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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
+       Fifth Edition
+
+Author: H. Barber
+
+Release Date: June 10, 2007 [EBook #21791]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE AEROPLANE SPEAKS ***
+
+
+
+
+Produced by Jonathan Ingram, Marvin A. Hodges, David Garcia
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+[Illustration: THE FLIGHT FOLK.]
+
+
+
+
+
+
+THE AEROPLANE SPEAKS
+
+BY H. BARBER, A.F.Ae.S.
+
+(CAPTAIN, ROYAL FLYING CORPS)
+
+
+WITH 36 FULL PAGES OF "TYPES OF AEROPLANES" AND 87 SKETCHES AND DIAGRAMS
+
+_FIFTH EDITION_
+
+
+LONDON
+
+McBRIDE, NAST & CO., LTD.
+
+
+
+
+THE AEROPLANE SPEAKS.
+
+  _First edition--December, 1916_
+  _Second edition--February, 1917_
+  _Third edition--April, 1917_
+  _Fourth edition--July, 1917_
+  _Fifth edition--December, 1917_
+
+
+FIRST REVIEWS:
+
+  =C. G. G. in the AEROPLANE:= "One hopes that the Subaltern
+  Flying Officer will appreciate the gift which the author has
+  given him out of his own vast store of experience, for the book
+  contains the concentrated knowledge of many expensive years in
+  tabloid form, or perhaps one should say in condensed milk form,
+  seeing that it is easy to swallow and agreeable to the taste,
+  as well as wholesome and nourishing. And, besides the young
+  service aviator, there are thousands of young men, and women
+  also, now employed in the aircraft industry, who will appreciate
+  far better the value of the finicky little jobs they are doing
+  if they will read this book and see how vital is their work to
+  the man who flies."
+
+  =THE FIELD:= "Entirely different from any other text-book on
+  the subject, not merely in its form, but in its capacity to convey
+  a knowledge of the principles and practice of flying. Undoubtedly
+  it is the best book on its subject."
+
+  =THE UNITED SERVICE GAZETTE:= "Should be in the hands of every
+  person interested in aviation."
+
+  =THE OUTLOOK:= "As amusing as it is instructive."
+
+  =THE MORNING POST:= "Should be read and re-read by the would
+  be and even the experienced pilot."
+
+
+
+  PRINTED IN ENGLAND BY
+  BILLING AND SONS, LIMITED
+  GUILDFORD
+
+
+
+
+  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_
+
+                                                                    PAGE
+
+    _PART I.--THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES_          1
+        _II.--THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES,
+                FINISH THE JOB_                                       15
+       _III.--THE GREAT TEST_                                         27
+        _IV.--CROSS COUNTRY_                                          38
+
+  CHAPTER I.--FLIGHT                                                  55
+         II.--STABILITY AND CONTROL                                   70
+        III.--RIGGING                                                 90
+         IV.--PROPELLERS                                             115
+          V.--MAINTENANCE                                            126
+
+  TYPES OF AEROPLANES                                                130
+
+  GLOSSARY                                                           133
+
+
+
+
+
+
+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 an illustration thus:
+
+[Illustration]
+
+"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.
+
+[Illustration: The action of the surface upon the air.]
+
+"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 and consequent Force 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."
+
+[Illustration]
+
+"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
+or 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."
+
+[Illustration]
+
+"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."
+
+[Illustration]
+
+"That's not practical politics," said the Surface. "The way you talk one
+would think you were drawing £400 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:
+
+[Illustration]
+
+"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:
+
+[Illustration]
+
+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.
+
+[Illustration: Maximum Climb. Maximum Speed.]
+
+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."
+
+[Illustration]
+
+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
+counter-balance 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:
+
+[Illustration]
+
+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."
+
+[Illustration: The angles shown above are only roughly approximate, as
+they vary with different types of aeroplanes.]
+
+"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."
+
+[Footnote 1: Propeller Slip: As the propeller screws through the air,
+the latter to a certain extent gives back to the thrust of the propeller
+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 propeller
+will feel the slip as a strong draught of air.]
+
+[Footnote 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.]
+
+[Footnote 3: Pancakes: Pilot's slang for stalling an aeroplane and
+dropping like a pancake.]
+
+
+
+
+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."
+
+[Illustration]
+
+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" (see illustration A,
+over-leaf).
+
+"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."
+
+[Illustration]
+
+"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 _rôles_ have changed one with
+the other, and I'm then the Elevator and the Elevator is me!"
+
+[Illustration]
+
+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 a certain axis of the
+machine, and the Elevator is a Controlling Surface designed to turn the
+Aeroplane about another 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:
+
+[Illustration: H.E., Horizontal equivalent.]
+
+"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 total 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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+"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."
+
+[Illustration]
+
+"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."
+
+[Illustration: "Wash out" on both sides.]
+
+"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."
+
+[Illustration]
+
+"Well, I hope that's all as it should be," she concluded, "for to-morrow
+the Great Test in the air is due."
+
+[Footnote 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.]
+
+[Footnote 5: Skin friction is that part of the drift due to the friction
+of the air with roughness upon the surface of the aeroplane.]
+
+[Footnote 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.]
+
+[Footnote 7: An explanation of the way in which the wash-out is combined
+with a wash-in to offset propeller torque will be found on p. 82.]
+
+
+
+
+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 "stream-lined" 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
+Pitot Air Speed Indicator, and, adjusting it to zero, smiles as he
+hears the Pitot-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 Pitot 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--a
+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.
+
+"On 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 Triplex 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, crowned 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 lower 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.
+
+[Footnote 8: A.M.'s: Air Mechanics.]
+
+[Footnote 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.]
+
+[Footnote 10: Deviation Curve: A curved line indicating any errors in the
+compass.]
+
+[Footnote 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.]
+
+[Footnote 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.]
+
+[Footnote 13: Box-kite. The first crude form of biplane.]
+
+
+
+
+PART IV
+
+'CROSS COUNTRY
+
+
+The Aeroplane had been designed and built, and tested in the air, and
+now it 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 too 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 very 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, "Tanks 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
+(see p. 40).
+
+"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.
+
+[Illustration:
+  A--B, 150 miles,
+  A--C, 50 miles; direction and miles per hour of wind.
+  C--D, 100 miles; airspeed of aeroplane.
+  A--D, Distance covered by aeroplane in one hour.
+  A--E, Compass course.]
+
+"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.
+
+[Illustration: The Pilot's Cock-pit.]
+
+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 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 a 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 place it
+appears, nestling down between the hills and 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 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. A high tail
+means 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:
+
+[Illustration:
+  The Pilot's Aeroplane.
+  The Change of Design He Would Like.]
+
+"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," says someone, "and that they haven't had
+difficulties with the fog. It rolled up very quickly, you know."
+
+"Never fear," remarks 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," says
+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 who 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 walk 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.
+
+[Illustration]
+
+_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.
+
+[Illustration]
+
+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, if, as seems reasonable, we regard the surface from the point
+of view of the neutral lift line. 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 criticized adversely
+is then applicable. Flat lifting surfaces are, however, never used.
+
+The surface acts upon the air in the following manner:
+
+[Illustration]
+
+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^2.
+
+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.
+
+[Illustration]
+
+The direction of the reaction is, at efficient angles of incidence,
+approximately at right-angles to the neutral lift line 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 roughness 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.
+
+[Illustration]
+
+_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 over the top of the surface 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. Also, too great an
+angle for the velocity will result in the underside of the surface
+tending to compress the air against which it is driven rather than
+accelerate it _downwards_, and that will tend to produce drift rather
+than the _upwards_ reaction, or lift.
+
+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 stream-line 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
+thickness 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 higher the
+aspect ratio, the greater the reaction. It is obvious, I think, that the
+greater the span, the greater the mass of undisturbed air engaged, and,
+as already explained, the reaction is partly the result of the mass of
+air engaged. I say "undisturbed" advisedly, for otherwise it might be
+argued that, whatever the shape of the surface, the same mass of air
+would be engaged. The word "undisturbed" makes all the difference, for
+it must be remembered that the rear part of the underside of the surface
+engages air most of which has been deflected downwards by the surface
+in front of it. That being so, the rear part of the surface has not the
+same opportunity of forcing; the air downwards (since it is already
+flowing downwards) and securing there from an upwards, reaction as has
+the surface in front of it. It is therefore of less value for its area
+than the front part of the surface, since it does less work and secures
+less reaction--_i.e._, lift. Again, the rarefied area over the top of
+the surface is most rare towards the front of it, as, owing to eddies,
+the rear of such area tends to become denser.
+
+[Illustration]
+
+Thus, you see, the front part of the surface is the most valuable from
+the point of view of securing an upwards reaction from the air; and so,
+by increasing the proportion of front, or "span," to chord, we increase
+the amount of reaction for a given velocity and area of surface. That
+means a better proportion of reaction to weight of surface, though the
+designer must not forget the drift of struts and wires necessary to
+brace up a surface of high aspect ratio.
+
+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 higher 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 about 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.
+
+[Illustration: H.E., Horizontal equivalent. D., Dihedral angle.]
+
+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.
+
+[Illustration]
+
+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 requisite 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.
+
+[Illustration: 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.=
+
+
+SUMMARY.
+
+_Essentials for Maximum Climb._
+
+  1. Low velocity.
+  2. Large surface.
+  3. Large angle relative to propeller thrust.
+  4. Large angle relative to direction of motion.
+  5. Large camber.
+
+_Essentials for Maximum Velocity._
+
+  1. High velocity.
+  2. Small surface.
+  3. Small angle relative to propeller thrust.
+  4. Small angle relative to direction of motion.
+  5. Small camber.
+
+
+It is mechanically impossible to construct an aeroplane of reasonable
+weight of which it would be possible to vary 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. 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.
+
+[Footnote 14: See Newton's laws in the Glossary at the end of the book.]
+
+[Footnote 15: See "Aerofoil" in the Glossary.]
+
+
+
+
+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 everything 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.
+
+[Illustration]
+
+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.
+
+[Illustration]
+
+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.[17]
+
+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°.
+
+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°, the C.P. moves
+forward as in the case of flat surfaces (see B); but angles above 30° do
+not interest us, since they produce a very low ratio of lift to drift.
+
+[Illustration]
+
+Below angles of about 30° (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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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°. Such decrease applies to
+both main surface and stabilizer, since both are fixed rigidly to the
+aeroplane.
+
+The main surface, which had 12° angle, has now only 10°, _i.e._, a loss
+of _one-sixth_.
+
+The stabilizer, which had 4° angle, has now only 2°, _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 angle and
+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°. Then, in order to secure
+a sufficiency of longitudinal stability, it is necessary to set the
+forward stabilizer at about 15°. 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.
+
+[Illustration]
+
+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, fitted to a "pusher" aeroplane, 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. When fitted to a
+"tractor" aeroplane, the engine is reversed so that a reverse condition
+results. In modern aeroplanes this tendency is not sufficiently
+important to bother about, except in the matter of spiral descents
+(see section headed "Spinning"). In the old days of crudely designed
+and under-powered "pusher" 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:
+
+[Illustration]
+
+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.
+
+[Illustration:
+    R, Direction of reaction of wing indicated.
+  R R, Resultant direction of reaction of both wings.
+    M, Horizontal (sideway) component of reaction.
+    L, Vertical component of reaction (lift).]
+
+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. This
+produces a little lateral stability without any marked pendulum effect.
+
+_Propeller torque_ affects lateral stability. An aeroplane tends to turn
+over sideways in the opposite direction to which the propeller revolves.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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.
+
+[Illustration:  Note: Observe that the inclination of the ailerons to
+the surface is the same in each case.]
+
+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:
+
+[Illustration: "Wash Out" on both sides relative to the Centre.]
+
+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." _Experientia 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.
+
+[Illustration]
+
+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 and/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.
+
+[Illustration: Nose Dive Spin.]
+
+In this connection every pilot of an aeroplane fitted with a rotary
+engine should bear in mind the gyroscopic effect of such engine. In the
+case of such an engine fitted to a "pusher" aeroplane, its effect when a
+left-hand turn is made is to depress the nose of the machine. If fitted
+to a "tractor" it is reversed, so the effect is to depress the nose
+if a right-hand turn is made. The sharper the turn, the greater such
+effect--an effect which may render the aeroplane unmanageable if the
+spiral is one of very small radius and the engine is revolving with
+sufficient speed to produce a material gyroscopic effect. Such
+gyroscopic effect should, however, slightly _assist_ the pilot to
+navigate a small spiral if he will remember to (1) make _right-hand_
+spirals in the case of a "pusher," (2) make _left-hand_ spirals in the
+case of a "tractor." The effect will then be to keep the nose up and
+prevent a nose-dive. I say "slightly" assist because the engine is, of
+course, throttled down for a spiral descent, and its lesser revolutions
+will produce a lesser gyroscopic effect.
+
+On the other hand, it might be argued that if the aeroplane gets into a
+"spin," anything tending to depress the nose of the machine is of value,
+since it is often claimed that the best way to get out of a spin is
+to put the machine into a nose-dive--the great velocity of the dive
+rendering the controls more efficient and better enabling the pilot to
+regain control. It is, however, a very contentious point, and few are
+able to express opinions based on practice, since pilots indulging in
+nose-dive spins are either not heard of again or have usually but a hazy
+recollection of exactly what happened to them.
+
+GLIDING DESCENT WITHOUT PROPELLER THRUST.--All aeroplanes are, or should
+be, designed to assume their correct 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.
+
+[Illustration]
+
+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. (see illustration above).
+
+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.
+
+[Illustration: Position A. Path B. Path C. Path D.]
+
+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.
+
+[Footnote 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.]
+
+[Footnote 17: The reason the C.P. of an inclined surface is forward of
+the centre of the surface is because the front of the surface does most
+of the work, as explained on p. 62.]
+
+
+
+
+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.
+
+[Illustration: Cross Sectional area]
+
+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.
+
+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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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.
+
+[Illustration]
+
+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._
+
+[Illustration]
+
+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.
+
+[Illustration: Strut straight. Wires and gap correctly adjusted. Strut
+bent throwing wires and gap out of adjustment.]
+
+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.
+
+[Illustration: Strut bedded properly. Strut bedded badly.]
+
+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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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 roughness 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 roughness 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.
+
+[Illustration: Distortion of upper wing caused by auxiliary lift wire
+being too tight.]
+
+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 much as possible. The rules to be observed are as
+follows:
+
+[Illustration: Wrong shape. Result of wrong shape. Right Shape.]
+
+(_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.
+
+[Illustration: Position in which controlling surface must be rigged. It
+will be its position during flight.]
+
+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.
+
+[Illustration: Position during flight. Position in which controlling
+surface must be rigged.]
+
+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:
+
+[Illustration]
+
+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:
+
+[Illustration: FRONT ELEVATION and PLAN.]
+
+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 their ends to struts. 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:
+
+[Illustration: Points A, B, and C, must be the same fixed distances
+from the butt as are Points D, E, and F. Distances 1 and 2 must equal
+distances 3 and 4.]
+
+The above applies to both front and rear bays.
+
+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 beforehand. 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:
+
+[Illustration]
+
+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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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 inch or more to the stagger,
+with the certain result that the aeroplane 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.
+
+[Illustration: The dotted lines on the surface represent the spars
+within it.]
+
+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 (see illustration overleaf).
+
+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 roughness 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.
+
+_Aspect Ratio._--Ditto.
+
+_Camber._--Ditto.
+
+[Illustration:
+  M, Direction of motion of propeller (rotary).
+  R, Direction of reaction.
+  T, Direction of thrust.
+ AD, Direction of the resistance of the air to the passage of the
+     aeroplane, _i.e._, aeroplane drift.
+  D, Direction of propeller drift (rotary).
+  P, Engine power, opposed to propeller drift and transmitted to
+     the propeller through the propeller shaft.]
+
+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. = 5.903 feet.
+    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 then 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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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 line 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:
+
+[Illustration:
+  A, B, C, and D, Actual pitch at points tested.
+  I, Pitch angle at point tested nearest to centre of propeller.
+  E, Circumference at I.
+  J, Pitch angle at point tested nearest to I.
+  F, Circumference at J.
+  K, Pitch angle at next point tested.
+  G, Circumference at K.
+  L, Pitch angle tested at point nearest tip of blade.
+  H, Circumference at L.]
+
+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:
+
+[Illustration]
+
+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 1/16 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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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.
+
+Weight B should equal weight E.
+
+Weight C should equal weight 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 calipers thus:
+
+[Illustration]
+
+The distance A--B should equal K--L.
+
+The distance C--D should equal I--J.
+
+The distance E--F should equal G--H.
+
+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,[18]
+but a fairly accurate idea of the concave camber can be secured by
+slowly passing a straight-edge along the blade, thus:
+
+[Illustration]
+
+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, surface area, or to bad mounting. 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.
+
+[Illustration:
+  SPIRAL COURSES OF TWO-BLADE TIPS.
+  SPIRAL COURSES OF FOUR-BLADE TIPS.
+  Pitch the same in each case.]
+
+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 (see illustration on preceding page).
+
+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.
+
+[Footnote 18: I have heard of temporary ones being made quickly by
+bending strips of lead over the convex side of the blade, but I should
+think it very difficult to secure a sufficient degree of accuracy in
+that way.]
+
+
+
+
+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 practised 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 symmetrical with it. 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 practised 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 ON 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 pull by
+means of wires.
+
+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.
+
+[Illustration]
+
+[Illustration]
+
+
+
+
+GLOSSARY
+
+_The numbers at the right-hand side of the page indicate the parts
+numbered in the preceding diagrams._
+
+
+=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 the 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 fitted 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 lift 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. [1]
+
+=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. [2]
+
+=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. [3]
+
+=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. [4]
+
+=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 the anti-lift wires are suspended. [5]
+
+=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." [6]
+
+=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. [7]
+
+=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 "eddies" and "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. [8]
+
+=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. [9]
+
+=Edge, Leading=--The front edge of a surface relative to its normal
+direction of motion. [10]
+
+=Edge, Trailing=--The rear edge of a surface relative to its normal
+direction of motion. [11]
+
+=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. [12]
+
+=Fin=--Additional keel-surface, usually mounted at the rear of an
+aeroplane. [13]
+
+=Flange= (_of a rib_)--That horizontal part of a rib which prevents it
+from bending sideways. [14]
+
+=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 aluminium, 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. [15]
+
+=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
+any two of its surfaces. [16]
+
+=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. [17]
+
+=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. [18]
+
+=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/or pilot
+and passenger, and to which the tail-plane is not fixed. [19]
+
+=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. [20]
+
+=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. [21]
+
+=Power, Horse=--One horse-power represents a force sufficient to raise
+33,000 lb. 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. [22]
+
+=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. [23]
+
+=Rib, Compression=--Acts as an ordinary rib, besides bearing the stress of
+compression produced by the tension of the internal bracing wires. [24]
+
+=Rib, False=--A subsidiary rib, usually used to improve the camber of the
+front part of the surface. [25]
+
+=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. [26]
+
+=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. [28]
+
+=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.
+[28_a_]
+
+=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. [29]
+
+=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. [30]
+
+=Strut=--Any wooden member intended to take merely the stress of direct
+compression.
+
+=Strut, Interplane=--A strut holding the top and bottom surfaces
+apart. [31]
+
+=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_. [32]
+
+=Strut, Extension=--A strut supporting an "extension" when not in
+flight. It may also prevent the extension from collapsing upwards during
+flight. [33]
+
+=Strut, undercarriage=-- [33_a_]
+
+=Strut, Dope=--A strut within a surface, so placed as to prevent the
+tension of the doped fabric from distorting the framework. [34]
+
+=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_. [36]
+
+=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. [37_a_]
+
+=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. [38]
+
+=Wash-out=--A decreasing angle of incidence of a surface towards its
+wing-tip. [39]
+
+=Wing-tip=--The right or left-hand extremity of a surface. [40]
+
+=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. [41]
+
+=Wire, Anti-lift or Landing=--A wire opposed to the direction of gravity,
+and used to sustain a surface when it is at rest. [42]
+
+=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. [44]
+
+=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." [45]
+
+=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 described 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 fuselage, between top tail booms, or at the
+top of similar construction. [46]
+
+=Wire, Bottom Bracing=--Ditto, substituting "bottom" for "top." [47]
+
+=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. [48]
+
+=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. [49]
+
+=Wire, Control Bracing=--A wire preventing distortion of a controlling
+surface. [50]
+
+=Wire, Control=--A wire connecting a controlling surface with the pilot's
+control lever, wheel, or rudder-bar. [51]
+
+=Wire, Aileron Gap=--A wire connecting top and bottom ailerons. [52]
+
+=Wire, Aileron Balance=--A wire connecting the right- and left-hand top
+ailerons. Sometimes termed the "aileron compensating wire." [53]
+
+=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 × displacement.
+
+=Wind-Screen=--A small transparent screen mounted in front of the pilot
+to protect his face from the air pressure.
+
+
+
+
+Types of Aeroplanes.
+
+
+
+
+[Illustration: Plate I.]
+
+The first machine to fly--of which there is anything like authentic
+record--was the Ader "Avion," after which the more notable advances were
+made as shown above.
+
+
+[Illustration: Plate II.]
+
+The Henri Farman was the first widely used aeroplane. Above are shown
+the chief steps in its development.
+
+
+[Illustration: Plate III.]
+
+THE AVRO.--The aeroplane designed and built by Mr. A. V. Roe was the
+first successful heavier-than-air flying machine built by a British
+subject. Mr. Roe's progress may be followed in the picture, from his
+early "canard" biplane, through various triplanes, with 35 J.A.P. and 35
+h.p. Green engines, to his successful tractor biplane with the same 35
+h.p. Green, thence through the "totally enclosed" biplane 1912, with 60
+h.p. Green, to the biplane 1913-14, with 80 h.p. Gnome.
+
+
+[Illustration: Plate IV.]
+
+THE SOPWITH LAND-GOING BIPLANES.--The earliest was a pair of Wright
+planes with a fuselage added. Next was the famous tractor with 80 h.p.
+Gnome. Then the "tabloid" of 1913, which set a completely new fashion
+in aeroplane design. From this developed the Gordon-Bennett racer shown
+over date 1914. The gun-carrier was produced about the same time, and
+the later tractor biplane in a development of the famous 80 h.p. but
+with 100 h.p. monosoupape Gnome.
+
+
+[Illustration: Plate V.]
+
+THE MAURICE FARMAN.--First, 1909, the 50-60 h.p. Renault and coil-spring
+chassis. 1910, the same chassis with beginning of the characteristic
+bent-up skids. 1911 appeared the huge French Military Trials 3-seater;
+also the round-ended planes and tails and "Henry" type wheels. This
+developed, 1912, into the square-ended planes and upper tail, and long
+double-acting ailerons of the British Military Trials. The 1913 type had
+two rectangular tail-planes and better seating arrangements, known
+affectionately as the "mechanical cow"; the same year came the first
+"shorthorn," with two tail-planes and a low nacelle. This finally
+developed into the carefully streamlined "shorthorn" with the raised
+nacelle and a single tail-plane.
+
+
+[Illustration: Plate VI.]
+
+THE SHORT "PUSHERS."--In 1909 came the semi-Wright biplane, with 35 h.p.
+Green, on which Mr. Moore-Brabazon won the "Daily Mail's" £1000 prize
+for the first mile flight on a circuit on a British aeroplane. Then the
+first box-kite flown by Mr. Grace at Wolverhampton. Later the famous
+"extension" type on which the first Naval officers learned to fly. Then
+the "38" type with elevator on the nacelle, on which dozens of R.N.A.S.
+pilots were taught.
+
+
+[Illustration: Plate VII.]
+
+SHORT TRACTORS, 1911-1912.--They were all co-existent, but the first was
+the "tractor-pusher" (bottom of picture). Then came the "twin-tractor
+plus propeller" (at top). A development was the "triple-tractor" (on the
+right), with two 50 h.p. Gnomes, one immediately behind the other under
+the cowl, one driving the two chains, the other coupled direct. Later
+came the single-engined 80 h.p. tractor (on the left), the original of
+the famous Short seaplanes.
+
+
+[Illustration: Plate VIII.]
+
+THE VICKERS MACHINES: First the Vickers-R.E.P. of 1911, which developed
+into the full-bodied No. V. with R.E.P. engine, then the Military Trials
+"sociable" with Viale engine, and so to the big No. VII with a 100 h.p.
+Gnome. Contemporary with the No. V and No. VI were a number of school
+box-kites of ordinary Farman type, which developed into the curious
+"pumpkin" sociable, and the early "gun 'bus" of 1913. Thence arrived the
+gun-carrier with 100 h.p. monosoupape Gnome.
+
+
+[Illustration: Plate IX.]
+
+THE BRISTOL AEROPLANES.--First, 1910, Farman type box-kites familiar to
+all early pupils. Then the miniature Maurice-Farman type biplane of the
+"Circuit of Britain." Contemporaneous was the "floating tail" monoplane
+designed by Pierre Prier, and after it a similar machine with fixed
+tail. Then came the handsome but unfortunate monoplane designed by
+M. Coanda for the Military Trials, 1912.
+
+
+[Illustration: Plate X.]
+
+THE BRISTOL TRACTORS.--Late 1912 came the round fuselaged tractor, with
+Gnome engine, designed by Mr. Gordon England for Turkey. 1912-13 came
+the biplane built onto the Military Trials monoplane type fuselage,
+also with a Gnome, designed by M. Coanda for Roumania. Then the
+Renault-engined Coanda tractor 1913, followed by 80 h.p. Gnome-engined
+scout, designed by Messrs. Barnwell and Busteed, which with Gnomes,
+le Rhones and Clergets, has been one of the great successes. Almost
+contemporary was the two-seater Bristol.
+
+
+[Illustration: Plate XI.]
+
+THE MARTINSYDES.--1909, first experimental monoplane built with small
+4-cylinder engine. J.A.P.-engined machine, 1910, followed by the
+Gnome-engined machine, 1911. 1912, first big monoplane with Antoinette
+engine was built, followed by powerful Austro-Daimler monoplane, 1913.
+Then came the little Gnome-engined scout biplanes, 1914, some with,
+some without, skids.
+
+
+[Illustration: Plate XII.]
+
+THE CURTISS BIPLANES.--In 1909 came the "June-bug," the united product
+of Glen Curtiss, Dr. Graham Bell, and J. A. D. McCurdy. Then the
+box-kite type, 1909, on which Mr. Curtiss won the Gordon-Bennett Race at
+Reims. Next the "rear-elevator" pusher, 1912, followed by first tractor,
+1913, with an outside flywheel. All purely Curtiss machines to that
+date had independent ailerons intended to get away from Wright patents.
+Following these came tractors with engines varying from 70 to 160 h.p.,
+fitted with varying types of chassis. All these have ordinary ailerons.
+
+
+[Illustration: Plate XIII.]
+
+THE BLERIOT (1).--The first engine-driven machine was a "canard"
+monoplane. Then came the curious tractor monoplanes 1908-1909, in
+order shown. Famous "Type XI" was prototype of all Bleriot successes.
+"Type XII" was never a great success, though the ancestor of the popular
+"parasol" type. The big passenger carrier was a descendant of this type.
+
+
+[Illustration: Plate XIV.]
+
+THE BLERIOT (2):--1910, "Type XI," on which Mr. Grahame-White won
+Gordon-Bennett Race, with a 14-cylinder 100 h.p. Gnome. 1911 came the
+improved "Type XI," with large and effective elevator flaps. On this
+type, with a 50 h.p. Gnome, Lieut. de Conneau (M. Beaumont) won
+Paris-Rome Race and "Circuit of Britain." Same year saw experimental
+"Limousine" flown by M. Legagneux, and fast but dangerous "clipped-wing"
+Gordon-Bennett racer with the fish-tail, flown by Mr. Hamel. About the
+same time came the fish-tailed side-by-side two-seater, flown by Mr.
+Hamel at Hendon and by M. Perreyon in 1912 Military Trials. 1911, M.
+Bleriot produced the 100 h.p. three-seater which killed M. Desparmets in
+French Military Trials. 1912-13, M. Bleriot produced a quite promising
+experimental biplane, and a "monocoque" monoplane in which the passenger
+faced rearward.
+
+
+[Illustration: Plate XV.]
+
+THE BLERIOT (3)--1912 tandem two-seater proved one of the best machines
+of its day. 1913 "canard" lived up to its name. A "pusher" monoplane was
+built in which the propeller revolved on the top tail boom. This machine
+came to an untimely end, with the famous pilot, M. Perreyon. 1912
+"tandem" was developed in 1914 into the type shown in centre; almost
+simultaneously "parasol" tandem appeared. 1914, M. Bleriot built a
+monoplane embodying a most valuable idea never fully developed. The
+engine tanks and pilot were all inside an armoured casing. Behind them
+the fuselage was a "monocoque" of three-ply wood bolted onto the armour.
+And behind this all the tail surfaces were bolted on as a separate unit.
+
+
+[Illustration: Plate XVI.]
+
+THE CAUDRON.--1910, came the machine with ailerons and a 28 h.p. Anzani.
+1911 this was altered to warp control and a "star" Anzani was fitted.
+From this came the 35 h.p. type of 1912, one of the most successful
+of school machines. Small fast monoplane, 1912, was never further
+developed. 1913 appeared the familiar biplanes with 80 h.p. Gnomes,
+and 5-seater with 100 h.p. Anzani for French "Circuit of Anjou." 1914
+produced the "scout" biplane which won at Vienna. 1915 appeared the
+twin-engined type, the first successful "battle-plane."
+
+
+[Illustration: Plate XVII.]
+
+THE DEPERDUSSIN.--In 1911 the little monoplane with a Gyp. engine.
+Then the Gnome-engined machine of the "Circuit of Europe." In 1912 came
+the Navy's machine with 70 h.p. Gnome, and Prevost's Gordon-Bennett
+"Bullet," 135 miles in the hour. The last was the British-built
+"Thunder-Bug," familiar at Hendon.
+
+
+[Illustration: Plate XVIII.]
+
+THE BREGUET.--First to fly was the complicated but business-like machine
+of 1909. Then came the record passenger carrier, 1910 (which lifted 8
+passengers). 1911 the French Military Trials machine with geared-down
+100 h.p. Gnome appeared. 1912 produced the machine with 130 h.p. Salmson
+engine on which the late Mr. Moorhouse flew the Channel with Mrs.
+Moorhouse and Mr. Ledeboer as passengers; also the machine with 130 h.p.
+horizontal Salmson, known as the "Whitebait." The last before the war
+was the rigid wing machine with 200 h.p. Salmson.
+
+
+[Illustration: Plate XIX.]
+
+THE CODY.--First the Military Experiment of 1908, with an Antoinette
+engine, then improved type 1909 with a Green engine. Next the
+"Cathedral," 1910, with a Green engine, which won Michelin Prize. In
+1911 "Daily Mail" Circuit machine, also with a Green, won the Michelin.
+This was modified into 1912 type which won Military Competition and
+£5,000 in prizes, with an Austro-Daimler engine, and later the Michelin
+Circuit Prize, again with a Green. 1912 the only Cody Monoplane was
+built. 1913 a modified biplane on which the great pioneer was killed.
+
+
+[Illustration: Plate XX.]
+
+THE NIEUPORT.--The first Nieuport of 1909 was curiously like a monoplane
+version of a Caudron. In 1910 came the little two-cylinder machine with
+fixed tail-plane and universally jointed tail. In 1911 the French Trials
+machine was built with 100 h.p. 14 cylinder Gnome, and is typical of
+this make. Also the little two-cylinder record breaker. A modification
+of 1913 was the height record machine of the late M. Legagneux.
+
+
+[Illustration: Plate XXI.]
+
+THE R.E.P. MONOPLANES.--First came the curious and highly interesting
+experiments of 1907, 1908, 1909, and 1910. 1910-1911, the World's
+Distance Record breaker was produced; after it, the "European Circuit,"
+all with R.E.P. engines. In 1913-14 came the French military type with
+Gnome engine and finally the "parasol," 1915.
+
+
+[Illustration: Plate XXII.]
+
+THE MORANE: First the European Circuit and Paris-Madrid type. Then the
+1912 types, with taper wing and modern type wing. The 1913 types, the
+"clipped wing," flown by the late Mr. Hamel, one of the standard tandem
+types now in use. About the same time came the "parasol." 1914-15 came
+a little biplane like a Nieuport, and the "destroyer" type with a round
+section body, flown by Vedrines.
+
+
+[Illustration: Plate XXIII.]
+
+THE VOISIN.--1908, the first properly controlled flight on a European
+aeroplane was made on a Voisin of the type shown with fixed engine.
+Then followed the record breaker of 1909 with a Gnome engine. In 1909
+also the only Voisin tractor was produced. 1910 the Paris-Bordeaux type
+was built; 1911 the amphibious "canard" and the "military" type with
+extensions, and the type without an elevator. 1913 came the type with
+only two tail-booms and a geared-down engine, which developed into the
+big "gun" machine with a Salmson engine.
+
+
+[Illustration: Plate XXIV.]
+
+THE HANRIOT AND PONNIER MONOPLANES.--In 1909 came the first Hanriot with
+50 h.p. 6-cylinder Buchet engine, and in 1910 the famous "Henrietta"
+type with E.N.Vs. and stationary Clergets. 1911 came the Clerget
+two-seater entered in French Military Trials, and 1912 the 100 h.p.
+Hanriot-Pagny monoplane which took part in British Military Trials.
+Sister machines of the same year were the single seater with 50 h.p.
+Gnome and the 100 h.p. Gnome racer with stripped chassis. In 1913 the
+Ponnier-Pagny racing monoplane with 160 h.p. Le Rhone competed in the
+Gordon-Bennett race, doing about 130 miles in the hour. The 60 h.p.
+Ponnier biplane was the first successful French scout tractor biplane.
+
+
+[Illustration: Plate XXV.]
+
+THE WRIGHT BIPLANE.--The first power flights were made, 1903, on a
+converted glider fitted with 16 h.p. motor. The prone position of the
+pilot will be noted. By 1907 the machine had become reasonably practical
+with 40 h.p. motor. On this the first real flying in the world was done.
+In 1910 the miniature racing Wright was produced; also the type with a
+rear elevator in addition to one in front. Soon afterwards the front
+elevator disappeared, and the machine became the standard American
+exhibition and school machine for four years. In 1915 a machine with
+enclosed fuselage was produced.
+
+
+[Illustration: Plate XXVI.]
+
+THE BLACKBURN MONOPLANES.--In 1909 was built the curious four-wheeled
+parasol-type machine with 35 h.p. Green engine and chain transmission,
+on which flying was done at Saltburn. In 1911 the Isaacson-engined
+machine was built, together with a 50 h.p. Gnome single-seater on which
+Mr. Hucks started in the Circuit of Britain race. In 1912 another 50
+h.p. single-seater was built on which a good deal of school work was
+done. A more advanced machine appeared in 1913 and a two-seater with
+80 h.p. Gnome did a great deal of cross-country work in 1913-14.
+
+
+[Illustration: Plate XXVII.]
+
+In 1908 the first Antoinette monoplane was produced by MM. Gastambide
+and Mengin. Then followed a machine with central skids, a single wheel,
+and wing skids. In 1909 came the machine with four-wheeled chassis and
+ailerons and later an improved edition which reverted to the central
+skid idea. On this M. Latham made his first cross-channel attempt.
+The next machine shed the wing skids and widened its wheelbase.
+During 1910-11 the ailerons vanished, warp control was adopted and the
+king-post system of wing-bracing was used. In 1911 the curious machine
+with streamlined "pantalette" chassis, totally enclosed body and
+internal wing-bracing, was produced for French Military Trials. In 1912
+the three-wheeled machine was used to a certain extent in the French
+Army. Then the type disappeared.
+
+
+[Illustration: Plate XXVIII.]
+
+In 1908 and 1909 detached experimental machines in various countries
+attained a certain success. The late Capt. Ferber made a primitive
+tractor biplane 1908. The Odier-Vendome biplane was a curious bat-winged
+pusher biplane built 1909. The tailless Etrich monoplane, built in
+Austria, 1908, was an adaptation of the Zanonia leaf. M. Santos-Dumont
+made primitive parasol type monoplanes known as "Demoiselles," in which
+bamboo was largely used. 1909 type is seen above. A curious steel
+monoplane was built by the late John Moisant, 1909. The twin-pusher
+biplane, built by the Barnwell Bros. in Scotland, made one or two
+straight flights in 1909. The Clement-Bayard Co. in France constructed
+in 1909 a biplane which did fairly well. Hans Grade, the first German
+to fly, made his early efforts on a "Demoiselle" type machine, 1908.
+
+
+[Illustration: Plate XXIX.]
+
+In 1910 a number of novel machines were produced. The Avis with Anzani
+engine was flown by the Hon. Alan Boyle. Note the cruciform universally
+jointed tail. The Goupy with 50 h.p. Gnome was an early French tractor,
+notable for its hinging wing-tips. The Farman was a curious "knock-up"
+job, chiefly composed of standard box-kite fittings. The Sommer with
+50 h.p. Gnome was a development of the box-kite with a shock-breaking
+chassis. The Savary, also French, was one of the first twin tractors to
+fly. The model illustrated had an E.N.V. engine. Note position of the
+rudders on the wing tips. The Austrian Etrich was the first successful
+machine of the Taube class ever built.
+
+
+[Illustration: Plate XXX.]
+
+INTERESTING MACHINES, 1910.--The Werner monoplane with E.N.V. engine,
+combined shaft and chain drive, was a variant of the de Pischoff. The
+Macfie biplane was a conventional biplane with 50 h.p. Gnome and useful
+originalities. The Valkyrie monoplane, another British machine, was a
+"canard" monoplane with propeller behind the pilot and in front of main
+plane. The Weiss monoplane was a good British effort at inherent
+stability. The Tellier monoplane was a modified Bleriot with Antoinette
+proportions. The Howard Wright biplane was a pusher with large lifting
+monoplane tail. The Dunne biplane was another British attempt at
+inherent stability. The Jezzi biplane was an amateur built
+twin-propeller.
+
+
+[Illustration: Plate XXXI.]
+
+SOME INTERESTING MACHINES, 1911.--The Compton-Paterson biplane was very
+similar to the early Curtiss pusher; it had a 50 h.p. Gnome. The Sloan
+bicurve was a French attempt at inherent stability with 50 h.p. Gnome
+and tractor screw. The Paulhan biplane was an attempt at a machine for
+military purposes to fold up readily for transport. The Sanders was
+a British biplane intended for rough service. The Barnwell monoplane
+was the first Scottish machine to fly; it had a horizontally opposed
+Scottish engine. The Harlan monoplane was an early German effort; note
+position of petrol tank.
+
+
+[Illustration: Plate XXXII.]
+
+The Clement-Bayard monoplane, 1911, was convertible into a tractor
+biplane. The standard engine was a 50 h.p. Gnome. The machine was
+interesting, but never did much. The Zodiac was one of the earliest
+to employ staggered wings. With 50 h.p. Gnome engine it was badly
+underpowered, so never did itself justice. The Jezzi tractor biplane,
+1911, was a development of an earlier model built entirely by Mr. Jezzi,
+an amateur constructor. With a low-powered J.A.P. engine it developed
+an amazing turn of speed, and it may be regarded as a forerunner of the
+scout type and the properly streamlined aeroplane. The Paulhan-Tatin
+monoplane, 1911, was a brilliant attempt at high speed for low power;
+it presented certain advantages as a scout. A 50 h.p. Gnome, fitted
+behind the pilot's seat in the streamlined fuselage, was cooled through
+louvres. The propeller at the end of the tail was connected with the
+engine by a flexible coupling. This machine was, in its day, the fastest
+for its power in the world, doing 80 miles per hour. Viking 1 was a twin
+tractor biplane driven by a 50 h.p. Gnome engine through chains. It was
+built by the author at Hendon in 1912.
+
+
+[Illustration: Plate XXXIII.]
+
+Much ingenuity was exerted by the French designers in 1911 to
+produce machines for the Military Trials. Among them was the 100 h.p.
+Gnome-Borel monoplane with a four-wheeled chassis, and the Astra
+triplane with a 75 h.p. Renault engine. This last had a surface of about
+500 square feet and presented considerable possibilities. Its principal
+feature was its enormous wheels with large size tyres as an attempt to
+solve difficulties of the severe landing tests. The Clement-Bayard
+biplane was a further development of the Clement-Bayard monoplane;
+the type represented could be converted into a monoplane at will. The
+Lohner Arrow biplane with the Daimler engine was an early German tractor
+biplane built with a view to inherent stability, and proved very
+successful. The Pivot monoplane was of somewhat unconventional French
+construction, chiefly notable for the special spring chassis and pivoted
+ailerons at the main planes; this pivoting had nothing to do with the
+name of the machine, which was designed by M. Pivot.
+
+
+[Illustration: Plate XXXIV.]
+
+The Flanders monoplane, 1912, with 70 h.p. Renault engine, was one of
+the last fitted with king-post system of wing bracing. The Flanders
+biplane entered for British Military Trials. Notable features: the
+highly staggered planes, extremely low chassis and deep fuselage. Also,
+the upper plane was bigger in every dimension than the lower; about
+the first instance of this practice. The Bristol biplane, with 100 h.p.
+Gnome engine, was also entered for the Trials, but ultimately withdrawn.
+The Mars monoplane, later known as the "D.F.W.," was a successful
+machine of Taube type with 120 h.p. Austro-Daimler engine. The building
+of the engine into a cowl, complete with radiator in front, followed car
+practice very closely. The tail of the monoplane had a flexible trailing
+edge; its angle of incidence could be varied from the pilot's seat,
+so that perfect longitudinal balance was attained at all loadings and
+speeds. The Handley-Page monoplane, with 70 h.p. Gnome engine, was an
+early successful British attempt at inherent stability.
+
+
+[Illustration: Plate XXXV.]
+
+The Sommer monoplane, with 50 h.p. Gnome, was a 1911-12 machine; it did
+a good deal of cross-country flying. The Vendome monoplane of 1912, also
+with 50 h.p. Gnome engine, was notable chiefly for its large wheels
+and jointed fuselage, which enabled the machine to be taken down for
+transport. The Savary biplane took part in the French Military Trials,
+1911. It had a four-cylinder Labor aviation motor. Notable features are
+twin chain-driven propellers, rudders between the main planes, the broad
+wheel-base and the position of the pilot. The Paulhan triplane, which
+also figured in the French Military Trials, was a development of the
+Paulhan folding biplane. It had a 70 h.p. Renault engine. For practical
+purposes it was a failure. The R.E.P. biplane, with 60 h.p. R.E.P.
+engine, was a development of the famous R.E.P. monoplanes. Its spring
+chassis, with sliding joints, marked an advance. Like the monoplanes,
+it was built largely of steel.
+
+
+[Illustration: Plate XXXVI.]
+
+In 1912 came the first really successful Handley-Page monoplane, with
+50 h.p. Gnome engine. The Short monoplane, was built generally on
+Bleriot lines. Its chassis was an original feature. The Coventry
+Ordnance biplane was a two-seater tractor built for the British Military
+Trials. It had a 100 h.p. 14-cylinder Gnome engine, with propeller
+geared down through a chain drive. The machine was an interesting
+experiment, but not an unqualified success. The Moreau "Aerostable,"
+fitted with a 50 h.p. Gnome, was a French attempt to obtain automatic
+stability, but it only operated longitudinally. The pilot's nacelle was
+pivoted under the main planes, wires were attached to the control
+members so that the movements of the nacelle in its efforts to keep a
+level keel brought them into operation. The Mersey monoplane, an entrant
+for the British Military Trials, was designed to present a clear field
+of view and fire. The 45 h.p. Isaacson engine was connected by a shaft
+to a propeller mounted behind the nacelle on the top tail boom. It was a
+promising experiment, but came to grief. The Radley-Moorhouse monoplane
+was a sporting type machine on Bleriot lines, with 50 h.p. Gnome engine.
+It was notable for its streamlined body and disc wheels.
+
+
+
+
+
+
+End of the Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
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+    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
+<title>The Project Gutenberg eBook of
+    The Aeroplane Speaks,
+    by H. Barber.
+</title>
+<style type="text/css">
+/*<![CDATA[*/
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+    span.pagenum  { position: absolute; left: 1%; right: 91%; font-size: 8pt; color: gray; background-color: inherit; text-indent: 0; display: none; }
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+    td { vertical-align: top; }
+    td.tr { vertical-align: top; text-align: right; }
+    td.br { vertical-align: bottom; text-align: right; }
+    .indent { margin-left: 15%; }
+    .gindent { margin-left: 3em; }
+    li { margin-left: 3em; text-indent: -1em; }
+    p.glossary { margin-left: 3em; text-indent: -3em; }
+    p.quote { font-size: 80%; }
+    span.glosref { text-align: right; margin-left: 1em; }
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+<body>
+
+
+<pre>
+
+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
+       Fifth Edition
+
+Author: H. Barber
+
+Release Date: June 10, 2007 [EBook #21791]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE AEROPLANE SPEAKS ***
+
+
+
+
+Produced by Jonathan Ingram, Marvin A. Hodges, David Garcia
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+
+<div style="height: 6em;"><br /><br /><br /><br /><br /><br /></div>
+
+<span class="pagenum"><a id="pagei" name="pagei"></a>[i]</span>
+
+<a name="image-0000"><!--IMG--></a>
+<div class="figure">
+<a href="images/frontis.jpg"><img src="images/frontist.jpg" width="400" height="270"
+alt="THE FLIGHT FOLK." /></a>
+<br />
+THE FLIGHT FOLK.
+</div>
+
+<h1>
+    THE AEROPLANE SPEAKS
+</h1>
+
+<h2>
+<small>BY</small><br />
+H. BARBER, <span class="sc">A.F.Ae.S.</span>
+</h2>
+
+<p class="center">
+(CAPTAIN, ROYAL FLYING CORPS)
+</p>
+
+<h4>
+WITH 36 FULL PAGES OF "TYPES OF AEROPLANES"<br />
+AND 87 SKETCHES AND DIAGRAMS
+</h4>
+
+<h4 style="padding: 3em 0em 3em 0em;">
+<i>FIFTH EDITION</i>
+</h4>
+
+<p class="center">
+LONDON<br />
+McBRIDE, NAST &amp; CO., LTD.
+</p>
+
+<p><span class="pagenum"><a id="pageiia" name="pageiia"></a>[iia]</span></p>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h3>THE AEROPLANE SPEAKS.</h3>
+
+<p class="center">
+  <i>First edition&mdash;December, 1916</i><br />
+  <i>Second edition&mdash;February, 1917</i><br />
+  <i>Third edition&mdash;April, 1917</i><br />
+  <i>Fourth edition&mdash;July, 1917</i><br />
+  <i>Fifth edition&mdash;December, 1917</i><br />
+</p>
+
+
+<div style="width: 67%; margin:auto; border: thin solid black; padding: 1em;">
+<p class="center">
+FIRST REVIEWS:
+</p>
+<p class="quote">
+<b>C. G. G. in the AEROPLANE:</b> "One hopes that the Subaltern
+Flying Officer will appreciate the gift which the author has
+given him out of his own vast store of experience, for the book
+contains the concentrated knowledge of many expensive years in
+tabloid form, or perhaps one should say in condensed milk form,
+seeing that it is easy to swallow and agreeable to the taste,
+as well as wholesome and nourishing. And, besides the young
+service aviator, there are thousands of young men, and women
+also, now employed in the aircraft industry, who will appreciate
+far better the value of the finicky little jobs they are doing
+if they will read this book and see how vital is their work to
+the man who flies."
+</p>
+<p class="quote">
+<b>THE FIELD:</b> "Entirely different from any other text-book on
+the subject, not merely in its form, but in its capacity to convey
+a knowledge of the principles and practice of flying. Undoubtedly
+it is the best book on its subject."
+</p>
+<p class="quote">
+<b>THE UNITED SERVICE GAZETTE:</b> "Should be in the hands of every
+person interested in aviation."
+</p>
+<p class="quote">
+<b>THE OUTLOOK:</b> "As amusing as it is instructive."
+</p>
+<p class="quote">
+<b>THE MORNING POST:</b> "Should be read and re-read by the would
+be and even the experienced pilot."
+</p>
+</div>
+
+<p class="center"><small>
+  PRINTED IN ENGLAND BY<br />
+  BILLING AND SONS, LIMITED<br />
+  GUILDFORD</small>
+</p>
+
+
+<p><span class="pagenum"><a id="pageii" name="pageii"></a>[ii]</span></p>
+
+<h3 style="padding-top: 3em;">
+DEDICATED<br />
+<small>TO THE</small><br />
+SUBALTERN FLYING OFFICER
+</h3>
+
+<p><span class="pagenum"><a id="pageiii" name="pageiii"></a>[iii]</span></p>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    MOTIVE
+</h2>
+<p>
+The reasons impelling me to write this book, the maiden effort of
+my pen, are, firstly, a strong desire to help the ordinary man to
+understand the Aeroplane and the joys and troubles of its Pilot; and,
+secondly, to produce something of <i>practical</i> 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.
+</p>
+<p>
+The matter contained in the Prologue appeared in the <i>Field</i> of May 6th,
+13th, 20th, and 27th, 1916, and is now reprinted by the kind permission
+of the editor, Sir Theodore Cook.
+</p>
+<p>
+I have much pleasure in also acknowledging the kindness of Mr. C. G.
+Grey, editor of the <i>Aeroplane</i>, to whom I am indebted for the valuable
+illustrations reproduced at the end of this book.
+</p>
+
+<p><span class="pagenum"><a id="pageiv" name="pageiv"></a>[iv]</span></p>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    CONTENTS
+</h2>
+
+<table border="0" align="center" summary="Table of Contents">
+
+<tr><td colspan="3" align="center"><i>PROLOGUE</i></td></tr>
+
+<tr><td></td><td></td><td><small>PAGE</small></td></tr>
+
+<tr><td class="tr" width="25%"><i>PART I.&mdash;</i></td><td><i>THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES</i></td>                  <td class="br"><a href="#h2H_4_0004">1</a></td></tr>
+<tr><td class="tr"><i>    II.&mdash;</i></td><td><i>THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB</i></td><td class="br"><a href="#h2H_4_0005">15</a></td></tr>
+<tr><td class="tr"><i>   III.&mdash;</i></td><td><i>THE GREAT TEST</i></td><td class="br"><a href="#h2H_4_0006">27</a></td></tr>
+<tr><td class="tr"><i>    IV.&mdash;</i></td><td><i>CROSS COUNTRY</i> </td><td class="br"><a href="#h2H_4_0007">38</a></td></tr>
+
+<tr><td colspan="3"><hr style="width: 33%;" /></td></tr>
+
+<tr><td class="tr">CHAPTER I.&mdash;</td><td>FLIGHT               </td><td class="br"><a href="#h2HCH0001">55</a></td></tr>
+<tr><td class="tr">       II.&mdash;</td><td>STABILITY AND CONTROL</td><td class="br"><a href="#h2HCH0002">70</a></td></tr>
+<tr><td class="tr">      III.&mdash;</td><td>RIGGING              </td><td class="br"><a href="#h2HCH0003">90</a></td></tr>
+<tr><td class="tr">       IV.&mdash;</td><td>PROPELLERS           </td><td class="br"><a href="#h2HCH0004">115</a></td></tr>
+<tr><td class="tr">        V.&mdash;</td><td>MAINTENANCE          </td><td class="br"><a href="#h2HCH0005">126</a></td></tr>
+
+<tr><td colspan="2">    TYPES OF AEROPLANES            </td><td class="br"><a href="#h2H_4_0015">130</a></td></tr>
+
+<tr><td colspan="2">    GLOSSARY                       </td><td class="br"><a href="#h2H_GLOS">133</a></td></tr>
+</table>
+
+<p><span class="pagenum"><a id="page1" name="page1"></a>[1]</span></p>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<a name="h2H_4_0004" id="h2H_4_0004"><!-- H2 anchor --></a>
+
+<h1>
+    THE AEROPLANE SPEAKS
+</h1>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    PROLOGUE
+<br />
+    PART I
+</h2>
+<h3>
+THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+</h3>
+<p>
+The Lecture Hall at the Royal Flying Corps School for Officers was
+deserted. The pupils had dispersed, and the Officer Instructor, more
+fagged than any pupil, was out on the aerodrome watching the test of
+a new machine.
+</p>
+<p>
+Deserted, did I say? But not so. The lecture that day had been upon
+the Elementary Principles of Flight, and they lingered yet. Upon the
+Blackboard was an illustration thus:
+</p>
+
+<a name="image-0001"><!--IMG--></a>
+<div class="figure">
+<a href="images/p001.jpg"><img src="images/p001-t.jpg" width="320" height="133"
+alt="" /></a>
+</div>
+
+<p>
+"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."
+</p>
+<p>
+"Quite right," said the Angle. "That's me, and I'm the famous Angle
+of Incidence."
+</p>
+<p>
+"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."
+</p>
+<p>
+"This is where I come in," a thick, gruff voice was
+
+<span class="pagenum"><a id="page2" name="page2"></a>[2]</span>
+
+ 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.
+</p>
+
+<a name="image-0002"><!--IMG--></a>
+<div class="figure">
+<a href="images/p002.jpg"><img src="images/p002-t.jpg" width="320" height="219"
+alt="The action of the surface upon the air." /></a>
+<br />
+The action of the surface upon the air.
+</div>
+
+<p>
+"I act in the direction of the arrow R, that is, more or less, for the
+direction varies somewhat with the Angle of Incidence and the curvature
+of the Surface; and, strange but true, I'm stronger on the top of the
+Surface than at the bottom of it. The Wind Tunnel has proved that by
+exhaustive research&mdash;and don't forget how quickly I can grow! As the
+speed through the air increases my strength increases more rapidly than
+you might think&mdash;approximately, as the Square of the Speed; so you see
+that if the Speed of the Surface through the air is, for instance,
+doubled, then I am a good deal more than doubled. That's because I am
+the result of not only the mass of air displaced, but also the result of
+the Speed and consequent Force with which the Surface engages the Air. I
+am a product of those two factors, and at the speeds at which Aeroplanes
+fly to-day, and at the altitudes and consequent density of air they at
+present experience, I increase at about the Square of the Speed.
+</p>
+<p>
+<span class="pagenum"><a id="page3" name="page3"></a>[3]</span>
+</p>
+<p>
+"Oh, I'm a most complex and interesting personality, I assure you&mdash;in
+fact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr.
+Hyde. There's Lift, my vertical part or <i>component</i>, 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."
+</p>
+
+<a name="image-0003"><!--IMG--></a>
+<div class="figure">
+<a href="images/p003.jpg"><img src="images/p003-t.jpg" width="320" height="256"
+alt="" /></a>
+</div>
+
+<p>
+"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
+or Weight, then&mdash;there you are&mdash;Flight! And nothing mysterious about
+it at all."
+</p>
+<p>
+"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."
+</p>
+<p>
+"Well," eagerly replied the Lift and the Thrust, "let's
+
+<span class="pagenum"><a id="page4" name="page4"></a>[4]</span>
+
+ 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."
+</p>
+<p>
+"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."
+</p>
+
+<a name="image-0004"><!--IMG--></a>
+<div class="figure">
+<a href="images/p004.jpg"><img src="images/p004-t.jpg" width="320" height="238"
+alt="" /></a>
+</div>
+
+<p>
+"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.
+</p>
+<p>
+"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>i.e.</i>, distance
+from wing-tip to wing-tip, it then engages more
+
+<span class="pagenum"><a id="page5" name="page5"></a>[5]</span>
+
+ air and produces both
+a maximum Reaction and a better proportion of Lift to Drift.
+</p>
+<p>
+"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 <i>chord</i>, as they call it. That's all I require,
+I assure you, to make me quite perfect and of infinite service to Miss
+Efficiency."
+</p>
+
+<a name="image-0005"><!--IMG--></a>
+<div class="figure">
+<a href="images/p005.jpg"><img src="images/p005-t.jpg" width="320" height="143"
+alt="" /></a>
+</div>
+
+<p>
+"That's not practical politics," said the Surface. "The way you talk one
+would think you were drawing £400 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."
+</p>
+<p>
+"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."
+</p>
+<p>
+<span class="pagenum"><a id="page6" name="page6"></a>[6]</span>
+</p>
+<p>
+"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."
+</p>
+<p>
+"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."
+</p>
+<p>
+"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."
+</p>
+<p>
+"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:
+</p>
+
+<a name="image-0006"><!--IMG--></a>
+<div class="figure">
+<a href="images/p006.jpg"><img src="images/p006-t.jpg" width="320" height="116"
+alt="" /></a>
+</div>
+
+<p>
+"Well, what do you think of that?" they all cried to the Drift.
+</p>
+<p>
+"You think you are very clever," sneered the Drift. "But you are not
+helping Efficiency as much as you think.
+
+<span class="pagenum"><a id="page7" name="page7"></a>[7]</span>
+
+ 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."
+</p>
+<p>
+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:
+</p>
+
+<a name="image-0007"><!--IMG--></a>
+<div class="figure">
+<a href="images/p008.jpg"><img src="images/p008-t.jpg" width="320" height="111"
+alt="" /></a>
+</div>
+
+<p>
+Said the Blackboard, "That's not half bad! It really begins to look
+something like the real thing, eh?"
+</p>
+<p>
+"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.
+</p>
+<p>
+<span class="pagenum"><a id="page8" name="page8"></a>[8]</span>
+</p>
+<p>
+"Oh, dear! Oh, dear!" she cried. "I'm always getting into trouble. What
+<i>will</i> the Designer say?"
+</p>
+<p>
+"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."
+</p>
+<p>
+"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?"
+</p>
+<p>
+"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 <i>slow</i> 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 <i>large
+Surface</i> in order to secure a large lift relative to the weight to be
+carried. We shall also require a <i>large Angle of Incidence</i> 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 <i>large Camber</i>.
+</p>
+<p>
+"On the other hand, if we are thinking merely of Speed, then a <i>small
+Surface</i>, just enough to lift the weight off the ground, will be best;
+also a <i>small Angle</i> to cut the Drift down, and that, of course, means
+a relatively <i>small Camber</i>.
+</p>
+<p>
+<span class="pagenum"><a id="page9" name="page9"></a>[9]</span>
+</p>
+<p>
+"So you see the essentials for <i>Climb</i> or quick ascent and for <i>Speed</i>
+are diametrically opposed. Now which is it to be?"
+</p>
+<p>
+"Nothing but perfection for me," said Efficiency. "What I want is
+Maximum Climb and Maximum Speed for the Power the Engine produces."
+</p>
+<p>
+And each Principle fully agreed with her beautiful sentiments, but work
+together they would not.
+</p>
+<p>
+The Aspect Ratio wanted infinite Span, and hang the Chord.
+</p>
+
+<a name="image-0008"><!--IMG--></a>
+<div class="figure">
+Maximum Climb.<br />
+<a href="images/p009.jpg"><img src="images/p009-t.jpg" width="320" height="229"
+alt="" /></a>
+<br />
+Maximum Speed.
+</div>
+
+<p>
+The Angle of Incidence would have two Angles and two Cambers in one,
+which was manifestly absurd; the Surface insisted upon no thickness
+whatever, and would not hear of such things as Spars and Ribs; and the
+Thrust objected to anything at all likely to produce Drift, and very
+nearly wiped the whole thing off the Blackboard.
+</p>
+<p>
+There was, indeed, the makings of a very pretty quarrel when the Letter
+arrived. It was about a mile long, and began to talk at once.
+</p>
+<p>
+"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,
+
+<span class="pagenum"><a id="page10" name="page10"></a>[10]</span>
+
+ <span class="sc">vary</span> the area of the Surface, the
+Angle of Incidence, and the Camber! And there you are&mdash;Maximum Climb or
+Maximum Speed as required! How does that suit you?"
+</p>
+<p>
+"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?"
+</p>
+<p>
+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&mdash;&mdash;"
+</p>
+<p>
+"Look here," said a Strut, rather pointedly, "where do you think you
+are going, anyway?"
+</p>
+<p>
+"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."
+</p>
+<p>
+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.
+</p>
+<p>
+And a League was formed, and there were Directors with Fees, and several
+out-of-service Tin Hats, and the Man-who-takes-the-credit, and a fine
+fat Guinea-pig, and all the rest of them. And the Inventor paid his
+Tailor and had a Hair-Cut, and is now a recognized <i>Press</i> Expert&mdash;but
+he is still waiting for those Mechanics!
+</p>
+<p>
+"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."
+</p>
+<p>
+Thud! What was that?
+</p>
+<p>
+Efficiency had fainted dead away! The last blow had been too much for
+her. And the Principles gathered mournfully round, but with the aid of
+the Propeller Slip<a href="#note-1" name="noteref-1"><small>1</small></a> and a
+
+<span class="pagenum"><a id="page11" name="page11"></a>[11]</span>
+
+   friendly lift from the Surface she was at
+length revived and regained a more normal aspect.
+</p>
+<p>
+Said the Stagger with a raffish air, "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."
+</p>
+<p>
+"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."
+</p>
+
+<a name="image-0009"><!--IMG--></a>
+<div class="figure">
+<a href="images/p011.jpg"><img src="images/p011-t.jpg" width="320" height="195"
+alt="" /></a>
+</div>
+
+<p>
+So the Chalk drew a nice long slim body to hold the Engine and the
+tanks, etc., with room for the Pilot's and Passenger's seats, and placed
+it exactly in the middle of the Biplane. And he was careful to make its
+position such that the Centre of Gravity was a little in advance of the
+Centre of Lift, so that when the Engine was not running and there was
+consequently no Thrust, the Aeroplane should be "nose-heavy" just to the
+right degree, and so take up a natural glide to Earth&mdash;and this was to
+help the Pilot and relieve him of work and worry, should he find himself
+in a fog or a cloud. And so that this tendency to glide downwards should
+not be in evidence when the Engine was running and descent not desired,
+the Thrust was placed a little below the Centre of Drift or Resistance.
+In this way it would in
+
+<span class="pagenum"><a id="page12" name="page12"></a>[12]</span>
+
+  a measure pull the nose of the Aeroplane up and
+counter-balance the "nose-heavy" tendency.
+</p>
+<p>
+And the Engine was so mounted that when the Propeller-Thrust was
+horizontal, which is its most efficient position, the Angle of Incidence
+and the Area of the surfaces were just sufficient to give a Lift a
+little in excess of the Weight. And the Camber was such that, as far as
+it was concerned, the Lift-Drift Ratio should be the best possible for
+that Angle of Incidence. And a beautifully simple under-carriage was
+added, the outstanding features of which were simplicity, strength,
+light-weight, and minimum drift. And, last of all, there was the
+Elevator, of which you will hear more by-and-by. And this is what
+it looked like then:
+</p>
+
+<a name="image-0010"><!--IMG--></a>
+<div class="figure">
+<a href="images/p012.jpg"><img src="images/p012-t.jpg" width="320" height="90"
+alt="" /></a>
+</div>
+
+<p>
+And Efficiency, smiling, thought that it was not such a bad compromise
+after all, and that the Designer might well be satisfied.
+</p>
+<p>
+"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?"
+</p>
+<p>
+"Yes, indeed," spoke up the Propeller, "though it means that I must
+assume a most undignified attitude, for helicopters<a href="#note-2" name="noteref-2"><small>2</small></a> I never approved
+of. In order to ascend more quickly the Pilot will deflect the Elevator,
+which, by the way, you see hinged to the Tail. By that means he will
+force the whole Aeroplane to assume a greater Angle of Incidence. And
+with greater Angle, the Lift will increase,
+
+<span class="pagenum"><a id="page13" name="page13"></a>[13]</span>
+
+ 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."
+</p>
+
+<a name="image-0011"><!--IMG--></a>
+<div class="figure">
+<a href="images/p013.jpg"><img src="images/p013-t.jpg" width="320" height="205"
+alt="The angles shown above are only roughly approximate, as they vary with different types of aeroplanes." /></a>
+<br />
+The angles shown above are only roughly approximate, as they vary with different types of aeroplanes.
+</div>
+
+<p>
+"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."
+</p>
+<p>
+"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,
+
+<span class="pagenum"><a id="page14" name="page14"></a>[14]</span>
+
+ <i>i.e.</i>, less than the Weight, and then&mdash;what
+price pancakes.<a href="#note-3" name="noteref-3"><small>3</small></a> Eh?"
+</p>
+<p>
+"Thank you," from Efficiency, "that was all most informing. And now will
+you tell me, please, how the greatest Speed may be secured?"
+</p>
+<p>
+"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."
+</p>
+<p>
+"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."
+</p>
+<p>
+"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?"
+</p>
+<p>
+"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."
+</p>
+<a name="note-1"><!--Note--></a>
+<p class="foot">
+<u>1</u> (<a href="#noteref-1">return</a>)<br />
+Propeller Slip: As the propeller screws through the air,
+the latter to a certain extent gives back to the thrust of the propeller
+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 propeller
+will feel the slip as a strong draught of air.
+</p>
+<a name="note-2"><!--Note--></a>
+<p class="foot">
+<u>2</u> (<a href="#noteref-2">return</a>)<br />
+Helicopter: An air-screw revolving upon a vertical axis.
+If driven with sufficient power, it will lift vertically, but, having
+regard to the mechanical difficulties of such construction, it is a
+most inefficient way of securing lift compared with the arrangement
+of an inclined surface driven by a propeller revolving about a
+horizontal axis.
+</p>
+<a name="note-3"><!--Note--></a>
+<p class="foot">
+<u>3</u> (<a href="#noteref-3">return</a>)<br />
+Pancakes: Pilot's slang for stalling an aeroplane and
+dropping like a pancake.
+</p>
+<p>
+<span class="pagenum"><a id="page15" name="page15"></a>[15]</span>
+</p>
+<a name="h2H_4_0005" id="h2H_4_0005"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    PART II
+</h2>
+<h3>
+    THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
+</h3>
+<p>
+Another day had passed, and the Flight Folk had again gathered together
+and were awaiting the arrival of Efficiency who, as usual, was rather
+late in making an appearance.
+</p>
+<p>
+The crowd was larger than ever, and among the newcomers some of the most
+important were the three Stabilities, named Directional, Longitudinal,
+and Lateral, with their assistants, the Rudder, Elevator, and Ailerons.
+There was Centrifugal Force, too, who would not sit still and created a
+most unfavourable impression, and Keel-Surface, the Dihedral Angle, and
+several other lesser fry.
+</p>
+<p>
+"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&mdash;the more the
+better."
+</p>
+<p>
+"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."
+</p>
+<p>
+"Well, we shall see what we shall see," said the Force darkly. "But who
+in the name of blue sky is this?"
+</p>
+<p>
+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,<a href="#note-4" name="noteref-4"><small>4</small></a> 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,<a href="#note-5" name="noteref-5"><small>5</small></a> and insisted upon me
+changing my fabric for one of a smoother
+
+<span class="pagenum"><a id="page16" name="page16"></a>[16]</span>
+
+ 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.
+</p>
+<p>
+"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."
+</p>
+
+<a name="image-0012"><!--IMG--></a>
+<div class="figure">
+<a href="images/p016.jpg"><img src="images/p016-t.jpg" width="320" height="279"
+alt="" /></a>
+</div>
+
+<p>
+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&mdash;the sides of the body, struts, wires,
+etc."
+</p>
+<p>
+"Oh, now I begin to see light," said she; "but just exactly how does
+it work?"
+</p>
+<p>
+"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
+
+<span class="pagenum"><a id="page17" name="page17"></a>[17]</span>
+
+ 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."
+</p>
+<p>
+"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."
+</p>
+<p>
+"I see," said Efficiency, and, daintily holding the Chalk, she
+approached the Blackboard. "Is this what you mean?"
+</p>
+<p>
+"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."
+</p>
+<p>
+"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&mdash;probable result a bad side-slip" (see illustration A,
+over-leaf).
+</p>
+<p>
+"And what can the Pilot do to save such a situation as that?" said
+Efficiency.
+</p>
+<p>
+"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
+
+<span class="pagenum"><a id="page18" name="page18"></a>[18]</span>
+
+ 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."
+</p>
+
+<a name="image-0013"><!--IMG--></a>
+<div class="figure">
+<a href="images/p018.jpg"><img src="images/p018-t.jpg" width="320" height="413"
+alt="" /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page19" name="page19"></a>[19]</span>
+</p>
+<p>
+"Ah!" said the Rudder, looking wise, "it's in a case like that when I
+become the Elevator and the Elevator becomes me."
+</p>
+<p>
+"That's absurd nonsense," said the Blackboard, "due to looseness of
+thought and expression."
+</p>
+<p>
+"Well," replied the Rudder, "when the Aeroplane is in position A and I
+am used, then I depress or <i>elevate</i> 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 <i>rôles</i> have changed one with
+the other, and I'm then the Elevator and the Elevator is me!"
+</p>
+
+<a name="image-0014"><!--IMG--></a>
+<div class="figure">
+<a href="images/p019.jpg"><img src="images/p019-t.jpg" width="320" height="202"
+alt="" /></a>
+</div>
+
+<p>
+Said Lateral Stability to the Rudder, "That's altogether the wrong way
+of looking at it, though I admit"&mdash;and this rather sarcastically&mdash;"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 a certain axis of the
+machine, and the Elevator is a Controlling Surface designed to turn the
+Aeroplane about another 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."
+</p>
+<p>
+"Thanks," said Efficiency to Lateral Stability. "And now, please, will
+you explain your duties?"
+</p>
+<p>
+<span class="pagenum"><a id="page20" name="page20"></a>[20]</span>
+</p>
+<p>
+"My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.
+First of all, I sometimes arrange with the Rigger to <i>wash-out</i>, 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."
+</p>
+<p>
+"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?"
+</p>
+<p>
+"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."
+</p>
+<p>
+"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.
+</p>
+<p>
+"Well," he replied, rather miserably, "I'm not nearly so perfect as the
+Longitudinal and Directional Stabilities. The Dihedral Angle&mdash;that is,
+the upward inclination of the Surfaces towards their wing-tips&mdash;does
+what it can for me, but, in my opinion, it's a more or less futile
+effort. The Blackboard will show you the argument." And he at once
+showed them two Surfaces, each set at a Dihedral Angle like this:
+</p>
+
+<a name="image-0015"><!--IMG--></a>
+<div class="figure">
+<a href="images/p020.jpg"><img src="images/p020-t.jpg" width="320" height="220"
+alt="H.E., Horizontal equivalent." /></a>
+<br />
+H.E., Horizontal equivalent.
+</div>
+
+<p>
+<span class="pagenum"><a id="page21" name="page21"></a>[21]</span>
+</p>
+<p>
+"Please imagine," said the Blackboard, "that the top <b>V</b> is the front
+view of a Surface flying towards you. Now if a gust blows it into the
+position of the lower <b>V</b> 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 <b>V</b> 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."
+</p>
+<p>
+"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 total 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 <b>V</b> 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."
+</p>
+<p>
+And Efficiency, blushing very prettily at the compliment, then asked,
+"And how does the Centre of Gravity affect matters?"
+</p>
+<p>
+"That's easy," said Grandfather Gravity. "I'm so heavy that if I am too
+low down I act like a pendulum
+
+<span class="pagenum"><a id="page22" name="page22"></a>[22]</span>
+
+ and cause the Aeroplane to roll about
+sideways, and if I am too high I'm like a stick balanced on your finger,
+and then if I'm disturbed, over I go and the Aeroplane with me; and, in
+addition to that, there are the tricks I play with the Aeroplane when
+it's banked up,<a href="#note-6" name="noteref-6"><small>6</small></a> <i>i.e.</i>, 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.
+</p>
+<p>
+And the Blackboard had been busy, and now showed them a picture of the
+Aeroplane as far as they knew it, and you will see that there is a
+slight Dihedral Angle, and also, fixed to the tail, a vertical Keel
+Surface or <i>fin</i>, as is very often the case in order to ensure the
+greater effect of such surface being behind the vertical turning axis.
+</p>
+
+<a name="image-0016"><!--IMG--></a>
+<div class="figure">
+<a href="images/p022.jpg"><img src="images/p022-t.jpg" width="320" height="118"
+alt="" /></a>
+</div>
+
+<p>
+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!"
+</p>
+<p>
+"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 <i>in effect</i> at a much less Angle of
+Incidence than the Main Surface.
+
+<span class="pagenum"><a id="page23" name="page23"></a>[23]</span>
+
+ To explain we must trouble the
+Blackboard again," and this was his effort:
+</p>
+
+<a name="image-0017"><!--IMG--></a>
+<div class="figure">
+<a href="images/p023.jpg"><img src="images/p023-t.jpg" width="320" height="220"
+alt="" /></a>
+</div>
+
+<p>
+"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."
+</p>
+<p>
+"I'm afraid I'm very stupid," said Efficiency, "but please tell me why
+you lay stress upon the words '<i>in effect</i>.'"
+</p>
+<p>
+<span class="pagenum"><a id="page24" name="page24"></a>[24]</span>
+</p>
+<p>
+"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 <i>in effect</i> a less Angle of
+Incidence. I'll just show you on the Blackboard."
+</p>
+
+<a name="image-0018"><!--IMG--></a>
+<div class="figure">
+<a href="images/p024.jpg"><img src="images/p024-t.jpg" width="320" height="219"
+alt="" /></a>
+</div>
+
+<p>
+"And now," said Efficiency, "I have only to meet the Ailerons and the
+Rudder, haven't I?"
+</p>
+<p>
+"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
+
+<span class="pagenum"><a id="page25" name="page25"></a>[25]</span>
+
+ Incidence, and that side will then get a
+reverse Lift or downward thrust, and so we are able to tilt the
+Aeroplane sideways.
+</p>
+<p>
+"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
+<i>washed-out</i> towards the Wing-tips. The reason of that is that by the
+time the air reaches us it has been deflected downwards&mdash;the greater the
+Angle of Incidence the more it is driven downwards&mdash;and in order for
+us to secure a Reaction from it, we have to take such a large Angle of
+Incidence that we produce a poor proportion of Lift to Drift; but the
+smaller the Angle of the Surface in front of us the less the air is
+deflected downwards, and consequently the less Angle is required of us,
+and the better our proportion of Lift to Drift, which, of course, makes
+us much more effective Controls."
+</p>
+
+<a name="image-0019"><!--IMG--></a>
+<div class="figure">
+<a href="images/p025.jpg"><img src="images/p025-t.jpg" width="320" height="249"
+alt="'Wash out' on both sides." /></a>
+<br />
+"Wash out" on both sides.
+</div>
+
+<p>
+"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
+
+<span class="pagenum"><a id="page26" name="page26"></a>[26]</span>
+
+ means decreased Lift as well as Drift, and the
+Designer does not always care to pay the price."
+</p>
+<p>
+"Well," said the Ailerons, "if it's not done it will mean more work for
+the Rudder, and that won't please the Pilot."
+</p>
+<p>
+"Whatever do you mean?" asked Efficiency. "What can the Rudder have to
+do with you?"
+</p>
+<p>
+"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 <i>suction</i> effect (though that term is not academically correct)
+on the top of the Surface. If there is more Drift, <i>i.e.</i>, 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."
+</p>
+<p>
+"I think, then," said Efficiency, "I should prefer to have that
+wash-out,<a href="#note-7" name="noteref-7"><small>7</small></a> and my friend the Designer is so clever at producing
+strength of construction for light weight, I'm pretty sure he won't mind
+paying the price in Lift. And now let me see if I can sketch the
+completed Aeroplane."
+</p>
+
+<a name="image-0020"><!--IMG--></a>
+<div class="figure">
+<a href="images/p026.jpg"><img src="images/p026-t.jpg" width="320" height="102"
+alt="" /></a>
+</div>
+
+<p>
+"Well, I hope that's all as it should be," she concluded, "for to-morrow
+the Great Test in the air is due."
+</p>
+<a name="note-4"><!--Note--></a>
+<p class="foot">
+<u>4</u> (<a href="#noteref-4">return</a>)<br />
+Morane parasol: A type of Morane monoplane in which the
+lifting surfaces are raised above the pilot in order to afford him a
+good view of the earth.
+</p>
+<a name="note-5"><!--Note--></a>
+<p class="foot">
+<u>5</u> (<a href="#noteref-5">return</a>)<br />
+Skin friction is that part of the drift due to the friction
+of the air with roughness upon the surface of the aeroplane.
+</p>
+<a name="note-6"><!--Note--></a>
+<p class="foot">
+<u>6</u> (<a href="#noteref-6">return</a>)<br />
+Banking: When an aeroplane is turned to the left or the
+right the centrifugal force of its momentum causes it to skid sideways
+and outwards away from the centre of the turn. To minimize such action
+the pilot banks, <i>i.e.</i>, tilts, the aeroplane sideways in order to
+oppose the underside of the planes to the air. The aeroplane will not
+then skid outwards beyond the slight skid necessary to secure a
+sufficient pressure of air to balance the centrifugal force.
+</p>
+<a name="note-7"><!--Note--></a>
+<p class="foot">
+<u>7</u> (<a href="#noteref-7">return</a>)<br />
+An explanation of the way in which the wash-out is combined
+with a wash-in to offset propeller torque will be found on p. 82.
+</p>
+<p>
+<span class="pagenum"><a id="page27" name="page27"></a>[27]</span>
+</p>
+<a name="h2H_4_0006" id="h2H_4_0006"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    PART III
+</h2>
+<h3>
+    THE GREAT TEST
+</h3>
+<p>
+It is five o'clock of a fine calm morning, when the Aeroplane is wheeled
+out of its shed on to the greensward of the Military Aerodrome. There is
+every promise of a good flying day, and, although the sun has not yet
+risen, it is light enough to discern the motionless layer of fleecy
+clouds some five thousand feet high, and far, far above that a few filmy
+mottled streaks of vapour. Just the kind of morning beloved of pilots.
+</p>
+<p>
+A brand new, rakish, up-to-date machine it is, of highly polished,
+beautifully finished wood, fabric as tight as a drum, polished metal,
+and every part so perfectly "stream-lined" to minimize drift, which is
+the resistance of the air to the passage of the machine, that to the
+veriest tyro the remark of the Pilot is obviously justified.
+</p>
+<p>
+"Clean looking 'bus, looks almost alive and impatient to be off. Ought
+to have a turn for speed with those lines."
+</p>
+<p>
+"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."
+</p>
+<p>
+The A.M.'s<a href="#note-8" name="noteref-8"><small>8</small></a> have now placed the Aeroplane in position facing the
+gentle air that is just beginning to make itself evident; the engine
+Fitter, having made sure of a sufficiency of oil and petrol in the
+tanks, is standing by the Propeller; the Rigger, satisfied with a job
+well done, is critically "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.
+</p>
+<p>
+As the Pilot approaches the Aeroplane the Rigger springs
+
+<span class="pagenum"><a id="page28" name="page28"></a>[28]</span>
+
+ 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.
+</p>
+<p>
+The air of this early dawn is distinctly chilly, and the A.M.'s are
+beginning to stamp their cold feet upon the dewy grass, but very careful
+and circumspect is the Pilot, as he mutters to himself, "Don't worry and
+flurry, or you'll die in a hurry."
+</p>
+<p>
+At last he fumbles for his safety belt, but with a start remembers the
+Pitot Air Speed Indicator, and, adjusting it to zero, smiles as he
+hears the Pitot-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 Pitot Tube,
+whose mouth is open to the air to receive its pressure, stammer,
+"Oh Lor! I've got an earwig already&mdash;hope to goodness the Rigger blows
+me out when I come down&mdash;and this morning air simply fills me with
+moisture; I'll never keep the Liquid steady in the Gauge. I'm not sure
+of my rubber connections either."
+</p>
+<p>
+"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."
+</p>
+<p>
+"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
+
+<span class="pagenum"><a id="page29" name="page29"></a>[29]</span>
+
+ are at their ends bent over in the form of loops in
+order to connect with the metal fittings on the spars and elsewhere&mdash;a
+cheap and easy way of making connection.
+</p>
+<p>
+"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,"<a href="#note-9" name="noteref-9"><small>9</small></a> 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."
+</p>
+<p>
+"And who on earth are they?" asked the Loops, trembling for their
+troublesome little lives.
+</p>
+<p>
+"On 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&mdash;but all the same the Designer will get his way in the end. I'm
+his Best Girl, you know, and if we could only get rid of the Directors,
+the little Tin god, and the Man-who-takes-the-credit, we should be quite
+happy."
+</p>
+<p>
+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, <span class="sc">BOOM</span>! Nonsense! It <span class="sc">MUST</span> 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 <i>him</i>, and so there you are,
+and so poor beautiful Miss Efficiency was compromised.
+</p>
+<p>
+All this time the Pilot is carefully buckling his belt and making
+himself perfectly easy and comfortable, as all good pilots do. As he
+straightens himself up from a careful
+
+<span class="pagenum"><a id="page30" name="page30"></a>[30]</span>
+
+ inspection of the Deviation
+Curve<a href="#note-10" name="noteref-10"><small>10</small></a> of the Compass and takes command of the Controls, the Throttle
+and the Ignition, the voices grow fainter and fainter until there is
+nothing but a trembling of the Lift and Drift wires to indicate to his
+understanding eye their state of tension in expectancy of the Great
+Test.
+</p>
+<p>
+"Petrol on?" shouts the Fitter to the Pilot.
+</p>
+<p>
+"Petrol on," replies the Pilot.
+</p>
+<p>
+"Ignition off?"
+</p>
+<p>
+"Ignition off."
+</p>
+<p>
+Round goes the Propeller, the Engine sucking in the Petrol Vapour with
+satisfied gulps. And then&mdash;
+</p>
+<p>
+"Contact?" from the Fitter.
+</p>
+<p>
+"Contact," says the Pilot.
+</p>
+<p>
+Now one swing of the Propeller by the Fitter, and the Engine is awake
+and working. Slowly at first though, and in a weak voice demanding, "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."
+</p>
+<p>
+Faster and faster turn the Engine and Propeller, and the Aeroplane,
+trembling in all its parts, strains to jump the blocks and be off.
+Carefully the Pilot listens to what the Engine Revolution Indicator
+says. At last, "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.
+</p>
+<p>
+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.
+</p>
+<p>
+"Ah!" says the Wind-screen, "that's Discipline, that
+
+<span class="pagenum"><a id="page31" name="page31"></a>[31]</span>
+
+ is. Through my
+little Triplex 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."
+</p>
+<p>
+"Discipline is it?" complains the Under-carriage, as its wheels roll
+swiftly over the rather rough ground. "I'm <i>bump</i> getting it, and <i>bump</i>,
+<i>bump</i>, all I want, <i>bang</i>, <i>bump</i>, <i>rattle</i>, 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.
+</p>
+<p>
+Below is left the Earth with all its bumps and troubles. Up into the
+clean clear Air moves with incredible speed and steadiness this triumph
+of the Designer, the result of how much mental effort, imagination,
+trials and errors, failures and successes, and many a life lost in high
+endeavour.
+</p>
+<p>
+Now is the mighty voice of the Engine heard as he turns the Propeller
+nine hundred times a minute. Now does the Thrust fight the Drift for all
+it's worth, and the Air Speed Indicator gasps with delight "One hundred
+miles an hour!"
+</p>
+<p>
+And now does the burden of work fall upon the Lift and Drift Wires, and
+they scream to the Turnbuckles whose business it is to hold them in
+tension, "This is the limit! the Limit! <span class="sc">The Limit</span>! 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.
+</p>
+<p>
+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.
+</p>
+<p>
+And the Fabric's quite mixed in its mind, and ejaculates,
+
+<span class="pagenum"><a id="page32" name="page32"></a>[32]</span>
+
+ "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&mdash;&mdash; Yes, Irish,
+I said. I used to come from Egypt, but I've got naturalized since the
+War began."
+</p>
+<p>
+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.
+</p>
+<p>
+"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."
+</p>
+<p>
+And the Drift has hopes of reaching the Maximum Angle of Incidence and
+vanquishing the Thrust and the Lift. And he grows very bold as he
+strangles the Thrust; but the situation is saved by the Propeller, who
+is now bravely helicopting skywards, somewhat to the chagrin of
+Efficiency.
+</p>
+<p>
+"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.
+</p>
+<p>
+And lo! the Aeroplane has almost reached the clouds! But what's this? A
+sudden gust, and down sinks one wing and up goes the other. "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
+
+<span class="pagenum"><a id="page33" name="page33"></a>[33]</span>
+
+ 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."
+</p>
+<p>
+But the ever-watchful Pilot instantly depresses one aileron, elevating
+the other, with just a touch of the rudder to keep on the course, and
+the Planes welcome back their precious Lift as the Aeroplane flicks back
+to its normal position.
+</p>
+<p>
+"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.
+</p>
+<p>
+"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, crowned with white filmy lace-like
+streaks of vapour. And withal the increasing sense with altitude of
+vast, clean, silent solitudes of space.
+</p>
+<p>
+Lives there the man who can adequately describe this Wonder? "Never,"
+says the Pilot, who has seen it many times, but to whom it is ever new
+and more wonderful.
+</p>
+<p>
+Up, up, up, and still up, unfalteringly speeds the Pilot and his mount.
+Sweet the drone of the Engine and steady the Thrust as the Propeller
+exultingly battles with the Drift.
+</p>
+<p>
+And look! What is that bright silver streak all along the horizon? It
+puzzled the Pilot when first he saw it, but now he knows it for the Sea,
+full fifty miles away!
+</p>
+<p>
+And on his right is the brightness of the morn and the smiling Earth
+unveiling itself to the ardent rays of the Sun; and on his left, so high
+is he, there is yet black night, hiding
+
+<span class="pagenum"><a id="page34" name="page34"></a>[34]</span>
+
+ innumerable Cities, Towns,
+villages, and all those places where soon teeming multitudes of men
+shall awake, and by their unceasing toil and the spirit within them
+produce marvels of which the Aeroplane is but the harbinger.
+</p>
+<p>
+And the Pilot's soul is refreshed, and his vision, now exalted, sees the
+Earth a very garden, even as it appears at that height, with discord
+banished and a happy time come, when the Designer shall have at last
+captured Efficiency, and the Man-who-takes-the-credit is he who has
+earned it, and when kisses are the only things that go by favour.
+</p>
+<p>
+Now the Pilot anxiously scans the Barograph, which is an instrument much
+the same as the Altimeter; but in this case the expansion of the vacuum
+box causes a pen to trace a line upon a roll of paper. This paper is
+made by clockwork to pass over the point of the pen, and so a curved
+line is made which accurately registers the speed of the ascent in feet
+per minute. No longer is the ascent at the rate of a thousand feet a
+minute, and the Propeller complains to the Engine, "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."
+</p>
+<p>
+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."
+</p>
+<p>
+"We fully agree," said the dying Power and Thrust. "Only maintain Us and
+you shall be surprised at the result. For our enemy Drift <i>decreases in
+respect of distance with the increase of altitude and rarity of air</i>,
+and there is no limit to the speed through space if only our strength
+remains. And with oxygen for pilot and passengers and a steeper pitch<a href="#note-11" name="noteref-11"><small>11</small></a>
+
+<span class="pagenum"><a id="page35" name="page35"></a>[35]</span>
+
+ for the Propeller we may then circle the Earth in a day!"
+</p>
+<p>
+Ah, Reader, smile not unbelievingly, as you smiled but a few years past.
+There may be greater wonders yet. Consider that as the speed increases,
+so does the momentum or stored-up force in the mass of the aeroplane
+become terrific. And, bearing that in mind, remember that with altitude
+<i>gravity decreases</i>. There may yet be literally other worlds to
+conquer.<a href="#note-12" name="noteref-12"><small>12</small></a>
+</p>
+<p>
+Now at fifteen thousand feet the conditions are chilly and rare, and the
+Pilot, with thoughts of breakfast far below, exclaims, "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.
+</p>
+<p>
+A pilot, being only human, may be able to do two things at once, but
+three is a tall order, so was this pilot relieved
+
+<span class="pagenum"><a id="page36" name="page36"></a>[36]</span>
+
+ 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.
+</p>
+<p>
+But what is this stream of bad language from the Exhaust Pipe,
+accompanied by gouts of smoke and vapour? The engine, now revolving at
+no more than one-tenth its normal speed, has upset the proportion of
+petrol to air, and combustion is taking place intermittently or in the
+Exhaust Pipe, where it has no business to be. "Crash, Bang,
+Rattle&mdash;&mdash;!&mdash;&mdash;!&mdash;&mdash;!" and worse than that, yells the Exhaust, and the
+Aeroplane, who is a gentleman and not a box kite,<a href="#note-13" name="noteref-13"><small>13</small></a> remonstrates with
+the severity of a Senior Officer. "See the Medical Officer, you young
+Hun. Go and see a doctor. Vocal diarrh&oelig;a, 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!"
+</p>
+<p>
+Now the pilot decides to change the straight gliding descent to a spiral
+one, and, obedient to the Rudder, the Aeroplane turns to the left. But
+the Momentum (two tons at 100 miles per hour is no small affair) heavily
+resents this change of direction, and tries its level best to prevent it
+and to pull the machine sideways and outwards from its spiral
+course&mdash;that is, to make it "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 lower surfaces of the
+planes press up against the air until the pressure equals the
+centrifugal force of the Momentum, and the Aeroplane spirals steadily
+downwards.
+</p>
+<p>
+Down, down, down, and the air grows denser, and the Pilot gulps largely,
+filling his lungs with the heavier air to counteract the increasing
+pressure from without. Down through a gap in the clouds, and the
+Aerodrome springs into view, appearing no larger than a saucer, and the
+Pilot, having by now got the "feel" of the Controls, proceeds
+
+<span class="pagenum"><a id="page37" name="page37"></a>[37]</span>
+
+ 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!
+</p>
+<p>
+All the parts are now working well together. Such wires as were before
+in undue tension have secured relief by slightly elongating their loops,
+and each one is now doing its bit, and all are sharing the burden of
+work together.
+</p>
+<p>
+The Struts and the Spars, which felt so awkward at first, have bedded
+themselves in their sockets, and are taking the compression stresses
+uncomplainingly.
+</p>
+<p>
+The Control Cables of twisted wire, a bit tight before, have slightly
+lengthened by perhaps the eighth of an inch, and, the Controls instantly
+responding to the delicate touch of the Pilot, the Aeroplane, at the
+will of its Master, darts this way and that way, dives, loops, spirals,
+and at last, in one long, magnificent glide, lands gently in front of
+its shed.
+</p>
+<p>
+"Well, what result?" calls the Flight-Commander to the Pilot.
+</p>
+<p>
+"A hundred miles an hour and a thousand feet a minute," he briefly
+replies.
+</p>
+<p>
+"And a very good result too," says the Aeroplane, complacently, as he is
+carefully wheeled into his shed.
+</p>
+<hr />
+<p>
+That is the way Aeroplanes speak to those who love them and understand
+them. Lots of Pilots know all about it, and can spin you wonderful
+yarns, much better than this one, if you catch them in a confidential
+mood&mdash;on leave, for instance, and after a good dinner.
+</p>
+<a name="note-8"><!--Note--></a>
+<p class="foot">
+<u>8</u> (<a href="#noteref-8">return</a>)<br />
+A.M.'s: Air Mechanics.
+</p>
+<a name="note-9"><!--Note--></a>
+<p class="foot">
+<u>9</u> (<a href="#noteref-9">return</a>)<br />
+Butt means to thicken at the end. Screw means to machine a
+thread on the butt-end of the wire, and in this way the wire can make
+connection with the desired place by being screwed into a metal fitting,
+thus eliminating the disadvantage of the unsatisfactory loop.
+</p>
+<a name="note-10"><!--Note--></a>
+<p class="foot">
+<u>10</u> (<a href="#noteref-10">return</a>)<br />
+Deviation Curve: A curved line indicating any errors in the
+compass.
+</p>
+<a name="note-11"><!--Note--></a>
+<p class="foot">
+<u>11</u> (<a href="#noteref-11">return</a>)<br />
+A propeller screws through the air, and the distance it
+advances during one revolution, supposing the air to be solid, is known
+as the pitch. The pitch, which depends upon the angle of the propeller
+blades, must be equal to the speed of the aeroplane, plus the slip, and
+if, on account of the rarity of the air, the speed of the aeroplane
+increases, then the angle and pitch should be correspondingly increased.
+Propellers with a pitch capable of being varied by the pilot are the
+dream of propeller designers. For explanation of "slip" see Chapter IV.
+on propellers.
+</p>
+<a name="note-12"><!--Note--></a>
+<p class="foot">
+<u>12</u> (<a href="#noteref-12">return</a>)<br />
+Getting out of my depth? Invading the realms of fancy?
+Well, perhaps so, but at any rate it is possible that extraordinary
+speed through space may be secured if means are found to maintain the
+impulse of the engine and the thrust-drift efficiency of the propeller
+at great altitude.
+</p>
+<a name="note-13"><!--Note--></a>
+<p class="foot">
+<u>13</u> (<a href="#noteref-13">return</a>)<br />
+Box-kite. The first crude form of biplane.
+</p>
+<a name="h2H_4_0007" id="h2H_4_0007"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    PART IV
+</h2>
+<h3>
+    'CROSS COUNTRY
+</h3>
+<p>
+The Aeroplane had been designed and built, and tested in the air, and
+now it stood on the Aerodrome ready for its first 'cross-country flight.
+</p>
+<p>
+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.
+</p>
+<p>
+Somehow, and despite every conceivable obstacle, it had managed to
+muddle through, and now it was ready for its work. It was not perfect,
+for there were fifty different ways in which it might be improved, some
+of them shamefully obvious. But it was fairly sound mechanically, had a
+little inherent stability, was easily controlled, could climb a thousand
+feet a minute, and its speed was a hundred miles an hour. In short,
+quite a creditable machine, though of course the right man had not got
+the credit.
+</p>
+<p>
+It is rough, unsettled weather with a thirty mile an hour wind on the
+ground, and that means fifty more or less aloft. Lots of clouds at
+different altitudes to bother the Pilot, and the air none too clear for
+the observation of landmarks.
+</p>
+<p>
+As the Pilot and Observer approach the Aeroplane the former is clearly
+not in the best of tempers. "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!"
+</p>
+<p>
+<span class="pagenum"><a id="page39" name="page39"></a>[39]</span>
+</p>
+<p>
+"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 very 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!"
+</p>
+<p>
+And, as the Pilot approaches the Aeroplane, his face brightens and he
+soon forgets his troubles as he critically inspects the craft which is
+to transport him and the Observer over the hills and far away. Turning
+to the Flight-Sergeant he inquires, "Tanks full of petrol and oil?"
+</p>
+<p>
+"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."
+</p>
+<p>
+"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
+(see p. 40).
+</p>
+<p>
+"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&ndash;E, which is always a
+line parallel to C&ndash;D. That is, to be exact, it will be fourteen degrees
+off the C&ndash;D course, as, in this part of the globe, there is that much
+difference between the North and South lines on the
+
+<span class="pagenum"><a id="page40" name="page40"></a>[40]</span>
+
+ map and the magnetic
+North to which the compass needle points. If the compass has an error,
+as it may have of a few degrees, that, too, must be taken into account,
+and the deviation or error curve on the dashboard will indicate it.
+</p>
+
+<a name="image-0021"><!--IMG--></a>
+<div class="figure-al">
+<a href="images/p040.jpg"><img src="images/p040-t.jpg" width="320" height="432"
+alt="A--B, 150 miles,
+     A--C, 50 miles; direction and miles per hour of wind.
+     C--D, 100 miles; airspeed of aeroplane.
+     A--D, Distance covered by aeroplane in one hour.
+     A--E, Compass course." /></a>
+<br /><div>
+    A&ndash;B, 150 miles,<br />
+    A&ndash;C, 50 miles; direction and miles per hour of wind.<br />
+    C&ndash;D, 100 miles; airspeed of aeroplane.<br />
+    A&ndash;D, Distance covered by aeroplane in one hour.<br />
+    A&ndash;E, Compass course.</div>
+</div>
+
+<p>
+<span class="pagenum"><a id="page41" name="page41"></a>[41]</span>
+</p>
+<p>
+"The Aeroplane will then always be pointing in a direction parallel to
+A&ndash;E, but, owing to the side wind, it will be actually travelling over
+the course A&ndash;B, though in a rather sideways attitude to that course.
+</p>
+<p>
+"The distance we shall travel over the A&ndash;B course in one hour is A&ndash;D.
+That is nearly eighty-seven miles, so we ought to accomplish our journey
+of a hundred and fifty miles in about one and three-quarter hours.
+</p>
+<p>
+"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."
+</p>
+<p>
+"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?"
+</p>
+<p>
+"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."
+</p>
+<p>
+"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.
+</p>
+<p>
+Round buzzes the Propeller, and the Pilot, giving the official signal,
+the Aeroplane is released and rolls swiftly over the ground in the teeth
+of the gusty wind.
+</p>
+<p>
+In less than fifty yards it takes to the air and begins to climb rapidly
+upwards, but how different are the conditions to the calm morning of
+yesterday! If the air were visible it would be seen to be acting in the
+most extraordinary manner; crazily swirling, lifting and dropping, gusts
+viciously colliding&mdash;a mad phantasmagoria of forces!
+</p>
+<p>
+Wickedly it seizes and shakes the Aeroplane; then tries to turn it over
+sideways; then instantly changes its mind
+
+<span class="pagenum"><a id="page42" name="page42"></a>[42]</span>
+
+ and in a second drops it into
+a hole a hundred feet deep; and if it were not for his safety belt the
+Pilot might find his seat sinking away from beneath him.
+</p>
+<p>
+Gusts strike the front of the craft like so many slaps in the face; and
+others, with the motion of mountainous waves, sometimes lift it hundreds
+of feet in a few seconds, hoping to see it plunge over the summit in a
+death-dive&mdash;and so it goes on, but the Pilot, perfectly at one with his
+mount and instantly alert to its slightest motion, is skilfully and
+naturally making perhaps fifty movements a minute of hand and feet; the
+former lightly grasping the "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.
+</p>
+
+<a name="image-0022"><!--IMG--></a>
+<div class="figure">
+<a href="images/p042.jpg"><img src="images/p042-t.jpg" width="320" height="297"
+alt="The Pilot's Cock-pit." /></a>
+<br />
+The Pilot's Cock-pit.
+</div>
+
+<p>
+A strain on the Pilot? Not a bit of it, for this is his Work which he
+loves and excels in; and given a cool head, alert eye, and a sensitive
+touch for the controls, what
+
+<span class="pagenum"><a id="page43" name="page43"></a>[43]</span>
+
+ sport can compare with these ever-changing
+battles of the air?
+</p>
+<p>
+The Aeroplane has all this time been climbing in great wide circles, and
+is now some three thousand feet above the Aerodrome which from such
+height looks absurdly small. The buildings below now seem quite squat;
+the hills appear to have sunk away into the ground, and the whole
+country below, cut up into diminutive fields, has the appearance of
+having been lately tidied and thoroughly spring-cleaned! A doll's
+country it looks, with tiny horses and cows ornamenting the fields and
+little model motor-cars and carts stuck on the roads, the latter
+stretching away across country like ribbons accidentally dropped.
+</p>
+<p>
+At three thousand feet altitude the Pilot is satisfied that he is
+now sufficiently high to secure, in the event of engine failure,
+a long enough glide to earth to enable him to choose and reach a good
+landing-place; and, being furthermore content with the steady running of
+the engine, he decides to climb no more but to follow the course he has
+mapped out. Consulting the compass, he places the Aeroplane on the A&ndash;E
+course and, using the Elevator, he gives his craft its minimum angle of
+incidence at which it will just maintain horizontal flight and secure
+its maximum speed.
+</p>
+<p>
+Swiftly he speeds away, and few thoughts he has now for the changing
+panorama of country, cloud, and colour. Ever present in his mind are the
+three great 'cross-country queries. "Am I on my right course? Can I see
+a good landing-ground within gliding distance?" And "How is the Engine
+running?"
+</p>
+<p>
+Keenly both he and the Observer compare their maps with the country
+below. The roads, khaki-coloured ribbons, are easily seen but are not
+of much use, for there are so many of them and they all look alike from
+such an altitude.
+</p>
+<p>
+Now where can that lake be which the map shows so plainly? He feels that
+surely he should see it by now, and has an uncomfortable feeling that
+he is flying too far West. What pilot is there indeed who has not many
+times experienced such unpleasant sensation? Few things in the air can
+create greater anxiety. Wisely, however, he sticks
+
+<span class="pagenum"><a id="page44" name="page44"></a>[44]</span>
+
+ to his compass
+course, and the next minute he is rewarded by a sight of the lake,
+though indeed he now sees that the direction of his travel will not take
+him over it, as should be the case if he were flying over the shortest
+route to his destination. He must have slightly miscalculated the
+velocity or direction of the side-wind.
+</p>
+<p>
+"About ten degrees off," he mutters, and, using the Rudder, corrects his
+course accordingly.
+</p>
+<p>
+Now he feels happier and that he is well on his way. The gusts, too,
+have ceased to trouble him as, at this altitude, they are not nearly so
+bad as they were near the ground, the broken surface of which does much
+to produce them; and sometimes for miles he makes but a movement or two
+of the controls.
+</p>
+<p>
+The clouds just above race by with dizzy and uniform speed; the country
+below slowly unrolls, and the steady drone of the Engine is almost
+hypnotic in effect. "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&mdash;&mdash;&mdash;." But the Pilot
+pulls himself together with a start and peers far ahead in search of the
+next landmark. This time it is a little country town, red-roofed his map
+tells him, and roughly of cruciform shape; and, sure enough, there in
+the right direction are the broken outlines of a few red roofs peeping
+out from between the trees.
+</p>
+<p>
+Another minute and he can see this little town, a fairy place it
+appears, nestling down between the hills and its red roofs and
+picturesque shape, a glowing and lovely contrast with the dark green of
+the surrounding moors.
+</p>
+<p>
+So extraordinarily clean and tidy it looks from such a height, and laid
+out in such orderly fashion with perfectly defined squares, parks,
+avenues, and public buildings, it indeed appears hardly real, but rather
+as if it has this very day materialized from some delightful children's
+book!
+</p>
+<p>
+Every city and town you must know has its distinct individuality to the
+Pilot's eye. Some are not fairy places
+
+<span class="pagenum"><a id="page45" name="page45"></a>[45]</span>
+
+ at all, but great dark ugly blots
+upon the fair countryside, and with tall shafts belching forth murky
+columns of smoke to defile clean space. Others, melancholy-looking
+masses of grey, slate-roofed houses, are always sad and dispirited;
+never welcoming the glad sunshine, but ever calling for leaden skies and
+a weeping Heaven. Others again, little coquettes with village green,
+white palings everywhere, bright gravel roads, and an irrepressible air
+of brightness and gaiety.
+</p>
+<p>
+Then there are the rivers, silvery streaks peacefully winding far, far
+away to the distant horizon; they and the lakes the finest landmarks the
+Pilot can have. And the forests. How can I describe them? The trees
+cannot be seen separately, but merge altogether into enormous irregular
+dark green masses sprawling over the country, and sometimes with great
+ungainly arms half encircling some town or village; and the wind passing
+over the foliage at times gives the forest an almost living appearance,
+as of some great dragon of olden times rousing itself from slumber to
+devour the peaceful villages its arms encircle.
+</p>
+<p>
+And the Pilot and Observer fly on and on, seeing these things and many
+others which baffle my poor skill to describe&mdash;things, dear Reader, that
+you shall see, and poets sing of, and great artists paint in the days to
+come when the Designer has captured Efficiency. Then, and the time is
+near, shall you see this beautiful world as you have never seen it
+before, the garden it is, the peace it breathes, and the wonder of it.
+</p>
+<p>
+The Pilot, flying on, is now anxiously looking for the railway line
+which midway on his journey should point the course. Ah! There it is at
+last, but suddenly (and the map at fault) it plunges into the earth!
+Well the writer remembers when that happened to him on a long
+'cross-country flight in the early days of aviation. Anxiously he
+wondered "Are tunnels always straight?" and with what relief, keeping on
+a straight course, he picked up the line again some three miles farther
+on!
+</p>
+<p>
+Now at last the Pilot sees the sea, just a streak on the north-eastern
+horizon, and he knows that his flight is two-thirds over. Indeed, he
+should have seen it before, but the air is none too clear, and he is not
+yet able to discern the river which soon should cross his path. As he
+swiftly
+
+<span class="pagenum"><a id="page46" name="page46"></a>[46]</span>
+
+ 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 <i>duty</i>
+of a Pilot to fear fog, his deadliest enemy. Fog not only hides the
+landmarks by which he keeps his course, but makes the control of the
+Aeroplane a matter of the greatest difficulty. He may not realize it,
+but, in keeping his machine on an even keel, he is unconsciously
+balancing it against the horizon, and with the horizon gone he is lost
+indeed. Not only that, but it also prevents him from choosing his
+landing-place, and the chances are that, landing in a fog, he will smash
+into a tree, hedge, or building, with disastrous results. The best and
+boldest pilot 'wares a fog, and so this one, finding the conditions
+becoming worse and yet worse, and being forced to descend lower and
+lower in order to keep the earth within view, wisely decides to choose a
+landing-place while there is yet time to do so.
+</p>
+<p>
+Throttling down the power of the engine he spirals downwards, keenly
+observing the country below. There are plenty of green fields to lure
+him, and his great object is to avoid one in which the grass is long,
+for that would bring his machine to a stop so suddenly as to turn it
+over; or one of rough surface likely to break the under-carriage. Now is
+perfect eyesight and a cool head indispensable. He sees and decides upon
+a field and, knowing his job, he sticks to that field with no change of
+mind to confuse him. It is none too large, and gliding just over the
+trees and head on to the wind he skilfully "stalls" his machine; that
+is, the speed having decreased sufficiently to avoid such a man&oelig;uvre
+resulting in ascent, he, by means of the Elevator, gives the Aeroplane
+as large an angle of incidence as possible, and the undersides of the
+planes meeting the air at such a large angle act as an air-brake, and
+the Aeroplane, skimming over the ground, lessens its speed and finally
+stops just at the farther end of the field.
+</p>
+<p>
+Then, after driving the Aeroplane up to and under the lee of the hedge,
+he stops the engine, and quickly lashing the joy-stick fast in order
+to prevent the wind from blowing the controlling surfaces about and
+possibly damaging them,
+
+<span class="pagenum"><a id="page47" name="page47"></a>[47]</span>
+
+ 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."
+</p>
+<p>
+Working with a will they soon have the engine and the open part of the
+body which contains the seats, controls, and instruments snugly housed
+with their waterproof covers, and the Aeroplane is ready to weather the
+possible storm. 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?"
+</p>
+<p>
+"All right, you cut along and I'll stop here, for the Aeroplane must not
+be left alone. Get back as quickly as possible."
+</p>
+<p>
+And the Observer trots off, leaving the Pilot filling his pipe and
+anxiously scrutinizing the weather conditions. Very thick it is now, but
+the day is yet young, and he has hopes of the fog lifting sufficiently
+to enable the flight to be resumed. A little impatiently he awaits the
+return of his comrade, but with never a doubt of the result, for the
+hospitality
+
+<span class="pagenum"><a id="page48" name="page48"></a>[48]</span>
+
+ of the country house is proverbial among pilots! What old
+hand among them is there who cannot instance many a forced landing made
+pleasant by such hospitality? Never too late or too early to help with
+food, petrol, oil, tools, and assistants. Many a grateful thought has
+the writer for such kind help given in the days before the war (how long
+ago they seem!), when aeroplanes were still more imperfect than they are
+now, and involuntary descents often a part of 'cross-country flying.
+</p>
+<p>
+Ah! those early days! How fresh and inspiring they were! As one started
+off on one's first 'cross-country flight, on a machine the first of its
+design, and with everything yet to learn, and the wonders of the air yet
+to explore; then the joy of accomplishment, the dreams of Efficiency,
+the hard work and long hours better than leisure; and what a field of
+endeavour&mdash;the realms of space to conquer! And the battle still goes on
+with ever-increasing success. Who is bold enough to say what its limits
+shall be?
+</p>
+<p>
+So ruminates this Pilot-Designer, as he puffs at his pipe, until his
+reverie is abruptly disturbed by the return of the Observer.
+</p>
+<p>
+"Wake up, you <i>airman</i>," 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."
+</p>
+<p>
+"Well, that's splendid, but don't call me newspaper names or you'll
+spoil my appetite!"
+</p>
+<p>
+Then, with hunger such as only flying can produce, they appreciatively
+discuss their lunch, and with many a grateful thought for the
+donors&mdash;and they talk shop. They can't help it, and even golf is a poor
+second to flight talk. Says the Pilot, who must have his grievance,
+"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
+
+<span class="pagenum"><a id="page49" name="page49"></a>[49]</span>
+
+ existence in which the tail could not be
+raised several feet, and that would make all the difference. A high tail
+means 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:
+</p>
+
+<a name="image-0023"><!--IMG--></a>
+<div class="figure">
+<span class="sc">The Pilot's Aeroplane.</span><br />
+<a href="images/p049.jpg"><img src="images/p049-t.jpg" width="320" height="169"
+alt="THE PILOT'S AEROPLANE. THE CHANGE OF DESIGN HE WOULD LIKE." /></a>
+<br />
+<span class="sc">The Change of Design He Would Like.</span>
+</div>
+
+<p>
+"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?"
+</p>
+<p>
+"As regards the first part of your question I assure you that there's
+nothing in it, and I'll prove it to you as follows&mdash;&mdash;"
+</p>
+<p>
+"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.
+</p>
+<p>
+"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
+
+<span class="pagenum"><a id="page50" name="page50"></a>[50]</span>
+
+ thrown together and anything was good enough that could get
+off the ground."
+</p>
+<p>
+"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?"
+</p>
+<p>
+"I believe you're right. I am sure those hills over there could not be
+seen a few minutes ago, and look&mdash;there's sunshine over there. We'd
+better hurry up."
+</p>
+<p>
+Ten minutes' hard work and the covers are off, neatly folded and stowed
+aboard; the picketing wires are cast adrift, and the Pilot is once more
+in his seat. The Aeroplane has been turned to face the other end of the
+field, and, the Observer swinging round the propeller, the engine is
+awake again and slowly ticking over. Quickly the Observer climbs into
+his seat in front of the Pilot, and, the latter slightly opening the
+throttle, the Aeroplane leisurely rolls over the ground towards the
+other end of the field, from which the ascent will be made.
+</p>
+<p>
+Arriving there the Pilot turns the Aeroplane in order to face the wind
+and thus secure a quick "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&mdash;well, like an Aeroplane with an excellent
+margin of lift. Upwards it climbs with even and powerful lift, and the
+familiar scenes below again gladden the eyes of the Pilot. Smaller and
+more and more squat grow the houses and hills; more and more doll-like
+appear the fields which are clearly outlined by the hedges; and soon the
+country below is easily identified with the map. Now they can see the
+river before them and a bay of the sea which must be crossed or skirted.
+The fog still lingers along the course of the river and between the
+hills, but is fast rolling away in grey, ghost-like masses.
+
+<span class="pagenum"><a id="page51" name="page51"></a>[51]</span>
+
+ Out to sea
+it obscures the horizon, making it difficult to be sure where water ends
+and fog begins, and creating a strange, rather weird, effect by which
+ships at a certain distance appear to be floating in space.
+</p>
+<p>
+Now the Aeroplane is almost over the river, and the next instant it
+suddenly drops into a "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.
+</p>
+<p>
+Now the Aeroplane is over the bay and, the sea being calm, the Pilot
+looks down, down through the water, and clearly sees the bottom,
+hundreds of feet below the surface. Down through the reflection of the
+blue sky and clouds, and one might think that is all, but it isn't. Only
+those who fly know the beauties of the sea as viewed from above; its
+dappled pearly tints; its soft dark blue shadows; the beautiful
+contrasts of unusual shades of colour which are always differing and
+shifting with the changing sunshine and the ever moving position of the
+aerial observer. Ah! for some better pen than mine to describe these
+things! One with glowing words and a magic rhythm to express the wonders
+of the air and the beauty of the garden beneath&mdash;the immensity of the
+sea&mdash;the sense of space and of one's littleness there&mdash;the realization
+of the Power moving the multitudes below&mdash;the exaltation of spirit
+altitude produces&mdash;the joy of speed. A new world of sensation!
+</p>
+<p>
+<span class="pagenum"><a id="page52" name="page52"></a>[52]</span>
+</p>
+<p>
+Now the bay is almost crossed and the Aerodrome at B. can be
+distinguished....
+</p>
+<hr />
+<p>
+On the Aerodrome is a little crowd waiting and watching for the arrival
+of the Aeroplane, for it is of a new and improved type and its first
+'cross-country performance is of keen interest to these men; men who
+really know something about flight.
+</p>
+<p>
+There is the Squadron Commander who has done some real flying in his
+time; several well-seasoned Flight-Commanders; a dozen or more
+Flight-Lieutenants; a knowledgeable Flight-Sergeant; a number of Air
+Mechanics, and, a little on one side and almost unnoticed, the Designer.
+</p>
+<p>
+"I hope they are all right," says someone, "and that they haven't had
+difficulties with the fog. It rolled up very quickly, you know."
+</p>
+<p>
+"Never fear," remarks a Flight-Commander. "I know the Pilot well and
+he's a good 'un; far too good to carry on into a fog."
+</p>
+<p>
+"They say the machine is really something out of the ordinary," says
+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."
+</p>
+<p>
+"Ah! my boy. You do a bit more flying and you'll discover that things
+are not always as they appear from a distance!"
+</p>
+<p>
+"There she is, sir!" cries the Flight-Sergeant. "Just a speck over the
+silvery corner of that cloud."
+</p>
+<p>
+A tiny speck it looks, some six miles distant and three thousand feet
+high; but, racing along, it rapidly appears larger and soon its outlines
+can be traced and the sunlight be seen playing upon the whirling
+propeller.
+</p>
+<p>
+Now the distant drone of the engine can be heard, but not for long, for
+suddenly it ceases and, the nose of the Aeroplane sinking, the craft
+commences gliding downwards.
+</p>
+<p>
+"Surely too far away," says a subaltern. "It will be
+
+<span class="pagenum"><a id="page53" name="page53"></a>[53]</span>
+
+ 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!
+</p>
+<p>
+Swiftly the machine glides downwards towards them, and it can now be
+seen how surprisingly little it is affected by the rough weather and
+gusts; so much so that a little chorus of approval is heard.
+</p>
+<p>
+"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."
+</p>
+<p>
+Then the Aeroplane with noiseless engine glides over the boundary of the
+Aerodrome, and, with just a soft soughing sound from the air it cleaves,
+lands gently not fifty yards from the onlookers.
+</p>
+<p>
+"Glad to see you," says the Squadron Commander to the Pilot. "How do you
+like the machine?" And the Pilot replies:
+</p>
+<p>
+"I never want a better one, sir. It almost flies itself!"
+</p>
+<p>
+And the Designer turns his face homewards and towards his beloved
+drawing-office; well satisfied, but still dreaming dreams of the future
+and ... looking far ahead who should he see but Efficiency at last
+coming towards him! And to him she is all things. In her hair is the
+morning sunshine; her eyes hold the blue of the sky, and on her cheeks
+is the pearly tint of the clouds as seen from above. The passion of
+speed, the lure of space, the sense of power, and the wonder of the
+future ... all these things she holds for him.
+</p>
+<p>
+"Ah!" he cries. "You'll never leave me now, when at last there is no one
+between us?"
+</p>
+<p>
+And Efficiency, smiling and blushing, but practical as ever, says:
+</p>
+<p>
+"And you will never throw those Compromises in my face?"
+</p>
+<p>
+"My dear, I love you for them! Haven't they been my life ever since I
+began striving for you ten long years ago?"
+</p>
+<p>
+<span class="pagenum"><a id="page54" name="page54"></a>[54]</span>
+</p>
+<p>
+And so they walk off very happily, arm-in-arm together; and if this
+hasn't bored you and you'd like some more of the same sort of thing, I'd
+just love to tell you some day of the wonderful things they accomplish
+together, and of what they dream the future holds in store.
+</p>
+
+<a name="image-0024"><!--IMG--></a>
+<div class="figure">
+<a href="images/p054.jpg"><img src="images/p054-t.jpg" width="320" height="142"
+alt="" /></a>
+</div>
+
+<p class="center">
+<i>And that's the end of the Prologue.</i>
+</p>
+
+<p><span class="pagenum"><a id="page55" name="page55"></a>[55]</span></p>
+
+<a name="h2HCH0001" id="h2HCH0001"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    CHAPTER I
+</h2>
+<h3>
+    FLIGHT
+</h3>
+<p>
+Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum.
+It therefore obeys Newton's laws<a href="#note-14" name="noteref-14"><small>14</small></a> and resists movement. It is that
+resistance or reaction which makes flight possible.
+</p>
+<p>
+Flight is secured by driving through the air a surface<a href="#note-15" name="noteref-15"><small>15</small></a> inclined
+upwards and towards the direction of motion.
+</p>
+
+<a name="image-0025"><!--IMG--></a>
+<div class="figure">
+<a href="images/p055.jpg"><img src="images/p055-t.jpg" width="320" height="56"
+alt="S = Side view of surface. M = Direction of motion." /></a>
+<br />
+</div>
+
+<p>
+S = Side view of surface.
+</p>
+<p>
+M = Direction of motion.
+</p>
+<p>
+<span class="sc">Chord.</span>&mdash;The Chord is, for practical purposes, taken to be a straight
+line from the leading edge of the surface to its trailing edge.
+</p>
+<p>
+N = A line through the surface starting from its trailing edge. The
+position of this line, which I call the <i>Neutral Lift Line</i>, 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 <i>above</i> the line of motion. If it is coincident with
+M, there is no lift. If it makes an angle with M and <i>below</i> it, then
+there is a pressure tending to force the surface down.
+</p>
+<p>
+I = Angle of Incidence. This angle is generally defined as the angle the
+chord makes with the direction of motion, but that is a bad definition,
+as it leads to misconception. The angle of incidence is best described
+as the angle the
+
+<span class="pagenum"><a id="page56" name="page56"></a>[56]</span>
+
+ 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, if, as seems reasonable, we regard the surface from the point
+of view of the neutral lift line. 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 criticized adversely
+is then applicable. Flat lifting surfaces are, however, never used.
+</p>
+<p>
+The surface acts upon the air in the following manner:
+</p>
+
+<a name="image-0026"><!--IMG--></a>
+<div class="figure">
+<a href="images/p056.jpg"><img src="images/p056-t.jpg" width="320" height="119"
+alt="" /></a>
+</div>
+
+<p>
+As the bottom of the surface meets the air, it compresses it and
+accelerates it <i>downwards</i>. As a result of this definite action there
+is, of course, an equal and opposite reaction <i>upwards</i>.
+</p>
+<p>
+The top surface, in moving forward, tends to leave the air behind it,
+thus creating a semi-vacuum or rarefied area over the top of the
+surface. Consequently the pressure of
+
+<span class="pagenum"><a id="page57" name="page57"></a>[57]</span>
+
+ air on the top of the surface is
+decreased, thus assisting the reaction below to lift the surface
+<i>upwards</i>.
+</p>
+<p>
+The reaction increases approximately as the square of the velocity. It
+is the result of (1) the mass of air engaged, and (2) the velocity and
+consequent force with which the surface engages the air. If the reaction
+was produced by only one of those factors it would increase in direct
+proportion to the velocity, but, since it is the product of both
+factors, it increases as V<sup>2</sup>.
+</p>
+<p>
+Approximately three-fifths of the reaction is due to the decrease of
+density (and consequent decrease of downward pressure) on the top of the
+surface; and only some two-fifths is due to the upward reaction secured
+by the action of the bottom surface upon the air. A practical point in
+respect of this is that, in the event of the fabric covering the surface
+getting into bad condition, it is more likely to strip off the top than
+off the bottom.
+</p>
+
+<a name="image-0027"><!--IMG--></a>
+<div class="figure">
+<a href="images/p057.jpg"><img src="images/p057-t.jpg" width="320" height="131"
+alt="" /></a>
+</div>
+
+<p>
+The direction of the reaction is, at efficient angles of incidence,
+approximately at right-angles to the neutral lift line of the surface,
+as illustrated above; and it is, in considering flight, convenient to
+divide it into two component parts or values, thus:
+</p>
+<p>
+1. The vertical component of the reaction, <i>i.e.</i>, Lift, which is
+opposed to Gravity, <i>i.e.</i>, the weight of the aeroplane.
+</p>
+<p>
+2. The horizontal component, <i>i.e.</i>, Drift (sometimes called
+Resistance), to which is opposed the thrust of the propeller.
+</p>
+<p>
+The direction of the reaction is, of course, the resultant of the forces
+Lift and Drift. The Lift is the useful part of the reaction, for it
+lifts the weight of the aeroplane.
+</p>
+<p>
+<span class="pagenum"><a id="page58" name="page58"></a>[58]</span>
+</p>
+<p>
+The Drift is the villain of the piece, and must be overcome by the
+Thrust in order to secure the necessary velocity to produce the
+requisite lift for flight.
+</p>
+<p>
+<span class="sc">Drift</span>.&mdash;The drift of the whole aeroplane (we have considered only the
+lifting surface heretofore) may be conveniently divided into three
+parts, as follows:
+</p>
+<p>
+<i>Active Drift</i>, which, is the drift produced by the lifting surfaces.
+</p>
+<p>
+<i>Passive Drift</i>, which is the drift produced by all the rest of the
+aeroplane&mdash;the struts, wires, fuselage, under-carriage, etc., all of
+which is known as "detrimental surface."
+</p>
+<p>
+<i>Skin Friction</i>, which is the drift produced by the friction of the air
+with roughness of surface. The latter is practically negligible having
+regard to the smooth surface of the modern aeroplane, and its
+comparatively slow velocity compared with, for instance, the velocity of
+a propeller blade.
+</p>
+<p>
+<span class="sc">Lift-Drift Ratio.</span>&mdash;The proportion of lift to drift is known as the
+lift-drift ratio, and is of paramount importance, for it expresses <i>the
+efficiency of the aeroplane</i> (as distinct from engine and propeller).
+A knowledge of the factors governing the lift-drift ratio is, as will
+be seen later, <i>an absolute necessity</i> to anyone responsible for the
+rigging of an aeroplane, and the maintenance of it in an efficient and
+safe condition.
+</p>
+<p>
+Those factors are as follows:
+</p>
+<p class="indent" style="text-indent: -5%;">
+1. <i>Velocity</i>.&mdash;The greater the velocity the greater the proportion of
+drift to lift, and consequently the less the efficiency. Considering the
+lifting surfaces alone, both the lift and the (active) drift, being
+component parts of the reaction, increase as the square of the velocity,
+and the efficiency remains the same at all speeds. But, considering the
+whole aeroplane, we must remember the passive drift. It also increases
+as the square of the velocity (with no attendant lift), and, adding
+itself to the active drift, results in increasing the proportion of
+total drift (active + passive) to lift.
+</p>
+<p class="indent">
+But for the increase in passive drift the efficiency of the aeroplane
+would not fall with increasing
+
+<span class="pagenum"><a id="page59" name="page59"></a>[59]</span>
+
+ velocity, and it would be possible, by
+doubling the thrust, to approximately double the speed or lift&mdash;a happy
+state of affairs which can never be, but which we may, in a measure,
+approach by doing everything possible to diminish the passive drift.
+</p>
+<p class="indent">
+Every effort is then made to decrease it by "stream-lining," <i>i.e.</i>, 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.
+</p>
+
+<a name="image-0028"><!--IMG--></a>
+<div class="figure">
+<a href="images/p059.jpg"><img src="images/p059-t.jpg" width="320" height="104"
+alt="" /></a>
+</div>
+
+<p class="indent">
+<i>Head-resistance</i> is a term often applied to passive drift, but it is
+apt to convey a wrong impression, as the drift is not nearly so much the
+result of the head or forward part of struts, wires, etc., as it is of
+the rarefied area behind.
+</p>
+<p class="indent">
+Above is illustrated the flow of air round two objects moving in the
+direction of the arrow M.
+</p>
+<p class="indent">
+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
+<i>decreasing</i> DD.
+</p>
+<p class="indent">
+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
+
+<span class="pagenum"><a id="page60" name="page60"></a>[60]</span>
+
+ pressure, then, the better is head-resistance D able to
+get its work in, and the more thrust will be required to overcome it.
+</p>
+<p class="indent">
+The "fineness" of the stream-line shape, <i>i.e.</i>, the proportion of
+length to width, is determined by the velocity&mdash;the greater the
+velocity, the greater the fineness. The best degree of fineness for any
+given velocity is found by means of wind-tunnel research.
+</p>
+<p class="indent">
+The practical application of all this is, from a rigging point of view,
+the importance of adjusting all stream-line parts to be dead-on in the
+line of flight, but more of that later on.
+</p>
+<p class="indent" style="text-indent: -5%;">
+2. <i>Angle of Incidence</i>.&mdash;The most efficient angle of incidence varies
+with the thrust at the disposal of the designer, the weight to be
+carried, and the climb-velocity ratio desired.
+</p>
+<p class="indent">
+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 over the top of the surface 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. Also, too great an
+angle for the velocity will result in the underside of the surface
+tending to compress the air against which it is driven rather than
+accelerate it <i>downwards</i>, and that will tend to produce drift rather
+than the <i>upwards</i> reaction, or lift.
+</p>
+<p class="indent">
+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.
+
+<span class="pagenum"><a id="page61" name="page61"></a>[61]</span>
+
+ Any deviation will adversely affect the lift-drift ratio, <i>i.e.</i>,
+the efficiency.
+</p>
+<p class="indent" style="text-indent: -5%;">
+3. <i>Camber</i>.&mdash;(Refer to the second illustration in this chapter.) The
+lifting surfaces are cambered, <i>i.e.</i>, curved, in order to decrease the
+horizontal component of the reaction, <i>i.e.</i>, the drift.
+</p>
+<p class="indent">
+<i>The bottom camber</i>: If the bottom of the surface was flat, every
+particle of air meeting it would do so with a shock, and such shock
+would produce a very considerable horizontal reaction or drift. By
+curving it such shock is diminished, and the curve should be such
+as to produce a uniform (not necessarily constant) acceleration and
+compression of the air from the leading edge to the trailing edge.
+Any unevenness in the acceleration and compression of the air produces
+drift.
+</p>
+<p class="indent">
+<i>The top camber</i>: If this was flat it would produce a rarefied area of
+irregular shape. I have already explained the bad effect this has upon
+the lift-drift ratio. The top surface is then curved to produce a
+rarefied area the shape of which shall be as stream-line and free from
+attendant eddies as possible.
+</p>
+<p class="indent">
+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 stream-line form&mdash;<i>i.e.</i>, of infinite
+fineness. This is, of course, carrying theory to absurdity as the
+surface would then cease to exist.
+</p>
+<p class="indent">
+The best cambers for varying velocities, angles of incidence, and
+thickness of surface, are found by means of wind-tunnel research. The
+practical application of all this is in taking the greatest care to
+prevent the surface from becoming distorted and thus spoiling the camber
+and consequently the lift-drift ratio.
+</p>
+<p>
+<span class="pagenum"><a id="page62" name="page62"></a>[62]</span>
+</p>
+<p class="indent" style="text-indent: -5%;">
+4. <i>Aspect Ratio</i>.&mdash;This is the proportion of span to chord. Thus, if
+the span is, for instance, 50 feet and the chord 5 feet, the surface
+would be said to have an aspect ratio of 10 to 1.
+</p>
+<p class="indent">
+For <i>a given velocity</i> and <i>a given area</i> of surface, the higher the
+aspect ratio, the greater the reaction. It is obvious, I think, that the
+greater the span, the greater the mass of undisturbed air engaged, and,
+as already explained, the reaction is partly the result of the mass of
+air engaged. I say "undisturbed" advisedly, for otherwise it might be
+argued that, whatever the shape of the surface, the same mass of air
+would be engaged. The word "undisturbed" makes all the difference, for
+it must be remembered that the rear part of the underside of the surface
+engages air most of which has been deflected downwards by the surface
+in front of it. That being so, the rear part of the surface has not the
+same opportunity of forcing; the air downwards (since it is already
+flowing downwards) and securing there from an upwards, reaction as has
+the surface in front of it. It is therefore of less value for its area
+than the front part of the surface, since it does less work and secures
+less reaction&mdash;<i>i.e.</i>, lift. Again, the rarefied area over the top of
+the surface is most rare towards the front of it, as, owing to eddies,
+the rear of such area tends to become denser.
+</p>
+
+<a name="image-0029"><!--IMG--></a>
+<div class="figure">
+<a href="images/p062.jpg"><img src="images/p062-t.jpg" width="320" height="141"
+alt="" /></a>
+</div>
+
+<p class="indent">
+Thus, you see, the front part of the surface is the most valuable from
+the point of view of securing an upwards reaction from the air; and so,
+by increasing the proportion of front, or "span," to chord, we increase
+the amount of reaction for a
+
+<span class="pagenum"><a id="page63" name="page63"></a>[63]</span>
+
+ given velocity and area of surface. That
+means a better proportion of reaction to weight of surface, though the
+designer must not forget the drift of struts and wires necessary to
+brace up a surface of high aspect ratio.
+</p>
+<p class="indent">
+Not only that, but, <i>provided</i> the chord is not decreased to an extent
+making it impossible to secure the best camber owing to the thickness of
+the surface, the higher 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.
+</p>
+<p class="indent" style="text-indent: -5%;">
+5. <i>Stagger</i>.&mdash;This is the advancement of the top surface relative to
+the bottom surface, and is not, of course, applicable to a single
+surface, <i>i.e.</i>, 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 about 1-1/2 times the chord. If less than that, the air engaged by
+the bottom of the top surface will have a tendency to be drawn into the
+rarefied area over the top of the bottom surface, with the result that
+the surfaces will not secure as good a reaction as would otherwise be
+the case.
+</p>
+<p class="indent">
+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
+
+<span class="pagenum"><a id="page64" name="page64"></a>[64]</span>
+
+ and engages undisturbed air, with the result that the efficiency
+can in this way be increased by about 5 per cent. Theoretically the
+top plane should be staggered forward for a distance equal to about 30
+per cent. of the chord, the exact distance depending upon the velocity
+and angle of incidence; but this is not always possible to arrange
+in designing an aeroplane, owing to difficulties of balance, desired
+position, and view of pilot, observer, etc.
+</p>
+
+<a name="image-0030"><!--IMG--></a>
+<div class="figure">
+<a href="images/p063.jpg"><img src="images/p063-t.jpg" width="320" height="217"
+alt="H.E., Horizontal equivalent. D., Dihedral angle." /></a>
+<br />
+H.E., Horizontal equivalent.<br /> D., Dihedral angle.
+</div>
+
+<p class="indent" style="text-indent: -5%;">
+6. <i>Horizontal Equivalent.</i>-The vertical component of the reaction,
+<i>i.e.</i>, lift, varies as the horizontal equivalent (H.E.) of the surface,
+but the drift remains the same. Then it follows that if H.E. grows less,
+the ratio of lift to drift must do the same.
+</p>
+<p class="indent">
+A, B, and C are front views of three surfaces.
+</p>
+<p class="indent">
+A has its full H.E., and therefore, from the point of view from which we
+are at the moment considering efficiency, it has its best lift-drift
+ratio.
+</p>
+<p class="indent">
+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
+
+<span class="pagenum"><a id="page65" name="page65"></a>[65]</span>
+
+ same amount of surface as in A to produce it), the lift-drift ratio
+falls.
+</p>
+<p class="indent">
+<span class="sc">The Margin of Power</span> is the power available above that necessary to
+maintain horizontal flight.
+</p>
+<p class="indent">
+<span class="sc">The Margin of Lift</span> is the height an aeroplane can gain in a given time
+and starting from a given altitude. As an example, thus: 1,000 feet the
+first minute, and starting from an altitude of 500 feet above sea-level.
+</p>
+<p class="indent">
+The margin of lift decreases with altitude, owing to the decrease in the
+density of the air, which adversely affects the engine. Provided the
+engine maintained its impulse with altitude, then, if we ignore the
+problem of the propeller, which I will go into later on, the margin of
+lift would not disappear. Moreover, greater velocity for a given power
+would be secured at a greater altitude, owing to the decreased density
+of air to be overcome. After reading that you may like to light your
+pipe and indulge in dreams of the wonderful possibilities which may
+become realities if some brilliant genius shows us some day how to
+secure a constant power with increasing altitude. I am afraid, however,
+that will always remain impossible; but it is probable that some very
+interesting steps may be taken in that direction.
+</p>
+<p class="indent">
+<span class="sc">The Minimum Angle of Incidence</span> is the smallest angle at which, for a
+given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+</p>
+<p class="indent">
+<span class="sc">The Maximum Angle of Incidence</span> is the greatest angle at which, for a
+given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+</p>
+<p class="indent">
+<span class="sc">The Optimum Angle of Incidence</span> is the angle at which the lift-drift
+ratio is highest. In modern aeroplanes it is that angle of incidence
+possessed by the surface when the axis of the propeller is horizontal.
+</p>
+<p class="indent">
+<span class="sc">The Best Climbing Angle</span> is approximately half-way between the maximum
+and the optimum angles.
+</p>
+<p>
+<span class="pagenum"><a id="page66" name="page66"></a>[66]</span>
+</p>
+<p class="indent">
+All present-day aeroplanes are a compromise between Climb and horizontal
+Velocity. We will compare the essentials for two aeroplanes, one
+designed for maximum climb, and the other for maximum velocity.
+</p>
+<p>
+<span class="sc">Essentials for Maximum Climb:</span>
+</p>
+<p class="indent" style="text-indent: -5%;">
+1. <i>Low velocity</i>, in order to secure the best lift-drift ratio.
+</p>
+<p class="indent" style="text-indent: -5%;">
+2. Having a low velocity, <i>a large surface</i> will be necessary
+in order to engage the necessary mass of air to secure the
+requisite lift.
+</p>
+
+<a name="image-0031"><!--IMG--></a>
+<div class="figure">
+<a href="images/p065.jpg"><img src="images/p065-t.jpg" width="320" height="150"
+alt="" /></a>
+</div>
+
+<p class="indent" style="text-indent: -5%;">
+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 <i>large angle relative to the direction of
+the thrust</i> will be necessary in order to secure the requisite
+angle relative to the direction of motion.
+</p>
+
+<p>
+The propeller thrust should be always horizontal, because the most
+efficient flying-machine (having regard to climb or velocity) has, so
+far, been found to be an arrangement of an inclined surface driven by a
+<i>horizontal</i> thrust&mdash;the surface lifting the weight, and the thrust
+overcoming the drift. This is, in practice, a far more efficient
+arrangement than the helicopter, <i>i.e.</i>, 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,
+
+<span class="pagenum"><a id="page67" name="page67"></a>[67]</span>
+
+ in order to absorb the power of the engine, to revolve the
+comparatively small-surfaced air screw at an immensely greater velocity
+than that of the aeroplane's surface. As already explained, the
+lift-drift ratio falls with velocity on account of the increase in
+passive drift. This applies to a blade of a propeller or air-screw which
+is nothing but a revolving surface set at angle of incidence, and which
+it is impossible to construct without a good deal of detrimental surface
+near the central boss.
+</p>
+<p class="indent" style="text-indent: -5%;">
+4. The velocity being low, then it follows that for that reason
+also <i>the angle of incidence should be comparatively large</i>.
+</p>
+<p class="indent" style="text-indent: -5%;">
+5. <i>Camber</i>.&mdash;Since such an aeroplane would be of low velocity,
+and therefore possess a large angle of incidence, a <i>large
+camber</i> would be necessary.
+</p>
+<p>
+Let us now consider the essentials for an aeroplane of maximum velocity
+for its power, and possessing merely enough lift to get off the ground,
+but no margin of lift.
+</p>
+<p class="indent" style="text-indent: -5%;">
+1. Comparatively <i>high velocity</i>.
+</p>
+<p class="indent" style="text-indent: -5%;">
+2. A comparatively <i>small surface</i>, 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 requisite lift.
+</p>
+<p class="indent" style="text-indent: -5%;">
+3. <i>A small angle relative to the propeller thrust</i>, since the latter
+coincides with the direction of motion.
+</p>
+<p class="indent" style="text-indent: -5%;">
+4. A comparatively <i>small angle of incidence</i> by reason of the high
+velocity.
+</p>
+<p class="indent" style="text-indent: -5%;">
+5. A comparatively <i>small camber</i> follows as a result of the small
+angle of incidence.
+</p>
+
+<a name="image-0032"><!--IMG--></a>
+<div class="figure">
+<a href="images/p067.jpg"><img src="images/p067-t.jpg" width="320" height="130"
+alt="" /></a>
+</div>
+
+<p><span class="pagenum"><a id="page68" name="page68"></a>[68]</span></p>
+
+<p class="center">
+ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE DESIGNED
+AS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIGHT
+MARGIN OF LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS HORIZONTAL.
+</p>
+
+<a name="image-0033"><!--IMG--></a>
+<a name="image-0034"><!--IMG--></a>
+<a name="image-0035"><!--IMG--></a>
+<a name="image-0036"><!--IMG--></a>
+<div class="figure">
+<a href="images/p068.jpg"><img src="images/p068-t.jpg" width="400" height="75"
+alt="" /></a>
+</div>
+
+<p>
+<span class="sc">Minimum Angle.</span>
+</p>
+<p>
+This gives the greatest velocity during horizontal flight at a low
+altitude. Greater velocity would be secured if the surface, angle, and
+camber were smaller and designed to just maintain horizontal flight with
+a horizontal thrust. Also, in such case, the propeller would not be
+thrusting downwards, but along a horizontal line which is obviously a
+more efficient arrangement if we regard the aeroplane merely from one
+point of view, <i>i.e.</i>, either with reference to velocity or climb.
+</p>
+<p>
+<span class="sc">Optimum Angle.</span>
+(Thrust horizontal).
+</p>
+<p>
+The velocity is less than at the smaller minimum angle, and, as
+aeroplanes are designed to-day, the area and angle of incidence of the
+surface is such as to secure a slight ascent at a low altitude. The
+camber of the surface is designed for this angle of incidence and
+velocity. The lift-drift ratio is best at this angle.
+</p>
+<p>
+<span class="sc">Best Climbing Angle.</span>
+</p>
+<p>
+The velocity is now still less by reason of the increased angle
+producing increase of drift. Less velocity at a given angle produces
+less lift, but the increased angle more or less offsets the loss of lift
+due to the decreased velocity; and, in addition, the thrust is now
+hauling the aeroplane upwards.
+</p>
+<p>
+<span class="sc">Maximum Angle.</span>
+</p>
+<p>
+The greater angle has now produced so much drift as to lessen the
+velocity to a point where the combined lifts from the surface and from
+the thrust are only just able to maintain horizontal flight. Any greater
+angle will result in a still lower lift-drift ratio. The lift will then
+become less than the weight and the aeroplane will consequently fall.
+Such a fall is known as "stalling" or "pancaking."
+</p>
+<p>
+<b>NOTE.&mdash;The golden rule for beginners: Never exceed the Best Climbing
+Angle. Always maintain the flying speed of the aeroplane.</b>
+</p>
+<p>
+<span class="pagenum"><a id="page69" name="page69"></a>[69]</span>
+</p>
+
+<p class="center">
+<span class="sc">SUMMARY.</span>
+</p>
+
+<table border="0" summary="summary">
+
+<tr><td>
+<p>
+<i>Essentials for Maximum Climb.</i>
+</p>
+</td><td>
+<p>
+<i>Essentials for Maximum Velocity.</i>
+</p>
+</td></tr>
+<tr><td>
+<ul style="list-style: none;">
+<li>    1. Low velocity.</li>
+<li>    2. Large surface.</li>
+<li>    3. Large angle relative to propeller thrust.</li>
+<li>    4. Large angle relative to direction of motion.</li>
+<li>    5. Large camber.</li>
+</ul>
+</td><td>
+<ul style="list-style: none;">
+<li>    1. High velocity.</li>
+<li>    2. Small surface.</li>
+<li>    3. Small angle relative to propeller thrust.</li>
+<li>    4. Small angle relative to direction of motion.</li>
+<li>    5. Small camber.</li>
+</ul>
+</td></tr>
+</table>
+
+<p>
+It is mechanically impossible to construct an aeroplane of reasonable
+weight of which it would be possible to vary the above opposing
+essentials. Therefore, all aeroplanes are designed as a compromise
+between Climb and Velocity.
+</p>
+<p>
+As a rule aeroplanes are designed to have at low altitude a slight
+margin of lift when the propeller thrust is horizontal. By this means,
+when the altitude is reached where the margin of lift disappears (on
+account of loss of engine power), and which is, consequently, the
+altitude where it is just possible to maintain horizontal flight, the
+aeroplane is flying with its thrust horizontal and with maximum
+efficiency (as distinct from engine and propeller efficiency).
+</p>
+<p>
+The margin of lift at low altitude, and when the thrust is horizontal,
+should then be such that the higher altitude at which the margin of lift
+is lost is that altitude at which most of the aeroplane's horizontal
+flight work is done. That ensures maximum velocity when most required.
+</p>
+<p>
+Unfortunately, where aeroplanes designed for fighting are concerned, the
+altitude where most of the work is done is that at which both maximum
+velocity and maximum margin of lift for power are required.
+</p>
+<p>
+Perhaps some day a brilliant inventor will design an aeroplane of
+reasonable weight and drift of which it will be possible for the pilot
+to vary at will the above-mentioned opposing essentials. Then we shall
+get maximum velocity, or maximum margin of lift, for power as required.
+Until then the design of the aeroplane must remain a compromise between
+Velocity and Climb.
+</p>
+<a name="note-14"><!--Note--></a>
+<p class="foot">
+<u>14</u> (<a href="#noteref-14">return</a>)<br />
+See Newton's laws in the Glossary at the end of the book.
+</p>
+<a name="note-15"><!--Note--></a>
+<p class="foot">
+<u>15</u> (<a href="#noteref-15">return</a>)<br />
+See "Aerofoil" in the Glossary.
+</p>
+<p>
+<span class="pagenum"><a id="page70" name="page70"></a>[70]</span>
+</p>
+<a name="h2HCH0002" id="h2HCH0002"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    CHAPTER II
+</h2>
+<h3>
+    STABILITY AND CONTROL
+</h3>
+<p>
+<span class="sc">Stability</span> is a condition whereby an object disturbed has a natural
+tendency to return to its first and normal position. Example: a weight
+suspended by a cord.
+</p>
+<p>
+<span class="sc">Instability</span> is a condition whereby an object disturbed has a natural
+tendency to move as far as possible away from its first position, with
+no tendency to return. Example: a stick balanced vertically upon your
+finger.
+</p>
+<p>
+<span class="sc">Neutral Instability</span> is a condition whereby an object disturbed has no
+tendency to move farther than displaced by the force of the disturbance,
+and no tendency to return to its first position.
+</p>
+<p>
+In order that an aeroplane may be reasonably controllable, it is
+necessary for it to possess some degree of stability longitudinally,
+laterally, and directionally.
+</p>
+<p>
+<span class="sc">Longitudinal Stability</span> in an aeroplane is its stability about an axis
+transverse to the direction of normal horizontal flight, and without
+which it would pitch and toss.
+</p>
+<p>
+<span class="sc">Lateral Stability</span> is its stability about its longitudinal axis, and
+without which it would roll sideways.
+</p>
+<p>
+<span class="sc">Directional Stability</span> is its stability about its vertical axis, and
+without which it would have no tendency to keep its course.
+</p>
+<p>
+For such directional stability to exist there must be, in effect,<a href="#note-16" name="noteref-16"><small>16</small></a>
+more "keel-surface" behind the vertical axis than there is in front of
+it. By keel-surface I mean everything
+
+<span class="pagenum"><a id="page71" name="page71"></a>[71]</span>
+
+ to be seen when looking at an
+aeroplane from the side of it&mdash;the sides of the body, undercarriage,
+struts, wires, etc. The same thing applies to a weathercock. You know
+what would happen if there was insufficient keel-surface behind the
+vertical axis upon which it is pivoted. It would turn off its proper
+course, which is opposite to the direction of the wind. It is very much
+the same in the case of an aeroplane.
+</p>
+
+<a name="image-0037"><!--IMG--></a>
+<div class="figure">
+<a href="images/p071.jpg"><img src="images/p071-t.jpg" width="320" height="174"
+alt="" /></a>
+</div>
+
+<p>
+The above illustration represents an aeroplane (directionally stable)
+flying along the course B. A gust striking it as indicated acts upon the
+greater proportion of keel-surface behind the turning axis and throws it
+into the new course. It does not, however, travel along the new course,
+owing to its momentum in the direction B. It travels, as long as such
+momentum lasts, in a direction which is the resultant of the two forces
+Thrust and Momentum. But the centre line of the aeroplane is pointing in
+the direction of the new course. Therefore its attitude, relative to the
+direction of motion, is more or less sideways, and it consequently
+receives an air pressure in the direction C. Such pressure, acting upon
+the keel-surface, presses the tail back towards its first position in
+which the aeroplane is upon its course B.
+</p>
+<p>
+What I have described is continually going on during flight, but in a
+well-designed aeroplane such stabilizing movements are, most of the
+time, so slight as to be imperceptible to the pilot.
+</p>
+<p>
+If an aeroplane was not stabilized in this way, it would
+
+<span class="pagenum"><a id="page72" name="page72"></a>[72]</span>
+
+ 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.
+</p>
+<p>
+<span class="sc">Longitudinal Stability.</span>&mdash;Flat surfaces are longitudinally stable owing
+to the fact that with decreasing angles of incidence the centre line of
+pressure (C.P.) moves forward.
+</p>
+<p>
+The C.P. is a line taken across the surface, transverse to the direction
+of motion, and about which all the air forces may be said to balance, or
+through which they may be said to act.
+</p>
+
+<a name="image-0038"><!--IMG--></a>
+<div class="figure">
+<a href="images/p072.jpg"><img src="images/p072-t.jpg" width="320" height="102"
+alt="D. C. B. A." /></a>
+</div>
+
+<p>
+Imagine A to be a flat surface, attitude vertical, travelling through
+the air in the direction of motion M. Its C.P. is then obviously along
+the exact centre line of the surface as illustrated. 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.<a href="#note-17" name="noteref-17"><small>17</small></a>
+</p>
+<p>
+Now, should some gust or eddy tend to make the surface decrease the
+angle, <i>i.e.</i>, dive, then the C.P. moves forward and pushes the front
+of the surface up. Should the surface tend to assume too large an
+angle, then the reverse happens&mdash;the C.P. moves back and pushes the
+rear of the surface up. Flat surfaces are, then, theoretically stable
+longitudinally. They are not, however, used, on account of their poor
+lift-drift ratio.
+</p>
+<p>
+<span class="pagenum"><a id="page73" name="page73"></a>[73]</span>
+</p>
+<p>
+As already explained, cambered surfaces are used, and these are
+longitudinally unstable at those angles of incidence producing a
+reasonable lift-drift ratio, <i>i.e.</i>, at angles below about 12°.
+</p>
+<p>
+A is a cambered surface, attitude approximately vertical, moving through
+the air in the direction M. Obviously the C.P. coincides with the
+transverse centre line of the surface.
+</p>
+<p>
+With decreasing angles, down to angles of about 30°, the C.P. moves
+forward as in the case of flat surfaces (see B); but angles above 30° do
+not interest us, since they produce a very low ratio of lift to drift.
+</p>
+
+<a name="image-0039"><!--IMG--></a>
+<div class="figure">
+<a href="images/p073.jpg"><img src="images/p073-t.jpg" width="320" height="104"
+alt="C. B. A." /></a>
+</div>
+
+<p>
+Below angles of about 30° (see C) the dipping front part of the surface
+assumes a negative angle of incidence resulting in the <i>downward</i> 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>i.e.</i>, 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."
+</p>
+<p>
+It is therefore necessary to find a means of stabilizing the naturally
+unstable cambered surface. This is usually secured by means of a
+stabilizing surface fixed some distance in the rear of the main surface,
+and it is a necessary condition that the neutral lift lines of the two
+surfaces, when projected
+
+<span class="pagenum"><a id="page74" name="page74"></a>[74]</span>
+
+ to meet each other, make a dihedral angle. In
+other words, the rear stabilizing surface must have a lesser angle of
+incidence than the main surface&mdash;certainly not more than one-third of
+that of the main surface. This is known as the longitudinal dihedral.
+</p>
+
+<a name="image-0040"><!--IMG--></a>
+<div class="figure">
+<a href="images/p074a.jpg"><img src="images/p074a-t.jpg" width="320" height="172"
+alt="" /></a>
+</div>
+
+<p>
+I may add that the tail-plane is sometimes mounted upon the aeroplane at
+the same angle as the main surface, but, in such cases, it attacks air
+which has received a downward deflection from the main surface, thus:
+</p>
+
+<a name="image-0041"><!--IMG--></a>
+<div class="figure">
+<a href="images/p074b.jpg"><img src="images/p074b-t.jpg" width="320" height="151"
+alt="" /></a>
+</div>
+
+<p>
+The angle at which the tail surface attacks the air (the angle of
+incidence) is therefore less than the angle of incidence of the main
+surface.
+</p>
+<p>
+<span class="pagenum"><a id="page75" name="page75"></a>[75]</span>
+</p>
+<p>
+I will now, by means of the following illustration, try to explain how
+the longitudinal dihedral secures stability:
+</p>
+
+<a name="image-0042"><!--IMG--></a>
+<div class="figure">
+<a href="images/p075.jpg"><img src="images/p075-t.jpg" width="320" height="204"
+alt="" /></a>
+</div>
+
+<p>
+First, imagine the aeroplane travelling in the direction of motion,
+which coincides with the direction of thrust T. The weight is, of
+course, balanced about a C.P., the resultant of the C.P. of the main
+surface and the C.P. of the stabilizing surface. For the sake of
+illustration, the stabilizing surface has been given an angle of
+incidence, and therefore has a lift and C.P. In practice the stabilizer
+is often set at no angle of incidence. In such case the proposition
+remains the same, but it is, perhaps, a little easier to illustrate
+it as above.
+</p>
+<p>
+Now, we will suppose that a gust or eddy throws the machine into the
+lower position. It no longer travels in the direction of T, since the
+momentum in the old direction pulls it off that course. M is now the
+resultant of the Thrust and the Momentum, and you will note that this
+results in a decrease in the angle our old friend the neutral lift line
+makes with M, <i>i.e.</i>, a decrease in the angle of incidence and therefore
+a decrease in lift.
+</p>
+<p>
+We will suppose that this decrease is 2°. Such decrease applies to
+both main surface and stabilizer, since both are fixed rigidly to the
+aeroplane.
+</p>
+<p>
+The main surface, which had 12° angle, has now only 10°, <i>i.e.</i>, a loss
+of <i>one-sixth</i>.
+</p>
+<p>
+<span class="pagenum"><a id="page76" name="page76"></a>[76]</span>
+</p>
+<p>
+The stabilizer, which had 4° angle, has now only 2°, <i>i.e.</i>, a loss of
+<i>one-half</i>.
+</p>
+<p>
+The latter has therefore lost a greater <i>proportion</i> of its angle of
+incidence, and consequently its lift, than has the main surface. It must
+then fall relative to the main surface. The tail falling, the aeroplane
+then assumes its first position, though at a slightly less altitude.
+</p>
+<p>
+Should a gust throw the nose of the aeroplane up, then the reverse
+happens. Both main surface and stabilizer increase their angles of
+incidence in the same amount, but the angle, and therefore the lift, of
+the stabilizer increases in greater proportion than does the angle and
+lift of the main surface, with the result that it lifts the tail. The
+aeroplane then assumes its first position, though at a slightly greater
+altitude.
+</p>
+<p>
+Do not fall into the widespread error that the angle of incidence varies
+as the angle of the aeroplane to the horizontal. It varies with such
+angle, but not as anything approaching it. Remember that the stabilizing
+effect of the longitudinal dihedral lasts only as long as there is
+momentum in the direction of the first course.
+</p>
+<p>
+These stabilizing movements are taking place all the time, even though
+imperceptible to the pilot.
+</p>
+<p>
+Aeroplanes have, in the past, been built with a stabilizing surface in
+front of the main surface instead of at the rear of it. In such design
+the main surface (which is then the tail surface as well as the
+principal lifting surface) must be set at a less angle than the forward
+stabilizing surface, in order to secure a longitudinal dihedral. The
+defect of such design lies in the fact that the main surface must have a
+certain angle to lift the weight&mdash;say 5°. Then, in order to secure a
+sufficiency of longitudinal stability, it is necessary to set the
+forward stabilizer at about 15°. 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.
+</p>
+<p>
+Aeroplanes of the Dunne and similar types possess no
+
+<span class="pagenum"><a id="page77" name="page77"></a>[77]</span>
+
+ stabilizing surface
+distinct from the main surface, but they have a longitudinal dihedral
+which renders them stable.
+</p>
+<p>
+The main surface towards the wing-tips is given a decreasing angle of
+incidence and corresponding camber. The wing-tips then act as
+longitudinal stabilizers.
+</p>
+
+<a name="image-0043"><!--IMG--></a>
+<div class="figure">
+<a href="images/p077.jpg"><img src="images/p077-t.jpg" width="320" height="153"
+alt="" /></a>
+</div>
+
+<p>
+This design of aeroplane, while very interesting, has not proved very
+practicable, owing to the following disadvantages: (1) The plan design
+is not, from a mechanical point of view, so sound as that of the
+ordinary aeroplane surface, which is, in plan, a parallelogram. It is,
+then, necessary to make the strength of construction greater than would
+otherwise be the case. That means extra weight. (2) The plan of the
+surface area is such that the aspect ratio is not so high as if the
+surface was arranged with its leading edges at right angles to the
+direction of motion. The lower the aspect ratio, then, the less the
+lift. This design, then, produces less lift for weight of surface than
+would the same surface if arranged as a parallelogram. (3) In order to
+secure the longitudinal dihedral, the angle of incidence has to be very
+much decreased towards the wing-tips. Then, in order that the lift-drift
+ratio may be preserved, there must be a corresponding decrease in the
+camber. That calls for surface ribs of varying cambers, and results in
+an expensive and lengthy job for the builder. (4) In order to secure
+directional stability, the surface is, in the centre, arranged to dip
+down in the form of a V, pointing towards the direction of motion.
+Should the aeroplane turn off its course, then its momentum in the
+direction of its first course causes it to move in a direction the
+resultant of the thrust and the momentum. It then moves in a more or
+less sideways attitude, which results
+
+<span class="pagenum"><a id="page78" name="page78"></a>[78]</span>
+
+ in an air pressure upon one side
+of the V, and which tends to turn the aeroplane back to its first
+course. This arrangement of the surface results in a bad drift. Vertical
+surfaces at the wing-tips may also be set at an angle producing the same
+stabilizing effect, but they also increase the drift.
+</p>
+<p>
+The gyroscopic action of a rotary engine will affect the longitudinal
+stability when an aeroplane is turned to right or left. In the case of a
+Gnome engine, fitted to a "pusher" aeroplane, 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. When fitted to a
+"tractor" aeroplane, the engine is reversed so that a reverse condition
+results. In modern aeroplanes this tendency is not sufficiently
+important to bother about, except in the matter of spiral descents (see
+section headed "Spinning"). In the old days of crudely designed and
+under-powered "pusher" 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."
+</p>
+<p>
+<span class="sc">Lateral Stability</span> 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>i.e.</i>, the
+upward inclination of the surface towards its wing-tips thus:
+</p>
+
+<a name="image-0044"><!--IMG--></a>
+<div class="figure">
+<a href="images/p078.jpg"><img src="images/p078-t.jpg" width="320" height="225"
+alt="" /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page79" name="page79"></a>[79]</span>
+</p>
+<p>
+Imagine the top <b>V</b>, illustrated opposite, to be the front view of a
+surface flying towards you. The horizontal equivalent (H.E.) of the left
+wing is the same as that of the right wing. Therefore, the lift of one
+wing is equal to the lift of the other, and the weight, being situated
+always in the centre, is balanced.
+</p>
+<p>
+If some movement of the air causes the surface to tilt sideways, as in
+the lower illustration, then you will note that the H.E. of the left
+wing increases, and the H.E. of the right wing decreases. The left wing
+then, having the greatest lift, rises; and the surface assumes its first
+and normal position.
+</p>
+<p>
+Unfortunately, however, the righting effect is not proportional to the
+difference between the right and left H.E.'s.
+</p>
+
+<a name="image-0045"><!--IMG--></a>
+<div class="figure-al">
+<a href="images/p079.jpg"><img src="images/p079-t.jpg" width="320" height="245"
+alt="    R, Direction of reaction of wing indicated.
+  R R, Resultant direction of reaction of both wings.
+    M, Horizontal (sideway) component of reaction.
+    L, Vertical component of reaction (lift)." /></a>
+<br /><div>
+    R, Direction of reaction of wing indicated.<br />
+  R R, Resultant direction of reaction of both wings.<br />
+    M, Horizontal (sideway) component of reaction.<br />
+    L, Vertical component of reaction (lift).</div>
+</div>
+
+<p>
+In the case of A, the resultant direction of the reaction of both wings
+is opposed to the direction of gravity or weight. The two forces R R
+and gravity are then evenly balanced, and the surface is in a state of
+equilibrium.
+</p>
+<p>
+<span class="pagenum"><a id="page80" name="page80"></a>[80]</span>
+</p>
+<p>
+In the case of B, you will note that the R R is not directly opposed to
+gravity. This results in the appearance of M, and so the resultant
+direction of motion of the aeroplane is no longer directly forward, but
+is along a line the resultant of the Thrust and M. In other words, it
+is, while flying forward, at the same time moving sideways in the
+direction M.
+</p>
+<p>
+In moving sideways, the keel-surface receives, of course, a pressure
+from the air equal and opposite to M. Since such surface is greatest in
+effect towards the tail, then the latter must be pushed sideways. That
+causes the aeroplane to turn; and, the highest wing being on the outside
+of the turn, it has a greater velocity than the lower wing. That
+produces greater lift, and tends to tilt the aeroplane over still more.
+Such tilting tendency is, however, opposed by the difference in the
+H.E.'s of the two wings.
+</p>
+<p>
+It then follows that, for the lateral dihedral angle to be effective,
+such angle must be large enough to produce, when the aeroplane tilts, a
+difference in the H.E.'s of the two wings, which difference must be
+sufficient to not only oppose the tilting tendency due to the aeroplane
+turning, but sufficient to also force the aeroplane back to its original
+position of equilibrium.
+</p>
+<p>
+It is now, I hope, clear to the reader that the lateral dihedral is not
+quite so effective as would appear at first sight. Some designers,
+indeed, prefer not to use it, since its effect is not very great, and
+since it must be paid for in loss of H.E. and consequently loss of lift,
+thus decreasing the lift-drift ratio, <i>i.e.</i>, 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.
+</p>
+<p>
+<i>The disposition of the keel-surface</i> affects the lateral stability. It
+should be, in effect, equally divided by the longitudinal turning axis
+of the aeroplane. If there is an excess of keel-surface above or below
+such axis, then a side gust striking it will tend to turn the aeroplane
+over sideways.
+</p>
+<p>
+<i>The position of the centre of gravity</i> affects lateral stability. If
+too low, it produces a pendulum effect and causes the aeroplane to roll
+sideways.
+</p>
+<p>
+If too high, it acts as a stick balanced vertically would act. If
+disturbed, it tends to travel to a position as far as
+
+<span class="pagenum"><a id="page81" name="page81"></a>[81]</span>
+
+ possible from its
+original position. It would then tend, when moved, to turn the aeroplane
+over sideways and into an upside-down position.
+</p>
+<p>
+From the point of view of lateral stability, the best position for the
+centre of gravity is one a little below the centre of drift. This
+produces a little lateral stability without any marked pendulum effect.
+</p>
+<p>
+<i>Propeller torque</i> affects lateral stability. An aeroplane tends to turn
+over sideways in the opposite direction to which the propeller revolves.
+</p>
+
+<a name="image-0046"><!--IMG--></a>
+<div class="figure">
+<a href="images/p081a.jpg"><img src="images/p081a-t.jpg" width="320" height="71"
+alt="" /></a>
+</div>
+
+<p>
+This tendency is offset by increasing the angle of incidence (and
+consequently the lift) of the side tending to fall; and it is always
+advisable, if practical considerations allow it, to also decrease the
+angle upon the other side. In that way it is not necessary to depart so
+far from the normal angle of incidence at which the lift-drift ratio is
+highest.
+</p>
+<p>
+<i>Wash-in</i> is the term applied to the increased angle.
+</p>
+<p>
+<i>Wash-out</i> is the term applied to the decreased angle.
+</p>
+<p>
+Both lateral and directional stability may be improved by washing out
+the angle of incidence on both sides of the surface, thus:
+</p>
+
+<a name="image-0047"><!--IMG--></a>
+<div class="figure">
+<a href="images/p081b.jpg"><img src="images/p081b-t.jpg" width="320" height="75"
+alt="" /></a>
+</div>
+
+<p>
+The decreased angle decreases the drift and therefore the effect of
+gusts upon the wing-tips, which is just where they have the most effect
+upon the aeroplane, owing to the distance from the turning axis.
+</p>
+<p>
+The wash-out also renders the ailerons (lateral controlling services)
+more effective, as, in order to operate them, it is
+
+<span class="pagenum"><a id="page82" name="page82"></a>[82]</span>
+
+ not then necessary
+to give them such a large angle of incidence as would otherwise be
+required.
+</p>
+
+<a name="image-0048"><!--IMG--></a>
+<div class="figure">
+<a href="images/p082a.jpg"><img src="images/p082a-t.jpg" width="320" height="242"
+alt="Note: Observe that the inclination of the ailerons to the
+surface is the same in each case." /></a>
+</div>
+
+<p>
+The less the angle of incidence of the ailerons, the better their
+lift-drift ratio, i.e., their efficiency. You will note that, while the
+aileron attached to the surface with washed-out angle is operated to the
+same extent as the aileron illustrated above it, its angle of incidence
+is considerably less. Its efficiency is therefore greater.
+</p>
+<p>
+The advantages of the wash-in must, of course, be paid for in some loss
+of lift, as the lift decreases with the decreased angle.
+</p>
+<p>
+In order to secure all the above described advantages, a combination is
+sometimes effected, thus:
+</p>
+
+<a name="image-0049"><!--IMG--></a>
+<div class="figure">
+<a href="images/p082b.jpg"><img src="images/p082b-t.jpg" width="320" height="133"
+alt="'Wash Out' on both sides relative to the Centre." /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page83" name="page83"></a>[83]</span>
+</p>
+<p>
+<span class="sc">Banking.</span>&mdash;An aeroplane turned off its course to right or left does not
+at once proceed along its new course. Its momentum in the direction of
+its first course causes it to travel along a line the resultant of such
+momentum and the thrust. In other words, it more or less skids sideways
+and away from the centre of the turn. Its lifting surfaces do not then
+meet the air in their correct attitude, and the lift may fall to such an
+extent as to become less than the weight, in which case the aeroplane
+must fall. This bad effect is minimized by "banking," <i>i.e.</i>, tilting
+the aeroplane sideways. The bottom of the lifting surface is in that way
+opposed to the air through which it is moving in the direction of the
+momentum and receives an opposite air pressure. The rarefied area over
+the top of the surface is rendered still more rare, and this, of course,
+assists the air pressure in opposing the momentum.
+</p>
+<p>
+The velocity of the "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.
+</p>
+<p>
+The sharper the turn, the greater the effect of the centrifugal force,
+and therefore the steeper should be the "bank." <i>Experientia docet</i>.
+</p>
+<p>
+<i>The position of the centre of gravity</i> affects banking. A low C.G. will
+tend to swing outward from the centre of the turn, and will cause the
+aeroplane to bank&mdash;perhaps too much, in which case the pilot must remedy
+matters by operating the ailerons.
+</p>
+<p>
+A high C.G. also tends to swing outward from the centre of the turn. It
+will tend to make the aeroplane bank the wrong way, and such effect must
+be remedied by means of the ailerons.
+</p>
+<p>
+The pleasantest machine from a banking point of view is one in which the
+C.G. is a little below the centre of drift. It tends to bank the
+aeroplane the right way for the turn, and the pilot can, if necessary,
+perfect the bank by means of the ailerons.
+</p>
+<p>
+<i>The disposition of the keel-surface</i> 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
+
+<span class="pagenum"><a id="page84" name="page84"></a>[84]</span>
+
+ to bank, perhaps too much. An excess
+of keel-surface below the axis has the reverse effect.
+</p>
+<p>
+<span class="sc">Side-Slipping.</span>&mdash;This usually occurs as a result of over-banking. It is
+always the result of the aeroplane tilting sideways and thus decreasing
+the horizontal equivalent, and therefore the lift, of the surface. An
+excessive "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>i.e.</i>, side-slip.
+</p>
+
+<a name="image-0050"><!--IMG--></a>
+<div class="figure">
+<a href="images/p084.jpg"><img src="images/p084-t.jpg" width="320" height="263"
+alt="A. B. C." /></a>
+</div>
+
+<p>
+When making a very sharp turn it is necessary to bank very steeply
+indeed. If, at the same time, the longitudinal axis of the aeroplane
+remains approximately horizontal, then there must be a fall, and the
+direction of motion will be the resultant of the thrust and the fall as
+illustrated above in sketch A. The lifting surfaces and the controlling
+surfaces are not then meeting the air in the correct attitude, with the
+result that, in addition to falling, the aeroplane will probably become
+quite unmanageable.
+</p>
+<p>
+<span class="pagenum"><a id="page85" name="page85"></a>[85]</span>
+</p>
+<p>
+The pilot, however, prevents such a state of affairs from happening by
+"nosing-down," <i>i.e.</i>, 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, <i>but</i> its
+attitude relative to the direction of motion is correct and the
+controlling surfaces are all of them working efficiently. The recovery
+of a normal attitude relative to the Earth is then made as illustrated
+in sketch C.
+</p>
+<p>
+The pilot must then learn to know just the angle of bank at which the
+margin of lift is lost, and, if a sharp turn necessitates banking beyond
+that angle, he must "nose-down."
+</p>
+<p>
+In this matter of banking and nosing-down, and, indeed, regarding
+stability and control generally, the golden rule for all but very
+experienced pilots should be: <i>Keep the aeroplane in such an attitude
+that the air pressure is always directly in the pilot's face.</i> The
+aeroplane is then always engaging the air as designed to do so, and both
+lifting and controlling surfaces are acting efficiently. The only
+exception to this rule is a vertical dive, and I think that is obviously
+not an attitude for any but very experienced pilots to hanker after.
+</p>
+<p>
+<span class="sc">Spinning.</span>&mdash;This is the worst of all predicaments the pilot can find
+himself in. Fortunately it rarely happens.
+</p>
+<p>
+It is due to the combination of (1) a very steep spiral descent of small
+radius, and (2) insufficiency of keel-surface behind the vertical axis,
+or the jamming of the rudder and/or elevator into a position by which
+the aeroplane is forced into an increasingly steep and small spiral.
+</p>
+<p>
+Owing to the small radius of such a spiral, the mass of the aeroplane
+may gain a rotary momentum greater, in effect, than the air pressure of
+the keel-surface or controlling surfaces opposed to it; and, when once
+such a condition occurs, it is difficult to see what can be done by the
+pilot to remedy it. The sensible pilot will not go beyond reasonable
+limits of steepness and radius when executing spiral descents.
+</p>
+<p>
+<span class="pagenum"><a id="page86" name="page86"></a>[86]</span>
+</p>
+
+<a name="image-0051"><!--IMG--></a>
+<div class="figure">
+<a href="images/p086.jpg"><img src="images/p086-t.jpg" width="320" height="519"
+alt="Nose Dive Spin." /></a>
+</div>
+
+<p>
+In this connection every pilot of an aeroplane fitted with a rotary
+engine should bear in mind the gyroscopic effect of such engine. In the
+case of such an engine fitted to a "pusher" aeroplane, its effect when a
+left-hand turn is made
+
+<span class="pagenum"><a id="page87" name="page87"></a>[87]</span>
+
+ is to depress the nose of the machine. If fitted
+to a "tractor" it is reversed, so the effect is to depress the nose
+if a right-hand turn is made. The sharper the turn, the greater such
+effect&mdash;an effect which may render the aeroplane unmanageable if the
+spiral is one of very small radius and the engine is revolving with
+sufficient speed to produce a material gyroscopic effect. Such
+gyroscopic effect should, however, slightly <i>assist</i> the pilot to
+navigate a small spiral if he will remember to (1) make <i>right-hand</i>
+spirals in the case of a "pusher," (2) make <i>left-hand</i> spirals in the
+case of a "tractor." The effect will then be to keep the nose up and
+prevent a nose-dive. I say "slightly" assist because the engine is, of
+course, throttled down for a spiral descent, and its lesser revolutions
+will produce a lesser gyroscopic effect.
+</p>
+<p>
+On the other hand, it might be argued that if the aeroplane gets into a
+"spin," anything tending to depress the nose of the machine is of value,
+since it is often claimed that the best way to get out of a spin is to
+put the machine into a nose-dive&mdash;the great velocity of the dive
+rendering the controls more efficient and better enabling the pilot to
+regain control. It is, however, a very contentious point, and few are
+able to express opinions based on practice, since pilots indulging in
+nose-dive spins are either not heard of again or have usually but a hazy
+recollection of exactly what happened to them.
+</p>
+<p>
+<span class="sc">Gliding Descent Without Propeller Thrust.</span>&mdash;All aeroplanes are, or should
+be, designed to assume their correct gliding angle when the power and
+thrust is cut off. This relieves the pilot of work, worry, and danger
+should he find himself in a fog or cloud. The pilot, although he may not
+realize it, maintains the correct attitude of the aeroplane by observing
+its position relative to the horizon. Flying into a fog or cloud the
+horizon is lost to view, and he must then rely upon his instruments&mdash;(1)
+the compass for direction; (2) an inclinometer (arched spirit-level)
+mounted transversely to the longitudinal axis, for lateral stability;
+and (3) an inclinometer mounted parallel to the longitudinal axis, or
+the airspeed indicator, which will indicate a nose-down position by
+increase in air speed, and a tail-down position by decrease in air
+speed.
+</p>
+<p>
+<span class="pagenum"><a id="page88" name="page88"></a>[88]</span>
+</p>
+<p>
+The pilot is then under the necessity of watching three instruments
+and manipulating his three controls to keep the instruments indicating
+longitudinal, lateral, and directional stability. That is a feat beyond
+the capacity of the ordinary man. If, however, by the simple movement
+of throttling down the power and thrust, he can be relieved of looking
+after the longitudinal stability, he then has only two instruments to
+watch. That is no small job in itself, but it is, at any rate, fairly
+practicable.
+</p>
+
+<a name="image-0052"><!--IMG--></a>
+<div class="figure">
+<a href="images/p087.jpg"><img src="images/p087-t.jpg" width="320" height="159"
+alt="" /></a>
+</div>
+
+<p>
+Aeroplanes are, then, designed, or should be, so that the centre of
+gravity is slightly forward of centre of lift. The aeroplane is then,
+as a glider, nose-heavy&mdash;and the distance the C.G. is placed in advance
+of the C.L. should be such as to ensure a gliding angle producing a
+velocity the same as the normal flying speed (for which the strength
+of construction has been designed).
+</p>
+<p>
+In order that this nose-heavy tendency should not exist when the thrust
+is working and descent not required, the centre of thrust is placed a
+little below the centre of drift or resistance, and thus tends to pull
+up the nose of the aeroplane.
+</p>
+<p>
+The distance the centre of thrust is placed below the centre of drift
+should be such as to produce a force equal and opposite to that due to
+the C.G. being forward of the C.L. (see illustration above).
+</p>
+<p>
+<span class="sc">Looping and Upside-Down Flying.</span>&mdash;If a loop is desired, it is best to
+throttle the engine down at point A. The C.G. being forward of the C.P.,
+then causes the aeroplane to nose
+
+<span class="pagenum"><a id="page89" name="page89"></a>[89]</span>
+
+ down, and assists the pilot in making
+a reasonably small loop along the course C and in securing a quick
+recovery. If the engine is not throttled down, then the aeroplane may be
+expected to follow the course D, which results in a longer nose dive
+than in the case of the course C.
+</p>
+
+<a name="image-0053"><!--IMG--></a>
+<div class="figure">
+<a href="images/p088.jpg"><img src="images/p088-t.jpg" width="320" height="348"
+alt="Position A. Path B. Path C. Path D." /></a>
+</div>
+
+<p>
+A steady, gentle movement of the elevator is necessary. A jerky movement
+may change the direction of motion so suddenly as to produce dangerous
+air stresses upon the surfaces, in which case there is a possibility of
+collapse.
+</p>
+<p>
+If an upside-down flight is desired, the engine may, or may not, be
+throttled down at point A. If not throttled down, then the elevator must
+be operated to secure a course approximately in the direction B. If it
+is throttled down, then the course must be one of a steeper angle than
+B, or there will be danger of stalling.
+</p>
+<a name="note-16"><!--Note--></a>
+<p class="foot">
+<u>16</u> (<a href="#noteref-16">return</a>)<br />
+"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.
+</p>
+<a name="note-17"><!--Note--></a>
+<p class="foot">
+<u>17</u> (<a href="#noteref-17">return</a>)<br />
+The reason the C.P. of an inclined surface is forward of
+the centre of the surface is because the front of the surface does most
+of the work, as explained on <a href="#page62">p. 62</a>.
+</p>
+<p>
+<span class="pagenum"><a id="page90" name="page90"></a>[90]</span>
+</p>
+<a name="h2HCH0003" id="h2HCH0003"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    CHAPTER III
+</h2>
+<h3>
+    RIGGING
+</h3>
+<p>
+In order to rig an aeroplane intelligently, and to maintain it in an
+efficient and safe condition, it is necessary to possess a knowledge of
+the stresses it is called upon to endure, and the strains likely to
+appear.
+</p>
+<p>
+<span class="sc">Stress</span> is the load or burden a body is called upon to bear. It is
+usually expressed by the result found by dividing the load by the number
+of superficial square inches contained in the cross-sectional area of
+the body.
+</p>
+
+<a name="image-0054"><!--IMG--></a>
+<div class="figure">
+<a href="images/p090.jpg"><img src="images/p090-t.jpg" width="320" height="150"
+alt="Cross Sectional area" /></a>
+</div>
+
+<p>
+Thus, if, for instance, the object illustrated above contains 4 square
+inches of cross-sectional area, and the total load it is called upon to
+endure is 10 tons, the stress would be expressed as 2-1/2 tons.
+</p>
+<p>
+<span class="sc">Strain</span> is the deformation produced by stress.
+</p>
+<p>
+<span class="sc">The Factor of Safety</span> 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
+
+<span class="pagenum"><a id="page91" name="page91"></a>[91]</span>
+
+ at which it will collapse is 10 cwts., the factor of
+safety is then 5.
+</p>
+<p>
+<span class="sc">Compression.</span>&mdash;The simple stress of compression tends to produce a
+crushing strain. Example: the interplane and fuselage struts.
+</p>
+<p>
+<span class="sc">Tension.</span>&mdash;The simple stress of tension tends to produce the strain of
+elongation. Example: all the wires.
+</p>
+<p>
+<span class="sc">Bending.</span>&mdash;The compound stress of bending is a combination of compression
+and tension.
+</p>
+
+<a name="image-0055"><!--IMG--></a>
+<div class="figure">
+<a href="images/p091a.jpg"><img src="images/p091a-t.jpg" width="320" height="60"
+alt="" /></a>
+</div>
+
+<p>
+The above sketch illustrates a straight piece of wood of which the top,
+centre, and bottom lines are of equal length. We will now imagine it
+bent to form a circle, thus:
+</p>
+
+<a name="image-0056"><!--IMG--></a>
+<div class="figure">
+<a href="images/p091b.jpg"><img src="images/p091b-t.jpg" width="320" height="191"
+alt="" /></a>
+</div>
+
+<p>
+The centre line is still the same length as before being bent; but the
+top line, being farther from the centre of the circle, is now longer
+than the centre line. That can be due only to the strain of elongation
+produced by the stress of tension. The wood between the centre line and
+the top
+
+<span class="pagenum"><a id="page92" name="page92"></a>[92]</span>
+
+ line is then in tension; and the farther from the centre, the
+greater the strain, and consequently the greater the tension.
+</p>
+<p>
+The bottom line, being nearest to the centre of the circle, is now
+shorter than the centre line. That can be due only to the strain of
+crushing produced by the stress of compression. The wood between the
+centre and bottom lines is then in compression; and the nearer the
+centre of the circle, the greater the strain, and consequently the
+greater the compression.
+</p>
+<p>
+It then follows that there is neither tension nor compression, <i>i.e.</i>,
+no stress, at the centre line, and that the wood immediately surrounding
+it is under considerably less stress than the wood farther away. This
+being so, the wood in the centre may be hollowed out without unduly
+weakening struts and spars. In this way 25 to 33 per cent. is saved in
+the weight of wood in an aeroplane.
+</p>
+<p>
+The strength of wood is in its fibres, which should, as far as possible,
+run without break from one end of a strut or spar to the other end. A
+point to remember is that the outside fibres, being farthest removed
+from the centre line, are doing by far the greatest work.
+</p>
+<p>
+<span class="sc">Shear Stress</span> is such that, when material collapses under it, one part
+slides over the other. Example: all the locking pins.
+</p>
+
+<a name="image-0057"><!--IMG--></a>
+<div class="figure">
+<a href="images/p092.jpg"><img src="images/p092-t.jpg" width="320" height="126"
+alt="" /></a>
+</div>
+
+<p>
+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.
+</p>
+<p>
+<span class="pagenum"><a id="page93" name="page93"></a>[93]</span>
+</p>
+<p>
+<span class="sc">Torsion.</span>&mdash;This is a twisting stress compounded of compression, tension,
+and shear stresses. Example: the propeller shaft.
+</p>
+<p>
+<span class="sc">Nature of Wood under Stress.</span>&mdash;Wood, for its weight, takes the stress of
+compression far better than any other stress. For instance: a
+walking-stick of less than 1 lb. in weight will, if kept perfectly
+straight, probably stand up to a compression stress of a ton or more
+before crushing; whereas, if the same stick is put under a bending
+stress, it will probably collapse to a stress of not more than about 50
+lb. That is a very great difference, and, since weight is of the
+greatest importance, the design of an aeroplane is always such as to, as
+far as possible, keep the various wooden parts of its construction in
+direct compression. Weight being of such vital importance, and designers
+all trying to outdo each other in saving weight, it follows that the
+factor of safety is rather low in an aeroplane. The parts in direct
+compression will, however, take the stresses safely provided the
+following conditions are carefully observed.
+</p>
+<p>
+<span class="sc">Conditions to be Observed:</span>
+</p>
+<p class="indent" style="text-indent: -5%;">
+1. <i>All the spars and struts must be perfectly straight.</i>
+</p>
+
+<a name="image-0058"><!--IMG--></a>
+<div class="figure">
+<a href="images/p093.jpg"><img src="images/p093-t.jpg" width="320" height="142"
+alt="" /></a>
+</div>
+
+<p class="indent" style="text-indent: 0;">
+The above sketch illustrates a section through an interplane
+strut. If the strut is to be kept straight, <i>i.e.</i>, 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
+
+<span class="pagenum"><a id="page94" name="page94"></a>[94]</span>
+
+ and tension on the other side,
+in which case it may be forced to take a bending stress for
+which it is not designed. Even if it does not collapse it will,
+in effect, become shorter, and thus throw out of adjustment
+the gap and all the wires attached to the top and bottom of
+the strut, with the result that the flight efficiency of the
+aeroplane will be spoiled.
+</p>
+
+<a name="image-0059"><!--IMG--></a>
+<a name="image-0060"><!--IMG--></a>
+<div class="figure">
+<a href="images/p094.jpg"><img src="images/p094-t.jpg" width="320" height="178"
+alt="Strut straight. Wires and gap correctly adjusted. Strut bent throwing wires and gap out of adjustment." /></a>
+</div>
+
+<p class="indent">
+The only exception to the above condition is what is known as
+the Arch. For instance, in the case of the Maurice Farman, the
+spars of the centre-section plane, which have to take the weight
+of the nacelle, are arched upwards. If this was not done, it is
+possible that rough landings might result in the weight causing
+the spars to become slightly distorted downwards. That would
+produce a dangerous bending stress, but, as long as the wood is
+arched, or, at any rate, kept from bending downwards, it will
+remain in direct compression and no danger can result.
+</p>
+<p class="indent" style="text-indent: -5%;">
+2. <i>Struts and spars must be symmetrical.</i> By that I mean that
+the cross-sectional dimensions must be correct, as otherwise
+there will be bulging places on the outside, with the result
+that the stress will not be evenly disposed about the centre
+of strength, and a bending stress may be produced.
+</p>
+<p class="indent" style="text-indent: -5%;">
+3. <i>Struts, spars, etc., must be undamaged.</i> Remember
+
+<span class="pagenum"><a id="page95" name="page95"></a>[95]</span>
+
+ that, from
+what I have already explained about bending stresses, the outside
+fibres of the wood are doing by far the most work. If these get
+bruised or scored, then the strut or spar suffers in strength
+much more than one might think at first sight; and, if it ever
+gets a tendency to bend, it is likely to collapse at that point.
+</p>
+<p class="indent" style="text-indent: -5%;">
+4. <i>The wood must have a good, clear grain with no cross-grain,
+knots, or shakes.</i> Such blemishes produce weak places and, if a
+tendency to bend appears, then it may collapse at such a point.
+</p>
+
+<a name="image-0061"><!--IMG--></a>
+<div class="figure">
+<a href="images/p095.jpg"><img src="images/p095-t.jpg" width="320" height="174"
+alt="Strut bedded properly. Strut bedded badly." /></a>
+</div>
+
+<p class="indent" style="text-indent: -5%;">
+5. <i>The struts, spars, etc., must be properly bedded into their
+sockets or fittings.</i> To begin with, they must be of good pushing
+or gentle tapping fit. They must never be driven in with a heavy
+hammer. Then again, a strut must bed well down all over its
+cross-sectional area as illustrated above; otherwise the stress
+of compression will not be evenly disposed about the centre of
+strength, and that may produce a bending stress. The bottom of
+the strut or spar should be covered with some sort of paint,
+bedded into the socket or fitting, and then withdrawn to see if
+the paint has stuck all over the bed.
+</p>
+<p class="indent" style="text-indent: -5%;">
+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
+
+<span class="pagenum"><a id="page96" name="page96"></a>[96]</span>
+
+ uniformly, but becomes unsymmetrical, <i>i.e.</i>, distorted.
+I have already explained the danger of that in condition 2. This
+should be minimized by <i>well varnishing the wood</i> to keep the
+moisture out of it.
+</p>
+<p>
+<span class="sc">Function of Interplane Struts.</span>&mdash;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.
+</p>
+
+<a name="image-0062"><!--IMG--></a>
+<div class="figure">
+<a href="images/p096.jpg"><img src="images/p096-t.jpg" width="320" height="136"
+alt="" /></a>
+</div>
+
+<p>
+Sometimes, as illustrated above, the rear spar is not so thick as the
+main spar, and it is then necessary to make up for that difference by
+making the rear struts correspondingly longer. If that is not done,
+then the top and bottom chords will not be parallel, and the top and
+bottom planes will have different angles of incidence. Also, the sockets
+or fittings, or even the spars upon which they are placed, sometimes
+vary in thickness owing to faulty manufacture. This must be offset by
+altering the length of the struts. The best way to proceed is to measure
+the distance between the top and bottom spars by the side of each strut,
+and if that distance, or "gap" as it is called, is not as stated in the
+aeroplane's specifications, then make it correct by
+
+<span class="pagenum"><a id="page97" name="page97"></a>[97]</span>
+
+ changing the length
+of the strut. This applies to both front and rear interplane struts.
+When measuring the gap, always be careful to measure from the centre of
+the spar, as it may be set at an angle, and the rear of it may be
+considerably lower than its front.
+</p>
+<p>
+<span class="sc">Boring Holes in Wood.</span>&mdash;It should be a strict rule that no spar be used
+which has an unnecessary hole in it. Before boring a hole, its position
+should be confirmed by whoever is in charge of the workshop. A bolt-hole
+should be of a size to enable the bolt to be pushed in, or, at any rate,
+not more than gently tapped in. Bolts should not be hammered in, as that
+may split the spar. On the other hand, a bolt should not be slack in its
+hole, as, in such a case, it may work sideways and split the spar, not
+to speak of throwing out of adjustment the wires leading from the lug
+or socket under the bolt-head.
+</p>
+<p>
+<span class="sc">Washers.</span>&mdash;Under the bolt-head, and also under the nut, a washer must be
+placed&mdash;a very large washer compared with the size which would be used
+in all-metal construction. This is to disperse the stress over a large
+area; otherwise the washer may be pulled into the wood and weaken it,
+besides possibly throwing out of adjustment the wires attached to the
+bolt or the fitting it is holding to the spar.
+</p>
+<p>
+<span class="sc">Locking.</span>&mdash;Now as regards locking the bolts. If split pins are used, be
+sure to see that they are used in such a way that the nut cannot
+possibly unscrew at all. The split pin should be passed through the bolt
+as near as possible to the nut. It should not be passed through both nut
+and bolt.
+</p>
+<p>
+If it is locked by burring over the edge of the bolt, do not use a heavy
+hammer and try to spread the whole head of the bolt. That might damage
+the woodwork inside the fabric-covered surface. Use a small, light
+hammer, and gently tap round the edge of the bolt until it is burred
+over.
+</p>
+<p>
+<span class="sc">Turnbuckles.</span>&mdash;A turnbuckle is composed of a central barrel into each end
+of which is screwed an eye-bolt. Wires are taken from the eyes of the
+eye-bolt, and so, by turning the barrel, they can be adjusted to their
+proper tension. Eye-bolts must be a good fit in the barrel; that is to
+say, not slack and not very tight. Theoretically it is not necessary
+
+<span class="pagenum"><a id="page98" name="page98"></a>[98]</span>
+
+ to screw the eye-bolt into the barrel for a distance greater than the
+diameter of the bolt, but, in practice, it is better to screw it in
+for a considerably greater distance than that if a reasonable degree
+of safety is to be secured.
+</p>
+<p>
+Now about turning the barrel to secure the right adjustment. The barrel
+looks solid, but, as a matter of fact, it is hollow and much more frail
+than it appears. For that reason it should not be turned by seizing
+it with pliers, as that may distort it and spoil the bore within it.
+The best method is to pass a piece of wire through the hole in its
+centre, and to use that as a lever. When the correct adjustment has been
+secured, the turnbuckle must be locked to prevent it from unscrewing.
+It is quite possible to lock it in such a way as to allow it to unscrew
+a quarter or a half turn, and that would throw the wires out of the very
+fine adjustment necessary. The proper way is to use the locking wire so
+that its direction is such as to oppose the tendency of the barrel to
+unscrew, thus:
+</p>
+
+<a name="image-0063"><!--IMG--></a>
+<div class="figure">
+<a href="images/p098.jpg"><img src="images/p098-t.jpg" width="320" height="69"
+alt="" /></a>
+</div>
+
+<p>
+<span class="sc">Wires.</span>&mdash;The following points should be carefully observed where wire is
+concerned:
+</p>
+<p>
+1. <i>Quality.</i>&mdash;It must not be too hard or too soft. An easy practical
+way of learning to know the approximate quality of wire is as follows:
+</p>
+<p>
+Take three pieces, all of the same gauge, and each about a foot in
+length. One piece should be too soft, another too hard, and the third
+piece of the right quality. Fix them in a vice, about an inch apart and
+in a vertical position, and with the light from a window shining upon
+them. Burnish them if necessary, and you will see a band of light
+reflected from each wire.
+</p>
+<p>
+Now bend the wires over as far as possible and away from the light.
+Where the soft wire is concerned, it will squash out at the bend, and
+this will be indicated by the band of light, which will broaden at that
+point. In the case of the wire which is too hard, the band of light will
+broaden very
+
+<span class="pagenum"><a id="page99" name="page99"></a>[99]</span>
+
+ little at the turn, but, if you look carefully, you will
+see some little roughness 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 roughness of surface.
+</p>
+<p>
+By making this experiment two or three times one can soon learn to know
+really bad wire from good, and also learn to know the strength of hand
+necessary to bend the right quality.
+</p>
+<p>
+2. <i>It must not be damaged.</i> That is to say, it must be unkinked,
+rustless, and unscored.
+</p>
+<p>
+3. Now as regards keeping wire in good condition. Where outside wires
+are concerned, they should be kept <i>well greased or oiled</i>, especially
+where bent over at the ends. Internal bracing wires cannot be reached
+for the purpose of regreasing them, as they are inside fabric-covered
+surfaces. They should be prevented from rusting by being painted with
+an anti-rust mixture. Great care should be taken to see that the wire
+is perfectly clean and dry before being painted. A greasy finger-mark
+is sufficient to stop the paint from sticking to the wire. In such
+a case there will be a little space between the paint and the wire.
+Air may enter there and cause the wire to rust.
+</p>
+<p>
+4. <i>Tension of Wires.</i>&mdash;The tension to which the wires are adjusted is
+of the greatest importance. All the wires should be of the same tension
+when the aeroplane is supported in such a way as to throw no stress upon
+them. If some wires are in greater tension than others, the aeroplane
+will quickly become distorted and lose its efficiency.
+</p>
+<p>
+In order to secure the same tension of all wires, the aeroplane,
+when being rigged, should be supported by packing underneath the lower
+surfaces as well as by packing underneath the fuselage or nacelle. In
+this way the anti-lift wires are relieved of the weight, and there is
+no stress upon any of the wires.
+</p>
+<p>
+As a general rule the wires of an aeroplane are tensioned too much.
+The tension should be sufficient to keep the framework rigid. Anything
+more than that lowers the factor of safety, throws various parts of the
+framework into undue compression, pulls the fittings into the wood, and
+will, in the end, distort the whole framework of the aeroplane.
+</p>
+<p>
+<span class="pagenum"><a id="page100" name="page100"></a>[100]</span>
+</p>
+<p>
+Only experience will teach the rigger what tension to employ. Much may
+be done by learning the construction of the various types of aeroplanes,
+the work the various parts do, and in cultivating a touch for tensioning
+wires by constantly handling them.
+</p>
+<p>
+5. <i>Wires with no Opposition Wires.</i>&mdash;In some few cases wires will be
+found which have no opposition wires pulling in the opposite direction.
+For instance, an auxiliary lift wire may run from the bottom of a strut
+to a spar in the top plane at a point between struts. In such a case
+great care should be taken not to tighten the wire beyond barely taking
+up the slack.
+</p>
+
+<a name="image-0064"><!--IMG--></a>
+<div class="figure">
+<a href="images/p100.jpg"><img src="images/p100-t.jpg" width="320" height="147"
+alt="Distortion of upper wing caused by auxiliary lift wire
+being too tight." /></a>
+</div>
+
+<p>
+Such a wire must be a little slack, or, as illustrated above, it will
+distort the framework. That, in the example given, will spoil the camber
+(curvature) of the surface, and result in changing both the lift and
+the drift at that part of the surface. Such a condition will cause the
+aeroplane to lose its directional stability and also to fly one wing
+down.
+</p>
+<p>
+I cannot impress this matter of tension upon the reader too strongly.
+It is of the utmost importance. When this, and also accuracy in securing
+the various adjustments, has been learned, one is on the way to becoming
+a good rigger.
+</p>
+<p>
+6. <i>Wire Loops.</i>&mdash;Wire is often bent over at its end in the form of a
+loop, in order to connect with a turnbuckle or fitting. These loops,
+even when made as perfectly as possible, have a tendency to elongate,
+thus spoiling the adjustment of the wires. Great care should be taken
+to minimize this
+
+<span class="pagenum"><a id="page101" name="page101"></a>[101]</span>
+
+ as much as possible. The rules to be observed are as
+follows:
+</p>
+
+<a name="image-0065"><!--IMG--></a>
+<div class="figure">
+<a href="images/p101.jpg"><img src="images/p101-t.jpg" width="320" height="165"
+alt="Wrong shape. Result of wrong shape. Right Shape." /></a>
+</div>
+
+<p class="indent" style="text-indent: -5%;">
+(<i>a</i>) The size of the loop should be as small as possible within
+reason. By that I mean it should not be so small as to create
+the possibility of the wire breaking.
+</p>
+<p class="indent" style="text-indent: -5%;">
+(<i>b</i>) The shape of the loop should be symmetrical.
+</p>
+<p class="indent" style="text-indent: -5%;">
+(<i>c</i>) It should have well-defined shoulders in order to prevent the
+ferrule from slipping up. At the same time, a shoulder should
+not have an angular place.
+</p>
+<p class="indent" style="text-indent: -5%;">
+(<i>d</i>) When the loop is finished it should be undamaged, and it
+should not be, as is often the case, badly scored.
+</p>
+<p>
+7. <i>Stranded Wire Cable.</i>&mdash;No splice should be served with twine until
+it has been inspected by whoever is in charge of the workshop. The
+serving may cover bad work.
+</p>
+<p>
+Should a strand become broken, then the cable should be replaced at once
+by another one.
+</p>
+<p>
+Control cables have a way of wearing out and fraying wherever they pass
+round pulleys. Every time an aeroplane comes down from flight the rigger
+should carefully examine the cables, especially where they pass round
+pulleys. If he finds a strand broken, he should replace the cable.
+</p>
+<p>
+The ailerons' balance cable on the top of the top plane is often
+forgotten, since it is necessary to fetch a high pair of steps in order
+to examine it. Don't slack this, or some gusty day the pilot may
+unexpectedly find himself minus the aileron control.
+</p>
+<p>
+<span class="pagenum"><a id="page102" name="page102"></a>[102]</span>
+</p>
+<p>
+<span class="sc">Controlling Surfaces.</span>&mdash;The greatest care should be exercised in rigging
+the aileron, rudder, and elevator properly, for the pilot entirely
+depends upon them in managing the aeroplane.
+</p>
+
+<a name="image-0066"><!--IMG--></a>
+<div class="figure">
+<a href="images/p102a.jpg"><img src="images/p102a-t.jpg" width="320" height="69"
+alt="Position in which controlling surface must be rigged. It
+will be its position during flight." /></a>
+</div>
+
+<p>
+The ailerons and elevator should be rigged so that, when the aeroplane
+is in flight, they are in a fair true line with the surface in front and
+to which they are hinged.
+</p>
+
+<a name="image-0067"><!--IMG--></a>
+<div class="figure">
+<a href="images/p102b.jpg"><img src="images/p102b-t.jpg" width="320" height="80"
+alt="Position during flight. Position in which controlling
+surface must be rigged." /></a>
+</div>
+
+<p>
+If the surface to which they are hinged is not a lifting surface, then
+they should be rigged to be in a fair true line with it as illustrated
+above.
+</p>
+<p>
+If the controlling surface is, as illustrated, hinged to the back of a
+lifting surface, then it should be rigged a little below the position it
+would occupy if in a fair true line with the surface in front. This is
+because, in such a case, it is set at an angle of incidence. This angle
+will, during flight, cause it to lift a little above the position in
+which it has been rigged. It is able to lift owing to a certain amount
+of slack in the control wire holding it&mdash;and one cannot adjust the
+control wire to have no slack, because that would cause it to bind
+against the pulleys and make the operation of it too hard for the pilot.
+It is therefore necessary to rig it a little below the position it would
+occupy if it was rigged in a fair true line with the surface in front.
+Remember that this only applies when it is hinged to a lifting surface.
+The greater the angle of incidence (and therefore the lift) of the
+surface in front, then the more the controlling surface will have to be
+rigged down.
+</p>
+<p>
+As a general rule it is safe to rig it down so that its trailing
+
+<span class="pagenum"><a id="page103" name="page103"></a>[103]</span>
+
+ edge is
+1/2 to 3/4 inch below the position it would occupy if in a fair line
+with the surface in front; or about 1/2 inch down for every 18 inches of
+chord of the controlling surface.
+</p>
+<p>
+When making these adjustments the pilot's control levers should be in
+their neutral positions. It is not sufficient to lash them. They should
+be rigidly blocked into position with wood packing.
+</p>
+<p>
+The surfaces must not be distorted in any way. If they are held true by
+bracing wires, then such wires must be carefully adjusted. If they are
+distorted and there are no bracing wires with which to true them, then
+some of the internal framework will probably have to be replaced.
+</p>
+<p>
+The controlling surfaces should never be adjusted with a view to
+altering the stability of the aeroplane. Nothing can be accomplished in
+that way. The only result will be to spoil the control of the aeroplane.
+</p>
+<p>
+<span class="sc">Fabric-Covered Surfaces.</span>&mdash;First of all make sure that there is no
+distortion of spars or ribs, and that they are perfectly sound. Then
+adjust the internal bracing wires so that the ribs are parallel to the
+direction of flight. The ribs usually cause the fabric to make a ridge
+where they occur, and, if such ridge is not parallel to the direction of
+flight, it will produce excessive drift. As a rule the ribs are at right
+angles to both main and rear spars.
+</p>
+<p>
+The tension of the internal bracing wires should be just sufficient to
+give rigidity to the framework. They should not be tensioned above that
+unless the wires are, at their ends, bent to form loops. In that case a
+little extra tension may be given to offset the probable elongation of
+the loops.
+</p>
+<p>
+The turnbuckles must now be generously greased, and served round with
+adhesive tape. The wires must be rendered perfectly dry and clean, and
+then painted with an anti-rust mixture. The woodwork must be well
+varnished.
+</p>
+<p>
+If it is necessary to bore holes in the spars for the purpose of
+receiving, for instance, socket bolts, then their places should be
+marked before being bored and their positions confirmed by whoever is in
+charge of the workshop. All is now ready for the sail-maker to cover the
+surface with fabric.
+</p>
+<p>
+<span class="pagenum"><a id="page104" name="page104"></a>[104]</span>
+</p>
+<p>
+<span class="sc">Adjustment of Control Cables.</span>&mdash;The adjustment of the control cables is
+quite an art, and upon it will depend to a large degree the quick and
+easy control of the aeroplane by the pilot.
+</p>
+<p>
+The method is as follows:
+</p>
+<p>
+After having rigged the controlling surfaces, and as far as possible
+secured the correct adjustment of the control cables, then remove the
+packing which has kept the control levers rigid. Then, sitting in the
+pilot's seat, move the control levers <i>smartly</i>. Tension the control
+cables so that when the levers are smartly moved there is no perceptible
+snatch or lag. Be careful not to tension the cables more than necessary
+to take out the snatch. If tensioned too much they will (1) bind round
+the pulleys and result in hard work for the pilot; (2) throw dangerous
+stresses upon the controlling surfaces, which are of rather flimsy
+construction; and (3) cause the cables to fray round the pulleys quicker
+than would otherwise be the case.
+</p>
+<p>
+Now, after having tensioned the cables sufficiently to take out the
+snatch, place the levers in their neutral positions, and move them to
+and fro about 1/8 inch either side of such positions. If the adjustment
+is correct, it should be possible to see the controlling surfaces move.
+If they do not move, then the control cables are too slack.
+</p>
+<p>
+<span class="sc">Flying Position.</span>&mdash;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."
+</p>
+<p>
+In the case of an aeroplane fitted with a stationary engine this is
+secured by packing up the machine so that the engine foundations are
+perfectly horizontal both longitudinally and laterally. This position is
+found by placing a straight-edge and a spirit-level across the engine
+foundations (both longitudinally and laterally), and great care should
+be taken to see that the bubble is exactly in the centre of the level.
+The slightest error will assume magnitude towards the extremities of the
+aeroplane. Great care should be taken to block up the aeroplane rigidly.
+In case it gets accidentally disturbed while the work is going on, it is
+well to constantly verify the flying position by running the
+straight-edge and spirit-level
+
+<span class="pagenum"><a id="page105" name="page105"></a>[105]</span>
+
+ 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. <i>This should never be omitted.</i>
+</p>
+<p>
+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.
+</p>
+<p>
+<span class="sc">Angle of Incidence.</span>&mdash;One method of finding the angle of incidence is
+as follows:
+</p>
+
+<a name="image-0068"><!--IMG--></a>
+<div class="figure">
+<a href="images/p105.jpg"><img src="images/p105-t.jpg" width="320" height="151"
+alt="" /></a>
+</div>
+
+<p>
+First place the aeroplane in its flying position. The corner of the
+straight-edge must be placed underneath and against the <i>centre</i> of the
+rear spar, and held in a horizontal position parallel to the ribs. This
+is secured by using a spirit-level. The set measurement will then be
+from the top of the straight-edge to the centre of the bottom surface of
+the main spar, or it may be from the top of the straight-edge to the
+lowest part of the leading edge. Care should be taken to measure from
+the centre of the spar and to see that the bubble is exactly in the
+centre of the level. Remember that all this will be useless if the
+aeroplane has not been placed accurately in its flying position.
+</p>
+<p>
+This method of finding the angle of incidence must be
+
+<span class="pagenum"><a id="page106" name="page106"></a>[106]</span>
+
+ used under every
+part of the lower surface where struts occur. It should not be used
+between the struts, because, in such places, the spars may have taken
+a slight permanent set up or down; not, perhaps, sufficiently bad to
+make any material difference to the flying of the machine, but quite
+bad enough to throw out the angle of incidence, which cannot be
+corrected at such a place.
+</p>
+<p>
+If the angle is wrong, it should then be corrected as follows:
+</p>
+<p>
+If it is too great, then the rear spar must be warped up until it is
+right, and this is done by slackening <i>all</i> the wires going to the top
+of the strut, and then tightening <i>all</i> the wires going to the bottom
+of the strut.
+</p>
+<p>
+If the angle is too small, then slacken <i>all</i> the wires going to the
+bottom of the strut, and tighten <i>all</i> the wires going to the top of the
+strut, until the correct adjustment is secured.
+</p>
+<p>
+Never attempt to adjust the angle by warping the main spar.
+</p>
+<p>
+The set measurement, which is of course stated in the aeroplane's
+specifications, should be accurate to 1/16 inch.
+</p>
+<p>
+<span class="sc">Lateral Dihedral Angle.</span>&mdash;One method of securing this is as follows, and
+this method will, at the same time, secure the correct angle of
+incidence:
+</p>
+
+<a name="image-0069"><!--IMG--></a>
+<a name="image-0070"><!--IMG--></a>
+<div class="figure">
+<a href="images/p106.jpg"><img src="images/p106-t.jpg" width="320" height="236"
+alt="FRONT ELEVATION and PLAN." /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page107" name="page107"></a>[107]</span>
+</p>
+<p>
+The strings, drawn very tight, must be taken over both the main and
+rear spars of the top surface. They must run between points on the
+spars just inside the outer struts. The set measurement (which should
+be accurate to 1/16 inch or less) is then from the strings down to four
+points on the main and rear spars of the centre-section surface. These
+points should be just inside the four centre-section struts; that is
+to say, as far as possible away from the centre of the centre-section.
+Do not attempt to take the set measurement near the centre of the
+centre-section.
+</p>
+<p>
+The strings should be as tight as possible, and, if it can be arranged,
+the best way to accomplish that is as shown in the above illustration,
+<i>i.e.</i>, by weighting the strings down to the spars by means of weights
+and tying their ends to struts. This will give a tight and motionless
+string.
+</p>
+<p>
+However carefully the above adjustment is made, there is sure to be some
+slight error. This is of no great importance, provided it is divided
+equally between the left- and right-hand wings. In order to make sure
+of this, certain check measurements should be taken as follows:
+</p>
+<p>
+Each bay must be diagonally measured, and such measurements must be the
+same to within 1/16 inch on each side of the aeroplane. As a rule such
+diagonal measurements are taken from the bottom socket of one strut to
+the top socket of another strut, but this is bad practice, because of
+possible inaccuracies due to faulty manufacture.
+</p>
+<p>
+The points between which the diagonal measurements are taken should be
+at fixed distances from the butts of the spars, such distances being the
+same on each side of the aeroplane, thus:
+</p>
+
+<a name="image-0071"><!--IMG--></a>
+<div class="figure">
+<a href="images/p107.jpg"><img src="images/p107-t.jpg" width="320" height="95"
+alt="Points A, B, and C, must be the same fixed distances
+from the butt as are Points D, E, and F. Distances 1 and 2 must equal
+distances 3 and 4." /></a>
+<br />
+Points A, B, and C, must be the same fixed distances
+from the butt as are Points D, E, and F. Distances 1 and 2 must equal
+distances 3 and 4.
+</div>
+
+<p>
+The above applies to both front and rear bays.
+</p>
+<p>
+<span class="pagenum"><a id="page108" name="page108"></a>[108]</span>
+</p>
+<p>
+It would be better to use the centre line of the aeroplane rather than
+the butts of the spars. It is not practicable to do so, however, as the
+centre line probably runs through the petrol tanks, etc.
+</p>
+<p>
+<span class="sc">The Dihedral Board.</span>&mdash;Another method of securing the dihedral angle, and
+also the angle of incidence, is by means of the dihedral board. It is a
+light handy thing to use, but leads to many errors, and should not be
+used unless necessary. The reasons are as follows:
+</p>
+<p>
+The dihedral board is probably not true. If it must be used, then it
+should be very carefully tested for truth beforehand. Another reason
+against its use is that it has to be placed on the spars in a position
+between the struts, and that is just where the spars may have a little
+permanent set up or down, or some inaccuracy of surface which will, of
+course, throw out the accuracy of the adjustment. The method of using
+it is as follows:
+</p>
+
+<a name="image-0072"><!--IMG--></a>
+<div class="figure">
+<a href="images/p108.jpg"><img src="images/p108-t.jpg" width="320" height="122"
+alt="" /></a>
+</div>
+
+<p>
+The board is cut to the same angle as that specified for the upward
+inclination of the surface towards its wing-tips. It is placed on the
+spar as indicated above, and it is provided with two short legs to raise
+it above the flanges of the ribs (which cross over the spars), as they
+may vary in depth. A spirit-level is then placed on the board, and the
+wires must be adjusted to give the surface such an inclination as to
+result in the bubble being in the centre of the level. This operation
+must be performed in respect of each bay both front and rear. The bays
+must then be diagonally measured as already explained.
+</p>
+<p>
+<span class="sc">Yet Another Method</span> of finding the dihedral angle, and at the same time
+the angle of incidence, is as follows:
+</p>
+<p>
+<span class="pagenum"><a id="page109" name="page109"></a>[109]</span>
+</p>
+<p>
+A horizontal line is taken from underneath the butt of each spar,
+and the set measurement is either the angle it makes with the spar,
+or a fixed measurement from the line to the spar taken at a specified
+distance from the butt. This operation must be performed in respect of
+both main and rear spars, and all the bays must be measured diagonally
+afterwards.
+</p>
+
+<a name="image-0073"><!--IMG--></a>
+<div class="figure">
+<a href="images/p109a.jpg"><img src="images/p109a-t.jpg" width="320" height="102"
+alt="" /></a>
+</div>
+
+<p>
+Whichever method is used, be sure that after the job is done the spars
+are perfectly straight.
+</p>
+<p>
+<span class="sc">Stagger.</span>&mdash;The stagger is the distance the top surface is in advance of
+the bottom surface when the aeroplane is in flying position. The set
+measurement is obtained as follows:
+</p>
+
+<a name="image-0074"><!--IMG--></a>
+<div class="figure">
+<a href="images/p109b.jpg"><img src="images/p109b-t.jpg" width="320" height="162"
+alt="" /></a>
+</div>
+
+<p>
+Plumb-lines must be dropped over the leading edge of the top surface
+wherever struts occur, and also near the fuselage. The set measurement
+is taken from the front of the lower leading edge to the plumb-lines. It
+makes a difference whether the measurement is taken along a horizontal
+line
+
+<span class="pagenum"><a id="page110" name="page110"></a>[110]</span>
+
+ (which can be found by using a straight-edge and a spirit-level) or
+along a projection of the chord. The line along which the measurement
+should be taken is laid down in the aeroplane's specifications.
+</p>
+<p>
+If a mistake is made and the measurement taken along the wrong line, it
+may result in a difference of perhaps 1/4 inch or more to the stagger,
+with the certain result that the aeroplane will, in flight, be
+nose-heavy or tail-heavy.
+</p>
+<p>
+After the adjustments of the angles of incidence, dihedral, and stagger
+have been secured, it is as well to confirm all of them, as, in making
+the last adjustment, the first one may have been spoiled.
+</p>
+<p>
+<span class="sc">Over-All Adjustments.</span>&mdash;The following over-all check measurements should
+now be taken.
+</p>
+
+<a name="image-0075"><!--IMG--></a>
+<div class="figure">
+<a href="images/p110.jpg"><img src="images/p110-t.jpg" width="320" height="207"
+alt="The dotted lines on the surface represent the spars within it." /></a>
+<br />
+The dotted lines on the surface represent the spars within it.
+</div>
+
+<p>
+The straight lines AC and BC should be equal to within 1/8 inch. The
+point C is the centre of the propeller, or, in the case of a "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&mdash;two measurements on each side of the
+aeroplane.
+</p>
+<p>
+The two measurements FD and FE should be equal to
+
+<span class="pagenum"><a id="page111" name="page111"></a>[111]</span>
+
+ within 1/8 inch. F is
+the centre of the fuselage or rudder-post. D and E are points marked on
+both top and bottom rear spars, and each must be the same fixed distance
+from the butt of the spar. Two measurements on each side of the
+aeroplane.
+</p>
+<p>
+If these over-all measurements are not correct, then it is probably due
+to some of the drift or anti-drift wires being too tight or too slack.
+It may possibly be due to the fuselage being out of truth, but of course
+the rigger should have made quite sure that the fuselage was true before
+rigging the rest of the machine. Again, it may be due to the internal
+bracing wires within the lifting surfaces not being accurately adjusted,
+but of course this should have been seen to before covering the surfaces
+with fabric.
+</p>
+<p>
+<span class="sc">Fuselage.</span>&mdash;The method of truing the fuselage is laid down in the
+aeroplane's specifications. After it has been adjusted according to the
+specified directions, it should then be arranged on trestles in such
+a way as to make about three-quarters of it towards the tail stick
+out unsupported. In this way it will assume a condition as near as
+possible to flying conditions, and when it is in this position the set
+measurements should be confirmed. If this is not done it may be out of
+truth, but perhaps appear all right when supported by trestles at both
+ends, as, in such case, its weight may keep it true as long as it is
+resting upon the trestles.
+</p>
+<p>
+<span class="sc">The Tail-Plane</span> (<span class="sc">Empennage</span>).&mdash;The exact angle of incidence of the
+tail-plane is laid down in the aeroplane's specifications. It is
+necessary to make sure that the spars are horizontal when the aeroplane
+is in flying position and the tail unsupported as explained above under
+the heading of Fuselage. If the spars are tapered, then make sure that
+their centre lines are horizontal.
+</p>
+<p>
+<span class="sc">Undercarriage.</span>&mdash;The undercarriage must be very carefully aligned as laid
+down in the specifications.
+</p>
+<p>
+1. The aeroplane must be placed in its flying position and sufficiently
+high to ensure the wheels being off the ground when rigged. When in this
+position the axle must be horizontal
+
+<span class="pagenum"><a id="page112" name="page112"></a>[112]</span>
+
+ and the bracing wires adjusted to
+secure the various set measurements stated in the specifications.
+</p>
+<p>
+2. Make sure that the struts bed well down into their sockets.
+</p>
+<p>
+3. Make sure that the shock absorbers are of equal tension. In the case
+of rubber shock absorbers, both the number of turns and the lengths must
+be equal.
+</p>
+
+<p class="center">
+<span class="sc">How to Diagnose Faults in Flight, Stability, and Control.</span>
+</p>
+<p>
+<span class="sc">Directional Stability</span> will be badly affected if there is more drift
+(<i>i.e.</i>, resistance) on one side of the aeroplane than there is on the
+other side. The aeroplane will tend to turn towards the side having the
+most drift. This may be caused as follows:
+</p>
+<p>
+1. The angle of incidence of the main surface or the tail surface may be
+wrong. The greater the angle of incidence, the greater the drift. The
+less the angle, the less the drift.
+</p>
+<p>
+2. If the alignment of the fuselage, fin in front of the rudder, the
+struts or stream-line wires, or, in the case of the Maurice Farman, the
+front outriggers, are not absolutely correct&mdash;that is to say, if they
+are turned a little to the left or to the right instead of being in line
+with the direction of flight&mdash;then they will act as a rudder and cause
+the aeroplane to turn off its course.
+</p>
+<p>
+3. If any part of the surface is distorted, it will cause the aeroplane
+to turn off its course. The surface is cambered, <i>i.e.</i>, curved, to pass
+through the air with the least possible drift. If, owing perhaps to the
+leading edge, spars, or trailing edge becoming bent, the curvature is
+spoiled, that will result in changing the amount of drift on one side of
+the aeroplane, which will then have a tendency to turn off its course.
+</p>
+<p>
+<span class="sc">Lateral Instability</span> (<span class="sc">Flying One Wing Down</span>).&mdash;The only possible reason
+for such a condition is a difference in the lifts of right and left
+wings. That may be caused as follows:
+</p>
+<p>
+1. The angle of incidence may be wrong. If it is too great, it will
+produce more lift than on the other side of the aeroplane; and if too
+small, it will produce less lift than on
+
+<span class="pagenum"><a id="page113" name="page113"></a>[113]</span>
+
+ the other side&mdash;the result
+being that, in either case, the aeroplane will try to fly one wing down.
+</p>
+<p>
+2. <i>Distorted Surfaces.</i>&mdash;If some part of the surface is distorted, then
+its camber is spoiled, and the lift will not be the same on both sides
+of the aeroplane, and that, of course, will cause it to fly one wing
+down.
+</p>
+<p>
+Longitudinal Instability may be due to the following reasons:
+</p>
+<p>
+1. <i>The stagger may be wrong.</i> 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."
+</p>
+<p>
+A 1/4-inch area in the stagger will make a very considerable difference
+to the longitudinal stability.
+</p>
+<p>
+2. If <i>the angle of incidence</i> of the main surface is not right, it will
+have a bad effect, especially in the case of an aeroplane with a lifting
+tail-plane.
+</p>
+<p>
+If the angle is too great, it will produce an excess of lift, and that
+may lift up the nose of the aeroplane and result in a tendency to fly
+"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."
+</p>
+<p>
+3. <i>The fuselage</i> 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."
+</p>
+<p>
+4. (The least likely reason.) <i>The tail-plane</i> 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
+
+<span class="pagenum"><a id="page114" name="page114"></a>[114]</span>
+
+ smaller angle of incidence than is the main surface, and
+if that difference is decreased too much, the aeroplane will become
+uncontrollable longitudinally. Sometimes the tail-plane is mounted on
+the aeroplane at the same angle as the main surface, but it actually
+engages the air at a lesser angle, owing to the air being deflected
+downwards by the main surface. There is then, in effect, a longitudinal
+dihedral as explained and illustrated in Chapter I.
+</p>
+<p>
+<span class="sc">Climbs Badly.</span>&mdash;Such a condition is, apart from engine or propeller
+trouble, probably due to (1) distorted surfaces, or (2) too small an
+angle of incidence.
+</p>
+<p>
+<span class="sc">Flight Speed Poor.</span>&mdash;Such a condition is, apart from engine or propeller
+trouble, probably due to (1) distorted surfaces, (2) too great an angle
+of incidence, or (3) dirt or mud, and consequently excessive
+skin-friction.
+</p>
+<p>
+<span class="sc">Inefficient Control</span> is probably due to (1) wrong setting of control
+surfaces, (2) distortion of control surfaces, or (3) control cables
+being badly tensioned.
+</p>
+<p>
+<span class="sc">Will not "Taxi" Straight.</span>&mdash;If the aeroplane is uncontrollable on the
+ground, it is probably due to (1) alignment of undercarriage being
+wrong, or (2) unequal tension of shock absorbers.
+</p>
+<p>
+<span class="pagenum"><a id="page115" name="page115"></a>[115]</span>
+</p>
+<a name="h2HCH0004" id="h2HCH0004"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    CHAPTER IV
+</h2>
+<h3>
+    THE PROPELLER, OR "AIR-SCREW"
+</h3>
+<p>
+The sole object of the propeller is to translate the power of the engine
+into thrust.
+</p>
+<p>
+The propeller screws through the air, and its blades, being set at an
+angle inclined to the direction of motion, secure a reaction, as in the
+case of the aeroplane's lifting surface.
+</p>
+<p>
+This reaction may be conveniently divided into two component parts or
+values, namely, Thrust and Drift (see illustration overleaf).
+</p>
+<p>
+The Thrust is opposed to the Drift of the aeroplane, and must be equal
+and opposite to it at flying speed. If it falls off in power, then the
+flying speed must decrease to a velocity, at which the aeroplane drift
+equals the decreased thrust. The Drift of the propeller may be
+conveniently divided into the following component values:
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Active Drift</i>, produced by the useful thrusting part of the
+propeller.
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Passive Drift</i>, produced by all the rest of the propeller,
+<i>i.e.</i>, by its detrimental surface.
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Skin-Friction</i>, produced by the friction of the air with
+roughness of surface.
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Eddies</i> attending the movement of the air caused by the action
+of the propeller.
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Cavitation</i> (very marked at excessive speed of revolution).
+A tendency of the propeller to produce a cavity or semi-vacuum
+in which it revolves, the thrust decreasing with increase of
+speed and cavitation.
+</p>
+<p>
+<span class="sc">Thrust-Drift Ratio.</span>&mdash;The proportion of thrust to drift is of paramount
+importance, for it expresses the efficiency of the propeller. It is
+affected by the following factors:
+</p>
+<p>
+<span class="pagenum"><a id="page116" name="page116"></a>[116]</span>
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Speed of Revolution.</i>&mdash;The greater the speed, the greater the
+proportion of drift to thrust. This is due to the increase with
+speed of the passive drift, which carries with it no increase
+in thrust. For this reason propellers are often geared down to
+revolve at a lower speed than that of the engine.
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Angle of Incidence.</i>&mdash;The same reasons as in the case of the
+aeroplane surface.
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Aspect Ratio.</i>&mdash;Ditto.
+</p>
+<p class="indent" style="text-indent: -5%;">
+<i>Camber.</i>&mdash;Ditto.
+</p>
+
+<a name="image-0076"><!--IMG--></a>
+<div class="figure-al">
+<a href="images/p116.jpg"><img src="images/p116-t.jpg" width="320" height="416"
+alt="" /></a>
+<br />
+<div>
+  M, Direction of motion of propeller (rotary).<br />
+  R, Direction of reaction. <br />
+  T, Direction of thrust. <br />
+ AD, Direction of the resistance of the air to the passage of the
+     aeroplane, <i>i.e.</i>, aeroplane drift. <br />
+  D, Direction of propeller drift (rotary). <br />
+  P, Engine power, opposed to propeller drift and transmitted to
+     the propeller through the propeller shaft.</div>
+</div>
+
+
+
+<p>
+<span class="pagenum"><a id="page117" name="page117"></a>[117]</span>
+</p>
+<p>
+In addition to the above factors there are, when it comes to actually
+designing a propeller, mechanical difficulties to consider. For
+instance, the blades must be of a certain strength and consequent
+thickness. That, in itself, limits the aspect ratio, for it will
+necessitate a chord long enough in proportion to the thickness to make
+a good camber possible. Again, the diameter of the propeller must be
+limited, having regard to the fact that greater diameters than those
+used to-day would not only result in excessive weight of construction,
+but would also necessitate a very high undercarriage to keep the
+propeller off the ground, and such undercarriage would not only produce
+excessive drift, but would also tend to make the aeroplane stand on
+its nose when alighting. The latter difficulty cannot be overcome by
+mounting the propeller higher, as the centre of its thrust must be
+approximately coincident with the centre of aeroplane drift.
+</p>
+
+<p class="center">
+<span class="sc">Maintenance of Efficiency.</span>
+</p>
+
+<p>
+The following conditions must be observed:
+</p>
+<p>
+1. <span class="sc">Pitch Angle.</span>&mdash;The angle, at any given point on the propeller, at
+which the blade is set is known as the pitch angle, and it must be
+correct to half a degree if reasonable efficiency is to be maintained.
+</p>
+<p>
+This angle secures the "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 <i>Slip</i>. If a propeller has a pitch of, say, 10 feet, but
+actually advances, say, only 8 feet owing to slip, then it will be said
+to possess 20 per cent. slip.
+</p>
+<p>
+Thus, the pitch must equal the flying speed of the aeroplane plus the
+slip of the propeller. For example, let us find the pitch of a
+propeller, given the following conditions:
+</p>
+
+<table border="0" align="center" summary="Conditions">
+<tr><td>        Flying speed         </td><td>...</td><td>70 miles per hour. </td></tr>
+<tr><td>        Propeller revolutions</td><td>...</td><td>1,200 per minute.  </td></tr>
+<tr><td>        Slip                 </td><td>...</td><td>15 per cent.       </td></tr>
+</table>
+
+<p>
+<span class="pagenum"><a id="page118" name="page118"></a>[118]</span>
+</p>
+
+<p>
+First find the distance in feet the aeroplane will travel forward in one
+minute. That is&mdash;
+</p>
+
+<table border="0" align="center" summary="Equation for distance in feet" cellspacing="0" cellpadding="0">
+<tr><td align="center"> 369,600 feet (70 miles)</td></tr>
+<tr><td> <hr class="full" /></td><td>&nbsp;= 6,160 feet per minute.</td></tr>
+<tr><td align="center"> 60    "    (minutes)</td></tr>
+</table>
+
+<p>
+Now divide the feet per minute by the propeller revolutions per minute,
+add 15 per cent. for the slip, and the result will be the propeller
+pitch:
+</p>
+
+<table border="0" align="center" summary="Equation for distance in feet" cellspacing="0" cellpadding="0">
+<tr><td align="center">        6,160</td></tr>
+<tr><td> <hr class="full" /></td><td>&nbsp;+ 15 per cent. = 5.903 feet.</td></tr>
+<tr><td align="center">        1,200</td></tr>
+</table>
+
+<p>
+In order to secure a constant pitch from root to tip of blade, the pitch
+angle decreases towards the tip. This is necessary, since the end of the
+blade travels faster than its root, and yet must advance forward at the
+same speed as the rest of the propeller. For example, two men ascending
+a hill. One prefers to walk fast and the other slowly, but they wish to
+arrive at the top of the hill simultaneously. Then the fast walker must
+travel a farther distance than the slow one, and his angle of path
+(pitch angle) must then be smaller than the angle of path taken by the
+slow walker. Their pitch angles are different, but their pitch (in this
+case altitude reached in a given time) is the same.
+</p>
+
+<a name="image-0077"><!--IMG--></a>
+<div class="figure">
+<a href="images/p118.jpg"><img src="images/p118-t.jpg" width="320" height="177"
+alt="" /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page119" name="page119"></a>[119]</span>
+</p>
+<p>
+In order to test the pitch angle, the propeller must be mounted upon
+a shaft at right angles to a beam the face of which must be perfectly
+level, thus:
+</p>
+
+<a name="image-0078"><!--IMG--></a>
+<div class="figure">
+<a href="images/p119a.jpg"><img src="images/p119a-t.jpg" width="320" height="134"
+alt="" /></a>
+</div>
+
+<p>
+First select a point on the blade at some distance (say about 2 feet)
+from the centre of the propeller. At that point find, by means of a
+protractor, the angle a projection of the chord makes with the face of
+the beam. That angle is the pitch angle of the blade at that point.
+</p>
+<p>
+Now lay out the angle on paper, thus:
+</p>
+
+<a name="image-0079"><!--IMG--></a>
+<div class="figure">
+<a href="images/p119b.jpg"><img src="images/p119b-t.jpg" width="320" height="97"
+alt="" /></a>
+</div>
+
+<p>
+The line above and parallel to the circumference line must be placed
+in a position making the distance between the two lines equal to the
+specified pitch, which is, or should be, marked upon the boss of the
+propeller.
+</p>
+<p>
+Now find the circumference of the propeller where the pitch angle is
+being tested. For example, if that place is 2 feet radius from the
+centre, then the circumference will be 2 feet x 2 = 4 feet diameter,
+which, if multiplied by 3.1416 = 15.56 feet circumference.
+</p>
+<p>
+Now mark off the circumference distance, which is represented above by
+A&ndash;B, and reduce it in scale for convenience.
+</p>
+<p>
+The distance a vertical line makes between B and the chord line is the
+pitch at the point where the angle is being
+
+<span class="pagenum"><a id="page120" name="page120"></a>[120]</span>
+
+ tested, and it should
+coincide with the specified pitch.
+</p>
+<p>
+You will note, from the above illustration, that the actual pitch line
+should meet the junction of the chord line and top line.
+</p>
+<p>
+The propeller should be tested at several points, about a foot apart, on
+each blade; and the diagram, provided the propeller is not faulty, will
+then look like this:
+</p>
+
+<a name="image-0080"><!--IMG--></a>
+<div class="figure-al">
+<a href="images/p120a.jpg"><img src="images/p120a-t.jpg" width="320" height="148"
+alt="" /></a>
+<br />
+<div>
+A, B, C, and D, Actual pitch at points tested.<br />
+I, Pitch angle at point tested nearest to centre of propeller.<br />
+E, Circumference at I.<br />
+J, Pitch angle at point tested nearest to I.<br />
+F, Circumference at J.<br />
+K, Pitch angle at next point tested.<br />
+G, Circumference at K.<br />
+L, Pitch angle tested at point nearest tip of blade.<br />
+H, Circumference at L.
+</div>
+</div>
+
+<p>
+At each point tested the actual pitch coincides with the specified
+pitch: a satisfactory condition.
+</p>
+<p>
+A faulty propeller will produce a diagram something like this:
+</p>
+
+<a name="image-0081"><!--IMG--></a>
+<div class="figure">
+<a href="images/p120b.jpg"><img src="images/p120b-t.jpg" width="319" height="106"
+alt="" /></a>
+</div>
+
+<p>
+At every point tested the pitch angle is wrong, for nowhere does the
+actual pitch coincide with the specified pitch. Angles A, C, and D, are
+too large, and B is too small. The angle should be correct to half a
+degree if reasonable efficiency is to be maintained.
+</p>
+<p>
+<span class="pagenum"><a id="page121" name="page121"></a>[121]</span>
+</p>
+<p>
+A fault in the pitch angle may be due to (1) faulty manufacture, (2)
+distortion, or (3) the shaft hole through the boss being out of
+position.
+</p>
+<p>
+2. <span class="sc">Straightness.</span>&mdash;To test for straightness the propeller must be mounted
+upon a shaft. Now bring the tip of one blade round to graze some fixed
+object. Mark the point it grazes. Now bring the other tip round, and it
+should come within 1/8 inch of the mark. If it does not do so, it is due
+to (1) faulty manufacture, (2) distortion, or (3) to the hole through
+the boss being out of position.
+</p>
+<p>
+3. <span class="sc">Length.</span>&mdash;The blades should be of equal length to 1/16 inch.
+</p>
+<p>
+4. <span class="sc">Balance.</span>&mdash;The usual method of testing a propeller for balance is as
+follows: Mount it upon a shaft, which must be on ball-bearings. Place
+the propeller in a horizontal position, and it should remain in that
+position. If a weight of a trifle over an ounce placed in a bolt-hole on
+one side of the boss fails to disturb the balance, then the propeller is
+usually regarded as unfit for use.
+</p>
+
+<a name="image-0082"><!--IMG--></a>
+<div class="figure">
+<a href="images/p121a.jpg"><img src="images/p121a-t.jpg" width="322" height="68"
+alt="" /></a>
+</div>
+
+<p>
+The above method is rather futile, as it does not test for the balance
+of centrifugal force, which comes into play as soon as the propeller
+revolves. It can be tested as follows:
+</p>
+
+<a name="image-0083"><!--IMG--></a>
+<div class="figure">
+<a href="images/p121b.jpg"><img src="images/p121b-t.jpg" width="321" height="115"
+alt="" /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page122" name="page122"></a>[122]</span>
+</p>
+<p>
+The propeller must be in a horizontal position, and then weighed at
+fixed points, such as A, B, C, D, E, and F, and the weights noted. The
+points A, B, and C must, of course, be at the same fixed distances from
+the centre of the propeller as the points D, E, and F. Now reverse the
+propeller and weigh at each point again. Note the results. The first
+series of weights should correspond to the second series, thus:
+</p>
+<div>
+<p class="i4">     Weight A should equal weight F. </p>
+<p class="i4">     Weight B should equal weight E. </p>
+<p class="i4">     Weight C should equal weight D. </p>
+</div>
+
+<p>
+There is no standard practice as to the degree of error permissible, but
+if there are any appreciable differences the propeller is unfit for use.
+</p>
+<p>
+5. <span class="sc">Surface Area.</span>&mdash;The surface area of the blades should be equal. Test
+with calipers thus:
+</p>
+
+<a name="image-0084"><!--IMG--></a>
+<div class="figure">
+<a href="images/p122.jpg"><img src="images/p122-t.jpg" width="320" height="73"
+alt="" /></a>
+</div>
+
+<div>
+<p class="i4">     The distance A&ndash;B should equal K&ndash;L. </p>
+<p class="i4">     The distance C&ndash;D should equal I&ndash;J. </p>
+<p class="i4">     The distance E&ndash;F should equal G&ndash;H. </p>
+</div>
+
+<p>
+The points between which the distances are taken must, of course, be at
+the same distance from the centre in the case of each blade.
+</p>
+<p>
+There is no standard practice as to the degree of error permissible. If,
+however, there is an error of over 1/8 inch, the propeller is really
+unfit for use.
+</p>
+<p>
+6. <span class="sc">Camber.</span>&mdash;The camber (curvature) of the blades should be (1) equal,
+(2) decrease evenly towards the tips of the blades, and (3) the greatest
+depth of the curve should, at any point of the blade, be approximately
+at the same percentage of the chord from the leading edge as at other
+points.
+</p>
+<p>
+It is difficult to test the top camber without a set of templates,<a href="#note-18" name="noteref-18"><small>18</small></a>
+but a fairly accurate idea of the concave camber
+
+<span class="pagenum"><a id="page123" name="page123"></a>[123]</span>
+
+ can be secured by
+slowly passing a straight-edge along the blade, thus:
+</p>
+
+<a name="image-0085"><!--IMG--></a>
+<div class="figure">
+<a href="images/p123.jpg"><img src="images/p123-t.jpg" width="320" height="90"
+alt="" /></a>
+</div>
+
+<p>
+The camber can now be easily seen, and as the straight-edge is passed
+along the blade, the observer should look for any irregularities of the
+curvature, which should gradually and evenly decrease towards the tip of
+the blade.
+</p>
+<p>
+7. <span class="sc">The Joints.</span>&mdash;The usual method for testing the glued joints is by
+revolving the propeller at greater speed than it will be called upon to
+make during flight, and then carefully examining the joints to see if
+they have opened. It is not likely, however, that the reader will have
+the opportunity of making this test. He should, however, examine all the
+joints very carefully, trying by hand to see if they are quite sound.
+Suspect a propeller of which the joints appear to hold any thickness of
+glue. Sometimes the joints in the boss open a little, but this is not
+dangerous unless they extend to the blades, as the bolts will hold the
+laminations together.
+</p>
+<p>
+8. <span class="sc">Condition of Surface.</span>&mdash;The surface should be very smooth, especially
+towards the tips of the blades. Some propeller tips have a speed of over
+30,000 feet a minute, and any roughness will produce a bad drift or
+resistance and lower the efficiency.
+</p>
+<p>
+9. <span class="sc">Mounting.</span>&mdash;Great care should be taken to see that the propeller is
+mounted quite straight on its shaft. Test in the same way as for
+straightness. If it is not straight, it is possibly due to some of the
+propeller bolts being too slack or to others having been pulled up too
+tightly.
+</p>
+<p>
+<span class="sc">Flutter.</span>&mdash;Propeller "flutter," or vibration, may be due to faulty pitch
+angle, balance, camber, surface area, or to bad mounting. It causes a
+condition sometimes mistaken for engine trouble, and one which may
+easily lead to the collapse of the propeller.
+</p>
+<p>
+<span class="pagenum"><a id="page124" name="page124"></a>[124]</span>
+</p>
+<p>
+<span class="sc">Care of Propellers.</span>&mdash;The care of propellers is of the greatest
+importance, as they become distorted very easily.
+</p>
+<p class="indent" style="text-indent: -5%;">
+1. Do not store them in a very damp or a very dry place.
+</p>
+<p class="indent" style="text-indent: -5%;">
+2. Do not store them where the sun will shine upon them.
+</p>
+<p class="indent" style="text-indent: -5%;">
+3. Never leave them long in a horizontal position or leaning up
+against a wall.
+</p>
+<p class="indent" style="text-indent: -5%;">
+4. They should be hung on horizontal pegs, and the position of
+the propellers should be vertical.
+</p>
+<p>
+If the points I have impressed upon you in these notes are not attended
+to, you may be sure of the following results:
+</p>
+<p class="indent" style="text-indent: -5%;">
+1. Lack of efficiency, resulting in less aeroplane speed and climb
+than would otherwise be the case.
+</p>
+<p class="indent" style="text-indent: -5%;">
+2. Propeller "flutter" and possible collapse.
+</p>
+<p class="indent" style="text-indent: -5%;">
+3. A bad stress upon the propeller shaft and its bearings.
+</p>
+<p>
+<span class="sc">Tractor.</span>&mdash;A propeller mounted in front of the main surface.
+</p>
+<p>
+<span class="sc">Pusher.</span>&mdash;A propeller mounted behind the main surface.
+</p>
+<p>
+<span class="sc">Four-Bladed Propellers.</span>&mdash;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.
+</p>
+
+<a name="image-0086"><!--IMG--></a>
+<div class="figure">
+<span class="sc">Spiral Courses of Two-Blade Tips.</span><br />
+<span class="sc">Spiral Courses of Four-Blade Tips.</span>
+<br />
+<a href="images/p124.jpg"><img src="images/p124-t.jpg" width="320" height="156"
+alt="SPIRAL COURSES OF TWO-BLADE TIPS. SPIRAL COURSES OF FOUR-BLADE TIPS. Pitch the same in each case." /></a>
+<br />
+Pitch the same in each case.
+</div>
+
+<p>
+<span class="pagenum"><a id="page125" name="page125"></a>[125]</span>
+</p>
+<p>
+The smaller the pitch, the less the "gap," <i>i.e.</i>, the distance,
+measured in the direction of the thrust, between the spiral courses of
+the blades (see illustration on preceding page).
+</p>
+<p>
+If the gap is too small, then the following blade will engage air which
+the preceding blade has put into motion, with the result that the
+following blade will not secure as good a reaction as would otherwise be
+the case. It is very much the same as in the case of the aeroplane gap.
+</p>
+<p>
+For a given pitch, the gap of a four-bladed propeller is only half
+that of a two-bladed one. Therefore the four-bladed propeller is only
+suitable for large pitch, as such pitch produces spirals with a large
+gap, thus offsetting the decrease in gap caused by the numerous blades.
+</p>
+<p>
+The greater the speed of rotation, the less the pitch for a given
+aeroplane speed. Then, in order to secure a large pitch and consequently
+a good gap, the four-bladed propeller is usually geared to rotate at a
+lower speed than would be the case if directly attached to the engine
+crank-shaft.
+</p>
+
+<a name="note-18"><!--Note--></a>
+<p class="foot">
+<u>18</u> (<a href="#noteref-18">return</a>)<br />
+I have heard of temporary ones being made quickly by
+bending strips of lead over the convex side of the blade, but I should
+think it very difficult to secure a sufficient degree of accuracy in
+that way.
+</p>
+
+<p>
+<span class="pagenum"><a id="page126" name="page126"></a>[126]</span>
+</p>
+<a name="h2HCH0005" id="h2HCH0005"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    CHAPTER V
+</h2>
+<h3>
+    MAINTENANCE
+</h3>
+<p>
+<span class="sc">Cleanliness.</span>&mdash;The fabric must be kept clean and free from oil, as that
+will rot it. To take out dirt or oily patches, try acetone. If that will
+not remedy matters, then try petrol, but use it sparingly, as otherwise
+it will take off an unnecessary amount of dope. If that will not remove
+the dirt, then hot water and soap will do so, but, in that case, be
+sure to use soap having no alkali in it, as otherwise it may injure the
+fabric. Use the water sparingly, or it may get inside the planes and
+rust the internal bracing wires, or cause some of the wooden framework
+to swell.
+</p>
+<p>
+The wheels of the undercarriage have a way of throwing up mud on to
+the lower surface. This should, if possible, be taken off while wet.
+It should never be scraped off when dry, as that may injure the fabric.
+If dry, then it should be moistened before being removed.
+</p>
+<p>
+Measures should be taken to prevent dirt from collecting upon any
+part of the aeroplane, as, otherwise, excessive skin-friction will be
+produced with resultant loss of flight speed. The wires, being greasy,
+collect dirt very easily.
+</p>
+<p>
+<span class="sc">Control Cables.</span>&mdash;After every flight the rigger should pass his hand over
+the control cables and carefully examine them near pulleys. Removal of
+grease may be necessary to make a close inspection possible. If only one
+strand is broken the wire should be replaced. Do not forget the aileron
+balance wire on the top surface.
+</p>
+<p>
+Once a day try the tension of the control cables by smartly moving the
+control levers about as explained elsewhere.
+</p>
+<p>
+<span class="sc">Wires.</span>&mdash;All the wires should be kept well greased or oiled, and in the
+correct tension. When examining the wires, it is necessary to place the
+aeroplane on level ground, as otherwise it may be twisted, thus throwing
+some wires into
+
+<span class="pagenum"><a id="page127" name="page127"></a>[127]</span>
+
+ undue tension and slackening others. The best way, if
+there is time, is to pack the machine up into its "flying position."
+</p>
+<p>
+If you see a slack wire, do not jump to the conclusion that it must
+be tensioned. Perhaps its opposition wire is too tight, in which case
+slacken it, and possibly you will find that will tighten the slack wire.
+</p>
+<p>
+Carefully examine all wires and their connections near the propeller,
+and be sure that they are snaked round with safety wire, so that the
+latter may keep them out of the way of the propeller if they come
+adrift.
+</p>
+<p>
+The wires inside the fuselage should be cleaned and regreased about once
+a fortnight.
+</p>
+<p>
+<span class="sc">Struts and Sockets.</span>&mdash;These should be carefully examined to see if any
+splitting has occurred.
+</p>
+<p>
+<span class="sc">Distortion.</span>&mdash;Carefully examine all surfaces, including the controlling
+surfaces, to see whether any distortion has occurred. If distortion can
+be corrected by the adjustment of wires, well and good; but if not, then
+some of the internal framework probably requires replacement.
+</p>
+<p>
+<span class="sc">Adjustments.</span>&mdash;Verify the angles of incidence, dihedral, and stagger, and
+the rigging position of the controlling surfaces, as often as possible.
+</p>
+<p>
+<span class="sc">Undercarriage.</span>&mdash;Constantly examine the alignment and fittings of the
+undercarriage, and the condition of tyres and shock absorbers. The
+latter, when made of rubber, wear quickest underneath. Inspect axles and
+skids to see if there are any signs of them becoming bent. The wheels
+should be taken off occasionally and greased.
+</p>
+<p>
+<span class="sc">Locking Arrangements.</span>&mdash;Constantly inspect the locking arrangements of
+turnbuckles, bolts, etc. Pay particular attention to the control cable
+connections, and to all moving parts in respect of the controls.
+</p>
+<p>
+<span class="sc">Lubrication.</span>&mdash;Keep all moving parts, such as pulleys, control levers,
+and hinges of controlling surfaces, well greased.
+</p>
+<p>
+<span class="sc">Special Inspection.</span>&mdash;Apart from constantly examining the aeroplane with
+reference to the above points I have made, I think that, in the case of
+an aeroplane in constant use,
+
+<span class="pagenum"><a id="page128" name="page128"></a>[128]</span>
+
+ it is an excellent thing to make a special
+inspection of every part, say, once a week. This will take from two to
+three hours according to the type of aeroplane. In order to carry it out
+methodically, the rigger should have a list of every part down to the
+smallest split-pin. He can then check the parts as he examines them,
+and nothing will be passed over. This, I know from experience, greatly
+increases the confidence of the pilot, and tends to produce good work
+in the air.
+</p>
+<p>
+<span class="sc">Windy Weather.</span>&mdash;The aeroplane, when on the ground, should face the wind;
+and it is advisable to lash the control lever fast, so that the
+controlling surfaces may not be blown about and possibly damaged.
+</p>
+<p>
+<span class="sc">"Vetting" by Eye.</span>&mdash;This should be practised at every opportunity, and,
+if persevered in, it is possible to become quite expert in diagnosing by
+eye faults in flight efficiency, stability, and control.
+</p>
+<p>
+The aeroplane should be standing upon level ground, or, better than
+that, packed up into its "flying position."
+</p>
+<p>
+Now stand in front of it and line up the leading edge with the main
+spar, rear spar, and trailing edge. Their shadows can usually be seen
+through the fabric. Allowance must, of course, be made for wash-in and
+wash-out; otherwise, the parts I have specified should be parallel with
+each other.
+</p>
+<p>
+Now line up the centre part of the main-plane with the tail-plane. The
+latter should be symmetrical with it. Next, sight each interplane front
+strut with its rear strut. They should be parallel.
+</p>
+<p>
+Then, standing on one side of the aeroplane, sight all the front struts.
+The one nearest to you should cover all the others. This applies to the
+rear struts also.
+</p>
+<p>
+Look for distortion of leading edges, main and rear spars, trailing
+edges, tail-plane, and controlling surfaces.
+</p>
+<p>
+This sort of thing, if practised constantly, will not only develop
+an expert eye for diagnosis of faults, but will also greatly assist in
+impressing upon the memory the characteristics and possible troubles
+of the various types of aeroplanes.
+</p>
+<p>
+<span class="sc">Mishandling on the Ground.</span>&mdash;This is the cause of a lot of unnecessary
+damage. The golden rule to observe is, <span class="sc">Produce no Bending Stresses.</span>
+</p>
+<p>
+<span class="pagenum"><a id="page129" name="page129"></a>[129]</span>
+</p>
+<p>
+Nearly all the wood in an aeroplane is designed to take merely the
+stress of direct compression, and it cannot be bent safely. Therefore,
+in packing an aeroplane up from the ground, or in pulling or pushing it
+about, be careful to stress it in such a way as to produce, as far as
+possible, only direct compression stresses. For instance, if it is
+necessary to support the lifting surface, then the packing should be
+arranged to come directly under the struts so that they may take the
+stress in the form of compression for which they are designed. Such
+supports should be covered with soft packing in order to prevent the
+fabric from becoming damaged.
+</p>
+<p>
+When pulling an aeroplane along, if possible, pull from the top of the
+undercarriage struts. If necessary to pull from elsewhere, then do so by
+grasping the interplane struts as low down as possible. Never pull by
+means of wires.
+</p>
+<p>
+Never lay fabric-covered parts upon a concrete floor. Any slight
+movement will cause the fabric to scrape over the floor with resultant
+damage.
+</p>
+<p>
+Struts, spars, etc., should never be left about the floor, as in such
+position they are likely to become scored. I have already explained the
+importance of protecting the outside fibres of the wood. Remember also
+that wood becomes distorted easily. This particularly applies to
+interplane struts. If there are no proper racks to stand them in, then
+the best plan is to lean them up against the wall in as near a vertical
+position as possible.
+</p>
+<p>
+<span class="sc">Time.</span>&mdash;Learn to know the time necessary to complete any of the various
+rigging jobs. This is really important. Ignorance of this will lead
+to bitter disappointments in civil life; and, where Service flying is
+concerned, it will, to say the least of it, earn unpopularity with
+senior officers, and fail to develop respect and good work where men
+are concerned.
+</p>
+<p>
+<span class="sc">The Aeroplane Shed.</span>&mdash;This should be kept as clean and orderly as
+possible. A clean, smart shed produces briskness, energy, and pride of
+work. A dirty, disorderly shed nearly always produces slackness and poor
+quality of work, lost tools, and mislaid material.
+</p>
+
+<p>
+<span class="pagenum"><a id="page130" name="page130"></a>[130]</span>
+</p>
+
+<a name="image-0087"><!--IMG--></a>
+<div class="figure">
+<a href="images/p130.jpg"><img src="images/p130-t.jpg" width="320" height="564"
+alt="" /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page131" name="page131"></a>[131]</span>
+</p>
+
+<a name="image-0088"><!--IMG--></a>
+<div class="figure">
+<a href="images/p131.jpg"><img src="images/p131-t.jpg" width="320" height="233"
+alt="" /></a>
+</div>
+
+<p>
+<span class="pagenum"><a id="page132" name="page132"></a>[132]</span>
+</p>
+
+<p><br /><!--[Blank Page]--></p>
+
+<p>
+<span class="pagenum"><a id="page133" name="page133"></a>[133]</span>
+</p>
+<a name="h2H_GLOS" id="h2H_GLOS"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    GLOSSARY
+</h2>
+<p>
+
+<i>The numbers at the right-hand side of the page indicate the parts
+numbered in the preceding diagrams.</i>
+</p>
+
+<p class="glossary">
+<b>Aeronautics</b>&mdash;The science of aerial navigation.
+</p>
+<p class="glossary">
+<b>Aerofoil</b>&mdash;A rigid structure, of large superficial area relative to its
+thickness, designed to obtain, when driven through the air at an
+angle inclined to the direction of motion, a reaction from the air
+approximately at right angles to its surface. Always cambered when
+intended to secure a reaction in one direction only. As the term
+"aerofoil" is hardly ever used in practical aeronautics, I have,
+throughout this book, used the term <span class="sc">SURFACE</span>, which, while academically
+incorrect, since it does not indicate thickness, is the term usually
+used to describe the cambered lifting surfaces, <i>i.e.</i>, the "planes"
+or "wings," and the stabilizers and the controlling aerofoils.
+</p>
+<p class="glossary">
+<b>Aerodrome</b>&mdash;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.
+</p>
+<p class="glossary">
+<b>Aeroplane</b>&mdash;A power-driven aerofoil fitted with stabilizing and
+controlling surfaces.
+</p>
+<p class="glossary">
+<b>Acceleration</b>&mdash;The rate of change of velocity.
+</p>
+<p class="glossary">
+<b>Angle of Incidence</b>&mdash;The angle at which the "neutral lift line" of
+a surface attacks the air.
+</p>
+<p class="glossary">
+<b>Angle of Incidence, Rigger's</b>&mdash;The angle the chord of a surface makes
+with a line parallel to the axis of the propeller.
+</p>
+<p class="glossary">
+<b>Angle of Incidence, Maximum</b>&mdash;The greatest angle of incidence at which,
+for a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+</p>
+<p class="glossary">
+<b>Angle of Incidence, Minimum</b>&mdash;The smallest angle of incidence at which,
+for a given power, surface (including detrimental surface), and weight,
+horizontal flight can be maintained.
+</p>
+<p class="glossary">
+<b>Angle of Incidence, Best Climbing</b>&mdash;That angle of incidence at which an
+aeroplane ascends quickest. An angle approximately halfway between the
+maximum and optimum angles.
+</p>
+<p class="glossary">
+<b>Angle of Incidence, Optimum</b>&mdash;The angle of incidence at which the
+lift-drift ratio is the highest.
+</p>
+<p>
+<span class="pagenum"><a id="page134" name="page134"></a>[134]</span>
+</p>
+<p class="glossary">
+<b>Angle, Gliding</b>&mdash;The angle between the horizontal and the path along
+which an aeroplane, at normal flying speed, but not under engine power,
+descends in still air.
+</p>
+<p class="glossary">
+<b>Angle, Dihedral</b>&mdash;The angle between two planes.
+</p>
+<p class="glossary">
+<b>Angle, Lateral Dihedral</b>&mdash;The lifting surface of an aeroplane is said to
+be at a lateral dihedral angle when it is inclined upward towards its
+wing-tips.
+</p>
+<p class="glossary">
+<b>Angle, Longitudinal Dihedral</b>&mdash;The main surface and tail surface are said
+to be at a longitudinal dihedral angle when the projections of their
+neutral lift lines meet and produce an angle above them.
+</p>
+<p class="glossary">
+<b>Angle, Rigger's Longitudinal Dihedral</b>&mdash;Ditto, but substituting "chords"
+for "neutral lift lines."
+</p>
+<p class="glossary">
+<b>Angle, Pitch</b>&mdash;The angle at any given point of a propeller, at which the
+blade is inclined to the direction of motion when the propeller is
+revolving but the aeroplane stationary.
+</p>
+<p class="glossary">
+<b>Altimeter</b>&mdash;An instrument used for measuring height.
+</p>
+<p class="glossary">
+<b>Air-Speed Indicator</b>&mdash;An instrument used for measuring air pressures or
+velocities. It consequently indicates whether the surface is securing
+the requisite reaction for flight. Usually calibrated in miles per hour,
+in which case it indicates the correct number of miles per hour at only
+one altitude. This is owing to the density of the air decreasing with
+increase of altitude and necessitating a greater speed through space to
+secure the same air pressure as would be secured by less speed at a
+lower altitude. It would be more correct to calibrate it in units of air
+pressure. <span class="glosref">[1]</span>
+</p>
+<p class="glossary">
+<b>Air Pocket</b>&mdash;A local movement or condition of the air causing an
+aeroplane to drop or lose its correct attitude.
+</p>
+<p class="glossary">
+<b>Aspect-Ratio</b>&mdash;The proportion of span to chord of a surface.
+</p>
+<p class="glossary">
+<b>Air-Screw (Propeller)</b>&mdash;A surface so shaped that its rotation about an
+axis produces a force (thrust) in the direction of its axis. <span class="glosref">[2]</span>
+</p>
+<p class="glossary">
+<b>Aileron</b>&mdash;A controlling surface, usually situated at the wing-tip, the
+operation of which turns an aeroplane about its longitudinal axis;
+causes an aeroplane to tilt sideways. <span class="glosref">[3]</span>
+</p>
+<p class="glossary">
+<b>Aviation</b>&mdash;The art of driving an aeroplane.
+</p>
+<p class="glossary">
+<b>Aviator</b>&mdash;The driver of an aeroplane.
+</p>
+<p class="glossary">
+<b>Barograph</b>&mdash;A recording barometer, the charts of which can be calibrated
+for showing air density or height.
+</p>
+<p class="glossary">
+<b>Barometer</b>&mdash;An instrument used for indicating the density of air.
+</p>
+<p class="glossary">
+<b>Bank, to</b>&mdash;To turn an aeroplane about its longitudinal axis (to tilt
+sideways) when turning to left or right.
+</p>
+<p class="glossary">
+<b>Biplane</b>&mdash;An aeroplane of which the main lifting surface consists of a
+surface or pair of wings mounted above another surface or pair of wings.
+</p>
+<p>
+<span class="pagenum"><a id="page135" name="page135"></a>[135]</span>
+</p>
+<p class="glossary">
+<b>Bay</b>&mdash;The space enclosed by two struts and whatever they are fixed to.
+</p>
+<p class="glossary">
+<b>Boom</b>&mdash;A term usually applied to the long spars joining the tail of a
+"pusher" aeroplane to its main lifting surface. <span class="glosref">[4]</span>
+</p>
+<p class="glossary">
+<b>Bracing</b>&mdash;A system of struts and tie wires to transfer a force from one
+point to another.
+</p>
+<p class="glossary">
+<b>Canard</b>&mdash;Literally "duck." The name which was given to a type of
+aeroplane of which the longitudinal stabilizing surface (<i>empennage</i>)
+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 <i>empennage</i>.
+</p>
+<p class="glossary">
+<b>Cabre</b>&mdash;To fly or glide at an excessive angle of incidence; tail down.
+</p>
+<p class="glossary">
+<b>Camber</b>&mdash;Curvature.
+</p>
+<p class="glossary">
+<b>Chord</b>&mdash;Usually taken to be a straight line between the trailing and
+leading edges of a surface.
+</p>
+<p class="glossary">
+<b>Cell</b>&mdash;The whole of the lower surface, that part of the upper surface
+directly over it, together with the struts and wires holding them
+together.
+</p>
+<p class="glossary">
+<b>Centre (Line) of Pressure</b>&mdash;A line running from wing-tip to wing-tip, and
+through which all the air forces acting upon the surface may be said to
+act, or about which they may be said to balance.
+</p>
+<p class="glossary">
+<b>Centre (Line) of Pressure, Resultant</b>&mdash;A line transverse to the
+longitudinal axis, and the position of which is the resultant of the
+centres of pressure of two or more surfaces.
+</p>
+<p class="glossary">
+<b>Centre of Gravity</b>&mdash;The centre of weight.
+</p>
+<p class="glossary">
+<b>Cabane</b>&mdash;A combination of two pylons, situated over the fuselage, and
+from which the anti-lift wires are suspended. <span class="glosref">[5]</span>
+</p>
+<p class="glossary">
+<b>Cloche</b>&mdash;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.
+</p>
+<p class="glossary">
+<b>Centrifugal Force</b>&mdash;Every body which moves in a curved path is urged
+outwards from the centre of the curve by a force termed "centrifugal."
+</p>
+<p class="glossary">
+<b>Control Lever</b>&mdash;A lever by means of which the controlling surfaces are
+operated. It usually operates the ailerons and elevator. The
+"joy-stick." <span class="glosref">[6]</span>
+</p>
+<p class="glossary">
+<b>Cavitation, Propeller</b>&mdash;The tendency to produce a cavity in the air.
+</p>
+<p class="glossary">
+<b>Distance Piece</b>&mdash;A long, thin piece of wood (sometimes tape) passing
+through and attached to all the ribs in order to prevent them from
+rolling over sideways. <span class="glosref">[7]</span>
+</p>
+<p class="glossary">
+<b>Displacement</b>&mdash;Change of position.
+</p>
+<p>
+<span class="pagenum"><a id="page136" name="page136"></a>[136]</span>
+</p>
+<p class="glossary">
+<b>Drift</b> (<i>of an aeroplane as distinct from the propeller</i>)&mdash;The horizontal
+component of the reaction produced by the action of driving through the
+air a surface inclined upwards and towards its direction of motion
+<i>plus</i> the horizontal component of the reaction produced by the
+"detrimental" surface <i>plus</i> resistance due to "skin-friction."
+Sometimes termed "head-resistance."
+</p>
+<p class="glossary">
+<b>Drift, Active</b>&mdash;Drift produced by the lifting surface.
+</p>
+<p class="glossary">
+<b>Drift, Passive</b>&mdash;Drift produced by the detrimental surface.
+</p>
+<p class="glossary">
+<b>Drift</b> (<i>of a propeller</i>)&mdash;Analogous to the drift of an aeroplane. It is
+convenient to include "eddies" and "cavitation" within this term.
+</p>
+<p class="glossary">
+<b>Drift, to</b>&mdash;To be carried by a current of air; to make leeway.
+</p>
+<p class="glossary">
+<b>Dive, to</b>&mdash;To descend so steeply as to produce a speed greater than the
+normal flying speed.
+</p>
+<p class="glossary">
+<b>Dope, to</b>&mdash;To paint a fabric with a special fluid for the purpose of
+tightening and protecting it.
+</p>
+<p class="glossary">
+<b>Density</b>&mdash;Mass of unit volume; for instance, pounds per cubic foot.
+</p>
+<p class="glossary">
+<b>Efficiency</b>&mdash;
+</p>
+
+<table border="0" align="center" summary="" cellspacing="0" cellpadding="0">
+<tr><td align="center"> Output </td></tr>
+<tr><td> <hr class="full" /></td></tr>
+<tr><td align="center"> Input. </td></tr>
+</table>
+
+<p class="glossary">
+<b>Efficiency</b> (<i>of an aeroplane as distinct from engine and propeller</i>)&mdash;
+</p>
+
+<table border="0" align="center" summary="" cellspacing="0" cellpadding="0">
+<tr><td align="center">  Lift and Velocity </td></tr>
+<tr><td> <hr class="full" /></td></tr>
+<tr><td align="center">  Thrust (= aeroplane drift). </td></tr>
+</table>
+
+<p class="glossary">
+<b>Efficiency, Engine</b>&mdash;
+</p>
+
+<table border="0" align="center" summary="" cellspacing="0" cellpadding="0">
+<tr><td align="center">Brake horse-power </td></tr>
+<tr><td> <hr class="full" /></td></tr>
+<tr><td align="center">Indicated horse-power. </td></tr>
+</table>
+
+<p class="glossary">
+<b>Efficiency, Propeller</b>&mdash;
+</p>
+
+<table border="0" align="center" summary="" cellspacing="0" cellpadding="0">
+<tr><td align="center"> Thrust horse-power</td></tr>
+<tr><td> <hr class="full" /></td></tr>
+<tr><td align="center"> Horse-power received from engine<br />
+          (= propeller drift).</td></tr>
+</table>
+
+<p class="indent" >
+  <span class="sc">Note.</span>&mdash;The above terms can, of course, be expressed in foot-pounds. It
+  is then only necessary to divide the upper term by the lower one to find
+  the measure of efficiency.
+</p>
+<p class="glossary">
+<b>Elevator</b>&mdash;A controlling surface, usually hinged to the rear of the
+tail-plane, the operation of which turns an aeroplane about an axis
+which is transverse to the direction of normal horizontal flight. <span class="glosref">[8]</span>
+</p>
+<p class="glossary">
+<b>Empennage</b>&mdash;See "Tail-plane."
+</p>
+<p class="glossary">
+<b>Energy</b>&mdash;Stored work. For instance, a given weight of coal or petroleum
+stores a given quantity of energy which may be expressed in foot-pounds.
+</p>
+<p class="glossary">
+<b>Extension</b>&mdash;That part of the upper surface extending beyond the span of
+the lower surface. <span class="glosref">[9]</span>
+</p>
+<p class="glossary">
+<b>Edge, Leading</b>&mdash;The front edge of a surface relative to its normal
+direction of motion. <span class="glosref">[10]</span>
+</p>
+<p class="glossary">
+<b>Edge, Trailing</b>&mdash;The rear edge of a surface relative to its normal
+direction of motion. <span class="glosref">[11]</span>
+</p>
+<p>
+<span class="pagenum"><a id="page137" name="page137"></a>[137]</span>
+</p>
+<p class="glossary">
+<b>Factor of Safety</b>&mdash;Usually taken to mean the result found by dividing the
+stress at which a body will collapse by the maximum stress it will be
+called upon to bear.
+</p>
+<p class="glossary">
+<b>Fineness</b> (<i>of stream-line</i>)&mdash;The proportion of length to maximum width.
+</p>
+<p class="glossary">
+<b>Flying Position</b>&mdash;A special position in which an aeroplane must be placed
+when rigging it or making adjustments. It varies with different types of
+aeroplanes. Would be more correctly described as "rigging position."
+</p>
+<p class="glossary">
+<b>Fuselage</b>&mdash;That part of an aeroplane containing the pilot, and to which
+is fixed the tail-plane. <span class="glosref">[12]</span>
+</p>
+<p class="glossary">
+<b>Fin</b>&mdash;Additional keel-surface, usually mounted at the rear of an
+aeroplane. <span class="glosref">[13]</span>
+</p>
+<p class="glossary">
+<b>Flange</b> (<i>of a rib</i>)&mdash;That horizontal part of a rib which prevents it
+from bending sideways. <span class="glosref">[14]</span>
+</p>
+<p class="glossary">
+<b>Flight</b>&mdash;The sustenance of a body heavier than air by means of its action
+upon the air.
+</p>
+<p class="glossary">
+<b>Foot-pound</b>&mdash;A measure of work representing the weight of 1 lb. raised 1
+foot.
+</p>
+<p class="glossary">
+<b>Fairing</b>&mdash;Usually made of thin sheet aluminium, 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. <span class="glosref">[15]</span>
+</p>
+<p class="glossary">
+<b>Gravity</b>&mdash;Is the force of the Earth's attraction upon a body. It
+decreases with increase of distance from the Earth. See "Weight."
+</p>
+<p class="glossary">
+<b>Gravity, Specific</b>&mdash;
+</p>
+<table border="0" align="center" summary="" cellspacing="0" cellpadding="0">
+<tr><td align="center">    Density of substance</td></tr>
+<tr><td> <hr class="full" /></td></tr>
+<tr><td align="center">    Density of water.</td></tr>
+</table>
+
+<p class="gindent" style="text-indent: 0;">
+    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>i.e.</i>, 0.7.
+</p>
+<p class="glossary">
+<b>Gap</b> (<i>of an aeroplane</i>)&mdash;The distance between the upper and lower
+surfaces of a biplane. In a triplane or multiplane, the distance between
+any two of its surfaces. <span class="glosref">[16]</span>
+</p>
+<p class="glossary">
+<b>Gap, Propeller</b>&mdash;The distance, measured in the direction of the thrust,
+between the spiral courses of the blades.
+</p>
+<p class="glossary">
+<b>Girder</b>&mdash;A structure designed to resist bending, and to combine lightness
+and strength.
+</p>
+<p class="glossary">
+<b>Gyroscope</b>&mdash;A heavy circular wheel revolving at high speed, the effect of
+which is a tendency to maintain its plane of rotation against disturbing
+forces.
+</p>
+<p class="glossary">
+<b>Hangar</b>&mdash;An aeroplane shed.
+</p>
+<p class="glossary">
+<b>Head-resistance</b>&mdash;Drift. The resistance of the air to the passage of
+a body.
+</p>
+<p class="glossary">
+<b>Helicopter</b>&mdash;An air-screw revolving about a vertical axis, the direction
+of its thrust being opposed to gravity.
+</p>
+<p>
+<span class="pagenum"><a id="page138" name="page138"></a>[138]</span>
+</p>
+<p class="glossary">
+<b>Horizontal Equivalent</b>&mdash;The plan view of a body whatever its attitude
+may be.
+</p>
+<p class="glossary">
+<b>Impulse</b>&mdash;A force causing a body to gain or lose momentum.
+</p>
+<p class="glossary">
+<b>Inclinometer</b>&mdash;A curved form of spirit-level used for indicating the
+attitude of a body relative to the horizontal.
+</p>
+<p class="glossary">
+<b>Instability</b>&mdash;An inherent tendency of a body, which, if the body is
+disturbed, causes it to move into a position as far as possible away
+from its first position.
+</p>
+<p class="glossary">
+<b>Instability, Neutral</b>&mdash;An inherent tendency of a body to remain in the
+position given it by the force of a disturbance, with no tendency to
+move farther or to return to its first position.
+</p>
+<p class="glossary">
+<b>Inertia</b>&mdash;The inherent resistance to displacement of a body as distinct
+from resistance the result of an external force.
+</p>
+<p class="glossary">
+<b>Joy-Stick</b>&mdash;See "Control Lever."
+</p>
+<p class="glossary">
+<b>Keel-Surface</b>&mdash;Everything to be seen when viewing an aeroplane from the
+side of it.
+</p>
+<p class="glossary">
+<b>King-Post</b>&mdash;A bracing strut; in an aeroplane, usually passing through a
+surface and attached to the main spar, and from the end or ends of which
+wires are taken to spar, surface, or other part of the construction in
+order to prevent distortion. When used in connection with a controlling
+surface, it usually performs the additional function of a lever, control
+cables connecting its ends with the pilot's control lever. <span class="glosref">[17]</span>
+</p>
+<p class="glossary">
+<b>Lift</b>&mdash;The vertical component of the reaction produced by the action of
+driving through the air a surface inclined upwards and towards its
+direction of motion.
+</p>
+<p class="glossary">
+<b>Lift, Margin of</b>&mdash;The height an aeroplane can gain in a given time and
+starting from a given altitude.
+</p>
+<p class="glossary">
+<b>Lift-Drift Ratio</b>&mdash;The proportion of lift to drift.
+</p>
+<p class="glossary">
+<b>Loading</b>&mdash;The weight carried by an aerofoil. Usually expressed in pounds
+per square foot of superficial area.
+</p>
+<p class="glossary">
+<b>Longeron</b>&mdash;The term usually applied to any long spar running length-ways
+of a fuselage. <span class="glosref">[18]</span>
+</p>
+<p class="glossary">
+<b>Mass</b>&mdash;The mass of a body is a measure of the quantity of material in it.
+</p>
+<p class="glossary">
+<b>Momentum</b>&mdash;The product of the mass and velocity of a body is known as
+"momentum."
+</p>
+<p class="glossary">
+<b>Monoplane</b>&mdash;An aeroplane of which the main lifting surface consists of
+one surface or one pair of wings.
+</p>
+<p class="glossary">
+<b>Multiplane</b>&mdash;An aeroplane of which the main lifting surface consists of
+numerous surfaces or pairs of wings mounted one above the other.
+</p>
+<p>
+<span class="pagenum"><a id="page139" name="page139"></a>[139]</span>
+</p>
+<p class="glossary">
+<b>Montant</b>&mdash;Fuselage strut.
+</p>
+<p class="glossary">
+<b>Nacelle</b>&mdash;That part of an aeroplane containing the engine and/or pilot
+and passenger, and to which the tail-plane is not fixed. <span class="glosref">[19]</span>
+</p>
+<p class="glossary">
+<b>Neutral Lift Line</b>&mdash;A line taken through a surface in a forward direction
+relative to its direction of motion, and starting from its trailing
+edge. If the attitude of the surface is such as to make the said line
+coincident with the direction of motion, it results in no lift, the
+reaction then consisting solely of drift. The position of the neutral
+lift line, <i>i.e.</i>, the angle it makes with the chord, varies with
+differences of camber, and it is found by means of wind-tunnel research.
+</p>
+<p class="glossary">
+<b>Newton's Laws of Motion</b>&mdash;1. If a body be at rest, it will remain at
+rest; or, if in motion, it will move uniformly in a straight line until
+acted upon by some force.
+</p>
+<p class="gindent">
+2. The rate of change of the quantity of motion (momentum) is
+proportional to the force which causes it, and takes place in the
+direction of the straight line in which the force acts. If a body be
+acted upon by several forces, it will obey each as though the others
+did not exist, and this whether the body be at rest or in motion.
+</p>
+<p class="gindent">
+3. To every action there is opposed an equal and opposite reaction.
+</p>
+<p class="glossary">
+<b>Ornithopter (or Orthopter)</b>&mdash;A flapping wing design of aircraft intended
+to imitate the flight of a bird.
+</p>
+<p class="glossary">
+<b>Outrigger</b>&mdash;This term is usually applied to the framework connecting the
+main surface with an elevator placed in advance of it. Sometimes applied
+to the "tail-boom" framework connecting the tail-plane with the main
+lifting surface. <span class="glosref">[20]</span>
+</p>
+<p class="glossary">
+<b>Pancake, to</b>&mdash;To "stall."
+</p>
+<p class="glossary">
+<b>Plane</b>&mdash;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.
+</p>
+<p class="glossary">
+<b>Propeller</b>&mdash;See "Air-Screw."
+</p>
+<p class="glossary">
+<b>Propeller, Tractor</b>&mdash;An air-screw mounted in front of the main lifting
+surface.
+</p>
+<p class="glossary">
+<b>Propeller, Pusher</b>&mdash;An air-screw mounted behind the main lifting surface.
+</p>
+<p class="glossary">
+<b>Pusher</b>&mdash;An aeroplane of which the propeller is mounted behind the main
+lifting surface.
+</p>
+<p class="glossary">
+<b>Pylon</b>&mdash;Any V-shaped construction from the point of which wires are
+taken.
+</p>
+<p class="glossary">
+<b>Power</b>&mdash;Rate of working. <span class="glosref">[21]</span>
+</p>
+<p class="glossary">
+<b>Power, Horse</b>&mdash;One horse-power represents a force sufficient to raise
+33,000 lb. 1 foot in a minute.
+</p>
+<p>
+<span class="pagenum"><a id="page140" name="page140"></a>[140]</span>
+</p>
+<p class="glossary">
+<b>Power, Indicated Horse</b>&mdash;The I.H.P. of an engine is a measure of the rate
+at which work is done by the pressure upon the piston or pistons, as
+distinct from the rate at which the engine does work. The latter is
+usually termed "brake horse-power," since it may be measured by an
+absorption brake.
+</p>
+<p class="glossary">
+<b>Power, Margin of</b>&mdash;The available quantity of power above that necessary
+to maintain horizontal flight at the optimum angle.
+</p>
+<p class="glossary">
+<b>Pitot Tube</b>&mdash;A form of air-speed indicator consisting of a tube with open
+end facing the wind, which, combined with a static pressure or suction
+tube, is used in conjunction with a gauge for measuring air pressures or
+velocities. (<i>No. 1 in diagram.</i>)
+</p>
+<p class="glossary">
+<b>Pitch, Propeller</b>&mdash;The distance a propeller advances during one
+revolution supposing the air to be solid.
+</p>
+<p class="glossary">
+<b>Pitch, to</b>&mdash;To plunge nose-down.
+</p>
+<p class="glossary">
+<b>Reaction</b>&mdash;A force, equal and opposite to the force of the action
+producing it.
+</p>
+<p class="glossary">
+<b>Rudder</b>&mdash;A controlling surface, usually hinged to the tail, the operation
+of which turns an aeroplane about an axis which is vertical in normal
+horizontal flight; causes an aeroplane to turn to left or right of the
+pilot. <span class="glosref">[22]</span>
+</p>
+<p class="glossary">
+<b>Roll, to</b>&mdash;To turn about the longitudinal axis.
+</p>
+<p class="glossary">
+<b>Rib, Ordinary</b>&mdash;A light curved wooden part mounted in a fore and aft
+direction within a surface. The ordinary ribs give the surface its
+camber, carry the fabric, and transfer the lift from the fabric to the
+spars. <span class="glosref">[23]</span>
+</p>
+<p class="glossary">
+<b>Rib, Compression</b>&mdash;Acts as an ordinary rib, besides bearing the stress of
+compression produced by the tension of the internal bracing wires. <span class="glosref">[24]</span>
+</p>
+<p class="glossary">
+<b>Rib, False</b>&mdash;A subsidiary rib, usually used to improve the camber of the
+front part of the surface. <span class="glosref">[25]</span>
+</p>
+<p class="glossary">
+<b>Right and Left Hand</b>&mdash;Always used relative to the position of the pilot.
+When observing an aeroplane from the front of it, the right hand side of
+it is then on the left hand of the observer.
+</p>
+<p class="glossary">
+<b>Remou</b>&mdash;A local movement or condition of the air which may cause
+displacement of an aeroplane.
+</p>
+<p class="glossary">
+<b>Rudder-Bar</b>&mdash;A control lever moved by the pilot's feet, and operating the
+rudder. <span class="glosref">[26]</span>
+</p>
+<p class="glossary">
+<b>Surface</b>&mdash;See "Aerofoil."
+</p>
+<p class="glossary">
+<b>Surface, Detrimental</b>&mdash;All exterior parts of an aeroplane including the
+propeller, but excluding the (aeroplane) lifting and (propeller)
+thrusting surfaces.
+</p>
+<p class="glossary">
+<b>Surface, Controlling</b>&mdash;A surface the operation of which turns an
+aeroplane about one of its axes.
+</p>
+<p>
+<span class="pagenum"><a id="page141" name="page141"></a>[141]</span>
+</p>
+<p class="glossary">
+<b>Skin-Friction</b>&mdash;The friction of the air with roughness of surface. A form
+of drift.
+</p>
+<p class="glossary">
+<b>Span</b>&mdash;The distance from wing-tip to wing-tip.
+</p>
+<p class="glossary">
+<b>Stagger</b>&mdash;The distance the upper surface is forward of the lower surface
+when the axis of the propeller is horizontal.
+</p>
+<p class="glossary">
+<b>Stability</b>&mdash;The inherent tendency of a body, when disturbed, to return to
+its normal position.
+</p>
+<p class="glossary">
+<b>Stability, Directional</b>&mdash;The stability about an axis which is vertical
+during normal horizontal flight, and without which an aeroplane has no
+natural tendency to remain upon its course.
+</p>
+<p class="glossary">
+<b>Stability, Longitudinal</b>&mdash;The stability of an aeroplane about an axis
+transverse to the direction of normal horizontal flight, and without
+which it has no tendency to oppose pitching and tossing.
+</p>
+<p class="glossary">
+<b>Stability, Lateral</b>&mdash;The stability of an aeroplane about its longitudinal
+axis, and without which it has no tendency to oppose sideways rolling.
+</p>
+<p class="glossary">
+<b>Stabilizer</b>&mdash;A surface, such as fin or tail-plane, designed to give an
+aeroplane inherent stability.
+</p>
+<p class="glossary">
+<b>Stall, to</b>&mdash;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>i.e.</i>, "stall" or "pancake."
+</p>
+<p class="glossary">
+<b>Stress</b>&mdash;Burden or load.
+</p>
+<p class="glossary">
+<b>Strain</b>&mdash;Deformation produced by stress.
+</p>
+<p class="glossary">
+<b>Side-Slip, to</b>&mdash;To fall as a result of an excessive "bank" or "roll."
+</p>
+<p class="glossary">
+<b>Skid, to</b>&mdash;To be carried sideways by centrifugal force when turning to
+left or right.
+</p>
+<p class="glossary">
+<b>Skid, Undercarriage</b>&mdash;A spar, mounted in a fore and aft direction, and to
+which the wheels of the undercarriage are sometimes attached. Should a
+wheel give way the skid is then supposed to act like the runner of a
+sleigh and to support the aeroplane. <span class="glosref">[28]</span>
+</p>
+<p class="glossary">
+<b>Skid, Tail</b>&mdash;A piece of wood or other material, orientable, and fitted
+with shock absorbers, situated under the tail of an aeroplane in order
+to support it upon the ground and to absorb the shock of alighting. <span class="glosref">[28<i>a</i>]</span>
+</p>
+<p class="glossary">
+<b>Section</b>&mdash;Any separate part of the top surface, that part of the bottom
+surface immediately underneath it, with their struts and wires.
+</p>
+<p class="glossary">
+<b>Spar</b>&mdash;Any long piece of wood or other material.
+</p>
+<p class="glossary">
+<b>Spar, Main</b>&mdash;A spar within a surface and to which all the ribs are
+attached, such spar being the one situated nearest to the centre of
+pressure. It transfers more than half the lift from the ribs to the
+bracing. <span class="glosref">[29]</span>
+</p>
+<p>
+<span class="pagenum"><a id="page142" name="page142"></a>[142]</span>
+</p>
+<p class="glossary">
+<b>Spar, Rear</b>&mdash;A spar within a surface, and to which all the ribs are
+attached, such spar being situated at the rear of the centre of pressure
+and at a greater distance from it than is the main spar. It transfers
+less than half of the lift from the ribs to the bracing. <span class="glosref">[30]</span>
+</p>
+<p class="glossary">
+<b>Strut</b>&mdash;Any wooden member intended to take merely the stress of direct
+compression.
+</p>
+<p class="glossary">
+<b>Strut, Interplane</b>&mdash;A strut holding the top and bottom surfaces
+apart. <span class="glosref">[31]</span>
+</p>
+<p class="glossary">
+<b>Strut, Fuselage</b>&mdash;A strut holding the <i>fuselage longerons</i> apart. It
+should be stated whether top, bottom, or side. If side, then it should
+be stated whether right or left hand. <i>Montant</i>. <span class="glosref">[32]</span>
+</p>
+<p class="glossary">
+<b>Strut, Extension</b>&mdash;A strut supporting an "extension" when not in flight.
+It may also prevent the extension from collapsing upwards during
+flight. <span class="glosref">[33]</span>
+</p>
+<p class="glossary">
+<b>Strut, undercarriage</b>&mdash; <span class="glosref">[33<i>a</i>]</span>
+</p>
+<p class="glossary">
+<b>Strut, Dope</b>&mdash;A strut within a surface, so placed as to prevent the
+tension of the doped fabric from distorting the framework. <span class="glosref">[34]</span>
+</p>
+<p class="glossary">
+<b>Serving</b>&mdash;To bind round with wire, cord, or similar material. Usually
+used in connection with wood joints and wire cable splices.
+</p>
+<p class="glossary">
+<b>Slip, Propeller</b>&mdash;The pitch less the distance the propeller advances
+during one revolution.
+</p>
+<p class="glossary">
+<b>Stream-Line</b>&mdash;A form or shape of detrimental surface designed to produce
+minimum drift.
+</p>
+<p class="glossary">
+<b>Toss, to</b>&mdash;To plunge tail-down.
+</p>
+<p class="glossary">
+<b>Torque, Propeller</b>&mdash;The tendency of a propeller to turn an aeroplane
+about its longitudinal axis in a direction opposite to that in which the
+propeller revolves.
+</p>
+<p class="glossary">
+<b>Tail-Slide</b>&mdash;A fall whereby the tail of an aeroplane leads.
+</p>
+<p class="glossary">
+<b>Tractor</b>&mdash;An aeroplane of which the propeller is mounted in front of the
+main lifting surface.
+</p>
+<p class="glossary">
+<b>Triplane</b>&mdash;An aeroplane of which the main lifting surface consists of
+three surfaces or pairs of wings mounted one above the other.
+</p>
+<p class="glossary">
+<b>Tail-Plane</b>&mdash;A horizontal stabilizing surface mounted at some distance
+behind the main lifting surface. <i>Empennage</i>. <span class="glosref">[36]</span>
+</p>
+<p class="glossary">
+<b>Turnbuckle</b>&mdash;A form of wire-tightener, consisting of a barrel into each
+end of which is screwed an eyebolt. Wires are attached to the eyebolts
+and the required degree of tension is secured by means of rotating the
+barrel.
+</p>
+<p class="glossary">
+<b>Thrust, Propeller</b>&mdash;See "Air-Screw."
+</p>
+<p class="glossary">
+<b>Undercarriage</b>&mdash;That part of an aeroplane beneath the <i>fuselage</i> or
+<i>nacelle</i>, and intended to support the aeroplane when at rest, and to
+absorb the shock of alighting.
+</p>
+<p>
+<span class="pagenum"><a id="page143" name="page143"></a>[143]</span>
+</p>
+<p class="glossary">
+<b>Velocity</b>&mdash;Rate of displacement; speed.
+</p>
+<p class="glossary">
+<b>Volplane</b>&mdash;A gliding descent.
+</p>
+<p class="glossary">
+<b>Weight</b>&mdash;Is a measure of the force of the Earth's attraction (gravity)
+upon a body. The standard unit of weight in this country is 1 lb., and
+is the force of the Earth's attraction on a piece of platinum called
+<i>the standard pound</i>, deposited with the Board of Trade in London. At
+the centre of the Earth a body will be attracted with equal force in
+every direction. It will therefore have no weight, though its mass is
+unchanged. Gravity, of which weight is a measure, decreases with
+increase of altitude.
+</p>
+<p class="glossary">
+<b>Web</b> (<i>of a rib</i>)&mdash;That vertical part of a rib which prevents it from
+bending upwards. <span class="glosref">[37<i>a</i>]</span>
+</p>
+<p class="glossary">
+<b>Warp, to</b>&mdash;To distort a surface in order to vary its angle of incidence.
+To vary the angle of incidence of a controlling surface.
+</p>
+<p class="glossary">
+<b>Wash</b>&mdash;The disturbance of air produced by the flight of an aeroplane.
+</p>
+<p class="glossary">
+<b>Wash-in</b>&mdash;An increasing angle of incidence of a surface towards its
+wing-tip. <span class="glosref">[38]</span>
+</p>
+<p class="glossary">
+<b>Wash-out</b>&mdash;A decreasing angle of incidence of a surface towards its
+wing-tip. <span class="glosref">[39]</span>
+</p>
+<p class="glossary">
+<b>Wing-tip</b>&mdash;The right or left-hand extremity of a surface. <span class="glosref">[40]</span>
+</p>
+<p class="glossary">
+<b>Wire</b>&mdash;A wire is, in Aeronautics, always known by the name of its
+function.
+</p>
+<p class="glossary">
+<b>Wire, Lift or Flying</b>&mdash;A wire opposed to the direction of lift, and used
+to prevent a surface from collapsing upward during flight. <span class="glosref">[41]</span>
+</p>
+<p class="glossary">
+<b>Wire, Anti-lift or Landing</b>&mdash;A wire opposed to the direction of gravity,
+and used to sustain a surface when it is at rest. <span class="glosref">[42]</span>
+</p>
+<p class="glossary">
+<b>Wire, Drift</b>&mdash;A wire opposed to the direction of drift, and used to
+prevent a surface from collapsing backwards during flight.
+</p>
+<p class="glossary">
+<b>Wire, Anti-drift</b>&mdash;A wire opposed to the tension of a drift wire, and
+used to prevent such tension from distorting the framework. <span class="glosref">[44]</span>
+</p>
+<p class="glossary">
+<b>Wire, Incidence</b>&mdash;A wire running from the top of an interplane strut to
+the bottom of the interplane strut in front of or behind it. It
+maintains the "stagger" and assists in maintaining the angle of
+incidence. Sometimes termed "stagger wire." <span class="glosref">[45]</span>
+</p>
+<p class="glossary">
+<b>Wire, Bracing</b>&mdash;Any wire holding together the framework of any part of an
+aeroplane. It is not, however, usually applied to the wires described
+above unless the function performed includes a function additional to
+those described above. Thus, a lift wire, while strictly speaking a
+bracing wire, is not usually described as one unless it performs the
+additional function of bracing some well-defined part such as the
+undercarriage. It will then be said to be an "undercarriage bracing lift
+wire." It might, perhaps, be acting as a drift wire also, in which case
+it will then be described
+
+<span class="pagenum"><a id="page144" name="page144"></a>[144]</span>
+
+ 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.
+</p>
+<p class="glossary">
+<b>Wire, Internal Bracing</b>&mdash;A bracing wire (usually drift or anti-drift)
+within a surface.
+</p>
+<p class="glossary">
+<b>Wire, Top Bracing</b>&mdash;A bracing wire, approximately horizontal and situated
+between the top longerons of fuselage, between top tail booms, or at the
+top of similar construction. <span class="glosref">[46]</span>
+</p>
+<p class="glossary">
+<b>Wire, Bottom Bracing</b>&mdash;Ditto, substituting "bottom" for "top." <span class="glosref">[47]</span>
+</p>
+<p class="glossary">
+<b>Wire, Side Bracing</b>&mdash;A bracing wire crossing diagonally a side bay of
+fuselage, tail boom bay, undercarriage side bay or centre-section side
+bay. This term is not usually used with reference to incidence wires,
+although they cross diagonally the side bays of the cell. It should be
+stated whether right- or left-hand. <span class="glosref">[48]</span>
+</p>
+<p class="glossary">
+<b>Wire, Cross Bracing</b>&mdash;A bracing wire, the position of which is diagonal
+from right to left when viewing it from the front of an aeroplane. <span class="glosref">[49]</span>
+</p>
+<p class="glossary">
+<b>Wire, Control Bracing</b>&mdash;A wire preventing distortion of a controlling
+surface. <span class="glosref">[50]</span>
+</p>
+<p class="glossary">
+<b>Wire, Control</b>&mdash;A wire connecting a controlling surface with the pilot's
+control lever, wheel, or rudder-bar. <span class="glosref">[51]</span>
+</p>
+<p class="glossary">
+<b>Wire, Aileron Gap</b>&mdash;A wire connecting top and bottom ailerons. <span class="glosref">[52]</span>
+</p>
+<p class="glossary">
+<b>Wire, Aileron Balance</b>&mdash;A wire connecting the right- and left-hand top
+ailerons. Sometimes termed the "aileron compensating wire." <span class="glosref">[53]</span>
+</p>
+<p class="glossary">
+<b>Wire, Snaking</b>&mdash;A wire, usually of soft metal, wound spirally or tied
+round another wire, and attached at each end to the framework. Used to
+prevent the wire round which it is "snaked" from becoming, in the event
+of its displacement, entangled with the propeller.
+</p>
+<p class="glossary">
+<b>Wire, Locking</b>&mdash;A wire used to prevent a turnbuckle barrel or other
+fitting from losing its adjustment.
+</p>
+<p class="glossary">
+<b>Wing</b>&mdash;Strictly speaking, a wing is one of the surfaces of an
+ornithopter. The term is, however, often applied to the lifting surface
+of an aeroplane when such surface is divided into two parts, one being
+the left-hand "wing," and the other the right-hand "wing."
+</p>
+<p class="glossary">
+<b>Wind-Tunnel</b>&mdash;A large tube used for experimenting with surfaces and
+models, and through which a current of air is made to flow by artificial
+means.
+</p>
+<p class="glossary">
+<b>Work</b>&mdash;Force × displacement.
+</p>
+<p class="glossary">
+<b>Wind-Screen</b>&mdash;A small transparent screen mounted in front of the pilot to
+protect his face from the air pressure.
+</p>
+
+<p>
+<span class="pagenum"><a id="page145" name="page145"></a>[145]</span>
+</p>
+<a name="h2H_4_0015" id="h2H_4_0015"><!-- H2 anchor --></a>
+
+<div style="height: 4em;"><br /><br /><br /><br /></div>
+
+<h2>
+    Types of Aeroplanes.
+</h2>
+
+<p>
+<span class="pagenum"><a id="page146" name="page146"></a>[146]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page147" name="page147"></a>[147]</span>
+</p>
+
+<a name="image-0089"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate01.jpg"><img src="images/plate01t.jpg" width="400" height="280"
+alt="Plate I." /></a>
+<br />
+<span class="sc">Plate I.</span>
+</div>
+
+<p class="plate">
+The first machine to fly&mdash;of which there is anything like authentic
+record&mdash;was the Ader "Avion," after which the more notable advances were
+made as shown above.
+</p>
+
+<p>
+<span class="pagenum"><a id="page148" name="page148"></a>[148]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+<p>
+<span class="pagenum"><a id="page149" name="page149"></a>[149]</span>
+</p>
+
+<a name="image-0090"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate02.jpg"><img src="images/plate02t.jpg" width="400" height="290"
+alt="Plate II." /></a>
+<br />
+<span class="sc">Plate II.</span>
+</div>
+
+<p class="plate">
+The Henri Farman was the first widely used aeroplane. Above are shown
+the chief steps in its development.
+</p>
+
+<p>
+<span class="pagenum"><a id="page150" name="page150"></a>[150]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+<p>
+<span class="pagenum"><a id="page151" name="page151"></a>[151]</span>
+</p>
+
+<a name="image-0091"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate03.jpg"><img src="images/plate03t.jpg" width="400" height="255"
+alt="Plate III." /></a>
+<br />
+<span class="sc">Plate III.</span>
+</div>
+
+<p class="plate">
+THE AVRO.&mdash;The aeroplane designed and built by Mr. A. V. Roe was the
+first successful heavier-than-air flying machine built by a British
+subject. Mr. Roe's progress may be followed in the picture, from his
+early "canard" biplane, through various triplanes, with 35 J.A.P. and 35
+h.p. Green engines, to his successful tractor biplane with the same 35
+h.p. Green, thence through the "totally enclosed" biplane 1912, with 60
+h.p. Green, to the biplane 1913-14, with 80 h.p. Gnome.
+</p>
+
+<p>
+<span class="pagenum"><a id="page152" name="page152"></a>[152]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page153" name="page153"></a>[153]</span>
+</p>
+
+<a name="image-0092"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate04.jpg"><img src="images/plate04t.jpg" width="400" height="265"
+alt="Plate IV." /></a>
+<br />
+<span class="sc">Plate IV.</span>
+</div>
+
+<p class="plate">
+THE SOPWITH LAND-GOING BIPLANES.&mdash;The earliest was a pair of Wright
+planes with a fuselage added. Next was the famous tractor with 80 h.p.
+Gnome. Then the "tabloid" of 1913, which set a completely new fashion in
+aeroplane design. From this developed the Gordon-Bennett racer shown
+over date 1914. The gun-carrier was produced about the same time, and
+the later tractor biplane in a development of the famous 80 h.p. but
+with 100 h.p. monosoupape Gnome.
+</p>
+
+<p>
+<span class="pagenum"><a id="page154" name="page154"></a>[154]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page155" name="page155"></a>[155]</span>
+</p>
+
+<a name="image-0092b"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate05.jpg"><img src="images/plate05t.jpg" width="400" height="265"
+alt="Plate V." /></a>
+<br />
+<span class="sc">Plate V.</span>
+</div>
+
+<p class="plate">
+THE MAURICE FARMAN.&mdash;First, 1909, the 50-60 h.p. Renault and coil-spring
+chassis. 1910, the same chassis with beginning of the characteristic
+bent-up skids. 1911 appeared the huge French Military Trials 3-seater;
+also the round-ended planes and tails and "Henry" type wheels. This
+developed, 1912, into the square-ended planes and upper tail, and long
+double-acting ailerons of the British Military Trials. The 1913 type had
+two rectangular tail-planes and better seating arrangements, known
+affectionately as the "mechanical cow"; the same year came the first
+"shorthorn," with two tail-planes and a low nacelle. This finally
+developed into the carefully streamlined "shorthorn" with the raised
+nacelle and a single tail-plane.
+</p>
+
+<p>
+<span class="pagenum"><a id="page156" name="page156"></a>[156]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page157" name="page157"></a>[157]</span>
+</p>
+
+<a name="image-0093"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate06.jpg"><img src="images/plate06t.jpg" width="400" height="250"
+alt="Plate VI." /></a>
+<br />
+<span class="sc">Plate VI.</span>
+</div>
+
+<p class="plate">
+THE SHORT "PUSHERS."&mdash;In 1909 came the semi-Wright biplane, with 35 h.p.
+Green, on which Mr. Moore-Brabazon won the "Daily Mail's" £1000 prize
+for the first mile flight on a circuit on a British aeroplane. Then the
+first box-kite flown by Mr. Grace at Wolverhampton. Later the famous
+"extension" type on which the first Naval officers learned to fly. Then
+the "38" type with elevator on the nacelle, on which dozens of R.N.A.S.
+pilots were taught.
+</p>
+
+<p>
+<span class="pagenum"><a id="page158" name="page158"></a>[158]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page159" name="page159"></a>[159]</span>
+</p>
+
+<a name="image-0094"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate07.jpg"><img src="images/plate07t.jpg" width="400" height="255"
+alt="Plate VII." /></a>
+<br />
+<span class="sc">Plate VII.</span>
+</div>
+
+<p class="plate">
+SHORT TRACTORS, 1911&ndash;1912.&mdash;They were all co-existent, but the first was
+the "tractor-pusher" (bottom of picture). Then came the "twin-tractor
+plus propeller" (at top). A development was the "triple-tractor" (on the
+right), with two 50 h.p. Gnomes, one immediately behind the other under
+the cowl, one driving the two chains, the other coupled direct. Later
+came the single-engined 80 h.p. tractor (on the left), the original of
+the famous Short seaplanes.
+</p>
+
+<p>
+<span class="pagenum"><a id="page160" name="page160"></a>[160]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page161" name="page161"></a>[161]</span>
+</p>
+
+<a name="image-0095"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate08.jpg"><img src="images/plate08t.jpg" width="400" height="275"
+alt="Plate VIII." /></a>
+<br />
+<span class="sc">Plate VIII.</span>
+</div>
+
+<p class="plate">
+THE VICKERS MACHINES: First the Vickers-R.E.P. of 1911, which developed
+into the full-bodied No. V. with R.E.P. engine, then the Military Trials
+"sociable" with Viale engine, and so to the big No. VII with a 100 h.p.
+Gnome. Contemporary with the No. V and No. VI were a number of school
+box-kites of ordinary Farman type, which developed into the curious
+"pumpkin" sociable, and the early "gun 'bus" of 1913. Thence arrived the
+gun-carrier with 100 h.p. monosoupape Gnome.
+</p>
+
+<p>
+<span class="pagenum"><a id="page162" name="page162"></a>[162]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page163" name="page163"></a>[163]</span>
+</p>
+
+<a name="image-0096"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate09.jpg"><img src="images/plate09t.jpg" width="400" height="250"
+alt="Plate IX." /></a>
+<br />
+<span class="sc">Plate IX.</span>
+</div>
+
+<p class="plate">
+THE BRISTOL AEROPLANES.&mdash;First, 1910, Farman type box-kites familiar to
+all early pupils. Then the miniature Maurice-Farman type biplane of the
+"Circuit of Britain." Contemporaneous was the "floating tail" monoplane
+designed by Pierre Prier, and after it a similar machine with fixed
+tail. Then came the handsome but unfortunate monoplane designed by M.
+Coanda for the Military Trials, 1912.
+</p>
+
+<p>
+<span class="pagenum"><a id="page164" name="page164"></a>[164]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page165" name="page165"></a>[165]</span>
+</p>
+
+<a name="image-0097"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate10.jpg"><img src="images/plate10t.jpg" width="400" height="270"
+alt="Plate X." /></a>
+<br />
+<span class="sc">Plate X.</span>
+</div>
+
+<p class="plate">
+THE BRISTOL TRACTORS.&mdash;Late 1912 came the round fuselaged tractor, with
+Gnome engine, designed by Mr. Gordon England for Turkey. 1912-13 came
+the biplane built onto the Military Trials monoplane type fuselage, also
+with a Gnome, designed by M. Coanda for Roumania. Then the
+Renault-engined Coanda tractor 1913, followed by 80 h.p. Gnome-engined
+scout, designed by Messrs. Barnwell and Busteed, which with Gnomes, le
+Rhones and Clergets, has been one of the great successes. Almost
+contemporary was the two-seater Bristol.
+</p>
+
+<p>
+<span class="pagenum"><a id="page166" name="page166"></a>[166]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page167" name="page167"></a>[167]</span>
+</p>
+
+<a name="image-0098"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate11.jpg"><img src="images/plate11t.jpg" width="400" height="275"
+alt="Plate XI." /></a>
+<br />
+<span class="sc">Plate XI.</span>
+</div>
+
+<p class="plate">
+THE MARTINSYDES.&mdash;1909, first experimental monoplane built with small
+4-cylinder engine. J.A.P.-engined machine, 1910, followed by the
+Gnome-engined machine, 1911. 1912, first big monoplane with Antoinette
+engine was built, followed by powerful Austro-Daimler monoplane, 1913.
+Then came the little Gnome-engined scout biplanes, 1914, some with, some
+without, skids.
+</p>
+
+<p>
+<span class="pagenum"><a id="page168" name="page168"></a>[168]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page169" name="page169"></a>[169]</span>
+</p>
+
+<a name="image-0099"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate12.jpg"><img src="images/plate12t.jpg" width="400" height="250"
+alt="Plate XII." /></a>
+<br />
+<span class="sc">Plate XII.</span>
+</div>
+
+<p class="plate">
+THE CURTISS BIPLANES.&mdash;In 1909 came the "June-bug," the united product
+of Glen Curtiss, Dr. Graham Bell, and J. A. D. McCurdy. Then the
+box-kite type, 1909, on which Mr. Curtiss won the Gordon-Bennett Race at
+Reims. Next the "rear-elevator" pusher, 1912, followed by first tractor,
+1913, with an outside flywheel. All purely Curtiss machines to that date
+had independent ailerons intended to get away from Wright patents.
+Following these came tractors with engines varying from 70 to 160 h.p.,
+fitted with varying types of chassis. All these have ordinary ailerons.
+</p>
+
+<p>
+<span class="pagenum"><a id="page170" name="page170"></a>[170]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page171" name="page171"></a>[171]</span>
+</p>
+
+<a name="image-0100"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate13.jpg"><img src="images/plate13t.jpg" width="400" height="250"
+alt="Plate XIII." /></a>
+<br />
+<span class="sc">Plate XIII.</span>
+</div>
+
+<p class="plate">
+THE BLERIOT (1).&mdash;The first engine-driven machine was a "canard"
+monoplane. Then came the curious tractor monoplanes 1908&ndash;1909, in order
+shown. Famous "Type XI" was prototype of all Bleriot successes. "Type
+XII" was never a great success, though the ancestor of the popular
+"parasol" type. The big passenger carrier was a descendant of this type.
+</p>
+
+<p>
+<span class="pagenum"><a id="page172" name="page172"></a>[172]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page173" name="page173"></a>[173]</span>
+</p>
+
+<a name="image-0101"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate14.jpg"><img src="images/plate14t.jpg" width="400" height="250"
+alt="Plate XIV." /></a>
+<br />
+<span class="sc">Plate XIV.</span>
+</div>
+
+<p class="plate">
+THE BLERIOT (2):&mdash;1910, "Type XI," on which Mr. Grahame-White won
+Gordon-Bennett Race, with a 14-cylinder 100 h.p. Gnome. 1911 came the
+improved "Type XI," with large and effective elevator flaps. On this
+type, with a 50 h.p. Gnome, Lieut. de Conneau (M. Beaumont) won
+Paris-Rome Race and "Circuit of Britain." Same year saw experimental
+"Limousine" flown by M. Legagneux, and fast but dangerous "clipped-wing"
+Gordon-Bennett racer with the fish-tail, flown by Mr. Hamel. About the
+same time came the fish-tailed side-by-side two-seater, flown by Mr.
+Hamel at Hendon and by M. Perreyon in 1912 Military Trials. 1911, M.
+Bleriot produced the 100 h.p. three-seater which killed M. Desparmets in
+French Military Trials. 1912-13, M. Bleriot produced a quite promising
+experimental biplane, and a "monocoque" monoplane in which the passenger
+faced rearward.
+</p>
+
+<p>
+<span class="pagenum"><a id="page174" name="page174"></a>[174]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page175" name="page175"></a>[175]</span>
+</p>
+
+<a name="image-0102"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate15.jpg"><img src="images/plate15t.jpg" width="400" height="250"
+alt="Plate XV." /></a>
+<br />
+<span class="sc">Plate XV.</span>
+</div>
+
+<p class="plate">
+THE BLERIOT (3)&mdash;1912 tandem two-seater proved one of the best machines
+of its day. 1913 "canard" lived up to its name. A "pusher" monoplane was
+built in which the propeller revolved on the top tail boom. This machine
+came to an untimely end, with the famous pilot, M. Perreyon. 1912
+"tandem" was developed in 1914 into the type shown in centre; almost
+simultaneously "parasol" tandem appeared. 1914, M. Bleriot built a
+monoplane embodying a most valuable idea never fully developed. The
+engine tanks and pilot were all inside an armoured casing. Behind them
+the fuselage was a "monocoque" of three-ply wood bolted onto the armour.
+And behind this all the tail surfaces were bolted on as a separate unit.
+</p>
+
+<p>
+<span class="pagenum"><a id="page176" name="page176"></a>[176]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page177" name="page177"></a>[177]</span>
+</p>
+
+<a name="image-0103"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate16.jpg"><img src="images/plate16t.jpg" width="400" height="280"
+alt="Plate XVI." /></a>
+<br />
+<span class="sc">Plate XVI.</span>
+</div>
+
+<p class="plate">
+THE CAUDRON.&mdash;1910, came the machine with ailerons and a 28 h.p. Anzani.
+1911 this was altered to warp control and a "star" Anzani was fitted.
+From this came the 35 h.p. type of 1912, one of the most successful
+of school machines. Small fast monoplane, 1912, was never further
+developed. 1913 appeared the familiar biplanes with 80 h.p. Gnomes,
+and 5-seater with 100 h.p. Anzani for French "Circuit of Anjou." 1914
+produced the "scout" biplane which won at Vienna. 1915 appeared the
+twin-engined type, the first successful "battle-plane."
+</p>
+
+<p>
+<span class="pagenum"><a id="page178" name="page178"></a>[178]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page179" name="page179"></a>[179]</span>
+</p>
+
+<a name="image-0104"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate17.jpg"><img src="images/plate17t.jpg" width="400" height="215"
+alt="Plate XVII." /></a>
+<br />
+<span class="sc">Plate XVII.</span>
+</div>
+
+<p class="plate">
+THE DEPERDUSSIN.&mdash;In 1911 the little monoplane with a Gyp. engine.
+Then the Gnome-engined machine of the "Circuit of Europe." In 1912 came
+the Navy's machine with 70 h.p. Gnome, and Prevost's Gordon-Bennett
+"Bullet," 135 miles in the hour. The last was the British-built
+"Thunder-Bug," familiar at Hendon.
+</p>
+
+<p>
+<span class="pagenum"><a id="page180" name="page180"></a>[180]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page181" name="page181"></a>[181]</span>
+</p>
+
+<a name="image-0105"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate18.jpg"><img src="images/plate18t.jpg" width="400" height="215"
+alt="Plate XVIII." /></a>
+<br />
+<span class="sc">Plate XVIII.</span>
+</div>
+
+<p class="plate">
+THE BREGUET.&mdash;First to fly was the complicated but business-like machine
+of 1909. Then came the record passenger carrier, 1910 (which lifted 8
+passengers). 1911 the French Military Trials machine with geared-down
+100 h.p. Gnome appeared. 1912 produced the machine with 130 h.p. Salmson
+engine on which the late Mr. Moorhouse flew the Channel with Mrs.
+Moorhouse and Mr. Ledeboer as passengers; also the machine with 130 h.p.
+horizontal Salmson, known as the "Whitebait." The last before the war
+was the rigid wing machine with 200 h.p. Salmson.
+</p>
+
+<p>
+<span class="pagenum"><a id="page182" name="page182"></a>[182]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page183" name="page183"></a>[183]</span>
+</p>
+
+<a name="image-0106"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate19.jpg"><img src="images/plate19t.jpg" width="400" height="215"
+alt="Plate XIX." /></a>
+<br />
+<span class="sc">Plate XIX.</span>
+</div>
+
+<p class="plate">
+THE CODY.&mdash;First the Military Experiment of 1908, with an Antoinette
+engine, then improved type 1909 with a Green engine. Next the
+"Cathedral," 1910, with a Green engine, which won Michelin Prize. In
+1911 "Daily Mail" Circuit machine, also with a Green, won the Michelin.
+This was modified into 1912 type which won Military Competition and
+£5,000 in prizes, with an Austro-Daimler engine, and later the Michelin
+Circuit Prize, again with a Green. 1912 the only Cody Monoplane was
+built. 1913 a modified biplane on which the great pioneer was killed.
+</p>
+
+<p>
+<span class="pagenum"><a id="page184" name="page184"></a>[184]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page185" name="page185"></a>[185]</span>
+</p>
+
+<a name="image-0107"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate20.jpg"><img src="images/plate20t.jpg" width="400" height="215"
+alt="Plate XX." /></a>
+<br />
+<span class="sc">Plate XX.</span>
+</div>
+
+<p class="plate">
+THE NIEUPORT.&mdash;The first Nieuport of 1909 was curiously like a monoplane
+version of a Caudron. In 1910 came the little two-cylinder machine with
+fixed tail-plane and universally jointed tail. In 1911 the French Trials
+machine was built with 100 h.p. 14 cylinder Gnome, and is typical of
+this make. Also the little two-cylinder record breaker. A modification
+of 1913 was the height record machine of the late M. Legagneux.
+</p>
+
+<p>
+<span class="pagenum"><a id="page186" name="page186"></a>[186]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page187" name="page187"></a>[187]</span>
+</p>
+
+<a name="image-0108"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate21.jpg"><img src="images/plate21t.jpg" width="400" height="215"
+alt="Plate XXI." /></a>
+<br />
+<span class="sc">Plate XXI.</span>
+</div>
+
+<p class="plate">
+THE R.E.P. MONOPLANES.&mdash;First came the curious and highly interesting
+experiments of 1907, 1908, 1909, and 1910. 1910&ndash;1911, the World's
+Distance Record breaker was produced; after it, the "European Circuit,"
+all with R.E.P. engines. In 1913-14 came the French military type with
+Gnome engine and finally the "parasol," 1915.
+</p>
+
+<p>
+<span class="pagenum"><a id="page188" name="page188"></a>[188]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page189" name="page189"></a>[189]</span>
+</p>
+
+<a name="image-0109"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate22.jpg"><img src="images/plate22t.jpg" width="400" height="215"
+alt="Plate XXII." /></a>
+<br />
+<span class="sc">Plate XXII.</span>
+</div>
+
+<p class="plate">
+THE MORANE: First the European Circuit and Paris-Madrid type. Then the
+1912 types, with taper wing and modern type wing. The 1913 types, the
+"clipped wing," flown by the late Mr. Hamel, one of the standard tandem
+types now in use. About the same time came the "parasol." 1914-15 came
+a little biplane like a Nieuport, and the "destroyer" type with a round
+section body, flown by Vedrines.
+</p>
+
+<p>
+<span class="pagenum"><a id="page190" name="page190"></a>[190]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page191" name="page191"></a>[191]</span>
+</p>
+
+<a name="image-0110"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate23.jpg"><img src="images/plate23t.jpg" width="400" height="215"
+alt="Plate XXIII." /></a>
+<br />
+<span class="sc">Plate XXIII.</span>
+</div>
+
+<p class="plate">
+THE VOISIN.&mdash;1908, the first properly controlled flight on a European
+aeroplane was made on a Voisin of the type shown with fixed engine. Then
+followed the record breaker of 1909 with a Gnome engine. In 1909 also
+the only Voisin tractor was produced. 1910 the Paris-Bordeaux type was
+built; 1911 the amphibious "canard" and the "military" type with
+extensions, and the type without an elevator. 1913 came the type with
+only two tail-booms and a geared-down engine, which developed into the
+big "gun" machine with a Salmson engine.
+</p>
+
+<p>
+<span class="pagenum"><a id="page192" name="page192"></a>[192]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page193" name="page193"></a>[193]</span>
+</p>
+
+<a name="image-0111"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate24.jpg"><img src="images/plate24t.jpg" width="400" height="215"
+alt="Plate XXIV." /></a>
+<br />
+<span class="sc">Plate XXIV.</span>
+</div>
+
+<p class="plate">
+THE HANRIOT AND PONNIER MONOPLANES.&mdash;In 1909 came the first Hanriot with
+50 h.p. 6-cylinder Buchet engine, and in 1910 the famous "Henrietta"
+type with E.N.Vs. and stationary Clergets. 1911 came the Clerget
+two-seater entered in French Military Trials, and 1912 the 100 h.p.
+Hanriot-Pagny monoplane which took part in British Military Trials.
+Sister machines of the same year were the single seater with 50 h.p.
+Gnome and the 100 h.p. Gnome racer with stripped chassis. In 1913 the
+Ponnier-Pagny racing monoplane with 160 h.p. Le Rhone competed in the
+Gordon-Bennett race, doing about 130 miles in the hour. The 60 h.p.
+Ponnier biplane was the first successful French scout tractor biplane.
+</p>
+
+<p>
+<span class="pagenum"><a id="page194" name="page194"></a>[194]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page195" name="page195"></a>[195]</span>
+</p>
+
+<a name="image-0112"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate25.jpg"><img src="images/plate25t.jpg" width="400" height="215"
+alt="Plate XXV." /></a>
+<br />
+<span class="sc">Plate XXV.</span>
+</div>
+
+<p class="plate">
+THE WRIGHT BIPLANE.&mdash;The first power flights were made, 1903, on a
+converted glider fitted with 16 h.p. motor. The prone position of the
+pilot will be noted. By 1907 the machine had become reasonably practical
+with 40 h.p. motor. On this the first real flying in the world was done.
+In 1910 the miniature racing Wright was produced; also the type with a
+rear elevator in addition to one in front. Soon afterwards the front
+elevator disappeared, and the machine became the standard American
+exhibition and school machine for four years. In 1915 a machine with
+enclosed fuselage was produced.
+</p>
+
+<p>
+<span class="pagenum"><a id="page196" name="page196"></a>[196]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page197" name="page197"></a>[197]</span>
+</p>
+
+<a name="image-0113"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate26.jpg"><img src="images/plate26t.jpg" width="400" height="215"
+alt="Plate XXVI." /></a>
+<br />
+<span class="sc">Plate XXVI.</span>
+</div>
+
+<p class="plate">
+THE BLACKBURN MONOPLANES.&mdash;In 1909 was built the curious four-wheeled
+parasol-type machine with 35 h.p. Green engine and chain transmission,
+on which flying was done at Saltburn. In 1911 the Isaacson-engined
+machine was built, together with a 50 h.p. Gnome single-seater on which
+Mr. Hucks started in the Circuit of Britain race. In 1912 another 50
+h.p. single-seater was built on which a good deal of school work was
+done. A more advanced machine appeared in 1913 and a two-seater with
+80 h.p. Gnome did a great deal of cross-country work in 1913-14.
+</p>
+
+<p>
+<span class="pagenum"><a id="page198" name="page198"></a>[198]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page199" name="page199"></a>[199]</span>
+</p>
+
+<a name="image-0114"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate27.jpg"><img src="images/plate27t.jpg" width="400" height="215"
+alt="Plate XXVII." /></a>
+<br />
+<span class="sc">Plate XXVII.</span>
+</div>
+
+<p class="plate">
+In 1908 the first Antoinette monoplane was produced by MM. Gastambide
+and Mengin. Then followed a machine with central skids, a single wheel,
+and wing skids. In 1909 came the machine with four-wheeled chassis and
+ailerons and later an improved edition which reverted to the central
+skid idea. On this M. Latham made his first cross-channel attempt.
+The next machine shed the wing skids and widened its wheelbase.
+During 1910-11 the ailerons vanished, warp control was adopted and the
+king-post system of wing-bracing was used. In 1911 the curious machine
+with streamlined "pantalette" chassis, totally enclosed body and
+internal wing-bracing, was produced for French Military Trials. In 1912
+the three-wheeled machine was used to a certain extent in the French
+Army. Then the type disappeared.
+</p>
+
+<p>
+<span class="pagenum"><a id="page200" name="page200"></a>[200]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page201" name="page201"></a>[201]</span>
+</p>
+
+<a name="image-0115"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate28.jpg"><img src="images/plate28t.jpg" width="400" height="215"
+alt="Plate XXVIII." /></a>
+<br />
+<span class="sc">Plate XXVIII.</span>
+</div>
+
+<p class="plate">
+In 1908 and 1909 detached experimental machines in various countries
+attained a certain success. The late Capt. Ferber made a primitive
+tractor biplane 1908. The Odier-Vendome biplane was a curious bat-winged
+pusher biplane built 1909. The tailless Etrich monoplane, built in
+Austria, 1908, was an adaptation of the Zanonia leaf. M. Santos-Dumont
+made primitive parasol type monoplanes known as "Demoiselles," in which
+bamboo was largely used. 1909 type is seen above. A curious steel
+monoplane was built by the late John Moisant, 1909. The twin-pusher
+biplane, built by the Barnwell Bros. in Scotland, made one or two
+straight flights in 1909. The Clement-Bayard Co. in France constructed
+in 1909 a biplane which did fairly well. Hans Grade, the first German
+to fly, made his early efforts on a "Demoiselle" type machine, 1908.
+</p>
+
+<p>
+<span class="pagenum"><a id="page202" name="page202"></a>[202]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page203" name="page203"></a>[203]</span>
+</p>
+
+<a name="image-0116"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate29.jpg"><img src="images/plate29t.jpg" width="400" height="215"
+alt="Plate XXIX." /></a>
+<br />
+<span class="sc">Plate XXIX.</span>
+</div>
+
+<p class="plate">
+In 1910 a number of novel machines were produced. The Avis with Anzani
+engine was flown by the Hon. Alan Boyle. Note the cruciform universally
+jointed tail. The Goupy with 50 h.p. Gnome was an early French tractor,
+notable for its hinging wing-tips. The Farman was a curious "knock-up"
+job, chiefly composed of standard box-kite fittings. The Sommer with
+50 h.p. Gnome was a development of the box-kite with a shock-breaking
+chassis. The Savary, also French, was one of the first twin tractors to
+fly. The model illustrated had an E.N.V. engine. Note position of the
+rudders on the wing tips. The Austrian Etrich was the first successful
+machine of the Taube class ever built.
+</p>
+
+<p>
+<span class="pagenum"><a id="page204" name="page204"></a>[204]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page205" name="page205"></a>[205]</span>
+</p>
+
+<a name="image-0117"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate30.jpg"><img src="images/plate30t.jpg" width="400" height="215"
+alt="Plate XXX." /></a>
+<br />
+<span class="sc">Plate XXX.</span>
+</div>
+
+<p class="plate">
+INTERESTING MACHINES, 1910.&mdash;The Werner monoplane with E.N.V. engine,
+combined shaft and chain drive, was a variant of the de Pischoff. The
+Macfie biplane was a conventional biplane with 50 h.p. Gnome and useful
+originalities. The Valkyrie monoplane, another British machine, was a
+"canard" monoplane with propeller behind the pilot and in front of main
+plane. The Weiss monoplane was a good British effort at inherent
+stability. The Tellier monoplane was a modified Bleriot with Antoinette
+proportions. The Howard Wright biplane was a pusher with large lifting
+monoplane tail. The Dunne biplane was another British attempt at
+inherent stability. The Jezzi biplane was an amateur built
+twin-propeller.
+</p>
+
+<p>
+<span class="pagenum"><a id="page206" name="page206"></a>[206]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page207" name="page207"></a>[207]</span>
+</p>
+
+<a name="image-0118"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate31.jpg"><img src="images/plate31t.jpg" width="400" height="215"
+alt="Plate XXXI." /></a>
+<br />
+<span class="sc">Plate XXXI.</span>
+</div>
+
+<p class="plate">
+SOME INTERESTING MACHINES, 1911.&mdash;The Compton-Paterson biplane was very
+similar to the early Curtiss pusher; it had a 50 h.p. Gnome. The Sloan
+bicurve was a French attempt at inherent stability with 50 h.p. Gnome
+and tractor screw. The Paulhan biplane was an attempt at a machine for
+military purposes to fold up readily for transport. The Sanders was
+a British biplane intended for rough service. The Barnwell monoplane
+was the first Scottish machine to fly; it had a horizontally opposed
+Scottish engine. The Harlan monoplane was an early German effort; note
+position of petrol tank.
+</p>
+
+<p>
+<span class="pagenum"><a id="page208" name="page208"></a>[208]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page209" name="page209"></a>[209]</span>
+</p>
+
+<a name="image-0119"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate32.jpg"><img src="images/plate32t.jpg" width="400" height="215"
+alt="Plate XXXII." /></a>
+<br />
+<span class="sc">Plate XXXII.</span>
+</div>
+
+<p class="plate">
+The Clement-Bayard monoplane, 1911, was convertible into a tractor
+biplane. The standard engine was a 50 h.p. Gnome. The machine was
+interesting, but never did much. The Zodiac was one of the earliest to
+employ staggered wings. With 50 h.p. Gnome engine it was badly
+underpowered, so never did itself justice. The Jezzi tractor biplane,
+1911, was a development of an earlier model built entirely by Mr. Jezzi,
+an amateur constructor. With a low-powered J.A.P. engine it developed an
+amazing turn of speed, and it may be regarded as a forerunner of the
+scout type and the properly streamlined aeroplane. The Paulhan-Tatin
+monoplane, 1911, was a brilliant attempt at high speed for low power; it
+presented certain advantages as a scout. A 50 h.p. Gnome, fitted behind
+the pilot's seat in the streamlined fuselage, was cooled through
+louvres. The propeller at the end of the tail was connected with the
+engine by a flexible coupling. This machine was, in its day, the fastest
+for its power in the world, doing 80 miles per hour. Viking 1 was a twin
+tractor biplane driven by a 50 h.p. Gnome engine through chains. It was
+built by the author at Hendon in 1912.
+</p>
+
+<p>
+<span class="pagenum"><a id="page210" name="page210"></a>[210]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page211" name="page211"></a>[211]</span>
+</p>
+
+<a name="image-0120"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate33.jpg"><img src="images/plate33t.jpg" width="400" height="215"
+alt="Plate XXXIII." /></a>
+<br />
+<span class="sc">Plate XXXIII.</span>
+</div>
+
+<p class="plate">
+Much ingenuity was exerted by the French designers in 1911 to produce
+machines for the Military Trials. Among them was the 100 h.p.
+Gnome-Borel monoplane with a four-wheeled chassis, and the Astra
+triplane with a 75 h.p. Renault engine. This last had a surface of about
+500 square feet and presented considerable possibilities. Its principal
+feature was its enormous wheels with large size tyres as an attempt to
+solve difficulties of the severe landing tests. The Clement-Bayard
+biplane was a further development of the Clement-Bayard monoplane; the
+type represented could be converted into a monoplane at will. The Lohner
+Arrow biplane with the Daimler engine was an early German tractor
+biplane built with a view to inherent stability, and proved very
+successful. The Pivot monoplane was of somewhat unconventional French
+construction, chiefly notable for the special spring chassis and pivoted
+ailerons at the main planes; this pivoting had nothing to do with the
+name of the machine, which was designed by M. Pivot.
+</p>
+
+<p>
+<span class="pagenum"><a id="page212" name="page212"></a>[212]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page213" name="page213"></a>[213]</span>
+</p>
+
+<a name="image-0121"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate34.jpg"><img src="images/plate34t.jpg" width="400" height="215"
+alt="Plate XXXIV." /></a>
+<br />
+<span class="sc">Plate XXXIV.</span>
+</div>
+
+<p class="plate">
+The Flanders monoplane, 1912, with 70 h.p. Renault engine, was one of
+the last fitted with king-post system of wing bracing. The Flanders
+biplane entered for British Military Trials. Notable features: the
+highly staggered planes, extremely low chassis and deep fuselage. Also,
+the upper plane was bigger in every dimension than the lower; about
+the first instance of this practice. The Bristol biplane, with 100 h.p.
+Gnome engine, was also entered for the Trials, but ultimately withdrawn.
+The Mars monoplane, later known as the "D.F.W.," was a successful
+machine of Taube type with 120 h.p. Austro-Daimler engine. The building
+of the engine into a cowl, complete with radiator in front, followed car
+practice very closely. The tail of the monoplane had a flexible trailing
+edge; its angle of incidence could be varied from the pilot's seat,
+so that perfect longitudinal balance was attained at all loadings and
+speeds. The Handley-Page monoplane, with 70 h.p. Gnome engine, was an
+early successful British attempt at inherent stability.
+</p>
+
+<p>
+<span class="pagenum"><a id="page214" name="page214"></a>[214]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page215" name="page215"></a>[215]</span>
+</p>
+
+<a name="image-0122"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate35.jpg"><img src="images/plate35t.jpg" width="400" height="215"
+alt="Plate XXXV." /></a>
+<br />
+<span class="sc">Plate XXXV.</span>
+</div>
+
+<p class="plate">
+The Sommer monoplane, with 50 h.p. Gnome, was a 1911-12 machine; it did
+a good deal of cross-country flying. The Vendome monoplane of 1912, also
+with 50 h.p. Gnome engine, was notable chiefly for its large wheels
+and jointed fuselage, which enabled the machine to be taken down for
+transport. The Savary biplane took part in the French Military Trials,
+1911. It had a four-cylinder Labor aviation motor. Notable features are
+twin chain-driven propellers, rudders between the main planes, the broad
+wheel-base and the position of the pilot. The Paulhan triplane, which
+also figured in the French Military Trials, was a development of the
+Paulhan folding biplane. It had a 70 h.p. Renault engine. For practical
+purposes it was a failure. The R.E.P. biplane, with 60 h.p. R.E.P.
+engine, was a development of the famous R.E.P. monoplanes. Its spring
+chassis, with sliding joints, marked an advance. Like the monoplanes,
+it was built largely of steel.
+</p>
+
+<p>
+<span class="pagenum"><a id="page216" name="page216"></a>[216]</span>
+</p>
+
+<div style="height: 2em;"><br /><br /></div>
+
+<p>
+<span class="pagenum"><a id="page217" name="page217"></a>[217]</span>
+</p>
+
+<a name="image-0123"><!--IMG--></a>
+<div class="figure">
+<a href="images/plate36.jpg"><img src="images/plate36t.jpg" width="400" height="215"
+alt="Plate XXXVI." /></a>
+<br />
+<span class="sc">Plate XXXVI.</span>
+</div>
+
+<p class="plate">
+In 1912 came the first really successful Handley-Page monoplane, with
+50 h.p. Gnome engine. The Short monoplane, was built generally on
+Bleriot lines. Its chassis was an original feature. The Coventry
+Ordnance biplane was a two-seater tractor built for the British Military
+Trials. It had a 100 h.p. 14-cylinder Gnome engine, with propeller
+geared down through a chain drive. The machine was an interesting
+experiment, but not an unqualified success. The Moreau "Aerostable,"
+fitted with a 50 h.p. Gnome, was a French attempt to obtain automatic
+stability, but it only operated longitudinally. The pilot's nacelle was
+pivoted under the main planes, wires were attached to the control
+members so that the movements of the nacelle in its efforts to keep a
+level keel brought them into operation. The Mersey monoplane, an entrant
+for the British Military Trials, was designed to present a clear field
+of view and fire. The 45 h.p. Isaacson engine was connected by a shaft
+to a propeller mounted behind the nacelle on the top tail boom. It was a
+promising experiment, but came to grief. The Radley-Moorhouse monoplane
+was a sporting type machine on Bleriot lines, with 50 h.p. Gnome engine.
+It was notable for its streamlined body and disc wheels.
+</p>
+
+<div style="height: 6em;"><br /><br /><br /><br /><br /><br /></div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
+
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+</pre>
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+</body>
+</html>
+
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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
+       Fifth Edition
+
+Author: H. Barber
+
+Release Date: June 10, 2007 [EBook #21791]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE AEROPLANE SPEAKS ***
+
+
+
+
+Produced by Jonathan Ingram, Marvin A. Hodges, David Garcia
+and the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+[Illustration: THE FLIGHT FOLK.]
+
+
+
+
+
+
+THE AEROPLANE SPEAKS
+
+BY H. BARBER, A.F.Ae.S.
+
+(CAPTAIN, ROYAL FLYING CORPS)
+
+
+WITH 36 FULL PAGES OF "TYPES OF AEROPLANES" AND 87 SKETCHES AND DIAGRAMS
+
+_FIFTH EDITION_
+
+
+LONDON
+
+McBRIDE, NAST & CO., LTD.
+
+
+
+
+THE AEROPLANE SPEAKS.
+
+  _First edition--December, 1916_
+  _Second edition--February, 1917_
+  _Third edition--April, 1917_
+  _Fourth edition--July, 1917_
+  _Fifth edition--December, 1917_
+
+
+FIRST REVIEWS:
+
+  =C. G. G. in the AEROPLANE:= "One hopes that the Subaltern
+  Flying Officer will appreciate the gift which the author has
+  given him out of his own vast store of experience, for the book
+  contains the concentrated knowledge of many expensive years in
+  tabloid form, or perhaps one should say in condensed milk form,
+  seeing that it is easy to swallow and agreeable to the taste,
+  as well as wholesome and nourishing. And, besides the young
+  service aviator, there are thousands of young men, and women
+  also, now employed in the aircraft industry, who will appreciate
+  far better the value of the finicky little jobs they are doing
+  if they will read this book and see how vital is their work to
+  the man who flies."
+
+  =THE FIELD:= "Entirely different from any other text-book on
+  the subject, not merely in its form, but in its capacity to convey
+  a knowledge of the principles and practice of flying. Undoubtedly
+  it is the best book on its subject."
+
+  =THE UNITED SERVICE GAZETTE:= "Should be in the hands of every
+  person interested in aviation."
+
+  =THE OUTLOOK:= "As amusing as it is instructive."
+
+  =THE MORNING POST:= "Should be read and re-read by the would
+  be and even the experienced pilot."
+
+
+
+  PRINTED IN ENGLAND BY
+  BILLING AND SONS, LIMITED
+  GUILDFORD
+
+
+
+
+  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_
+
+                                                                    PAGE
+
+    _PART I.--THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES_          1
+        _II.--THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES,
+                FINISH THE JOB_                                       15
+       _III.--THE GREAT TEST_                                         27
+        _IV.--CROSS COUNTRY_                                          38
+
+  CHAPTER I.--FLIGHT                                                  55
+         II.--STABILITY AND CONTROL                                   70
+        III.--RIGGING                                                 90
+         IV.--PROPELLERS                                             115
+          V.--MAINTENANCE                                            126
+
+  TYPES OF AEROPLANES                                                130
+
+  GLOSSARY                                                           133
+
+
+
+
+
+
+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 an illustration thus:
+
+[Illustration]
+
+"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.
+
+[Illustration: The action of the surface upon the air.]
+
+"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 and consequent Force 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."
+
+[Illustration]
+
+"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
+or 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."
+
+[Illustration]
+
+"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."
+
+[Illustration]
+
+"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:
+
+[Illustration]
+
+"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:
+
+[Illustration]
+
+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.
+
+[Illustration: Maximum Climb. Maximum Speed.]
+
+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."
+
+[Illustration]
+
+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
+counter-balance 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:
+
+[Illustration]
+
+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."
+
+[Illustration: The angles shown above are only roughly approximate, as
+they vary with different types of aeroplanes.]
+
+"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."
+
+[Footnote 1: Propeller Slip: As the propeller screws through the air,
+the latter to a certain extent gives back to the thrust of the propeller
+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 propeller
+will feel the slip as a strong draught of air.]
+
+[Footnote 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.]
+
+[Footnote 3: Pancakes: Pilot's slang for stalling an aeroplane and
+dropping like a pancake.]
+
+
+
+
+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."
+
+[Illustration]
+
+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" (see illustration A,
+over-leaf).
+
+"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."
+
+[Illustration]
+
+"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!"
+
+[Illustration]
+
+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 a certain axis of the
+machine, and the Elevator is a Controlling Surface designed to turn the
+Aeroplane about another 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:
+
+[Illustration: H.E., Horizontal equivalent.]
+
+"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 total 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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+"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."
+
+[Illustration]
+
+"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."
+
+[Illustration: "Wash out" on both sides.]
+
+"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."
+
+[Illustration]
+
+"Well, I hope that's all as it should be," she concluded, "for to-morrow
+the Great Test in the air is due."
+
+[Footnote 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.]
+
+[Footnote 5: Skin friction is that part of the drift due to the friction
+of the air with roughness upon the surface of the aeroplane.]
+
+[Footnote 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.]
+
+[Footnote 7: An explanation of the way in which the wash-out is combined
+with a wash-in to offset propeller torque will be found on p. 82.]
+
+
+
+
+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 "stream-lined" 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
+Pitot Air Speed Indicator, and, adjusting it to zero, smiles as he
+hears the Pitot-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 Pitot 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--a
+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.
+
+"On 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 Triplex 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, crowned 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 lower 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.
+
+[Footnote 8: A.M.'s: Air Mechanics.]
+
+[Footnote 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.]
+
+[Footnote 10: Deviation Curve: A curved line indicating any errors in the
+compass.]
+
+[Footnote 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.]
+
+[Footnote 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.]
+
+[Footnote 13: Box-kite. The first crude form of biplane.]
+
+
+
+
+PART IV
+
+'CROSS COUNTRY
+
+
+The Aeroplane had been designed and built, and tested in the air, and
+now it 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 too 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 very 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, "Tanks 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
+(see p. 40).
+
+"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.
+
+[Illustration:
+  A--B, 150 miles,
+  A--C, 50 miles; direction and miles per hour of wind.
+  C--D, 100 miles; airspeed of aeroplane.
+  A--D, Distance covered by aeroplane in one hour.
+  A--E, Compass course.]
+
+"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.
+
+[Illustration: The Pilot's Cock-pit.]
+
+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 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 a 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 place it
+appears, nestling down between the hills and 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 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. A high tail
+means 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:
+
+[Illustration:
+  The Pilot's Aeroplane.
+  The Change of Design He Would Like.]
+
+"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," says someone, "and that they haven't had
+difficulties with the fog. It rolled up very quickly, you know."
+
+"Never fear," remarks 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," says
+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 who 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 walk 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.
+
+[Illustration]
+
+_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.
+
+[Illustration]
+
+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, if, as seems reasonable, we regard the surface from the point
+of view of the neutral lift line. 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 criticized adversely
+is then applicable. Flat lifting surfaces are, however, never used.
+
+The surface acts upon the air in the following manner:
+
+[Illustration]
+
+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^2.
+
+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.
+
+[Illustration]
+
+The direction of the reaction is, at efficient angles of incidence,
+approximately at right-angles to the neutral lift line 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 roughness 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.
+
+[Illustration]
+
+_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 over the top of the surface 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. Also, too great an
+angle for the velocity will result in the underside of the surface
+tending to compress the air against which it is driven rather than
+accelerate it _downwards_, and that will tend to produce drift rather
+than the _upwards_ reaction, or lift.
+
+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 stream-line 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
+thickness 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 higher the
+aspect ratio, the greater the reaction. It is obvious, I think, that the
+greater the span, the greater the mass of undisturbed air engaged, and,
+as already explained, the reaction is partly the result of the mass of
+air engaged. I say "undisturbed" advisedly, for otherwise it might be
+argued that, whatever the shape of the surface, the same mass of air
+would be engaged. The word "undisturbed" makes all the difference, for
+it must be remembered that the rear part of the underside of the surface
+engages air most of which has been deflected downwards by the surface
+in front of it. That being so, the rear part of the surface has not the
+same opportunity of forcing; the air downwards (since it is already
+flowing downwards) and securing there from an upwards, reaction as has
+the surface in front of it. It is therefore of less value for its area
+than the front part of the surface, since it does less work and secures
+less reaction--_i.e._, lift. Again, the rarefied area over the top of
+the surface is most rare towards the front of it, as, owing to eddies,
+the rear of such area tends to become denser.
+
+[Illustration]
+
+Thus, you see, the front part of the surface is the most valuable from
+the point of view of securing an upwards reaction from the air; and so,
+by increasing the proportion of front, or "span," to chord, we increase
+the amount of reaction for a given velocity and area of surface. That
+means a better proportion of reaction to weight of surface, though the
+designer must not forget the drift of struts and wires necessary to
+brace up a surface of high aspect ratio.
+
+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 higher 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 about 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.
+
+[Illustration: H.E., Horizontal equivalent. D., Dihedral angle.]
+
+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.
+
+[Illustration]
+
+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 requisite 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.
+
+[Illustration: 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.=
+
+
+SUMMARY.
+
+_Essentials for Maximum Climb._
+
+  1. Low velocity.
+  2. Large surface.
+  3. Large angle relative to propeller thrust.
+  4. Large angle relative to direction of motion.
+  5. Large camber.
+
+_Essentials for Maximum Velocity._
+
+  1. High velocity.
+  2. Small surface.
+  3. Small angle relative to propeller thrust.
+  4. Small angle relative to direction of motion.
+  5. Small camber.
+
+
+It is mechanically impossible to construct an aeroplane of reasonable
+weight of which it would be possible to vary 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. 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.
+
+[Footnote 14: See Newton's laws in the Glossary at the end of the book.]
+
+[Footnote 15: See "Aerofoil" in the Glossary.]
+
+
+
+
+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 everything 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.
+
+[Illustration]
+
+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.
+
+[Illustration]
+
+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.[17]
+
+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 deg..
+
+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 deg., the C.P. moves
+forward as in the case of flat surfaces (see B); but angles above 30 deg. do
+not interest us, since they produce a very low ratio of lift to drift.
+
+[Illustration]
+
+Below angles of about 30 deg. (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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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 deg.. Such decrease applies to
+both main surface and stabilizer, since both are fixed rigidly to the
+aeroplane.
+
+The main surface, which had 12 deg. angle, has now only 10 deg., _i.e._, a loss
+of _one-sixth_.
+
+The stabilizer, which had 4 deg. angle, has now only 2 deg., _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 angle and
+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 deg.. Then, in order to secure
+a sufficiency of longitudinal stability, it is necessary to set the
+forward stabilizer at about 15 deg.. 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.
+
+[Illustration]
+
+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, fitted to a "pusher" aeroplane, 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. When fitted to a
+"tractor" aeroplane, the engine is reversed so that a reverse condition
+results. In modern aeroplanes this tendency is not sufficiently
+important to bother about, except in the matter of spiral descents
+(see section headed "Spinning"). In the old days of crudely designed
+and under-powered "pusher" 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:
+
+[Illustration]
+
+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.
+
+[Illustration:
+    R, Direction of reaction of wing indicated.
+  R R, Resultant direction of reaction of both wings.
+    M, Horizontal (sideway) component of reaction.
+    L, Vertical component of reaction (lift).]
+
+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. This
+produces a little lateral stability without any marked pendulum effect.
+
+_Propeller torque_ affects lateral stability. An aeroplane tends to turn
+over sideways in the opposite direction to which the propeller revolves.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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.
+
+[Illustration:  Note: Observe that the inclination of the ailerons to
+the surface is the same in each case.]
+
+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:
+
+[Illustration: "Wash Out" on both sides relative to the Centre.]
+
+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." _Experientia 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.
+
+[Illustration]
+
+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 and/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.
+
+[Illustration: Nose Dive Spin.]
+
+In this connection every pilot of an aeroplane fitted with a rotary
+engine should bear in mind the gyroscopic effect of such engine. In the
+case of such an engine fitted to a "pusher" aeroplane, its effect when a
+left-hand turn is made is to depress the nose of the machine. If fitted
+to a "tractor" it is reversed, so the effect is to depress the nose
+if a right-hand turn is made. The sharper the turn, the greater such
+effect--an effect which may render the aeroplane unmanageable if the
+spiral is one of very small radius and the engine is revolving with
+sufficient speed to produce a material gyroscopic effect. Such
+gyroscopic effect should, however, slightly _assist_ the pilot to
+navigate a small spiral if he will remember to (1) make _right-hand_
+spirals in the case of a "pusher," (2) make _left-hand_ spirals in the
+case of a "tractor." The effect will then be to keep the nose up and
+prevent a nose-dive. I say "slightly" assist because the engine is, of
+course, throttled down for a spiral descent, and its lesser revolutions
+will produce a lesser gyroscopic effect.
+
+On the other hand, it might be argued that if the aeroplane gets into a
+"spin," anything tending to depress the nose of the machine is of value,
+since it is often claimed that the best way to get out of a spin is
+to put the machine into a nose-dive--the great velocity of the dive
+rendering the controls more efficient and better enabling the pilot to
+regain control. It is, however, a very contentious point, and few are
+able to express opinions based on practice, since pilots indulging in
+nose-dive spins are either not heard of again or have usually but a hazy
+recollection of exactly what happened to them.
+
+GLIDING DESCENT WITHOUT PROPELLER THRUST.--All aeroplanes are, or should
+be, designed to assume their correct 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.
+
+[Illustration]
+
+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. (see illustration above).
+
+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.
+
+[Illustration: Position A. Path B. Path C. Path D.]
+
+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.
+
+[Footnote 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.]
+
+[Footnote 17: The reason the C.P. of an inclined surface is forward of
+the centre of the surface is because the front of the surface does most
+of the work, as explained on p. 62.]
+
+
+
+
+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.
+
+[Illustration: Cross Sectional area]
+
+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.
+
+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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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.
+
+[Illustration]
+
+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._
+
+[Illustration]
+
+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.
+
+[Illustration: Strut straight. Wires and gap correctly adjusted. Strut
+bent throwing wires and gap out of adjustment.]
+
+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.
+
+[Illustration: Strut bedded properly. Strut bedded badly.]
+
+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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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 roughness 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 roughness 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.
+
+[Illustration: Distortion of upper wing caused by auxiliary lift wire
+being too tight.]
+
+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 much as possible. The rules to be observed are as
+follows:
+
+[Illustration: Wrong shape. Result of wrong shape. Right Shape.]
+
+(_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.
+
+[Illustration: Position in which controlling surface must be rigged. It
+will be its position during flight.]
+
+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.
+
+[Illustration: Position during flight. Position in which controlling
+surface must be rigged.]
+
+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:
+
+[Illustration]
+
+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:
+
+[Illustration: FRONT ELEVATION and PLAN.]
+
+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 their ends to struts. 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:
+
+[Illustration: Points A, B, and C, must be the same fixed distances
+from the butt as are Points D, E, and F. Distances 1 and 2 must equal
+distances 3 and 4.]
+
+The above applies to both front and rear bays.
+
+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 beforehand. 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:
+
+[Illustration]
+
+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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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 inch or more to the stagger,
+with the certain result that the aeroplane 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.
+
+[Illustration: The dotted lines on the surface represent the spars
+within it.]
+
+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 (see illustration overleaf).
+
+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 roughness 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.
+
+_Aspect Ratio._--Ditto.
+
+_Camber._--Ditto.
+
+[Illustration:
+  M, Direction of motion of propeller (rotary).
+  R, Direction of reaction.
+  T, Direction of thrust.
+ AD, Direction of the resistance of the air to the passage of the
+     aeroplane, _i.e._, aeroplane drift.
+  D, Direction of propeller drift (rotary).
+  P, Engine power, opposed to propeller drift and transmitted to
+     the propeller through the propeller shaft.]
+
+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. = 5.903 feet.
+    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 then 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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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 line 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:
+
+[Illustration:
+  A, B, C, and D, Actual pitch at points tested.
+  I, Pitch angle at point tested nearest to centre of propeller.
+  E, Circumference at I.
+  J, Pitch angle at point tested nearest to I.
+  F, Circumference at J.
+  K, Pitch angle at next point tested.
+  G, Circumference at K.
+  L, Pitch angle tested at point nearest tip of blade.
+  H, Circumference at L.]
+
+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:
+
+[Illustration]
+
+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 1/16 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.
+
+[Illustration]
+
+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:
+
+[Illustration]
+
+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.
+
+Weight B should equal weight E.
+
+Weight C should equal weight 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 calipers thus:
+
+[Illustration]
+
+The distance A--B should equal K--L.
+
+The distance C--D should equal I--J.
+
+The distance E--F should equal G--H.
+
+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,[18]
+but a fairly accurate idea of the concave camber can be secured by
+slowly passing a straight-edge along the blade, thus:
+
+[Illustration]
+
+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, surface area, or to bad mounting. 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.
+
+[Illustration:
+  SPIRAL COURSES OF TWO-BLADE TIPS.
+  SPIRAL COURSES OF FOUR-BLADE TIPS.
+  Pitch the same in each case.]
+
+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 (see illustration on preceding page).
+
+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.
+
+[Footnote 18: I have heard of temporary ones being made quickly by
+bending strips of lead over the convex side of the blade, but I should
+think it very difficult to secure a sufficient degree of accuracy in
+that way.]
+
+
+
+
+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 practised 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 symmetrical with it. 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 practised 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 ON 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 pull by
+means of wires.
+
+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.
+
+[Illustration]
+
+[Illustration]
+
+
+
+
+GLOSSARY
+
+_The numbers at the right-hand side of the page indicate the parts
+numbered in the preceding diagrams._
+
+
+=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 the 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 fitted 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 lift 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. [1]
+
+=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. [2]
+
+=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. [3]
+
+=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. [4]
+
+=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 the anti-lift wires are suspended. [5]
+
+=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." [6]
+
+=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. [7]
+
+=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 "eddies" and "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. [8]
+
+=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. [9]
+
+=Edge, Leading=--The front edge of a surface relative to its normal
+direction of motion. [10]
+
+=Edge, Trailing=--The rear edge of a surface relative to its normal
+direction of motion. [11]
+
+=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. [12]
+
+=Fin=--Additional keel-surface, usually mounted at the rear of an
+aeroplane. [13]
+
+=Flange= (_of a rib_)--That horizontal part of a rib which prevents it
+from bending sideways. [14]
+
+=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 aluminium, 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. [15]
+
+=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
+any two of its surfaces. [16]
+
+=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. [17]
+
+=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. [18]
+
+=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/or pilot
+and passenger, and to which the tail-plane is not fixed. [19]
+
+=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. [20]
+
+=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. [21]
+
+=Power, Horse=--One horse-power represents a force sufficient to raise
+33,000 lb. 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. [22]
+
+=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. [23]
+
+=Rib, Compression=--Acts as an ordinary rib, besides bearing the stress of
+compression produced by the tension of the internal bracing wires. [24]
+
+=Rib, False=--A subsidiary rib, usually used to improve the camber of the
+front part of the surface. [25]
+
+=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. [26]
+
+=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. [28]
+
+=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.
+[28_a_]
+
+=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. [29]
+
+=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. [30]
+
+=Strut=--Any wooden member intended to take merely the stress of direct
+compression.
+
+=Strut, Interplane=--A strut holding the top and bottom surfaces
+apart. [31]
+
+=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_. [32]
+
+=Strut, Extension=--A strut supporting an "extension" when not in
+flight. It may also prevent the extension from collapsing upwards during
+flight. [33]
+
+=Strut, undercarriage=-- [33_a_]
+
+=Strut, Dope=--A strut within a surface, so placed as to prevent the
+tension of the doped fabric from distorting the framework. [34]
+
+=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_. [36]
+
+=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. [37_a_]
+
+=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. [38]
+
+=Wash-out=--A decreasing angle of incidence of a surface towards its
+wing-tip. [39]
+
+=Wing-tip=--The right or left-hand extremity of a surface. [40]
+
+=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. [41]
+
+=Wire, Anti-lift or Landing=--A wire opposed to the direction of gravity,
+and used to sustain a surface when it is at rest. [42]
+
+=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. [44]
+
+=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." [45]
+
+=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 described 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 fuselage, between top tail booms, or at the
+top of similar construction. [46]
+
+=Wire, Bottom Bracing=--Ditto, substituting "bottom" for "top." [47]
+
+=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. [48]
+
+=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. [49]
+
+=Wire, Control Bracing=--A wire preventing distortion of a controlling
+surface. [50]
+
+=Wire, Control=--A wire connecting a controlling surface with the pilot's
+control lever, wheel, or rudder-bar. [51]
+
+=Wire, Aileron Gap=--A wire connecting top and bottom ailerons. [52]
+
+=Wire, Aileron Balance=--A wire connecting the right- and left-hand top
+ailerons. Sometimes termed the "aileron compensating wire." [53]
+
+=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.
+
+
+
+
+Types of Aeroplanes.
+
+
+
+
+[Illustration: Plate I.]
+
+The first machine to fly--of which there is anything like authentic
+record--was the Ader "Avion," after which the more notable advances were
+made as shown above.
+
+
+[Illustration: Plate II.]
+
+The Henri Farman was the first widely used aeroplane. Above are shown
+the chief steps in its development.
+
+
+[Illustration: Plate III.]
+
+THE AVRO.--The aeroplane designed and built by Mr. A. V. Roe was the
+first successful heavier-than-air flying machine built by a British
+subject. Mr. Roe's progress may be followed in the picture, from his
+early "canard" biplane, through various triplanes, with 35 J.A.P. and 35
+h.p. Green engines, to his successful tractor biplane with the same 35
+h.p. Green, thence through the "totally enclosed" biplane 1912, with 60
+h.p. Green, to the biplane 1913-14, with 80 h.p. Gnome.
+
+
+[Illustration: Plate IV.]
+
+THE SOPWITH LAND-GOING BIPLANES.--The earliest was a pair of Wright
+planes with a fuselage added. Next was the famous tractor with 80 h.p.
+Gnome. Then the "tabloid" of 1913, which set a completely new fashion
+in aeroplane design. From this developed the Gordon-Bennett racer shown
+over date 1914. The gun-carrier was produced about the same time, and
+the later tractor biplane in a development of the famous 80 h.p. but
+with 100 h.p. monosoupape Gnome.
+
+
+[Illustration: Plate V.]
+
+THE MAURICE FARMAN.--First, 1909, the 50-60 h.p. Renault and coil-spring
+chassis. 1910, the same chassis with beginning of the characteristic
+bent-up skids. 1911 appeared the huge French Military Trials 3-seater;
+also the round-ended planes and tails and "Henry" type wheels. This
+developed, 1912, into the square-ended planes and upper tail, and long
+double-acting ailerons of the British Military Trials. The 1913 type had
+two rectangular tail-planes and better seating arrangements, known
+affectionately as the "mechanical cow"; the same year came the first
+"shorthorn," with two tail-planes and a low nacelle. This finally
+developed into the carefully streamlined "shorthorn" with the raised
+nacelle and a single tail-plane.
+
+
+[Illustration: Plate VI.]
+
+THE SHORT "PUSHERS."--In 1909 came the semi-Wright biplane, with 35 h.p.
+Green, on which Mr. Moore-Brabazon won the "Daily Mail's" L1000 prize
+for the first mile flight on a circuit on a British aeroplane. Then the
+first box-kite flown by Mr. Grace at Wolverhampton. Later the famous
+"extension" type on which the first Naval officers learned to fly. Then
+the "38" type with elevator on the nacelle, on which dozens of R.N.A.S.
+pilots were taught.
+
+
+[Illustration: Plate VII.]
+
+SHORT TRACTORS, 1911-1912.--They were all co-existent, but the first was
+the "tractor-pusher" (bottom of picture). Then came the "twin-tractor
+plus propeller" (at top). A development was the "triple-tractor" (on the
+right), with two 50 h.p. Gnomes, one immediately behind the other under
+the cowl, one driving the two chains, the other coupled direct. Later
+came the single-engined 80 h.p. tractor (on the left), the original of
+the famous Short seaplanes.
+
+
+[Illustration: Plate VIII.]
+
+THE VICKERS MACHINES: First the Vickers-R.E.P. of 1911, which developed
+into the full-bodied No. V. with R.E.P. engine, then the Military Trials
+"sociable" with Viale engine, and so to the big No. VII with a 100 h.p.
+Gnome. Contemporary with the No. V and No. VI were a number of school
+box-kites of ordinary Farman type, which developed into the curious
+"pumpkin" sociable, and the early "gun 'bus" of 1913. Thence arrived the
+gun-carrier with 100 h.p. monosoupape Gnome.
+
+
+[Illustration: Plate IX.]
+
+THE BRISTOL AEROPLANES.--First, 1910, Farman type box-kites familiar to
+all early pupils. Then the miniature Maurice-Farman type biplane of the
+"Circuit of Britain." Contemporaneous was the "floating tail" monoplane
+designed by Pierre Prier, and after it a similar machine with fixed
+tail. Then came the handsome but unfortunate monoplane designed by
+M. Coanda for the Military Trials, 1912.
+
+
+[Illustration: Plate X.]
+
+THE BRISTOL TRACTORS.--Late 1912 came the round fuselaged tractor, with
+Gnome engine, designed by Mr. Gordon England for Turkey. 1912-13 came
+the biplane built onto the Military Trials monoplane type fuselage,
+also with a Gnome, designed by M. Coanda for Roumania. Then the
+Renault-engined Coanda tractor 1913, followed by 80 h.p. Gnome-engined
+scout, designed by Messrs. Barnwell and Busteed, which with Gnomes,
+le Rhones and Clergets, has been one of the great successes. Almost
+contemporary was the two-seater Bristol.
+
+
+[Illustration: Plate XI.]
+
+THE MARTINSYDES.--1909, first experimental monoplane built with small
+4-cylinder engine. J.A.P.-engined machine, 1910, followed by the
+Gnome-engined machine, 1911. 1912, first big monoplane with Antoinette
+engine was built, followed by powerful Austro-Daimler monoplane, 1913.
+Then came the little Gnome-engined scout biplanes, 1914, some with,
+some without, skids.
+
+
+[Illustration: Plate XII.]
+
+THE CURTISS BIPLANES.--In 1909 came the "June-bug," the united product
+of Glen Curtiss, Dr. Graham Bell, and J. A. D. McCurdy. Then the
+box-kite type, 1909, on which Mr. Curtiss won the Gordon-Bennett Race at
+Reims. Next the "rear-elevator" pusher, 1912, followed by first tractor,
+1913, with an outside flywheel. All purely Curtiss machines to that
+date had independent ailerons intended to get away from Wright patents.
+Following these came tractors with engines varying from 70 to 160 h.p.,
+fitted with varying types of chassis. All these have ordinary ailerons.
+
+
+[Illustration: Plate XIII.]
+
+THE BLERIOT (1).--The first engine-driven machine was a "canard"
+monoplane. Then came the curious tractor monoplanes 1908-1909, in
+order shown. Famous "Type XI" was prototype of all Bleriot successes.
+"Type XII" was never a great success, though the ancestor of the popular
+"parasol" type. The big passenger carrier was a descendant of this type.
+
+
+[Illustration: Plate XIV.]
+
+THE BLERIOT (2):--1910, "Type XI," on which Mr. Grahame-White won
+Gordon-Bennett Race, with a 14-cylinder 100 h.p. Gnome. 1911 came the
+improved "Type XI," with large and effective elevator flaps. On this
+type, with a 50 h.p. Gnome, Lieut. de Conneau (M. Beaumont) won
+Paris-Rome Race and "Circuit of Britain." Same year saw experimental
+"Limousine" flown by M. Legagneux, and fast but dangerous "clipped-wing"
+Gordon-Bennett racer with the fish-tail, flown by Mr. Hamel. About the
+same time came the fish-tailed side-by-side two-seater, flown by Mr.
+Hamel at Hendon and by M. Perreyon in 1912 Military Trials. 1911, M.
+Bleriot produced the 100 h.p. three-seater which killed M. Desparmets in
+French Military Trials. 1912-13, M. Bleriot produced a quite promising
+experimental biplane, and a "monocoque" monoplane in which the passenger
+faced rearward.
+
+
+[Illustration: Plate XV.]
+
+THE BLERIOT (3)--1912 tandem two-seater proved one of the best machines
+of its day. 1913 "canard" lived up to its name. A "pusher" monoplane was
+built in which the propeller revolved on the top tail boom. This machine
+came to an untimely end, with the famous pilot, M. Perreyon. 1912
+"tandem" was developed in 1914 into the type shown in centre; almost
+simultaneously "parasol" tandem appeared. 1914, M. Bleriot built a
+monoplane embodying a most valuable idea never fully developed. The
+engine tanks and pilot were all inside an armoured casing. Behind them
+the fuselage was a "monocoque" of three-ply wood bolted onto the armour.
+And behind this all the tail surfaces were bolted on as a separate unit.
+
+
+[Illustration: Plate XVI.]
+
+THE CAUDRON.--1910, came the machine with ailerons and a 28 h.p. Anzani.
+1911 this was altered to warp control and a "star" Anzani was fitted.
+From this came the 35 h.p. type of 1912, one of the most successful
+of school machines. Small fast monoplane, 1912, was never further
+developed. 1913 appeared the familiar biplanes with 80 h.p. Gnomes,
+and 5-seater with 100 h.p. Anzani for French "Circuit of Anjou." 1914
+produced the "scout" biplane which won at Vienna. 1915 appeared the
+twin-engined type, the first successful "battle-plane."
+
+
+[Illustration: Plate XVII.]
+
+THE DEPERDUSSIN.--In 1911 the little monoplane with a Gyp. engine.
+Then the Gnome-engined machine of the "Circuit of Europe." In 1912 came
+the Navy's machine with 70 h.p. Gnome, and Prevost's Gordon-Bennett
+"Bullet," 135 miles in the hour. The last was the British-built
+"Thunder-Bug," familiar at Hendon.
+
+
+[Illustration: Plate XVIII.]
+
+THE BREGUET.--First to fly was the complicated but business-like machine
+of 1909. Then came the record passenger carrier, 1910 (which lifted 8
+passengers). 1911 the French Military Trials machine with geared-down
+100 h.p. Gnome appeared. 1912 produced the machine with 130 h.p. Salmson
+engine on which the late Mr. Moorhouse flew the Channel with Mrs.
+Moorhouse and Mr. Ledeboer as passengers; also the machine with 130 h.p.
+horizontal Salmson, known as the "Whitebait." The last before the war
+was the rigid wing machine with 200 h.p. Salmson.
+
+
+[Illustration: Plate XIX.]
+
+THE CODY.--First the Military Experiment of 1908, with an Antoinette
+engine, then improved type 1909 with a Green engine. Next the
+"Cathedral," 1910, with a Green engine, which won Michelin Prize. In
+1911 "Daily Mail" Circuit machine, also with a Green, won the Michelin.
+This was modified into 1912 type which won Military Competition and
+L5,000 in prizes, with an Austro-Daimler engine, and later the Michelin
+Circuit Prize, again with a Green. 1912 the only Cody Monoplane was
+built. 1913 a modified biplane on which the great pioneer was killed.
+
+
+[Illustration: Plate XX.]
+
+THE NIEUPORT.--The first Nieuport of 1909 was curiously like a monoplane
+version of a Caudron. In 1910 came the little two-cylinder machine with
+fixed tail-plane and universally jointed tail. In 1911 the French Trials
+machine was built with 100 h.p. 14 cylinder Gnome, and is typical of
+this make. Also the little two-cylinder record breaker. A modification
+of 1913 was the height record machine of the late M. Legagneux.
+
+
+[Illustration: Plate XXI.]
+
+THE R.E.P. MONOPLANES.--First came the curious and highly interesting
+experiments of 1907, 1908, 1909, and 1910. 1910-1911, the World's
+Distance Record breaker was produced; after it, the "European Circuit,"
+all with R.E.P. engines. In 1913-14 came the French military type with
+Gnome engine and finally the "parasol," 1915.
+
+
+[Illustration: Plate XXII.]
+
+THE MORANE: First the European Circuit and Paris-Madrid type. Then the
+1912 types, with taper wing and modern type wing. The 1913 types, the
+"clipped wing," flown by the late Mr. Hamel, one of the standard tandem
+types now in use. About the same time came the "parasol." 1914-15 came
+a little biplane like a Nieuport, and the "destroyer" type with a round
+section body, flown by Vedrines.
+
+
+[Illustration: Plate XXIII.]
+
+THE VOISIN.--1908, the first properly controlled flight on a European
+aeroplane was made on a Voisin of the type shown with fixed engine.
+Then followed the record breaker of 1909 with a Gnome engine. In 1909
+also the only Voisin tractor was produced. 1910 the Paris-Bordeaux type
+was built; 1911 the amphibious "canard" and the "military" type with
+extensions, and the type without an elevator. 1913 came the type with
+only two tail-booms and a geared-down engine, which developed into the
+big "gun" machine with a Salmson engine.
+
+
+[Illustration: Plate XXIV.]
+
+THE HANRIOT AND PONNIER MONOPLANES.--In 1909 came the first Hanriot with
+50 h.p. 6-cylinder Buchet engine, and in 1910 the famous "Henrietta"
+type with E.N.Vs. and stationary Clergets. 1911 came the Clerget
+two-seater entered in French Military Trials, and 1912 the 100 h.p.
+Hanriot-Pagny monoplane which took part in British Military Trials.
+Sister machines of the same year were the single seater with 50 h.p.
+Gnome and the 100 h.p. Gnome racer with stripped chassis. In 1913 the
+Ponnier-Pagny racing monoplane with 160 h.p. Le Rhone competed in the
+Gordon-Bennett race, doing about 130 miles in the hour. The 60 h.p.
+Ponnier biplane was the first successful French scout tractor biplane.
+
+
+[Illustration: Plate XXV.]
+
+THE WRIGHT BIPLANE.--The first power flights were made, 1903, on a
+converted glider fitted with 16 h.p. motor. The prone position of the
+pilot will be noted. By 1907 the machine had become reasonably practical
+with 40 h.p. motor. On this the first real flying in the world was done.
+In 1910 the miniature racing Wright was produced; also the type with a
+rear elevator in addition to one in front. Soon afterwards the front
+elevator disappeared, and the machine became the standard American
+exhibition and school machine for four years. In 1915 a machine with
+enclosed fuselage was produced.
+
+
+[Illustration: Plate XXVI.]
+
+THE BLACKBURN MONOPLANES.--In 1909 was built the curious four-wheeled
+parasol-type machine with 35 h.p. Green engine and chain transmission,
+on which flying was done at Saltburn. In 1911 the Isaacson-engined
+machine was built, together with a 50 h.p. Gnome single-seater on which
+Mr. Hucks started in the Circuit of Britain race. In 1912 another 50
+h.p. single-seater was built on which a good deal of school work was
+done. A more advanced machine appeared in 1913 and a two-seater with
+80 h.p. Gnome did a great deal of cross-country work in 1913-14.
+
+
+[Illustration: Plate XXVII.]
+
+In 1908 the first Antoinette monoplane was produced by MM. Gastambide
+and Mengin. Then followed a machine with central skids, a single wheel,
+and wing skids. In 1909 came the machine with four-wheeled chassis and
+ailerons and later an improved edition which reverted to the central
+skid idea. On this M. Latham made his first cross-channel attempt.
+The next machine shed the wing skids and widened its wheelbase.
+During 1910-11 the ailerons vanished, warp control was adopted and the
+king-post system of wing-bracing was used. In 1911 the curious machine
+with streamlined "pantalette" chassis, totally enclosed body and
+internal wing-bracing, was produced for French Military Trials. In 1912
+the three-wheeled machine was used to a certain extent in the French
+Army. Then the type disappeared.
+
+
+[Illustration: Plate XXVIII.]
+
+In 1908 and 1909 detached experimental machines in various countries
+attained a certain success. The late Capt. Ferber made a primitive
+tractor biplane 1908. The Odier-Vendome biplane was a curious bat-winged
+pusher biplane built 1909. The tailless Etrich monoplane, built in
+Austria, 1908, was an adaptation of the Zanonia leaf. M. Santos-Dumont
+made primitive parasol type monoplanes known as "Demoiselles," in which
+bamboo was largely used. 1909 type is seen above. A curious steel
+monoplane was built by the late John Moisant, 1909. The twin-pusher
+biplane, built by the Barnwell Bros. in Scotland, made one or two
+straight flights in 1909. The Clement-Bayard Co. in France constructed
+in 1909 a biplane which did fairly well. Hans Grade, the first German
+to fly, made his early efforts on a "Demoiselle" type machine, 1908.
+
+
+[Illustration: Plate XXIX.]
+
+In 1910 a number of novel machines were produced. The Avis with Anzani
+engine was flown by the Hon. Alan Boyle. Note the cruciform universally
+jointed tail. The Goupy with 50 h.p. Gnome was an early French tractor,
+notable for its hinging wing-tips. The Farman was a curious "knock-up"
+job, chiefly composed of standard box-kite fittings. The Sommer with
+50 h.p. Gnome was a development of the box-kite with a shock-breaking
+chassis. The Savary, also French, was one of the first twin tractors to
+fly. The model illustrated had an E.N.V. engine. Note position of the
+rudders on the wing tips. The Austrian Etrich was the first successful
+machine of the Taube class ever built.
+
+
+[Illustration: Plate XXX.]
+
+INTERESTING MACHINES, 1910.--The Werner monoplane with E.N.V. engine,
+combined shaft and chain drive, was a variant of the de Pischoff. The
+Macfie biplane was a conventional biplane with 50 h.p. Gnome and useful
+originalities. The Valkyrie monoplane, another British machine, was a
+"canard" monoplane with propeller behind the pilot and in front of main
+plane. The Weiss monoplane was a good British effort at inherent
+stability. The Tellier monoplane was a modified Bleriot with Antoinette
+proportions. The Howard Wright biplane was a pusher with large lifting
+monoplane tail. The Dunne biplane was another British attempt at
+inherent stability. The Jezzi biplane was an amateur built
+twin-propeller.
+
+
+[Illustration: Plate XXXI.]
+
+SOME INTERESTING MACHINES, 1911.--The Compton-Paterson biplane was very
+similar to the early Curtiss pusher; it had a 50 h.p. Gnome. The Sloan
+bicurve was a French attempt at inherent stability with 50 h.p. Gnome
+and tractor screw. The Paulhan biplane was an attempt at a machine for
+military purposes to fold up readily for transport. The Sanders was
+a British biplane intended for rough service. The Barnwell monoplane
+was the first Scottish machine to fly; it had a horizontally opposed
+Scottish engine. The Harlan monoplane was an early German effort; note
+position of petrol tank.
+
+
+[Illustration: Plate XXXII.]
+
+The Clement-Bayard monoplane, 1911, was convertible into a tractor
+biplane. The standard engine was a 50 h.p. Gnome. The machine was
+interesting, but never did much. The Zodiac was one of the earliest
+to employ staggered wings. With 50 h.p. Gnome engine it was badly
+underpowered, so never did itself justice. The Jezzi tractor biplane,
+1911, was a development of an earlier model built entirely by Mr. Jezzi,
+an amateur constructor. With a low-powered J.A.P. engine it developed
+an amazing turn of speed, and it may be regarded as a forerunner of the
+scout type and the properly streamlined aeroplane. The Paulhan-Tatin
+monoplane, 1911, was a brilliant attempt at high speed for low power;
+it presented certain advantages as a scout. A 50 h.p. Gnome, fitted
+behind the pilot's seat in the streamlined fuselage, was cooled through
+louvres. The propeller at the end of the tail was connected with the
+engine by a flexible coupling. This machine was, in its day, the fastest
+for its power in the world, doing 80 miles per hour. Viking 1 was a twin
+tractor biplane driven by a 50 h.p. Gnome engine through chains. It was
+built by the author at Hendon in 1912.
+
+
+[Illustration: Plate XXXIII.]
+
+Much ingenuity was exerted by the French designers in 1911 to
+produce machines for the Military Trials. Among them was the 100 h.p.
+Gnome-Borel monoplane with a four-wheeled chassis, and the Astra
+triplane with a 75 h.p. Renault engine. This last had a surface of about
+500 square feet and presented considerable possibilities. Its principal
+feature was its enormous wheels with large size tyres as an attempt to
+solve difficulties of the severe landing tests. The Clement-Bayard
+biplane was a further development of the Clement-Bayard monoplane;
+the type represented could be converted into a monoplane at will. The
+Lohner Arrow biplane with the Daimler engine was an early German tractor
+biplane built with a view to inherent stability, and proved very
+successful. The Pivot monoplane was of somewhat unconventional French
+construction, chiefly notable for the special spring chassis and pivoted
+ailerons at the main planes; this pivoting had nothing to do with the
+name of the machine, which was designed by M. Pivot.
+
+
+[Illustration: Plate XXXIV.]
+
+The Flanders monoplane, 1912, with 70 h.p. Renault engine, was one of
+the last fitted with king-post system of wing bracing. The Flanders
+biplane entered for British Military Trials. Notable features: the
+highly staggered planes, extremely low chassis and deep fuselage. Also,
+the upper plane was bigger in every dimension than the lower; about
+the first instance of this practice. The Bristol biplane, with 100 h.p.
+Gnome engine, was also entered for the Trials, but ultimately withdrawn.
+The Mars monoplane, later known as the "D.F.W.," was a successful
+machine of Taube type with 120 h.p. Austro-Daimler engine. The building
+of the engine into a cowl, complete with radiator in front, followed car
+practice very closely. The tail of the monoplane had a flexible trailing
+edge; its angle of incidence could be varied from the pilot's seat,
+so that perfect longitudinal balance was attained at all loadings and
+speeds. The Handley-Page monoplane, with 70 h.p. Gnome engine, was an
+early successful British attempt at inherent stability.
+
+
+[Illustration: Plate XXXV.]
+
+The Sommer monoplane, with 50 h.p. Gnome, was a 1911-12 machine; it did
+a good deal of cross-country flying. The Vendome monoplane of 1912, also
+with 50 h.p. Gnome engine, was notable chiefly for its large wheels
+and jointed fuselage, which enabled the machine to be taken down for
+transport. The Savary biplane took part in the French Military Trials,
+1911. It had a four-cylinder Labor aviation motor. Notable features are
+twin chain-driven propellers, rudders between the main planes, the broad
+wheel-base and the position of the pilot. The Paulhan triplane, which
+also figured in the French Military Trials, was a development of the
+Paulhan folding biplane. It had a 70 h.p. Renault engine. For practical
+purposes it was a failure. The R.E.P. biplane, with 60 h.p. R.E.P.
+engine, was a development of the famous R.E.P. monoplanes. Its spring
+chassis, with sliding joints, marked an advance. Like the monoplanes,
+it was built largely of steel.
+
+
+[Illustration: Plate XXXVI.]
+
+In 1912 came the first really successful Handley-Page monoplane, with
+50 h.p. Gnome engine. The Short monoplane, was built generally on
+Bleriot lines. Its chassis was an original feature. The Coventry
+Ordnance biplane was a two-seater tractor built for the British Military
+Trials. It had a 100 h.p. 14-cylinder Gnome engine, with propeller
+geared down through a chain drive. The machine was an interesting
+experiment, but not an unqualified success. The Moreau "Aerostable,"
+fitted with a 50 h.p. Gnome, was a French attempt to obtain automatic
+stability, but it only operated longitudinally. The pilot's nacelle was
+pivoted under the main planes, wires were attached to the control
+members so that the movements of the nacelle in its efforts to keep a
+level keel brought them into operation. The Mersey monoplane, an entrant
+for the British Military Trials, was designed to present a clear field
+of view and fire. The 45 h.p. Isaacson engine was connected by a shaft
+to a propeller mounted behind the nacelle on the top tail boom. It was a
+promising experiment, but came to grief. The Radley-Moorhouse monoplane
+was a sporting type machine on Bleriot lines, with 50 h.p. Gnome engine.
+It was notable for its streamlined body and disc wheels.
+
+
+
+
+
+
+End of the Project Gutenberg EBook of The Aeroplane Speaks, by H. Barber
+
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