greater Angle, the Lift will increase,
+though I'm sorry to say the Drift will increase also. Owing
+to the greater Drift, the Speed through the air will lessen,
+and I'm afraid that won't be helpful to the Lift; but I shall
+now be pointing upwards, and besides overcoming the Drift
+in a forward direction I shall be doing my best to haul
+the Aeroplane skywards. At a certain angle known as the
+Best Climbing Angle, we shall have our Maximum Margin
+of Lift, and I'm hoping that may be as much as almost a
+thousand feet altitude a minute.''
+
+
+[[2]] Helicopter. An air-screw revolving upon a vertical axis.
+If driven with sufficient power, it will lift vertically,
+but having regard to the mechanical difficulties of such construction,
+it is a most inefficient way of securing lift compared with the
+arrangement of an inclined surface driven by a propeller
+revolving about a horizontal axis.
+
+
+``Then, if the Pilot is green, my chance will come,'' said
+the Maximum Angle of Incidence. ``For if the Angle is
+increased over the Best Climbing Angle, the Drift will rush
+up; and the Speed, and with it the Lift, will, when my
+Angle is reached, drop to a point when the latter will be no
+more than the Weight. The Margin of Lift will have
+entirely disappeared, and there we shall be, staggering
+along at my tremendous angle, and only just maintaining
+horizontal flight.''
+
+``And then with luck I'll get my chance,'' said the Drift.
+``If he is a bit worse than green, he'll perhaps still further
+increase the Angle. Then the Drift, largely increasing, the
+Speed, and consequently the Lift, will become still less,
+i.e., less than the Weight, and then--what price pancakes,[[3]]
+eh?''
+
+
+[[3]] Pancakes: Pilot's slang for stalling an aeroplane
+and dropping like a pancake.
+
+
+``Thank you,'' from Efficiency, ``that was all most
+informing. And now will you tell me, please, how the
+greatest Speed may be secured?''
+
+``Certainly, now it's my turn,'' piped the Minimum Angle
+of Incidence. ``By means of the Elevator, the Pilot places
+the Aeroplane at my small Angle, at which the Lift only
+just equals the Weight, and, also, at which we shall make
+greater speed with no more Drift than before. Then we get
+our greatest Speed, just maintaining horizontal flight.''
+
+``Yes; though I'm out of the horizontal and thrusting
+downwards,'' grumbled the Propeller, ``and that's not
+efficient, though I suppose it's the best we can do until that
+Inventor fellow finds his Mechanics.''
+
+``Thank you so much,'' said Efficiency. ``I think I have
+now at any rate an idea of the Elementary Principles of
+Flight, and I don't know that I care to delve much deeper,
+for sums always give me a headache; but isn't there something
+about Stability and Control? Don't you think I ought
+to have a glimmering of them too?''
+
+``Well, I should smile,'' said a spruce Spar, who had come
+all the way from America. ``And that, as the Lecturer
+says, `will be the subject of our next lecture,' so be here
+again to-morrow, and you will be glad to hear that it will be
+distinctly more lively than the subject we have covered
+to-day.''
+
+
+
+PART II
+
+THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES,
+FINISH THE JOB
+
+Another day had passed, and the Flight Folk had again
+gathered together and were awaiting the arrival of Efficiency
+who, as usual, was rather late in making an appearance.
+
+The crowd was larger than ever, and among the newcomers
+some of the most important were the three Stabilities,
+named Directional, Longitudinal, and Lateral, with
+their assistants, the Rudder, Elevator, and Ailerons. There
+was Centrifugal Force, too, who would not sit still and
+created a most unfavourable impression, and Keel-Surface,
+ the Dihedral Angle, and several other lesser fry.
+
+``Well,'' said Centrifugal Force, ``I wish this Efficiency
+I've heard so much about would get a move on. Sitting
+still doesn't agree with me at all. Motion I believe in.
+There's nothing like motion--the more the better.''
+
+``We are entirely opposed to that,'' objected the three
+Stabilities, all in a breath. ``Unless it's in a perfectly
+straight line or a perfect circle. Nothing but perfectly
+straight lines or, upon occasion, perfect circles satisfy us,
+and we are strongly suspicious of your tendencies.''
+
+``Well, we shall see what we shall see,'' said the Force
+darkly. ``But who in the name of blue sky is this?''
+
+And in tripped Efficiency, in a beautifully ``doped''
+dress of the latest fashionable shade of khaki-coloured
+fabric, a perfectly stream-lined bonnet, and a bewitching
+little Morane parasol,[[4]] smiling as usual, and airily exclaiming,
+``I'm so sorry I'm late, but you see the Designer's
+such a funny man. He objects to skin friction,[[5]] and insisted
+upon me changing my fabric for one of a smoother
+surface, and that delayed me. Dear me, there are a lot
+more of us to-day, aren't there? I think I had better meet
+one at a time.'' And turning to Directional Stability, she
+politely asked him what he preferred to do.
+
+
+[[4]] Morane parasol: A type of Morane monoplane in which the
+lifting surfaces are raised above the pilot in order to afford
+him a good view of the earth.
+
+[[5]] Skin friction is that part of the drift due to the friction
+of the air with roughnesses upon the surface of the aeroplane.
+
+
+``My purpose in life, miss,'' said he, ``is to keep the Aeroplane
+on its course, and to achieve that there must be, in
+effect, more Keel-Surface behind the Vertical Turning Axis
+than there is in front of it.''
+
+Efficiency looking a little puzzled, he added: ``Just like
+a weathercock, and by Keel-Surface I mean everything
+you can see when you view the Aeroplane from the side of
+it--the sides of the body, struts, wires, etc.''
+
+``Oh, now I begin to see light,'' said she: ``but just
+exactly how does it work?''
+
+``I'll answer that,'' said Momentum. ``When perhaps
+by a gust of air the Aeroplane is blown out of its course
+and points in another direction, it doesn't immediately
+fly off on that new course. I'm so strong I pull it off the
+new course to a certain extent, and towards the direction
+of the old course. And so it travels, as long as my strength
+lasts, in a more or less sideways position.''
+
+``Then,'' said the Keel-Surface, ``I get a pressure of
+air all on one side, and as there is, in effect, most of me
+towards the tail, the latter gets pressed sideways, and the
+Aeroplane thus tends to assume its first position and course.''
+
+``I see,'' said Efficiency, and, daintily holding the Chalk,
+she approached the Blackboard. ``Is this what you mean?''
+
+``Yes, that's right enough,'' said the Keel-Surface, ``and
+you might remember, too, that I always make the Aeroplane
+nose into the gusts rather than away from them.''
+
+``If that was not the case,'' broke in Lateral Stability,
+and affecting the fashionable Flying Corps stammer, ``it
+would be a h-h-h-o-r-rible affair! If there were too much
+Keel-Surface in front, then that gust would blow the Aeroplane
+round the other way a very considerable distance.
+And the right-hand Surface being on the outside of the turn
+would have more speed, and consequently more Lift, than
+the Surface on the other side. That means a greater proportion
+of the Lift on that side, and before you could say
+Warp to the Ailerons over the Aeroplane would go--probable
+result a bad side-slip''
+
+``And what can the Pilot do to save such a situation as
+that?'' said Efficiency.
+
+``Well,'' replied Lateral Stability, ``he will try to turn
+the Aeroplane sideways and back to an even keel by means
+of warping the Ailerons or little wings which are hinged
+on to the Wing-tips, and about which you will hear more
+later on; but if the side-slip is very bad he may not be able
+to right the Aeroplane by means of the Ailerons, and then
+the only thing for him to do is to use the Rudder and to turn
+the nose of the Aeroplane down and head-on to the direction
+of motion. The Aeroplane will then be meeting the air in
+the direction it is designed to do so, and the Surfaces and
+also the controls (the Rudder, Ailerons, and Elevator) will
+be working efficiently; but its attitude relative to the earth
+will probably be more or less upside-down, for the action
+of turning the Aeroplane's nose down results, as you will
+see by the illustration B, in the right wing, which is on the
+outside of the circle. travelling through the air with greater
+speed than the left-hand wing. More Speed means more
+Lift, so that results in overturning the Aeroplane still more;
+but now it is, at any rate, meeting the air as it is designed
+to meet it, and everything is working properly. It is then
+only necessary to warp the Elevator, as shown in illustration
+C, in order to bring the Aeroplane into a proper attitude
+relative to the earth.''
+
+``Ah!'' said the Rudder, looking wise, ``it's in a case
+like that when I become the Elevator and the Elevator
+becomes me.''
+
+``That's absurd nonsense,'' said the Blackboard, ``due
+to looseness of thought and expression.''
+
+``Well,'' replied the Rudder, ``when 'the Aeroplane is
+in position A and I am used, then I depress or ELEVATE the nose
+of the machine; and, if the Elevator is used, then it turns
+the Aeroplane to right or left, which is normally my function.
+Surely our roles have changed one with the other, and I'm
+then the Elevator and the Elevator is me!''
+
+Said Lateral Stability to the Rudder, ``That's altogether
+the wrong way of looking at it, though I admit''--and
+this rather sarcastically--``that the way you put it sounds
+rather fine when you are talking of your experiences in
+the air to those `interested in aviation' but knowing little
+about it; but it won't go down here! You are a Controlling
+Surface designed to turn the Aeroplane about its vertical
+axis, and the Elevator is a Controlling Surface designed to
+turn the Aeroplane about its lateral axis. Those are your
+respective jobs, and you can't possibly change them about.
+Such talk only leads to confusion, and I hope we shall hear
+no more of it.''
+
+``Thanks,'' said Efficiency to Lateral Stability. ``And
+now, please, will you explain your duties?''
+
+``My duty is to keep the Aeroplane horizontal from
+Wing-tip to Wing-tip. First of all, I sometimes arrange
+with the Rigger to wash-out, that is decrease, the Angle
+of Incidence on one side of the Aeroplane, and to effect
+the reverse condition, if it is not too much trouble, on the
+other side.''
+
+``But,'' objected Efficiency, ``the Lift varies with the
+Angle of Incidence, and surely such a condition will result in
+one side of the Aeroplane lifting more than the other side?'
+
+``That's all right,'' said the Propeller, ``it's meant to
+off-set the tendency of the Aeroplane to turn over sideways
+in the opposite direction to which I revolve.''
+
+``That's quite clear, though rather unexpected; but how
+do you counteract the effect of the gusts when they try to
+overturn the Aeroplane sideways?'' said she, turning to
+Lateral Stability again.
+
+``Well,'' he replied, rather miserably, ``I'm not nearly
+so perfect as the Longitudinal and Directional Stabilities.
+The Dihedral Angle--that is, the upward inclination of the
+Surfaces towards their wing-tips--does what it can for me,
+but, in my opinion, it's a more or less futile effort. The
+Blackboard will show you the argument.'' And he at once
+showed them two Surfaces, each set at a Dihedral Angle
+like this:
+
+``Please imagine,'' said the Blackboard, ``that the top
+V is the front view of a Surface flying towards you. Now
+if a gust blows it into the position of the lower V you see
+that the horizontal equivalent of the Surface on one side
+becomes larger, and on the other side it becomes smaller.
+That results in more Lift on the lower side and less on the
+higher side, and if the V is large enough it should produce
+such a difference in the Lift of one side to the other as to
+quickly turn the Aeroplane back to its former and normal
+position.''
+
+``Yes,'' said the Dihedral Angle, ``that's what would
+happen if they would only make me large enough; but
+they won't do it because it would too greatly decrease the
+horizontal equivalent, and therefore the Lift, and incidentally
+it would, as Aeroplanes are built to-day, produce
+an excess of Keel Surface above the turning axis, and that
+in itself would spoil the Lateral Stability. The Keel Surface
+should be equally divided above and below the longitudinal
+turning axis (upon which the Aeroplane rolls sideways),
+or the side upon which there is an excess will get
+blown over by the gusts. It strikes me that my future
+isn't very promising, and about my only chance is when
+the Junior Draughtsman makes a mistake, as he did the
+other day. And just think of it, they call him a Designer
+now that he's got a job at the Factory! What did he do?
+Why, he calculated the weights wrong and got the Centre
+of Gravity too high, and they didn't discover it until the
+machine was built. Then all they could do was to give
+me a larger Angle. That dropped the bottom of the V
+lower down, and as that's the centre of the machine, where
+all the Weight is, of course that put the Centre of Gravity
+in its right place. But now there is too much Keel Surface
+above, and the whole thing's a Bad Compromise, not at all
+like Our Efficiency.''
+
+And Efficiency, blushing very prettily at the compliment,
+then asked, ``And how does the Centre of Gravity affect
+matters?''
+
+``That's easy,'' said Grandfather Gravity. ``I'm so
+heavy that if I am too low down I act like a pendulum
+and cause the Aeroplane to roll about sideways, and if I
+am too high I'm like a stick balanced on your finger, and
+then if I'm disturbed, over I go and the Aeroplane with
+me; and, in addition to that, there are the tricks I play
+with the Aeroplane when it's banked up,[[6]] i.e., tilted sideways
+for a turn, and Centrifugal Force sets me going the
+way I'm not wanted to go. No; I get on best with Lateral
+Stability when my Centre is right on the centre of Drift,
+or, at any rate, not much below it.'' And with that he
+settled back into the Lecturer's Chair and went sound
+asleep again, for he was so very, very old, in fact the father
+of all the Principles.
+
+
+[[6]] Banking: When an aeroplane is turned to the left or
+the right the centrifugal force of its momentum causes it to
+skid sideways and outwards away from the centre of the turn.
+To minimize such action the pilot banks, i.e., tilts, the aeroplane
+sideways in order to oppose the underside of the planes to the air.
+The aeroplane will not then skid outwards beyond the slight skid
+necessary to secure a sufficient pressure of air to balance the
+centrifugal force.
+
+
+And the Blackboard had been busy, and now showed
+them a picture of the Aeroplane as far as they knew it, and
+you will see that there is a slight Dihedral Angle, and
+also, fixed to the tail, a vertical Keel Surface or fin, as
+is very often the case in order to ensure the greater effect
+of such surface being behind the vertical turning axis.
+
+But Efficiency, growing rather critical with her newly
+gained knowledge, cried out: ``But where's the horizontal
+Tail Surface? It doesn't look right like that!''
+
+``This is when I have the pleasure of meeting you, my
+dear,'' said Longitudinal Stability. ``Here's the Tail Surface,''
+he said, ``and in order to help me it must be set IN
+EFFECT at a much less Angle of Incidence than the Main Surface.
+To explain we must trouble the Blackboard again,'' and
+this was his effort:
+
+``I have tried to make that as clear as possible,'' he
+said. ``It may appear a bit complicated at first, but if
+you will take the trouble to look at it for a minute you will find
+it quite simple. A is the normal and proper direction of
+motion of the Aeroplane, but, owing to a gust of air, it takes
+up the new nose-down position. Owing to Momentum,
+however, it does not fly straight along in that direction, but
+moves more or less in the direction B, which is the resultant
+of the two forces, Momentum and Thrust. And so you will
+note that the Angle of Incidence, which is the inclination
+of the Surfaces to the Direction of Motion, has decreased,
+and of course the Lift decreases with it. You will also see,
+and this is the point, that the Tail Surface has lost a higher
+proportion of its Angle, and consequently its Lift, than has
+the Main Surface. Then, such being the case, the Tail must
+fall and the Aeroplane assume its normal position again,
+though probably at a slightly lower altitude.''
+
+``I'm afraid I'm very stupid,'' said Efficiency, ``but
+please tell me why you lay stress upon the words `IN
+EFFECT.' ''
+
+``Ah! I was wondering if you would spot that,'' he
+replied. ``And there is a very good reason for it. You see,
+in some Aeroplanes the Tail Surface may be actually set
+at the same Angle on the machine as the Main Surface, but
+owing to the air being deflected downwards by the front
+Main Surface it meets the Tail Surface at a lesser angle,
+and indeed in some cases at no angle at all. The Tail is then
+for its surface getting less Lift than the Main Surface, although
+set at the same angle on the machine. It may then be
+said to have IN EFFECT a less Angle of Incidence. I'll just
+show you on the Blackboard.''
+
+``And now,'' said Efficiency, ``I have only to meet the
+Ailerons and the Rudder, haven't I?''
+
+``Here we are,'' replied the Ailerons, or little wings.
+``Please hinge us on to the back of the Main Surfaces, one
+of us at each Wing-tip, and join us up to the Pilot's joystick
+by means of the control cables. When the Pilot wishes to
+tilt the Aeroplane sideways, he will move the stick and depress
+us upon one side, thus giving us a larger Angle of Incidence
+and so creating more Lift on that side of the Aeroplane;
+and, by means of a cable connecting us with the Ailerons on
+the other side of the Aeroplane, we shall, as we are depressed,
+pull them up and give them a reverse or negative Angle of
+Incidence, and that side will then get a reverse Lift or downward
+thrust, and so we are able to tilt the Aeroplane sideways.
+
+``And we work best when the Angle of Incidence of the
+Surface in front of us is very small, for which reason it is
+sometimes decreased or washed-out towards the Wing-tips.
+The reason of that is that by the time the air reaches us
+it has been deflected downwards--the greater the Angle
+of Incidence the more it is driven downwards--and in order
+for us to secure a Reaction from it, we have to take such a
+large Angle of Incidence that we produce a poor proportion
+of Lift to Drift; but the smaller the Angle of the Surface in
+front of us the less the air is deflected downwards, and
+consequently the less Angle is required of us, and the better our
+proportion of Lift to Drift, which, of course, makes us much
+more effective Controls.''
+
+``Yes,'' said the Lateral and Directional Stabilities in
+one voice, ``that's so, and the wash-out helps us also, for
+then the Surfaces towards their Wing-tips have less Drift
+or `Head-Resistance,' and consequently the gusts will affect
+them and us less; but such decreased Angle of Incidence
+means decreased Lift as well as Drift, and the Designer does
+not always care to pay the price.''
+
+``Well,'' said the Ailerons, ``if it's not done it will mean
+more work for the Rudder, and that won't please the Pilot.''
+
+``Whatever do you mean?'' asked Efficiency. ``What
+can the Rudder have to do with you?''
+
+``It's like this,'' they replied: ``when we are deflected
+downwards we gain a larger Angle of Incidence and also
+enter an area of compressed air, and so produce more Drift
+than those of us on the other side of the Aeroplane, which
+are deflected upwards into an area of rarefied air due to
+the SUCTION effect (though that term is not academically
+correct) on the top of the Surface. If there is more Drift,
+i.e., Resistance, on one side of the Aeroplane than on the other
+side, then of course it will turn off its course, and if that
+difference in Drift is serious, as it will very likely be if there
+is no wash-out, then it will mean a good deal of work for the
+Rudder in keeping the Aeroplane on its course, besides
+creating extra Drift in doing so.''
+
+``I think, then,'' said Efficiency, ``I should prefer to
+have that wash-out,[[7]] and my friend the Designer is so clever
+at producing strength of construction for light weight, I'm
+pretty sure he won't mind paying the price in Lift. And
+now let me see if I can sketch the completed Aeroplane.''
+
+
+[[7]] An explanation of the way in which the wash-out is combined
+with a wash-in to offset propellor torque will be found on p. 82.
+
+
+``Well, I hope that's all as it should be,'' she concluded,
+``for to-morrow the Great Test in the air is due.''
+
+
+
+PART III
+
+THE GREAT TEST
+
+It is five o'clock of a fine calm morning, when the Aeroplane
+is wheeled out of its shed on to the greensward of the Military
+Aerodrome. There is every promise of a good flying day,
+and, although the sun has not yet risen, it is light enough to
+discern the motionless layer of fleecy clouds some five thousand
+feet high, and far, far above that a few filmy mottled streaks
+of vapour. Just the kind of morning beloved of pilots.
+
+A brand new, rakish, up-to-date machine it is, of highly
+polished, beautifully finished wood, fabric as tight as a
+drum, polished metal, and every part so perfectly ``streamlined''
+to minimize Drift, which is the resistance of the air
+to the passage of the machine, that to the veriest tyro the
+remark of the Pilot is obviously justified.
+
+``Clean looking 'bus, looks almost alive and impatient
+to be off. Ought to have a turn for speed with those
+lines.''
+
+``Yes,'' replies the Flight-Commander, ``it's the latest
+of its type and looks a beauty. Give it a good test. A
+special report is required on this machine.''
+
+The A.M.'s[[8]] have now placed the Aeroplane in position
+facing the gentle air that is just beginning to make itself
+evident; the engine Fitter, having made sure of a sufficiency
+of oil and petrol in the tanks, is standing by the Propeller;
+the Rigger, satisfied with a job well done, is critically ``vetting''
+the machine by eye, four A.M.'s are at their posts,
+ready to hold the Aeroplane from jumping the blocks which
+have been placed in front of the wheels; and the Flight-
+Sergeant is awaiting the Pilot's orders.
+
+
+[[8]] A.M.'s: Air Mechanics.
+
+
+As the Pilot approaches the Aeroplane the Rigger springs
+to attention and reports, ``All correct, sir,'' but the Fitter
+does not this morning report the condition of the Engine,
+for well he knows that this Pilot always personally looks
+after the preliminary engine test. The latter, in leathern
+kit, warm flying boots and goggled, climbs into his seat,
+and now, even more than before, has the Aeroplane an almost
+living appearance, as if straining to be off and away. First
+he moves the Controls to see that everything is clear, for
+sometimes when the Aeroplane is on the ground the control
+lever or ``joy-stick'' is lashed fast to prevent the wind
+from blowing the controlling surfaces about and possibly
+damaging them.
+
+The air of this early dawn is distinctly chilly, and the
+A.M.'s are beginning to stamp their cold feet upon the dewy
+grass, but very careful and circumspect is the Pilot, as he
+mutters to himself, ``Don't worry and flurry, or you'll die
+in a hurry.''
+
+At last he fumbles for his safety belt, but with a start
+remembers the Pilot Air Speed Indicator, and, adjusting
+it to zero, smiles as he hears the Pilot-head's gruff voice,
+``Well, I should think so, twenty miles an hour I was registering.
+That's likely to cause a green pilot to stall the Aeroplane.
+Pancake, they call it.'' And the Pilot, who is an
+old hand and has learned a lot of things in the air that mere
+earth-dwellers know nothing about, distinctly heard the
+Pilot Tube, whose mouth is open to the air to receive its
+pressure, stammer. ``Oh Lor! I've got an earwig already--
+hope to goodness the Rigger blows me out when I come
+down--and this morning air simply fills me with moisture;
+I'll never keep the Liquid steady in the Gauge. I'm not
+sure of my rubber connections either.''
+
+``Oh, shut up!'' cry all the Wires in unison, ``haven't
+we got our troubles too? We're in the most horrible state
+of tension. It's simply murdering our Factor of Safety,
+and how we can possibly stand it when we get the Lift only
+the Designer knows.''
+
+``That's all right,'' squeak all the little Wire loops,
+``we're that accommodating, we're sure to elongate a bit
+and so relieve your tension.'' For the whole Aeroplane is
+braced together with innumerable wires, many of which
+are at their ends bent over in the form of loops in order to
+connect with the metal fittings on the spars and elsewhere--
+cheap and easy way of making connection.
+
+``Elongate, you little devils, would you?'' fairly shout
+the Angles of Incidence, Dihedral and Stagger, amid a chorus
+of groans from all parts of the Aeroplane. ``What's going
+to happen to us then? How are we going to keep our
+adjustments upon which good flying depends?''
+
+``Butt us and screw us,''[[9]] wail the Wires. ``Butt us
+and screw us, and death to the Loops. That's what we
+sang to the Designer, but he only looked sad and scowled
+at the Directors.''
+
+
+[[9]] Butt means to thicken at the end. Screw means to machine a thread
+on the butt-end of the wire, and in this way the wire can make connection
+with the desired place by being screwed into a metal fitting,
+thus eliminating the disadvantage of the unsatisfactory loop.
+
+
+``And who on earth are they?'' asked the Loops, trembling
+for their troublesome little lives.
+
+``Oh earth indeed,'' sniffed Efficiency, who had not
+spoken before, having been rendered rather shy by being
+badly compromised in the Drawing Office. ``I'd like to
+get some of them up between Heaven and Earth, I would.
+I'd give 'em something to think of besides their Debits
+and Credits--but all the same the Designer will get his
+way in the end. I'm his Best Girl, you know, and if we
+could only get rid of the Directors, the little Tin god, and
+the Man-who-takes-the-credit, we should be quite happy.''
+Then she abruptly subsides, feeling that perhaps the less
+said the better until she has made a reputation in the Air.
+The matter of that Compromise still rankled, and indeed
+it does seem hardly fit that a bold bad Tin god should flirt
+with Efficiency. You see there was a little Tin god, and he
+said ``Boom, Boom BOOM! Nonsense! It MUST be done,''
+and things like that in a very loud voice, and the Designer
+tore his hair and was furious, but the Directors, who were
+thinking of nothing but Orders and Dividends, had the
+whip-hand of HIM, and so there you are, and so poor beautiful
+Miss Efficiency was compromised.
+
+All this time the Pilot is carefully buckling his belt and
+making himself perfectly easy and comfortable, as all good
+pilots do. As he straightens himself up from a careful
+inspection of the Deviation Curve[[10]] of the Compass and takes
+command of the Controls, the Throttle and the Ignition,
+the voices grow fainter and fainter until there is nothing
+but a trembling of the Lift and Drift wires to indicate to his
+understanding eye their state of tension in expectancy of
+the Great Test.
+
+
+[[10]] Deviation curve: A curved line indicating any errors in the compass.
+
+
+``Petrol on?'' shouts the Fitter to the Pilot.
+
+``Petrol on,'' replies the Pilot.
+
+``Ignition off?''
+
+``Ignition off.''
+
+Round goes the Propeller, the Engine sucking in the
+Petrol Vapour with satisfied gulps. And then--
+
+``Contact?'' from the Fitter.
+
+``Contact,'' says the Pilot.
+
+Now one swing of the Propeller by the Fitter, and the
+Engine is awake and working. Slowly at first though, and
+in a weak voice demanding, ``Not too much Throttle, please.
+I'm very cold and mustn't run fast until my Oil has thinned
+and is circulating freely. Three minutes slowly, as you love
+me, Pilot.''
+
+Faster and faster turn the Engine and Propeller, and
+the Aeroplane, trembling in all its parts, strains to jump
+the blocks and be off. Carefully the Pilot listens to what the
+Engine Revolution Indicator says. At last, ``Steady
+at 1,500 revs. and I'll pick up the rest in the Air.'' Then
+does he throttle down the Engine, carefully putting the
+lever back to the last notch to make sure that in such position
+the Throttle is still sufficiently open for the Engine to continue
+working, as otherwise it might lead to him ``losing'' his
+Engine in the air when throttling down the power for descent.
+Then, giving the official signal, he sees the blocks removed
+from the wheels, and the Flight-Sergeant saluting he knows
+that all is clear to ascend. One more signal, and all the
+A.M.'s run clear of the Aeroplane.
+
+Then gently, gently mind you, with none of the ``crashing
+on'' bad Pilots think so fine, he opens the Throttle
+and, the Propeller Thrust overcoming its enemy the Drift,
+the Aeroplane moves forward.
+
+``Ah!'' says the Wind-screen, ``that's Discipline, that
+is. Through my little window I see most things, and don't
+I just know that poor discipline always results in poor work
+in the air, and don't you forget it.''
+
+``Discipline is it?'' complains the Under-carriage, as
+its wheels roll swiftly over the rather rough ground. ``I'm
+bump getting it; and bump, bump, all I want, bang, bump,
+rattle, too!'' But, as the Lift increases with the Speed,
+the complaints of the Under-carriage are stilled, and then,
+the friendly Lift becoming greater than the Weight, the
+Aeroplane swiftly and easily takes to the air.
+
+Below is left the Earth with all its bumps and troubles.
+Up into the clean clear Air moves with incredible speed
+and steadiness this triumph of the Designer, the result of
+how much mental effort, imagination, trials and errors,
+failures and successes, and many a life lost in high
+endeavour.
+
+Now is the mighty voice of the Engine heard as he turns
+the Propeller nine hundred times a minute. Now does the
+Thrust fight the Drift for all it's worth, and the Air Speed
+Indicator gasps with delight, ``One hundred miles an hour!''
+
+And now does the burden of work fall upon the Lift and
+Drift Wires, and they scream to the Turnbuckles whose
+business it is to hold them in tension, ``This is the limit!
+the Limit! THE LIMIT! Release us, if only a quarter
+turn.'' But the Turnbuckles are locked too fast to turn
+their eyes or utter a word. Only the Locking Wires thus:
+``Ha! ha! the Rigger knew his job. He knew the trick, and
+there's no release here.'' For an expert rigger will always
+use the locking wire in such a way as to oppose the slightest
+tendency of the turnbuckle to unscrew. The other kind of
+rigger will often use the wire in such a way as to allow the
+turnbuckle, to the ``eyes'' of which the wires are attached,
+to unscrew a quarter of a turn or more, with the result that
+the correct adjustment of the wires may be lost; and upon
+their fine adjustment much depends.
+
+And the Struts and the Spars groan in compression and
+pray to keep straight, for once ``out of truth'' there is, in
+addition to possible collapse, the certainty that in bending
+they will throw many wires out of adjustment.
+
+And the Fabric's quite mixed in its mind, and ejaculates,
+``Now, who would have thought I got more Lift from the
+top of the Surface than its bottom?'' And then truculently
+to the Distance Pieces, which run from rib to rib, ``Just
+keep the Ribs from rolling, will you? or you'll see me strip.
+I'm an Irishman, I am, and if my coat comes off---- Yes,
+Irish, I said. I used to come from Egypt, but I've got
+naturalized since the War began.''
+
+Then the Air Speed Indicator catches the eye of the
+Pilot. ``Good enough,'' he says as he gently deflects the
+Elevator and points the nose of the Aeroplane upwards in
+search of the elusive Best Climbing Angle.
+
+``Ha! ha!'' shouts the Drift, growing stronger with the
+increased Angle of Incidence. ``Ha! ha!'' he laughs to
+the Thrust. ``Now I've got you. Now who's Master?''
+
+And the Propeller shrieks hysterically, ``Oh! look at
+me. I'm a helicopter. That's not fair. Where's Efficiency?''
+And she can only sadly reply, ``Yes, indeed, but
+you see we're a Compromise.''
+
+And the Drift has hopes of reaching the Maximum Angle
+of Incidence and vanquishing the Thrust and the Lift. And
+he grows very bold as he strangles the Thrust; but the situation
+is saved by the Propeller, who is now bravely helicopting
+skywards, somewhat to the chagrin of Efficiency.
+
+``Much ado about nothing,'' quotes the Aeroplane
+learnedly. ``Compromise or not, I'm climbing a thousand
+feet a minute. Ask the Altimeter. He'll confirm it.''
+
+And so indeed it was. The vacuum box of the Altimeter
+was steadily expanding under the decreased pressure of
+the rarefied air, and by means of its little levers and its
+wonderful chain no larger than a hair it was moving the
+needle round the gauge and indicating the ascent at the
+rate of a thousand feet a minute.
+
+And lo! the Aeroplane has almost reached the clouds!
+But what's this? A sudden gust, and down sinks one wing
+and up goes the other. ``Oh, my Horizontal Equivalent!''
+despairingly call the Planes: ``it's eloping with the Lift,
+and what in the name of Gravity will happen? Surely
+there was enough scandal in the Factory without this, too!''
+For the lift varies with the horizontal equivalent of the
+planes, so that if the aeroplane tilts sideways beyond a certain
+angle, the lift becomes less than the weight of the machine,
+which must then fall. A fall in such a position is known as
+a ``side-slip.''
+
+But the ever-watchful Pilot instantly depresses one aileron,
+elevating the other, with just a touch of the rudder to keep
+on the course, and the Planes welcome back their precious
+Lift as the Aeroplane flicks back to its normal position.
+
+``Bit bumpy here under these clouds,'' is all the Pilot
+says as he heads for a gap between them, and the next minute
+the Aeroplane shoots up into a new world of space.
+
+``My eye!'' ejaculates the Wind-screen, ``talk about a
+view!'' And indeed mere words will always fail to express
+the wonder of it. Six thousand feet up now, and look!
+The sun is rising quicker than ever mortal on earth witnessed
+its ascent. Far below is Mother Earth, wrapt in mists and
+deep blue shadows, and far above are those light, filmy,
+ethereal clouds now faintly tinged with pink And all
+about great mountains of cloud, lazily floating in space.
+The sun rises and they take on all colours, blending one
+with the other, from dazzling white to crimson and deep
+violet-blue. Lakes and rivers here and there in the enormous
+expanse of country below refract the level rays of the sun
+and, like so many immense diamonds, send dazzling shafts
+of light far upwards. The tops of the hills now laugh to the
+light of the sun, but the valleys are still mysterious dark
+blue caverns, clowned with white filmy lace-like streaks of
+vapour. And withal the increasing sense with altitude of
+vast, clean, silent solitudes of space.
+
+Lives there the man who can adequately describe this
+Wonder? ``Never,'' says the Pilot, who has seen it many
+times, but to whom it is ever new and more wonderful.
+
+Up, up, up, and still up, unfalteringly speeds the Pilot
+and his mount. Sweet the drone of the Engine and steady
+the Thrust as the Propeller exultingly battles with the Drift.
+
+And look! What is that bright silver streak all along
+the horizon? It puzzled the Pilot when first he saw it,
+but now he knows it for the Sea, full fifty miles away!
+
+And on his right is the brightness of the Morn and the
+smiling Earth unveiling itself to the ardent rays of the Sun;
+and on his left, so high is he, there is yet black Night, hiding
+innumerable Cities, Towns, Villages and all those places
+where soon teeming multitudes of men shall awake, and by
+their unceasing toil and the spirit within them produce
+marvels of which the Aeroplane is but the harbinger.
+
+And the Pilot's soul is refreshed, and his vision, now
+exalted, sees the Earth a very garden, even as it appears
+at that height, with discord banished and a happy time
+come, when the Designer shall have at last captured Efficiency,
+and the Man-who-takes-the-credit is he who has earned it,
+and when kisses are the only things that go by favour.
+
+Now the Pilot anxiously scans the Barograph, which is
+an instrument much the same as the Altimeter; but in this
+case the expansion of the vacuum box causes a pen to trace
+a line upon a roll of paper. This paper is made by clockwork
+to pass over the point of the pen, and so a curved line is
+made which accurately registers the speed of the ascent in
+feet per minute. No longer is the ascent at the rate of a
+thousand feet a minute, and the Propeller complains to the
+Engine, ``I'm losing my Revs. and the Thrust. Buck up
+with the Power, for the Lift is decreasing, though the Weight
+remains much the same.''
+
+Quoth the Engine: ``I strangle for Air. A certain proportion,
+and that of right density, I must have to one part
+of Petrol, in order to give me full power and compression,
+and here at an altitude of ten thousand feet the Air is only
+two-thirds as dense as at sea-level. Oh, where is he who
+will invent a contrivance to keep me supplied with Air of
+right density and quality? It should not be impossible
+within certain limits.''
+
+``We fully agree,'' said the dying Power and Thrust. ``Only
+maintain Us and you shall be surprised at the result. For
+our enemy Drift decreases in respect of distance with the increase
+of altitude and rarity of air, and there is no limit to the
+speed through space if only our strength remains. And
+with oxygen for Pilot and Passengers and a steeper pitch[[11]]
+for the Propeller we may then circle the Earth in a day!''
+
+
+[[11]] A propeller screws through the air, and the distance it advances
+during one revolution, supposing the air to be solid, is known as the pitch.
+The pitch, which depends upon the angle of the propeller blades, must be equal
+to the speed of the aeroplane, plus the slip, and if, on account of the rarity
+of the air the speed of the aeroplane increases, then the angle and pitch
+should be correspondingly increased. Propellers with a pitch capable of being
+varied by the pilot are the dream of propeller designers. For explanation of
+``slip'' see Chapter IV. on propellers.
+
+
+Ah, Reader, smile not unbelievingly, as you smiled but
+a few years past. There may be greater wonders yet. Consider
+that as the speed increases, so does the momentum
+or stored-up force in the mass of the aeroplane become
+terrific. And, bearing that in mind, remember that with
+altitude gravity decreases. There may yet be literally other
+worlds to conquer.[[12]]
+
+
+[[12]] Getting out of my depth? Invading the realms of fancy? Well,
+perhaps so, but at any rate it is possible that extraordinary speed through
+space may be secured if means are found to maintain the impulse of the engine
+and the thrust-drift efficiency of the propeller at great altitude.
+
+
+Now at fifteen thousand feet the conditions are chilly
+and rare, and the Pilot, with thoughts of breakfast far below,
+exclaims, ``High enough! I had better get on with the
+Test.'' And then, as he depresses the Elevator, the Aeroplane
+with relief assumes its normal horizontal position.
+Then, almost closing the Throttle, the Thrust dies away.
+Now, the nose of the Aeroplane should sink of its own volition,
+and the craft glide downward at flying speed, which is in
+this case a hundred miles an hour. That is what should
+happen if the Designer has carefully calculated the weight
+of every part and arranged for the centre of gravity to be just
+the right distance in front of the centre of lift. Thus is the
+Aeroplane ``nose-heavy'' as a glider, and just so to a degree
+ensuring a speed of glide equal to its flying speed. And the
+Air Speed Indicator is steady at one hundred miles an hour,
+and ``That's all right!'' exclaims the Pilot. ``And very
+useful, too, in a fog or a cloud,'' he reflects, for then he can
+safely leave the angle of the glide to itself, and give all his
+attention, and he will need it all, to keeping the Aeroplane
+horizontal from wing-tip to wing-tip, and to keeping it
+straight on its course. The latter he will manage with the
+rudder, controlled by his feet, and the Compass will tell him
+whether a straight course is kept. The former he will control
+by the Ailerons, or little wings hinged to the tips of the planes,
+and the bubble in the Inclinometer in front of him must be
+kept in the middle.
+
+A Pilot, being only human, may be able to do two things
+at once, but three is a tall order, so was this Pilot relieved
+to find the Design not at fault and his craft a ``natural
+glider.'' To correct this nose-heavy tendency when the
+Engine is running, and descent not required, the centre
+of Thrust is arranged to be a little below the centre of Drift
+or Resistance, and thus acts as a counter-balance.
+
+But what is this stream of bad language from the Exhaust
+Pipe, accompanied by gouts of smoke and vapour?
+The Engine, now revolving at no more than one-tenth its
+normal speed, has upset the proportion of petrol to air,
+and combustion is taking place intermittently or in the
+Exhaust Pipe, where it has no business to be.
+
+``Crash, Bang, Rattle----!----!----!'' and worse than
+that, yells the Exhaust, and the Aeroplane, who is a gentleman
+and not a box kite,[[13]] remonstrates with the severity
+of a Senior Officer. ``See the Medical Officer, you young
+Hun. Go and see a doctor. Vocal diarrhoea, that's your
+complaint, and a very nasty one too. Bad form, bad for
+discipline, and a nuisance in the Mess. What's your Regiment?
+Special Reserve, you say? Humph! Sounds like
+Secondhand Bicycle Trade to me!''
+
+
+[[13]] Box-kite. The first crude form of biplane.
+
+
+Now the Pilot decides to change the straight gliding
+descent to a spiral one, and, obedient to the Rudder, the
+Aeroplane turns to the left. But the Momentum (two tons
+at 100 miles per hour is no small affair) heavily resents this
+change of direction, and tries its level best to prevent it
+and to pull the machine sideways and outwards from its
+spiral course--that is, to make it ``side-skid'' outwards.
+But the Pilot deflects the Ailerons and ``banks'' up the planes
+to the correct angle, and, the Aeroplane skidding sideways
+and outwards, the lowest surfaces of the planes press up against
+the air until the pressure equals the centrifugal force of
+the Momentum, and the Aeroplane spirals steadily downwards.
+
+Down, down, down, and the air grows denser, and the
+Pilot gulps largely, filling his lungs with the heavier air to
+counteract the increasing pressure from without. Down
+through a gap in the clouds, and the Aerodrome springs
+into view, appearing no larger than a saucer, and the Pilot,
+having by now got the ``feel'' of the Controls, proceeds
+to put the Aeroplane through its paces. First at its Maximum
+Angle, staggering along tail-down and just maintaining
+horizontal flight; then a dive at far over flying speed, finishing
+with a perfect loop; then sharp turns with attendant
+vertical ``banks'' and then a wonderful switchback flight,
+speeding down at a hundred and fifty miles an hour with
+short, exhilarating ascents at the rate of two thousand feet
+a minute!
+
+All the parts are now working well together. Such
+wires as were before in undue tension have secured relief
+by slightly elongating their loops, and each one is now doing
+its bit, and all are sharing the burden of work together.
+
+The Struts and the Spars, which felt so awkward at first,
+have bedded themselves in their sockets, and are taking
+the compression stresses uncomplainingly.
+
+The Control Cables of twisted wire, a bit tight before,
+have slightly lengthened by perhaps the eighth of an inch,
+and, the Controls instantly responding to the delicate touch
+of the Pilot, the Aeroplane, at the will of its Master, darts
+this way and that way, dives, loops, spirals, and at last, in
+one long, magnificent glide, lands gently in front of its shed.
+
+``Well, what result?'' calls the Flight-Commander to
+the Pilot.
+
+``A hundred miles an hour and a thousand feet a minute,''
+he briefly replies.
+
+``And a very good result too,'' says the Aeroplane, complacently,
+as he is carefully wheeled into his shed.
+
+      
+
+That is the way Aeroplanes speak to those who love them
+and understand them. Lots of Pilots know all about it,
+and can spin you wonderful yarns, much better than this
+one, if you catch them in a confidential mood--on leave,
+for instance, and after a good dinner.
+
+
+
+PART IV
+
+'CROSS COUNTRY
+
+The Aeroplane had been designed and built, and tested in
+the air, and now stood on the Aerodrome ready for its first
+'cross-country flight.
+
+It had run the gauntlet of pseudo-designers, crank inventors,
+press ``experts,'' and politicians; of manufacturers
+keen on cheap work and large profits; of poor pilots who had
+funked it, and good pilots who had expected too much of
+it. Thousands of pounds had been wasted on it, many had
+gone bankrupt over it, and others it had provided with safe
+fat jobs.
+
+Somehow, and despite every conceivable obstacle, it had
+managed to muddle through, and now it was ready for its
+work. It was not perfect, for there were fifty different
+ways in which it might be improved, some of them shamefully
+obvious. But it was fairly sound mechanically, had a little
+inherent stability, was easily controlled, could climb a thousand
+feet a minute, and its speed was a hundred miles an
+hour. In short, quite a creditable machine, though of course
+the right man had not got the credit.
+
+It is rough, unsettled weather with a thirty mile an
+hour wind on the ground, and that means fifty more or
+less aloft. Lots of clouds at different altitudes to bother
+the Pilot, and the air none to clear for the observation of
+landmarks.
+
+As the Pilot and Observer approach the Aeroplane the
+former is clearly not in the best of tempers. ``It's rotten
+luck,'' he is saying, ``a blank shame that I should have
+to take this blessed 'bus and join X Reserve Squadron,
+stationed a hundred and fifty miles from anywhere; and
+just as I have licked my Flight into shape. Now some
+slack blighter will, I suppose, command it and get the credit
+of all my work!''
+
+``Shut up, you grouser,'' said the Observer. ``Do you
+think you're the only one with troubles? Haven't I been
+through it too? Oh! I know all about it! You're from
+the Special Reserve and your C.O. doesn't like your style
+of beauty, and you won't lick his boots, and you were a bit
+of a technical knut in civil life, but now you've jolly well
+got to know less than those senior to you. Well! It's a
+jolly good experience for most of us. Perhaps conceit won't
+be at quite such a premium after this war. And what's
+the use of grousing? That never helped anyone. So buck
+up, old chap. Your day will come yet. Here's our machine,
+and I must say it looks a beauty!''
+
+And, as the Pilot approaches the Aeroplane, his face
+brightens and he soon forgets his troubles as he critically
+inspects the craft which is to transport him and the Observer
+over the hills and far away. Turning to the Flight-Sergeant
+he inquires, ``Tank full of petrol and oil?''
+
+``Yes, sir,'' he replies, ``and everything else all correct.
+Propeller, engine, and body covers on board, sir; tool kit
+checked over and in the locker; engine and Aeroplane logbooks
+written up, signed, and under your seat; engine revs.
+up to mark, and all the control cables in perfect condition
+and tension.''
+
+``Very good,'' said the Pilot; and then turning to the
+Observer, ``Before we start you had better have a look
+at the course I have mapped out.
+
+``A is where we stand and we have to reach B, a hundred
+and fifty miles due North. I judge that, at the altitude
+we shall fly, there will be an East wind, for although it is
+not quite East on the ground it is probably about twenty
+degrees different aloft, the wind usually moving round clockways
+to about that extent. I think that it is blowing at the
+rate of about fifty miles an hour, and I therefore take a line
+on the map to C, fifty miles due West of A. The Aeroplane's
+speed is a hundred miles an hour, and so I take a line of one
+hundred miles from C to D. Our compass course will then
+be in the direction A--E, which is always a line parallel to
+C--D. That is, to be exact, it will be fourteen degrees off
+the C--D course, as, in this part of the globe, there is that
+much difference between the North and South lines on the
+map and the magnetic North to which the compass needle
+points. If the compass has an error, as it may have of a
+few degrees, that, too, must be taken into account, and the
+deviation or error curve on the dashboard will indicate it.
+
+``The Aeroplane will then always be pointing in a direction
+parallel to A--E, but, owing to the side wind, it will be actually
+travelling over the course A--B, though in a rather
+sideways attitude to that course.
+
+``The distance we shall travel over the A--B course
+in one hour is A--D. That is nearly eighty-seven miles,
+so we ought to accomplish our journey of a hundred and
+fifty miles in about one and three-quarter hours.
+
+``I hope that's quite clear to you. It's a very simple
+way of calculating the compass course, and I always do it
+like that.''
+
+``Yes, that's plain enough. You have drafted what
+engineers call `a parallelogram of forces'; but suppose you
+have miscalculated the velocity of the wind, or that it should
+change in velocity or direction?''
+
+``Well, that of course will more or less alter matters,''
+replies the Pilot. ``But there are any number of good
+landmarks such as lakes, rivers, towns, and railway lines.
+They will help to keep us on the right course, and the compass
+will, at any rate, prevent us from going far astray when
+between them.''
+
+``Well, we'd better be off, old chap. Hop aboard.''
+This from the Observer as he climbs into the front seat
+from which he will command a good view over the lower
+plane; and the Pilot takes his place in the rear seat, and,
+after making himself perfectly comfortable, fixing his safety
+belt, and moving the control levers to make sure that they
+are working freely, he gives the signal to the Engine Fitter
+to turn the propeller and so start the engine.
+
+Round buzzes the Propeller, and the Pilot, giving the
+official signal, the Aeroplane is released and rolls swiftly
+over the ground in the teeth of the gusty wind.
+
+In less than fifty yards it takes to the air and begins
+to climb rapidly upwards, but how different are the conditions
+to the calm morning of yesterday! If the air were
+visible it would be seen to be acting in the most extraordinary
+manner; crazily swirling, lifting and dropping, gusts viciously
+colliding--a mad phantasmagoria of forces!
+
+Wickedly it seizes and shakes the Aeroplane; then tries
+to turn it over sideways; then instantly changes its mind
+and in a second drops it into a hole a hundred feet deep,
+and if it were not for his safety belt the Pilot might find
+his seat sinking away from beneath him.
+
+Gusts strike the front of the craft like so many slaps in
+the face; and others, with the motion of mountainous waves,
+sometimes lift it hundreds of feet in a few seconds, hoping
+to see it plunge over the summit in a death-dive--and so it
+goes on, but the Pilot, perfectly at one with his mount and
+instantly alert to its slightest motion, is skilfully and naturally
+making perhaps fifty movements a minute of hand and feet;
+the former lightly grasping the ``joy-stick'' which controls
+the Elevator hinged to the tail, and also the Ailerons or little
+wings hinged to the wing-tips; and the latter moving the
+Rudder control-bar.
+
+A strain on the Pilot? Not a bit of it, for this is his
+Work which he loves and excels in; and given a cool head,
+alert eye, and a sensitive touch for the controls, what
+sport can compare with these ever-changing battles of
+the air?
+
+The Aeroplane has all this time been climbing in great
+wide circles, and is now some three thousand feet above
+the Aerodrome which from such height looks absurdly
+small. The buildings below now seem quite squat; the
+hills appear to have sunk away into the ground, and the
+whole country below, cut up into diminutive fields, has
+the appearance of having been lately tidied and thoroughly
+spring-cleaned! A doll's country it looks, with tiny horses
+and cows ornamenting the fields and little model motor-cars
+and carts stuck on the roads, the latter stretching away
+across the country like ribbons accidentally dropped.
+
+At three thousand feet altitude the Pilot is satisfied
+that he is now sufficiently high to secure, in the event of
+engine failure, a long enough glide to earth to enable him
+to choose and reach a good landing-place; and, being furthermore
+content with the steady running of the engine, he
+decides to climb no more but to follow the course he has
+mapped out. Consulting the compass, he places the Aeroplane
+on the A--E course and, using the Elevator, he gives
+his craft its minimum angle of incidence at which it will
+just maintain horizontal flight and secure its maximum
+speed.
+
+Swiftly he speeds away, and few thoughts he has now
+for the changing panorama of country, cloud, and colour.
+Ever present in his mind are the three great 'cross-country
+queries. ``Am I on my right course? Can I see a good
+landing-ground within gliding distance?'' And ``How is
+the Engine running?''
+
+Keenly both he and the Observer compare their maps
+with the country below. The roads, khaki-coloured ribbons,
+are easily seen but are not of much use, for there are so many
+of them and they all look alike from such an altitude.
+
+Now where can that lake be which the map shows so
+plainly? He feels that surely he should see it by now,
+and has an uncomfortable feeling that he is flying too far
+West. What pilot is there indeed who has not many times
+experienced such unpleasant sensation? Few things in the
+air can create greater anxiety. Wisely, however, he sticks
+to his compass course, and the next minute he is rewarded
+by the sight of the lake, though indeed he now sees that the
+direction of his travel will not take him over it, as should
+be the case if he were flying over the shortest route to his
+destination. He must have slightly miscalculated the velocity
+or direction of the side-wind.
+
+``About ten degrees off,'' he mutters, and, using the
+Rudder, corrects his course accordingly.
+
+Now he feels happier and that he is well on his way.
+The gusts, too, have ceased to trouble him as, at this altitude,
+they are not nearly so bad as they were near the ground
+the broken surface of which does much to produce them;
+and sometimes for miles he makes but a movement or two
+of the controls.
+
+The clouds just above race by with dizzy and uniform
+speed; the country below slowly unrolls, and the steady
+drone of the Engine is almost hypnotic in effect. ``Sleep,
+sleep, sleep,'' it insidiously suggests. ``Listen to me and
+watch the clouds; there's nothing else to do. Dream,
+dream, dream of speeding through space for ever, and ever,
+and ever; and rest, rest, rest to the sound of my rhythmical
+hum. Droning on and on, nothing whatever matters. All
+things now are merged into speed through space and a sleepy
+monotonous d-d-r-r-o-o-n-n-e - - - - -.'' But the Pilot pulls
+himself together with a start and peers far ahead in search
+of the next landmark. This time it is a little country town.
+red-roofed his map tells him, and roughly of cruciform shape;
+and, sure enough, there in the right direction are the broken
+outlines of a few red roofs peeping out from between the trees.
+
+Another minute and he can see this little town, a fairy
+town it appears, nestling down between the hills with its
+red roofs and picturesque shape, a glowing and lovely contrast
+with the dark green of the surrounding moors.
+
+So extraordinarily clean and tidy it looks from such a
+height, and laid out in such orderly fashion with perfectly
+defined squares, parks, avenues, and public buildings, it
+indeed appears hardly real, but rather as if it has this very
+day materialized from some delightful children's book!
+
+Every city and town you must know has its distinct
+individuality to the Pilot's eye. Some are not fairy places
+at all, but great dark ugly blots upon the fair countryside,
+and with tall shafts belching forth murky columns of smoke
+to defile clean space. Others, melancholy-looking masses
+of grey, slate-roofed houses, are always sad and dispirited;
+never welcoming the glad sunshine, but ever calling for leaden
+skies and a weeping Heaven. Others again, little coquettes
+with village green, white palings everywhere, bright gravel
+roads, and an irrepressible air of brightness and gaiety.
+
+Then there are the rivers, silvery streaks peacefully
+winding far, far away to the distant horizon; they and the
+lakes the finest landmarks the Pilot can have. And the
+forests. How can I describe them? The trees cannot be
+seen separately, but merge altogether into enormous irregular
+dark green masses sprawling over the country, and sometimes
+with great ungainly arms half encircling some town or village;
+and the wind passing over the foliage at times gives the forest
+an almost living appearance, as of some great dragon of olden
+times rousing itself from slumber to devour the peaceful
+villages which its arms encircle.
+
+And the Pilot and Observer fly on and on, seeing these
+things and many others which baffle my poor skill to describe--
+things, dear Reader, that you shall see, and poets sing of,
+and great artists paint in the days to come when the Designer
+has captured Efficiency. Then, and the time is near, shall
+you see this beautiful world as you have never seen it before,
+the garden it is, the peace it breathes, and the wonder of it.
+
+The Pilot, flying on, is now anxiously looking for the
+railway line which midway on his journey should point
+the course. Ah! There it is at last, but suddenly (and
+the map at fault) it plunges into the earth! Well the writer
+remembers when that happened to him on a long 'cross-
+country flight in the early days of aviation. Anxiously
+he wondered ``Are tunnels always straight?'' and with what
+relief, keeping on a straight course, he picked up the line
+again some three miles farther on!
+
+Now at last the Pilot sees the sea, just a streak on the
+north-eastern horizon, and he knows that his flight is two-
+thirds over. Indeed, he should have seen it before, but
+the air is none too clear, and he is not yet able to discern
+the river which soon should cross his path. As he swiftly
+speeds on the air becomes denser and denser with what he
+fears must be the beginning of a sea-fog, perhaps drifting
+inland along the course of the river. Now does he feel real
+anxiety, for it is the DUTY of a Pilot to fear fog, his deadliest
+enemy. Fog not only hides the landmarks by which he
+keeps his course, but makes the control of the Aeroplane
+a matter of the greatest difficulty. He may not realize
+it, but, in keeping his machine on an even keel, he is
+unconsciously balancing it against the horizon, and with the
+horizon gone he is lost indeed. Not only that, but it also
+prevents him from choosing his landing-place, and the
+chances are that, landing in a fog, he will smash into a tree,
+hedge, or building, with disastrous results. The best and
+boldest pilot 'wares a fog, and so this one, finding the
+conditions becoming worse and yet worse, and being forced to
+descend lower and lower in order to keep the earth within
+view, wisely decides to choose a landing-place while there is
+yet time to do so.
+
+Throttling down the power of the engine he spirals downwards,
+keenly observing the country below. There are
+plenty of green fields to lure him, and his great object is to
+avoid one in which the grass is long, for that would bring
+his machine to a stop so suddenly as to turn it over; or one
+of rough surface likely to break the under-carriage. Now
+is perfect eyesight and a cool head indispensable. He sees
+and decides upon a field and, knowing his job, he sticks to
+that field with no change of mind to confuse him. It is none
+too large, and gliding just over the trees and head on to the
+wind he skilfully ``stalls'' his machine; that is, the speed
+having decreased sufficiently to avoid such a manoeuvre
+resulting in ascent, he, by means of the Elevator, gives the
+Aeroplane as large an angle of incidence as possible. and the
+undersides of the planes meeting the air at such a large
+angle act as an air-brake, and the Aeroplane, skimming
+over the ground, lessens its speed and finally stops just at
+the farther end of the field.
+
+Then, after driving the Aeroplane up to and under the
+lee of the hedge, he stops the engine, and quickly lashing
+the joy-stick fast in order to prevent the wind from blowing
+the controlling surfaces about and possibly damaging them,
+he hurriedly alights. Now running to the tail he lifts it up
+on to his shoulder, for the wind has become rough indeed
+and there is danger of the Aeroplane becoming unmanageable.
+By this action he decreases the angle at which the planes
+are inclined to the wind and so minimizes the latter's effect
+upon them. Then to the Observer, ``Hurry up, old fellow,
+and try to find some rope, wire, or anything with which to
+picket the machine. The wind is rising and I shan't be able
+to hold the 'bus steady for long. Don't forget the wire-
+cutters. They're in the tool kit.'' And the Observer rushes
+off in frantic haste, before long triumphantly returning with
+a long length of wire from a neighbouring fence. Blocking
+up the tail with some debris at hand, they soon succeed,
+with the aid of the wire, in stoutly picketing the Aeroplane
+to the roots of the high hedge in front of it; done with much
+care, too, so that the wire shall not fray the fabric or set up
+dangerous bending-stresses in the woodwork. Their work
+is not done yet, for the Observer remarking, ``I don't like
+the look of this thick weather and rather fear a heavy rain-
+storm,'' the Pilot replies, ``Well, it's a fearful bore, but the
+first rule of our game is never to take an unnecessary risk,
+so out with the engine and body covers.''
+
+Working with a will they soon have the engine and the
+open part of the body which contains the seats, controls,
+and instruments snugly housed with their waterproof covers,
+and the Aeroplane is ready to weather the possible storm.
+
+Says the Observer, ``I'm remarkably peckish, and methinks
+I spy the towers of one of England's stately homes
+showing themselves just beyond that wood, less than a
+quarter of a mile away. What ho! for a raid. What do
+you say?''
+
+``All right, you cut along and I'll stop here, for the
+Aeroplane must not be left alone. Get back as quickly as
+possible.''
+
+And the Observer trots off, leaving the Pilot filling his
+pipe and anxiously scrutinizing the weather conditions.
+Very thick it is now, but the day is yet young, and he has
+hopes of the fog lifting sufficiently to enable the flight to be
+resumed. A little impatiently he awaits the return of his
+comrade, but with never a doubt of the result, for the hospi-
+tality of the country house is proverbial among pilots!
+What old hand among them is there who cannot instance
+many a forced landing made pleasant by such hospitality?
+Never too late or too early to help with food, petrol, oil,
+tools, and assistants. Many a grateful thought has the
+writer for such kind help given in the days before the war
+(how long ago they seem!), when aeroplanes were still more
+imperfect than they are now, and involuntary descents
+often a part of 'cross-country flying.
+
+Ah! those early days! How fresh and inspiring they
+were! As one started off on one's first 'cross-country flight,
+on a machine the first of its design, and with everything
+yet to learn, and the wonders of the air yet to explore; then
+the joy of accomplishment, the dreams of Efficiency, the
+hard work and long hours better than leisure; and what a
+field of endeavour--the realms of space to conquer! And
+the battle still goes on with ever-increasing success. Who
+is bold enough to say what its limits shall be?
+
+So ruminates this Pilot-Designer, as he puffs at his pipe,
+until his reverie is abruptly disturbed by the return of the
+Observer.
+
+``Wake up, you AIRMAN,'' the latter shouts. ``Here's
+the very thing the doctor ordered! A basket of first-class
+grub and something to keep the fog out, too.''
+
+``Well, that's splendid, but don't call me newspaper
+names or you'll spoil my appetite!''
+
+Then, with hunger such as only flying can produce, they
+appreciatively discuss their lunch, and with many a grateful
+thought for the donors--and they talk shop. They can't
+help it, and even golf is a poor second to flight talk. Says
+the Pilot, who must have his grievance, ``Just observe
+where I managed to stop the machine. Not twenty feet
+from this hedge! A little more and we should have been
+through it and into Kingdom Come! I stalled as well as
+one could, but the tail touched the ground and so I could
+not give the Aeroplane any larger angle of incidence. Could
+I have given it a larger angle, then the planes would have
+become a much more effective air-brake, and we should
+have come to rest in a much shorter distance. It's all the
+fault of the tail. There's hardly a type of Aeroplane in
+existence in which the tail could not be raised several feet,
+and that would make all the difference. High tails mean
+a large angle of incidence when the machine touches ground
+and, with enough angle, I'll guarantee to safely land the
+fastest machine in a five-acre field. You can, I am sure,
+imagine what a difference that would make where forced
+landings are concerned!'' Then rapidly sketching in his
+notebook, he shows the Observer the following illustration:
+
+``That's very pretty,'' said the Observer, ``but how
+about Mechanical Difficulties, and Efficiency in respect of
+Flight? And, anyway, why hasn't such an obvious thing
+been done already?''
+
+``As regards the first part of your question I assure
+you that there's nothing in it, and I'll prove it to you as
+follows----''
+
+``Oh! That's all right, old chap. I'll take your word
+for it,'' hurriedly replies the Observer, whose soul isn't tuned
+to a technical key.
+
+``As regards the latter part of your inquiry,'' went on
+the Pilot, a little nettled at having such a poor listener,
+``it's very simple. Aeroplanes have `just growed' like
+Topsy, and they consequently contain this and many another
+relic of early day design when Aeroplanes were more or less
+thrown together and anything was good enough that could
+get off the ground.''
+
+``By Jove,'' interrupts the Observer, ``I do believe the
+fog is lifting. Hadn't we better get the engine and body
+covers off, just in case it's really so?''
+
+``I believe you're right. I am sure those hills over there
+could not be seen a few minutes ago, and look--there's
+sunshine over there. We'd better hurry up.''
+
+Ten minutes' hard work and the covers are off, neatly
+folded and stowed aboard; the picketing wires are cast adrift,
+and the Pilot is once more in his seat. The Aeroplane has
+been turned to face the other end of the field, and, the Observer
+swinging round the propeller, the engine is awake
+again and slowly ticking over. Quickly the Observer climbs
+into his seat in front of the Pilot, and, the latter slightly
+opening the throttle, the Aeroplane leisurely rolls over the
+ground towards the other end of the field, from which the
+ascent will be made.
+
+Arriving there the Pilot turns the Aeroplane in order to
+face the wind and thus secure a quick ``get-off.'' Then he
+opens the throttle fully and the mighty voice of the Engine
+roars out ``Now see me clear that hedge!'' and the Aeroplane
+races forward at its minimum angle of incidence. Tail
+up, and with ever-increasing speed, it rushes towards the
+hedge under the lee of which it has lately been at rest; and
+then, just as the Observer involuntarily pulls back an imaginary
+``joy-stick,'' the Pilot moves the real one and places the
+machine at its best climbing angle. Like a living thing it
+responds, and instantly leaves the ground, clearing the hedge
+like a--well, like an Aeroplane with an excellent margin of
+lift. Upwards it climbs with even and powerful lift, and the
+familiar scenes below again gladden the eyes of the Pilot.
+Smaller and more and more squat grow the houses and hills;
+more and more doll-like appear the fields which are clearly
+outlined by the hedges; and soon the country below is easily
+identified with the map. Now they can see the river before
+them and a bay of the sea which must be crossed or skirted.
+The fog still lingers along the course of the river and between
+the hills, but is fast rolling away in grey, ghost-like masses.
+Out to sea it obscures the horizon, making it difficult to be
+sure where water ends and fog begins, and creating a strange,
+rather weird effect by which ships at a certain distance appear
+to be floating in space.
+
+Now the Aeroplane is almost over the river, and the
+next instant it suddenly drops into a ``hole in the air.''
+With great suddenness it happens, and for some two hundred
+feet it drops nose-down and tilted over sideways; but the
+Pilot is prepared and has put his craft on an even keel in
+less time than it takes to tell you about it; for well he knows
+that he must expect such conditions when passing over a
+shore or, indeed, any well-defined change in the composition
+of the earth's surface. Especially is this so on a hot and
+sunny day, for then the warm surface of the earth creates
+columns of ascending air, the speed of the ascent depending
+upon the composition of the surface. Sandy soil, for instance,
+such as borders this river produces a quickly ascending
+column of air, whereas water and forests have not such a
+marked effect. Thus, when our Aeroplane passed over the
+shore of the river, it suddenly lost the lift due to the ascending
+air produced by the warm sandy soil, and it consequently
+dropped just as if it had fallen into a hole.
+
+Now the Aeroplane is over the bay and, the sea being
+calm, the Pilot looks down, down through the water, and
+clearly sees the bottom, hundreds of feet below the surface.
+Down through the reflection of the blue sky and clouds,
+and one might think that is all, but it isn't. Only those
+who fly know the beauties of the sea as viewed from above;
+its dappled pearly tints; its soft dark blue shadows; the beautiful
+contrasts of unusual shades of colour which are always
+differing and shifting with the changing sunshine and the
+ever moving position of the aerial observer. Ah! for some
+better pen than mine to describe these things! One with
+glowing words and a magic rhythm to express the wonders
+of the air and the beauty of the garden beneath--the immensity
+of the sea--the sense of space and of one's littleness
+there--the realization of the Power moving the multitudes
+below--the exaltation of spirit altitude produces--the joy of
+speed. A new world of sensation!
+
+Now the bay is almost crossed and the Aerodrome at B
+can be distinguished.
+
+On the Aerodrome is a little crowd waiting and watching
+for the arrival of the Aeroplane, for it is of a new and improved
+type and its first 'cross-country performance is of
+keen interest to these men; men who really know something
+about flight.
+
+There is the Squadron Commander who has done some
+real flying in his time; several well-seasoned Flight-
+Commanders; a dozen or more Flight-Lieutenants; a
+knowledgeable Flight-Sergeant; a number of Air Mechanics,
+and, a little on one side and almost unnoticed, the
+Designer.
+
+``I hope they are all right,'' said someone, ``and that
+they haven't had difficulties with the fog. It rolled up very
+quickly, you know.''
+
+``Never fear,'' remarked a Flight-Commander. ``I know
+the Pilot well and he's a good 'un; far too good to carry on
+into a fog.''
+
+``They say the machine is really something out of the
+ordinary,'' said another, ``and that, for once, the Designer
+has been allowed full play; that he hasn't been forced to
+unduly standardize ribs, spars, struts, etc., and has more
+or less had his own way. I wonder who he is. It seems
+strange we hear so little of him.''
+
+``Ah! my boy. You do a bit more flying and you'll
+discover that things are not always as they appear from a
+distance!''
+
+``There she is, sir!'' cries the Flight-Sergeant. ``Just a
+speck over the silvery corner of that cloud.''
+
+A tiny speck it looks, some six miles distant and three
+thousand feet high; but, racing along, it rapidly appears
+larger and soon its outlines can be traced and the sunlight
+be seen playing upon the whirling propeller.
+
+Now the distant drone of the engine can be heard,
+but not for long, for suddenly it ceases and, the nose of
+the Aeroplane sinking, the craft commences gliding downwards.
+
+``Surely too far away,'' says a subaltern. It will be
+a wonderful machine if, from that distance and height, it
+can glide into the Aerodrome.'' And more than one express
+the opinion that it cannot be done; but the Designer smiles
+to himself, yet with a little anxiety, for his reputation is
+at stake, and Efficiency, the main reward he desires, is perhaps,
+or perhaps not, at last within his grasp!
+
+Swiftly the machine glides downwards towards them,
+and it can now be seen how surprisingly little it is affected
+by the rough weather and gusts; so much so that a little
+chorus of approval is heard.
+
+``Jolly good gliding angle,'' says someone; and another,
+``Beautifully quick controls, what?'' and from yet another,
+``By Jove! The Pilot must be sure of the machine. Look,
+he's stopped the engine entirely.''
+
+Then the Aeroplane with noiseless engine glides over
+the boundary of the Aerodrome, and, with just a soft soughing
+sound from the air it cleaves, lands gently not fifty yards from
+the onlookers.
+
+``Glad to see you,'' says the Squadron Commander to
+the Pilot. ``How do you like the machine?'' And the
+Pilot replies:
+
+``I never want a better one, sir. It almost flies itself!''
+
+And the Designer turns his face homewards and towards
+his beloved drawing-office; well satisfied, but still dreaming
+dreams of the future and . . . looking far ahead whom should
+he see but Efficiency at last coming towards him! And to
+him she is all things. In her hair is the morning sunshine;
+her eyes hold the blue of the sky, and on her cheeks is the
+pearly tint of the clouds as seen from above. The passion
+of speed, the lure of space, the sense of power, and the
+wonder of the future . . . all these things she holds for him.
+
+``Ah!'' he cries. ``You'll never leave me now, when
+at last there is no one between us?''
+
+And Efficiency, smiling and blushing, but practical as
+ever, says:
+
+``And you will never throw those Compromises in my
+face?''
+
+``My dear, I love you for them! Haven't they been
+my life ever since I began striving for you ten long years
+ago?''
+
+And so they walked off very happily, arm-in-arm together;
+and if this hasn't bored you and you'd like some more of the
+same sort of thing, I'd just love to tell you some day of the
+wonderful things they accomplish together, and of what
+they dream the future holds in store.
+
+And that's the end of the Prologue.
+
+
+
+CHAPTER I
+
+FLIGHT
+
+Air has weight (about 13 cubic feet = 1 lb.), inertia, and
+momentum. It therefore obeys Newton's laws[[14]] and resists
+movement. It is that resistance or reaction which makes
+flight possible.
+
+
+[[14]] See Newton's laws in the Glossary at the end of the book.
+
+
+Flight is secured by driving through the air a surface[[15]]
+inclined upwards and towards the direction of motion.
+
+
+[[15]] See ``Aerofoil'' in the Glossary.
+
+
+S = Side view of surface.
+
+M = Direction of motion.
+
+CHORD.--The Chord is, for practical purposes, taken to
+be a straight line from the leading edge of the surface to its
+trailing edge.
+
+N = A line through the surface starting from its trailing
+edge. The position of this line, which I call the Neutral
+Lift Line, is found by means of wind-tunnel research, and it
+varies with differences in the camber (curvature) of surfaces.
+In order to secure flight, the inclination of the surface must
+be such that the neutral lift line makes an angle with and
+ABOVE the line of motion. If it is coincident with M, there is
+no lift. If it makes an angle with M and BELOW it, then
+there is a pressure tending to force the surface down.
+
+I = Angle of Incidence. This angle is generally defined
+as the angle the chord makes with the direction of motion,
+but that is a bad definition, as it leads to misconception.
+The angle of incidence is best described as the angle the
+neutral lift line makes with the direction of motion relative
+to the air. You will, however, find that in nearly all rigging
+specifications the angle of incidence is taken to mean the
+angle the chord makes with a line parallel to the propeller
+thrust. This is necessary from the point of view of the
+practical mechanic who has to rig the aeroplane, for he could
+not find the neutral lift line, whereas he can easily find the
+chord. Again, he would certainly be in doubt as to ``the
+direction of motion relative to the air,'' whereas he can
+easily find a line parallel to the propeller thrust. It is a
+pity, however, that these practical considerations have
+resulted in a bad definition of the angle of incidence becoming
+prevalent, a consequence of which has been the widespread
+fallacy that flight may be secured with a negative
+inclination of the surface. Flight may conceivably be
+secured with a negative angle of chord, but never with a
+negative inclination of the surface. All this is only applicable
+to cambered surfaces. In the case of flat surfaces the neutral
+lift line coincides with the chord and the definition I have
+criticised adversely is then applicable. Flat lifting surfaces
+are, however, never used.
+
+The surface acts upon the air in the following manner:
+
+
+As the bottom of the surface meets the air, it compresses
+it and accelerates it DOWNWARDS. As a result of this definite
+action there is, of course, an equal and opposite reaction
+UPWARDS.
+
+The top surface, in moving forward, tends to leave the
+air behind it, thus creating a semi-vacuum or rarefied area
+over the top of the surface. Consequently the pressure of
+air on the top of the surface is decreased, thus assisting the
+reaction below to lift the surface UPWARDS.
+
+The reaction increases approximately as the square of
+the velocity. It is the result of (1) the mass of air engaged,
+and (2) the velocity and consequent force with which the
+surface engages the air. If the reaction was produced by
+only one of those factors it would increase in direct proportion
+to the velocity, but, since it is the product of both factors,
+it increases as V<2S>.
+
+Approximately three-fifths of the reaction is due to the
+decrease of density (and consequent decrease of downward
+pressure) on the top of the surface; and only some two-
+fifths is due to the upward reaction secured by the action
+of the bottom surface upon the air. A practical point in
+respect of this is that, in the event of the fabric covering the
+surface getting into bad condition, it is more likely to strip
+off the top than off the bottom.
+
+The direction of the reaction is approximately at right-
+angles to the chord of the surface, as illustrated above; and
+it is, in considering flight, convenient to divide it into two
+component parts or values, thus:
+
+1. The vertical component of the reaction, i.e., Lift,
+which is opposed to Gravity, i.e., the weight of the
+aeroplane.
+
+2. The horizontal component, i.e., Drift (sometimes
+called Resistance), to which is opposed the thrust of the
+propeller.
+
+The direction of the reaction is, of course, the resultant
+of the forces Lift and Drift.
+
+The Lift is the useful part of the reaction, for it lifts the
+weight of the aeroplane.
+
+The Drift is the villain of the piece, and must be overcome
+by the Thrust in order to secure the necessary velocity to
+produce the requisite Lift for flight.
+
+DRIFT.--The drift of the whole aeroplane (we have considered
+only the lifting surface heretofore) may be conveniently
+divided into three parts, as follows:
+
+Active Drift, which is the drift produced by the lifting
+surfaces.
+
+Passive Drift, which is the drift produced by all the rest
+of the aeroplane--the struts, wires, fuselage, under-carriage,
+etc., all of which is known as ``detrimental surface.''
+
+Skin Friction, which is the drift produced by the friction
+of the air with roughnesses of surface. The latter is practically
+negligible having regard to the smooth surface of the
+modern aeroplane, and its comparatively slow velocity
+compared with, for instance, the velocity of a propeller
+blade.
+
+LIFT-DRIFT RATIO.--The proportion of lift to drift is
+known as the lift-drift ratio, and is of paramount importance,
+for it expresses the efficiency of the aeroplane (as distinct
+from engine and propeller). A knowledge of the factors
+governing the lift-drift ratio is, as will be seen later, an
+absolute necessity to anyone responsible for the rigging of an
+aeroplane, and the maintenance of it in an efficient and safe
+condition.
+
+Those factors are as follows:
+
+1. Velocity.--The greater the velocity the greater the
+proportion of drift to lift, and consequently the
+less the efficiency. Considering the lifting surfaces
+alone, both the lift and the (active) drift, being
+component parts of the reaction, increase as the
+square of the velocity, and the efficiency remains
+the same at all speeds. But, considering the
+whole aeroplane, we must remember the passive
+drift. It also increases as the square of the
+velocity (with no attendant lift), and, adding
+itself to the active drift, results in increasing
+the proportion of total drift (active + passive) to
+lift.
+
+But for the increase in passive drift the efficiency
+of the aeroplane would not fall with increasing
+velocity, and it would be possible, by doubling
+the thrust, to approximately double the speed
+or lift--a happy state of affairs which can never
+be, but which we may, in a measure, approach
+by doing everything possible to diminish the passive
+drift.
+
+Every effort is then made to decrease it by
+``stream-lining,'' i.e., by giving all ``detrimental''
+parts of the aeroplane a form by which they will
+pass through the air with the least possible drift.
+Even the wires bracing the aeroplane together are,
+in many cases, stream-lined, and with a markedly
+good effect upon the lift-drift ratio. In the case
+of a certain well-known type of aeroplane the
+replacing of the ordinary wires by stream-lined
+wires added over five miles an hour to the flight
+speed.
+
+Head-resistance is a term often applied to passive
+drift, but it is apt to convey a wrong impression,
+as the drift is not nearly so much the result of
+the head or forward part of struts, wires, etc.,
+as it is of the rarefied area behind.
+
+Above is illustrated the flow of air round two
+objects moving in the direction of the arrow M.
+
+In the case of A, you will note that the rarefied
+area DD is of very considerable extent; whereas
+in the case of B, the air flows round it in such a
+way as to meet very closely to the rear of the
+object, thus DECREASING DD.
+
+The greater the rarefied area DD. then, the less
+the density, and, consequently, the less the pressure
+of air upon the rear of the object. The less such
+pressure, then, the better is head-resistance D
+able to get its work in, and the more thrust will
+be required to overcome it.
+
+The ``fineness'' of the stream-line shape, i.e.,
+the proportion of length to width, is determined
+by the velocity--the greater the velocity, the
+greater the fineness. The best degree of fineness
+for any given velocity is found by means of wind-
+tunnel research.
+
+The practical application of all this is, from a
+rigging point of view, the importance of adjusting
+all stream-line parts to be dead-on in the line of
+flight, but more of that later on.
+
+2. Angle of Incidence.--The most efficient angle of
+incidence varies with the thrust at the disposal
+of the designer, the weight to be carried, and the
+climb-velocity ratio desired.
+
+The best angles of incidence for these varying
+factors are found by means of wind-tunnel research
+and practical trial and error. Generally
+speaking, the greater the velocity the smaller
+should be the angle of incidence, in order to preserve
+a clean, stream-line shape of rarefied area
+and freedom from eddies. Should the angle be
+too great for the velocity, then the rarefied area
+becomes of irregular shape with attendant turbulent
+eddies. Such eddies possess no lift value,
+and since it has taken power to produce them,
+they represent drift and adversely affect the lift-
+drift ratio.
+
+From a rigging point of view, one must presume
+that every standard aeroplane has its lifting
+surface set at the most efficient angle, and the
+practical application of all this is in taking the
+greatest possible care to rig the surface at the
+correct angle and to maintain it at such angle.
+Any deviation will adversely affect the lift-drift
+ratio, i.e., the efficiency.
+
+3. Camber.--(Refer to the second illustration in this
+chapter.) The lifting surfaces are cambered, i.e.,
+curved, in order to decrease the horizontal component
+of the reaction, i.e., the drift.
+
+The bottom camber: If the bottom of the surface
+was flat, every particle of air meeting it would do
+so with a shock, and such shock would produce a
+very considerable horizontal reaction or drift. By
+curving it such shock is diminished, and the curve
+should be such as to produce a uniform (not
+necessarily constant) acceleration and compression
+of the air from the leading edge to the trailing
+edge. Any unevenness in the acceleration and
+compression of the air produces drift.
+
+The top camber: If this was flat it would produce
+a rarefied area of irregular shape. I have already
+explained the bad effect this has upon the lift-
+drift ratio. The top surface is then curved to
+produce a rarefied area the shape of which shall
+be as stream-line and free from attendant eddies
+as possible.
+
+The camber varies with the angle of incidence,
+the velocity, and the thickness of the surface.
+Generally speaking, the greater the velocity, the
+less the camber and angle of incidence. With
+infinite velocity the surface would be set at no
+angle of incidence (the neutral lift line coincident
+with the direction of motion relative to the air),
+and would be, top and bottom, of pure streamline
+form--i.e., of infinite fineness. This is, of
+course, carrying theory to absurdity as the surface
+would then cease to exist.
+
+The best cambers for varying velocities, angles
+of incidence, and thicknesses of surface, are found
+by means of wind-tunnel research. The practical
+application of all this is in taking the greatest
+care to prevent the surface from becoming distorted
+and thus spoiling the camber and consequently
+the lift-drift ratio.
+
+4. Aspect Ratio.--This is the proportion of span to
+chord. Thus, if the span is, for instance, 50 feet
+and the chord 5 feet, the surface would be said to
+have an aspect ratio of 10 to 1.
+
+For A GIVEN VELOCITY and A GIVEN AREA of surface,
+the greater the aspect ratio, the greater the reaction.
+It is obvious, I think, that the greater
+the span, the greater the mass of air engaged,
+and, as already explained, the reaction is partly
+the result of the mass of air engaged.
+
+Not only that, but, PROVIDED the chord is not
+decreased to an extent making it impossible to
+secure the best camber owing to the thickness of
+the surface, the greater the aspect ratio, the better
+the lift-drift ratio. The reason of this is rather
+obscure. It is sometimes advanced that it is
+owing to the ``spill'' of air from under the wing-
+tips. With a high aspect ratio the chord is less
+than would otherwise be the case. Less chord
+results in smaller wing-tips and consequently less
+``spill.'' This, however, appears to be a rather
+inadequate reason for the high aspect ratio producing
+the high lift-drift ratio. Other reasons
+are also advanced, but they are of such a contentious
+nature I do not think it well to go into them
+here. They are of interest to designers, but this
+is written for the practical pilot and rigger.
+
+5. Stagger.--This is the advancement of the top surface
+relative to the bottom surface, and is not, of course,
+applicable to a single surface, i.e., a monoplane.
+In the case of a biplane having no stagger, there
+will be ``interference'' and consequent loss of
+Efficiency unless the gap between the top and bottom
+surfaces is equal to not less than 1 1/2  times the
+chord. If less than that, the air engaged by the
+bottom of the top surface will have a tendency
+to be drawn into the rarefied area over the top
+of the bottom surface, with the result that the
+surfaces will not secure as good a reaction as would
+otherwise be the case.
+
+It is not practicable to have a gap of much
+more than a distance equal to the chord, owing
+to the drift produced by the great length of struts
+and wires such a large gap would necessitate.
+By staggering the top surface forward, however,
+
+it is removed from the action of the lower surface
+and engages undisturbed air, with the result that
+the efficiency can in this way be increased by
+about 5 per cent. Theoretically the top plane
+should be staggered forward for a distance equal
+to about 30 per cent. of the chord, the exact
+distance depending upon the velocity and angle
+of incidence; but this is not always possible to
+arrange in designing an aeroplane, owing to difficulties
+of balance, desired position, and view of
+pilot, observer, etc.
+
+6. Horizontal Equivalent.--The vertical component of
+the reaction, i.e., lift, varies as the horizontal
+equivalent (H.E.) of the surface, but the drift
+remains the same. Then it follows that if H.E. grows
+less, the ratio of lift to drift must do the same.
+
+A, B, and C are front views of three surfaces.
+
+A has its full H.E., and therefore, from the point
+of view from which we are at the moment considering
+efficiency, it has its best lift-drift ratio.
+
+B and C both possess the same surface as A,
+but one is inclined upwards from its centre and
+the other is straight but tilted. For these reasons
+their H.E.'s are, as illustrated, less than in the
+case of A. That means less vertical lift, and,
+the drift remaining the same (for there is the
+same amount of surface as in A to produce it),
+the lift-drift ratio falls.
+
+THE MARGIN OF POWER is the power available above
+that necessary to maintain horizontal flight.
+
+THE MARGIN OF LIFT is the height an aeroplane can gain
+in a given time and starting from a given altitude.
+As an example, thus: 1,000 feet the first minute,
+and starting from an altitude of 500 feet above
+sea-level.
+
+The margin of lift decreases with altitude, owing
+to the decrease in the density of the air, which
+adversely affects the engine. Provided the engine
+maintained its impulse with altitude, then, if we
+ignore the problem of the propeller, which I will
+go into later on, the margin of lift would not
+disappear. Moreover, greater velocity for a given
+power would be secured at a greater altitude, owing
+to the decreased density of air to be overcome.
+After reading that, you may like to light your pipe
+and indulge in dreams of the wonderful possibilities
+which may become realities if some brilliant genius
+shows us some day how to secure a constant power
+with increasing altitude. I am afraid, however,
+that will always remain impossible; but it is probable
+that some very interesting steps may be taken in
+that direction.
+
+THE MINIMUM ANGLE OF INCIDENCE is the smallest
+angle at which, for a given power, surface (including
+detrimental surface), and weight, horizontal flight
+can be maintained.
+
+THE MAXIMUM ANGLE OF INCIDENCE is the greatest
+angle at which, for a given power, surface (including
+detrimental surface), and weight, horizontal flight
+can be maintained.
+
+THE OPTIMUM ANGLE OF INCIDENCE is the angle at
+which the lift-drift ratio is highest. In modern
+aeroplanes it is that angle of incidence possessed by the
+surface when the axis of the propeller is horizontal.
+
+THE BEST CLIMBING ANGLE is approximately half-way
+between the maximum and the optimum angles.
+
+All present-day aeroplanes are a compromise between
+Climb and horizontal Velocity. We will compare
+the essentials for two aeroplanes, one designed for
+maximum climb, and the other for maximum velocity.
+
+ESSENTIALS FOR MAXIMUM CLIMB:
+
+1. Low velocity, in order to secure the best lift-drift
+ratio.
+
+2. Having a low velocity, a large surface will be
+necessary in order to engage the necessary mass
+of air to secure the requisite lift.
+
+3. Since (1) such a climbing machine will move
+along an upward sloping path, and (2) will climb
+with its propeller thrust horizontal, then a large
+angle relative to the direction of the thrust will be
+necessary in order to secure the requisite angle
+relative to the direction of motion.
+
+The propeller thrust should be always horizontal, because
+the most efficient flying-machine (having regard to climb OR
+velocity) has, so far, been found to be an arrangement of an
+inclined surface driven by a HORIZONTAL thrust--the surface
+lifting the weight, and the thrust overcoming the drift.
+This is, in practice, a far more efficient arrangement than
+the helicopter, i.e., the air-screw revolving about a vertical
+axis and producing a thrust opposed to gravity. If, when
+climbing, the propeller thrust is at such an angle as to tend
+to haul the aeroplane upwards, then it is, in a measure,
+acting as a helicopter, and that means inefficiency. The
+reason of a helicopter being inefficient in practice is due to
+the fact that, owing to mechanical difficulties, it is impossible
+to construct within a reasonable weight an air-screw of the
+requisite dimensions. That being so, it would be necessary,
+in order to absorb the power of the engine, to revolve the
+comparatively small-surfaced air screw at an immensely
+greater velocity than that of the aeroplane's surface. As
+already explained, the lift-drift ratio falls with velocity on
+account of the increase in passive drift. This applies to a
+blade of a propeller or air-screw, which is nothing but a
+revolving surface set at angle of incidence, and which it is
+impossible to construct without a good deal of detrimental
+surface near the central boss.
+
+4. The velocity being low, then it follows that for
+that reason also the angle of incidence should be
+comparatively large.
+
+5. Camber.--Since such an aeroplane would be of
+low velocity, and therefore possess a large angle
+of incidence, a large camber would be necessary.
+
+Let us now consider the essentials for an aeroplane of
+maximum velocity for its power, and possessing merely
+enough lift to get off the ground, but no margin of lift.
+
+1. Comparatively HIGH VELOCITY.
+
+2. A comparatively SMALL SURFACE, because, being
+of greater velocity than the maximum climber,
+a greater mass of air will be engaged for a given
+surface and time, and therefore a smaller surface
+will be sufficient to secure the requisit lift.
+
+3. A small angle relative to the propeller thrust, since
+the latter coincides with the direction of motion.
+
+4. A comparatively small angle of incidence by reason
+of the high velocity.
+
+5. A comparatively small camber follows as a result
+of the small angle of incidence.
+
+
+SUMMARY.
+
+Essentials for Maximum            Essentials for Maximum
+        Climb.                            Velocity
+
+1. Low velocity.                  High velocity.
+2. Large surface.                 Small surface.
+3. Large angle relative to        Small angle relative to
+   propeller thrust.                 propeller thrust.
+4. Large angle relative to        Small angle relative to direction
+   direction of motion.              of motion.
+5. Large camber.                  Small camber.
+
+
+It is mechanically impossible to construct an aeroplane
+of reasonable weight of which it would be possible to very
+the above opposing essentials. Therefore, all aeroplanes
+are designed as a compromise between Climb and Velocity.
+
+As a rule aeroplanes are designed to have at low altitude
+a slight margin of lift when the propeller thrust is horizontal.
+
+
+ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE
+DESIGNED AS A COMPROMISE BETWEEN
+VELOCITY AND CLIMB, AND POSSESSING A SLIGHT MARGIN OF LIFT AT A LOW
+ALTITUDE AND WHEN THE THRUST IS HORIZONTAL
+
+MINIMUM ANGLE.
+
+This gives the greatest velocity
+during horizontal flight at a low
+altitude. Greater velocity would
+be secured if the surface, angle,
+and camber were smaller and designed
+to just maintain horizontal
+flight with a horizontal thrust.
+Also, in such case, the propeller
+would not be thrusting downwards,
+but along a horizontal line
+which is obviously a more efficient
+arrangement if we regard
+the aeroplane merely from one
+point of view, i.e., either with
+reference to velocity OR climb.
+
+OPTIMUM ANGLE
+(Thrust horizontal)
+
+The velocity is less than at the
+smaller minimum angle, and, as
+aeroplanes are designed to-day, the
+area and angle of incidence of the
+surface is such as to secure a
+slight ascent at a low altitude. The
+camber of the surface is designed
+for this angle of incidence and
+velocity. The lift-drift ratio is
+best at this angle.
+
+BEST CLIMBING ANGLE
+
+The velocity is now still less by
+reason of the increased angle
+producing increase of drift. Less
+velocity at A GIVEN ANGLE produces
+less lift, but the increased angle
+more or less offsets the loss of lift
+due to the decreased velocity, and
+in addition, the thrust is now hauling
+the aeroplane upwards.
+
+MAXIMUM ANGLE
+
+The greater angle has now produced
+so much drift as to lessen
+the velocity to a point where the
+combined lifts from the surface
+and from the thrust are only just
+able to maintain horizontal flight.
+Any greater angle will result in a
+still lower lift-drift ratio. The lift
+will then become less than the
+weight and the aeroplane will
+consequently fall. Such a fall is
+known as ``stalling'' or ``pancaking.''
+
+NOTE.--The golden rule for beginners: Never exceed the Best Climbing Angle.
+Always maintain the flying speed of the aeroplane.
+
+
+By this means, when the altitude is reached where the margin
+of lift disappears (on account of loss of engine power), and
+which is, consequently, the altitude where it is just possible
+to maintain horizontal flight, the aeroplane is flying with
+its thrust horizontal and with maximum efficiency (as distinct
+from engine and propeller efficiency).
+
+The margin of lift at low altitude, and when the thrust
+is horizontal, should then be such that the higher altitude at
+which the margin of lift is lost is that altitude at which most
+of the aeroplane's horizontal flight work is done. That
+ensures maximum velocity when most required.
+
+Unfortunately, where aeroplanes designed for fighting
+are concerned, the altitude where most of the work is done
+is that at which both maximum velocity and maximum
+margin of lift for power are required.
+
+Perhaps some day a brilliant inventor will design an
+aeroplane of reasonable weight and drift of which it will be
+possible for the pilot to vary at will the above-mentioned
+opposing essentials. Then we shall get maximum velocity,
+or maximum margin of lift, for power as required. Until
+then the design of the aeroplane must remain a compromise
+between Velocity and Climb.
+
+
+
+CHAPTER II
+
+STABILITY AND CONTROL
+
+STABILITY is a condition whereby an object disturbed
+has a natural tendency to return to its first and normal
+position. Example: a weight suspended by a cord.
+
+INSTABILITY is a condition whereby an object disturbed
+has a natural tendency to move as far as possible away from
+its first position, with no tendency to return. Example:
+a stick balanced vertically upon your finger.
+
+NEUTRAL INSTABILITY is a condition whereby an object
+disturbed has no tendency to move farther than displaced
+by the force of the disturbance, and no tendency to return
+to its first position.
+
+In order that an aeroplane may be reasonably controllable,
+it is necessary for it to possess some degree of stability
+longitudinally, laterally, and directionally.
+
+LONGITUDINAL STABILITY in an aeroplane is its stability
+about an axis transverse to the direction of normal horizontal
+flight, and without which it would pitch and toss.
+
+LATERAL STABILITY is its stability about its longitudinal
+axis, and without which it would roll sideways.
+
+DIRECTIONAL STABILITY is its stability about its vertical
+axis, and without which it would have no tendency to keep
+its course.
+
+For such directional stability to exist there must be,
+in effect,[[16]] more ``keel-surface'' behind the vertical axis
+than there is in front of it. By keel-surface I mean every-
+thing to be seen when looking at an aeroplane from the side
+of it--the sides of the body, undercarriage, struts, wires, etc.
+The same thing applies to a weathercock. You know what
+would happen if there was insufficient keel-surface behind
+the vertical axis upon which it is pivoted. It would turn
+off its proper course, which is opposite to the direction of
+the wind. It is very much the same in the case of an aeroplane.
+
+[[16]] ``In effect'' because, although there may be actually the greatest proportion
+of keel-surface In front of the vertical axis, such surface may be much nearer to
+the axis than is the keel-surface towards the tail. The latter may then be actually
+less than the surface in front, but, being farther from the axis, it has a greater
+leverage, and consequently is greater in effect than the surface in front.
+
+The above illustration represents an aeroplane (directionally
+stable) flying along the course B. A gust striking it
+as indicated acts upon the greater proportion of keel-surface
+behind the turning axis and throws it into the new course.
+It does not, however, travel along the new course, owing to
+its momentum in the direction B. It travels, as long as
+such momentum lasts, in a direction which is the resultant
+of the two forces Thrust and Momentum. But the centre
+line of the aeroplane is pointing in the direction of the new
+course. Therefore its attitude, relative to the direction of
+motion, is more or less sideways, and it consequently receives
+an air pressure in the direction C. Such pressure, acting
+upon the keel-surface, presses the tail back towards its first
+position in which the aeroplane is upon its course B.
+
+What I have described is continually going on during
+flight, but in a well-designed aeroplane such stabilizing
+movements are, most of the time, so slight as to be imperceptible
+to the pilot.
+
+If an aeroplane was not stabilized in this way, it would
+not only be continually trying to leave its course, but it would
+also possess a dangerous tendency to ``nose away'' from the
+direction of the side gusts. In such case the gust shown in
+the above illustration would turn the aeroplane round the
+opposite way a very considerable distance; and the right
+wing, being on the outside of the turn, would travel with
+greater velocity than the left wing. Increased velocity
+means increased lift; and so, the right wing lifting, the
+aeroplane would turn over sideways very quickly.
+
+LONGITUDINAL STABILITY.--Flat surfaces are longitudinally
+stable owing to the fact that with decreasing angles of
+incidence the centre line of pressure (C.P.) moves forward.
+
+The C.P. is a line taken across the surface, transverse
+to the direction of motion, and about which all the air forces
+may be said to balance, or through which they may be said
+to act.
+
+
+Imagine A to be a flat surface, attitude vertical, travelling
+through the air in the direction of motion M. Its C.P. is
+then obviously along the exact centre line of the surface
+as illustrated.
+
+In B, C, and D the surfaces are shown with angles of
+incidence decreasing to nothing, and you will note that the
+C.P. moves forward with the decreasing angle.
+
+Now, should some gust or eddy tend to make the surface
+decrease the angle, i.e., dive, then the C.P. moves forward
+and pushes the front of the surface up. Should the surface
+tend to assume too large an angle, then the reverse
+happens--the C.P. moves back and pushes the rear of the
+surface up.
+
+Flat surfaces are, then, theoretically stable longitudinally.
+They are not, however, used, on account of their poor
+lift-drift ratio.
+
+As already explained, cambered surfaces are used, and
+these are longitudinally unstable at those angles of incidence
+producing a reasonable lift-drift ratio, i.e., at angles below:
+about 12 degrees.
+
+A is a cambered surface, attitude approximately vertical,
+moving through the air in the direction M. Obviously the C. P.
+coincides with the transverse centre line of the surface.
+
+With decreasing angles, down to angles of about 30 degrees,
+the C.P. moves forward as in the case of flat surfaces (see B),
+but angles above 30 degrees do not interest us, since they produce
+a very low ratio of lift to drift.
+
+Below angles of about 30 degrees (see C) the dipping front part
+of the surface assumes a negative angle of incidence resulting
+in the DOWNWARD air pressure D, and the more the angle of
+incidence is decreased, the greater such negative angle and its
+resultant pressure D. Since the C.P. is the resultant of all
+the air forces, its position is naturally affected by D, which
+causes it to move backwards. Now, should some gust or
+eddy tend to make the surface decrease its angle of incidence,
+i.e., dive, then the C.P. moves backwards, and, pushing up
+the rear of the surface, causes it to dive the more. Should
+the surface tend to assume too large an angle, then the reverse
+happens; the pressure D decreases, with the result
+that C.P. moves forward and pushes up the front of the surface,
+thus increasing the angle still further, the final result
+being a ``tail-slide.''
+
+It is therefore necessary to find a means of stabilizing
+the naturally unstable cambered surface. This is usually
+secured by means of a stabilizing surface fixed some distance
+in the rear of the main surface, and it is a necessary condition
+that the neutral lift lines of the two surfaces, when projected
+to meet each other, make a dihedral angle. In other words,
+the rear stabilizing surface must have a lesser angle of
+incidence than the main surface--certainly not more than
+one-third of that of the main surface. This is known as the
+longitudinal dihedral.
+
+I may add that the tail-plane is sometimes mounted upon
+the aeroplane at the same angle as the main surface, but,
+in such cases, it attacks air which has received a downward
+deflection from the main surface, thus:
+
+{illust.}
+
+
+The angle at which the tail surface attacks the air (the.
+angle of incidence) is therefore less than the angle of incidence
+of the main surface.
+
+I will now, by means of the following illustration, try
+to explain how the longitudinal dihedral secures stability:
+
+First, imagine the aeroplane travelling in the direction
+of motion, which coincides with the direction of thrust T.
+The weight is, of course, balanced about a C.P., the resultant
+of the C.P. of the main surface and the C.P. of the stabilizing
+surface. For the sake of illustration, the stabilizing surface
+has been given an angle of incidence, and therefore has a
+lift and C.P. In practice the stabilizer is often set at no
+angle of incidence. In such case the proposition remains
+the same, but it is, perhaps, a little easier to illustrate it
+as above.
+
+Now, we will suppose that a gust or eddy throws the
+machine into the lower position. It no longer travels in
+the direction of T, since the momentum in the old direction
+pulls it off that course. M is now the resultant of the Thrust
+and the Momentum, and you will note that this results in a
+decrease in the angle our old friend the neutral lift line makes
+with M, i.e., a decrease in the angle of incidence and therefore
+a decrease in lift.
+
+We will suppose that this decrease is 2 degrees. Such decrease
+applies to both main surface and stabilizer, since both are
+fixed rigidly to the aeroplane.
+
+The main surface, which had 12 degrees angle, has now only
+10 degrees, i.e., a loss of ONE-SIXTH.
+
+The stabilizer, which had 4 degrees angle, has now only 2 degrees,
+i.e., a loss of ONE-HALF.
+
+The latter has therefore lost a greater PROPORTION of its
+angle of incidence, and consequently its lift, than has the
+main surface. It must then fall relative to the main surface.
+The tail falling, the aeroplane then assumes its first position,
+though at a slightly less altitude.
+
+Should a gust throw the nose of the aeroplane up, then
+the reverse happens. Both main surface and stabilizer
+increase their angles of incidence in the same amount, but
+the angle, and therefore the lift, of the stabilizer increases
+in greater proportion than does the lift of the main surface,
+with the result that it lifts the tail. The aeroplane then
+assumes its first position, though at a slightly greater
+altitude.
+
+Do not fall into the widespread error that the angle of
+incidence varies as the angle of the aeroplane to the horizontal.
+It varies with such angle, but not as anything approaching it.
+Remember that the stabilizing effect of the longitudinal
+dihedral lasts only as long as there is momentum in the direction
+of the first course.
+
+These stabilizing movements are taking place all the
+time, even though imperceptible to the pilot.
+
+Aeroplanes have, in the past, been built with a stabilizing
+surface in front of the main surface instead of at the rear of
+it. In such design the main surface (which is then the tail
+surface as well as the principal lifting surface) must be set
+at a less angle than the forward stabilizing surface, in order
+to secure a longitudinal dihedral. The defect of such design
+lies in the fact that the main surface must have a certain angle
+to lift the weight--say 5 degrees. Then, in order to secure a
+sufficiency of longitudinal stability, it is necessary to set the
+forward stabilizer at about 15 degrees. Such a large angle of incidence
+results in a very poor lift-drift ratio (and consequently great
+loss of efficiency), except at very low velocities compared with
+the speed of modern aeroplanes. At the time such aeroplanes
+were built velocities were comparatively low, and this defect
+was; for that reason, not sufficiently appreciated. In the end
+it killed the ``canard'' or ``tail-first'' design.
+
+Aeroplanes of the Dunne and similar types possess no
+stabilizing surface distinct from the main surface, but they
+have a longitudinal dihedral which renders them stable.
+
+The main surface towards the wing-tips is given a
+decreasing angle of incidence and corresponding camber. The
+wing-tips then act as longitudinal stabilizers.
+
+This design of aeroplane, while very interesting, has
+not proved very practicable, owing to the following
+disadvantages: (1) The plan design is not, from a mechanical
+point of view, so sound as that of the ordinary aeroplane
+surface, which is, in plan, a parallelogram. It is, then,
+necessary to make the strength of construction greater than
+would otherwise be the case. That means extra weight.
+(2) The plan of the surface area is such that the aspect ratio
+is not so high as if the surface was arranged with its leading
+edges at right angles to the direction of motion. The lower
+the aspect ratio, then, the less the lift. This design, then,
+produces less lift for weight of surface than would the same
+surface if arranged as a parallelogram. (3) In order to secure
+the longitudinal dihedral, the angle of incidence has to be
+very much decreased towards the wing-tips. Then, in order
+that the lift-drift ratio may be preserved, there must be a
+corresponding decrease in the camber. That calls for surface
+ribs of varying cambers, and results in an expensive and
+lengthy job for the builder. (4) In order to secure directional
+stability, the surface is, in the centre, arranged to dip down
+in the form of a V, pointing towards the direction of motion.
+Should the aeroplane turn off its course, then its momentum
+in the direction of its first course causes it to move in a
+direction the resultant of the thrust and the momentum. It
+then moves in a more or less sideways attitude, which results
+in an air pressure upon one side of the V, and which tends to
+turn the aeroplane back to its first course. This arrangement
+of the surface results in a bad drift. Vertical surfaces at
+the wing-tips may also be set at an angle producing the same
+stabilizing effect, but they also increase the drift.
+
+The gyroscopic action of a rotary engine will affect the
+longitudinal stability when an aeroplane is turned to right
+or left. In the case of a Gnome engine, such gyroscopic
+action will tend to depress the nose of the aeroplane when it
+is turned to the left, and to elevate it when it is turned to
+the right. In modern aeroplanes this tendency is not sufficiently
+important to bother about. In the old days of crudely
+designed and under-powered aeroplanes this gyroscopic action
+was very marked, and led the majority of pilots to dislike
+turning an aeroplane to the right, since, in doing so, there
+was some danger of ``stalling.''
+
+
+LATERAL STABILITY is far more difficult for the designer
+to secure than is longitudinal or directional stability. Some
+degree of lateral stability may be secured by means of the
+``lateral dihedral,'' i.e., the upward inclination of the surface
+towards its wing-tips thus:
+
+Imagine the top V, illustrated opposite, to be the front
+view of a surface flying towards you. The horizontal equivalent
+(H.E.) of the left wing is the same as that of the right
+wing. Therefore, the lift of one wing is equal to the lift
+of the other, and the weight, being situated always in the
+centre, is balanced.
+
+If some movement of the air causes the surface to tilt
+sideways, as in the lower illustration, then you will note that
+the H.E. of the left wing increases, and the H.E. of the right
+wing decreases. The left wing then, having the greatest
+lift, rises; and the surface assumes its first and normal
+position.
+
+Unfortunately however, the righting effect is not proportional
+to the difference between the right and left H.E.'s.
+
+
+
+In the case of A, the resultant direction of the reaction
+of both wings is opposed to the direction of gravity or weight.
+The two forces R R and gravity are then evenly balanced,
+and the surface is in a state of equilibrium.
+
+In the case of B, you will note that the R R is not directly
+opposed to gravity. This results in the appearance of M,
+and so the resultant direction of motion of the aeroplane
+is no longer directly forward, but is along a line the resultant
+of the thrust and M. In other words, it is, while flying
+forward, at the same time moving sideways in the direction M.
+
+In moving sideways, the keel-surface receives, of course,
+a pressure from the air equal and opposite to M. Since
+such surface is greatest in effect towards the tail, then the
+latter must be pushed sideways. That causes the aeroplane
+to turn; and, the highest wing being on the outside of the
+turn, it has a greater velocity than the lower wing. That
+produces greater lift, and tends to tilt the aeroplane over
+still more. Such tilting tendency is, however, opposed by
+the difference in the H.E.'s of the two wings.
+
+It then follows that, for the lateral dihedral angle to
+be effective, such angle must be large enough to produce,
+when the aeroplane tilts, a difference in the H.E.'s of the
+two wings, which difference must be sufficient to not only
+oppose the tilting tendency due to the aeroplane turning,
+but sufficient to also force the aeroplane back to its original
+position of equilibrium.
+
+It is now, I hope, clear to the reader that the lateral
+dihedral is not quite so effective as would appear at first
+sight. Some designers, indeed, prefer not to use it, since its
+effect is not very great, and since it must be paid for in loss
+of H.E. and consequently loss of lift, thus decreasing the lift-
+drift ratio, i.e., the efficiency. Also, it is sometimes advanced
+that the lateral dihedral increases the ``spill'' of air from the
+wing-tips and that this adversely affects the lift-drift ratio.
+
+The disposition of the keel-surface affects the lateral
+stability. It should be, in effect, equally divided by the
+longitudinal turning axis of the aeroplane. If there is an
+excess of keel-surface above or below such axis, then a side
+gust striking it will tend to turn the aeroplane over sideways.
+
+The position of the centre of gravity affects lateral stability.
+If too low, it produces a pendulum effect and causes the
+aeroplane to roll sideways.
+
+If too high, it acts as a stick balanced vertically would
+act. If disturbed, it tends to travel to a position as far as
+possible from its original position. It would then tend,
+when moved, to turn the aeroplane over sideways and into
+an upside-down position.
+
+From the point of view of lateral stability, the best
+position for the centre of gravity is one a little below the
+centre of drift.
+
+Propeller torque affects lateral stability. An aeroplane
+tends to turn over sideways in the opposite direction to which
+the propeller revolves.
+ 
+This tendency is offset by increasing the angle of incidence
+(and consequently the lift) of the side tending to fall; and it
+is always advisable, if practical considerations allow it, to
+also decrease the angle upon the other side. In that way
+it is not necessary to depart so far from the normal angle
+of incidence at which the lift-drift ratio is highest.
+
+Wash-in is the term applied to the increased angle.
+
+Wash-out is the term applied to the decreased angle.
+
+Both lateral and directional stability may be improved
+by washing out the angle of incidence on both sides of the
+surface, thus:
+
+The decreased angle decreases the drift and therefore the
+effect of gusts upon the wing-tips which is just where they
+have the most effect upon the aeroplane, owing to the distance
+from the turning axis.
+
+The wash-out also renders the ailerons (lateral controlling
+services) more effective, as, in order to operate them, it is
+not then necessary to give them such a large angle of incidence
+as would otherwise be required.
+
+
+
+The less the angle of incidence of the ailerons, the better
+their lift-drift ratio, i.e., their efficiency. You will note
+that, while the aileron attached to the surface with washed-out
+angle is operated to the same extent as the aileron illustrated
+above it, its angle of incidence is considerably less. Its efficiency
+is therefore greater.
+
+The advantages of the wash-in must, of course be paid for
+in some loss of lift, as the lift decreases with the decreased angle.
+
+In order to secure all the above described advantages,
+a combination is sometimes effected, thus:
+
+BANKING.--An aeroplane turned off its course to right
+or left does not at once proceed along its new course. Its
+momentum in the direction of its first course causes it to
+travel along a line the resultant of such momentum and the
+thrust. In other words, it more or less skids sideways and
+away from the centre of the turn. Its lifting surfaces do
+not then meet the air in their correct attitude, and the lift
+may fall to such an extent as to become less than the weight,
+in which case the aeroplane must fall. This bad effect is
+minimized by ``banking,'' i.e., tilting the aeroplane sideways.
+The bottom of the lifting surface is in that way opposed to
+the air through which it is moving in the direction of the
+momentum and receives an opposite air pressure. The
+rarefied area over the top of the surface is rendered still more
+rare, and this, of course, assists the air pressure in opposing
+the momentum.
+
+The velocity of the ``skid,'' or sideways movement, is
+then only such as is necessary to secure an air pressure equal
+and opposite to the centrifugal force of the turn.
+
+The sharper the turn, the greater the effect of the centrifugal
+force, and therefore the steeper should be the ``bank.''
+Experentia docet.
+
+The position of the centre of gravity affects banking. A low
+C.G. will tend to swing outward from the centre of the turn,
+and will cause the aeroplane to bank--perhaps too much, in
+which case the pilot must remedy matters by operating the
+ailerons.
+
+A high C.G. also tends to swing outward from the centre
+of the turn. It will tend to make the aeroplane bank the
+wrong way, and such effect must be remedied by means of
+the ailerons.
+
+The pleasantest machine from a banking point of view is
+one in which the C.G. is a little below the centre of drift.
+It tends to bank the aeroplane the right way for the turn,
+and the pilot can, if necessary, perfect the bank by means
+of the ailerons.
+
+The disposition of the keel-surface affects banking. It
+should be, in effect, evenly divided by the longitudinal axis.
+An excess of keel-surface above the longitudinal axis will,
+when banking, receive an air pressure causing the aeroplane
+to bank, perhaps too much. An excess of keel-surface below
+the axis has the reverse effect.
+
+
+SIDE-SLIPPING.--This usually occurs as a result of over-
+banking. It is always the result of the aeroplane tilting
+sideways and thus decreasing the horizontal equivalent, and
+therefore the lift, of the surface. An excessive ``bank,''
+or sideways tilt, results in the H.E., and therefore the lift,
+becoming less than the weight, when, of course, the aeroplane
+must fall, i.e., side-slip.
+
+
+When making a very sharp turn it is necessary to bank
+very steeply indeed. If, at the same time, the longitudinal
+axis of the aeroplane remains approximately horizontal,
+then there must be a fall, and the direction of motion will be
+the resultant of the thrust and the fall as illustrated above
+in sketch A. The lifting surfaces and the controlling surfaces
+are not then meeting the air in the correct attitude,
+with the result that, in addition to falling, the aeroplane
+will probably become quite unmanageable.
+
+The Pilot, however, prevents such a state of affairs from
+happening by ``nosing-down,'' i.e., by operating the rudder
+to turn the nose of the aeroplane downward and towards
+the direction of motion as illustrated in sketch B. This
+results in the higher wing, which is on the outside of the turn,
+travelling with greater velocity, and therefore securing a
+greater reaction than the lower wing, thus tending to tilt
+the aeroplane over still more. The aeroplane is now almost
+upside-down, but its attitude relative to the direction of
+motion is correct and the controlling surfaces are all of them
+working efficiently. The recovery of a normal attitude
+relative to the Earth is then made as illustrated in sketch C.
+
+The Pilot must then learn to know just the angle of bank
+at which the margin of lift is lost, and, if a sharp turn
+necessitates banking beyond that angle, he must ``nose-down.''
+
+In this matter of banking and nosing-down, and, indeed,
+regarding stability and control generally, the golden rule
+for all but very experienced pilots should be: Keep the
+aeroplane in such an attitude that the air pressure is always
+directly in the pilot's face. The aeroplane is then always
+engaging the air as designed to do so, and both lifting and
+controlling surfaces are acting efficiently. The only exception
+to this rule is a vertical dive, and I think that is
+obviously not an attitude for any but very experienced
+pilots to hanker after.
+
+SPINNING.--This is the worst of all predicaments the
+pilot can find himself in. Fortunately it rarely happens.
+
+It is due to the combination of (1) a very steep spiral
+descent of small radius, and (2) insufficiency of keel-surface
+behind the vertical axis, or the jamming of the rudder
+end or elevator into a position by which the aeroplane is forced
+into an increasingly steep and small spiral.
+
+Owing to the small radius of such a spiral, the mass of
+the aeroplane may gain a rotary momentum greater, in effect,
+than the air pressure of the keel-surface or controlling surfaces
+opposed to it; and, when once such a condition occurs,
+it is difficult to see what can be done by the pilot to remedy
+it. The sensible pilot will not go beyond reasonable limits
+of steepness and radius when executing spiral descents.
+
+GLIDING DESCENT WITHOUT PROPELLER THRUST.--All
+aeroplanes are, or should be, designed to assume their gliding
+angle when the power and thrust is cut off. This relieves
+the pilot of work, worry, and danger should he find himself
+in a fog or cloud. The Pilot, although he may not realize
+it, maintains the correct attitude of the aeroplane by observing
+its position relative to the horizon. Flying into a
+fog or cloud the horizon is lost to view, and he must then rely
+upon his instruments--(1) the compass for direction; (2) an
+inclinometer (arched spirit-level) mounted transversely to
+the longitudinal axis, for lateral stability; and (3) an inclinometer
+mounted parallel to the longitudinal axis, or the airspeed
+indicator, which will indicate a nose-down position
+by increase in air speed, and a tail-down position by decrease
+in air speed.
+
+The pilot is then under the necessity of watching three
+instruments and manipulating his three controls to keep the
+instruments indicating longitudinal, lateral, and directional
+stability. That is a feat beyond the capacity of the ordinary
+man. If, however, by the simple movement of throttling
+down the power and thrust, he can be relieved of looking
+after the longitudinal stability, he then has only two instruments
+to watch. That is no small job in itself, but it is,
+at any rate, fairly practicable.
+
+Aeroplanes are, then, designed, or should be, so that the
+centre of gravity is slightly forward of centre of lift. The
+aeroplane is then, as a glider, nose-heavy--and the distance
+the C.G. is placed in advance of the C.L. should be such as
+to ensure a gliding angle producing a velocity the same as
+the normal flying speed (for which the strength of construction
+has been designed).
+
+In order that this nose-heavy tendency should not exist
+when the thrust is working and descent not required, the
+centre of thrust is placed a little below the centre of drift
+or resistance, and thus tends to pull up the nose of the
+aeroplane.
+
+The distance the centre of thrust is placed below the
+centre of drift should be such as to produce a force equal
+and opposite to that due to the C.G. being forward of the
+C.L.
+
+LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired,
+it is best to throttle the engine down at point A. The C.G.
+being forward of the C.P., then causes the aeroplane to nose-
+down, and assists the pilot in making a reasonably small
+loop along the course C and in securing a quick recovery.
+If the engine is not throttled down, then the aeroplane may
+be expected to follow the course D, which results in a longer
+nose dive than in the case of the course C.
+
+A steady, gentle movement of the elevator is necessary.
+A jerky movement may change the direction of motion so
+suddenly as to produce dangerous air stresses upon the surfaces,
+in which case there is a possibility of collapse.
+
+If an upside-down flight is desired, the engine may, or
+may not, be throttled down at point A. If not throttled
+down, then the elevator must be operated to secure a course
+approximately in the direction B. If it is throttled down,
+then the course must be one of a steeper angle than B, or
+there will be danger of stalling.
+
+
+Diagram p. 88.--This is not set at quite
+the correct angle. Path B should slope
+slightly downwards from Position A.
+
+
+
+CHAPTER III
+
+RIGGING
+
+In order to rig an aeroplane intelligently, and to maintain
+it in an efficient and safe condition, it is necessary to possess
+a knowledge of the stresses it is called upon to endure, and
+the strains likely to appear.
+
+
+STRESS is the load or burden a body is called upon to
+bear. It is usually expressed by the result found by dividing
+the load by the number of superficial square inches contained
+in the cross-sectional area of the body.
+
+Thus, if, for instance, the object illustrated above contains
+4 square inches of cross-sectional area, and the total load
+it is called upon to endure is 10 tons, the stress would be
+expressed as 2 1/2 tons.
+
+
+STRAIN is the deformation produced by stress.
+
+
+THE FACTOR OF SAFETY is usually expressed by the result
+found by dividing the stress at which it is known the body
+will collapse, by the maximum stress it will be called upon to
+endure. For instance, if a control wire be called upon to endure
+a maximum stress of 2 cwts., and the known stress at which
+it will collapse is 10 cwts., the factor of safety is then 5.
+
+[cwts. = centerweights = 100 pound units as in cent & century.
+Interestinly enough, this word only exists today in abbreviation
+form, probably of centreweights, but the dictionary entries, even
+from a hundred years ago do not list this as a word, but do list
+c. or C. as the previous popular abbreviation as in Roman Numerals]
+The word listed is "hundredweight.  Michael S. Hart, 1997]
+
+
+COMPRESSION.--The simple stress of compression tends
+to produce a crushing strain. Example: the interplane and
+fuselage struts.
+
+
+TENSION.--The simple stress of tension tends to produce
+the strain of elongation. Example: all the wires.
+
+
+BENDING.--The compound stress of bending is a combination
+of compression and tension.
+
+The above sketch illustrates a straight piece of wood of
+which the top, centre, and bottom lines are of equal length.
+We will now imagine it bent to form a circle, thus:
+
+The centre line is still the same length as before being
+bent; but the top line, being farther from the centre of the
+circle, is now longer than the centre line. That can be due
+only to the strain of elongation produced by the stress of
+tension. The wood between the centre line and the top
+line is then in tension; and the farther from the centre,
+the greater the strain, and consequently the greater the
+tension.
+
+The bottom line, being nearest to the centre of the circle,
+is now shorter than the centre line. That can be due only
+to the strain of crushing produced by the stress of compression.
+The wood between the centre and bottom lines is
+then in compression; and the nearer the centre of the circle,
+the greater the strain, and consequently the greater the
+compression.
+
+It then follows that there is neither tension nor compression,
+i.e., no stress, at the centre line, and that the wood
+immediately surrounding it is under considerably less stress
+than the wood farther away. This being so, the wood in
+the centre may be hollowed out without unduly weakening
+struts and spars. In this way 25 to 33 per cent. is saved in
+the weight of wood in an aeroplane.
+
+The strength of wood is in its fibres, which should, as far
+as possible, run without break from one end of a strut or
+spar to the other end. A point to remember is that the
+outside fibres, being farthest removed from the centre line,
+are doing by far the greatest work.
+
+
+SHEAR STRESS IS such that, when material collapses under it,
+one part slides over the other. Example: all the locking pins.
+
+Some of the bolts are also in shear or ``sideways'' stress,
+owing to lugs under their heads and from which wires are
+taken. Such a wire, exerting a sideways pull upon a bolt,
+tries to break it in such a way as to make one piece of the bolt
+slide over the other piece.
+
+TORSION.--This is a twisting stress compounded of compression,
+tension, and shear stresses. Example: the propeller shaft.
+
+
+NATURE OF WOOD UNDER STRESS.--Wood, for its weight,
+takes the stress of compression far better than any other
+stress. For instance: a walking-stick of less than 1 lb. in
+weight will, if kept perfectly straight, probably stand up to
+a compression stress of a ton or more before crushing; whereas,
+if the same stick is put under a bending stress, it will probably
+collapse to a stress of not more than about 50 lb. That is
+a very great difference, and, since weight is of the greatest
+importance, the design of an aeroplane is always such as to,
+as far as possible, keep the various wooden parts of its
+construction in direct compression. Weight being of such vital
+importance, and designers all trying to outdo each other in
+saving weight, it follows that the factor of safety is rather
+low in an aeroplane. The parts in direct compression will,
+however, take the stresses safely provided the following
+conditions are carefully observed.
+
+CONDITIONS TO BE OBSERVED:
+
+
+1. All the spars and struts must be perfectly straight.
+
+The above sketch illustrates a section through an
+interplane strut. If the strut is to be kept straight,
+i.e., prevented from bending, then the stress of
+compression must be equally disposed about the
+centre of strength. If it is not straight, then
+there will be more compression on one side of the
+centre of strength than on the other side. That
+is a step towards getting compression on one side
+and tension on the other side, in which case it
+may be forced to take a bending stress for which
+it is not designed. Even if it does not collapse
+it will, in effect, become shorter, and thus throw
+out of adjustment the gap and all the wires attached
+to the top and bottom of the strut, with the result
+that the flight efficiency of the aeroplane will be
+spoiled.
+
+The only exception to the above condition is
+what is known as the Arch. For instance, in the
+case of the Maurice Farman, the spars of the centre-
+section plane, which have to take the weight of
+the nacelle, are arched upwards. If this was not
+done, it is possible that rough landings might
+result in the weight causing the spars to become
+slightly distorted downwards. That would produce
+a dangerous bending stress, but, as long as
+the wood is arched, or, at any rate, kept from
+bending downwards, it will remain in direct
+compression and no danger can result.
+
+
+2. Struts and spars must be symmetrical. By that I mean
+that the cross-sectional dimensions must be correct,
+as otherwise there will be bulging places on the
+outside, with the result that the stress will not be
+evenly disposed about the centre of strength, and
+a bending stress may be produced.
+
+
+3. Struts, spars, etc., must be undamaged. Remember
+that, from what I have already explained about
+bending stresses, the outside fibres of the wood are
+doing by far the most work. If these get bruised
+or scored, then the strut or spar suffers in strength
+much more than one might think at first sight;
+and, if it ever gets a tendency to bend, it is likely
+to collapse at that point.
+
+
+4. The wood must have a good, clear grain with no cross-
+grain, knots, or shakes. Such blemishes produce
+weak places and, if a tendency to bend appears,
+then it may collapse at such a point.
+
+5. The struts, spars, etc., must be properly bedded into
+their sockets or fittings. To begin with, they must
+be of good pushing or gentle tapping fit. They
+must never be driven in with a heavy hammer.
+Then again, a strut must bed well down all over its
+cross-sectional area as illustrated above; otherwise
+the stress of compression will not be evenly disposed
+about the centre of strength, and that may
+produce a bending stress. The bottom of the strut
+or spar should be covered with some sort of
+paint, bedded into the socket or fitting, and then
+withdrawn to see if the paint has stuck all over the
+bed.
+
+
+6. The atmosphere is sometimes much damper than at
+other times, and this causes wood to expand and
+contract appreciably. This would not matter but
+for the fact that it does not expand and contract
+uniformly, but becomes unsymmetrical, i.e., distorted.
+I have already explained the danger of that in
+condition 2. This should be minimized by WELL
+VARNISHING THE WOOD to keep the moisture out of it.
+
+
+FUNCTION OF INTERPLANE STRUTS.--These struts have to
+keep the lifting surfaces or ``planes'' apart, but this is only
+part of their work. They must keep the planes apart, so
+that the latter are in their correct attitude. That is only so
+when the spars of the bottom plane are parallel with those of
+the top plane. Also, the chord of the top plane must be
+parallel with the chord of the bottom plane. If that is not
+so, then one plane will not have the same angle of incidence
+as the other one. At first sight one might think that all
+that is necessary is to cut all the struts to be the same length,
+but that is not the case.
+
+Sometimes, as illustrated above, the rear spar is not so
+thick as the main spar, and it is then necessary to make
+up for that difference by making the rear struts correspondingly
+longer. If that is not done, then the top and
+bottom chords will not be parallel, and the top and bottom
+planes will have different angles of incidence. Also, the
+sockets or fittings, or even the spars upon which they are
+placed, sometimes vary in thickness owing to faulty manufacture.
+This must be offset by altering the length of the
+struts. The best way to proceed is to measure the distance
+between the top and bottom spars by the side of each strut,
+and if that distance, or ``gap'' as it is called, is not as stated
+in the aeroplane's specifications, then make it correct by
+changing the length of the strut. This applies to both front
+and rear interplane struts. When measuring the gap, always
+be careful to measure from the centre of the spar, as it may
+be set at an angle, and the rear of it may be considerably
+lower than its front.
+
+
+BORING HOLES IN WOOD.--It should be a strict rule that
+no spar be used which has an unnecessary hole in it. Before
+boring a hole, its position should be confirmed by whoever
+is in charge of the workshop. A bolt-hole should be of a size
+to enable the bolt to be pushed in, or, at any rate, not more
+than gently tapped in. Bolts should not be hammered in, as
+that may split the spar. On the other hand, a bolt should not
+be slack in its hole, as, in such a case, it may work sideways and
+split the spar, not to speak of throwing out of adjustment
+the wires leading from the lug or socket under the bolt-head.
+
+
+WASHERS.--Under the bolt-head, and also under the nut,
+a washer must be placed--a very large washer compared
+with the size which would be used in all-metal construction.
+This is to disperse the stress over a large area; otherwise
+the washer may be pulled into the wood and weaken it,
+besides possibly throwing out of adjustment the wires
+attached to the bolt or the fitting it is holding to the spar.
+
+
+LOCKING.--Now as regards locking the bolts. If split
+pins are used, be sure to see that they are used in such a way
+that the nut cannot possibly unscrew at all. The split pin
+should be passed through the bolt as near as possible to the
+nut. It should not be passed through both nut and bolt.
+
+If it is locked by burring over the edge of the bolt, do not
+use a heavy hammer and try to spread the whole head of
+the bolt. That might damage the woodwork inside the
+fabric-covered surface. Use a small, light hammer, and gently
+tap round the edge of the bolt until it is burred over.
+
+
+TURNBUCKLES.--A turnbuckle is composed of a central
+barrel into each end of which is screwed an eye-bolt. Wires
+are taken from the eyes of the eye-bolt, and so, by turning
+the barrel, they can be adjusted to their proper tension.
+Eye-bolts must be a good fit in the barrel; that is to say,
+not slack and not very tight. Theoretically it is not neces-
+sary to screw the eye-bolt into the barrel for a distance
+greater than the diameter of the bolt, but, in practice, it is
+better to screw it in for a considerably greater distance than
+that if a reasonable degree of safety is to be secured.
+
+Now about turning the barrel to secure the right adjustment.
+The barrel looks solid, but, as a matter of fact, it
+is hollow and much more frail than it appears. For that
+reason it should not be turned by seizing it with pliers, as
+that may distort it and spoil the bore within it. The best
+method is to pass a piece of wire through the hole in its centre,
+and to use that as a lever. When the correct adjustment
+has been secured, the turnbuckle must be locked to prevent
+it from unscrewing. It is quite possible to lock it in such a
+way as to allow it to unscrew a quarter or a half turn, and
+that would throw the wires out of the very fine adjustment
+necessary. The proper way is to use the locking wire so
+that its direction is such as to oppose the tendency of the
+barrel to unscrew, thus:
+
+
+WIRES.--The following points should be carefully observed
+where wire is concerned:
+
+1. Quality.--It must not be too hard or too soft. An
+easy practical way of learning to know the approximate
+quality of wire is as follows:
+
+Take three pieces, all of the same gauge, and each about a
+foot in length. One piece should be too soft, another too hard,
+and the third piece of the right quality. Fix them in a vice,
+about an inch apart and in a vertical position, and with the light
+from a window shining upon them. Burnish them if necessary,
+and you will see a band of light reflected from each
+wire.
+
+Now bend the wires over as far as possible and away from
+the light. Where the soft wire is concerned, it will squash
+out at the bend, and this will be indicated by the band of
+light, which will broaden at that point. In the case of the
+wire which is too hard, the band of light will broaden very
+little at the turn, but, if you look carefully, you will see some
+little roughnesses of surface. In the case of the wire of the
+right quality, the band of light may broaden a very little
+at the turn, but there will be no roughnesses of surface.
+
+By making this experiment two or three times one can
+soon learn to know really bad wire from good, and also learn
+to know the strength of hand necessary to bend the right
+quality.
+
+2. It must not be damaged. That is to say, it must be
+unkinked, rustless, and unscored.
+
+3. Now as regards keeping wire in good condition. Where
+outside wires are concerned, they should be kept WELL GREASED
+OR OILED, especially where bent over at the ends. Internal
+bracing wires cannot be reached for the purpose of regreasing
+them, as they are inside fabric-covered surfaces. They should
+be prevented from rusting by being painted with an anti-rust
+mixture. Great care should be taken to see that the wire
+is perfectly clean and dry before being painted. A greasy
+finger-mark is sufficient to stop the paint from sticking to
+the wire. In such a case there will be a little space between
+the paint and the wire. Air may enter there and cause the
+wire to rust.
+
+4. Tension of Wires.--The tension to which the wires are
+adjusted is of the greatest importance. All the wires should
+be of the same tension when the aeroplane is supported in
+such a way as to throw no stress upon them. If some wires
+are in greater tension than others, the aeroplane will quickly
+become distorted and lose its efficiency.
+
+In order to secure the same tension of all wires, the aeroplane,
+when being rigged, should be supported by packing
+underneath the lower surfaces as well as by packing underneath
+the fuselage or nacelle. In this way the anti-lift wires
+are relieved of the weight, and there is no stress upon any
+of the wires.
+
+As a general rule the wires of an aeroplane are tensioned
+too much. The tension should be sufficient to keep the
+framework rigid. Anything more than that lowers the factor
+of safety, throws various parts of the framework into undue
+compression, pulls the fittings into the wood, and will, in
+the end, distort the whole framework of the aeroplane.
+
+Only experience will teach the rigger what tension to
+employ. Much may be done by learning the construction
+of the various types of aeroplanes, the work the various
+parts do, and in cultivating a touch for tensioning wires by
+constantly handling them.
+
+5. Wires with no Opposition Wires.--In some few cases
+wires will be found which have no opposition wires pulling
+in the opposite direction. For instance, an auxiliary lift
+wire may run from the bottom of a strut to a spar in the top
+plane at a point between struts. In such a case great care
+should be taken not to tighten the wire beyond barely taking
+up the slack.
+
+Such a wire must be a little slack, or, as illustrated above,
+it will distort the framework. That, in the example given,
+will spoil the camber (curvature) of the surface, and result
+in changing both the lift and the drift at that part of the surface.
+Such a condition will cause the aeroplane to lose its
+directional stability and also to fly one wing down.
+
+I cannot impress this matter of tension upon the reader
+too strongly. It is of the utmost importance. When this,
+and also accuracy in securing the various adjustments, has
+been learned, one is on the way to becoming a good
+rigger.
+
+6. Wire Loops.--Wire is often bent over at its end in the
+form of a loop, in order to connect with a turnbuckle or
+fitting. These loops, even when made as perfectly as possible,
+have a tendency to elongate, thus spoiling the adjustment
+of the wires Great care should be taken to minimize this
+as far as possible. The rules to be observed are as
+follows:
+
+(a) The size of the loop should be as small as possible
+within reason. By that I mean it should not be
+so small as to create the possibility of the wire
+breaking.
+
+
+(b) The shape of the loop should be symmetrical.
+
+
+(c) It should have well-defined shoulders in order to
+prevent the ferrule from slipping up. At the same
+time, a shoulder should not have an angular place.
+
+
+(d) When the loop is finished it should be undamaged,
+and it should not be, as is often the case, badly scored.
+
+
+7. Stranded Wire Cable.--No splice should be served with
+twine until it has been inspected by whoever is in charge of
+the workshop. The serving may cover bad work.
+
+Should a strand become broken, then the cable should be
+replaced at once by another one.
+
+Control cables have a way of wearing out and fraying
+wherever they pass round pulleys. Every time an aeroplane
+comes down from flight the rigger should carefully examine
+the cables, especially where they pass round pulleys. If
+he finds a strand broken, he should replace the cable.
+
+The ailerons' balance cable on the top of the top plane
+is often forgotten, since it is necessary to fetch a high pair
+of steps in order to examine it. Don't slack this, or some
+gusty day the pilot may unexpectedly find himself minus the
+aileron control.
+
+CONTROLLING SURFACES.--The greatest care should be
+exercised in rigging the aileron, rudder, and elevator properly,
+for the pilot entirely depends upon them in managing the
+aeroplane.
+
+The ailerons and elevator should be rigged so that, when
+the aeroplane is in flight, they are in a fair true line with the
+surface in front and to which they are hinged.
+
+If the surface to which they are hinged is not a lifting
+surface, then they should be rigged to be in a fair true line
+with it as illustrated above.
+
+If the controlling surface is, as illustrated, hinged to the
+back of a lifting surface, then it should be rigged a little below
+the position it would occupy if in a fair true line with the
+surface in front. This is because, in such a case, it is set
+at an angle of incidence. This angle will, during flight,
+cause it to lift a little above the position in which it has been
+rigged. It is able to lift owing to a certain amount of slack
+in the control wire holding it--and one cannot adjust the
+control wire to have no slack, because that would cause it
+to bind against the pulleys and make the operation of it too
+hard for the pilot. It is therefore necessary to rig it a little
+below the position it would occupy if it was rigged in a fair
+true line with the surface in front. Remember that this
+only applies when it is hinged to a lifting surface. The
+greater the angle of incidence (and therefore the lift) of the
+surface in front, then the more the controlling surface will
+have to be rigged down.
+
+As a general rule it is safe to rig it down so that its trailing
+
+edge is 1/2 to 3/4 inch below the position it would occupy if in
+a fair line with the surface in front; or about 1/2 inch down for
+every 18 inches of chord of the controlling surface.
+
+When making these adjustments the pilot's control levers
+should be in their neutral positions. It is not sufficient
+to lash them. They should be rigidly blocked into position
+with wood packing.
+
+The surfaces must not be distorted in any way. If
+they are held true by bracing wires, then such wires must be
+carefully adjusted. If they are distorted and there are no
+bracing wires with which to true them, then some of the
+internal framework will probably have to be replaced.
+
+The controlling surfaces should never be adjusted with
+a view to altering the stability of the aeroplane. Nothing
+can be accomplished in that way. The only result will be
+to spoil the control of the aeroplane.
+
+
+FABRIC-COVERED SURFACES.--First of all make sure
+that there is no distortion of spars or ribs, and that they are
+perfectly sound. Then adjust the internal bracing wires
+so that the ribs are parallel to the direction of flight. The
+ribs usually cause the fabric to make a ridge where they occur,
+and, if such ridge is not parallel to the direction of flight,
+it will produce excessive drift. As a rule the ribs are at
+right angles to both main and rear spars.
+
+The tension of the internal bracing wires should be just
+sufficient to give rigidity to the framework. They should
+not be tensioned above that unless the wires are, at their
+ends, bent to form loops. In that case a little extra tension
+may be given to offset the probable elongation of the
+loops.
+
+The turnbuckles must now be generously greased, and
+served round with adhesive tape. The wires must be rendered
+perfectly dry and clean, and then painted with an anti-rust
+mixture. The woodwork must be well varnished.
+
+If it is necessary to bore holes in the spars for the purpose
+of receiving, for instance, socket bolts, then their places
+should be marked before being bored and their positions
+confirmed by whoever is in charge of the workshop. All is
+now ready for the sail-maker to cover the surface with
+fabric.
+
+
+ADJUSTMENT OF CONTROL CABLES.--The adjustment of
+the control cables is quite an art, and upon it will depend to
+a large degree the quick and easy control of the aeroplane
+by the pilot.
+
+The method is as follows:
+
+After having rigged the controlling surfaces, and as far
+as possible secured the correct adjustment of the control
+cables, then remove the packing which has kept the control
+levers rigid. Then, sitting in the pilot's seat, move the
+control levers SMARTLY. Tension the control cables so that
+when the levers are smartly moved there is no perceptible
+snatch or lag. Be careful not to tension the cables more than
+necessary to take out the snatch. If tensioned too much
+they will (1) bind round the pulleys and result in hard work
+for the pilot; (2) throw dangerous stresses upon the controlling
+surfaces, which are of rather flimsy construction; and (3)
+cause the cables to fray round the pulleys quicker than would
+otherwise be the case.
+
+Now, after having tensioned the cables sufficiently to
+take out the snatch, place the levers in their neutral positions,
+and move them to and fro about 1/8 inch either side of such
+positions. If the adjustment is correct, it should be possible
+to see the controlling surfaces move. If they do not move,
+then the control cables are too slack.
+
+
+FLYING POSITION.--Before rigging an aeroplane or making
+any adjustments it is necessary to place it in what is known
+as its ``flying position.'' I may add that it would be better
+termed its ``rigging position.''
+
+In the case of an aeroplane fitted with a stationary engine
+this is secured by packing up the machine so that the engine
+foundations are perfectly horizontal both longitudinally and
+laterally. This position is found by placing a straight-edge
+and a spirit-level across the engine foundations (both
+longitudinally and laterally), and great care should be taken to
+see that the bubble is exactly in the centre of the level. The
+slightest error will assume magnitude towards the extremities
+of the aeroplane. Great care should be taken to block up
+the aeroplane rigidly. In case it gets accidentally disturbed
+while the work is going on, it is well to constantly verify the
+flying position by running the straight-edge and spirit-level
+over the engine foundations. The straight-edge should be
+carefully tested before being used, as, being generally made of
+wood, it will not remain true long. Place it lightly in a vice,
+and in such a position that a spirit-level on top shows the
+bubble exactly in the centre. Now slowly move the level
+along the straight-edge, and the bubble should remain exactly
+in the centre. If it does not do so, then the straight-edge
+is not true and must be corrected. THIS SHOULD NEVER BE
+OMITTED.
+
+In the case of aeroplanes fitted with engines of the rotary
+type, the ``flying position'' is some special attitude laid
+down in the aeroplane's specifications, and great care should
+be taken to secure accuracy.
+
+
+ANGLE OF INCIDENCE.--One method of finding the angle
+of incidence is as follows:
+
+First place the aeroplane in its flying position. The
+corner of the straight-edge must be placed underneath and
+against the CENTRE of the rear spar, and held in a horizontal
+position parallel to the ribs. This is secured by using a
+spirit-level. The set measurement will then be from the
+top of the straight-edge to the centre of the bottom surface
+of the main spar, or it may be from the top of the straight-
+edge to the lowest part of the leading edge. Care should be
+taken to measure from the centre of the spar and to see that
+the bubble is exactly in the centre of the level. Remember
+that all this will be useless if the aeroplane has not been placed
+accurately in its flying position.
+
+This method of finding the angle of incidence must be
+used under every part of the lower surface where struts
+occur. It should not be used between the struts, because,
+in such places, the spars may have taken a slight permanent
+set up or down; not, perhaps, sufficiently bad to make any
+material difference to the flying of the machine, but quite bad
+enough to throw out the angle of incidence, which cannot
+be corrected at such a place.
+
+If the angle is wrong, it should then be corrected as follows:
+
+If it is too great, then the rear spar must be warped up
+until it is right, and this is done by slackening ALL the wires
+going to the top of the strut, and then tightening ALL the
+wires going to the bottom of the strut.
+
+If the angle is too small, then slacken ALL the wires going
+to the bottom of the strut, and tighten ALL the wires going to
+the top of the strut, until the correct adjustment is secured.
+
+Never attempt to adjust the angle by warping the main spar.
+
+The set measurement, which is of course stated in the
+aeroplane's specifications, should be accurate to 1/16 inch.
+
+
+LATERAL DIHEDRAL ANGLE.--One method of securing
+this is as follows, and this method will, at the same time,
+secure the correct angle of incidence:
+
+The strings, drawn very tight, must be taken over both
+the main and rear spars of the top surface. They must run
+between points on the spars just inside the outer struts.
+The set measurement (which should be accurate to 1/16 inch
+or less) is then from the strings down to four points on the
+main and rear spars of the centre-section surface. These
+points should be just inside the four centre-section struts;
+that is to say, as far as possible away from the centre of the
+centre-section. Do not attempt to take the set measurement
+near the centre of the centre-section.
+
+The strings should be as tight as possible, and, if it can
+be arranged, the best way to accomplish that is as shown in
+the above illustration, i.e., by weighting the strings down to
+the spars by means of weights and tying each end of the strings
+to a strut. This will give a tight and motionless string.
+
+However carefully the above adjustment is made, there is
+sure to be some slight error. This is of no great importance,
+provided it is divided equally between the left- and right-
+hand wings. In order to make sure of this, certain check
+measurements should be taken as follows:
+
+Each bay must be diagonally measured, and such measurements
+must be the same to within 1/16 inch on each side of
+the aeroplane. As a rule such diagonal measurements are
+taken from the bottom socket of one strut to the top socket
+of another strut, but this is bad practice, because of possible
+inaccuracies due to faulty manufacture.
+
+The points between which the diagonal measurements
+are taken should be at fixed distances from the butts of the
+spars, such distances being the same on each side of the
+aeroplane, thus:
+
+
+It would be better to use the centre line of the aeroplane
+rather than the butts of the spars. It is not practicable
+to do so, however, as the centre line probably runs through
+the petrol tanks, etc.
+
+
+THE DIHEDRAL BOARD.--Another method of securing
+the dihedral angle, and also the angle of incidence, is by
+means of the dihedral board. It is a light handy thing to
+use, but leads to many errors, and should not be used unless
+necessary. The reasons are as follows:
+
+The dihedral board is probably not true. If it must be
+used, then it should be very carefully tested for truth before-
+hand. Another reason against its use is that it has to be
+placed on the spars in a position between the struts, and
+that is just where the spars may have a little permanent
+set up or down, or some inaccuracy of surface which will,
+of course, throw out the accuracy of the adjustment. The
+method of using it is as follows:
+
+The board is cut to the same angle as that specified for
+the upward inclination of the surface towards its wing-
+tips. It is placed on the spar as indicated above, and it
+is provided with two short legs to raise it above the flanges
+of the ribs (which cross over the spars), as they may vary
+in depth. A spirit-level is then placed on the board, and the
+wires must be adjusted to give the surface such an inclination
+as to result in the bubble being in the centre of the level.
+This operation must be performed in respect of each bay
+both front and rear. The bays must then be diagonally
+measured as already explained.
+
+
+YET ANOTHER METHOD of finding the dihedral angle,
+and at the same time the angle of incidence, is as follows:
+
+A horizontal line is taken from underneath the butt of
+each spar, and the set measurement is either the angle it makes
+with the spar, or a fixed measurement from the line to the
+spar taken at a specified distance from the butt. This operation
+must be performed in respect of both main and rear
+spars, and all the bays must be measured diagonally afterwards.
+
+
+
+Whichever method is used, be sure that after the job is
+done the spars are perfectly straight.
+
+
+STAGGER.--The stagger is the distance the top surface
+is in advance of the bottom surface when the aeroplane
+is in flying position. The set measurement is obtained as
+follows:
+
+Plumb-lines must be dropped over the leading edge of
+the top surface wherever struts occur, and also near the
+fuselage. The set measurement is taken from the front of the
+lower leading edge to the plumb-lines. It makes a difference
+whether the measurement is taken along a horizontal line
+(which can be found by using a straight-edge and a spirit-
+level) or along a projection of the chord. The line along
+which the measurement should be taken is laid down in the
+aeroplane's specifications.
+
+If a mistake is made and the measurement taken along
+the wrong line, it may result in a difference of perhaps 1/4
+will, in flight, be nose-heavy or tail-heavy.
+
+After the adjustments of the angles of incidence, dihedral,
+and stagger have been secured, it is as well to confirm all of
+them, as, in making the last adjustment, the first one may
+have been spoiled.
+
+
+OVER-ALL ADJUSTMENTS.--The following over-all check
+measurements should now be taken.
+
+The straight lines AC and BC should be equal to within
+1/8 inch. The point C is the centre of the propeller, or, in the
+case of a ``pusher'' aeroplane, the centre of the nacelle.
+The points A and B are marked on the main spar, and must
+in each case be the same distance from the butt of the spar.
+The rigger should not attempt to make A and B merely the
+sockets of the outer struts, as they may not have been placed
+quite accurately by the manufacturer. The lines AC and BC
+must be taken from both top and bottom spars--two measurements
+on each side of the aeroplane.
+
+The two measurements FD and FE should be equal to
+within 1/8 inch. F is the centre of the fuselage or rudder-
+post. D and E are points marked on both top and bottom
+rear spars, and each must be the same fixed distance from
+the butt of the spar. Two measurements on each side of the
+aeroplane.
+
+If these over-all measurements are not correct, then it
+is probably due to some of the drift or anti-drift wires being
+too tight or too slack. It may possibly be due to the fuselage
+being out of truth, but of course the rigger should have made
+quite sure that the fuselage was true before rigging the rest
+of the machine. Again, it may be due to the internal bracing
+wires within the lifting surfaces not being accurately adjusted,
+but of course this should have been seen to before covering the
+surfaces with fabric.
+
+
+FUSELAGE.--The method of truing the fuselage is laid
+down in the aeroplane's specifications. After it has been
+adjusted according to the specified directions, it should then
+be arranged on trestles in such a way as to make about three-
+quarters of it towards the tail stick out unsupported. In
+this way it will assume a condition as near as possible to
+flying conditions, and when it is in this position the set
+measurements should be confirmed. If this is not done it
+may be out of truth, but perhaps appear all right when
+supported by trestles at both ends, as, in such case, its
+weight may keep it true as long as it is resting upon the
+trestles.
+
+
+THE TAIL-PLANE (EMPENNAGE).--The exact angle of
+incidence of the tail-plane is laid down in the aeroplane's
+specifications. It is necessary to make sure that the spars
+are horizontal when the aeroplane is in flying position and
+the tail unsupported as explained above under the heading
+of Fuselage. If the spars are tapered, then make sure that
+their centre lines are horizontal.
+
+
+UNDERCARRIAGE.--The undercarriage must be very carefully
+aligned as laid down in the specifications.
+
+1. The aeroplane must be placed in its flying position
+and sufficiently high to ensure the wheels being off the ground
+when rigged. When in this position the axle must be hori-
+
+nontal and the bracing wires adjusted to secure the various
+set measurements stated in the specifications.
+
+2. Make sure that the struts bed well down into their
+sockets.
+
+3. Make sure that the shock absorbers are of equal
+tension. In the case of rubber shock absorbers, both the
+number of turns and the lengths must be equal.
+
+
+HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.
+
+
+DIRECTIONAL STABILITY will be badly affected if there is
+more drift (i.e., resistance) on one side of the aeroplane than
+there is on the other side. The aeroplane will tend to turn
+towards the side having the most drift. This may be caused
+as follows:
+
+1. The angle of incidence of the main surface or the tail
+surface may be wrong. The greater the angle of incidence,
+the greater the drift. The less the angle, the less the drift.
+
+2. If the alignment of the fuselage, fin in front of the
+rudder, the struts or stream-line wires, or, in the case of
+the Maurice Farman, the front outriggers, are not absolutely
+correct--that is to say, if they are turned a little to the
+left or to the right instead of being in line with the direction
+of flight--then they will act as a rudder and cause the aeroplane
+to turn off its course.
+
+3. If any part of the surface is distorted, it will cause
+the aeroplane to turn off its course. The surface is cambered,
+i.e., curved, to pass through the air with the least possible
+drift. If, owing perhaps to the leading edge, spars, or trailing
+edge becoming bent, the curvature is spoiled, that will
+result in changing the amount of drift on one side of the aeroplane,
+which will then have a tendency to turn off its course.
+
+
+LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible
+reason for such a condition is a difference in the lifts
+of right and left wings. That may be caused as follows:
+
+1. The angle of incidence may be wrong. If it is too
+great, it will produce more lift than on the other side of the
+aeroplane; and if too small, it will produce less lift than on
+the other side--the result being that, in either case, the aeroplane
+will try to fly one wing down.
+
+2. Distorted Surfaces.--If some part of the surface is
+distorted, then its camber is spoiled, and the lift will not be
+the same on both sides of the aeroplane, and that, of course,
+will cause it to fly one wing down.
+
+
+LONGITUDINAL INSTABILITY may be due to the following reasons:
+
+1. The stagger may be wrong. The top surface may have
+drifted back a little owing to some of the wires, probably
+the incidence wires, having elongated their loops or having
+pulled the fittings into the wood. If the top surface is not
+staggered forward to the correct degree, then consequently
+the whole of its lift is too far back, and it will then have a
+tendency to lift up the tail of the machine too much. The
+aeroplane would then be said to be ``nose-heavy.''
+
+A 1/4-inch area in the stagger will make a very considerable
+difference to the longitudinal stability.
+
+2. If the angle of incidence of the main surface is not right,
+it will have a bad effect, especially in the case of an aeroplane
+with a lifting tail-plane.
+
+If the angle is too great, it will produce an excess of lift,
+and that may lift up the nose of the aeroplane and result in
+a tendency to fly ``tail-down.'' If the angle is too small,
+it will produce a decreased lift, and the aeroplane may have a
+tendency to fly ``nose-down.''
+
+3. The fuselage may have become warped upward or
+downward, thus giving the tail-plane an incorrect angle of
+incidence. If it has too much angle, it will lift too much,
+and the aeroplane will be ``nose-heavy.'' If it has too little
+angle, then it will not lift enough, and the aeroplane will be
+``tail-heavy.''
+
+4. (The least likely reason.) The tail-plane may be
+mounted upon the fuselage at a wrong angle of incidence,
+in which case it must be corrected. If nose-heavy, it should
+be given a smaller angle of incidence. If tail-heavy, it should
+be given a larger angle; but care should be taken not to give
+it too great an angle, because the longitudinal stability
+entirely depends upon the tail-plane being set at a much
+smaller angle of incidence than is the main surface, and if
+that difference is decreased too much, the aeroplane will
+become uncontrollable longitudinally. Sometimes the tail-
+plane is mounted on the aeroplane at the same angle as the
+main surface, but it actually engages the air at a lesser angle,
+owing to the air being deflected downwards by the main
+surface. There is then, in effect, a longitudinal dihedral
+as explained and illustrated in Chapter I.
+
+
+CLIMBS BADLY.--Such a condition is, apart from engine
+or propeller trouble, probably due to (1) distorted surfaces,
+or (2) too small an angle of incidence.
+
+
+FLIGHT SPEED POOR.--Such a condition is, apart from
+engine or propeller trouble, probably due to (1) distorted
+surfaces, (2) too great an angle of incidence, or (3) dirt or
+mud, and consequently excessive skin-friction.
+
+
+INEFFICIENT CONTROL is probably due to (1) wrong setting
+of control surfaces, (2) distortion of control surfaces, or
+(3) control cables being badly tensioned.
+
+
+WILL NOT TAXI STRAIGHT.--If the aeroplane is uncontrollable
+on the ground, it is probably due to (1) alignment
+of undercarriage being wrong, or (2) unequal tension of shock
+absorbers.
+
+
+
+CHAPTER IV
+
+THE PROPELLER, OR ``AIR-SCREW''
+
+The sole object of the propeller is to translate the power
+of the engine into thrust.
+
+The propeller screws through the air, and its blades, being
+set at an angle inclined to the direction of motion, secure
+a reaction, as in the case of the aeroplane's lifting surface.
+
+This reaction may be conveniently divided into two
+component parts or values, namely, Thrust and Drift.
+
+The Thrust is opposed to the Drift of the aeroplane, and
+must be equal and opposite to it at flying speed. If it falls
+off in power, then the flying speed must decrease to a velocity,
+at which the aeroplane drift equals the decreased thrust.
+
+The Drift of the propeller may be conveniently divided
+into the following component values:
+
+
+Active Drift, produced by the useful thrusting part of the propeller.
+
+
+Passive Drift, produced by all the rest of the propeller,
+i.e., by its detrimental surface.
+
+
+Skin Friction, produced by the friction of the air with
+roughnesses of surface.
+
+
+Eddies attending the movement of the air caused by
+the action of the propeller.
+
+
+Cavitation (very marked at excessive speed of revolution).
+A tendency of the propeller to produce a
+cavity or semi-vacuum in which it revolves, the
+thrust decreasing with increase of speed and
+cavitation.
+
+
+THRUST-DRIFT RATIO.--The proportion of thrust to drift
+is of paramount importance, for it expresses the efficiency
+of the propeller. It is affected by the following factors:
+Speed of Revolution.--The greater the speed, the greater
+the proportion of drift to thrust. This is due to
+the increase with speed of the passive drift, which
+carries with it no increase in thrust. For this
+reason propellers are often geared down to revolve
+at a lower speed than that of the engine.
+
+
+Angle of Incidence.--The same reasons as in the case of
+the aeroplane surface.
+
+Surface Area.--Ditto.
+
+Aspect Ratio.--Ditto.
+
+Camber.--Ditto.
+
+In addition to the above factors there are, when it comes
+to actually designing a propeller, mechanical difficulties to
+consider. For instance, the blades must be of a certain
+strength and consequent thickness. That, in itself, limits
+the aspect ratio, for it will necessitate a chord long enough
+in proportion to the thickness to make a good camber possible.
+Again, the diameter of the propeller must be limited, having
+regard to the fact that greater diameters than those used
+to-day would not only result in excessive weight of construction,
+but would also necessitate a very high undercarriage
+to keep the propeller off the ground, and such undercarriage
+would not only produce excessive drift, but would also tend
+to make the aeroplane stand on its nose when alighting.
+The latter difficulty cannot be overcome by mounting the
+propeller higher, as the centre of its thrust must be approximately
+coincident with the centre of aeroplane drift.
+
+
+MAINTENANCE OF EFFICIENCY.
+
+
+The following conditions must be observed:
+
+
+1. PITCH ANGLE.--The angle, at any given point on the
+propeller, at which the blade is set is known as the pitch
+angle, and it must be correct to half a degree if reasonable
+efficiency is to be maintained.
+
+This angle secures the ``pitch,'' which is the distance the
+propeller advances during one revolution, supposing the air
+to be solid. The air, as a matter of fact, gives back to the
+thrust of the blades just as the pebbles slip back as one
+ascends a shingle beach. Such ``give-back'' is known as
+Slip. If a propeller has a pitch of, say, 10 feet, but actually
+advances, say, only 8 feet owing to slip, then it will be said
+to possess 20 per cent. slip.
+
+Thus, the pitch must equal the flying speed of the
+aeroplane plus the slip of the propeller. For example,
+let us find the pitch of a propeller, given the following
+conditions:
+          Flying speed .............. 70 miles per hour.
+          Propeller revolutions ..... 1,200 per minute.
+          Slip ...................... 15 per cent.
+
+First find the distance in feet the aeroplane will travel
+forward in one minute. That is--
+
+   369,600 feet (70 miles)
+   ------------------------ = 6,160 feet per minute.
+      60     `` (minutes)
+
+
+Now divide the feet per minute by the propeller revolutions
+per minute, add 15 per cent. for the slip, and the result
+will be the propeller pitch:
+
+     6,160 
+     ----- + 15 per cent. = 5 feet 1 3/5 inches.
+     1,200
+
+
+In order to secure a constant pitch from root to tip of
+blade, the pitch angle decreases towards the tip. This is
+necessary, since the end of the blade travels faster than its
+root, and yet must advance forward at the same speed as
+the rest of the propeller. For example, two men ascending
+a hill. One prefers to walk fast and the other slowly, but they
+wish to arrive at the top of the hill simultaneously. Then
+the fast walker must travel a farther distance than the slow
+one, and his angle of path (pitch angle) must be smaller
+than the angle of path taken by the slow walker. Their
+pitch angles are different, but their pitch (in this case altitude
+reached in a given time) is the same.
+
+In order to test the pitch angle, the propeller must be
+mounted upon a shaft at right angles to a beam the face of
+which must be perfectly level, thus:
+
+First select a point on the blade at some distance (say
+about 2 feet) from the centre of the propeller. At that
+point find, by means of a protractor, the angle a projection
+of the chord makes with the face of the beam. That angle
+is the pitch angle of the blade at that point.
+
+Now lay out the angle on paper, thus:
+
+The line above and parallel to the circumference line must
+be placed in a position making the distance between the
+two lines equal to the specified pitch, which is, or should be,
+marked upon the boss of the propeller.
+
+Now find the circumference of the propeller where the
+pitch angle is being tested. For example, if that place is
+2 feet radius from the centre, then the circumference will
+be 2 feet X 2 = 4 feet diameter, which, if multiplied by
+3.1416 = 15.56 feet circumference.
+
+Now mark off the circumference distance, which is
+represented above by A-B, and reduce it in scale for convenience.
+
+The distance a vertical line makes between B and the
+chord dine is the pitch at the point where the angle is being
+tested, and it should coincide with the specified pitch. You
+will note, from the above illustration, that the actual pitch
+line should meet the junction of the chord line and top
+line.
+
+The propeller should be tested at several points, about
+a foot apart, on each blade; and the diagram, provided the
+propeller is not faulty, will then look like this:
+
+At each point tested the actual pitch coincides with the
+specified pitch: a satisfactory condition.
+
+A faulty propeller will produce a diagram something
+like this:
+
+
+At every point tested the pitch angle is wrong, for nowhere
+does the actual pitch coincide with the specified pitch.
+Angles A, C, and D, are too large, and B is too small. The
+angle should be correct to half a degree if reasonable efficiency
+is to be maintained.
+
+A fault in the pitch angle may be due to (1) faulty manufacture,
+(2) distortion, or (3) the shaft hole through the boss
+being out of position.
+
+
+2. STRAIGHTNESS.--To test for straightness the propeller
+must be mounted upon a shaft. Now bring the tip of one
+blade round to graze some fixed object. Mark the point it
+grazes. Now bring the other tip round, and it should come
+within 1/8 inch of the mark. If it does not do so, it is due to
+(1) faulty manufacture, (2) distortion, or (3) to the hole
+through the boss being out of position.
+
+
+3. LENGTH.--The blades should be of equal length to
+inch.
+
+
+4. BALANCE.--The usual method of testing a propeller
+for balance is as follows: Mount it upon a shaft, which must
+be on ball-bearings. Place the propeller in a horizontal
+position, and it should remain in that position. If a weight
+of a trifle over an ounce placed in a bolt-hole on one side of
+the boss fails to disturb the balance, then the propeller is
+usually regarded as unfit for use.
+
+The above method is rather futile, as it does not test for
+the balance of centrifugal force, which comes into play as
+soon as the propeller revolves. It can be tested as follows:
+
+
+The propeller must be in a horizontal position, and then
+weighed at fixed points, such as A, B, C, D, E, and F, and
+the weights noted. The points A, B, and C must, of course,
+be at the same fixed distances from the centre of the propeller
+as the points D, E, and F. Now reverse the propeller and
+weigh at each point again. Note the results. The first
+series of weights should correspond to the second series,
+thus:
+
+      Weight A should equal weight F.
+         ``  B   ``     ``     ``  E.
+         ``  C   ``     ``     ``  D.
+
+
+There is no standard practice as to the degree of error
+permissible, but if there are any appreciable differences the
+propeller is unfit for use.
+
+
+5. SURFACE AREA.--The surface area of the blades should
+be equal. Test with callipers thus:
+
+The points between which the distances are taken must,
+of course, be at the same distance from the centre in the
+case of each blade.
+
+There is no standard practice as to the degree of error
+permissible. If, however, there is an error of over 1/8 inch,
+the propeller is really unfit for use.
+
+
+6. CAMBER.--The camber (curvature) of the blades should
+be (1) equal, (2) decrease evenly towards the tips of the blades,
+and (3) the greatest depth of the curve should, at any point
+of the blade, be approximately at the same percentage of
+the chord from the leading edge as at other points.
+
+It is difficult to test the top camber without a set of
+templates, but a fairly accurate idea of the concave camber
+can be secured by slowly passing a straight-edge along the
+blade, thus:
+
+The camber can now be easily seen, and as the straight-
+edge is passed along the blade, the observer should look for
+any irregularities of the curvature, which should gradually
+and evenly decrease towards the tip of the blade.
+
+
+7. THE JOINTS.--The usual method for testing the glued
+joints is by revolving the propeller at greater speed than it
+will be called upon to make during flight, and then carefully
+examining the joints to see if they have opened. It is not
+likely, however, that the reader will have the opportunity
+of making this test. He should, however, examine all the
+joints very carefully, trying by hand to see if they are quite
+sound. Suspect a propeller of which the joints appear to
+hold any thickness of glue. Sometimes the joints in the
+boss open a little, but this is not dangerous unless they extend
+to the blades, as the bolts will hold the laminations together.
+
+
+8. CONDITION OF SURFACE.--The surface should be very
+smooth, especially towards the tips of the blades. Some
+propeller tips have a speed of over 30,000 feet a minute,
+and any roughness will produce a bad drift or resistance
+and lower the efficiency.
+
+
+9. MOUNTING.--Great care should be taken to see that
+the propeller is mounted quite straight on its shaft. Test in
+the same way as for straightness. If it is not straight, it
+is possibly due to some of the propeller bolts being too slack
+or to others having been pulled up too tightly.
+
+
+FLUTTER.--Propeller ``flutter,'' or vibration, may be due
+to faulty pitch angle, balance, camber, or surface area. It
+causes a condition sometimes mistaken for engine trouble,
+and one which may easily lead to the collapse of the propeller.
+
+
+CARE OF PROPELLERS.--The care of propellers is of the
+greatest importance, as they become distorted very easily.
+
+
+1. Do not store them in a very damp or a very dry place.
+
+
+2. Do not store them where the sun will shine upon them.
+
+
+3. Never leave them long in a horizontal position or
+leaning up against a wall.
+
+
+4. They should be hung on horizontal pegs, and the
+position of the propellers should be vertical.
+
+
+If the points I have impressed upon you in these notes
+are not attended to, you may be sure of the following results:
+
+
+1. Lack of efficiency, resulting in less aeroplane speed
+and climb than would otherwise be the case.
+
+
+2. Propeller ``flutter'' and possible collapse.
+
+
+3. A bad stress upon the propeller shaft and its bearings.
+
+
+TRACTOR.--A propeller mounted in front of the main
+surface.
+
+
+PUSHER.--A propeller mounted behind the main surface.
+
+
+FOUR-BLADED PROPELLERS.--Four- bladed propellers are
+suitable only when the pitch is comparatively large.
+
+For a given pitch, and having regard to ``interference,''
+they are not so efficient as two-bladed propellers.
+
+The smaller the pitch, the less the ``gap,'' i.e., the distance,
+measured in the direction of the thrust, between the
+spiral courses of the blades.
+
+If the gap is too small, then the following blade will
+engage air which the preceding blade has put into motion,
+with the result that the following blade will not secure as
+good a reaction as would otherwise be the case. It is very
+much the same as in the case of the aeroplane gap.
+
+For a given pitch, the gap of a four-bladed propeller is
+only half that of a two-bladed one. Therefore the four-
+bladed propeller is only suitable for large pitch, as such
+pitch produces spirals with a large gap, thus offsetting the
+decrease in gap caused by the numerous blades.
+
+The greater the speed of rotation, the less the pitch for
+a given aeroplane speed. Then, in order to secure a large
+pitch and consequently a good gap, the four-bladed propeller
+is usually geared to rotate at a lower speed than would be
+the case if directly attached to the engine crank-shaft.
+
+
+
+CHAPTER V
+
+MAINTENANCE
+
+CLEANLINESS.--The fabric must be kept clean and free
+from oil, as that will rot it. To take out dirt or oily patches,
+try acetone. If that will not remedy matters, then try
+petrol, but use it sparingly, as otherwise it will take off an
+unnecessary amount of dope. If that will not remove the
+dirt, then hot water and soap will do so, but, in that case,
+be sure to use soap having no alkali in it, as otherwise it may
+injure the fabric. Use the water sparingly, or it may get
+inside the planes and rust the internal bracing wires, or cause
+some of the wooden framework to swell.
+
+The wheels of the undercarriage have a way of throwing
+up mud on to the lower surface. This should, if possible, be
+taken off while wet. It should never be scraped off when
+dry, as that may injure the fabric. If dry, then it should
+be moistened before being removed.
+
+Measures should be taken to prevent dirt from collecting
+upon any part of the aeroplane, as, otherwise, excessive skin-
+friction will be produced with resultant loss of flight speed.
+The wires, being greasy, collect dirt very easily.
+
+
+CONTROL CABLES.--After every flight the rigger should
+pass his hand over the control cables and carefully examine
+them near pulleys. Removal of grease may be necessary
+to make a close inspection possible. If only one strand is
+broken the wire should be replaced. Do not forget the aileron
+balance wire on the top surface.
+
+Once a day try the tension of the control cables by smartly
+moving the control levers about as explained elsewhere.
+
+
+WIRES.--All the wires should be kept well greased or
+oiled, and in the correct tension. When examining the wires,
+it is necessary to place the aeroplane on level ground, as
+otherwise it may be twisted, thus throwing some wires into
+undue tension and slackening others. The best way, if there
+is time, is to pack the machine up into its ``flying position.''
+
+If you see a slack wire, do not jump to the conclusion
+that it must be tensioned. Perhaps its opposition wire is
+too tight, in which case slacken it, and possibly you will
+find that will tighten the slack wire.
+
+Carefully examine all wires and their connections near
+the propeller, and be sure that they are snaked round with
+safety wire, so that the latter may keep them out of the way
+of the propeller if they come adrift.
+
+The wires inside the fuselage should be cleaned and regreased
+about once a fortnight.
+
+
+STRUTS AND SOCKETS.--These should be carefully examined
+to see if any splitting has occurred.
+
+
+DISTORTION.--Carefully examine all surfaces, including
+the controlling surfaces, to see whether any distortion has
+occurred. If distortion can be corrected by the adjustment
+of wires, well and good; but if not, then some of the internal
+framework probably requires replacement.
+
+
+ADJUSTMENTS.--Verify the angles of incidence; dihedral,
+and stagger, and the rigging position of the controlling-
+surfaces, as often as possible.
+
+
+UNDERCARRIAGE.--Constantly examine the alignment and
+fittings of the undercarriage, and the condition of tyres and
+shock absorbers. The latter, when made of rubber, wear
+quickest underneath. Inspect axles and skids to see if
+there are any signs of them becoming bent. The wheels
+should be taken off occasionally and greased.
+
+
+LOCKING ARRANGEMENTS.--Constantly inspect the locking
+arrangements of turnbuckles, bolts, etc. Pay particular
+attention to the control cable connections, and to all moving
+parts in respect of the controls.
+
+
+LUBRICATION.--Keep all moving parts, such as pulleys,
+control levers, and hinges of controlling surfaces, well greased.
+
+
+SPECIAL INSPECTION.--Apart from constantly examining
+the aeroplane with reference to the above points I have made,
+I think that, in the case of an aeroplane in constant use
+it is an excellent thing to make a special inspection of every
+part, say once a week. This will take from two to three
+hours, according to the type of aeroplane. In order to carry
+it out methodically, the rigger should have a list of every part
+down to the smallest split-pin. He can then check the parts
+as he examines them, and nothing will be passed over. This,
+I know from experience, greatly increases the confidence of
+the pilot, and tends to produce good work in the air.
+
+
+WINDY WEATHER.--The aeroplane, when on the ground,
+should face the wind; and it is advisable to lash the control
+lever fast, so that the controlling surfaces may not be blown
+about and possibly damaged.
+
+
+``VETTING'' BY EYE.--This should be practiced at every
+opportunity, and, if persevered in, it is possible to become
+quite expert in diagnosing by eye faults in flight efficiency,
+stability and control.
+
+The aeroplane should be standing upon level ground, or,
+better than that, packed up into its ``flying position.''
+
+Now stand in front of it and line up the leading edge
+with the main spar, rear spar, and trailing edge. Their
+shadows can usually be seen through the fabric. Allowance
+must, of course, be made for wash-in and wash-out; otherwise,
+the parts I have specified should be parallel with each other.
+
+Now line up the centre part of the main-plane with the
+tail-plane. The latter should be horizontal.
+
+Next, sight each interplane front strut with its rear
+strut. They should be parallel.
+
+Then, standing on one side of the aeroplane, sight all
+the front struts. The one nearest to you should cover all
+the others. This applies to the rear struts also.
+
+Look for distortion of leading edges, main and rear spars,
+trailing edges, tail-plane and controlling surfaces.
+
+This sort of thing, if practiced constantly, will not only
+develop an expert eye for diagnosis of faults, but will also
+greatly assist in impressing upon the memory the characteristics
+and possible troubles of the various types of aeroplanes.
+
+
+MISHANDLING OF THE GROUND.--This is the cause of a
+lot of unnecessary damage. The golden rule to observe is:
+PRODUCE NO BENDING STRESSES.
+
+Nearly all the wood in an aeroplane is designed to take
+merely the stress of direct compression, and it cannot be bent
+safely. Therefore, in packing an aeroplane up from the
+ground, or in pulling or pushing it about, be careful to stress
+it in such a way as to produce, as far as possible, only direct
+compression stresses. For instance, if it is necessary to
+support the lifting surface, then the packing should be
+arranged to come directly under the struts so that they may
+take the stress in the form of compression for which they are
+designed. Such supports should be covered with soft packing
+in order to prevent the fabric from becoming damaged.
+
+When pulling an aeroplane along, if possible, pull from
+the top of the undercarriage struts. If necessary to pull
+from elsewhere, then do so by grasping the interplane struts
+as low down as possible.
+
+Never lay fabric-covered parts upon a concrete floor.
+Any slight movement will cause the fabric to scrape over the
+floor with resultant damage.
+
+Struts, spars, etc., should never be left about the floor,
+as in such position they are likely to become scored. I
+have already explained the importance of protecting the outside
+fibres of the wood. Remember also that wood becomes
+distorted easily. This particularly applies to interplane
+struts. If there are no proper racks to stand them in, then
+the best plan is to lean them up against the wall in as near a
+vertical position as possible.
+
+
+TIME.--Learn to know the time necessary to complete
+any of the various rigging jobs. This is really important.
+Ignorance of this will lead to bitter disappointments in civil
+life; and, where Service flying is concerned, it will, to say the
+least of it, earn unpopularity with senior officers, and fail to
+develop respect and good work where men are concerned.
+
+
+THE AEROPLANE SHED.--This should be kept as clean and
+orderly as possible. A clean, smart shed produces briskness,
+energy, and pride of work. A dirty, disorderly shed nearly
+always produces slackness and poor quality of work, lost
+tools and mislaid material.
+
+
+GLOSSARY
+
+Aeronautics--The science of aerial navigation.
+
+Aerofoil--A rigid structure, of large superficial area relative to its
+thickness, designed to obtain, when driven through the air at an
+angle inclined to the direction of motion, a reaction from the air
+approximately at right angles to its surface. Always cambered
+when intended to secure a reaction in one direction only. As the
+term ``aerofoil'' is hardly ever used in practical aeronautics,
+I have, throughout this book, used the term SURFACE, which,
+while academically incorrect, since it does not indicate thickness,
+is a term usually used to describe the cambered lifting surfaces,
+i.e., the ``planes'' or ``wings,'' and the stabilizers and the
+controlling aerofoils.
+
+Aerodrome--The name usually applied to a ground used for the
+practice of aviation. It really means ``flying machine,'' but is
+never used in that sense nowadays.
+
+Aeroplane--A power-driven aerofoil with stabilizing and controlling
+surfaces.
+
+Acceleration--The rate of change of velocity.
+
+Angle of Incidence--The angle at which the ``neutral lift line'' of
+a surface attacks the air.
+
+Angle of Incidence, Rigger's--The angle the chord of a surface makes
+with a line parallel to the axis of the propeller.
+
+Angle of Incidence, Maximum--The greatest angle of incidence at
+which, for a given power, surface (including detrimental surface),
+and weight, horizontal flight can be maintained.
+
+Angle of Incidence, Minimum--The smallest angle of incidence at
+which, for a given power, surface (including detrimental surface),
+and weight, horizontal flight can be maintained.
+
+Angle of Incidence, Best Climbing--That angle of incidence at which
+an aeroplane ascends quickest. An angle approximately halfway
+between the maximum and optimum angles.
+
+Angle of Incidence, Optimum--The angle of incidence at which the
+lift-drift ratio is the highest.
+
+
+Angle, Gliding--The angle between the horizontal and the path along
+which an aeroplane at normal flying speed, but not under engine
+power, descends in still air.
+
+Angle, Dihedral--The angle between two planes.
+
+Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
+be at a lateral dihedral angle when it is inclined upward towards
+its wing-tips.
+
+Angle, Longitudinal Dihedral--The main surface and tail surface are
+said to be at a longitudinal dihedral angle when the projections
+of their neutral lift lines meet and produce an angle above them.
+
+Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting
+``chords'' for ``neutral life lines.''
+
+Angle, Pitch--The angle at any given point of a propeller, at which
+the blade is inclined to the direction of motion when the propeller
+is revolving but the aeroplane stationary.
+
+Altimeter--An instrument used for measuring height.
+
+Air-Speed Indicator--An instrument used for measuring air pressures
+or velocities. It consequently indicates whether the surface is
+securing the requisite reaction for flight. Usually calibrated in
+miles per hour, in which case it indicates the correct number of
+miles per hour at only one altitude. This is owing to the density
+of the air decreasing with increase of altitude and necessitating
+a greater speed through space to secure the same air pressure
+as would be secured by less speed at a lower altitude. It would
+be more correct to calibrate it in units of air pressure.
+
+Air Pocket--A local movement or condition of the air causing an
+aeroplane to drop or lose its correct attitude.
+
+Aspect-Ratio--The proportion of span to chord of a surface.
+
+Air-Screw (Propeller)--A surface so shaped that its rotation about
+an axis produces a force (thrust) in the direction of its axis.
+
+Aileron--A controlling surface, usually situated at the wing-tip, the
+operation of which turns an aeroplane about its longitudinal axis;
+causes an aeroplane to tilt sideways.
+
+
+Aviation--The art of driving an aeroplane.
+
+Aviator--The driver of an aeroplane.
+
+Barograph--A recording barometer, the charts of which can be calibrated
+for showing air density or height.
+
+Barometer--An instrument used for indicating the density of air.
+
+Bank, to--To turn an aeroplane about its longitudinal axis (to tilt
+sideways) when turning to left or right.
+
+Biplane--An aeroplane of which the main lifting surface consists
+of a surface or pair of wings mounted above another surface or
+pair of wings.
+
+Bay--The space enclosed by two struts and whatever they are fixed to.
+
+Boom--A term usually applied to the long spars joining the tail of a
+``pusher'' aeroplane to its main lifting surface.
+
+Bracing--A system of struts and tie wires to transfer a force from
+one point to another.
+
+Canard--Literally ``duck.'' The name which was given to a type of
+aeroplane of which the longitudinal stabilizing surface (empennage)
+was mounted in front of the main lifting surface. Sometimes
+termed ``tail-first'' aeroplanes, but such term is erroneous,
+as in such a design the main lifting surface acts as, and is, the
+empennage.
+
+Cabre--To fly or glide at an excessive angle of incidence; tail down.
+
+Camber--Curvature.
+
+Chord--Usually taken to be a straight line between the trailing and
+leading edges of a surface.
+
+Cell--The whole of the lower surface, that part of the upper surface
+directly over it, together with the struts and wires holding them
+together.
+
+Centre (Line) of Pressure--A line running from wing-tip to wing-tip,
+and through which all the air forces acting upon the surface may
+be said to act, or about which they may be said to balance.
+
+Centre (Line) of Pressure, Resultant--A line transverse to the
+longitudinal axis, and the position of which is the resultant of the
+centres of pressure of two or more surfaces.
+
+Centre of Gravity--The centre of weight.
+
+Cabane--A combination of two pylons, situated over the fuselage,
+and from which anti-lift wires are suspended. 
+
+Cloche--Literally ``bell.'' Is applied to the bell-shaped construction
+which forms the lower part of the pilot's control lever in
+a Bleriot monoplane, and to which the control cables are
+attached.
+
+Centrifugal Force--Every body which moves in a curved path is
+urged outwards from the centre of the curve by a force termed
+``centrifugal.''
+
+Control Lever--A lever by means of which the controlling surfaces
+are operated. It usually operates the ailerons and elevator. The
+``joy-stick".
+
+Cavitation, Propeller--The tendency to produce a cavity in the air.
+
+Distance Piece--A long, thin piece of wood (sometimes tape) passing
+through and attached to all the ribs in order to prevent them from
+rolling over sideways.
+
+Displacement--Change of position.
+
+Drift (of an aeroplane as distinct from the propeller)--The horizontal
+component of the reaction produced by the action of driving
+through the air a surface inclined upwards and towards its direction
+of motion PLUS the horizontal component of the reaction produced
+by the ``detrimental'' surface PLUS resistance due to
+``skin-friction.'' Sometimes termed ``head-resistance.''
+
+Drift, Active--Drift produced by the lifting surface.
+
+Drift, Passive--Drift produced by the detrimental surface.
+
+Drift (of a propeller)--Analogous to the drift of an aeroplane. It is
+convenient to include ``cavitation'' within this term.
+
+Drift, to--To be carried by a current of air; to make leeway.
+
+Dive, to--To descend so steeply as to produce a speed greater than the
+normal flying speed.
+
+Dope, to--To paint a fabric with a special fluid for the purpose of
+tightening and protecting it.
+
+Density--Mass of unit volume, for instance, pounds per cubic foot.
+
+Efficiency--Output
+            Input
+
+Efficiency (of an aeroplane as distinct from engine and propeller)--
+         Lift and Velocity
+    Thrust (= aeroplane drift)
+
+Efficiency, Engine--Brake horse-power
+                    Indicated horse-power
+
+Efficiency, Propeller--        Thrust horse-power
+                       Horse-power received from engine
+                               (= propeller drift)
+
+NOTE.--The above terms can, of course, be expressed in foot-
+pounds. It is then only necessary to divide the upper term by
+the lower one to find the measure of efficiency.
+
+Elevator--A controlling surface, usually hinged to the rear of the tail-
+plane, the operation of which turns an aeroplane about an axis
+which is transverse to the direction of normal horizontal flight.
+
+Empennage--See ``Tail-plane.''
+
+Energy--Stored work. For instance, a given weight of coal or petroleum
+stores a given quantity of energy which may be expressed
+in foot-pounds.
+
+Extension--That part of the upper surface extending beyond the
+span of the lower surface.
+
+Edge, Leading--The front edge of a surface relative to its normal
+direction of motion.
+
+Edge, Trailing--The rear edge of a surface relative to its normal
+direction of motion.
+
+Factor of Safety--Usually taken to mean the result found by dividing
+the stress at which a body will collapse by the maximum stress
+it will be called upon to bear.
+
+Fineness (of stream-line)--The proportion of length to maximum width.
+
+Flying Position--A special position in which an aeroplane must be
+placed when rigging it or making adjustments. It varies with
+different types of aeroplanes. Would be more correctly described
+as ``rigging position.''
+
+Fuselage--That part of an aeroplane containing the pilot, and to which
+is fixed the tail-plane.
+
+Fin--Additional keel-surface, usually mounted at the rear of an
+aeroplane.
+
+Flange (of a rib)--That horizontal part of a rib which prevents it
+from bending sideways.
+
+Flight--The sustenance of a body heavier than air by means of its
+action upon the air.
+
+Foot-pound--A measure of work representing the weight of 1 lb.
+raised 1 foot.
+
+Fairing--Usually made of thin sheet aluminum, wood, or a light
+construction of wood and fabric; and bent round detrimental
+surface in order to give it a ``fair'' or ``stream-like'' shape.
+
+Gravity--Is the force of the Earth's attraction upon a body. It
+decreases with increase of distance from the Earth. See ``Weight.''
+
+Gravity, Specific--Density of substance
+                   Density of water.
+Thus, if the density of water is 10 lb. per unit volume, the same
+unit volume of petrol, if weighing 7 lb., would be said to have a
+specific gravity of  7/10, i.e., 0.7.
+
+Gap (of an aeroplane)--The distance between the upper and lower
+surfaces of a biplane. In a triplane or multiplane, the distance
+between a surface and the one first above it.
+
+Gap, Propeller--The distance, measured in the direction of the thrust,
+between the spiral courses of the blades.
+
+Girder--A structure designed to resist bending, and to combine lightness
+and strength.
+
+Gyroscope--A heavy circular wheel revolving at high speed, the effect
+of which is a tendency to maintain its plane of rotation against
+disturbing forces.
+
+Hangar--An aeroplane shed.
+
+Head-Resistance--Drift. The resistance of the air to the passage of
+a body.
+
+Helicopter--An air-screw revolving about a vertical axis, the direction
+of its thrust being opposed to gravity.
+
+Horizontal Equivalent--The plan view of a body whatever its attitude
+may be.
+
+Impulse--A force causing a body to gain or lose momentum.
+
+Inclinometer--A curved form of spirit-level used for indicating the
+attitude of a body relative to the horizontal.
+
+Instability--An inherent tendency of a body, which, if the body is
+disturbed, causes it to move into a position as far as possible away
+from its first position.
+
+Instability, Neutral--An inherent tendency of a body to remain in the
+position given it by the force of a disturbance, with no tendency
+to move farther or to return to its first position.
+
+Inertia--The inherent resistance to displacement of a body as distinct
+from resistance the result of an external force.
+
+Joy-Stick--See ``Control Lever.''
+
+Keel-Surface--Everything to be seen when viewing an aeroplane from
+the side of it.
+
+King-Post--A bracing strut; in an aeroplane, usually passing through
+a surface and attached to the main spar, and from the end or ends
+of which wires are taken to spar, surface, or other part of the
+construction in order to prevent distortion. When used in connection
+with a controlling surface, it usually performs the additional
+function of a lever, control cables connecting its ends with the
+pilot's control lever.
+
+Lift--The vertical component of the reaction produced by the action
+of driving through the air a surface inclined upwards and towards
+its direction of motion.
+
+Lift, Margin of--The height an aeroplane can gain in a given time and
+starting from a given altitude.
+
+Lift-Drift Ratio--The proportion of lift to drift.
+
+Loading--The weight carried by an aerofoil. Usually expressed in
+pounds per square foot of superficial area.
+
+Longeron--The term usually applied to any long spar running length-
+ways of a fuselage.
+
+Mass--The mass of a body is a measure of the quantity of material
+in it.
+
+Momentum--The product of the mass and velocity of a body is known
+as ``momentum.''
+
+Monoplane--An aeroplane of which the main lifting surface consists
+of one surface or one pair of wings.
+
+Multiplane--An aeroplane of which the main lifting surface consists
+of numerous surfaces or pairs of wings mounted one above the
+other.
+
+Montant--Fuselage strut.
+
+Nacelle--That part of an aeroplane containing the engine and
+pilot and passenger, and to which the tail plane is not fixed.
+
+Neutral Lift Line--A line taken through a surface in a forward direction
+relative to its direction of motion, and starting from its
+trailing edge. If the attitude of the surface is such as to make
+the said line coincident with the direction of motion, it results
+in no lift, the reaction then consisting solely of drift. The position
+of the neutral lift line, i.e., the angle it makes with the chord,
+varies with differences of camber, and it is found by means of
+wind-tunnel research.
+
+Newton's Laws of Motion--1. If a body be at rest, it will remain at
+rest; or, if in motion, it will move uniformly in a straight line
+until acted upon by some force.
+
+2. The rate of change of the quantity of motion (momentum) is
+proportional to the force which causes it, and takes place in the
+direction of the straight line in which the force acts. If a body
+be acted upon by several forces, it will obey each as though the
+others did not exist, and this whether the body be at rest or in
+motion.
+
+3. To every action there is opposed an equal and opposite
+reaction.
+
+Ornithopter (or Orthopter)--A flapping wing design of aircraft intended
+to imitate the flight of a bird.
+
+Outrigger--This term is usually applied to the framework connecting
+the main surface with an elevator placed in advance of it. Sometimes
+applied to the ``tail-boom'' framework connecting the
+tail-plane with the main lifting surface.
+
+Pancake, to--To ``stall ''
+
+Plane--This term is often applied to a lifting surface. Such application
+is not quite correct, since ``plane'' indicates a flat surface,
+and the lifting surfaces are always cambered.
+
+Propeller--See ``Air-Screw.''
+
+Propeller, Tractor--An air-screw mounted in front of the main lifting
+surface.
+
+Propeller, Pusher--An air-screw mounted behind the main lifting surface.
+
+Pusher--An aeroplane of which the propeller is mounted behind the
+main lifting surface.
+
+Pylon--Any V-shaped construction from the point of which wires
+are taken.
+
+Power--Rate of working.
+
+Power, Horse--One horse-power represents a force sufficient to raise
+33,000 lbs. 1 foot in a minute.
+
+Power, Indicated Horse--The I.H.P. of an engine is a measure of the
+rate at which work is done by the pressure upon the piston or
+pistons, as distinct from the rate at which the engine does work.
+The latter is usually termed ``brake horse-power,'' since it may be
+measured by an absorption brake.
+
+Power, Margin of--The available quantity of power above that necessary
+to maintain horizontal flight at the optimum angle.
+
+Pitot Tube--A form of air-speed indicator consisting of a tube with
+open end facing the wind, which, combined with a static pressure
+or suction tube, is used in conjunction with a gauge for measuring
+air pressures or velocities. (No. 1 in diagram.)
+
+Pitch, Propeller--The distance a propeller advances during one revolution
+supposing the air to be solid.
+
+Pitch, to--To plunge nose-down.
+
+Reaction--A force, equal and opposite to the force of the action producing
+it.
+
+Rudder--A controlling surface, usually hinged to the tail, the operation
+of which turns an aeroplane about an axis which is vertical in
+normal horizontal flight; causes an aeroplane to turn to left or
+right of the pilot.
+
+Roll, to--To turn about the longitudinal axis.
+
+Rib, Ordinary--A light curved wooden part mounted in a fore and aft
+direction within a surface. The ordinary ribs give the surface
+its camber, carry the fabric, and transfer the lift from the fabric
+to the spars.
+
+Rib, Compression--Acts as an ordinary rib, besides bearing the stress
+of compression produced by the tension of the internal bracing
+wires.
+
+Rib, False--A subsidiary rib, usually used to improve the camber of
+the front part of the surface.
+
+Right and Left Hand--Always used relative to the position of the
+pilot. When observing an aeroplane from the front of it, the
+right hand side of it is then on the left hand of the observer.
+
+Remou--A local movement or condition of the air which may cause
+displacement of an aeroplane.
+
+Rudder-Bar--A control lever moved by the pilot's feet, and operating
+the rudder.
+
+Surface--See ``Aerofoil.''
+
+Surface, Detrimental--All exterior parts of an aeroplane including
+the propeller, but excluding the (aeroplane) lifting and (propeller)
+thrusting surfaces.
+
+Surface, Controlling--A surface the operation of which turns an aeroplane
+about one of its axes.
+
+Skin-Friction--The friction of the air with roughness of surface. A
+form of drift.
+
+Span---The distance from wing-tip to wing-tip.
+
+Stagger--The distance the upper surface is forward of the lower surface
+when the axis of the propeller is horizontal.
+
+Stability--The inherent tendency of a body, when disturbed, to return
+to its normal position.
+
+Stability, Directional--The stability about an axis which is vertical
+during normal horizontal flight, and without which an aeroplane
+has no natural tendency to remain upon its course.
+
+Stability, Longitudinal--The stability of an aeroplane about an axis
+transverse to the direction of normal horizontal flight, and without
+which it has no tendency to oppose pitching and tossing.
+
+Stability, Lateral--The stability of an aeroplane about its longitudinal
+axis, and without which it has no tendency to oppose sideways
+rolling.
+
+Stabilizer--A surface, such as fin or tail-plane, designed to give an
+aeroplane inherent stability.
+
+Stall, to--To give or allow an aeroplane an angle of incidence greater
+than the ``maximum'' angle, the result being a fall in the lift-
+drift ratio, the lift consequently becoming less than the weight of
+the aeroplane, which must then fall, i.e., ``stall'' or ``pancake.''
+
+Stress--Burden or load.
+
+Strain--Deformation produced by stress.
+
+Side-Slip, to--To fall as a result of an excessive ``bank'' or ``roll.''
+
+Skid, to--To be carried sideways by centrifugal force when turning
+to left or right.
+
+Skid, Undercarriage--A spar, mounted in a fore and aft direction, and
+to which the wheels of the undercarriage are sometimes attached.
+Should a wheel give way the skid is then supposed to act like the
+runner of a sleigh and to support the aeroplane.
+
+Skid, Tail--A piece of wood or other material, orientable, and fitted
+with shock absorbers, situated under the tail of an aeroplane in
+order to support it upon the ground and to absorb the shock of
+alighting. 
+
+
+Section--Any separate part of the top surface, that part of the bottom
+surface immediately underneath it, with their struts and wires.
+
+Spar--Any long piece of wood or other material.
+
+Spar, Main--A spar within a surface and to which all the ribs are
+attached, such spar being the one situated nearest to the centre
+of pressure. It transfers more than half the lift from the ribs
+to the bracing. 
+
+Spar, Rear--A spar within a surface, and to which all the ribs are
+attached, such spar being situated at the rear of the centre of
+pressure and at a greater distance from it than is the main spar.
+It transfers less than half of the lift from the ribs to the bracing.
+
+Strut--Any wooden member intended to take merely the stress of
+direct compression.
+
+Strut, Interplane--A strut holding the top and bottom surfaces apart.
+
+Strut, Fuselage--A strut holding the fuselage longerons apart. It
+should be stated whether top, bottom, or side. If side, then it
+should be stated whether right or left hand. Montant. 
+
+Strut, Extension--A strut supporting an ``extension'' when not in
+flight. It may also prevent the extension from collapsing upwards
+during flight.
+
+Strut, Undercarriage--
+
+Strut, Dope--A strut within a surface, so placed as to prevent the
+tension of the doped fabric from distorting the framework.
+
+Serving--To bind round with wire, cord, or similar material. Usually
+used in connection with wood joints and wire cable splices.
+
+Slip, Propeller--The pitch less the distance the propeller advances
+during one revolution.
+
+Stream-Line--A form or shape of detrimental surface designed to
+produce minimum drift.
+
+Toss, to--To plunge tail-down.
+
+Torque, Propeller--The tendency of a propeller to turn an aeroplane
+about its longitudinal axis in a direction opposite to that in which
+the propeller revolves.
+
+Tail-Slide--A fall whereby the tail of an aeroplane leads.
+
+Tractor--An aeroplane of which the propeller is mounted in front of
+the main lifting surface.
+
+Triplane--An aeroplane of which the main lifting surface consists of
+three surfaces or pairs of wings mounted one above the other.
+
+Tail-Plane--A horizontal stabilizing surface mounted at some distance
+behind the main lifting surface. Empennage. 
+
+Turnbuckle--A form of wire-tightener, consisting of a barrel into each
+end of which is screwed an eyebolt. Wires are attached to the
+eyebolts and the required degree of tension is secured by means
+of rotating the barrel.
+
+Thrust, Propeller--See ``Air-Screw.''
+
+Undercarriage--That part of an aeroplane beneath the fuselage or
+nacelle, and intended to support the aeroplane when at rest, and
+to absorb the shock of alighting. 
+
+Velocity--Rate of displacement; speed.
+
+Volplane--A gliding descent.
+
+Weight--Is a measure of the force of the Earth's attraction (gravity)
+upon a body. The standard unit of weight in this country is
+1 lb., and is the force of the Earth's attraction on a piece of platinum
+called the standard pound, deposited with the Board of Trade
+in London. At the centre of the Earth a body will be attracted
+with equal force in every direction. It will therefore have no
+weight, though its mass is unchanged. Gravity, of which weight
+is a measure, decreases with increase of altitude.
+
+Web (of a rib)--That vertical part of a rib which prevents it from
+bending upwards.
+
+Warp, to--To distort a surface in order to vary its angle of incidence.
+To vary the angle of incidence of a controlling surface.
+
+Wash--The disturbance of air produced by the flight of an aeroplane.
+
+Wash-in--An increasing angle of incidence of a surface towards its
+wing-tip.
+
+Wash-out--A decreasing angle of incidence of a surface towards its
+wing-tip.
+
+Wing-tip--The right- or left-hand extremity of a surface.
+
+Wire--A wire is, in Aeronautics, always known by the name of its
+function.
+
+Wire, Lift or Flying--A wire opposed to the direction of lift, and used
+to prevent a surface from collapsing upward during flight.
+
+Wire, Anti-lift or Landing--A wire opposed to the direction of gravity,
+and used to sustain a surface when it is at rest.
+
+Wire, Drift--A wire opposed to the direction of drift, and used to
+prevent a surface from collapsing backwards during flight.
+
+Wire, Anti-drift--A wire opposed to the tension of a drift wire, and
+used to prevent such tension from distorting the framework.
+
+Wire, Incidence--A wire running from the top of an interplane strut to
+the bottom of the interplane strut in front of or behind it. It
+maintains the ``stagger'' and assists in maintaining the angle
+of incidence. Sometimes termed ``stagger wire.''
+
+Wire, Bracing--Any wire holding together the framework of any part
+of an aeroplane. It is not, however, usually applied to the wires
+described above unless the function performed includes a function
+additional to those described above. Thus, a lift wire, while
+strictly speaking a bracing wire, is not usually described as one
+unless it performs the additional function of bracing some well-
+defined part such as the undercarriage. It will then be said to
+be an ``undercarriage bracing lift wire.'' It might, perhaps,
+be acting as a drift wire also, in which case it will then be de-
+scribed as an ``undercarriage bracing lift-drift wire.'' It should
+always be stated whether a bracing wire is (1) top, (2) bottom,
+(3) cross, or (4) side. If a ``side bracing wire,'' then it should be
+stated whether right- or left-hand.
+
+Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)
+within a surface.
+
+Wire, Top Bracing--A bracing wire, approximately horizontal and
+situated between the top longerons of fuselate, between top tail
+booms, or at the top of similar construction.
+
+Wire, Bottom Bracing--Ditto, substituting ``bottom'' for ``top.''
+
+Wire, Side Bracing--A bracing wire crossing diagonally a side bay
+of fuselage, tail boom bay, undercarriage side bay or centre-section
+side bay. This term is not usually used with reference to incidence
+wires, although they cross diagonally the side bays of the
+cell. It should be stated whether right- or left-hand.
+
+Wire, Cross Bracing--A bracing wire, the position of which is diagonal
+from right to left when viewing it from the front of an aeroplane.
+
+Wire, Control Bracing--A wire preventing distortion of a controlling
+surface.
+
+Wire, Control--A wire connecting a controlling surface with the pilot's
+control lever, wheel, or rudder-bar.
+
+Wire, Aileron Gap--A wire connecting top and bottom ailerons.
+
+Wire, Aileron Balance--A wire connecting the right- and left-hand top
+ailerons. Sometimes termed the ``aileron compensating wire.''
+
+Wire, Snaking--A wire, usually of soft metal, wound spirally or tied
+round another wire, and attached at each end to the framework.
+Used to prevent the wire round which it is ``snaked'' from becoming,
+in the event of its displacement, entangled with the
+propeller.
+
+Wire, Locking--A wire used to prevent a turnbuckle barrel or other
+fitting from losing its adjustment.
+
+Wing--Strictly speaking, a wing is one of the surfaces of an ornithopter.
+The term is, however, often applied to the lifting surface of
+an aeroplane when such surface is divided into two parts, one being
+the left-hand ``wing,'' and the other the right-hand ``wing.''
+
+Wind-Tunnel--A large tube used for experimenting with surfaces and
+models, and through which a current of air is made to flow by
+artificial means.
+
+Work--Force X displacement.
+
+Wind-Screen--A small transparent screen mounted in front of the
+pilot to protect his face from the air pressure.
+
+
+
+
+
+End of The Project Gutenberg Etext of The Aeroplane Speaks, by Barber
+
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