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+*The Project Gutenberg Etext of The Aeroplane Speaks, by Barber*
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+The Aeroplane Speaks
+
+by H. Barber
+Captain, Royal Flying Corps
+
+February, 1997  [Etext #818]
+
+
+*The Project Gutenberg Etext of The Aeroplane Speaks, by Barber*
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+
+THE AEROPLANE SPEAKS
+
+BY H. BARBER
+(CAPTAIN, ROYAL FLYING CORPS)
+
+
+
+DEDICATED TO THE SUBALTERN FLYING OFFICER
+
+
+
+
+MOTIVE
+
+The reasons impelling me to write this book, the maiden
+effort of my pen, are, firstly, a strong desire to help the
+ordinary man to understand the Aeroplane and the joys
+and troubles of its Pilot; and, secondly, to produce something
+of PRACTICAL assistance to the Pilot and his invaluable assistant
+the Rigger. Having had some eight years' experience in
+designing, building, and flying aeroplanes, I have hopes
+that the practical knowledge I have gained may offset the
+disadvantage of a hand more used to managing the ``joy-
+stick'' than the dreadful haltings, the many side-slips, the
+irregular speed, and, in short, the altogether disconcerting
+ways of a pen.
+
+The matter contained in the Prologue appeared in the
+Field of May 6th, 13th, 20th, and 27th, 1916, and is now
+reprinted by the kind permission of the editor, Sir Theodore
+Cook.
+
+I have much pleasure in also acknowledging the kindness
+of Mr. C. G. Grey, editor of the Aeroplane, to whom I am
+indebted for the valuable illustrations reproduced at the
+end of this book.
+
+
+
+CONTENTS
+
+PROLOGUE
+
+PART
+I.   THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+II.  THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
+III. THE GREAT TEST
+IV.  CROSS COUNTRY 
+
+
+
+CHAPTER
+I.   FLIGHT
+II.  STABILITY AND CONTROL
+III. RIGGING
+IV.  PROPELLERS 
+V.   MAINTENANCE
+
+
+
+TYPES OF AEROPLANES
+
+GLOSSARY
+
+
+
+
+THE AEROPLANE SPEAKS
+
+PROLOGUE
+
+PART I
+
+THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
+
+The Lecture Hall at the Royal Flying Corps School for
+Officers was deserted. The pupils had dispersed, and the
+Officer Instructor, more fagged than any pupil, was out on
+the aerodrome watching the test of a new machine.
+
+Deserted, did I say? But not so. The lecture that day
+had been upon the Elementary Principles of Flight, and
+they lingered yet. Upon the Blackboard was the illustration
+you see in the frontispiece.
+
+``I am the side view of a Surface,'' it said, mimicking
+the tones of the lecturer. ``Flight is secured by driving me
+through the air at an angle inclined to the direction of
+motion.''
+
+``Quite right,'' said the Angle. ``That's me, and I'm
+the famous Angle of Incidence.''
+
+``And,'' continued the Surface, ``my action is to deflect
+the air downwards, and also, by fleeing from the air behind,
+to create a semi-vacuum or rarefied area over most of the
+top of my surface.''
+
+``This is where I come in,'' a thick, gruff voice was
+heard, and went on: ``I'm the Reaction. You can't have
+action without me. I'm a very considerable force, and my
+direction is at right-angles to you,'' and he looked heavily
+at the Surface. ``Like this,'' said he, picking up the chalk
+with his Lift, and drifting to the Blackboard.
+
+``I act in the direction of the arrow R, that is, more or
+less, for the direction varies somewhat with the Angle of
+Incidence and the curvature of the Surface; and, strange
+but true, I'm stronger on the top of the Surface than at
+the bottom of it. The Wind Tunnel has proved that by
+exhaustive research--and don't forget how quickly I can
+grow! As the speed through the air increases my strength
+increases more rapidly than you might think--approximately,
+as the Square of the Speed; so you see that if the Speed of
+the Surface through the air is, for instance, doubled, then
+I am a good deal more than doubled. That's because I
+am the result of not only the mass of air displaced, but also
+the result of the Speed with which the Surface engages
+the Air. I am a product of those two factors, and at the
+speeds at which Aeroplanes fly to-day, and at the altitudes
+and consequent density of air they at present experience,
+I increase at about the Square of the Speed.
+
+``Oh, I'm a most complex and interesting personality, I
+assure you--in fact, a dual personality, a sort of aeronautical
+Dr. Jekyll and Mr. Hyde. There's Lift, my vertical part or
+COMPONENT, as those who prefer long words would say; he
+always acts vertically upwards, and hates Gravity like poison.
+He's the useful and admirable part of me. Then there's Drift,
+my horizontal component, sometimes, though rather erroneously,
+called Head Resistance; he's a villain of the deepest
+dye, and must be overcome before flight can be secured.''
+
+``And I,'' said the Propeller, ``I screw through the air and
+produce the Thrust. I thrust the Aeroplane through the air
+and overcome the Drift; and the Lift increases with the Speed
+and when it equals the Gravity of Weight, then--there you
+are--Flight! And nothing mysterious about it at all.''
+
+``I hope you'll excuse me interrupting,'' said a very
+beautiful young lady, ``my name is Efficiency, and, while
+no doubt, all you have said is quite true, and that, as my
+young man the Designer says, `You can make a tea-tray
+fly if you slap on Power enough,' I can assure you that I'm
+not to be won quite so easily.''
+
+``Well,'' eagerly replied the Lift and the Thrust, ``let's
+be friends. Do tell us what we can do to help you to overcome
+Gravity and Drift with the least possible Power. That
+obviously seems the game to play, for more Power means
+heavier engines, and that in a way plays into the hands of
+our enemy, Gravity, besides necessitating a larger Surface
+or Angle to lift the Weight, and that increases the Drift.''
+
+``Very well,'' from Efficiency, ``I'll do my best, though
+I'm so shy, and I've just had such a bad time at the Factory,
+and I'm terribly afraid you'll find it awefully dry.''
+
+``Buck up, old dear!'' This from several new-comers,
+who had just appeared. ``We'll help you,'' and one of
+them, so lean and long that he took up the whole height of
+the lecture room, introduced himself.
+
+``I'm the High Aspect Ratio,'' he said, ``and what we
+have got to do to help this young lady is to improve the
+proportion of Lift to Drift. The more Lift we can get for a
+certain area of Surface, the greater the Weight the latter
+can carry; and the less the Drift, then the less Thrust and
+Power required to overcome it. Now it is a fact that, if
+the Surface is shaped to have the greatest possible span,
+i.e., distance from wing-tip to wing-tip, it then engages more
+air and produces both a maximum Reaction and a better
+proportion of Lift to Drift.
+
+``That being so, we can then well afford to lose a little
+Reaction by reducing the Angle of Incidence to a degree
+giving a still better proportion of Lift to Drift than would
+otherwise be the case; for you must understand that the
+Lift-Drift Ratio depends very much upon the size of the
+Angle of Incidence, which should be as small as possible
+within certain limits. So what I say is, make the surface of
+Infinite Span with no width or chord, as they call it. That's
+all I require, I assure you, to make me quite perfect and of
+infinite service to Miss Efficiency.''
+
+``That's not practical politics,'' said the Surface. ``The
+way you talk one would think you were drawing L400 a
+year at Westminster, and working up a reputation as an
+Aeronautical Expert. I must have some depth and chord
+to take my Spars and Ribs, and again, I must have a certain
+chord to make it possible for my Camber (that's curvature)
+to be just right for the Angle of Incidence. If that's not
+right the air won't get a nice uniform compression and
+downward acceleration from my underside, and the rarefied
+`suction' area over the top of me will not be as even and clean
+in effect as it might be. That would spoil the Lift-Drift Ratio
+more than you can help it. Just thrust that chalk along, will
+you? and the Blackboard will show you what I mean.''
+
+``Well,'' said the Aspect Ratio, ``have it your own way,
+though I'm sorry to see a pretty young lady like Efficiency
+compromised so early in the game.''
+
+``Look here,'' exclaimed a number of Struts, ``we have
+got a brilliant idea for improving the Aspect Ratio,'' and
+with that they hopped up on to the Spars. ``Now,'' excitedly,
+``place another Surface on top of us. Now do you
+see? There is double the Surface, and that being so, the
+proportion of Weight to Surface area is halved. That's
+less burden of work for the Surface, and so the Spars need
+not be so strong and so deep, which results in not so thick
+a Surface. That means the Chord can be proportionately
+decreased without adversely affecting the Camber. With
+the Chord decreased, the Span becomes relatively greater,
+and so produces a splendid Aspect Ratio, and an excellent
+proportion of Lift to Drift.''
+
+``I don't deny that they have rather got me there,''
+said the Drift, ``but all the same, don't forget my increase
+due to the drift of the Struts and their bracing wires.''
+
+``Yes, I dare say,'' replied the Surface, ``but remember
+that my Spars are less deep than before, and consequently I
+am not so thick now, and shall for that reason also be able
+to go through the air with a less proportion of Drift to Lift.''
+
+``Remember me also, please,'' croaked the Angle of
+Incidence. ``Since the Surface has now less weight to carry
+for its area, I may be set at a still lesser and finer Angle.
+That means less Drift again. We are certainly getting on
+splendidly! Show us how it looks now, Blackboard.'' And
+the Blackboard obligingly showed them as follows:
+
+``Well, what do you think of that?'' they all cried to the
+Drift.
+
+``You think you are very clever,'' sneered the Drift.
+``But you are not helping Efficiency as much as you think.
+The suction effect on the top of the lower Surface will give
+a downward motion to the air above it and the result will
+be that the bottom of the top Surface will not secure as good
+a Reaction from the air as would otherwise be the case,
+and that means loss of Lift; and you can't help matters
+by increasing the gap between the surfaces because that
+means longer Struts and Wires, and that in itself would
+help me, not to speak of increasing the Weight. You see
+it's not quite so easy as you thought.''
+
+At this moment a hiccough was heard, and a rather fast
+and rakish-looking chap, named Stagger, spoke up. ``How
+d'ye do, miss,'' he said politely to Efficiency, with a side
+glance out of his wicked old eye. ``I'm a bit of a knut,
+and without the slightest trouble I can easily minimize
+the disadvantage that old reprobate Drift has been frightening
+you with. I just stagger the top Surface a bit forward,
+and no longer is that suction effect dead under it. At the
+same time I'm sure the top Surface will kindly extend its
+Span for such distance as its Spars will support it without
+the aid of Struts. Such extension will be quite useful, as
+there will be no Surface at all underneath it to interfere
+with the Reaction above.'' And the Stagger leaned
+forward and picked up the Chalk, and this is the picture
+he drew:
+
+Said the Blackboard, ``That's not half bad! It really
+begins to look something like the real thing, eh?''
+
+``The real thing, is it?'' grumbled Drift. ``Just consider
+that contraption in the light of any one Principle, and I
+warrant you will not find one of them applied to perfection.
+The whole thing is nothing but a Compromise.'' And he
+glared fixedly at poor Efficiency.
+
+``Oh, dear! Oh, dear!'' she cried. ``I'm always getting
+into trouble. What WILL the Designer say?''
+
+``Never mind, my dear,'' said the Lift-Drift Ratio,
+consolingly. ``You are improving rapidly, and quite useful
+enough now to think of doing a job of work.''
+
+``Well, that's good news,'' and Efficiency wiped her eyes
+with her Fabric and became almost cheerful. ``Suppose
+we think about finishing it now? There will have to be an
+Engine and Propeller, won't there? And a body to fix
+them in, and tanks for oil and petrol, and a tail, and,'' archly,
+``one of those dashing young Pilots, what?''
+
+``Well, we are getting within sight of those interesting
+Factors,'' said the Lift-Drift Ratio, ``but first of all we
+had better decide upon the Area of the Surfaces, their Angle
+of Incidence and Camber. If we are to ascend as quickly
+as possible the Aeroplane must be SLOW in order to secure
+the best possible Lift-Drift Ratio, for the drift of the struts
+wires, body, etc., increases approximately as the square
+of the speed, but it carries with it no lift as it does in the
+case of the Surface. The less speed then, the less such
+drift, and the better the Aeroplane's proportion of lift to
+drift; and, being slow, we shall require a LARGE SURFACE in
+order to secure a large lift relative to the weight to be carried.
+We shall also require a LARGE ANGLE OF INCIDENCE relative to
+the horizontal, in order to secure a proper inclination of
+the Surface to the direction of motion, for you must remember
+that, while we shall fly upon an even keel and with
+the propeller thrust horizontal (which is its most efficient
+attitude), our flight path, which is our direction of motion,
+will be sloping upwards, and it will therefore be necessary
+to fix the Surface to the Aeroplane at a very considerable
+angle relative to the horizontal Propeller Thrust in order to
+secure a proper angle to the upwards direction of motion.
+Apart from that, we shall require a larger Angle of Incidence
+than in the case of a machine designed purely for speed,
+and that means a correspondingly LARGE CAMBER.
+
+``On the other hand, if we are thinking merely of Speed,
+then a SMALL SURFACE, just enough to lift the weight off the
+ground, will be best, also a SMALL ANGLE to cut the Drift down
+and that, of course, means a relatively SMALL CAMBER.
+
+``So you see the essentials for CLIMB or quick ascent and
+for SPEED are diametrically opposed. Now which is it to be?''
+
+``Nothing but perfection for me,'' said Efficiency. ``What
+I want is Maximum Climb and Maximum Speed for the
+Power the Engine produces.''
+
+And each Principle fully agreed with her beautiful
+sentiments, but work together they would not.
+
+The Aspect Ratio wanted infinite Span, and hang the
+Chord.
+
+The Angle of Incidence would have two Angles and two
+Cambers in one, which was manifestly absurd; the Surface
+insisted upon no thickness whatever, and would not hear
+of such things as Spars and Ribs; and the Thrust objected
+to anything at all likely to produce Drift, and very nearly
+wiped the whole thing off the Blackboard.
+
+There was, indeed, the makings of a very pretty quarrel
+when the Letter arrived. It was about a mile long, and
+began to talk at once.
+
+``I'm from the Inventor,'' he said, and hope rose in the
+heart of each heated Principle. ``It's really absurdly simple.
+All the Pilot has to do is to touch a button, and at his will,
+VARY the area of the Surface, the Angle of Incidence,
+and the Camber! And there you are--Maximum Climb or
+Maximum Speed as required! How does that suit you?''
+
+``That suits us very well,'' said the Surface, ``but, excuse
+me asking, how is it done without apparatus increasing the
+Drift and the Weight out of all reason? You won't mind
+showing us your Calculations, Working Drawings, Stress
+Diagrams, etc., will you?''
+
+Said the Letter with dignity, ``I come from an Inventor
+so brilliantly clever as to be far above the unimportant
+matters you mention. He is no common working man,
+sir! He leaves such things to Mechanics. The point is, you
+press a button and----''
+
+``Look here,'' said a Strut, rather pointedly, ``where do
+you think you are going, anyway?''
+
+``Well,'' from the Letter, ``as a matter of fact, I'm not
+addressed yet, but, of course, there's no doubt I shall reach
+the very highest quarters and absolutely revolutionize Flight
+when I get there.''
+
+Said the Chalk, ``I'll address you, if that's all you want;
+now drift along quickly!'' And off went the Letter to The
+Technical Editor, ``Daily Mauler,'' London.
+
+And a League was formed, and there were Directors with
+Fees, and several out-of-service Tin Hats, and the Man-who-
+takes-the-credit, and a fine fat Guinea-pig, and all the rest
+of them. And the Inventor paid his Tailor and had a Hair-
+Cut, and is now a recognized Press Expert--but he is still
+waiting for those Mechanics!
+
+``I'm afraid,'' said the Slide-rule, who had been busy
+making those lightning-like automatic calculations for which
+he is so famous, ``it's quite impossible to fully satisfy all of
+you, and it is perfectly plain to me that we shall have to effect
+a Compromise and sacrifice some of the Lift for Speed.''
+
+Thud! What was that?
+
+Efficiency had fainted dead away! The last blow had
+been too much for her. And the Principles gathered mournfully
+round, but with the aid of the Propeller Slip[[1]] and a
+friendly lift from the Surface she was at length revived and
+regained a more normal aspect.
+
+
+[[1]] Propeller Slip: As the propeller screws through the air,
+the latter to a certain extent gives back to the thrust of the
+propellor blades, just as the shingle on the beach slips back
+as you ascend it. Such ``give-back'' is known as ``slip,''
+and anyone behind the propellor will feel the slip as a
+strong draught of air.
+
+
+Said the Stagger with a raffish air, ``My dear young lady,
+I assure you that from the experiences of a varied career,
+I have learned that perfection is impossible, and I am sure
+the Designer will be quite satisfied if you become the Most
+Efficient Compromise.''
+
+``Well, that sounds so common sense,'' sighed Efficiency,
+``I suppose it must be true, and if the Designer is satisfied,
+that's all I really care about. Now do let's get on with the job.''
+
+So the Chalk drew a nice long slim body to hold the
+Engine and the tanks, etc., with room for the Pilot's and
+Passenger's seats, and placed it exactly in the middle of the
+Biplane. And he was careful to make its position such that
+the Centre of Gravity was a little in advance of the Centre
+of Lift, so that when the Engine was not running and there
+was consequently no Thrust, the Aeroplane should be ``nose-
+heavy'' just to the right degree, and so take up a natural
+glide to Earth--and this was to help the Pilot and relieve
+him of work and worry, should he find himself in a fog or
+a cloud. And so that this tendency to glide downwards
+should not be in evidence when the Engine was running and
+descent not desired, the Thrust was placed a little below
+the Centre of Drift or Resistance. In this way it would in
+a measure pull the nose of the Aeroplane up and counterbalance
+the ``nose-heavy'' tendency.
+
+And the Engine was so mounted that when the Propeller-
+Thrust was horizontal, which is its most efficient position,
+the Angle of Incidence and the Area of the surfaces were
+just sufficient to give a Lift a little in excess of the Weight.
+And the Camber was such that, as far as it was concerned,
+the Lift-Drift Ratio should be the best possible for that Angle
+of Incidence. And a beautifully simple under-carriage was
+added, the outstanding features of which were simplicity,
+strength, light-weight, and minimum drift. And, last of
+all, there was the Elevator, of which you will hear more
+by-and-by. And this is what it looked like then:
+
+And Efficiency, smiling, thought that it was not such a
+bad compromise after all and that the Designer might well
+be satisfied.
+
+``Now,'' said she, ``there's just one or two points I'm
+a bit hazy about. It appears that when the Propeller shaft
+is horizontal and so working in its most efficient attitude,
+I shall have a Lift from the Surfaces slightly in excess of the
+Weight. That means I shall ascend slightly, at the same time
+making nearly maximum speed for the power and thrust.
+Can't I do better than that?''
+
+``Yes, indeed,'' spoke up the Propeller, ``though it means
+that I must assume a most undignified attitude, for helicopters[[2]]
+I never approved of. In order to ascend more
+quickly the Pilot will deflect the Elevator, which, by the
+way, you see hinged to the Tail. By that means he will
+force the whole Aeroplane to assume a greater Angle of
+Incidence. And with greater Angle, the Lift will increase,
+though I'm sorry to say the Drift will increase also. Owing
+to the greater Drift, the Speed through the air will lessen,
+and I'm afraid that won't be helpful to the Lift; but I shall
+now be pointing upwards, and besides overcoming the Drift
+in a forward direction I shall be doing my best to haul
+the Aeroplane skywards. At a certain angle known as the
+Best Climbing Angle, we shall have our Maximum Margin
+of Lift, and I'm hoping that may be as much as almost a
+thousand feet altitude a minute.''
+
+
+[[2]] Helicopter. An air-screw revolving upon a vertical axis.
+If driven with sufficient power, it will lift vertically,
+but having regard to the mechanical difficulties of such construction,
+it is a most inefficient way of securing lift compared with the
+arrangement of an inclined surface driven by a propeller
+revolving about a horizontal axis.
+
+
+``Then, if the Pilot is green, my chance will come,'' said
+the Maximum Angle of Incidence. ``For if the Angle is
+increased over the Best Climbing Angle, the Drift will rush
+up; and the Speed, and with it the Lift, will, when my
+Angle is reached, drop to a point when the latter will be no
+more than the Weight. The Margin of Lift will have
+entirely disappeared, and there we shall be, staggering
+along at my tremendous angle, and only just maintaining
+horizontal flight.''
+
+``And then with luck I'll get my chance,'' said the Drift.
+``If he is a bit worse than green, he'll perhaps still further
+increase the Angle. Then the Drift, largely increasing, the
+Speed, and consequently the Lift, will become still less,
+i.e., less than the Weight, and then--what price pancakes,[[3]]
+eh?''
+
+
+[[3]] Pancakes: Pilot's slang for stalling an aeroplane
+and dropping like a pancake.
+
+
+``Thank you,'' from Efficiency, ``that was all most
+informing. And now will you tell me, please, how the
+greatest Speed may be secured?''
+
+``Certainly, now it's my turn,'' piped the Minimum Angle
+of Incidence. ``By means of the Elevator, the Pilot places
+the Aeroplane at my small Angle, at which the Lift only
+just equals the Weight, and, also, at which we shall make
+greater speed with no more Drift than before. Then we get
+our greatest Speed, just maintaining horizontal flight.''
+
+``Yes; though I'm out of the horizontal and thrusting
+downwards,'' grumbled the Propeller, ``and that's not
+efficient, though I suppose it's the best we can do until that
+Inventor fellow finds his Mechanics.''
+
+``Thank you so much,'' said Efficiency. ``I think I have
+now at any rate an idea of the Elementary Principles of
+Flight, and I don't know that I care to delve much deeper,
+for sums always give me a headache; but isn't there something
+about Stability and Control? Don't you think I ought
+to have a glimmering of them too?''
+
+``Well, I should smile,'' said a spruce Spar, who had come
+all the way from America. ``And that, as the Lecturer
+says, `will be the subject of our next lecture,' so be here
+again to-morrow, and you will be glad to hear that it will be
+distinctly more lively than the subject we have covered
+to-day.''
+
+
+
+PART II
+
+THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES,
+FINISH THE JOB
+
+Another day had passed, and the Flight Folk had again
+gathered together and were awaiting the arrival of Efficiency
+who, as usual, was rather late in making an appearance.
+
+The crowd was larger than ever, and among the newcomers
+some of the most important were the three Stabilities,
+named Directional, Longitudinal, and Lateral, with
+their assistants, the Rudder, Elevator, and Ailerons. There
+was Centrifugal Force, too, who would not sit still and
+created a most unfavourable impression, and Keel-Surface,
+ the Dihedral Angle, and several other lesser fry.
+
+``Well,'' said Centrifugal Force, ``I wish this Efficiency
+I've heard so much about would get a move on. Sitting
+still doesn't agree with me at all. Motion I believe in.
+There's nothing like motion--the more the better.''
+
+``We are entirely opposed to that,'' objected the three
+Stabilities, all in a breath. ``Unless it's in a perfectly
+straight line or a perfect circle. Nothing but perfectly
+straight lines or, upon occasion, perfect circles satisfy us,
+and we are strongly suspicious of your tendencies.''
+
+``Well, we shall see what we shall see,'' said the Force
+darkly. ``But who in the name of blue sky is this?''
+
+And in tripped Efficiency, in a beautifully ``doped''
+dress of the latest fashionable shade of khaki-coloured
+fabric, a perfectly stream-lined bonnet, and a bewitching
+little Morane parasol,[[4]] smiling as usual, and airily exclaiming,
+``I'm so sorry I'm late, but you see the Designer's
+such a funny man. He objects to skin friction,[[5]] and insisted
+upon me changing my fabric for one of a smoother
+surface, and that delayed me. Dear me, there are a lot
+more of us to-day, aren't there? I think I had better meet
+one at a time.'' And turning to Directional Stability, she
+politely asked him what he preferred to do.
+
+
+[[4]] Morane parasol: A type of Morane monoplane in which the
+lifting surfaces are raised above the pilot in order to afford
+him a good view of the earth.
+
+[[5]] Skin friction is that part of the drift due to the friction
+of the air with roughnesses upon the surface of the aeroplane.
+
+
+``My purpose in life, miss,'' said he, ``is to keep the Aeroplane
+on its course, and to achieve that there must be, in
+effect, more Keel-Surface behind the Vertical Turning Axis
+than there is in front of it.''
+
+Efficiency looking a little puzzled, he added: ``Just like
+a weathercock, and by Keel-Surface I mean everything
+you can see when you view the Aeroplane from the side of
+it--the sides of the body, struts, wires, etc.''
+
+``Oh, now I begin to see light,'' said she: ``but just
+exactly how does it work?''
+
+``I'll answer that,'' said Momentum. ``When perhaps
+by a gust of air the Aeroplane is blown out of its course
+and points in another direction, it doesn't immediately
+fly off on that new course. I'm so strong I pull it off the
+new course to a certain extent, and towards the direction
+of the old course. And so it travels, as long as my strength
+lasts, in a more or less sideways position.''
+
+``Then,'' said the Keel-Surface, ``I get a pressure of
+air all on one side, and as there is, in effect, most of me
+towards the tail, the latter gets pressed sideways, and the
+Aeroplane thus tends to assume its first position and course.''
+
+``I see,'' said Efficiency, and, daintily holding the Chalk,
+she approached the Blackboard. ``Is this what you mean?''
+
+``Yes, that's right enough,'' said the Keel-Surface, ``and
+you might remember, too, that I always make the Aeroplane
+nose into the gusts rather than away from them.''
+
+``If that was not the case,'' broke in Lateral Stability,
+and affecting the fashionable Flying Corps stammer, ``it
+would be a h-h-h-o-r-rible affair! If there were too much
+Keel-Surface in front, then that gust would blow the Aeroplane
+round the other way a very considerable distance.
+And the right-hand Surface being on the outside of the turn
+would have more speed, and consequently more Lift, than
+the Surface on the other side. That means a greater proportion
+of the Lift on that side, and before you could say
+Warp to the Ailerons over the Aeroplane would go--probable
+result a bad side-slip''
+
+``And what can the Pilot do to save such a situation as
+that?'' said Efficiency.
+
+``Well,'' replied Lateral Stability, ``he will try to turn
+the Aeroplane sideways and back to an even keel by means
+of warping the Ailerons or little wings which are hinged
+on to the Wing-tips, and about which you will hear more
+later on; but if the side-slip is very bad he may not be able
+to right the Aeroplane by means of the Ailerons, and then
+the only thing for him to do is to use the Rudder and to turn
+the nose of the Aeroplane down and head-on to the direction
+of motion. The Aeroplane will then be meeting the air in
+the direction it is designed to do so, and the Surfaces and
+also the controls (the Rudder, Ailerons, and Elevator) will
+be working efficiently; but its attitude relative to the earth
+will probably be more or less upside-down, for the action
+of turning the Aeroplane's nose down results, as you will
+see by the illustration B, in the right wing, which is on the
+outside of the circle. travelling through the air with greater
+speed than the left-hand wing. More Speed means more
+Lift, so that results in overturning the Aeroplane still more;
+but now it is, at any rate, meeting the air as it is designed
+to meet it, and everything is working properly. It is then
+only necessary to warp the Elevator, as shown in illustration
+C, in order to bring the Aeroplane into a proper attitude
+relative to the earth.''
+
+``Ah!'' said the Rudder, looking wise, ``it's in a case
+like that when I become the Elevator and the Elevator
+becomes me.''
+
+``That's absurd nonsense,'' said the Blackboard, ``due
+to looseness of thought and expression.''
+
+``Well,'' replied the Rudder, ``when 'the Aeroplane is
+in position A and I am used, then I depress or ELEVATE the nose
+of the machine; and, if the Elevator is used, then it turns
+the Aeroplane to right or left, which is normally my function.
+Surely our roles have changed one with the other, and I'm
+then the Elevator and the Elevator is me!''
+
+Said Lateral Stability to the Rudder, ``That's altogether
+the wrong way of looking at it, though I admit''--and
+this rather sarcastically--``that the way you put it sounds
+rather fine when you are talking of your experiences in
+the air to those `interested in aviation' but knowing little
+about it; but it won't go down here! You are a Controlling
+Surface designed to turn the Aeroplane about its vertical
+axis, and the Elevator is a Controlling Surface designed to
+turn the Aeroplane about its lateral axis. Those are your
+respective jobs, and you can't possibly change them about.
+Such talk only leads to confusion, and I hope we shall hear
+no more of it.''
+
+``Thanks,'' said Efficiency to Lateral Stability. ``And
+now, please, will you explain your duties?''
+
+``My duty is to keep the Aeroplane horizontal from
+Wing-tip to Wing-tip. First of all, I sometimes arrange
+with the Rigger to wash-out, that is decrease, the Angle
+of Incidence on one side of the Aeroplane, and to effect
+the reverse condition, if it is not too much trouble, on the
+other side.''
+
+``But,'' objected Efficiency, ``the Lift varies with the
+Angle of Incidence, and surely such a condition will result in
+one side of the Aeroplane lifting more than the other side?'
+
+``That's all right,'' said the Propeller, ``it's meant to
+off-set the tendency of the Aeroplane to turn over sideways
+in the opposite direction to which I revolve.''
+
+``That's quite clear, though rather unexpected; but how
+do you counteract the effect of the gusts when they try to
+overturn the Aeroplane sideways?'' said she, turning to
+Lateral Stability again.
+
+``Well,'' he replied, rather miserably, ``I'm not nearly
+so perfect as the Longitudinal and Directional Stabilities.
+The Dihedral Angle--that is, the upward inclination of the
+Surfaces towards their wing-tips--does what it can for me,
+but, in my opinion, it's a more or less futile effort. The
+Blackboard will show you the argument.'' And he at once
+showed them two Surfaces, each set at a Dihedral Angle
+like this:
+
+``Please imagine,'' said the Blackboard, ``that the top
+V is the front view of a Surface flying towards you. Now
+if a gust blows it into the position of the lower V you see
+that the horizontal equivalent of the Surface on one side
+becomes larger, and on the other side it becomes smaller.
+That results in more Lift on the lower side and less on the
+higher side, and if the V is large enough it should produce
+such a difference in the Lift of one side to the other as to
+quickly turn the Aeroplane back to its former and normal
+position.''
+
+``Yes,'' said the Dihedral Angle, ``that's what would
+happen if they would only make me large enough; but
+they won't do it because it would too greatly decrease the
+horizontal equivalent, and therefore the Lift, and incidentally
+it would, as Aeroplanes are built to-day, produce
+an excess of Keel Surface above the turning axis, and that
+in itself would spoil the Lateral Stability. The Keel Surface
+should be equally divided above and below the longitudinal
+turning axis (upon which the Aeroplane rolls sideways),
+or the side upon which there is an excess will get
+blown over by the gusts. It strikes me that my future
+isn't very promising, and about my only chance is when
+the Junior Draughtsman makes a mistake, as he did the
+other day. And just think of it, they call him a Designer
+now that he's got a job at the Factory! What did he do?
+Why, he calculated the weights wrong and got the Centre
+of Gravity too high, and they didn't discover it until the
+machine was built. Then all they could do was to give
+me a larger Angle. That dropped the bottom of the V
+lower down, and as that's the centre of the machine, where
+all the Weight is, of course that put the Centre of Gravity
+in its right place. But now there is too much Keel Surface
+above, and the whole thing's a Bad Compromise, not at all
+like Our Efficiency.''
+
+And Efficiency, blushing very prettily at the compliment,
+then asked, ``And how does the Centre of Gravity affect
+matters?''
+
+``That's easy,'' said Grandfather Gravity. ``I'm so
+heavy that if I am too low down I act like a pendulum
+and cause the Aeroplane to roll about sideways, and if I
+am too high I'm like a stick balanced on your finger, and
+then if I'm disturbed, over I go and the Aeroplane with
+me; and, in addition to that, there are the tricks I play
+with the Aeroplane when it's banked up,[[6]] i.e., tilted sideways
+for a turn, and Centrifugal Force sets me going the
+way I'm not wanted to go. No; I get on best with Lateral
+Stability when my Centre is right on the centre of Drift,
+or, at any rate, not much below it.'' And with that he
+settled back into the Lecturer's Chair and went sound
+asleep again, for he was so very, very old, in fact the father
+of all the Principles.
+
+
+[[6]] Banking: When an aeroplane is turned to the left or
+the right the centrifugal force of its momentum causes it to
+skid sideways and outwards away from the centre of the turn.
+To minimize such action the pilot banks, i.e., tilts, the aeroplane
+sideways in order to oppose the underside of the planes to the air.
+The aeroplane will not then skid outwards beyond the slight skid
+necessary to secure a sufficient pressure of air to balance the
+centrifugal force.
+
+
+And the Blackboard had been busy, and now showed
+them a picture of the Aeroplane as far as they knew it, and
+you will see that there is a slight Dihedral Angle, and
+also, fixed to the tail, a vertical Keel Surface or fin, as
+is very often the case in order to ensure the greater effect
+of such surface being behind the vertical turning axis.
+
+But Efficiency, growing rather critical with her newly
+gained knowledge, cried out: ``But where's the horizontal
+Tail Surface? It doesn't look right like that!''
+
+``This is when I have the pleasure of meeting you, my
+dear,'' said Longitudinal Stability. ``Here's the Tail Surface,''
+he said, ``and in order to help me it must be set IN
+EFFECT at a much less Angle of Incidence than the Main Surface.
+To explain we must trouble the Blackboard again,'' and
+this was his effort:
+
+``I have tried to make that as clear as possible,'' he
+said. ``It may appear a bit complicated at first, but if
+you will take the trouble to look at it for a minute you will find
+it quite simple. A is the normal and proper direction of
+motion of the Aeroplane, but, owing to a gust of air, it takes
+up the new nose-down position. Owing to Momentum,
+however, it does not fly straight along in that direction, but
+moves more or less in the direction B, which is the resultant
+of the two forces, Momentum and Thrust. And so you will
+note that the Angle of Incidence, which is the inclination
+of the Surfaces to the Direction of Motion, has decreased,
+and of course the Lift decreases with it. You will also see,
+and this is the point, that the Tail Surface has lost a higher
+proportion of its Angle, and consequently its Lift, than has
+the Main Surface. Then, such being the case, the Tail must
+fall and the Aeroplane assume its normal position again,
+though probably at a slightly lower altitude.''
+
+``I'm afraid I'm very stupid,'' said Efficiency, ``but
+please tell me why you lay stress upon the words `IN
+EFFECT.' ''
+
+``Ah! I was wondering if you would spot that,'' he
+replied. ``And there is a very good reason for it. You see,
+in some Aeroplanes the Tail Surface may be actually set
+at the same Angle on the machine as the Main Surface, but
+owing to the air being deflected downwards by the front
+Main Surface it meets the Tail Surface at a lesser angle,
+and indeed in some cases at no angle at all. The Tail is then
+for its surface getting less Lift than the Main Surface, although
+set at the same angle on the machine. It may then be
+said to have IN EFFECT a less Angle of Incidence. I'll just
+show you on the Blackboard.''
+
+``And now,'' said Efficiency, ``I have only to meet the
+Ailerons and the Rudder, haven't I?''
+
+``Here we are,'' replied the Ailerons, or little wings.
+``Please hinge us on to the back of the Main Surfaces, one
+of us at each Wing-tip, and join us up to the Pilot's joystick
+by means of the control cables. When the Pilot wishes to
+tilt the Aeroplane sideways, he will move the stick and depress
+us upon one side, thus giving us a larger Angle of Incidence
+and so creating more Lift on that side of the Aeroplane;
+and, by means of a cable connecting us with the Ailerons on
+the other side of the Aeroplane, we shall, as we are depressed,
+pull them up and give them a reverse or negative Angle of
+Incidence, and that side will then get a reverse Lift or downward
+thrust, and so we are able to tilt the Aeroplane sideways.
+
+``And we work best when the Angle of Incidence of the
+Surface in front of us is very small, for which reason it is
+sometimes decreased or washed-out towards the Wing-tips.
+The reason of that is that by the time the air reaches us
+it has been deflected downwards--the greater the Angle
+of Incidence the more it is driven downwards--and in order
+for us to secure a Reaction from it, we have to take such a
+large Angle of Incidence that we produce a poor proportion
+of Lift to Drift; but the smaller the Angle of the Surface in
+front of us the less the air is deflected downwards, and
+consequently the less Angle is required of us, and the better our
+proportion of Lift to Drift, which, of course, makes us much
+more effective Controls.''
+
+``Yes,'' said the Lateral and Directional Stabilities in
+one voice, ``that's so, and the wash-out helps us also, for
+then the Surfaces towards their Wing-tips have less Drift
+or `Head-Resistance,' and consequently the gusts will affect
+them and us less; but such decreased Angle of Incidence
+means decreased Lift as well as Drift, and the Designer does
+not always care to pay the price.''
+
+``Well,'' said the Ailerons, ``if it's not done it will mean
+more work for the Rudder, and that won't please the Pilot.''
+
+``Whatever do you mean?'' asked Efficiency. ``What
+can the Rudder have to do with you?''
+
+``It's like this,'' they replied: ``when we are deflected
+downwards we gain a larger Angle of Incidence and also
+enter an area of compressed air, and so produce more Drift
+than those of us on the other side of the Aeroplane, which
+are deflected upwards into an area of rarefied air due to
+the SUCTION effect (though that term is not academically
+correct) on the top of the Surface. If there is more Drift,
+i.e., Resistance, on one side of the Aeroplane than on the other
+side, then of course it will turn off its course, and if that
+difference in Drift is serious, as it will very likely be if there
+is no wash-out, then it will mean a good deal of work for the
+Rudder in keeping the Aeroplane on its course, besides
+creating extra Drift in doing so.''
+
+``I think, then,'' said Efficiency, ``I should prefer to
+have that wash-out,[[7]] and my friend the Designer is so clever
+at producing strength of construction for light weight, I'm
+pretty sure he won't mind paying the price in Lift. And
+now let me see if I can sketch the completed Aeroplane.''
+
+
+[[7]] An explanation of the way in which the wash-out is combined
+with a wash-in to offset propellor torque will be found on p. 82.
+
+
+``Well, I hope that's all as it should be,'' she concluded,
+``for to-morrow the Great Test in the air is due.''
+
+
+
+PART III
+
+THE GREAT TEST
+
+It is five o'clock of a fine calm morning, when the Aeroplane
+is wheeled out of its shed on to the greensward of the Military
+Aerodrome. There is every promise of a good flying day,
+and, although the sun has not yet risen, it is light enough to
+discern the motionless layer of fleecy clouds some five thousand
+feet high, and far, far above that a few filmy mottled streaks
+of vapour. Just the kind of morning beloved of pilots.
+
+A brand new, rakish, up-to-date machine it is, of highly
+polished, beautifully finished wood, fabric as tight as a
+drum, polished metal, and every part so perfectly ``streamlined''
+to minimize Drift, which is the resistance of the air
+to the passage of the machine, that to the veriest tyro the
+remark of the Pilot is obviously justified.
+
+``Clean looking 'bus, looks almost alive and impatient
+to be off. Ought to have a turn for speed with those
+lines.''
+
+``Yes,'' replies the Flight-Commander, ``it's the latest
+of its type and looks a beauty. Give it a good test. A
+special report is required on this machine.''
+
+The A.M.'s[[8]] have now placed the Aeroplane in position
+facing the gentle air that is just beginning to make itself
+evident; the engine Fitter, having made sure of a sufficiency
+of oil and petrol in the tanks, is standing by the Propeller;
+the Rigger, satisfied with a job well done, is critically ``vetting''
+the machine by eye, four A.M.'s are at their posts,
+ready to hold the Aeroplane from jumping the blocks which
+have been placed in front of the wheels; and the Flight-
+Sergeant is awaiting the Pilot's orders.
+
+
+[[8]] A.M.'s: Air Mechanics.
+
+
+As the Pilot approaches the Aeroplane the Rigger springs
+to attention and reports, ``All correct, sir,'' but the Fitter
+does not this morning report the condition of the Engine,
+for well he knows that this Pilot always personally looks
+after the preliminary engine test. The latter, in leathern
+kit, warm flying boots and goggled, climbs into his seat,
+and now, even more than before, has the Aeroplane an almost
+living appearance, as if straining to be off and away. First
+he moves the Controls to see that everything is clear, for
+sometimes when the Aeroplane is on the ground the control
+lever or ``joy-stick'' is lashed fast to prevent the wind
+from blowing the controlling surfaces about and possibly
+damaging them.
+
+The air of this early dawn is distinctly chilly, and the
+A.M.'s are beginning to stamp their cold feet upon the dewy
+grass, but very careful and circumspect is the Pilot, as he
+mutters to himself, ``Don't worry and flurry, or you'll die
+in a hurry.''
+
+At last he fumbles for his safety belt, but with a start
+remembers the Pilot Air Speed Indicator, and, adjusting
+it to zero, smiles as he hears the Pilot-head's gruff voice,
+``Well, I should think so, twenty miles an hour I was registering.
+That's likely to cause a green pilot to stall the Aeroplane.
+Pancake, they call it.'' And the Pilot, who is an
+old hand and has learned a lot of things in the air that mere
+earth-dwellers know nothing about, distinctly heard the
+Pilot Tube, whose mouth is open to the air to receive its
+pressure, stammer. ``Oh Lor! I've got an earwig already--
+hope to goodness the Rigger blows me out when I come
+down--and this morning air simply fills me with moisture;
+I'll never keep the Liquid steady in the Gauge. I'm not
+sure of my rubber connections either.''
+
+``Oh, shut up!'' cry all the Wires in unison, ``haven't
+we got our troubles too? We're in the most horrible state
+of tension. It's simply murdering our Factor of Safety,
+and how we can possibly stand it when we get the Lift only
+the Designer knows.''
+
+``That's all right,'' squeak all the little Wire loops,
+``we're that accommodating, we're sure to elongate a bit
+and so relieve your tension.'' For the whole Aeroplane is
+braced together with innumerable wires, many of which
+are at their ends bent over in the form of loops in order to
+connect with the metal fittings on the spars and elsewhere--
+cheap and easy way of making connection.
+
+``Elongate, you little devils, would you?'' fairly shout
+the Angles of Incidence, Dihedral and Stagger, amid a chorus
+of groans from all parts of the Aeroplane. ``What's going
+to happen to us then? How are we going to keep our
+adjustments upon which good flying depends?''
+
+``Butt us and screw us,''[[9]] wail the Wires. ``Butt us
+and screw us, and death to the Loops. That's what we
+sang to the Designer, but he only looked sad and scowled
+at the Directors.''
+
+
+[[9]] Butt means to thicken at the end. Screw means to machine a thread
+on the butt-end of the wire, and in this way the wire can make connection
+with the desired place by being screwed into a metal fitting,
+thus eliminating the disadvantage of the unsatisfactory loop.
+
+
+``And who on earth are they?'' asked the Loops, trembling
+for their troublesome little lives.
+
+``Oh earth indeed,'' sniffed Efficiency, who had not
+spoken before, having been rendered rather shy by being
+badly compromised in the Drawing Office. ``I'd like to
+get some of them up between Heaven and Earth, I would.
+I'd give 'em something to think of besides their Debits
+and Credits--but all the same the Designer will get his
+way in the end. I'm his Best Girl, you know, and if we
+could only get rid of the Directors, the little Tin god, and
+the Man-who-takes-the-credit, we should be quite happy.''
+Then she abruptly subsides, feeling that perhaps the less
+said the better until she has made a reputation in the Air.
+The matter of that Compromise still rankled, and indeed
+it does seem hardly fit that a bold bad Tin god should flirt
+with Efficiency. You see there was a little Tin god, and he
+said ``Boom, Boom BOOM! Nonsense! It MUST be done,''
+and things like that in a very loud voice, and the Designer
+tore his hair and was furious, but the Directors, who were
+thinking of nothing but Orders and Dividends, had the
+whip-hand of HIM, and so there you are, and so poor beautiful
+Miss Efficiency was compromised.
+
+All this time the Pilot is carefully buckling his belt and
+making himself perfectly easy and comfortable, as all good
+pilots do. As he straightens himself up from a careful
+inspection of the Deviation Curve[[10]] of the Compass and takes
+command of the Controls, the Throttle and the Ignition,
+the voices grow fainter and fainter until there is nothing
+but a trembling of the Lift and Drift wires to indicate to his
+understanding eye their state of tension in expectancy of
+the Great Test.
+
+
+[[10]] Deviation curve: A curved line indicating any errors in the compass.
+
+
+``Petrol on?'' shouts the Fitter to the Pilot.
+
+``Petrol on,'' replies the Pilot.
+
+``Ignition off?''
+
+``Ignition off.''
+
+Round goes the Propeller, the Engine sucking in the
+Petrol Vapour with satisfied gulps. And then--
+
+``Contact?'' from the Fitter.
+
+``Contact,'' says the Pilot.
+
+Now one swing of the Propeller by the Fitter, and the
+Engine is awake and working. Slowly at first though, and
+in a weak voice demanding, ``Not too much Throttle, please.
+I'm very cold and mustn't run fast until my Oil has thinned
+and is circulating freely. Three minutes slowly, as you love
+me, Pilot.''
+
+Faster and faster turn the Engine and Propeller, and
+the Aeroplane, trembling in all its parts, strains to jump
+the blocks and be off. Carefully the Pilot listens to what the
+Engine Revolution Indicator says. At last, ``Steady
+at 1,500 revs. and I'll pick up the rest in the Air.'' Then
+does he throttle down the Engine, carefully putting the
+lever back to the last notch to make sure that in such position
+the Throttle is still sufficiently open for the Engine to continue
+working, as otherwise it might lead to him ``losing'' his
+Engine in the air when throttling down the power for descent.
+Then, giving the official signal, he sees the blocks removed
+from the wheels, and the Flight-Sergeant saluting he knows
+that all is clear to ascend. One more signal, and all the
+A.M.'s run clear of the Aeroplane.
+
+Then gently, gently mind you, with none of the ``crashing
+on'' bad Pilots think so fine, he opens the Throttle
+and, the Propeller Thrust overcoming its enemy the Drift,
+the Aeroplane moves forward.
+
+``Ah!'' says the Wind-screen, ``that's Discipline, that
+is. Through my little window I see most things, and don't
+I just know that poor discipline always results in poor work
+in the air, and don't you forget it.''
+
+``Discipline is it?'' complains the Under-carriage, as
+its wheels roll swiftly over the rather rough ground. ``I'm
+bump getting it; and bump, bump, all I want, bang, bump,
+rattle, too!'' But, as the Lift increases with the Speed,
+the complaints of the Under-carriage are stilled, and then,
+the friendly Lift becoming greater than the Weight, the
+Aeroplane swiftly and easily takes to the air.
+
+Below is left the Earth with all its bumps and troubles.
+Up into the clean clear Air moves with incredible speed
+and steadiness this triumph of the Designer, the result of
+how much mental effort, imagination, trials and errors,
+failures and successes, and many a life lost in high
+endeavour.
+
+Now is the mighty voice of the Engine heard as he turns
+the Propeller nine hundred times a minute. Now does the
+Thrust fight the Drift for all it's worth, and the Air Speed
+Indicator gasps with delight, ``One hundred miles an hour!''
+
+And now does the burden of work fall upon the Lift and
+Drift Wires, and they scream to the Turnbuckles whose
+business it is to hold them in tension, ``This is the limit!
+the Limit! THE LIMIT! Release us, if only a quarter
+turn.'' But the Turnbuckles are locked too fast to turn
+their eyes or utter a word. Only the Locking Wires thus:
+``Ha! ha! the Rigger knew his job. He knew the trick, and
+there's no release here.'' For an expert rigger will always
+use the locking wire in such a way as to oppose the slightest
+tendency of the turnbuckle to unscrew. The other kind of
+rigger will often use the wire in such a way as to allow the
+turnbuckle, to the ``eyes'' of which the wires are attached,
+to unscrew a quarter of a turn or more, with the result that
+the correct adjustment of the wires may be lost; and upon
+their fine adjustment much depends.
+
+And the Struts and the Spars groan in compression and
+pray to keep straight, for once ``out of truth'' there is, in
+addition to possible collapse, the certainty that in bending
+they will throw many wires out of adjustment.
+
+And the Fabric's quite mixed in its mind, and ejaculates,
+``Now, who would have thought I got more Lift from the
+top of the Surface than its bottom?'' And then truculently
+to the Distance Pieces, which run from rib to rib, ``Just
+keep the Ribs from rolling, will you? or you'll see me strip.
+I'm an Irishman, I am, and if my coat comes off---- Yes,
+Irish, I said. I used to come from Egypt, but I've got
+naturalized since the War began.''
+
+Then the Air Speed Indicator catches the eye of the
+Pilot. ``Good enough,'' he says as he gently deflects the
+Elevator and points the nose of the Aeroplane upwards in
+search of the elusive Best Climbing Angle.
+
+``Ha! ha!'' shouts the Drift, growing stronger with the
+increased Angle of Incidence. ``Ha! ha!'' he laughs to
+the Thrust. ``Now I've got you. Now who's Master?''
+
+And the Propeller shrieks hysterically, ``Oh! look at
+me. I'm a helicopter. That's not fair. Where's Efficiency?''
+And she can only sadly reply, ``Yes, indeed, but
+you see we're a Compromise.''
+
+And the Drift has hopes of reaching the Maximum Angle
+of Incidence and vanquishing the Thrust and the Lift. And
+he grows very bold as he strangles the Thrust; but the situation
+is saved by the Propeller, who is now bravely helicopting
+skywards, somewhat to the chagrin of Efficiency.
+
+``Much ado about nothing,'' quotes the Aeroplane
+learnedly. ``Compromise or not, I'm climbing a thousand
+feet a minute. Ask the Altimeter. He'll confirm it.''
+
+And so indeed it was. The vacuum box of the Altimeter
+was steadily expanding under the decreased pressure of
+the rarefied air, and by means of its little levers and its
+wonderful chain no larger than a hair it was moving the
+needle round the gauge and indicating the ascent at the
+rate of a thousand feet a minute.
+
+And lo! the Aeroplane has almost reached the clouds!
+But what's this? A sudden gust, and down sinks one wing
+and up goes the other. ``Oh, my Horizontal Equivalent!''
+despairingly call the Planes: ``it's eloping with the Lift,
+and what in the name of Gravity will happen? Surely
+there was enough scandal in the Factory without this, too!''
+For the lift varies with the horizontal equivalent of the
+planes, so that if the aeroplane tilts sideways beyond a certain
+angle, the lift becomes less than the weight of the machine,
+which must then fall. A fall in such a position is known as
+a ``side-slip.''
+
+But the ever-watchful Pilot instantly depresses one aileron,
+elevating the other, with just a touch of the rudder to keep
+on the course, and the Planes welcome back their precious
+Lift as the Aeroplane flicks back to its normal position.
+
+``Bit bumpy here under these clouds,'' is all the Pilot
+says as he heads for a gap between them, and the next minute
+the Aeroplane shoots up into a new world of space.
+
+``My eye!'' ejaculates the Wind-screen, ``talk about a
+view!'' And indeed mere words will always fail to express
+the wonder of it. Six thousand feet up now, and look!
+The sun is rising quicker than ever mortal on earth witnessed
+its ascent. Far below is Mother Earth, wrapt in mists and
+deep blue shadows, and far above are those light, filmy,
+ethereal clouds now faintly tinged with pink And all
+about great mountains of cloud, lazily floating in space.
+The sun rises and they take on all colours, blending one
+with the other, from dazzling white to crimson and deep
+violet-blue. Lakes and rivers here and there in the enormous
+expanse of country below refract the level rays of the sun
+and, like so many immense diamonds, send dazzling shafts
+of light far upwards. The tops of the hills now laugh to the
+light of the sun, but the valleys are still mysterious dark
+blue caverns, clowned with white filmy lace-like streaks of
+vapour. And withal the increasing sense with altitude of
+vast, clean, silent solitudes of space.
+
+Lives there the man who can adequately describe this
+Wonder? ``Never,'' says the Pilot, who has seen it many
+times, but to whom it is ever new and more wonderful.
+
+Up, up, up, and still up, unfalteringly speeds the Pilot
+and his mount. Sweet the drone of the Engine and steady
+the Thrust as the Propeller exultingly battles with the Drift.
+
+And look! What is that bright silver streak all along
+the horizon? It puzzled the Pilot when first he saw it,
+but now he knows it for the Sea, full fifty miles away!
+
+And on his right is the brightness of the Morn and the
+smiling Earth unveiling itself to the ardent rays of the Sun;
+and on his left, so high is he, there is yet black Night, hiding
+innumerable Cities, Towns, Villages and all those places
+where soon teeming multitudes of men shall awake, and by
+their unceasing toil and the spirit within them produce
+marvels of which the Aeroplane is but the harbinger.
+
+And the Pilot's soul is refreshed, and his vision, now
+exalted, sees the Earth a very garden, even as it appears
+at that height, with discord banished and a happy time
+come, when the Designer shall have at last captured Efficiency,
+and the Man-who-takes-the-credit is he who has earned it,
+and when kisses are the only things that go by favour.
+
+Now the Pilot anxiously scans the Barograph, which is
+an instrument much the same as the Altimeter; but in this
+case the expansion of the vacuum box causes a pen to trace
+a line upon a roll of paper. This paper is made by clockwork
+to pass over the point of the pen, and so a curved line is
+made which accurately registers the speed of the ascent in
+feet per minute. No longer is the ascent at the rate of a
+thousand feet a minute, and the Propeller complains to the
+Engine, ``I'm losing my Revs. and the Thrust. Buck up
+with the Power, for the Lift is decreasing, though the Weight
+remains much the same.''
+
+Quoth the Engine: ``I strangle for Air. A certain proportion,
+and that of right density, I must have to one part
+of Petrol, in order to give me full power and compression,
+and here at an altitude of ten thousand feet the Air is only
+two-thirds as dense as at sea-level. Oh, where is he who
+will invent a contrivance to keep me supplied with Air of
+right density and quality? It should not be impossible
+within certain limits.''
+
+``We fully agree,'' said the dying Power and Thrust. ``Only
+maintain Us and you shall be surprised at the result. For
+our enemy Drift decreases in respect of distance with the increase
+of altitude and rarity of air, and there is no limit to the
+speed through space if only our strength remains. And
+with oxygen for Pilot and Passengers and a steeper pitch[[11]]
+for the Propeller we may then circle the Earth in a day!''
+
+
+[[11]] A propeller screws through the air, and the distance it advances
+during one revolution, supposing the air to be solid, is known as the pitch.
+The pitch, which depends upon the angle of the propeller blades, must be equal
+to the speed of the aeroplane, plus the slip, and if, on account of the rarity
+of the air the speed of the aeroplane increases, then the angle and pitch
+should be correspondingly increased. Propellers with a pitch capable of being
+varied by the pilot are the dream of propeller designers. For explanation of
+``slip'' see Chapter IV. on propellers.
+
+
+Ah, Reader, smile not unbelievingly, as you smiled but
+a few years past. There may be greater wonders yet. Consider
+that as the speed increases, so does the momentum
+or stored-up force in the mass of the aeroplane become
+terrific. And, bearing that in mind, remember that with
+altitude gravity decreases. There may yet be literally other
+worlds to conquer.[[12]]
+
+
+[[12]] Getting out of my depth? Invading the realms of fancy? Well,
+perhaps so, but at any rate it is possible that extraordinary speed through
+space may be secured if means are found to maintain the impulse of the engine
+and the thrust-drift efficiency of the propeller at great altitude.
+
+
+Now at fifteen thousand feet the conditions are chilly
+and rare, and the Pilot, with thoughts of breakfast far below,
+exclaims, ``High enough! I had better get on with the
+Test.'' And then, as he depresses the Elevator, the Aeroplane
+with relief assumes its normal horizontal position.
+Then, almost closing the Throttle, the Thrust dies away.
+Now, the nose of the Aeroplane should sink of its own volition,
+and the craft glide downward at flying speed, which is in
+this case a hundred miles an hour. That is what should
+happen if the Designer has carefully calculated the weight
+of every part and arranged for the centre of gravity to be just
+the right distance in front of the centre of lift. Thus is the
+Aeroplane ``nose-heavy'' as a glider, and just so to a degree
+ensuring a speed of glide equal to its flying speed. And the
+Air Speed Indicator is steady at one hundred miles an hour,
+and ``That's all right!'' exclaims the Pilot. ``And very
+useful, too, in a fog or a cloud,'' he reflects, for then he can
+safely leave the angle of the glide to itself, and give all his
+attention, and he will need it all, to keeping the Aeroplane
+horizontal from wing-tip to wing-tip, and to keeping it
+straight on its course. The latter he will manage with the
+rudder, controlled by his feet, and the Compass will tell him
+whether a straight course is kept. The former he will control
+by the Ailerons, or little wings hinged to the tips of the planes,
+and the bubble in the Inclinometer in front of him must be
+kept in the middle.
+
+A Pilot, being only human, may be able to do two things
+at once, but three is a tall order, so was this Pilot relieved
+to find the Design not at fault and his craft a ``natural
+glider.'' To correct this nose-heavy tendency when the
+Engine is running, and descent not required, the centre
+of Thrust is arranged to be a little below the centre of Drift
+or Resistance, and thus acts as a counter-balance.
+
+But what is this stream of bad language from the Exhaust
+Pipe, accompanied by gouts of smoke and vapour?
+The Engine, now revolving at no more than one-tenth its
+normal speed, has upset the proportion of petrol to air,
+and combustion is taking place intermittently or in the
+Exhaust Pipe, where it has no business to be.
+
+``Crash, Bang, Rattle----!----!----!'' and worse than
+that, yells the Exhaust, and the Aeroplane, who is a gentleman
+and not a box kite,[[13]] remonstrates with the severity
+of a Senior Officer. ``See the Medical Officer, you young
+Hun. Go and see a doctor. Vocal diarrhoea, that's your
+complaint, and a very nasty one too. Bad form, bad for
+discipline, and a nuisance in the Mess. What's your Regiment?
+Special Reserve, you say? Humph! Sounds like
+Secondhand Bicycle Trade to me!''
+
+
+[[13]] Box-kite. The first crude form of biplane.
+
+
+Now the Pilot decides to change the straight gliding
+descent to a spiral one, and, obedient to the Rudder, the
+Aeroplane turns to the left. But the Momentum (two tons
+at 100 miles per hour is no small affair) heavily resents this
+change of direction, and tries its level best to prevent it
+and to pull the machine sideways and outwards from its
+spiral course--that is, to make it ``side-skid'' outwards.
+But the Pilot deflects the Ailerons and ``banks'' up the planes
+to the correct angle, and, the Aeroplane skidding sideways
+and outwards, the lowest surfaces of the planes press up against
+the air until the pressure equals the centrifugal force of
+the Momentum, and the Aeroplane spirals steadily downwards.
+
+Down, down, down, and the air grows denser, and the
+Pilot gulps largely, filling his lungs with the heavier air to
+counteract the increasing pressure from without. Down
+through a gap in the clouds, and the Aerodrome springs
+into view, appearing no larger than a saucer, and the Pilot,
+having by now got the ``feel'' of the Controls, proceeds
+to put the Aeroplane through its paces. First at its Maximum
+Angle, staggering along tail-down and just maintaining
+horizontal flight; then a dive at far over flying speed, finishing
+with a perfect loop; then sharp turns with attendant
+vertical ``banks'' and then a wonderful switchback flight,
+speeding down at a hundred and fifty miles an hour with
+short, exhilarating ascents at the rate of two thousand feet
+a minute!
+
+All the parts are now working well together. Such
+wires as were before in undue tension have secured relief
+by slightly elongating their loops, and each one is now doing
+its bit, and all are sharing the burden of work together.
+
+The Struts and the Spars, which felt so awkward at first,
+have bedded themselves in their sockets, and are taking
+the compression stresses uncomplainingly.
+
+The Control Cables of twisted wire, a bit tight before,
+have slightly lengthened by perhaps the eighth of an inch,
+and, the Controls instantly responding to the delicate touch
+of the Pilot, the Aeroplane, at the will of its Master, darts
+this way and that way, dives, loops, spirals, and at last, in
+one long, magnificent glide, lands gently in front of its shed.
+
+``Well, what result?'' calls the Flight-Commander to
+the Pilot.
+
+``A hundred miles an hour and a thousand feet a minute,''
+he briefly replies.
+
+``And a very good result too,'' says the Aeroplane, complacently,
+as he is carefully wheeled into his shed.
+
+      
+
+That is the way Aeroplanes speak to those who love them
+and understand them. Lots of Pilots know all about it,
+and can spin you wonderful yarns, much better than this
+one, if you catch them in a confidential mood--on leave,
+for instance, and after a good dinner.
+
+
+
+PART IV
+
+'CROSS COUNTRY
+
+The Aeroplane had been designed and built, and tested in
+the air, and now stood on the Aerodrome ready for its first
+'cross-country flight.
+
+It had run the gauntlet of pseudo-designers, crank inventors,
+press ``experts,'' and politicians; of manufacturers
+keen on cheap work and large profits; of poor pilots who had
+funked it, and good pilots who had expected too much of
+it. Thousands of pounds had been wasted on it, many had
+gone bankrupt over it, and others it had provided with safe
+fat jobs.
+
+Somehow, and despite every conceivable obstacle, it had
+managed to muddle through, and now it was ready for its
+work. It was not perfect, for there were fifty different
+ways in which it might be improved, some of them shamefully
+obvious. But it was fairly sound mechanically, had a little
+inherent stability, was easily controlled, could climb a thousand
+feet a minute, and its speed was a hundred miles an
+hour. In short, quite a creditable machine, though of course
+the right man had not got the credit.
+
+It is rough, unsettled weather with a thirty mile an
+hour wind on the ground, and that means fifty more or
+less aloft. Lots of clouds at different altitudes to bother
+the Pilot, and the air none to clear for the observation of
+landmarks.
+
+As the Pilot and Observer approach the Aeroplane the
+former is clearly not in the best of tempers. ``It's rotten
+luck,'' he is saying, ``a blank shame that I should have
+to take this blessed 'bus and join X Reserve Squadron,
+stationed a hundred and fifty miles from anywhere; and
+just as I have licked my Flight into shape. Now some
+slack blighter will, I suppose, command it and get the credit
+of all my work!''
+
+``Shut up, you grouser,'' said the Observer. ``Do you
+think you're the only one with troubles? Haven't I been
+through it too? Oh! I know all about it! You're from
+the Special Reserve and your C.O. doesn't like your style
+of beauty, and you won't lick his boots, and you were a bit
+of a technical knut in civil life, but now you've jolly well
+got to know less than those senior to you. Well! It's a
+jolly good experience for most of us. Perhaps conceit won't
+be at quite such a premium after this war. And what's
+the use of grousing? That never helped anyone. So buck
+up, old chap. Your day will come yet. Here's our machine,
+and I must say it looks a beauty!''
+
+And, as the Pilot approaches the Aeroplane, his face
+brightens and he soon forgets his troubles as he critically
+inspects the craft which is to transport him and the Observer
+over the hills and far away. Turning to the Flight-Sergeant
+he inquires, ``Tank full of petrol and oil?''
+
+``Yes, sir,'' he replies, ``and everything else all correct.
+Propeller, engine, and body covers on board, sir; tool kit
+checked over and in the locker; engine and Aeroplane logbooks
+written up, signed, and under your seat; engine revs.
+up to mark, and all the control cables in perfect condition
+and tension.''
+
+``Very good,'' said the Pilot; and then turning to the
+Observer, ``Before we start you had better have a look
+at the course I have mapped out.
+
+``A is where we stand and we have to reach B, a hundred
+and fifty miles due North. I judge that, at the altitude
+we shall fly, there will be an East wind, for although it is
+not quite East on the ground it is probably about twenty
+degrees different aloft, the wind usually moving round clockways
+to about that extent. I think that it is blowing at the
+rate of about fifty miles an hour, and I therefore take a line
+on the map to C, fifty miles due West of A. The Aeroplane's
+speed is a hundred miles an hour, and so I take a line of one
+hundred miles from C to D. Our compass course will then
+be in the direction A--E, which is always a line parallel to
+C--D. That is, to be exact, it will be fourteen degrees off
+the C--D course, as, in this part of the globe, there is that
+much difference between the North and South lines on the
+map and the magnetic North to which the compass needle
+points. If the compass has an error, as it may have of a
+few degrees, that, too, must be taken into account, and the
+deviation or error curve on the dashboard will indicate it.
+
+``The Aeroplane will then always be pointing in a direction
+parallel to A--E, but, owing to the side wind, it will be actually
+travelling over the course A--B, though in a rather
+sideways attitude to that course.
+
+``The distance we shall travel over the A--B course
+in one hour is A--D. That is nearly eighty-seven miles,
+so we ought to accomplish our journey of a hundred and
+fifty miles in about one and three-quarter hours.
+
+``I hope that's quite clear to you. It's a very simple
+way of calculating the compass course, and I always do it
+like that.''
+
+``Yes, that's plain enough. You have drafted what
+engineers call `a parallelogram of forces'; but suppose you
+have miscalculated the velocity of the wind, or that it should
+change in velocity or direction?''
+
+``Well, that of course will more or less alter matters,''
+replies the Pilot. ``But there are any number of good
+landmarks such as lakes, rivers, towns, and railway lines.
+They will help to keep us on the right course, and the compass
+will, at any rate, prevent us from going far astray when
+between them.''
+
+``Well, we'd better be off, old chap. Hop aboard.''
+This from the Observer as he climbs into the front seat
+from which he will command a good view over the lower
+plane; and the Pilot takes his place in the rear seat, and,
+after making himself perfectly comfortable, fixing his safety
+belt, and moving the control levers to make sure that they
+are working freely, he gives the signal to the Engine Fitter
+to turn the propeller and so start the engine.
+
+Round buzzes the Propeller, and the Pilot, giving the
+official signal, the Aeroplane is released and rolls swiftly
+over the ground in the teeth of the gusty wind.
+
+In less than fifty yards it takes to the air and begins
+to climb rapidly upwards, but how different are the conditions
+to the calm morning of yesterday! If the air were
+visible it would be seen to be acting in the most extraordinary
+manner; crazily swirling, lifting and dropping, gusts viciously
+colliding--a mad phantasmagoria of forces!
+
+Wickedly it seizes and shakes the Aeroplane; then tries
+to turn it over sideways; then instantly changes its mind
+and in a second drops it into a hole a hundred feet deep,
+and if it were not for his safety belt the Pilot might find
+his seat sinking away from beneath him.
+
+Gusts strike the front of the craft like so many slaps in
+the face; and others, with the motion of mountainous waves,
+sometimes lift it hundreds of feet in a few seconds, hoping
+to see it plunge over the summit in a death-dive--and so it
+goes on, but the Pilot, perfectly at one with his mount and
+instantly alert to its slightest motion, is skilfully and naturally
+making perhaps fifty movements a minute of hand and feet;
+the former lightly grasping the ``joy-stick'' which controls
+the Elevator hinged to the tail, and also the Ailerons or little
+wings hinged to the wing-tips; and the latter moving the
+Rudder control-bar.
+
+A strain on the Pilot? Not a bit of it, for this is his
+Work which he loves and excels in; and given a cool head,
+alert eye, and a sensitive touch for the controls, what
+sport can compare with these ever-changing battles of
+the air?
+
+The Aeroplane has all this time been climbing in great
+wide circles, and is now some three thousand feet above
+the Aerodrome which from such height looks absurdly
+small. The buildings below now seem quite squat; the
+hills appear to have sunk away into the ground, and the
+whole country below, cut up into diminutive fields, has
+the appearance of having been lately tidied and thoroughly
+spring-cleaned! A doll's country it looks, with tiny horses
+and cows ornamenting the fields and little model motor-cars
+and carts stuck on the roads, the latter stretching away
+across the country like ribbons accidentally dropped.
+
+At three thousand feet altitude the Pilot is satisfied
+that he is now sufficiently high to secure, in the event of
+engine failure, a long enough glide to earth to enable him
+to choose and reach a good landing-place; and, being furthermore
+content with the steady running of the engine, he
+decides to climb no more but to follow the course he has
+mapped out. Consulting the compass, he places the Aeroplane
+on the A--E course and, using the Elevator, he gives
+his craft its minimum angle of incidence at which it will
+just maintain horizontal flight and secure its maximum
+speed.
+
+Swiftly he speeds away, and few thoughts he has now
+for the changing panorama of country, cloud, and colour.
+Ever present in his mind are the three great 'cross-country
+queries. ``Am I on my right course? Can I see a good
+landing-ground within gliding distance?'' And ``How is
+the Engine running?''
+
+Keenly both he and the Observer compare their maps
+with the country below. The roads, khaki-coloured ribbons,
+are easily seen but are not of much use, for there are so many
+of them and they all look alike from such an altitude.
+
+Now where can that lake be which the map shows so
+plainly? He feels that surely he should see it by now,
+and has an uncomfortable feeling that he is flying too far
+West. What pilot is there indeed who has not many times
+experienced such unpleasant sensation? Few things in the
+air can create greater anxiety. Wisely, however, he sticks
+to his compass course, and the next minute he is rewarded
+by the sight of the lake, though indeed he now sees that the
+direction of his travel will not take him over it, as should
+be the case if he were flying over the shortest route to his
+destination. He must have slightly miscalculated the velocity
+or direction of the side-wind.
+
+``About ten degrees off,'' he mutters, and, using the
+Rudder, corrects his course accordingly.
+
+Now he feels happier and that he is well on his way.
+The gusts, too, have ceased to trouble him as, at this altitude,
+they are not nearly so bad as they were near the ground
+the broken surface of which does much to produce them;
+and sometimes for miles he makes but a movement or two
+of the controls.
+
+The clouds just above race by with dizzy and uniform
+speed; the country below slowly unrolls, and the steady
+drone of the Engine is almost hypnotic in effect. ``Sleep,
+sleep, sleep,'' it insidiously suggests. ``Listen to me and
+watch the clouds; there's nothing else to do. Dream,
+dream, dream of speeding through space for ever, and ever,
+and ever; and rest, rest, rest to the sound of my rhythmical
+hum. Droning on and on, nothing whatever matters. All
+things now are merged into speed through space and a sleepy
+monotonous d-d-r-r-o-o-n-n-e - - - - -.'' But the Pilot pulls
+himself together with a start and peers far ahead in search
+of the next landmark. This time it is a little country town.
+red-roofed his map tells him, and roughly of cruciform shape;
+and, sure enough, there in the right direction are the broken
+outlines of a few red roofs peeping out from between the trees.
+
+Another minute and he can see this little town, a fairy
+town it appears, nestling down between the hills with its
+red roofs and picturesque shape, a glowing and lovely contrast
+with the dark green of the surrounding moors.
+
+So extraordinarily clean and tidy it looks from such a
+height, and laid out in such orderly fashion with perfectly
+defined squares, parks, avenues, and public buildings, it
+indeed appears hardly real, but rather as if it has this very
+day materialized from some delightful children's book!
+
+Every city and town you must know has its distinct
+individuality to the Pilot's eye. Some are not fairy places
+at all, but great dark ugly blots upon the fair countryside,
+and with tall shafts belching forth murky columns of smoke
+to defile clean space. Others, melancholy-looking masses
+of grey, slate-roofed houses, are always sad and dispirited;
+never welcoming the glad sunshine, but ever calling for leaden
+skies and a weeping Heaven. Others again, little coquettes
+with village green, white palings everywhere, bright gravel
+roads, and an irrepressible air of brightness and gaiety.
+
+Then there are the rivers, silvery streaks peacefully
+winding far, far away to the distant horizon; they and the
+lakes the finest landmarks the Pilot can have. And the
+forests. How can I describe them? The trees cannot be
+seen separately, but merge altogether into enormous irregular
+dark green masses sprawling over the country, and sometimes
+with great ungainly arms half encircling some town or village;
+and the wind passing over the foliage at times gives the forest
+an almost living appearance, as of some great dragon of olden
+times rousing itself from slumber to devour the peaceful
+villages which its arms encircle.
+
+And the Pilot and Observer fly on and on, seeing these
+things and many others which baffle my poor skill to describe--
+things, dear Reader, that you shall see, and poets sing of,
+and great artists paint in the days to come when the Designer
+has captured Efficiency. Then, and the time is near, shall
+you see this beautiful world as you have never seen it before,
+the garden it is, the peace it breathes, and the wonder of it.
+
+The Pilot, flying on, is now anxiously looking for the
+railway line which midway on his journey should point
+the course. Ah! There it is at last, but suddenly (and
+the map at fault) it plunges into the earth! Well the writer
+remembers when that happened to him on a long 'cross-
+country flight in the early days of aviation. Anxiously
+he wondered ``Are tunnels always straight?'' and with what
+relief, keeping on a straight course, he picked up the line
+again some three miles farther on!
+
+Now at last the Pilot sees the sea, just a streak on the
+north-eastern horizon, and he knows that his flight is two-
+thirds over. Indeed, he should have seen it before, but
+the air is none too clear, and he is not yet able to discern
+the river which soon should cross his path. As he swiftly
+speeds on the air becomes denser and denser with what he
+fears must be the beginning of a sea-fog, perhaps drifting
+inland along the course of the river. Now does he feel real
+anxiety, for it is the DUTY of a Pilot to fear fog, his deadliest
+enemy. Fog not only hides the landmarks by which he
+keeps his course, but makes the control of the Aeroplane
+a matter of the greatest difficulty. He may not realize
+it, but, in keeping his machine on an even keel, he is
+unconsciously balancing it against the horizon, and with the
+horizon gone he is lost indeed. Not only that, but it also
+prevents him from choosing his landing-place, and the
+chances are that, landing in a fog, he will smash into a tree,
+hedge, or building, with disastrous results. The best and
+boldest pilot 'wares a fog, and so this one, finding the
+conditions becoming worse and yet worse, and being forced to
+descend lower and lower in order to keep the earth within
+view, wisely decides to choose a landing-place while there is
+yet time to do so.
+
+Throttling down the power of the engine he spirals downwards,
+keenly observing the country below. There are
+plenty of green fields to lure him, and his great object is to
+avoid one in which the grass is long, for that would bring
+his machine to a stop so suddenly as to turn it over; or one
+of rough surface likely to break the under-carriage. Now
+is perfect eyesight and a cool head indispensable. He sees
+and decides upon a field and, knowing his job, he sticks to
+that field with no change of mind to confuse him. It is none
+too large, and gliding just over the trees and head on to the
+wind he skilfully ``stalls'' his machine; that is, the speed
+having decreased sufficiently to avoid such a manoeuvre
+resulting in ascent, he, by means of the Elevator, gives the
+Aeroplane as large an angle of incidence as possible. and the
+undersides of the planes meeting the air at such a large
+angle act as an air-brake, and the Aeroplane, skimming
+over the ground, lessens its speed and finally stops just at
+the farther end of the field.
+
+Then, after driving the Aeroplane up to and under the
+lee of the hedge, he stops the engine, and quickly lashing
+the joy-stick fast in order to prevent the wind from blowing
+the controlling surfaces about and possibly damaging them,
+he hurriedly alights. Now running to the tail he lifts it up
+on to his shoulder, for the wind has become rough indeed
+and there is danger of the Aeroplane becoming unmanageable.
+By this action he decreases the angle at which the planes
+are inclined to the wind and so minimizes the latter's effect
+upon them. Then to the Observer, ``Hurry up, old fellow,
+and try to find some rope, wire, or anything with which to
+picket the machine. The wind is rising and I shan't be able
+to hold the 'bus steady for long. Don't forget the wire-
+cutters. They're in the tool kit.'' And the Observer rushes
+off in frantic haste, before long triumphantly returning with
+a long length of wire from a neighbouring fence. Blocking
+up the tail with some debris at hand, they soon succeed,
+with the aid of the wire, in stoutly picketing the Aeroplane
+to the roots of the high hedge in front of it; done with much
+care, too, so that the wire shall not fray the fabric or set up
+dangerous bending-stresses in the woodwork. Their work
+is not done yet, for the Observer remarking, ``I don't like
+the look of this thick weather and rather fear a heavy rain-
+storm,'' the Pilot replies, ``Well, it's a fearful bore, but the
+first rule of our game is never to take an unnecessary risk,
+so out with the engine and body covers.''
+
+Working with a will they soon have the engine and the
+open part of the body which contains the seats, controls,
+and instruments snugly housed with their waterproof covers,
+and the Aeroplane is ready to weather the possible storm.
+
+Says the Observer, ``I'm remarkably peckish, and methinks
+I spy the towers of one of England's stately homes
+showing themselves just beyond that wood, less than a
+quarter of a mile away. What ho! for a raid. What do
+you say?''
+
+``All right, you cut along and I'll stop here, for the
+Aeroplane must not be left alone. Get back as quickly as
+possible.''
+
+And the Observer trots off, leaving the Pilot filling his
+pipe and anxiously scrutinizing the weather conditions.
+Very thick it is now, but the day is yet young, and he has
+hopes of the fog lifting sufficiently to enable the flight to be
+resumed. A little impatiently he awaits the return of his
+comrade, but with never a doubt of the result, for the hospi-
+tality of the country house is proverbial among pilots!
+What old hand among them is there who cannot instance
+many a forced landing made pleasant by such hospitality?
+Never too late or too early to help with food, petrol, oil,
+tools, and assistants. Many a grateful thought has the
+writer for such kind help given in the days before the war
+(how long ago they seem!), when aeroplanes were still more
+imperfect than they are now, and involuntary descents
+often a part of 'cross-country flying.
+
+Ah! those early days! How fresh and inspiring they
+were! As one started off on one's first 'cross-country flight,
+on a machine the first of its design, and with everything
+yet to learn, and the wonders of the air yet to explore; then
+the joy of accomplishment, the dreams of Efficiency, the
+hard work and long hours better than leisure; and what a
+field of endeavour--the realms of space to conquer! And
+the battle still goes on with ever-increasing success. Who
+is bold enough to say what its limits shall be?
+
+So ruminates this Pilot-Designer, as he puffs at his pipe,
+until his reverie is abruptly disturbed by the return of the
+Observer.
+
+``Wake up, you AIRMAN,'' the latter shouts. ``Here's
+the very thing the doctor ordered! A basket of first-class
+grub and something to keep the fog out, too.''
+
+``Well, that's splendid, but don't call me newspaper
+names or you'll spoil my appetite!''
+
+Then, with hunger such as only flying can produce, they
+appreciatively discuss their lunch, and with many a grateful
+thought for the donors--and they talk shop. They can't
+help it, and even golf is a poor second to flight talk. Says
+the Pilot, who must have his grievance, ``Just observe
+where I managed to stop the machine. Not twenty feet
+from this hedge! A little more and we should have been
+through it and into Kingdom Come! I stalled as well as
+one could, but the tail touched the ground and so I could
+not give the Aeroplane any larger angle of incidence. Could
+I have given it a larger angle, then the planes would have
+become a much more effective air-brake, and we should
+have come to rest in a much shorter distance. It's all the
+fault of the tail. There's hardly a type of Aeroplane in
+existence in which the tail could not be raised several feet,
+and that would make all the difference. High tails mean
+a large angle of incidence when the machine touches ground
+and, with enough angle, I'll guarantee to safely land the
+fastest machine in a five-acre field. You can, I am sure,
+imagine what a difference that would make where forced
+landings are concerned!'' Then rapidly sketching in his
+notebook, he shows the Observer the following illustration:
+
+``That's very pretty,'' said the Observer, ``but how
+about Mechanical Difficulties, and Efficiency in respect of
+Flight? And, anyway, why hasn't such an obvious thing
+been done already?''
+
+``As regards the first part of your question I assure
+you that there's nothing in it, and I'll prove it to you as
+follows----''
+
+``Oh! That's all right, old chap. I'll take your word
+for it,'' hurriedly replies the Observer, whose soul isn't tuned
+to a technical key.
+
+``As regards the latter part of your inquiry,'' went on
+the Pilot, a little nettled at having such a poor listener,
+``it's very simple. Aeroplanes have `just growed' like
+Topsy, and they consequently contain this and many another
+relic of early day design when Aeroplanes were more or less
+thrown together and anything was good enough that could
+get off the ground.''
+
+``By Jove,'' interrupts the Observer, ``I do believe the
+fog is lifting. Hadn't we better get the engine and body
+covers off, just in case it's really so?''
+
+``I believe you're right. I am sure those hills over there
+could not be seen a few minutes ago, and look--there's
+sunshine over there. We'd better hurry up.''
+
+Ten minutes' hard work and the covers are off, neatly
+folded and stowed aboard; the picketing wires are cast adrift,
+and the Pilot is once more in his seat. The Aeroplane has
+been turned to face the other end of the field, and, the Observer
+swinging round the propeller, the engine is awake
+again and slowly ticking over. Quickly the Observer climbs
+into his seat in front of the Pilot, and, the latter slightly
+opening the throttle, the Aeroplane leisurely rolls over the
+ground towards the other end of the field, from which the
+ascent will be made.
+
+Arriving there the Pilot turns the Aeroplane in order to
+face the wind and thus secure a quick ``get-off.'' Then he
+opens the throttle fully and the mighty voice of the Engine
+roars out ``Now see me clear that hedge!'' and the Aeroplane
+races forward at its minimum angle of incidence. Tail
+up, and with ever-increasing speed, it rushes towards the
+hedge under the lee of which it has lately been at rest; and
+then, just as the Observer involuntarily pulls back an imaginary
+``joy-stick,'' the Pilot moves the real one and places the
+machine at its best climbing angle. Like a living thing it
+responds, and instantly leaves the ground, clearing the hedge
+like a--well, like an Aeroplane with an excellent margin of
+lift. Upwards it climbs with even and powerful lift, and the
+familiar scenes below again gladden the eyes of the Pilot.
+Smaller and more and more squat grow the houses and hills;
+more and more doll-like appear the fields which are clearly
+outlined by the hedges; and soon the country below is easily
+identified with the map. Now they can see the river before
+them and a bay of the sea which must be crossed or skirted.
+The fog still lingers along the course of the river and between
+the hills, but is fast rolling away in grey, ghost-like masses.
+Out to sea it obscures the horizon, making it difficult to be
+sure where water ends and fog begins, and creating a strange,
+rather weird effect by which ships at a certain distance appear
+to be floating in space.
+
+Now the Aeroplane is almost over the river, and the
+next instant it suddenly drops into a ``hole in the air.''
+With great suddenness it happens, and for some two hundred
+feet it drops nose-down and tilted over sideways; but the
+Pilot is prepared and has put his craft on an even keel in
+less time than it takes to tell you about it; for well he knows
+that he must expect such conditions when passing over a
+shore or, indeed, any well-defined change in the composition
+of the earth's surface. Especially is this so on a hot and
+sunny day, for then the warm surface of the earth creates
+columns of ascending air, the speed of the ascent depending
+upon the composition of the surface. Sandy soil, for instance,
+such as borders this river produces a quickly ascending
+column of air, whereas water and forests have not such a
+marked effect. Thus, when our Aeroplane passed over the
+shore of the river, it suddenly lost the lift due to the ascending
+air produced by the warm sandy soil, and it consequently
+dropped just as if it had fallen into a hole.
+
+Now the Aeroplane is over the bay and, the sea being
+calm, the Pilot looks down, down through the water, and
+clearly sees the bottom, hundreds of feet below the surface.
+Down through the reflection of the blue sky and clouds,
+and one might think that is all, but it isn't. Only those
+who fly know the beauties of the sea as viewed from above;
+its dappled pearly tints; its soft dark blue shadows; the beautiful
+contrasts of unusual shades of colour which are always
+differing and shifting with the changing sunshine and the
+ever moving position of the aerial observer. Ah! for some
+better pen than mine to describe these things! One with
+glowing words and a magic rhythm to express the wonders
+of the air and the beauty of the garden beneath--the immensity
+of the sea--the sense of space and of one's littleness
+there--the realization of the Power moving the multitudes
+below--the exaltation of spirit altitude produces--the joy of
+speed. A new world of sensation!
+
+Now the bay is almost crossed and the Aerodrome at B
+can be distinguished.
+
+On the Aerodrome is a little crowd waiting and watching
+for the arrival of the Aeroplane, for it is of a new and improved
+type and its first 'cross-country performance is of
+keen interest to these men; men who really know something
+about flight.
+
+There is the Squadron Commander who has done some
+real flying in his time; several well-seasoned Flight-
+Commanders; a dozen or more Flight-Lieutenants; a
+knowledgeable Flight-Sergeant; a number of Air Mechanics,
+and, a little on one side and almost unnoticed, the
+Designer.
+
+``I hope they are all right,'' said someone, ``and that
+they haven't had difficulties with the fog. It rolled up very
+quickly, you know.''
+
+``Never fear,'' remarked a Flight-Commander. ``I know
+the Pilot well and he's a good 'un; far too good to carry on
+into a fog.''
+
+``They say the machine is really something out of the
+ordinary,'' said another, ``and that, for once, the Designer
+has been allowed full play; that he hasn't been forced to
+unduly standardize ribs, spars, struts, etc., and has more
+or less had his own way. I wonder who he is. It seems
+strange we hear so little of him.''
+
+``Ah! my boy. You do a bit more flying and you'll
+discover that things are not always as they appear from a
+distance!''
+
+``There she is, sir!'' cries the Flight-Sergeant. ``Just a
+speck over the silvery corner of that cloud.''
+
+A tiny speck it looks, some six miles distant and three
+thousand feet high; but, racing along, it rapidly appears
+larger and soon its outlines can be traced and the sunlight
+be seen playing upon the whirling propeller.
+
+Now the distant drone of the engine can be heard,
+but not for long, for suddenly it ceases and, the nose of
+the Aeroplane sinking, the craft commences gliding downwards.
+
+``Surely too far away,'' says a subaltern. It will be
+a wonderful machine if, from that distance and height, it
+can glide into the Aerodrome.'' And more than one express
+the opinion that it cannot be done; but the Designer smiles
+to himself, yet with a little anxiety, for his reputation is
+at stake, and Efficiency, the main reward he desires, is perhaps,
+or perhaps not, at last within his grasp!
+
+Swiftly the machine glides downwards towards them,
+and it can now be seen how surprisingly little it is affected
+by the rough weather and gusts; so much so that a little
+chorus of approval is heard.
+
+``Jolly good gliding angle,'' says someone; and another,
+``Beautifully quick controls, what?'' and from yet another,
+``By Jove! The Pilot must be sure of the machine. Look,
+he's stopped the engine entirely.''
+
+Then the Aeroplane with noiseless engine glides over
+the boundary of the Aerodrome, and, with just a soft soughing
+sound from the air it cleaves, lands gently not fifty yards from
+the onlookers.
+
+``Glad to see you,'' says the Squadron Commander to
+the Pilot. ``How do you like the machine?'' And the
+Pilot replies:
+
+``I never want a better one, sir. It almost flies itself!''
+
+And the Designer turns his face homewards and towards
+his beloved drawing-office; well satisfied, but still dreaming
+dreams of the future and . . . looking far ahead whom should
+he see but Efficiency at last coming towards him! And to
+him she is all things. In her hair is the morning sunshine;
+her eyes hold the blue of the sky, and on her cheeks is the
+pearly tint of the clouds as seen from above. The passion
+of speed, the lure of space, the sense of power, and the
+wonder of the future . . . all these things she holds for him.
+
+``Ah!'' he cries. ``You'll never leave me now, when
+at last there is no one between us?''
+
+And Efficiency, smiling and blushing, but practical as
+ever, says:
+
+``And you will never throw those Compromises in my
+face?''
+
+``My dear, I love you for them! Haven't they been
+my life ever since I began striving for you ten long years
+ago?''
+
+And so they walked off very happily, arm-in-arm together;
+and if this hasn't bored you and you'd like some more of the
+same sort of thing, I'd just love to tell you some day of the
+wonderful things they accomplish together, and of what
+they dream the future holds in store.
+
+And that's the end of the Prologue.
+
+
+
+CHAPTER I
+
+FLIGHT
+
+Air has weight (about 13 cubic feet = 1 lb.), inertia, and
+momentum. It therefore obeys Newton's laws[[14]] and resists
+movement. It is that resistance or reaction which makes
+flight possible.
+
+
+[[14]] See Newton's laws in the Glossary at the end of the book.
+
+
+Flight is secured by driving through the air a surface[[15]]
+inclined upwards and towards the direction of motion.
+
+
+[[15]] See ``Aerofoil'' in the Glossary.
+
+
+S = Side view of surface.
+
+M = Direction of motion.
+
+CHORD.--The Chord is, for practical purposes, taken to
+be a straight line from the leading edge of the surface to its
+trailing edge.
+
+N = A line through the surface starting from its trailing
+edge. The position of this line, which I call the Neutral
+Lift Line, is found by means of wind-tunnel research, and it
+varies with differences in the camber (curvature) of surfaces.
+In order to secure flight, the inclination of the surface must
+be such that the neutral lift line makes an angle with and
+ABOVE the line of motion. If it is coincident with M, there is
+no lift. If it makes an angle with M and BELOW it, then
+there is a pressure tending to force the surface down.
+
+I = Angle of Incidence. This angle is generally defined
+as the angle the chord makes with the direction of motion,
+but that is a bad definition, as it leads to misconception.
+The angle of incidence is best described as the angle the
+neutral lift line makes with the direction of motion relative
+to the air. You will, however, find that in nearly all rigging
+specifications the angle of incidence is taken to mean the
+angle the chord makes with a line parallel to the propeller
+thrust. This is necessary from the point of view of the
+practical mechanic who has to rig the aeroplane, for he could
+not find the neutral lift line, whereas he can easily find the
+chord. Again, he would certainly be in doubt as to ``the
+direction of motion relative to the air,'' whereas he can
+easily find a line parallel to the propeller thrust. It is a
+pity, however, that these practical considerations have
+resulted in a bad definition of the angle of incidence becoming
+prevalent, a consequence of which has been the widespread
+fallacy that flight may be secured with a negative
+inclination of the surface. Flight may conceivably be
+secured with a negative angle of chord, but never with a
+negative inclination of the surface. All this is only applicable
+to cambered surfaces. In the case of flat surfaces the neutral
+lift line coincides with the chord and the definition I have
+criticised adversely is then applicable. Flat lifting surfaces
+are, however, never used.
+
+The surface acts upon the air in the following manner:
+
+
+As the bottom of the surface meets the air, it compresses
+it and accelerates it DOWNWARDS. As a result of this definite
+action there is, of course, an equal and opposite reaction
+UPWARDS.
+
+The top surface, in moving forward, tends to leave the
+air behind it, thus creating a semi-vacuum or rarefied area
+over the top of the surface. Consequently the pressure of
+air on the top of the surface is decreased, thus assisting the
+reaction below to lift the surface UPWARDS.
+
+The reaction increases approximately as the square of
+the velocity. It is the result of (1) the mass of air engaged,
+and (2) the velocity and consequent force with which the
+surface engages the air. If the reaction was produced by
+only one of those factors it would increase in direct proportion
+to the velocity, but, since it is the product of both factors,
+it increases as V<2S>.
+
+Approximately three-fifths of the reaction is due to the
+decrease of density (and consequent decrease of downward
+pressure) on the top of the surface; and only some two-
+fifths is due to the upward reaction secured by the action
+of the bottom surface upon the air. A practical point in
+respect of this is that, in the event of the fabric covering the
+surface getting into bad condition, it is more likely to strip
+off the top than off the bottom.
+
+The direction of the reaction is approximately at right-
+angles to the chord of the surface, as illustrated above; and
+it is, in considering flight, convenient to divide it into two
+component parts or values, thus:
+
+1. The vertical component of the reaction, i.e., Lift,
+which is opposed to Gravity, i.e., the weight of the
+aeroplane.
+
+2. The horizontal component, i.e., Drift (sometimes
+called Resistance), to which is opposed the thrust of the
+propeller.
+
+The direction of the reaction is, of course, the resultant
+of the forces Lift and Drift.
+
+The Lift is the useful part of the reaction, for it lifts the
+weight of the aeroplane.
+
+The Drift is the villain of the piece, and must be overcome
+by the Thrust in order to secure the necessary velocity to
+produce the requisite Lift for flight.
+
+DRIFT.--The drift of the whole aeroplane (we have considered
+only the lifting surface heretofore) may be conveniently
+divided into three parts, as follows:
+
+Active Drift, which is the drift produced by the lifting
+surfaces.
+
+Passive Drift, which is the drift produced by all the rest
+of the aeroplane--the struts, wires, fuselage, under-carriage,
+etc., all of which is known as ``detrimental surface.''
+
+Skin Friction, which is the drift produced by the friction
+of the air with roughnesses of surface. The latter is practically
+negligible having regard to the smooth surface of the
+modern aeroplane, and its comparatively slow velocity
+compared with, for instance, the velocity of a propeller
+blade.
+
+LIFT-DRIFT RATIO.--The proportion of lift to drift is
+known as the lift-drift ratio, and is of paramount importance,
+for it expresses the efficiency of the aeroplane (as distinct
+from engine and propeller). A knowledge of the factors
+governing the lift-drift ratio is, as will be seen later, an
+absolute necessity to anyone responsible for the rigging of an
+aeroplane, and the maintenance of it in an efficient and safe
+condition.
+
+Those factors are as follows:
+
+1. Velocity.--The greater the velocity the greater the
+proportion of drift to lift, and consequently the
+less the efficiency. Considering the lifting surfaces
+alone, both the lift and the (active) drift, being
+component parts of the reaction, increase as the
+square of the velocity, and the efficiency remains
+the same at all speeds. But, considering the
+whole aeroplane, we must remember the passive
+drift. It also increases as the square of the
+velocity (with no attendant lift), and, adding
+itself to the active drift, results in increasing
+the proportion of total drift (active + passive) to
+lift.
+
+But for the increase in passive drift the efficiency
+of the aeroplane would not fall with increasing
+velocity, and it would be possible, by doubling
+the thrust, to approximately double the speed
+or lift--a happy state of affairs which can never
+be, but which we may, in a measure, approach
+by doing everything possible to diminish the passive
+drift.
+
+Every effort is then made to decrease it by
+``stream-lining,'' i.e., by giving all ``detrimental''
+parts of the aeroplane a form by which they will
+pass through the air with the least possible drift.
+Even the wires bracing the aeroplane together are,
+in many cases, stream-lined, and with a markedly
+good effect upon the lift-drift ratio. In the case
+of a certain well-known type of aeroplane the
+replacing of the ordinary wires by stream-lined
+wires added over five miles an hour to the flight
+speed.
+
+Head-resistance is a term often applied to passive
+drift, but it is apt to convey a wrong impression,
+as the drift is not nearly so much the result of
+the head or forward part of struts, wires, etc.,
+as it is of the rarefied area behind.
+
+Above is illustrated the flow of air round two
+objects moving in the direction of the arrow M.
+
+In the case of A, you will note that the rarefied
+area DD is of very considerable extent; whereas
+in the case of B, the air flows round it in such a
+way as to meet very closely to the rear of the
+object, thus DECREASING DD.
+
+The greater the rarefied area DD. then, the less
+the density, and, consequently, the less the pressure
+of air upon the rear of the object. The less such
+pressure, then, the better is head-resistance D
+able to get its work in, and the more thrust will
+be required to overcome it.
+
+The ``fineness'' of the stream-line shape, i.e.,
+the proportion of length to width, is determined
+by the velocity--the greater the velocity, the
+greater the fineness. The best degree of fineness
+for any given velocity is found by means of wind-
+tunnel research.
+
+The practical application of all this is, from a
+rigging point of view, the importance of adjusting
+all stream-line parts to be dead-on in the line of
+flight, but more of that later on.
+
+2. Angle of Incidence.--The most efficient angle of
+incidence varies with the thrust at the disposal
+of the designer, the weight to be carried, and the
+climb-velocity ratio desired.
+
+The best angles of incidence for these varying
+factors are found by means of wind-tunnel research
+and practical trial and error. Generally
+speaking, the greater the velocity the smaller
+should be the angle of incidence, in order to preserve
+a clean, stream-line shape of rarefied area
+and freedom from eddies. Should the angle be
+too great for the velocity, then the rarefied area
+becomes of irregular shape with attendant turbulent
+eddies. Such eddies possess no lift value,
+and since it has taken power to produce them,
+they represent drift and adversely affect the lift-
+drift ratio.
+
+From a rigging point of view, one must presume
+that every standard aeroplane has its lifting
+surface set at the most efficient angle, and the
+practical application of all this is in taking the
+greatest possible care to rig the surface at the
+correct angle and to maintain it at such angle.
+Any deviation will adversely affect the lift-drift
+ratio, i.e., the efficiency.
+
+3. Camber.--(Refer to the second illustration in this
+chapter.) The lifting surfaces are cambered, i.e.,
+curved, in order to decrease the horizontal component
+of the reaction, i.e., the drift.
+
+The bottom camber: If the bottom of the surface
+was flat, every particle of air meeting it would do
+so with a shock, and such shock would produce a
+very considerable horizontal reaction or drift. By
+curving it such shock is diminished, and the curve
+should be such as to produce a uniform (not
+necessarily constant) acceleration and compression
+of the air from the leading edge to the trailing
+edge. Any unevenness in the acceleration and
+compression of the air produces drift.
+
+The top camber: If this was flat it would produce
+a rarefied area of irregular shape. I have already
+explained the bad effect this has upon the lift-
+drift ratio. The top surface is then curved to
+produce a rarefied area the shape of which shall
+be as stream-line and free from attendant eddies
+as possible.
+
+The camber varies with the angle of incidence,
+the velocity, and the thickness of the surface.
+Generally speaking, the greater the velocity, the
+less the camber and angle of incidence. With
+infinite velocity the surface would be set at no
+angle of incidence (the neutral lift line coincident
+with the direction of motion relative to the air),
+and would be, top and bottom, of pure streamline
+form--i.e., of infinite fineness. This is, of
+course, carrying theory to absurdity as the surface
+would then cease to exist.
+
+The best cambers for varying velocities, angles
+of incidence, and thicknesses of surface, are found
+by means of wind-tunnel research. The practical
+application of all this is in taking the greatest
+care to prevent the surface from becoming distorted
+and thus spoiling the camber and consequently
+the lift-drift ratio.
+
+4. Aspect Ratio.--This is the proportion of span to
+chord. Thus, if the span is, for instance, 50 feet
+and the chord 5 feet, the surface would be said to
+have an aspect ratio of 10 to 1.
+
+For A GIVEN VELOCITY and A GIVEN AREA of surface,
+the greater the aspect ratio, the greater the reaction.
+It is obvious, I think, that the greater
+the span, the greater the mass of air engaged,
+and, as already explained, the reaction is partly
+the result of the mass of air engaged.
+
+Not only that, but, PROVIDED the chord is not
+decreased to an extent making it impossible to
+secure the best camber owing to the thickness of
+the surface, the greater the aspect ratio, the better
+the lift-drift ratio. The reason of this is rather
+obscure. It is sometimes advanced that it is
+owing to the ``spill'' of air from under the wing-
+tips. With a high aspect ratio the chord is less
+than would otherwise be the case. Less chord
+results in smaller wing-tips and consequently less
+``spill.'' This, however, appears to be a rather
+inadequate reason for the high aspect ratio producing
+the high lift-drift ratio. Other reasons
+are also advanced, but they are of such a contentious
+nature I do not think it well to go into them
+here. They are of interest to designers, but this
+is written for the practical pilot and rigger.
+
+5. Stagger.--This is the advancement of the top surface
+relative to the bottom surface, and is not, of course,
+applicable to a single surface, i.e., a monoplane.
+In the case of a biplane having no stagger, there
+will be ``interference'' and consequent loss of
+Efficiency unless the gap between the top and bottom
+surfaces is equal to not less than 1 1/2  times the
+chord. If less than that, the air engaged by the
+bottom of the top surface will have a tendency
+to be drawn into the rarefied area over the top
+of the bottom surface, with the result that the
+surfaces will not secure as good a reaction as would
+otherwise be the case.
+
+It is not practicable to have a gap of much
+more than a distance equal to the chord, owing
+to the drift produced by the great length of struts
+and wires such a large gap would necessitate.
+By staggering the top surface forward, however,
+
+it is removed from the action of the lower surface
+and engages undisturbed air, with the result that
+the efficiency can in this way be increased by
+about 5 per cent. Theoretically the top plane
+should be staggered forward for a distance equal
+to about 30 per cent. of the chord, the exact
+distance depending upon the velocity and angle
+of incidence; but this is not always possible to
+arrange in designing an aeroplane, owing to difficulties
+of balance, desired position, and view of
+pilot, observer, etc.
+
+6. Horizontal Equivalent.--The vertical component of
+the reaction, i.e., lift, varies as the horizontal
+equivalent (H.E.) of the surface, but the drift
+remains the same. Then it follows that if H.E. grows
+less, the ratio of lift to drift must do the same.
+
+A, B, and C are front views of three surfaces.
+
+A has its full H.E., and therefore, from the point
+of view from which we are at the moment considering
+efficiency, it has its best lift-drift ratio.
+
+B and C both possess the same surface as A,
+but one is inclined upwards from its centre and
+the other is straight but tilted. For these reasons
+their H.E.'s are, as illustrated, less than in the
+case of A. That means less vertical lift, and,
+the drift remaining the same (for there is the
+same amount of surface as in A to produce it),
+the lift-drift ratio falls.
+
+THE MARGIN OF POWER is the power available above
+that necessary to maintain horizontal flight.
+
+THE MARGIN OF LIFT is the height an aeroplane can gain
+in a given time and starting from a given altitude.
+As an example, thus: 1,000 feet the first minute,
+and starting from an altitude of 500 feet above
+sea-level.
+
+The margin of lift decreases with altitude, owing
+to the decrease in the density of the air, which
+adversely affects the engine. Provided the engine
+maintained its impulse with altitude, then, if we
+ignore the problem of the propeller, which I will
+go into later on, the margin of lift would not
+disappear. Moreover, greater velocity for a given
+power would be secured at a greater altitude, owing
+to the decreased density of air to be overcome.
+After reading that, you may like to light your pipe
+and indulge in dreams of the wonderful possibilities
+which may become realities if some brilliant genius
+shows us some day how to secure a constant power
+with increasing altitude. I am afraid, however,
+that will always remain impossible; but it is probable
+that some very interesting steps may be taken in
+that direction.
+
+THE MINIMUM ANGLE OF INCIDENCE is the smallest
+angle at which, for a given power, surface (including
+detrimental surface), and weight, horizontal flight
+can be maintained.
+
+THE MAXIMUM ANGLE OF INCIDENCE is the greatest
+angle at which, for a given power, surface (including
+detrimental surface), and weight, horizontal flight
+can be maintained.
+
+THE OPTIMUM ANGLE OF INCIDENCE is the angle at
+which the lift-drift ratio is highest. In modern
+aeroplanes it is that angle of incidence possessed by the
+surface when the axis of the propeller is horizontal.
+
+THE BEST CLIMBING ANGLE is approximately half-way
+between the maximum and the optimum angles.
+
+All present-day aeroplanes are a compromise between
+Climb and horizontal Velocity. We will compare
+the essentials for two aeroplanes, one designed for
+maximum climb, and the other for maximum velocity.
+
+ESSENTIALS FOR MAXIMUM CLIMB:
+
+1. Low velocity, in order to secure the best lift-drift
+ratio.
+
+2. Having a low velocity, a large surface will be
+necessary in order to engage the necessary mass
+of air to secure the requisite lift.
+
+3. Since (1) such a climbing machine will move
+along an upward sloping path, and (2) will climb
+with its propeller thrust horizontal, then a large
+angle relative to the direction of the thrust will be
+necessary in order to secure the requisite angle
+relative to the direction of motion.
+
+The propeller thrust should be always horizontal, because
+the most efficient flying-machine (having regard to climb OR
+velocity) has, so far, been found to be an arrangement of an
+inclined surface driven by a HORIZONTAL thrust--the surface
+lifting the weight, and the thrust overcoming the drift.
+This is, in practice, a far more efficient arrangement than
+the helicopter, i.e., the air-screw revolving about a vertical
+axis and producing a thrust opposed to gravity. If, when
+climbing, the propeller thrust is at such an angle as to tend
+to haul the aeroplane upwards, then it is, in a measure,
+acting as a helicopter, and that means inefficiency. The
+reason of a helicopter being inefficient in practice is due to
+the fact that, owing to mechanical difficulties, it is impossible
+to construct within a reasonable weight an air-screw of the
+requisite dimensions. That being so, it would be necessary,
+in order to absorb the power of the engine, to revolve the
+comparatively small-surfaced air screw at an immensely
+greater velocity than that of the aeroplane's surface. As
+already explained, the lift-drift ratio falls with velocity on
+account of the increase in passive drift. This applies to a
+blade of a propeller or air-screw, which is nothing but a
+revolving surface set at angle of incidence, and which it is
+impossible to construct without a good deal of detrimental
+surface near the central boss.
+
+4. The velocity being low, then it follows that for
+that reason also the angle of incidence should be
+comparatively large.
+
+5. Camber.--Since such an aeroplane would be of
+low velocity, and therefore possess a large angle
+of incidence, a large camber would be necessary.
+
+Let us now consider the essentials for an aeroplane of
+maximum velocity for its power, and possessing merely
+enough lift to get off the ground, but no margin of lift.
+
+1. Comparatively HIGH VELOCITY.
+
+2. A comparatively SMALL SURFACE, because, being
+of greater velocity than the maximum climber,
+a greater mass of air will be engaged for a given
+surface and time, and therefore a smaller surface
+will be sufficient to secure the requisit lift.
+
+3. A small angle relative to the propeller thrust, since
+the latter coincides with the direction of motion.
+
+4. A comparatively small angle of incidence by reason
+of the high velocity.
+
+5. A comparatively small camber follows as a result
+of the small angle of incidence.
+
+
+SUMMARY.
+
+Essentials for Maximum            Essentials for Maximum
+        Climb.                            Velocity
+
+1. Low velocity.                  High velocity.
+2. Large surface.                 Small surface.
+3. Large angle relative to        Small angle relative to
+   propeller thrust.                 propeller thrust.
+4. Large angle relative to        Small angle relative to direction
+   direction of motion.              of motion.
+5. Large camber.                  Small camber.
+
+
+It is mechanically impossible to construct an aeroplane
+of reasonable weight of which it would be possible to very
+the above opposing essentials. Therefore, all aeroplanes
+are designed as a compromise between Climb and Velocity.
+
+As a rule aeroplanes are designed to have at low altitude
+a slight margin of lift when the propeller thrust is horizontal.
+
+
+ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE
+DESIGNED AS A COMPROMISE BETWEEN
+VELOCITY AND CLIMB, AND POSSESSING A SLIGHT MARGIN OF LIFT AT A LOW
+ALTITUDE AND WHEN THE THRUST IS HORIZONTAL
+
+MINIMUM ANGLE.
+
+This gives the greatest velocity
+during horizontal flight at a low
+altitude. Greater velocity would
+be secured if the surface, angle,
+and camber were smaller and designed
+to just maintain horizontal
+flight with a horizontal thrust.
+Also, in such case, the propeller
+would not be thrusting downwards,
+but along a horizontal line
+which is obviously a more efficient
+arrangement if we regard
+the aeroplane merely from one
+point of view, i.e., either with
+reference to velocity OR climb.
+
+OPTIMUM ANGLE
+(Thrust horizontal)
+
+The velocity is less than at the
+smaller minimum angle, and, as
+aeroplanes are designed to-day, the
+area and angle of incidence of the
+surface is such as to secure a
+slight ascent at a low altitude. The
+camber of the surface is designed
+for this angle of incidence and
+velocity. The lift-drift ratio is
+best at this angle.
+
+BEST CLIMBING ANGLE
+
+The velocity is now still less by
+reason of the increased angle
+producing increase of drift. Less
+velocity at A GIVEN ANGLE produces
+less lift, but the increased angle
+more or less offsets the loss of lift
+due to the decreased velocity, and
+in addition, the thrust is now hauling
+the aeroplane upwards.
+
+MAXIMUM ANGLE
+
+The greater angle has now produced
+so much drift as to lessen
+the velocity to a point where the
+combined lifts from the surface
+and from the thrust are only just
+able to maintain horizontal flight.
+Any greater angle will result in a
+still lower lift-drift ratio. The lift
+will then become less than the
+weight and the aeroplane will
+consequently fall. Such a fall is
+known as ``stalling'' or ``pancaking.''
+
+NOTE.--The golden rule for beginners: Never exceed the Best Climbing Angle.
+Always maintain the flying speed of the aeroplane.
+
+
+By this means, when the altitude is reached where the margin
+of lift disappears (on account of loss of engine power), and
+which is, consequently, the altitude where it is just possible
+to maintain horizontal flight, the aeroplane is flying with
+its thrust horizontal and with maximum efficiency (as distinct
+from engine and propeller efficiency).
+
+The margin of lift at low altitude, and when the thrust
+is horizontal, should then be such that the higher altitude at
+which the margin of lift is lost is that altitude at which most
+of the aeroplane's horizontal flight work is done. That
+ensures maximum velocity when most required.
+
+Unfortunately, where aeroplanes designed for fighting
+are concerned, the altitude where most of the work is done
+is that at which both maximum velocity and maximum
+margin of lift for power are required.
+
+Perhaps some day a brilliant inventor will design an
+aeroplane of reasonable weight and drift of which it will be
+possible for the pilot to vary at will the above-mentioned
+opposing essentials. Then we shall get maximum velocity,
+or maximum margin of lift, for power as required. Until
+then the design of the aeroplane must remain a compromise
+between Velocity and Climb.
+
+
+
+CHAPTER II
+
+STABILITY AND CONTROL
+
+STABILITY is a condition whereby an object disturbed
+has a natural tendency to return to its first and normal
+position. Example: a weight suspended by a cord.
+
+INSTABILITY is a condition whereby an object disturbed
+has a natural tendency to move as far as possible away from
+its first position, with no tendency to return. Example:
+a stick balanced vertically upon your finger.
+
+NEUTRAL INSTABILITY is a condition whereby an object
+disturbed has no tendency to move farther than displaced
+by the force of the disturbance, and no tendency to return
+to its first position.
+
+In order that an aeroplane may be reasonably controllable,
+it is necessary for it to possess some degree of stability
+longitudinally, laterally, and directionally.
+
+LONGITUDINAL STABILITY in an aeroplane is its stability
+about an axis transverse to the direction of normal horizontal
+flight, and without which it would pitch and toss.
+
+LATERAL STABILITY is its stability about its longitudinal
+axis, and without which it would roll sideways.
+
+DIRECTIONAL STABILITY is its stability about its vertical
+axis, and without which it would have no tendency to keep
+its course.
+
+For such directional stability to exist there must be,
+in effect,[[16]] more ``keel-surface'' behind the vertical axis
+than there is in front of it. By keel-surface I mean every-
+thing to be seen when looking at an aeroplane from the side
+of it--the sides of the body, undercarriage, struts, wires, etc.
+The same thing applies to a weathercock. You know what
+would happen if there was insufficient keel-surface behind
+the vertical axis upon which it is pivoted. It would turn
+off its proper course, which is opposite to the direction of
+the wind. It is very much the same in the case of an aeroplane.
+
+[[16]] ``In effect'' because, although there may be actually the greatest proportion
+of keel-surface In front of the vertical axis, such surface may be much nearer to
+the axis than is the keel-surface towards the tail. The latter may then be actually
+less than the surface in front, but, being farther from the axis, it has a greater
+leverage, and consequently is greater in effect than the surface in front.
+
+The above illustration represents an aeroplane (directionally
+stable) flying along the course B. A gust striking it
+as indicated acts upon the greater proportion of keel-surface
+behind the turning axis and throws it into the new course.
+It does not, however, travel along the new course, owing to
+its momentum in the direction B. It travels, as long as
+such momentum lasts, in a direction which is the resultant
+of the two forces Thrust and Momentum. But the centre
+line of the aeroplane is pointing in the direction of the new
+course. Therefore its attitude, relative to the direction of
+motion, is more or less sideways, and it consequently receives
+an air pressure in the direction C. Such pressure, acting
+upon the keel-surface, presses the tail back towards its first
+position in which the aeroplane is upon its course B.
+
+What I have described is continually going on during
+flight, but in a well-designed aeroplane such stabilizing
+movements are, most of the time, so slight as to be imperceptible
+to the pilot.
+
+If an aeroplane was not stabilized in this way, it would
+not only be continually trying to leave its course, but it would
+also possess a dangerous tendency to ``nose away'' from the
+direction of the side gusts. In such case the gust shown in
+the above illustration would turn the aeroplane round the
+opposite way a very considerable distance; and the right
+wing, being on the outside of the turn, would travel with
+greater velocity than the left wing. Increased velocity
+means increased lift; and so, the right wing lifting, the
+aeroplane would turn over sideways very quickly.
+
+LONGITUDINAL STABILITY.--Flat surfaces are longitudinally
+stable owing to the fact that with decreasing angles of
+incidence the centre line of pressure (C.P.) moves forward.
+
+The C.P. is a line taken across the surface, transverse
+to the direction of motion, and about which all the air forces
+may be said to balance, or through which they may be said
+to act.
+
+
+Imagine A to be a flat surface, attitude vertical, travelling
+through the air in the direction of motion M. Its C.P. is
+then obviously along the exact centre line of the surface
+as illustrated.
+
+In B, C, and D the surfaces are shown with angles of
+incidence decreasing to nothing, and you will note that the
+C.P. moves forward with the decreasing angle.
+
+Now, should some gust or eddy tend to make the surface
+decrease the angle, i.e., dive, then the C.P. moves forward
+and pushes the front of the surface up. Should the surface
+tend to assume too large an angle, then the reverse
+happens--the C.P. moves back and pushes the rear of the
+surface up.
+
+Flat surfaces are, then, theoretically stable longitudinally.
+They are not, however, used, on account of their poor
+lift-drift ratio.
+
+As already explained, cambered surfaces are used, and
+these are longitudinally unstable at those angles of incidence
+producing a reasonable lift-drift ratio, i.e., at angles below:
+about 12 degrees.
+
+A is a cambered surface, attitude approximately vertical,
+moving through the air in the direction M. Obviously the C. P.
+coincides with the transverse centre line of the surface.
+
+With decreasing angles, down to angles of about 30 degrees,
+the C.P. moves forward as in the case of flat surfaces (see B),
+but angles above 30 degrees do not interest us, since they produce
+a very low ratio of lift to drift.
+
+Below angles of about 30 degrees (see C) the dipping front part
+of the surface assumes a negative angle of incidence resulting
+in the DOWNWARD air pressure D, and the more the angle of
+incidence is decreased, the greater such negative angle and its
+resultant pressure D. Since the C.P. is the resultant of all
+the air forces, its position is naturally affected by D, which
+causes it to move backwards. Now, should some gust or
+eddy tend to make the surface decrease its angle of incidence,
+i.e., dive, then the C.P. moves backwards, and, pushing up
+the rear of the surface, causes it to dive the more. Should
+the surface tend to assume too large an angle, then the reverse
+happens; the pressure D decreases, with the result
+that C.P. moves forward and pushes up the front of the surface,
+thus increasing the angle still further, the final result
+being a ``tail-slide.''
+
+It is therefore necessary to find a means of stabilizing
+the naturally unstable cambered surface. This is usually
+secured by means of a stabilizing surface fixed some distance
+in the rear of the main surface, and it is a necessary condition
+that the neutral lift lines of the two surfaces, when projected
+to meet each other, make a dihedral angle. In other words,
+the rear stabilizing surface must have a lesser angle of
+incidence than the main surface--certainly not more than
+one-third of that of the main surface. This is known as the
+longitudinal dihedral.
+
+I may add that the tail-plane is sometimes mounted upon
+the aeroplane at the same angle as the main surface, but,
+in such cases, it attacks air which has received a downward
+deflection from the main surface, thus:
+
+{illust.}
+
+
+The angle at which the tail surface attacks the air (the.
+angle of incidence) is therefore less than the angle of incidence
+of the main surface.
+
+I will now, by means of the following illustration, try
+to explain how the longitudinal dihedral secures stability:
+
+First, imagine the aeroplane travelling in the direction
+of motion, which coincides with the direction of thrust T.
+The weight is, of course, balanced about a C.P., the resultant
+of the C.P. of the main surface and the C.P. of the stabilizing
+surface. For the sake of illustration, the stabilizing surface
+has been given an angle of incidence, and therefore has a
+lift and C.P. In practice the stabilizer is often set at no
+angle of incidence. In such case the proposition remains
+the same, but it is, perhaps, a little easier to illustrate it
+as above.
+
+Now, we will suppose that a gust or eddy throws the
+machine into the lower position. It no longer travels in
+the direction of T, since the momentum in the old direction
+pulls it off that course. M is now the resultant of the Thrust
+and the Momentum, and you will note that this results in a
+decrease in the angle our old friend the neutral lift line makes
+with M, i.e., a decrease in the angle of incidence and therefore
+a decrease in lift.
+
+We will suppose that this decrease is 2 degrees. Such decrease
+applies to both main surface and stabilizer, since both are
+fixed rigidly to the aeroplane.
+
+The main surface, which had 12 degrees angle, has now only
+10 degrees, i.e., a loss of ONE-SIXTH.
+
+The stabilizer, which had 4 degrees angle, has now only 2 degrees,
+i.e., a loss of ONE-HALF.
+
+The latter has therefore lost a greater PROPORTION of its
+angle of incidence, and consequently its lift, than has the
+main surface. It must then fall relative to the main surface.
+The tail falling, the aeroplane then assumes its first position,
+though at a slightly less altitude.
+
+Should a gust throw the nose of the aeroplane up, then
+the reverse happens. Both main surface and stabilizer
+increase their angles of incidence in the same amount, but
+the angle, and therefore the lift, of the stabilizer increases
+in greater proportion than does the lift of the main surface,
+with the result that it lifts the tail. The aeroplane then
+assumes its first position, though at a slightly greater
+altitude.
+
+Do not fall into the widespread error that the angle of
+incidence varies as the angle of the aeroplane to the horizontal.
+It varies with such angle, but not as anything approaching it.
+Remember that the stabilizing effect of the longitudinal
+dihedral lasts only as long as there is momentum in the direction
+of the first course.
+
+These stabilizing movements are taking place all the
+time, even though imperceptible to the pilot.
+
+Aeroplanes have, in the past, been built with a stabilizing
+surface in front of the main surface instead of at the rear of
+it. In such design the main surface (which is then the tail
+surface as well as the principal lifting surface) must be set
+at a less angle than the forward stabilizing surface, in order
+to secure a longitudinal dihedral. The defect of such design
+lies in the fact that the main surface must have a certain angle
+to lift the weight--say 5 degrees. Then, in order to secure a
+sufficiency of longitudinal stability, it is necessary to set the
+forward stabilizer at about 15 degrees. Such a large angle of incidence
+results in a very poor lift-drift ratio (and consequently great
+loss of efficiency), except at very low velocities compared with
+the speed of modern aeroplanes. At the time such aeroplanes
+were built velocities were comparatively low, and this defect
+was; for that reason, not sufficiently appreciated. In the end
+it killed the ``canard'' or ``tail-first'' design.
+
+Aeroplanes of the Dunne and similar types possess no
+stabilizing surface distinct from the main surface, but they
+have a longitudinal dihedral which renders them stable.
+
+The main surface towards the wing-tips is given a
+decreasing angle of incidence and corresponding camber. The
+wing-tips then act as longitudinal stabilizers.
+
+This design of aeroplane, while very interesting, has
+not proved very practicable, owing to the following
+disadvantages: (1) The plan design is not, from a mechanical
+point of view, so sound as that of the ordinary aeroplane
+surface, which is, in plan, a parallelogram. It is, then,
+necessary to make the strength of construction greater than
+would otherwise be the case. That means extra weight.
+(2) The plan of the surface area is such that the aspect ratio
+is not so high as if the surface was arranged with its leading
+edges at right angles to the direction of motion. The lower
+the aspect ratio, then, the less the lift. This design, then,
+produces less lift for weight of surface than would the same
+surface if arranged as a parallelogram. (3) In order to secure
+the longitudinal dihedral, the angle of incidence has to be
+very much decreased towards the wing-tips. Then, in order
+that the lift-drift ratio may be preserved, there must be a
+corresponding decrease in the camber. That calls for surface
+ribs of varying cambers, and results in an expensive and
+lengthy job for the builder. (4) In order to secure directional
+stability, the surface is, in the centre, arranged to dip down
+in the form of a V, pointing towards the direction of motion.
+Should the aeroplane turn off its course, then its momentum
+in the direction of its first course causes it to move in a
+direction the resultant of the thrust and the momentum. It
+then moves in a more or less sideways attitude, which results
+in an air pressure upon one side of the V, and which tends to
+turn the aeroplane back to its first course. This arrangement
+of the surface results in a bad drift. Vertical surfaces at
+the wing-tips may also be set at an angle producing the same
+stabilizing effect, but they also increase the drift.
+
+The gyroscopic action of a rotary engine will affect the
+longitudinal stability when an aeroplane is turned to right
+or left. In the case of a Gnome engine, such gyroscopic
+action will tend to depress the nose of the aeroplane when it
+is turned to the left, and to elevate it when it is turned to
+the right. In modern aeroplanes this tendency is not sufficiently
+important to bother about. In the old days of crudely
+designed and under-powered aeroplanes this gyroscopic action
+was very marked, and led the majority of pilots to dislike
+turning an aeroplane to the right, since, in doing so, there
+was some danger of ``stalling.''
+
+
+LATERAL STABILITY is far more difficult for the designer
+to secure than is longitudinal or directional stability. Some
+degree of lateral stability may be secured by means of the
+``lateral dihedral,'' i.e., the upward inclination of the surface
+towards its wing-tips thus:
+
+Imagine the top V, illustrated opposite, to be the front
+view of a surface flying towards you. The horizontal equivalent
+(H.E.) of the left wing is the same as that of the right
+wing. Therefore, the lift of one wing is equal to the lift
+of the other, and the weight, being situated always in the
+centre, is balanced.
+
+If some movement of the air causes the surface to tilt
+sideways, as in the lower illustration, then you will note that
+the H.E. of the left wing increases, and the H.E. of the right
+wing decreases. The left wing then, having the greatest
+lift, rises; and the surface assumes its first and normal
+position.
+
+Unfortunately however, the righting effect is not proportional
+to the difference between the right and left H.E.'s.
+
+
+
+In the case of A, the resultant direction of the reaction
+of both wings is opposed to the direction of gravity or weight.
+The two forces R R and gravity are then evenly balanced,
+and the surface is in a state of equilibrium.
+
+In the case of B, you will note that the R R is not directly
+opposed to gravity. This results in the appearance of M,
+and so the resultant direction of motion of the aeroplane
+is no longer directly forward, but is along a line the resultant
+of the thrust and M. In other words, it is, while flying
+forward, at the same time moving sideways in the direction M.
+
+In moving sideways, the keel-surface receives, of course,
+a pressure from the air equal and opposite to M. Since
+such surface is greatest in effect towards the tail, then the
+latter must be pushed sideways. That causes the aeroplane
+to turn; and, the highest wing being on the outside of the
+turn, it has a greater velocity than the lower wing. That
+produces greater lift, and tends to tilt the aeroplane over
+still more. Such tilting tendency is, however, opposed by
+the difference in the H.E.'s of the two wings.
+
+It then follows that, for the lateral dihedral angle to
+be effective, such angle must be large enough to produce,
+when the aeroplane tilts, a difference in the H.E.'s of the
+two wings, which difference must be sufficient to not only
+oppose the tilting tendency due to the aeroplane turning,
+but sufficient to also force the aeroplane back to its original
+position of equilibrium.
+
+It is now, I hope, clear to the reader that the lateral
+dihedral is not quite so effective as would appear at first
+sight. Some designers, indeed, prefer not to use it, since its
+effect is not very great, and since it must be paid for in loss
+of H.E. and consequently loss of lift, thus decreasing the lift-
+drift ratio, i.e., the efficiency. Also, it is sometimes advanced
+that the lateral dihedral increases the ``spill'' of air from the
+wing-tips and that this adversely affects the lift-drift ratio.
+
+The disposition of the keel-surface affects the lateral
+stability. It should be, in effect, equally divided by the
+longitudinal turning axis of the aeroplane. If there is an
+excess of keel-surface above or below such axis, then a side
+gust striking it will tend to turn the aeroplane over sideways.
+
+The position of the centre of gravity affects lateral stability.
+If too low, it produces a pendulum effect and causes the
+aeroplane to roll sideways.
+
+If too high, it acts as a stick balanced vertically would
+act. If disturbed, it tends to travel to a position as far as
+possible from its original position. It would then tend,
+when moved, to turn the aeroplane over sideways and into
+an upside-down position.
+
+From the point of view of lateral stability, the best
+position for the centre of gravity is one a little below the
+centre of drift.
+
+Propeller torque affects lateral stability. An aeroplane
+tends to turn over sideways in the opposite direction to which
+the propeller revolves.
+ 
+This tendency is offset by increasing the angle of incidence
+(and consequently the lift) of the side tending to fall; and it
+is always advisable, if practical considerations allow it, to
+also decrease the angle upon the other side. In that way
+it is not necessary to depart so far from the normal angle
+of incidence at which the lift-drift ratio is highest.
+
+Wash-in is the term applied to the increased angle.
+
+Wash-out is the term applied to the decreased angle.
+
+Both lateral and directional stability may be improved
+by washing out the angle of incidence on both sides of the
+surface, thus:
+
+The decreased angle decreases the drift and therefore the
+effect of gusts upon the wing-tips which is just where they
+have the most effect upon the aeroplane, owing to the distance
+from the turning axis.
+
+The wash-out also renders the ailerons (lateral controlling
+services) more effective, as, in order to operate them, it is
+not then necessary to give them such a large angle of incidence
+as would otherwise be required.
+
+
+
+The less the angle of incidence of the ailerons, the better
+their lift-drift ratio, i.e., their efficiency. You will note
+that, while the aileron attached to the surface with washed-out
+angle is operated to the same extent as the aileron illustrated
+above it, its angle of incidence is considerably less. Its efficiency
+is therefore greater.
+
+The advantages of the wash-in must, of course be paid for
+in some loss of lift, as the lift decreases with the decreased angle.
+
+In order to secure all the above described advantages,
+a combination is sometimes effected, thus:
+
+BANKING.--An aeroplane turned off its course to right
+or left does not at once proceed along its new course. Its
+momentum in the direction of its first course causes it to
+travel along a line the resultant of such momentum and the
+thrust. In other words, it more or less skids sideways and
+away from the centre of the turn. Its lifting surfaces do
+not then meet the air in their correct attitude, and the lift
+may fall to such an extent as to become less than the weight,
+in which case the aeroplane must fall. This bad effect is
+minimized by ``banking,'' i.e., tilting the aeroplane sideways.
+The bottom of the lifting surface is in that way opposed to
+the air through which it is moving in the direction of the
+momentum and receives an opposite air pressure. The
+rarefied area over the top of the surface is rendered still more
+rare, and this, of course, assists the air pressure in opposing
+the momentum.
+
+The velocity of the ``skid,'' or sideways movement, is
+then only such as is necessary to secure an air pressure equal
+and opposite to the centrifugal force of the turn.
+
+The sharper the turn, the greater the effect of the centrifugal
+force, and therefore the steeper should be the ``bank.''
+Experentia docet.
+
+The position of the centre of gravity affects banking. A low
+C.G. will tend to swing outward from the centre of the turn,
+and will cause the aeroplane to bank--perhaps too much, in
+which case the pilot must remedy matters by operating the
+ailerons.
+
+A high C.G. also tends to swing outward from the centre
+of the turn. It will tend to make the aeroplane bank the
+wrong way, and such effect must be remedied by means of
+the ailerons.
+
+The pleasantest machine from a banking point of view is
+one in which the C.G. is a little below the centre of drift.
+It tends to bank the aeroplane the right way for the turn,
+and the pilot can, if necessary, perfect the bank by means
+of the ailerons.
+
+The disposition of the keel-surface affects banking. It
+should be, in effect, evenly divided by the longitudinal axis.
+An excess of keel-surface above the longitudinal axis will,
+when banking, receive an air pressure causing the aeroplane
+to bank, perhaps too much. An excess of keel-surface below
+the axis has the reverse effect.
+
+
+SIDE-SLIPPING.--This usually occurs as a result of over-
+banking. It is always the result of the aeroplane tilting
+sideways and thus decreasing the horizontal equivalent, and
+therefore the lift, of the surface. An excessive ``bank,''
+or sideways tilt, results in the H.E., and therefore the lift,
+becoming less than the weight, when, of course, the aeroplane
+must fall, i.e., side-slip.
+
+
+When making a very sharp turn it is necessary to bank
+very steeply indeed. If, at the same time, the longitudinal
+axis of the aeroplane remains approximately horizontal,
+then there must be a fall, and the direction of motion will be
+the resultant of the thrust and the fall as illustrated above
+in sketch A. The lifting surfaces and the controlling surfaces
+are not then meeting the air in the correct attitude,
+with the result that, in addition to falling, the aeroplane
+will probably become quite unmanageable.
+
+The Pilot, however, prevents such a state of affairs from
+happening by ``nosing-down,'' i.e., by operating the rudder
+to turn the nose of the aeroplane downward and towards
+the direction of motion as illustrated in sketch B. This
+results in the higher wing, which is on the outside of the turn,
+travelling with greater velocity, and therefore securing a
+greater reaction than the lower wing, thus tending to tilt
+the aeroplane over still more. The aeroplane is now almost
+upside-down, but its attitude relative to the direction of
+motion is correct and the controlling surfaces are all of them
+working efficiently. The recovery of a normal attitude
+relative to the Earth is then made as illustrated in sketch C.
+
+The Pilot must then learn to know just the angle of bank
+at which the margin of lift is lost, and, if a sharp turn
+necessitates banking beyond that angle, he must ``nose-down.''
+
+In this matter of banking and nosing-down, and, indeed,
+regarding stability and control generally, the golden rule
+for all but very experienced pilots should be: Keep the
+aeroplane in such an attitude that the air pressure is always
+directly in the pilot's face. The aeroplane is then always
+engaging the air as designed to do so, and both lifting and
+controlling surfaces are acting efficiently. The only exception
+to this rule is a vertical dive, and I think that is
+obviously not an attitude for any but very experienced
+pilots to hanker after.
+
+SPINNING.--This is the worst of all predicaments the
+pilot can find himself in. Fortunately it rarely happens.
+
+It is due to the combination of (1) a very steep spiral
+descent of small radius, and (2) insufficiency of keel-surface
+behind the vertical axis, or the jamming of the rudder
+end or elevator into a position by which the aeroplane is forced
+into an increasingly steep and small spiral.
+
+Owing to the small radius of such a spiral, the mass of
+the aeroplane may gain a rotary momentum greater, in effect,
+than the air pressure of the keel-surface or controlling surfaces
+opposed to it; and, when once such a condition occurs,
+it is difficult to see what can be done by the pilot to remedy
+it. The sensible pilot will not go beyond reasonable limits
+of steepness and radius when executing spiral descents.
+
+GLIDING DESCENT WITHOUT PROPELLER THRUST.--All
+aeroplanes are, or should be, designed to assume their gliding
+angle when the power and thrust is cut off. This relieves
+the pilot of work, worry, and danger should he find himself
+in a fog or cloud. The Pilot, although he may not realize
+it, maintains the correct attitude of the aeroplane by observing
+its position relative to the horizon. Flying into a
+fog or cloud the horizon is lost to view, and he must then rely
+upon his instruments--(1) the compass for direction; (2) an
+inclinometer (arched spirit-level) mounted transversely to
+the longitudinal axis, for lateral stability; and (3) an inclinometer
+mounted parallel to the longitudinal axis, or the airspeed
+indicator, which will indicate a nose-down position
+by increase in air speed, and a tail-down position by decrease
+in air speed.
+
+The pilot is then under the necessity of watching three
+instruments and manipulating his three controls to keep the
+instruments indicating longitudinal, lateral, and directional
+stability. That is a feat beyond the capacity of the ordinary
+man. If, however, by the simple movement of throttling
+down the power and thrust, he can be relieved of looking
+after the longitudinal stability, he then has only two instruments
+to watch. That is no small job in itself, but it is,
+at any rate, fairly practicable.
+
+Aeroplanes are, then, designed, or should be, so that the
+centre of gravity is slightly forward of centre of lift. The
+aeroplane is then, as a glider, nose-heavy--and the distance
+the C.G. is placed in advance of the C.L. should be such as
+to ensure a gliding angle producing a velocity the same as
+the normal flying speed (for which the strength of construction
+has been designed).
+
+In order that this nose-heavy tendency should not exist
+when the thrust is working and descent not required, the
+centre of thrust is placed a little below the centre of drift
+or resistance, and thus tends to pull up the nose of the
+aeroplane.
+
+The distance the centre of thrust is placed below the
+centre of drift should be such as to produce a force equal
+and opposite to that due to the C.G. being forward of the
+C.L.
+
+LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired,
+it is best to throttle the engine down at point A. The C.G.
+being forward of the C.P., then causes the aeroplane to nose-
+down, and assists the pilot in making a reasonably small
+loop along the course C and in securing a quick recovery.
+If the engine is not throttled down, then the aeroplane may
+be expected to follow the course D, which results in a longer
+nose dive than in the case of the course C.
+
+A steady, gentle movement of the elevator is necessary.
+A jerky movement may change the direction of motion so
+suddenly as to produce dangerous air stresses upon the surfaces,
+in which case there is a possibility of collapse.
+
+If an upside-down flight is desired, the engine may, or
+may not, be throttled down at point A. If not throttled
+down, then the elevator must be operated to secure a course
+approximately in the direction B. If it is throttled down,
+then the course must be one of a steeper angle than B, or
+there will be danger of stalling.
+
+
+Diagram p. 88.--This is not set at quite
+the correct angle. Path B should slope
+slightly downwards from Position A.
+
+
+
+CHAPTER III
+
+RIGGING
+
+In order to rig an aeroplane intelligently, and to maintain
+it in an efficient and safe condition, it is necessary to possess
+a knowledge of the stresses it is called upon to endure, and
+the strains likely to appear.
+
+
+STRESS is the load or burden a body is called upon to
+bear. It is usually expressed by the result found by dividing
+the load by the number of superficial square inches contained
+in the cross-sectional area of the body.
+
+Thus, if, for instance, the object illustrated above contains
+4 square inches of cross-sectional area, and the total load
+it is called upon to endure is 10 tons, the stress would be
+expressed as 2 1/2 tons.
+
+
+STRAIN is the deformation produced by stress.
+
+
+THE FACTOR OF SAFETY is usually expressed by the result
+found by dividing the stress at which it is known the body
+will collapse, by the maximum stress it will be called upon to
+endure. For instance, if a control wire be called upon to endure
+a maximum stress of 2 cwts., and the known stress at which
+it will collapse is 10 cwts., the factor of safety is then 5.
+
+[cwts. = centerweights = 100 pound units as in cent & century.
+Interestinly enough, this word only exists today in abbreviation
+form, probably of centreweights, but the dictionary entries, even
+from a hundred years ago do not list this as a word, but do list
+c. or C. as the previous popular abbreviation as in Roman Numerals]
+The word listed is "hundredweight.  Michael S. Hart, 1997]
+
+
+COMPRESSION.--The simple stress of compression tends
+to produce a crushing strain. Example: the interplane and
+fuselage struts.
+
+
+TENSION.--The simple stress of tension tends to produce
+the strain of elongation. Example: all the wires.
+
+
+BENDING.--The compound stress of bending is a combination
+of compression and tension.
+
+The above sketch illustrates a straight piece of wood of
+which the top, centre, and bottom lines are of equal length.
+We will now imagine it bent to form a circle, thus:
+
+The centre line is still the same length as before being
+bent; but the top line, being farther from the centre of the
+circle, is now longer than the centre line. That can be due
+only to the strain of elongation produced by the stress of
+tension. The wood between the centre line and the top
+line is then in tension; and the farther from the centre,
+the greater the strain, and consequently the greater the
+tension.
+
+The bottom line, being nearest to the centre of the circle,
+is now shorter than the centre line. That can be due only
+to the strain of crushing produced by the stress of compression.
+The wood between the centre and bottom lines is
+then in compression; and the nearer the centre of the circle,
+the greater the strain, and consequently the greater the
+compression.
+
+It then follows that there is neither tension nor compression,
+i.e., no stress, at the centre line, and that the wood
+immediately surrounding it is under considerably less stress
+than the wood farther away. This being so, the wood in
+the centre may be hollowed out without unduly weakening
+struts and spars. In this way 25 to 33 per cent. is saved in
+the weight of wood in an aeroplane.
+
+The strength of wood is in its fibres, which should, as far
+as possible, run without break from one end of a strut or
+spar to the other end. A point to remember is that the
+outside fibres, being farthest removed from the centre line,
+are doing by far the greatest work.
+
+
+SHEAR STRESS IS such that, when material collapses under it,
+one part slides over the other. Example: all the locking pins.
+
+Some of the bolts are also in shear or ``sideways'' stress,
+owing to lugs under their heads and from which wires are
+taken. Such a wire, exerting a sideways pull upon a bolt,
+tries to break it in such a way as to make one piece of the bolt
+slide over the other piece.
+
+TORSION.--This is a twisting stress compounded of compression,
+tension, and shear stresses. Example: the propeller shaft.
+
+
+NATURE OF WOOD UNDER STRESS.--Wood, for its weight,
+takes the stress of compression far better than any other
+stress. For instance: a walking-stick of less than 1 lb. in
+weight will, if kept perfectly straight, probably stand up to
+a compression stress of a ton or more before crushing; whereas,
+if the same stick is put under a bending stress, it will probably
+collapse to a stress of not more than about 50 lb. That is
+a very great difference, and, since weight is of the greatest
+importance, the design of an aeroplane is always such as to,
+as far as possible, keep the various wooden parts of its
+construction in direct compression. Weight being of such vital
+importance, and designers all trying to outdo each other in
+saving weight, it follows that the factor of safety is rather
+low in an aeroplane. The parts in direct compression will,
+however, take the stresses safely provided the following
+conditions are carefully observed.
+
+CONDITIONS TO BE OBSERVED:
+
+
+1. All the spars and struts must be perfectly straight.
+
+The above sketch illustrates a section through an
+interplane strut. If the strut is to be kept straight,
+i.e., prevented from bending, then the stress of
+compression must be equally disposed about the
+centre of strength. If it is not straight, then
+there will be more compression on one side of the
+centre of strength than on the other side. That
+is a step towards getting compression on one side
+and tension on the other side, in which case it
+may be forced to take a bending stress for which
+it is not designed. Even if it does not collapse
+it will, in effect, become shorter, and thus throw
+out of adjustment the gap and all the wires attached
+to the top and bottom of the strut, with the result
+that the flight efficiency of the aeroplane will be
+spoiled.
+
+The only exception to the above condition is
+what is known as the Arch. For instance, in the
+case of the Maurice Farman, the spars of the centre-
+section plane, which have to take the weight of
+the nacelle, are arched upwards. If this was not
+done, it is possible that rough landings might
+result in the weight causing the spars to become
+slightly distorted downwards. That would produce
+a dangerous bending stress, but, as long as
+the wood is arched, or, at any rate, kept from
+bending downwards, it will remain in direct
+compression and no danger can result.
+
+
+2. Struts and spars must be symmetrical. By that I mean
+that the cross-sectional dimensions must be correct,
+as otherwise there will be bulging places on the
+outside, with the result that the stress will not be
+evenly disposed about the centre of strength, and
+a bending stress may be produced.
+
+
+3. Struts, spars, etc., must be undamaged. Remember
+that, from what I have already explained about
+bending stresses, the outside fibres of the wood are
+doing by far the most work. If these get bruised
+or scored, then the strut or spar suffers in strength
+much more than one might think at first sight;
+and, if it ever gets a tendency to bend, it is likely
+to collapse at that point.
+
+
+4. The wood must have a good, clear grain with no cross-
+grain, knots, or shakes. Such blemishes produce
+weak places and, if a tendency to bend appears,
+then it may collapse at such a point.
+
+5. The struts, spars, etc., must be properly bedded into
+their sockets or fittings. To begin with, they must
+be of good pushing or gentle tapping fit. They
+must never be driven in with a heavy hammer.
+Then again, a strut must bed well down all over its
+cross-sectional area as illustrated above; otherwise
+the stress of compression will not be evenly disposed
+about the centre of strength, and that may
+produce a bending stress. The bottom of the strut
+or spar should be covered with some sort of
+paint, bedded into the socket or fitting, and then
+withdrawn to see if the paint has stuck all over the
+bed.
+
+
+6. The atmosphere is sometimes much damper than at
+other times, and this causes wood to expand and
+contract appreciably. This would not matter but
+for the fact that it does not expand and contract
+uniformly, but becomes unsymmetrical, i.e., distorted.
+I have already explained the danger of that in
+condition 2. This should be minimized by WELL
+VARNISHING THE WOOD to keep the moisture out of it.
+
+
+FUNCTION OF INTERPLANE STRUTS.--These struts have to
+keep the lifting surfaces or ``planes'' apart, but this is only
+part of their work. They must keep the planes apart, so
+that the latter are in their correct attitude. That is only so
+when the spars of the bottom plane are parallel with those of
+the top plane. Also, the chord of the top plane must be
+parallel with the chord of the bottom plane. If that is not
+so, then one plane will not have the same angle of incidence
+as the other one. At first sight one might think that all
+that is necessary is to cut all the struts to be the same length,
+but that is not the case.
+
+Sometimes, as illustrated above, the rear spar is not so
+thick as the main spar, and it is then necessary to make
+up for that difference by making the rear struts correspondingly
+longer. If that is not done, then the top and
+bottom chords will not be parallel, and the top and bottom
+planes will have different angles of incidence. Also, the
+sockets or fittings, or even the spars upon which they are
+placed, sometimes vary in thickness owing to faulty manufacture.
+This must be offset by altering the length of the
+struts. The best way to proceed is to measure the distance
+between the top and bottom spars by the side of each strut,
+and if that distance, or ``gap'' as it is called, is not as stated
+in the aeroplane's specifications, then make it correct by
+changing the length of the strut. This applies to both front
+and rear interplane struts. When measuring the gap, always
+be careful to measure from the centre of the spar, as it may
+be set at an angle, and the rear of it may be considerably
+lower than its front.
+
+
+BORING HOLES IN WOOD.--It should be a strict rule that
+no spar be used which has an unnecessary hole in it. Before
+boring a hole, its position should be confirmed by whoever
+is in charge of the workshop. A bolt-hole should be of a size
+to enable the bolt to be pushed in, or, at any rate, not more
+than gently tapped in. Bolts should not be hammered in, as
+that may split the spar. On the other hand, a bolt should not
+be slack in its hole, as, in such a case, it may work sideways and
+split the spar, not to speak of throwing out of adjustment
+the wires leading from the lug or socket under the bolt-head.
+
+
+WASHERS.--Under the bolt-head, and also under the nut,
+a washer must be placed--a very large washer compared
+with the size which would be used in all-metal construction.
+This is to disperse the stress over a large area; otherwise
+the washer may be pulled into the wood and weaken it,
+besides possibly throwing out of adjustment the wires
+attached to the bolt or the fitting it is holding to the spar.
+
+
+LOCKING.--Now as regards locking the bolts. If split
+pins are used, be sure to see that they are used in such a way
+that the nut cannot possibly unscrew at all. The split pin
+should be passed through the bolt as near as possible to the
+nut. It should not be passed through both nut and bolt.
+
+If it is locked by burring over the edge of the bolt, do not
+use a heavy hammer and try to spread the whole head of
+the bolt. That might damage the woodwork inside the
+fabric-covered surface. Use a small, light hammer, and gently
+tap round the edge of the bolt until it is burred over.
+
+
+TURNBUCKLES.--A turnbuckle is composed of a central
+barrel into each end of which is screwed an eye-bolt. Wires
+are taken from the eyes of the eye-bolt, and so, by turning
+the barrel, they can be adjusted to their proper tension.
+Eye-bolts must be a good fit in the barrel; that is to say,
+not slack and not very tight. Theoretically it is not neces-
+sary to screw the eye-bolt into the barrel for a distance
+greater than the diameter of the bolt, but, in practice, it is
+better to screw it in for a considerably greater distance than
+that if a reasonable degree of safety is to be secured.
+
+Now about turning the barrel to secure the right adjustment.
+The barrel looks solid, but, as a matter of fact, it
+is hollow and much more frail than it appears. For that
+reason it should not be turned by seizing it with pliers, as
+that may distort it and spoil the bore within it. The best
+method is to pass a piece of wire through the hole in its centre,
+and to use that as a lever. When the correct adjustment
+has been secured, the turnbuckle must be locked to prevent
+it from unscrewing. It is quite possible to lock it in such a
+way as to allow it to unscrew a quarter or a half turn, and
+that would throw the wires out of the very fine adjustment
+necessary. The proper way is to use the locking wire so
+that its direction is such as to oppose the tendency of the
+barrel to unscrew, thus:
+
+
+WIRES.--The following points should be carefully observed
+where wire is concerned:
+
+1. Quality.--It must not be too hard or too soft. An
+easy practical way of learning to know the approximate
+quality of wire is as follows:
+
+Take three pieces, all of the same gauge, and each about a
+foot in length. One piece should be too soft, another too hard,
+and the third piece of the right quality. Fix them in a vice,
+about an inch apart and in a vertical position, and with the light
+from a window shining upon them. Burnish them if necessary,
+and you will see a band of light reflected from each
+wire.
+
+Now bend the wires over as far as possible and away from
+the light. Where the soft wire is concerned, it will squash
+out at the bend, and this will be indicated by the band of
+light, which will broaden at that point. In the case of the
+wire which is too hard, the band of light will broaden very
+little at the turn, but, if you look carefully, you will see some
+little roughnesses of surface. In the case of the wire of the
+right quality, the band of light may broaden a very little
+at the turn, but there will be no roughnesses of surface.
+
+By making this experiment two or three times one can
+soon learn to know really bad wire from good, and also learn
+to know the strength of hand necessary to bend the right
+quality.
+
+2. It must not be damaged. That is to say, it must be
+unkinked, rustless, and unscored.
+
+3. Now as regards keeping wire in good condition. Where
+outside wires are concerned, they should be kept WELL GREASED
+OR OILED, especially where bent over at the ends. Internal
+bracing wires cannot be reached for the purpose of regreasing
+them, as they are inside fabric-covered surfaces. They should
+be prevented from rusting by being painted with an anti-rust
+mixture. Great care should be taken to see that the wire
+is perfectly clean and dry before being painted. A greasy
+finger-mark is sufficient to stop the paint from sticking to
+the wire. In such a case there will be a little space between
+the paint and the wire. Air may enter there and cause the
+wire to rust.
+
+4. Tension of Wires.--The tension to which the wires are
+adjusted is of the greatest importance. All the wires should
+be of the same tension when the aeroplane is supported in
+such a way as to throw no stress upon them. If some wires
+are in greater tension than others, the aeroplane will quickly
+become distorted and lose its efficiency.
+
+In order to secure the same tension of all wires, the aeroplane,
+when being rigged, should be supported by packing
+underneath the lower surfaces as well as by packing underneath
+the fuselage or nacelle. In this way the anti-lift wires
+are relieved of the weight, and there is no stress upon any
+of the wires.
+
+As a general rule the wires of an aeroplane are tensioned
+too much. The tension should be sufficient to keep the
+framework rigid. Anything more than that lowers the factor
+of safety, throws various parts of the framework into undue
+compression, pulls the fittings into the wood, and will, in
+the end, distort the whole framework of the aeroplane.
+
+Only experience will teach the rigger what tension to
+employ. Much may be done by learning the construction
+of the various types of aeroplanes, the work the various
+parts do, and in cultivating a touch for tensioning wires by
+constantly handling them.
+
+5. Wires with no Opposition Wires.--In some few cases
+wires will be found which have no opposition wires pulling
+in the opposite direction. For instance, an auxiliary lift
+wire may run from the bottom of a strut to a spar in the top
+plane at a point between struts. In such a case great care
+should be taken not to tighten the wire beyond barely taking
+up the slack.
+
+Such a wire must be a little slack, or, as illustrated above,
+it will distort the framework. That, in the example given,
+will spoil the camber (curvature) of the surface, and result
+in changing both the lift and the drift at that part of the surface.
+Such a condition will cause the aeroplane to lose its
+directional stability and also to fly one wing down.
+
+I cannot impress this matter of tension upon the reader
+too strongly. It is of the utmost importance. When this,
+and also accuracy in securing the various adjustments, has
+been learned, one is on the way to becoming a good
+rigger.
+
+6. Wire Loops.--Wire is often bent over at its end in the
+form of a loop, in order to connect with a turnbuckle or
+fitting. These loops, even when made as perfectly as possible,
+have a tendency to elongate, thus spoiling the adjustment
+of the wires Great care should be taken to minimize this
+as far as possible. The rules to be observed are as
+follows:
+
+(a) The size of the loop should be as small as possible
+within reason. By that I mean it should not be
+so small as to create the possibility of the wire
+breaking.
+
+
+(b) The shape of the loop should be symmetrical.
+
+
+(c) It should have well-defined shoulders in order to
+prevent the ferrule from slipping up. At the same
+time, a shoulder should not have an angular place.
+
+
+(d) When the loop is finished it should be undamaged,
+and it should not be, as is often the case, badly scored.
+
+
+7. Stranded Wire Cable.--No splice should be served with
+twine until it has been inspected by whoever is in charge of
+the workshop. The serving may cover bad work.
+
+Should a strand become broken, then the cable should be
+replaced at once by another one.
+
+Control cables have a way of wearing out and fraying
+wherever they pass round pulleys. Every time an aeroplane
+comes down from flight the rigger should carefully examine
+the cables, especially where they pass round pulleys. If
+he finds a strand broken, he should replace the cable.
+
+The ailerons' balance cable on the top of the top plane
+is often forgotten, since it is necessary to fetch a high pair
+of steps in order to examine it. Don't slack this, or some
+gusty day the pilot may unexpectedly find himself minus the
+aileron control.
+
+CONTROLLING SURFACES.--The greatest care should be
+exercised in rigging the aileron, rudder, and elevator properly,
+for the pilot entirely depends upon them in managing the
+aeroplane.
+
+The ailerons and elevator should be rigged so that, when
+the aeroplane is in flight, they are in a fair true line with the
+surface in front and to which they are hinged.
+
+If the surface to which they are hinged is not a lifting
+surface, then they should be rigged to be in a fair true line
+with it as illustrated above.
+
+If the controlling surface is, as illustrated, hinged to the
+back of a lifting surface, then it should be rigged a little below
+the position it would occupy if in a fair true line with the
+surface in front. This is because, in such a case, it is set
+at an angle of incidence. This angle will, during flight,
+cause it to lift a little above the position in which it has been
+rigged. It is able to lift owing to a certain amount of slack
+in the control wire holding it--and one cannot adjust the
+control wire to have no slack, because that would cause it
+to bind against the pulleys and make the operation of it too
+hard for the pilot. It is therefore necessary to rig it a little
+below the position it would occupy if it was rigged in a fair
+true line with the surface in front. Remember that this
+only applies when it is hinged to a lifting surface. The
+greater the angle of incidence (and therefore the lift) of the
+surface in front, then the more the controlling surface will
+have to be rigged down.
+
+As a general rule it is safe to rig it down so that its trailing
+
+edge is 1/2 to 3/4 inch below the position it would occupy if in
+a fair line with the surface in front; or about 1/2 inch down for
+every 18 inches of chord of the controlling surface.
+
+When making these adjustments the pilot's control levers
+should be in their neutral positions. It is not sufficient
+to lash them. They should be rigidly blocked into position
+with wood packing.
+
+The surfaces must not be distorted in any way. If
+they are held true by bracing wires, then such wires must be
+carefully adjusted. If they are distorted and there are no
+bracing wires with which to true them, then some of the
+internal framework will probably have to be replaced.
+
+The controlling surfaces should never be adjusted with
+a view to altering the stability of the aeroplane. Nothing
+can be accomplished in that way. The only result will be
+to spoil the control of the aeroplane.
+
+
+FABRIC-COVERED SURFACES.--First of all make sure
+that there is no distortion of spars or ribs, and that they are
+perfectly sound. Then adjust the internal bracing wires
+so that the ribs are parallel to the direction of flight. The
+ribs usually cause the fabric to make a ridge where they occur,
+and, if such ridge is not parallel to the direction of flight,
+it will produce excessive drift. As a rule the ribs are at
+right angles to both main and rear spars.
+
+The tension of the internal bracing wires should be just
+sufficient to give rigidity to the framework. They should
+not be tensioned above that unless the wires are, at their
+ends, bent to form loops. In that case a little extra tension
+may be given to offset the probable elongation of the
+loops.
+
+The turnbuckles must now be generously greased, and
+served round with adhesive tape. The wires must be rendered
+perfectly dry and clean, and then painted with an anti-rust
+mixture. The woodwork must be well varnished.
+
+If it is necessary to bore holes in the spars for the purpose
+of receiving, for instance, socket bolts, then their places
+should be marked before being bored and their positions
+confirmed by whoever is in charge of the workshop. All is
+now ready for the sail-maker to cover the surface with
+fabric.
+
+
+ADJUSTMENT OF CONTROL CABLES.--The adjustment of
+the control cables is quite an art, and upon it will depend to
+a large degree the quick and easy control of the aeroplane
+by the pilot.
+
+The method is as follows:
+
+After having rigged the controlling surfaces, and as far
+as possible secured the correct adjustment of the control
+cables, then remove the packing which has kept the control
+levers rigid. Then, sitting in the pilot's seat, move the
+control levers SMARTLY. Tension the control cables so that
+when the levers are smartly moved there is no perceptible
+snatch or lag. Be careful not to tension the cables more than
+necessary to take out the snatch. If tensioned too much
+they will (1) bind round the pulleys and result in hard work
+for the pilot; (2) throw dangerous stresses upon the controlling
+surfaces, which are of rather flimsy construction; and (3)
+cause the cables to fray round the pulleys quicker than would
+otherwise be the case.
+
+Now, after having tensioned the cables sufficiently to
+take out the snatch, place the levers in their neutral positions,
+and move them to and fro about 1/8 inch either side of such
+positions. If the adjustment is correct, it should be possible
+to see the controlling surfaces move. If they do not move,
+then the control cables are too slack.
+
+
+FLYING POSITION.--Before rigging an aeroplane or making
+any adjustments it is necessary to place it in what is known
+as its ``flying position.'' I may add that it would be better
+termed its ``rigging position.''
+
+In the case of an aeroplane fitted with a stationary engine
+this is secured by packing up the machine so that the engine
+foundations are perfectly horizontal both longitudinally and
+laterally. This position is found by placing a straight-edge
+and a spirit-level across the engine foundations (both
+longitudinally and laterally), and great care should be taken to
+see that the bubble is exactly in the centre of the level. The
+slightest error will assume magnitude towards the extremities
+of the aeroplane. Great care should be taken to block up
+the aeroplane rigidly. In case it gets accidentally disturbed
+while the work is going on, it is well to constantly verify the
+flying position by running the straight-edge and spirit-level
+over the engine foundations. The straight-edge should be
+carefully tested before being used, as, being generally made of
+wood, it will not remain true long. Place it lightly in a vice,
+and in such a position that a spirit-level on top shows the
+bubble exactly in the centre. Now slowly move the level
+along the straight-edge, and the bubble should remain exactly
+in the centre. If it does not do so, then the straight-edge
+is not true and must be corrected. THIS SHOULD NEVER BE
+OMITTED.
+
+In the case of aeroplanes fitted with engines of the rotary
+type, the ``flying position'' is some special attitude laid
+down in the aeroplane's specifications, and great care should
+be taken to secure accuracy.
+
+
+ANGLE OF INCIDENCE.--One method of finding the angle
+of incidence is as follows:
+
+First place the aeroplane in its flying position. The
+corner of the straight-edge must be placed underneath and
+against the CENTRE of the rear spar, and held in a horizontal
+position parallel to the ribs. This is secured by using a
+spirit-level. The set measurement will then be from the
+top of the straight-edge to the centre of the bottom surface
+of the main spar, or it may be from the top of the straight-
+edge to the lowest part of the leading edge. Care should be
+taken to measure from the centre of the spar and to see that
+the bubble is exactly in the centre of the level. Remember
+that all this will be useless if the aeroplane has not been placed
+accurately in its flying position.
+
+This method of finding the angle of incidence must be
+used under every part of the lower surface where struts
+occur. It should not be used between the struts, because,
+in such places, the spars may have taken a slight permanent
+set up or down; not, perhaps, sufficiently bad to make any
+material difference to the flying of the machine, but quite bad
+enough to throw out the angle of incidence, which cannot
+be corrected at such a place.
+
+If the angle is wrong, it should then be corrected as follows:
+
+If it is too great, then the rear spar must be warped up
+until it is right, and this is done by slackening ALL the wires
+going to the top of the strut, and then tightening ALL the
+wires going to the bottom of the strut.
+
+If the angle is too small, then slacken ALL the wires going
+to the bottom of the strut, and tighten ALL the wires going to
+the top of the strut, until the correct adjustment is secured.
+
+Never attempt to adjust the angle by warping the main spar.
+
+The set measurement, which is of course stated in the
+aeroplane's specifications, should be accurate to 1/16 inch.
+
+
+LATERAL DIHEDRAL ANGLE.--One method of securing
+this is as follows, and this method will, at the same time,
+secure the correct angle of incidence:
+
+The strings, drawn very tight, must be taken over both
+the main and rear spars of the top surface. They must run
+between points on the spars just inside the outer struts.
+The set measurement (which should be accurate to 1/16 inch
+or less) is then from the strings down to four points on the
+main and rear spars of the centre-section surface. These
+points should be just inside the four centre-section struts;
+that is to say, as far as possible away from the centre of the
+centre-section. Do not attempt to take the set measurement
+near the centre of the centre-section.
+
+The strings should be as tight as possible, and, if it can
+be arranged, the best way to accomplish that is as shown in
+the above illustration, i.e., by weighting the strings down to
+the spars by means of weights and tying each end of the strings
+to a strut. This will give a tight and motionless string.
+
+However carefully the above adjustment is made, there is
+sure to be some slight error. This is of no great importance,
+provided it is divided equally between the left- and right-
+hand wings. In order to make sure of this, certain check
+measurements should be taken as follows:
+
+Each bay must be diagonally measured, and such measurements
+must be the same to within 1/16 inch on each side of
+the aeroplane. As a rule such diagonal measurements are
+taken from the bottom socket of one strut to the top socket
+of another strut, but this is bad practice, because of possible
+inaccuracies due to faulty manufacture.
+
+The points between which the diagonal measurements
+are taken should be at fixed distances from the butts of the
+spars, such distances being the same on each side of the
+aeroplane, thus:
+
+
+It would be better to use the centre line of the aeroplane
+rather than the butts of the spars. It is not practicable
+to do so, however, as the centre line probably runs through
+the petrol tanks, etc.
+
+
+THE DIHEDRAL BOARD.--Another method of securing
+the dihedral angle, and also the angle of incidence, is by
+means of the dihedral board. It is a light handy thing to
+use, but leads to many errors, and should not be used unless
+necessary. The reasons are as follows:
+
+The dihedral board is probably not true. If it must be
+used, then it should be very carefully tested for truth before-
+hand. Another reason against its use is that it has to be
+placed on the spars in a position between the struts, and
+that is just where the spars may have a little permanent
+set up or down, or some inaccuracy of surface which will,
+of course, throw out the accuracy of the adjustment. The
+method of using it is as follows:
+
+The board is cut to the same angle as that specified for
+the upward inclination of the surface towards its wing-
+tips. It is placed on the spar as indicated above, and it
+is provided with two short legs to raise it above the flanges
+of the ribs (which cross over the spars), as they may vary
+in depth. A spirit-level is then placed on the board, and the
+wires must be adjusted to give the surface such an inclination
+as to result in the bubble being in the centre of the level.
+This operation must be performed in respect of each bay
+both front and rear. The bays must then be diagonally
+measured as already explained.
+
+
+YET ANOTHER METHOD of finding the dihedral angle,
+and at the same time the angle of incidence, is as follows:
+
+A horizontal line is taken from underneath the butt of
+each spar, and the set measurement is either the angle it makes
+with the spar, or a fixed measurement from the line to the
+spar taken at a specified distance from the butt. This operation
+must be performed in respect of both main and rear
+spars, and all the bays must be measured diagonally afterwards.
+
+
+
+Whichever method is used, be sure that after the job is
+done the spars are perfectly straight.
+
+
+STAGGER.--The stagger is the distance the top surface
+is in advance of the bottom surface when the aeroplane
+is in flying position. The set measurement is obtained as
+follows:
+
+Plumb-lines must be dropped over the leading edge of
+the top surface wherever struts occur, and also near the
+fuselage. The set measurement is taken from the front of the
+lower leading edge to the plumb-lines. It makes a difference
+whether the measurement is taken along a horizontal line
+(which can be found by using a straight-edge and a spirit-
+level) or along a projection of the chord. The line along
+which the measurement should be taken is laid down in the
+aeroplane's specifications.
+
+If a mistake is made and the measurement taken along
+the wrong line, it may result in a difference of perhaps 1/4
+will, in flight, be nose-heavy or tail-heavy.
+
+After the adjustments of the angles of incidence, dihedral,
+and stagger have been secured, it is as well to confirm all of
+them, as, in making the last adjustment, the first one may
+have been spoiled.
+
+
+OVER-ALL ADJUSTMENTS.--The following over-all check
+measurements should now be taken.
+
+The straight lines AC and BC should be equal to within
+1/8 inch. The point C is the centre of the propeller, or, in the
+case of a ``pusher'' aeroplane, the centre of the nacelle.
+The points A and B are marked on the main spar, and must
+in each case be the same distance from the butt of the spar.
+The rigger should not attempt to make A and B merely the
+sockets of the outer struts, as they may not have been placed
+quite accurately by the manufacturer. The lines AC and BC
+must be taken from both top and bottom spars--two measurements
+on each side of the aeroplane.
+
+The two measurements FD and FE should be equal to
+within 1/8 inch. F is the centre of the fuselage or rudder-
+post. D and E are points marked on both top and bottom
+rear spars, and each must be the same fixed distance from
+the butt of the spar. Two measurements on each side of the
+aeroplane.
+
+If these over-all measurements are not correct, then it
+is probably due to some of the drift or anti-drift wires being
+too tight or too slack. It may possibly be due to the fuselage
+being out of truth, but of course the rigger should have made
+quite sure that the fuselage was true before rigging the rest
+of the machine. Again, it may be due to the internal bracing
+wires within the lifting surfaces not being accurately adjusted,
+but of course this should have been seen to before covering the
+surfaces with fabric.
+
+
+FUSELAGE.--The method of truing the fuselage is laid
+down in the aeroplane's specifications. After it has been
+adjusted according to the specified directions, it should then
+be arranged on trestles in such a way as to make about three-
+quarters of it towards the tail stick out unsupported. In
+this way it will assume a condition as near as possible to
+flying conditions, and when it is in this position the set
+measurements should be confirmed. If this is not done it
+may be out of truth, but perhaps appear all right when
+supported by trestles at both ends, as, in such case, its
+weight may keep it true as long as it is resting upon the
+trestles.
+
+
+THE TAIL-PLANE (EMPENNAGE).--The exact angle of
+incidence of the tail-plane is laid down in the aeroplane's
+specifications. It is necessary to make sure that the spars
+are horizontal when the aeroplane is in flying position and
+the tail unsupported as explained above under the heading
+of Fuselage. If the spars are tapered, then make sure that
+their centre lines are horizontal.
+
+
+UNDERCARRIAGE.--The undercarriage must be very carefully
+aligned as laid down in the specifications.
+
+1. The aeroplane must be placed in its flying position
+and sufficiently high to ensure the wheels being off the ground
+when rigged. When in this position the axle must be hori-
+
+nontal and the bracing wires adjusted to secure the various
+set measurements stated in the specifications.
+
+2. Make sure that the struts bed well down into their
+sockets.
+
+3. Make sure that the shock absorbers are of equal
+tension. In the case of rubber shock absorbers, both the
+number of turns and the lengths must be equal.
+
+
+HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.
+
+
+DIRECTIONAL STABILITY will be badly affected if there is
+more drift (i.e., resistance) on one side of the aeroplane than
+there is on the other side. The aeroplane will tend to turn
+towards the side having the most drift. This may be caused
+as follows:
+
+1. The angle of incidence of the main surface or the tail
+surface may be wrong. The greater the angle of incidence,
+the greater the drift. The less the angle, the less the drift.
+
+2. If the alignment of the fuselage, fin in front of the
+rudder, the struts or stream-line wires, or, in the case of
+the Maurice Farman, the front outriggers, are not absolutely
+correct--that is to say, if they are turned a little to the
+left or to the right instead of being in line with the direction
+of flight--then they will act as a rudder and cause the aeroplane
+to turn off its course.
+
+3. If any part of the surface is distorted, it will cause
+the aeroplane to turn off its course. The surface is cambered,
+i.e., curved, to pass through the air with the least possible
+drift. If, owing perhaps to the leading edge, spars, or trailing
+edge becoming bent, the curvature is spoiled, that will
+result in changing the amount of drift on one side of the aeroplane,
+which will then have a tendency to turn off its course.
+
+
+LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible
+reason for such a condition is a difference in the lifts
+of right and left wings. That may be caused as follows:
+
+1. The angle of incidence may be wrong. If it is too
+great, it will produce more lift than on the other side of the
+aeroplane; and if too small, it will produce less lift than on
+the other side--the result being that, in either case, the aeroplane
+will try to fly one wing down.
+
+2. Distorted Surfaces.--If some part of the surface is
+distorted, then its camber is spoiled, and the lift will not be
+the same on both sides of the aeroplane, and that, of course,
+will cause it to fly one wing down.
+
+
+LONGITUDINAL INSTABILITY may be due to the following reasons:
+
+1. The stagger may be wrong. The top surface may have
+drifted back a little owing to some of the wires, probably
+the incidence wires, having elongated their loops or having
+pulled the fittings into the wood. If the top surface is not
+staggered forward to the correct degree, then consequently
+the whole of its lift is too far back, and it will then have a
+tendency to lift up the tail of the machine too much. The
+aeroplane would then be said to be ``nose-heavy.''
+
+A 1/4-inch area in the stagger will make a very considerable
+difference to the longitudinal stability.
+
+2. If the angle of incidence of the main surface is not right,
+it will have a bad effect, especially in the case of an aeroplane
+with a lifting tail-plane.
+
+If the angle is too great, it will produce an excess of lift,
+and that may lift up the nose of the aeroplane and result in
+a tendency to fly ``tail-down.'' If the angle is too small,
+it will produce a decreased lift, and the aeroplane may have a
+tendency to fly ``nose-down.''
+
+3. The fuselage may have become warped upward or
+downward, thus giving the tail-plane an incorrect angle of
+incidence. If it has too much angle, it will lift too much,
+and the aeroplane will be ``nose-heavy.'' If it has too little
+angle, then it will not lift enough, and the aeroplane will be
+``tail-heavy.''
+
+4. (The least likely reason.) The tail-plane may be
+mounted upon the fuselage at a wrong angle of incidence,
+in which case it must be corrected. If nose-heavy, it should
+be given a smaller angle of incidence. If tail-heavy, it should
+be given a larger angle; but care should be taken not to give
+it too great an angle, because the longitudinal stability
+entirely depends upon the tail-plane being set at a much
+smaller angle of incidence than is the main surface, and if
+that difference is decreased too much, the aeroplane will
+become uncontrollable longitudinally. Sometimes the tail-
+plane is mounted on the aeroplane at the same angle as the
+main surface, but it actually engages the air at a lesser angle,
+owing to the air being deflected downwards by the main
+surface. There is then, in effect, a longitudinal dihedral
+as explained and illustrated in Chapter I.
+
+
+CLIMBS BADLY.--Such a condition is, apart from engine
+or propeller trouble, probably due to (1) distorted surfaces,
+or (2) too small an angle of incidence.
+
+
+FLIGHT SPEED POOR.--Such a condition is, apart from
+engine or propeller trouble, probably due to (1) distorted
+surfaces, (2) too great an angle of incidence, or (3) dirt or
+mud, and consequently excessive skin-friction.
+
+
+INEFFICIENT CONTROL is probably due to (1) wrong setting
+of control surfaces, (2) distortion of control surfaces, or
+(3) control cables being badly tensioned.
+
+
+WILL NOT TAXI STRAIGHT.--If the aeroplane is uncontrollable
+on the ground, it is probably due to (1) alignment
+of undercarriage being wrong, or (2) unequal tension of shock
+absorbers.
+
+
+
+CHAPTER IV
+
+THE PROPELLER, OR ``AIR-SCREW''
+
+The sole object of the propeller is to translate the power
+of the engine into thrust.
+
+The propeller screws through the air, and its blades, being
+set at an angle inclined to the direction of motion, secure
+a reaction, as in the case of the aeroplane's lifting surface.
+
+This reaction may be conveniently divided into two
+component parts or values, namely, Thrust and Drift.
+
+The Thrust is opposed to the Drift of the aeroplane, and
+must be equal and opposite to it at flying speed. If it falls
+off in power, then the flying speed must decrease to a velocity,
+at which the aeroplane drift equals the decreased thrust.
+
+The Drift of the propeller may be conveniently divided
+into the following component values:
+
+
+Active Drift, produced by the useful thrusting part of the propeller.
+
+
+Passive Drift, produced by all the rest of the propeller,
+i.e., by its detrimental surface.
+
+
+Skin Friction, produced by the friction of the air with
+roughnesses of surface.
+
+
+Eddies attending the movement of the air caused by
+the action of the propeller.
+
+
+Cavitation (very marked at excessive speed of revolution).
+A tendency of the propeller to produce a
+cavity or semi-vacuum in which it revolves, the
+thrust decreasing with increase of speed and
+cavitation.
+
+
+THRUST-DRIFT RATIO.--The proportion of thrust to drift
+is of paramount importance, for it expresses the efficiency
+of the propeller. It is affected by the following factors:
+Speed of Revolution.--The greater the speed, the greater
+the proportion of drift to thrust. This is due to
+the increase with speed of the passive drift, which
+carries with it no increase in thrust. For this
+reason propellers are often geared down to revolve
+at a lower speed than that of the engine.
+
+
+Angle of Incidence.--The same reasons as in the case of
+the aeroplane surface.
+
+Surface Area.--Ditto.
+
+Aspect Ratio.--Ditto.
+
+Camber.--Ditto.
+
+In addition to the above factors there are, when it comes
+to actually designing a propeller, mechanical difficulties to
+consider. For instance, the blades must be of a certain
+strength and consequent thickness. That, in itself, limits
+the aspect ratio, for it will necessitate a chord long enough
+in proportion to the thickness to make a good camber possible.
+Again, the diameter of the propeller must be limited, having
+regard to the fact that greater diameters than those used
+to-day would not only result in excessive weight of construction,
+but would also necessitate a very high undercarriage
+to keep the propeller off the ground, and such undercarriage
+would not only produce excessive drift, but would also tend
+to make the aeroplane stand on its nose when alighting.
+The latter difficulty cannot be overcome by mounting the
+propeller higher, as the centre of its thrust must be approximately
+coincident with the centre of aeroplane drift.
+
+
+MAINTENANCE OF EFFICIENCY.
+
+
+The following conditions must be observed:
+
+
+1. PITCH ANGLE.--The angle, at any given point on the
+propeller, at which the blade is set is known as the pitch
+angle, and it must be correct to half a degree if reasonable
+efficiency is to be maintained.
+
+This angle secures the ``pitch,'' which is the distance the
+propeller advances during one revolution, supposing the air
+to be solid. The air, as a matter of fact, gives back to the
+thrust of the blades just as the pebbles slip back as one
+ascends a shingle beach. Such ``give-back'' is known as
+Slip. If a propeller has a pitch of, say, 10 feet, but actually
+advances, say, only 8 feet owing to slip, then it will be said
+to possess 20 per cent. slip.
+
+Thus, the pitch must equal the flying speed of the
+aeroplane plus the slip of the propeller. For example,
+let us find the pitch of a propeller, given the following
+conditions:
+          Flying speed .............. 70 miles per hour.
+          Propeller revolutions ..... 1,200 per minute.
+          Slip ...................... 15 per cent.
+
+First find the distance in feet the aeroplane will travel
+forward in one minute. That is--
+
+   369,600 feet (70 miles)
+   ------------------------ = 6,160 feet per minute.
+      60     `` (minutes)
+
+
+Now divide the feet per minute by the propeller revolutions
+per minute, add 15 per cent. for the slip, and the result
+will be the propeller pitch:
+
+     6,160 
+     ----- + 15 per cent. = 5 feet 1 3/5 inches.
+     1,200
+
+
+In order to secure a constant pitch from root to tip of
+blade, the pitch angle decreases towards the tip. This is
+necessary, since the end of the blade travels faster than its
+root, and yet must advance forward at the same speed as
+the rest of the propeller. For example, two men ascending
+a hill. One prefers to walk fast and the other slowly, but they
+wish to arrive at the top of the hill simultaneously. Then
+the fast walker must travel a farther distance than the slow
+one, and his angle of path (pitch angle) must be smaller
+than the angle of path taken by the slow walker. Their
+pitch angles are different, but their pitch (in this case altitude
+reached in a given time) is the same.
+
+In order to test the pitch angle, the propeller must be
+mounted upon a shaft at right angles to a beam the face of
+which must be perfectly level, thus:
+
+First select a point on the blade at some distance (say
+about 2 feet) from the centre of the propeller. At that
+point find, by means of a protractor, the angle a projection
+of the chord makes with the face of the beam. That angle
+is the pitch angle of the blade at that point.
+
+Now lay out the angle on paper, thus:
+
+The line above and parallel to the circumference line must
+be placed in a position making the distance between the
+two lines equal to the specified pitch, which is, or should be,
+marked upon the boss of the propeller.
+
+Now find the circumference of the propeller where the
+pitch angle is being tested. For example, if that place is
+2 feet radius from the centre, then the circumference will
+be 2 feet X 2 = 4 feet diameter, which, if multiplied by
+3.1416 = 15.56 feet circumference.
+
+Now mark off the circumference distance, which is
+represented above by A-B, and reduce it in scale for convenience.
+
+The distance a vertical line makes between B and the
+chord dine is the pitch at the point where the angle is being
+tested, and it should coincide with the specified pitch. You
+will note, from the above illustration, that the actual pitch
+line should meet the junction of the chord line and top
+line.
+
+The propeller should be tested at several points, about
+a foot apart, on each blade; and the diagram, provided the
+propeller is not faulty, will then look like this:
+
+At each point tested the actual pitch coincides with the
+specified pitch: a satisfactory condition.
+
+A faulty propeller will produce a diagram something
+like this:
+
+
+At every point tested the pitch angle is wrong, for nowhere
+does the actual pitch coincide with the specified pitch.
+Angles A, C, and D, are too large, and B is too small. The
+angle should be correct to half a degree if reasonable efficiency
+is to be maintained.
+
+A fault in the pitch angle may be due to (1) faulty manufacture,
+(2) distortion, or (3) the shaft hole through the boss
+being out of position.
+
+
+2. STRAIGHTNESS.--To test for straightness the propeller
+must be mounted upon a shaft. Now bring the tip of one
+blade round to graze some fixed object. Mark the point it
+grazes. Now bring the other tip round, and it should come
+within 1/8 inch of the mark. If it does not do so, it is due to
+(1) faulty manufacture, (2) distortion, or (3) to the hole
+through the boss being out of position.
+
+
+3. LENGTH.--The blades should be of equal length to
+inch.
+
+
+4. BALANCE.--The usual method of testing a propeller
+for balance is as follows: Mount it upon a shaft, which must
+be on ball-bearings. Place the propeller in a horizontal
+position, and it should remain in that position. If a weight
+of a trifle over an ounce placed in a bolt-hole on one side of
+the boss fails to disturb the balance, then the propeller is
+usually regarded as unfit for use.
+
+The above method is rather futile, as it does not test for
+the balance of centrifugal force, which comes into play as
+soon as the propeller revolves. It can be tested as follows:
+
+
+The propeller must be in a horizontal position, and then
+weighed at fixed points, such as A, B, C, D, E, and F, and
+the weights noted. The points A, B, and C must, of course,
+be at the same fixed distances from the centre of the propeller
+as the points D, E, and F. Now reverse the propeller and
+weigh at each point again. Note the results. The first
+series of weights should correspond to the second series,
+thus:
+
+      Weight A should equal weight F.
+         ``  B   ``     ``     ``  E.
+         ``  C   ``     ``     ``  D.
+
+
+There is no standard practice as to the degree of error
+permissible, but if there are any appreciable differences the
+propeller is unfit for use.
+
+
+5. SURFACE AREA.--The surface area of the blades should
+be equal. Test with callipers thus:
+
+The points between which the distances are taken must,
+of course, be at the same distance from the centre in the
+case of each blade.
+
+There is no standard practice as to the degree of error
+permissible. If, however, there is an error of over 1/8 inch,
+the propeller is really unfit for use.
+
+
+6. CAMBER.--The camber (curvature) of the blades should
+be (1) equal, (2) decrease evenly towards the tips of the blades,
+and (3) the greatest depth of the curve should, at any point
+of the blade, be approximately at the same percentage of
+the chord from the leading edge as at other points.
+
+It is difficult to test the top camber without a set of
+templates, but a fairly accurate idea of the concave camber
+can be secured by slowly passing a straight-edge along the
+blade, thus:
+
+The camber can now be easily seen, and as the straight-
+edge is passed along the blade, the observer should look for
+any irregularities of the curvature, which should gradually
+and evenly decrease towards the tip of the blade.
+
+
+7. THE JOINTS.--The usual method for testing the glued
+joints is by revolving the propeller at greater speed than it
+will be called upon to make during flight, and then carefully
+examining the joints to see if they have opened. It is not
+likely, however, that the reader will have the opportunity
+of making this test. He should, however, examine all the
+joints very carefully, trying by hand to see if they are quite
+sound. Suspect a propeller of which the joints appear to
+hold any thickness of glue. Sometimes the joints in the
+boss open a little, but this is not dangerous unless they extend
+to the blades, as the bolts will hold the laminations together.
+
+
+8. CONDITION OF SURFACE.--The surface should be very
+smooth, especially towards the tips of the blades. Some
+propeller tips have a speed of over 30,000 feet a minute,
+and any roughness will produce a bad drift or resistance
+and lower the efficiency.
+
+
+9. MOUNTING.--Great care should be taken to see that
+the propeller is mounted quite straight on its shaft. Test in
+the same way as for straightness. If it is not straight, it
+is possibly due to some of the propeller bolts being too slack
+or to others having been pulled up too tightly.
+
+
+FLUTTER.--Propeller ``flutter,'' or vibration, may be due
+to faulty pitch angle, balance, camber, or surface area. It
+causes a condition sometimes mistaken for engine trouble,
+and one which may easily lead to the collapse of the propeller.
+
+
+CARE OF PROPELLERS.--The care of propellers is of the
+greatest importance, as they become distorted very easily.
+
+
+1. Do not store them in a very damp or a very dry place.
+
+
+2. Do not store them where the sun will shine upon them.
+
+
+3. Never leave them long in a horizontal position or
+leaning up against a wall.
+
+
+4. They should be hung on horizontal pegs, and the
+position of the propellers should be vertical.
+
+
+If the points I have impressed upon you in these notes
+are not attended to, you may be sure of the following results:
+
+
+1. Lack of efficiency, resulting in less aeroplane speed
+and climb than would otherwise be the case.
+
+
+2. Propeller ``flutter'' and possible collapse.
+
+
+3. A bad stress upon the propeller shaft and its bearings.
+
+
+TRACTOR.--A propeller mounted in front of the main
+surface.
+
+
+PUSHER.--A propeller mounted behind the main surface.
+
+
+FOUR-BLADED PROPELLERS.--Four- bladed propellers are
+suitable only when the pitch is comparatively large.
+
+For a given pitch, and having regard to ``interference,''
+they are not so efficient as two-bladed propellers.
+
+The smaller the pitch, the less the ``gap,'' i.e., the distance,
+measured in the direction of the thrust, between the
+spiral courses of the blades.
+
+If the gap is too small, then the following blade will
+engage air which the preceding blade has put into motion,
+with the result that the following blade will not secure as
+good a reaction as would otherwise be the case. It is very
+much the same as in the case of the aeroplane gap.
+
+For a given pitch, the gap of a four-bladed propeller is
+only half that of a two-bladed one. Therefore the four-
+bladed propeller is only suitable for large pitch, as such
+pitch produces spirals with a large gap, thus offsetting the
+decrease in gap caused by the numerous blades.
+
+The greater the speed of rotation, the less the pitch for
+a given aeroplane speed. Then, in order to secure a large
+pitch and consequently a good gap, the four-bladed propeller
+is usually geared to rotate at a lower speed than would be
+the case if directly attached to the engine crank-shaft.
+
+
+
+CHAPTER V
+
+MAINTENANCE
+
+CLEANLINESS.--The fabric must be kept clean and free
+from oil, as that will rot it. To take out dirt or oily patches,
+try acetone. If that will not remedy matters, then try
+petrol, but use it sparingly, as otherwise it will take off an
+unnecessary amount of dope. If that will not remove the
+dirt, then hot water and soap will do so, but, in that case,
+be sure to use soap having no alkali in it, as otherwise it may
+injure the fabric. Use the water sparingly, or it may get
+inside the planes and rust the internal bracing wires, or cause
+some of the wooden framework to swell.
+
+The wheels of the undercarriage have a way of throwing
+up mud on to the lower surface. This should, if possible, be
+taken off while wet. It should never be scraped off when
+dry, as that may injure the fabric. If dry, then it should
+be moistened before being removed.
+
+Measures should be taken to prevent dirt from collecting
+upon any part of the aeroplane, as, otherwise, excessive skin-
+friction will be produced with resultant loss of flight speed.
+The wires, being greasy, collect dirt very easily.
+
+
+CONTROL CABLES.--After every flight the rigger should
+pass his hand over the control cables and carefully examine
+them near pulleys. Removal of grease may be necessary
+to make a close inspection possible. If only one strand is
+broken the wire should be replaced. Do not forget the aileron
+balance wire on the top surface.
+
+Once a day try the tension of the control cables by smartly
+moving the control levers about as explained elsewhere.
+
+
+WIRES.--All the wires should be kept well greased or
+oiled, and in the correct tension. When examining the wires,
+it is necessary to place the aeroplane on level ground, as
+otherwise it may be twisted, thus throwing some wires into
+undue tension and slackening others. The best way, if there
+is time, is to pack the machine up into its ``flying position.''
+
+If you see a slack wire, do not jump to the conclusion
+that it must be tensioned. Perhaps its opposition wire is
+too tight, in which case slacken it, and possibly you will
+find that will tighten the slack wire.
+
+Carefully examine all wires and their connections near
+the propeller, and be sure that they are snaked round with
+safety wire, so that the latter may keep them out of the way
+of the propeller if they come adrift.
+
+The wires inside the fuselage should be cleaned and regreased
+about once a fortnight.
+
+
+STRUTS AND SOCKETS.--These should be carefully examined
+to see if any splitting has occurred.
+
+
+DISTORTION.--Carefully examine all surfaces, including
+the controlling surfaces, to see whether any distortion has
+occurred. If distortion can be corrected by the adjustment
+of wires, well and good; but if not, then some of the internal
+framework probably requires replacement.
+
+
+ADJUSTMENTS.--Verify the angles of incidence; dihedral,
+and stagger, and the rigging position of the controlling-
+surfaces, as often as possible.
+
+
+UNDERCARRIAGE.--Constantly examine the alignment and
+fittings of the undercarriage, and the condition of tyres and
+shock absorbers. The latter, when made of rubber, wear
+quickest underneath. Inspect axles and skids to see if
+there are any signs of them becoming bent. The wheels
+should be taken off occasionally and greased.
+
+
+LOCKING ARRANGEMENTS.--Constantly inspect the locking
+arrangements of turnbuckles, bolts, etc. Pay particular
+attention to the control cable connections, and to all moving
+parts in respect of the controls.
+
+
+LUBRICATION.--Keep all moving parts, such as pulleys,
+control levers, and hinges of controlling surfaces, well greased.
+
+
+SPECIAL INSPECTION.--Apart from constantly examining
+the aeroplane with reference to the above points I have made,
+I think that, in the case of an aeroplane in constant use
+it is an excellent thing to make a special inspection of every
+part, say once a week. This will take from two to three
+hours, according to the type of aeroplane. In order to carry
+it out methodically, the rigger should have a list of every part
+down to the smallest split-pin. He can then check the parts
+as he examines them, and nothing will be passed over. This,
+I know from experience, greatly increases the confidence of
+the pilot, and tends to produce good work in the air.
+
+
+WINDY WEATHER.--The aeroplane, when on the ground,
+should face the wind; and it is advisable to lash the control
+lever fast, so that the controlling surfaces may not be blown
+about and possibly damaged.
+
+
+``VETTING'' BY EYE.--This should be practiced at every
+opportunity, and, if persevered in, it is possible to become
+quite expert in diagnosing by eye faults in flight efficiency,
+stability and control.
+
+The aeroplane should be standing upon level ground, or,
+better than that, packed up into its ``flying position.''
+
+Now stand in front of it and line up the leading edge
+with the main spar, rear spar, and trailing edge. Their
+shadows can usually be seen through the fabric. Allowance
+must, of course, be made for wash-in and wash-out; otherwise,
+the parts I have specified should be parallel with each other.
+
+Now line up the centre part of the main-plane with the
+tail-plane. The latter should be horizontal.
+
+Next, sight each interplane front strut with its rear
+strut. They should be parallel.
+
+Then, standing on one side of the aeroplane, sight all
+the front struts. The one nearest to you should cover all
+the others. This applies to the rear struts also.
+
+Look for distortion of leading edges, main and rear spars,
+trailing edges, tail-plane and controlling surfaces.
+
+This sort of thing, if practiced constantly, will not only
+develop an expert eye for diagnosis of faults, but will also
+greatly assist in impressing upon the memory the characteristics
+and possible troubles of the various types of aeroplanes.
+
+
+MISHANDLING OF THE GROUND.--This is the cause of a
+lot of unnecessary damage. The golden rule to observe is:
+PRODUCE NO BENDING STRESSES.
+
+Nearly all the wood in an aeroplane is designed to take
+merely the stress of direct compression, and it cannot be bent
+safely. Therefore, in packing an aeroplane up from the
+ground, or in pulling or pushing it about, be careful to stress
+it in such a way as to produce, as far as possible, only direct
+compression stresses. For instance, if it is necessary to
+support the lifting surface, then the packing should be
+arranged to come directly under the struts so that they may
+take the stress in the form of compression for which they are
+designed. Such supports should be covered with soft packing
+in order to prevent the fabric from becoming damaged.
+
+When pulling an aeroplane along, if possible, pull from
+the top of the undercarriage struts. If necessary to pull
+from elsewhere, then do so by grasping the interplane struts
+as low down as possible.
+
+Never lay fabric-covered parts upon a concrete floor.
+Any slight movement will cause the fabric to scrape over the
+floor with resultant damage.
+
+Struts, spars, etc., should never be left about the floor,
+as in such position they are likely to become scored. I
+have already explained the importance of protecting the outside
+fibres of the wood. Remember also that wood becomes
+distorted easily. This particularly applies to interplane
+struts. If there are no proper racks to stand them in, then
+the best plan is to lean them up against the wall in as near a
+vertical position as possible.
+
+
+TIME.--Learn to know the time necessary to complete
+any of the various rigging jobs. This is really important.
+Ignorance of this will lead to bitter disappointments in civil
+life; and, where Service flying is concerned, it will, to say the
+least of it, earn unpopularity with senior officers, and fail to
+develop respect and good work where men are concerned.
+
+
+THE AEROPLANE SHED.--This should be kept as clean and
+orderly as possible. A clean, smart shed produces briskness,
+energy, and pride of work. A dirty, disorderly shed nearly
+always produces slackness and poor quality of work, lost
+tools and mislaid material.
+
+
+GLOSSARY
+
+Aeronautics--The science of aerial navigation.
+
+Aerofoil--A rigid structure, of large superficial area relative to its
+thickness, designed to obtain, when driven through the air at an
+angle inclined to the direction of motion, a reaction from the air
+approximately at right angles to its surface. Always cambered
+when intended to secure a reaction in one direction only. As the
+term ``aerofoil'' is hardly ever used in practical aeronautics,
+I have, throughout this book, used the term SURFACE, which,
+while academically incorrect, since it does not indicate thickness,
+is a term usually used to describe the cambered lifting surfaces,
+i.e., the ``planes'' or ``wings,'' and the stabilizers and the
+controlling aerofoils.
+
+Aerodrome--The name usually applied to a ground used for the
+practice of aviation. It really means ``flying machine,'' but is
+never used in that sense nowadays.
+
+Aeroplane--A power-driven aerofoil with stabilizing and controlling
+surfaces.
+
+Acceleration--The rate of change of velocity.
+
+Angle of Incidence--The angle at which the ``neutral lift line'' of
+a surface attacks the air.
+
+Angle of Incidence, Rigger's--The angle the chord of a surface makes
+with a line parallel to the axis of the propeller.
+
+Angle of Incidence, Maximum--The greatest angle of incidence at
+which, for a given power, surface (including detrimental surface),
+and weight, horizontal flight can be maintained.
+
+Angle of Incidence, Minimum--The smallest angle of incidence at
+which, for a given power, surface (including detrimental surface),
+and weight, horizontal flight can be maintained.
+
+Angle of Incidence, Best Climbing--That angle of incidence at which
+an aeroplane ascends quickest. An angle approximately halfway
+between the maximum and optimum angles.
+
+Angle of Incidence, Optimum--The angle of incidence at which the
+lift-drift ratio is the highest.
+
+
+Angle, Gliding--The angle between the horizontal and the path along
+which an aeroplane at normal flying speed, but not under engine
+power, descends in still air.
+
+Angle, Dihedral--The angle between two planes.
+
+Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
+be at a lateral dihedral angle when it is inclined upward towards
+its wing-tips.
+
+Angle, Longitudinal Dihedral--The main surface and tail surface are
+said to be at a longitudinal dihedral angle when the projections
+of their neutral lift lines meet and produce an angle above them.
+
+Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting
+``chords'' for ``neutral life lines.''
+
+Angle, Pitch--The angle at any given point of a propeller, at which
+the blade is inclined to the direction of motion when the propeller
+is revolving but the aeroplane stationary.
+
+Altimeter--An instrument used for measuring height.
+
+Air-Speed Indicator--An instrument used for measuring air pressures
+or velocities. It consequently indicates whether the surface is
+securing the requisite reaction for flight. Usually calibrated in
+miles per hour, in which case it indicates the correct number of
+miles per hour at only one altitude. This is owing to the density
+of the air decreasing with increase of altitude and necessitating
+a greater speed through space to secure the same air pressure
+as would be secured by less speed at a lower altitude. It would
+be more correct to calibrate it in units of air pressure.
+
+Air Pocket--A local movement or condition of the air causing an
+aeroplane to drop or lose its correct attitude.
+
+Aspect-Ratio--The proportion of span to chord of a surface.
+
+Air-Screw (Propeller)--A surface so shaped that its rotation about
+an axis produces a force (thrust) in the direction of its axis.
+
+Aileron--A controlling surface, usually situated at the wing-tip, the
+operation of which turns an aeroplane about its longitudinal axis;
+causes an aeroplane to tilt sideways.
+
+
+Aviation--The art of driving an aeroplane.
+
+Aviator--The driver of an aeroplane.
+
+Barograph--A recording barometer, the charts of which can be calibrated
+for showing air density or height.
+
+Barometer--An instrument used for indicating the density of air.
+
+Bank, to--To turn an aeroplane about its longitudinal axis (to tilt
+sideways) when turning to left or right.
+
+Biplane--An aeroplane of which the main lifting surface consists
+of a surface or pair of wings mounted above another surface or
+pair of wings.
+
+Bay--The space enclosed by two struts and whatever they are fixed to.
+
+Boom--A term usually applied to the long spars joining the tail of a
+``pusher'' aeroplane to its main lifting surface.
+
+Bracing--A system of struts and tie wires to transfer a force from
+one point to another.
+
+Canard--Literally ``duck.'' The name which was given to a type of
+aeroplane of which the longitudinal stabilizing surface (empennage)
+was mounted in front of the main lifting surface. Sometimes
+termed ``tail-first'' aeroplanes, but such term is erroneous,
+as in such a design the main lifting surface acts as, and is, the
+empennage.
+
+Cabre--To fly or glide at an excessive angle of incidence; tail down.
+
+Camber--Curvature.
+
+Chord--Usually taken to be a straight line between the trailing and
+leading edges of a surface.
+
+Cell--The whole of the lower surface, that part of the upper surface
+directly over it, together with the struts and wires holding them
+together.
+
+Centre (Line) of Pressure--A line running from wing-tip to wing-tip,
+and through which all the air forces acting upon the surface may
+be said to act, or about which they may be said to balance.
+
+Centre (Line) of Pressure, Resultant--A line transverse to the
+longitudinal axis, and the position of which is the resultant of the
+centres of pressure of two or more surfaces.
+
+Centre of Gravity--The centre of weight.
+
+Cabane--A combination of two pylons, situated over the fuselage,
+and from which anti-lift wires are suspended. 
+
+Cloche--Literally ``bell.'' Is applied to the bell-shaped construction
+which forms the lower part of the pilot's control lever in
+a Bleriot monoplane, and to which the control cables are
+attached.
+
+Centrifugal Force--Every body which moves in a curved path is
+urged outwards from the centre of the curve by a force termed
+``centrifugal.''
+
+Control Lever--A lever by means of which the controlling surfaces
+are operated. It usually operates the ailerons and elevator. The
+``joy-stick".
+
+Cavitation, Propeller--The tendency to produce a cavity in the air.
+
+Distance Piece--A long, thin piece of wood (sometimes tape) passing
+through and attached to all the ribs in order to prevent them from
+rolling over sideways.
+
+Displacement--Change of position.
+
+Drift (of an aeroplane as distinct from the propeller)--The horizontal
+component of the reaction produced by the action of driving
+through the air a surface inclined upwards and towards its direction
+of motion PLUS the horizontal component of the reaction produced
+by the ``detrimental'' surface PLUS resistance due to
+``skin-friction.'' Sometimes termed ``head-resistance.''
+
+Drift, Active--Drift produced by the lifting surface.
+
+Drift, Passive--Drift produced by the detrimental surface.
+
+Drift (of a propeller)--Analogous to the drift of an aeroplane. It is
+convenient to include ``cavitation'' within this term.
+
+Drift, to--To be carried by a current of air; to make leeway.
+
+Dive, to--To descend so steeply as to produce a speed greater than the
+normal flying speed.
+
+Dope, to--To paint a fabric with a special fluid for the purpose of
+tightening and protecting it.
+
+Density--Mass of unit volume, for instance, pounds per cubic foot.
+
+Efficiency--Output
+            Input
+
+Efficiency (of an aeroplane as distinct from engine and propeller)--
+         Lift and Velocity
+    Thrust (= aeroplane drift)
+
+Efficiency, Engine--Brake horse-power
+                    Indicated horse-power
+
+Efficiency, Propeller--        Thrust horse-power
+                       Horse-power received from engine
+                               (= propeller drift)
+
+NOTE.--The above terms can, of course, be expressed in foot-
+pounds. It is then only necessary to divide the upper term by
+the lower one to find the measure of efficiency.
+
+Elevator--A controlling surface, usually hinged to the rear of the tail-
+plane, the operation of which turns an aeroplane about an axis
+which is transverse to the direction of normal horizontal flight.
+
+Empennage--See ``Tail-plane.''
+
+Energy--Stored work. For instance, a given weight of coal or petroleum
+stores a given quantity of energy which may be expressed
+in foot-pounds.
+
+Extension--That part of the upper surface extending beyond the
+span of the lower surface.
+
+Edge, Leading--The front edge of a surface relative to its normal
+direction of motion.
+
+Edge, Trailing--The rear edge of a surface relative to its normal
+direction of motion.
+
+Factor of Safety--Usually taken to mean the result found by dividing
+the stress at which a body will collapse by the maximum stress
+it will be called upon to bear.
+
+Fineness (of stream-line)--The proportion of length to maximum width.
+
+Flying Position--A special position in which an aeroplane must be
+placed when rigging it or making adjustments. It varies with
+different types of aeroplanes. Would be more correctly described
+as ``rigging position.''
+
+Fuselage--That part of an aeroplane containing the pilot, and to which
+is fixed the tail-plane.
+
+Fin--Additional keel-surface, usually mounted at the rear of an
+aeroplane.
+
+Flange (of a rib)--That horizontal part of a rib which prevents it
+from bending sideways.
+
+Flight--The sustenance of a body heavier than air by means of its
+action upon the air.
+
+Foot-pound--A measure of work representing the weight of 1 lb.
+raised 1 foot.
+
+Fairing--Usually made of thin sheet aluminum, wood, or a light
+construction of wood and fabric; and bent round detrimental
+surface in order to give it a ``fair'' or ``stream-like'' shape.
+
+Gravity--Is the force of the Earth's attraction upon a body. It
+decreases with increase of distance from the Earth. See ``Weight.''
+
+Gravity, Specific--Density of substance
+                   Density of water.
+Thus, if the density of water is 10 lb. per unit volume, the same
+unit volume of petrol, if weighing 7 lb., would be said to have a
+specific gravity of  7/10, i.e., 0.7.
+
+Gap (of an aeroplane)--The distance between the upper and lower
+surfaces of a biplane. In a triplane or multiplane, the distance
+between a surface and the one first above it.
+
+Gap, Propeller--The distance, measured in the direction of the thrust,
+between the spiral courses of the blades.
+
+Girder--A structure designed to resist bending, and to combine lightness
+and strength.
+
+Gyroscope--A heavy circular wheel revolving at high speed, the effect
+of which is a tendency to maintain its plane of rotation against
+disturbing forces.
+
+Hangar--An aeroplane shed.
+
+Head-Resistance--Drift. The resistance of the air to the passage of
+a body.
+
+Helicopter--An air-screw revolving about a vertical axis, the direction
+of its thrust being opposed to gravity.
+
+Horizontal Equivalent--The plan view of a body whatever its attitude
+may be.
+
+Impulse--A force causing a body to gain or lose momentum.
+
+Inclinometer--A curved form of spirit-level used for indicating the
+attitude of a body relative to the horizontal.
+
+Instability--An inherent tendency of a body, which, if the body is
+disturbed, causes it to move into a position as far as possible away
+from its first position.
+
+Instability, Neutral--An inherent tendency of a body to remain in the
+position given it by the force of a disturbance, with no tendency
+to move farther or to return to its first position.
+
+Inertia--The inherent resistance to displacement of a body as distinct
+from resistance the result of an external force.
+
+Joy-Stick--See ``Control Lever.''
+
+Keel-Surface--Everything to be seen when viewing an aeroplane from
+the side of it.
+
+King-Post--A bracing strut; in an aeroplane, usually passing through
+a surface and attached to the main spar, and from the end or ends
+of which wires are taken to spar, surface, or other part of the
+construction in order to prevent distortion. When used in connection
+with a controlling surface, it usually performs the additional
+function of a lever, control cables connecting its ends with the
+pilot's control lever.
+
+Lift--The vertical component of the reaction produced by the action
+of driving through the air a surface inclined upwards and towards
+its direction of motion.
+
+Lift, Margin of--The height an aeroplane can gain in a given time and
+starting from a given altitude.
+
+Lift-Drift Ratio--The proportion of lift to drift.
+
+Loading--The weight carried by an aerofoil. Usually expressed in
+pounds per square foot of superficial area.
+
+Longeron--The term usually applied to any long spar running length-
+ways of a fuselage.
+
+Mass--The mass of a body is a measure of the quantity of material
+in it.
+
+Momentum--The product of the mass and velocity of a body is known
+as ``momentum.''
+
+Monoplane--An aeroplane of which the main lifting surface consists
+of one surface or one pair of wings.
+
+Multiplane--An aeroplane of which the main lifting surface consists
+of numerous surfaces or pairs of wings mounted one above the
+other.
+
+Montant--Fuselage strut.
+
+Nacelle--That part of an aeroplane containing the engine and
+pilot and passenger, and to which the tail plane is not fixed.
+
+Neutral Lift Line--A line taken through a surface in a forward direction
+relative to its direction of motion, and starting from its
+trailing edge. If the attitude of the surface is such as to make
+the said line coincident with the direction of motion, it results
+in no lift, the reaction then consisting solely of drift. The position
+of the neutral lift line, i.e., the angle it makes with the chord,
+varies with differences of camber, and it is found by means of
+wind-tunnel research.
+
+Newton's Laws of Motion--1. If a body be at rest, it will remain at
+rest; or, if in motion, it will move uniformly in a straight line
+until acted upon by some force.
+
+2. The rate of change of the quantity of motion (momentum) is
+proportional to the force which causes it, and takes place in the
+direction of the straight line in which the force acts. If a body
+be acted upon by several forces, it will obey each as though the
+others did not exist, and this whether the body be at rest or in
+motion.
+
+3. To every action there is opposed an equal and opposite
+reaction.
+
+Ornithopter (or Orthopter)--A flapping wing design of aircraft intended
+to imitate the flight of a bird.
+
+Outrigger--This term is usually applied to the framework connecting
+the main surface with an elevator placed in advance of it. Sometimes
+applied to the ``tail-boom'' framework connecting the
+tail-plane with the main lifting surface.
+
+Pancake, to--To ``stall ''
+
+Plane--This term is often applied to a lifting surface. Such application
+is not quite correct, since ``plane'' indicates a flat surface,
+and the lifting surfaces are always cambered.
+
+Propeller--See ``Air-Screw.''
+
+Propeller, Tractor--An air-screw mounted in front of the main lifting
+surface.
+
+Propeller, Pusher--An air-screw mounted behind the main lifting surface.
+
+Pusher--An aeroplane of which the propeller is mounted behind the
+main lifting surface.
+
+Pylon--Any V-shaped construction from the point of which wires
+are taken.
+
+Power--Rate of working.
+
+Power, Horse--One horse-power represents a force sufficient to raise
+33,000 lbs. 1 foot in a minute.
+
+Power, Indicated Horse--The I.H.P. of an engine is a measure of the
+rate at which work is done by the pressure upon the piston or
+pistons, as distinct from the rate at which the engine does work.
+The latter is usually termed ``brake horse-power,'' since it may be
+measured by an absorption brake.
+
+Power, Margin of--The available quantity of power above that necessary
+to maintain horizontal flight at the optimum angle.
+
+Pitot Tube--A form of air-speed indicator consisting of a tube with
+open end facing the wind, which, combined with a static pressure
+or suction tube, is used in conjunction with a gauge for measuring
+air pressures or velocities. (No. 1 in diagram.)
+
+Pitch, Propeller--The distance a propeller advances during one revolution
+supposing the air to be solid.
+
+Pitch, to--To plunge nose-down.
+
+Reaction--A force, equal and opposite to the force of the action producing
+it.
+
+Rudder--A controlling surface, usually hinged to the tail, the operation
+of which turns an aeroplane about an axis which is vertical in
+normal horizontal flight; causes an aeroplane to turn to left or
+right of the pilot.
+
+Roll, to--To turn about the longitudinal axis.
+
+Rib, Ordinary--A light curved wooden part mounted in a fore and aft
+direction within a surface. The ordinary ribs give the surface
+its camber, carry the fabric, and transfer the lift from the fabric
+to the spars.
+
+Rib, Compression--Acts as an ordinary rib, besides bearing the stress
+of compression produced by the tension of the internal bracing
+wires.
+
+Rib, False--A subsidiary rib, usually used to improve the camber of
+the front part of the surface.
+
+Right and Left Hand--Always used relative to the position of the
+pilot. When observing an aeroplane from the front of it, the
+right hand side of it is then on the left hand of the observer.
+
+Remou--A local movement or condition of the air which may cause
+displacement of an aeroplane.
+
+Rudder-Bar--A control lever moved by the pilot's feet, and operating
+the rudder.
+
+Surface--See ``Aerofoil.''
+
+Surface, Detrimental--All exterior parts of an aeroplane including
+the propeller, but excluding the (aeroplane) lifting and (propeller)
+thrusting surfaces.
+
+Surface, Controlling--A surface the operation of which turns an aeroplane
+about one of its axes.
+
+Skin-Friction--The friction of the air with roughness of surface. A
+form of drift.
+
+Span---The distance from wing-tip to wing-tip.
+
+Stagger--The distance the upper surface is forward of the lower surface
+when the axis of the propeller is horizontal.
+
+Stability--The inherent tendency of a body, when disturbed, to return
+to its normal position.
+
+Stability, Directional--The stability about an axis which is vertical
+during normal horizontal flight, and without which an aeroplane
+has no natural tendency to remain upon its course.
+
+Stability, Longitudinal--The stability of an aeroplane about an axis
+transverse to the direction of normal horizontal flight, and without
+which it has no tendency to oppose pitching and tossing.
+
+Stability, Lateral--The stability of an aeroplane about its longitudinal
+axis, and without which it has no tendency to oppose sideways
+rolling.
+
+Stabilizer--A surface, such as fin or tail-plane, designed to give an
+aeroplane inherent stability.
+
+Stall, to--To give or allow an aeroplane an angle of incidence greater
+than the ``maximum'' angle, the result being a fall in the lift-
+drift ratio, the lift consequently becoming less than the weight of
+the aeroplane, which must then fall, i.e., ``stall'' or ``pancake.''
+
+Stress--Burden or load.
+
+Strain--Deformation produced by stress.
+
+Side-Slip, to--To fall as a result of an excessive ``bank'' or ``roll.''
+
+Skid, to--To be carried sideways by centrifugal force when turning
+to left or right.
+
+Skid, Undercarriage--A spar, mounted in a fore and aft direction, and
+to which the wheels of the undercarriage are sometimes attached.
+Should a wheel give way the skid is then supposed to act like the
+runner of a sleigh and to support the aeroplane.
+
+Skid, Tail--A piece of wood or other material, orientable, and fitted
+with shock absorbers, situated under the tail of an aeroplane in
+order to support it upon the ground and to absorb the shock of
+alighting. 
+
+
+Section--Any separate part of the top surface, that part of the bottom
+surface immediately underneath it, with their struts and wires.
+
+Spar--Any long piece of wood or other material.
+
+Spar, Main--A spar within a surface and to which all the ribs are
+attached, such spar being the one situated nearest to the centre
+of pressure. It transfers more than half the lift from the ribs
+to the bracing. 
+
+Spar, Rear--A spar within a surface, and to which all the ribs are
+attached, such spar being situated at the rear of the centre of
+pressure and at a greater distance from it than is the main spar.
+It transfers less than half of the lift from the ribs to the bracing.
+
+Strut--Any wooden member intended to take merely the stress of
+direct compression.
+
+Strut, Interplane--A strut holding the top and bottom surfaces apart.
+
+Strut, Fuselage--A strut holding the fuselage longerons apart. It
+should be stated whether top, bottom, or side. If side, then it
+should be stated whether right or left hand. Montant. 
+
+Strut, Extension--A strut supporting an ``extension'' when not in
+flight. It may also prevent the extension from collapsing upwards
+during flight.
+
+Strut, Undercarriage--
+
+Strut, Dope--A strut within a surface, so placed as to prevent the
+tension of the doped fabric from distorting the framework.
+
+Serving--To bind round with wire, cord, or similar material. Usually
+used in connection with wood joints and wire cable splices.
+
+Slip, Propeller--The pitch less the distance the propeller advances
+during one revolution.
+
+Stream-Line--A form or shape of detrimental surface designed to
+produce minimum drift.
+
+Toss, to--To plunge tail-down.
+
+Torque, Propeller--The tendency of a propeller to turn an aeroplane
+about its longitudinal axis in a direction opposite to that in which
+the propeller revolves.
+
+Tail-Slide--A fall whereby the tail of an aeroplane leads.
+
+Tractor--An aeroplane of which the propeller is mounted in front of
+the main lifting surface.
+
+Triplane--An aeroplane of which the main lifting surface consists of
+three surfaces or pairs of wings mounted one above the other.
+
+Tail-Plane--A horizontal stabilizing surface mounted at some distance
+behind the main lifting surface. Empennage. 
+
+Turnbuckle--A form of wire-tightener, consisting of a barrel into each
+end of which is screwed an eyebolt. Wires are attached to the
+eyebolts and the required degree of tension is secured by means
+of rotating the barrel.
+
+Thrust, Propeller--See ``Air-Screw.''
+
+Undercarriage--That part of an aeroplane beneath the fuselage or
+nacelle, and intended to support the aeroplane when at rest, and
+to absorb the shock of alighting. 
+
+Velocity--Rate of displacement; speed.
+
+Volplane--A gliding descent.
+
+Weight--Is a measure of the force of the Earth's attraction (gravity)
+upon a body. The standard unit of weight in this country is
+1 lb., and is the force of the Earth's attraction on a piece of platinum
+called the standard pound, deposited with the Board of Trade
+in London. At the centre of the Earth a body will be attracted
+with equal force in every direction. It will therefore have no
+weight, though its mass is unchanged. Gravity, of which weight
+is a measure, decreases with increase of altitude.
+
+Web (of a rib)--That vertical part of a rib which prevents it from
+bending upwards.
+
+Warp, to--To distort a surface in order to vary its angle of incidence.
+To vary the angle of incidence of a controlling surface.
+
+Wash--The disturbance of air produced by the flight of an aeroplane.
+
+Wash-in--An increasing angle of incidence of a surface towards its
+wing-tip.
+
+Wash-out--A decreasing angle of incidence of a surface towards its
+wing-tip.
+
+Wing-tip--The right- or left-hand extremity of a surface.
+
+Wire--A wire is, in Aeronautics, always known by the name of its
+function.
+
+Wire, Lift or Flying--A wire opposed to the direction of lift, and used
+to prevent a surface from collapsing upward during flight.
+
+Wire, Anti-lift or Landing--A wire opposed to the direction of gravity,
+and used to sustain a surface when it is at rest.
+
+Wire, Drift--A wire opposed to the direction of drift, and used to
+prevent a surface from collapsing backwards during flight.
+
+Wire, Anti-drift--A wire opposed to the tension of a drift wire, and
+used to prevent such tension from distorting the framework.
+
+Wire, Incidence--A wire running from the top of an interplane strut to
+the bottom of the interplane strut in front of or behind it. It
+maintains the ``stagger'' and assists in maintaining the angle
+of incidence. Sometimes termed ``stagger wire.''
+
+Wire, Bracing--Any wire holding together the framework of any part
+of an aeroplane. It is not, however, usually applied to the wires
+described above unless the function performed includes a function
+additional to those described above. Thus, a lift wire, while
+strictly speaking a bracing wire, is not usually described as one
+unless it performs the additional function of bracing some well-
+defined part such as the undercarriage. It will then be said to
+be an ``undercarriage bracing lift wire.'' It might, perhaps,
+be acting as a drift wire also, in which case it will then be de-
+scribed as an ``undercarriage bracing lift-drift wire.'' It should
+always be stated whether a bracing wire is (1) top, (2) bottom,
+(3) cross, or (4) side. If a ``side bracing wire,'' then it should be
+stated whether right- or left-hand.
+
+Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)
+within a surface.
+
+Wire, Top Bracing--A bracing wire, approximately horizontal and
+situated between the top longerons of fuselate, between top tail
+booms, or at the top of similar construction.
+
+Wire, Bottom Bracing--Ditto, substituting ``bottom'' for ``top.''
+
+Wire, Side Bracing--A bracing wire crossing diagonally a side bay
+of fuselage, tail boom bay, undercarriage side bay or centre-section
+side bay. This term is not usually used with reference to incidence
+wires, although they cross diagonally the side bays of the
+cell. It should be stated whether right- or left-hand.
+
+Wire, Cross Bracing--A bracing wire, the position of which is diagonal
+from right to left when viewing it from the front of an aeroplane.
+
+Wire, Control Bracing--A wire preventing distortion of a controlling
+surface.
+
+Wire, Control--A wire connecting a controlling surface with the pilot's
+control lever, wheel, or rudder-bar.
+
+Wire, Aileron Gap--A wire connecting top and bottom ailerons.
+
+Wire, Aileron Balance--A wire connecting the right- and left-hand top
+ailerons. Sometimes termed the ``aileron compensating wire.''
+
+Wire, Snaking--A wire, usually of soft metal, wound spirally or tied
+round another wire, and attached at each end to the framework.
+Used to prevent the wire round which it is ``snaked'' from becoming,
+in the event of its displacement, entangled with the
+propeller.
+
+Wire, Locking--A wire used to prevent a turnbuckle barrel or other
+fitting from losing its adjustment.
+
+Wing--Strictly speaking, a wing is one of the surfaces of an ornithopter.
+The term is, however, often applied to the lifting surface of
+an aeroplane when such surface is divided into two parts, one being
+the left-hand ``wing,'' and the other the right-hand ``wing.''
+
+Wind-Tunnel--A large tube used for experimenting with surfaces and
+models, and through which a current of air is made to flow by
+artificial means.
+
+Work--Force X displacement.
+
+Wind-Screen--A small transparent screen mounted in front of the
+pilot to protect his face from the air pressure.
+
+
+
+
+
+End of The Project Gutenberg Etext of The Aeroplane Speaks, by Barber
+
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