diff options
Diffstat (limited to '21791.txt')
| -rw-r--r-- | 21791.txt | 5984 |
1 files changed, 5984 insertions, 0 deletions
diff --git a/21791.txt b/21791.txt new file mode 100644 index 0000000..1052f5e --- /dev/null +++ b/21791.txt @@ -0,0 +1,5984 @@ +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 + +*** END OF THIS PROJECT GUTENBERG EBOOK THE AEROPLANE SPEAKS *** + +***** This file should be named 21791.txt or 21791.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/2/1/7/9/21791/ + +Produced by Jonathan Ingram, Marvin A. Hodges, David Garcia +and the Online Distributed Proofreading Team at +http://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. Special rules, +set forth in the General Terms of Use part of this license, apply to +copying and distributing Project Gutenberg-tm electronic works to +protect the PROJECT GUTENBERG-tm concept and trademark. Project +Gutenberg is a registered trademark, and may not be used if you +charge for the eBooks, unless you receive specific permission. If you +do not charge anything for copies of this eBook, complying with the +rules is very easy. You may use this eBook for nearly any purpose +such as creation of derivative works, reports, performances and +research. They may be modified and printed and given away--you may do +practically ANYTHING with public domain eBooks. Redistribution is +subject to the trademark license, especially commercial +redistribution. + + + +*** START: FULL LICENSE *** + +THE FULL PROJECT GUTENBERG LICENSE +PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK + +To protect the Project Gutenberg-tm mission of promoting the free +distribution of electronic works, by using or distributing this work +(or any other work associated in any way with the phrase "Project +Gutenberg"), you agree to comply with all the terms of the Full Project +Gutenberg-tm License (available with this file or online at +http://gutenberg.org/license). + + +Section 1. General Terms of Use and Redistributing Project Gutenberg-tm +electronic works + +1.A. By reading or using any part of this Project Gutenberg-tm +electronic work, you indicate that you have read, understand, agree to +and accept all the terms of this license and intellectual property +(trademark/copyright) agreement. If you do not agree to abide by all +the terms of this agreement, you must cease using and return or destroy +all copies of Project Gutenberg-tm electronic works in your possession. +If you paid a fee for obtaining a copy of or access to a Project +Gutenberg-tm electronic work and you do not agree to be bound by the +terms of this agreement, you may obtain a refund from the person or +entity to whom you paid the fee as set forth in paragraph 1.E.8. + +1.B. "Project Gutenberg" is a registered trademark. It may only be +used on or associated in any way with an electronic work by people who +agree to be bound by the terms of this agreement. There are a few +things that you can do with most Project Gutenberg-tm electronic works +even without complying with the full terms of this agreement. See +paragraph 1.C below. There are a lot of things you can do with Project +Gutenberg-tm electronic works if you follow the terms of this agreement +and help preserve free future access to Project Gutenberg-tm electronic +works. See paragraph 1.E below. + +1.C. The Project Gutenberg Literary Archive Foundation ("the Foundation" +or PGLAF), owns a compilation copyright in the collection of Project +Gutenberg-tm electronic works. Nearly all the individual works in the +collection are in the public domain in the United States. If an +individual work is in the public domain in the United States and you are +located in the United States, we do not claim a right to prevent you from +copying, distributing, performing, displaying or creating derivative +works based on the work as long as all references to Project Gutenberg +are removed. Of course, we hope that you will support the Project +Gutenberg-tm mission of promoting free access to electronic works by +freely sharing Project Gutenberg-tm works in compliance with the terms of +this agreement for keeping the Project Gutenberg-tm name associated with +the work. You can easily comply with the terms of this agreement by +keeping this work in the same format with its attached full Project +Gutenberg-tm License when you share it without charge with others. + +1.D. The copyright laws of the place where you are located also govern +what you can do with this work. Copyright laws in most countries are in +a constant state of change. If you are outside the United States, check +the laws of your country in addition to the terms of this agreement +before downloading, copying, displaying, performing, distributing or +creating derivative works based on this work or any other Project +Gutenberg-tm work. The Foundation makes no representations concerning +the copyright status of any work in any country outside the United +States. + +1.E. Unless you have removed all references to Project Gutenberg: + +1.E.1. The following sentence, with active links to, or other immediate +access to, the full Project Gutenberg-tm License must appear prominently +whenever any copy of a Project Gutenberg-tm work (any work on which the +phrase "Project Gutenberg" appears, or with which the phrase "Project +Gutenberg" is associated) is accessed, displayed, performed, viewed, +copied or distributed: + +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 + +1.E.2. If an individual Project Gutenberg-tm electronic work is derived +from the public domain (does not contain a notice indicating that it is +posted with permission of the copyright holder), the work can be copied +and distributed to anyone in the United States without paying any fees +or charges. If you are redistributing or providing access to a work +with the phrase "Project Gutenberg" associated with or appearing on the +work, you must comply either with the requirements of paragraphs 1.E.1 +through 1.E.7 or obtain permission for the use of the work and the +Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or +1.E.9. + +1.E.3. If an individual Project Gutenberg-tm electronic work is posted +with the permission of the copyright holder, your use and distribution +must comply with both paragraphs 1.E.1 through 1.E.7 and any additional +terms imposed by the copyright holder. Additional terms will be linked +to the Project Gutenberg-tm License for all works posted with the +permission of the copyright holder found at the beginning of this work. + +1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm +License terms from this work, or any files containing a part of this +work or any other work associated with Project Gutenberg-tm. + +1.E.5. Do not copy, display, perform, distribute or redistribute this +electronic work, or any part of this electronic work, without +prominently displaying the sentence set forth in paragraph 1.E.1 with +active links or immediate access to the full terms of the Project +Gutenberg-tm License. + +1.E.6. You may convert to and distribute this work in any binary, +compressed, marked up, nonproprietary or proprietary form, including any +word processing or hypertext form. However, if you provide access to or +distribute copies of a Project Gutenberg-tm work in a format other than +"Plain Vanilla ASCII" or other format used in the official version +posted on the official Project Gutenberg-tm web site (www.gutenberg.org), +you must, at no additional cost, fee or expense to the user, provide a +copy, a means of exporting a copy, or a means of obtaining a copy upon +request, of the work in its original "Plain Vanilla ASCII" or other +form. Any alternate format must include the full Project Gutenberg-tm +License as specified in paragraph 1.E.1. + +1.E.7. Do not charge a fee for access to, viewing, displaying, +performing, copying or distributing any Project Gutenberg-tm works +unless you comply with paragraph 1.E.8 or 1.E.9. + +1.E.8. You may charge a reasonable fee for copies of or providing +access to or distributing Project Gutenberg-tm electronic works provided +that + +- You pay a royalty fee of 20% of the gross profits you derive from + the use of Project Gutenberg-tm works calculated using the method + you already use to calculate your applicable taxes. The fee is + owed to the owner of the Project Gutenberg-tm trademark, but he + has agreed to donate royalties under this paragraph to the + Project Gutenberg Literary Archive Foundation. Royalty payments + must be paid within 60 days following each date on which you + prepare (or are legally required to prepare) your periodic tax + returns. Royalty payments should be clearly marked as such and + sent to the Project Gutenberg Literary Archive Foundation at the + address specified in Section 4, "Information about donations to + the Project Gutenberg Literary Archive Foundation." + +- You provide a full refund of any money paid by a user who notifies + you in writing (or by e-mail) within 30 days of receipt that s/he + does not agree to the terms of the full Project Gutenberg-tm + License. You must require such a user to return or + destroy all copies of the works possessed in a physical medium + and discontinue all use of and all access to other copies of + Project Gutenberg-tm works. + +- You provide, in accordance with paragraph 1.F.3, a full refund of any + money paid for a work or a replacement copy, if a defect in the + electronic work is discovered and reported to you within 90 days + of receipt of the work. + +- You comply with all other terms of this agreement for free + distribution of Project Gutenberg-tm works. + +1.E.9. If you wish to charge a fee or distribute a Project Gutenberg-tm +electronic work or group of works on different terms than are set +forth in this agreement, you must obtain permission in writing from +both the Project Gutenberg Literary Archive Foundation and Michael +Hart, the owner of the Project Gutenberg-tm trademark. Contact the +Foundation as set forth in Section 3 below. + +1.F. + +1.F.1. Project Gutenberg volunteers and employees expend considerable +effort to identify, do copyright research on, transcribe and proofread +public domain works in creating the Project Gutenberg-tm +collection. Despite these efforts, Project Gutenberg-tm electronic +works, and the medium on which they may be stored, may contain +"Defects," such as, but not limited to, incomplete, inaccurate or +corrupt data, transcription errors, a copyright or other intellectual +property infringement, a defective or damaged disk or other medium, a +computer virus, or computer codes that damage or cannot be read by +your equipment. + +1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right +of Replacement or Refund" described in paragraph 1.F.3, the Project +Gutenberg Literary Archive Foundation, the owner of the Project +Gutenberg-tm trademark, and any other party distributing a Project +Gutenberg-tm electronic work under this agreement, disclaim all +liability to you for damages, costs and expenses, including legal +fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT +LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE +PROVIDED IN PARAGRAPH F3. YOU AGREE THAT THE FOUNDATION, THE +TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE +LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR +INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH +DAMAGE. + +1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a +defect in this electronic work within 90 days of receiving it, you can +receive a refund of the money (if any) you paid for it by sending a +written explanation to the person you received the work from. If you +received the work on a physical medium, you must return the medium with +your written explanation. The person or entity that provided you with +the defective work may elect to provide a replacement copy in lieu of a +refund. If you received the work electronically, the person or entity +providing it to you may choose to give you a second opportunity to +receive the work electronically in lieu of a refund. If the second copy +is also defective, you may demand a refund in writing without further +opportunities to fix the problem. + +1.F.4. Except for the limited right of replacement or refund set forth +in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER +WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO +WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE. + +1.F.5. Some states do not allow disclaimers of certain implied +warranties or the exclusion or limitation of certain types of damages. +If any disclaimer or limitation set forth in this agreement violates the +law of the state applicable to this agreement, the agreement shall be +interpreted to make the maximum disclaimer or limitation permitted by +the applicable state law. The invalidity or unenforceability of any +provision of this agreement shall not void the remaining provisions. + +1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the +trademark owner, any agent or employee of the Foundation, anyone +providing copies of Project Gutenberg-tm electronic works in accordance +with this agreement, and any volunteers associated with the production, +promotion and distribution of Project Gutenberg-tm electronic works, +harmless from all liability, costs and expenses, including legal fees, +that arise directly or indirectly from any of the following which you do +or cause to occur: (a) distribution of this or any Project Gutenberg-tm +work, (b) alteration, modification, or additions or deletions to any +Project Gutenberg-tm work, and (c) any Defect you cause. + + +Section 2. Information about the Mission of Project Gutenberg-tm + +Project Gutenberg-tm is synonymous with the free distribution of +electronic works in formats readable by the widest variety of computers +including obsolete, old, middle-aged and new computers. It exists +because of the efforts of hundreds of volunteers and donations from +people in all walks of life. + +Volunteers and financial support to provide volunteers with the +assistance they need, is critical to reaching Project Gutenberg-tm's +goals and ensuring that the Project Gutenberg-tm collection will +remain freely available for generations to come. In 2001, the Project +Gutenberg Literary Archive Foundation was created to provide a secure +and permanent future for Project Gutenberg-tm and future generations. +To learn more about the Project Gutenberg Literary Archive Foundation +and how your efforts and donations can help, see Sections 3 and 4 +and the Foundation web page at http://www.pglaf.org. + + +Section 3. Information about the Project Gutenberg Literary Archive +Foundation + +The Project Gutenberg Literary Archive Foundation is a non profit +501(c)(3) educational corporation organized under the laws of the +state of Mississippi and granted tax exempt status by the Internal +Revenue Service. The Foundation's EIN or federal tax identification +number is 64-6221541. Its 501(c)(3) letter is posted at +http://pglaf.org/fundraising. Contributions to the Project Gutenberg +Literary Archive Foundation are tax deductible to the full extent +permitted by U.S. federal laws and your state's laws. + +The Foundation's principal office is located at 4557 Melan Dr. S. +Fairbanks, AK, 99712., but its volunteers and employees are scattered +throughout numerous locations. Its business office is located at +809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email +business@pglaf.org. Email contact links and up to date contact +information can be found at the Foundation's web site and official +page at http://pglaf.org + +For additional contact information: + Dr. Gregory B. Newby + Chief Executive and Director + gbnewby@pglaf.org + + +Section 4. Information about Donations to the Project Gutenberg +Literary Archive Foundation + +Project Gutenberg-tm depends upon and cannot survive without wide +spread public support and donations to carry out its mission of +increasing the number of public domain and licensed works that can be +freely distributed in machine readable form accessible by the widest +array of equipment including outdated equipment. Many small donations +($1 to $5,000) are particularly important to maintaining tax exempt +status with the IRS. + +The Foundation is committed to complying with the laws regulating +charities and charitable donations in all 50 states of the United +States. Compliance requirements are not uniform and it takes a +considerable effort, much paperwork and many fees to meet and keep up +with these requirements. We do not solicit donations in locations +where we have not received written confirmation of compliance. To +SEND DONATIONS or determine the status of compliance for any +particular state visit http://pglaf.org + +While we cannot and do not solicit contributions from states where we +have not met the solicitation requirements, we know of no prohibition +against accepting unsolicited donations from donors in such states who +approach us with offers to donate. + +International donations are gratefully accepted, but we cannot make +any statements concerning tax treatment of donations received from +outside the United States. U.S. laws alone swamp our small staff. + +Please check the Project Gutenberg Web pages for current donation +methods and addresses. Donations are accepted in a number of other +ways including checks, online payments and credit card donations. +To donate, please visit: http://pglaf.org/donate + + +Section 5. General Information About Project Gutenberg-tm electronic +works. + +Professor Michael S. Hart is the originator of the Project Gutenberg-tm +concept of a library of electronic works that could be freely shared +with anyone. For thirty years, he produced and distributed Project +Gutenberg-tm eBooks with only a loose network of volunteer support. + + +Project Gutenberg-tm eBooks are often created from several printed +editions, all of which are confirmed as Public Domain in the U.S. +unless a copyright notice is included. Thus, we do not necessarily +keep eBooks in compliance with any particular paper edition. + + +Most people start at our Web site which has the main PG search facility: + + http://www.gutenberg.org + +This Web site includes information about Project Gutenberg-tm, +including how to make donations to the Project Gutenberg Literary +Archive Foundation, how to help produce our new eBooks, and how to +subscribe to our email newsletter to hear about new eBooks. |