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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.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
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