The Pilot's Cock-pit.The Pilot's Cock-pit.
A strain on the Pilot? Not a bit of it, for this is his Work which he loves and excels in; and given a cool head, alert eye, and a sensitive touch for the controls, whatsport can compare with these ever-changing battles of the air?
The Aeroplane has all this time been climbing in great wide circles, and is now some three thousand feet above the Aerodrome which from such height looks absurdly small. The buildings below now seem quite squat; the hills appear to have sunk away into the ground, and the whole country below, cut up into diminutive fields, has the appearance of having been lately tidied and thoroughly spring-cleaned! A doll's country it looks, with tiny horses and cows ornamenting the fields and little model motor-cars and carts stuck on the roads, the latter stretching away across country like ribbons accidentally dropped.
At three thousand feet altitude the Pilot is satisfied that he is now sufficiently high to secure, in the event of engine failure, a long enough glide to earth to enable him to choose and reach a good landing-place; and, being furthermore content with the steady running of the engine, he decides to climb no more but to follow the course he has mapped out. Consulting the compass, he places the Aeroplane on the A–E course and, using the Elevator, he gives his craft its minimum angle of incidence at which it will just maintain horizontal flight and secure its maximum speed.
Swiftly he speeds away, and few thoughts he has now for the changing panorama of country, cloud, and colour. Ever present in his mind are the three great 'cross-country queries. "Am I on my right course? Can I see a good landing-ground within gliding distance?" And "How is the Engine running?"
Keenly both he and the Observer compare their maps with the country below. The roads, khaki-coloured ribbons, are easily seen but are not of much use, for there are so many of them and they all look alike from such an altitude.
Now where can that lake be which the map shows so plainly? He feels that surely he should see it by now, and has an uncomfortable feeling that he is flying too far West. What pilot is there indeed who has not many times experienced such unpleasant sensation? Few things in the air can create greater anxiety. Wisely, however, he sticksto his compass course, and the next minute he is rewarded by a sight of the lake, though indeed he now sees that the direction of his travel will not take him over it, as should be the case if he were flying over the shortest route to his destination. He must have slightly miscalculated the velocity or direction of the side-wind.
"About ten degrees off," he mutters, and, using the Rudder, corrects his course accordingly.
Now he feels happier and that he is well on his way. The gusts, too, have ceased to trouble him as, at this altitude, they are not nearly so bad as they were near the ground, the broken surface of which does much to produce them; and sometimes for miles he makes but a movement or two of the controls.
The clouds just above race by with dizzy and uniform speed; the country below slowly unrolls, and the steady drone of the Engine is almost hypnotic in effect. "Sleep, sleep, sleep," it insidiously suggests. "Listen to me and watch the clouds; there's nothing else to do. Dream, dream, dream of speeding through space for ever, and ever, and ever; and rest, rest, rest to the sound of my rhythmical hum. Droning on and on, nothing whatever matters. All things now are merged into speed through space and a sleepy monotonous d-d-r-r-o-o-n-n-e———." But the Pilot pulls himself together with a start and peers far ahead in search of the next landmark. This time it is a little country town, red-roofed his map tells him, and roughly of cruciform shape; and, sure enough, there in the right direction are the broken outlines of a few red roofs peeping out from between the trees.
Another minute and he can see this little town, a fairy place it appears, nestling down between the hills and its red roofs and picturesque shape, a glowing and lovely contrast with the dark green of the surrounding moors.
So extraordinarily clean and tidy it looks from such a height, and laid out in such orderly fashion with perfectly defined squares, parks, avenues, and public buildings, it indeed appears hardly real, but rather as if it has this very day materialized from some delightful children's book!
Every city and town you must know has its distinct individuality to the Pilot's eye. Some are not fairy placesat all, but great dark ugly blots upon the fair countryside, and with tall shafts belching forth murky columns of smoke to defile clean space. Others, melancholy-looking masses of grey, slate-roofed houses, are always sad and dispirited; never welcoming the glad sunshine, but ever calling for leaden skies and a weeping Heaven. Others again, little coquettes with village green, white palings everywhere, bright gravel roads, and an irrepressible air of brightness and gaiety.
Then there are the rivers, silvery streaks peacefully winding far, far away to the distant horizon; they and the lakes the finest landmarks the Pilot can have. And the forests. How can I describe them? The trees cannot be seen separately, but merge altogether into enormous irregular dark green masses sprawling over the country, and sometimes with great ungainly arms half encircling some town or village; and the wind passing over the foliage at times gives the forest an almost living appearance, as of some great dragon of olden times rousing itself from slumber to devour the peaceful villages its arms encircle.
And the Pilot and Observer fly on and on, seeing these things and many others which baffle my poor skill to describe—things, dear Reader, that you shall see, and poets sing of, and great artists paint in the days to come when the Designer has captured Efficiency. Then, and the time is near, shall you see this beautiful world as you have never seen it before, the garden it is, the peace it breathes, and the wonder of it.
The Pilot, flying on, is now anxiously looking for the railway line which midway on his journey should point the course. Ah! There it is at last, but suddenly (and the map at fault) it plunges into the earth! Well the writer remembers when that happened to him on a long 'cross-country flight in the early days of aviation. Anxiously he wondered "Are tunnels always straight?" and with what relief, keeping on a straight course, he picked up the line again some three miles farther on!
Now at last the Pilot sees the sea, just a streak on the north-eastern horizon, and he knows that his flight is two-thirds over. Indeed, he should have seen it before, but the air is none too clear, and he is not yet able to discern the river which soon should cross his path. As he swiftlyspeeds 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 thedutyof 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 manœuvre 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 hospitalityof 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, youairman," 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 inexistence in which the tail could not be raised several feet, and that would make all the difference. A high tail means a large angle of incidence when the machine touches ground and, with enough angle, I'll guarantee to safely land the fastest machine in a five-acre field. You can, I am sure, imagine what a difference that would make where forced landings are concerned!" Then rapidly sketching in his notebook, he shows the Observer the following illustration:
The Pilot's Aeroplane.THE PILOT'S AEROPLANE. THE CHANGE OF DESIGN HE WOULD LIKE.The Change of Design He Would Like.
"That's very pretty," said the Observer, "but how about Mechanical Difficulties, and Efficiency in respect of Flight? And, anyway, why hasn't such an obvious thing been done already?"
"As regards the first part of your question I assure you that there's nothing in it, and I'll prove it to you as follows——"
"Oh! That's all right, old chap. I'll take your word for it," hurriedly replies the Observer, whose soul isn't tuned to a technical key.
"As regards the latter part of your inquiry," went on the Pilot, a little nettled at having such a poor listener, "it's very simple. Aeroplanes have 'just growed' like Topsy, and they consequently contain this and many another relic of early day design when Aeroplanes were more or lessthrown together and anything was good enough that could get off the ground."
"By Jove," interrupts the Observer, "I do believe the fog is lifting. Hadn't we better get the engine and body covers off, just in case it's really so?"
"I believe you're right. I am sure those hills over there could not be seen a few minutes ago, and look—there's sunshine over there. We'd better hurry up."
Ten minutes' hard work and the covers are off, neatly folded and stowed aboard; the picketing wires are cast adrift, and the Pilot is once more in his seat. The Aeroplane has been turned to face the other end of the field, and, the Observer swinging round the propeller, the engine is awake again and slowly ticking over. Quickly the Observer climbs into his seat in front of the Pilot, and, the latter slightly opening the throttle, the Aeroplane leisurely rolls over the ground towards the other end of the field, from which the ascent will be made.
Arriving there the Pilot turns the Aeroplane in order to face the wind and thus secure a quick "get-off." Then he opens the throttle fully and the mighty voice of the Engine roars out "Now see me clear that hedge!" and the Aeroplane races forward at its minimum angle of incidence. Tail up, and with ever-increasing speed, it rushes towards the hedge under the lee of which it has lately been at rest; and then, just as the Observer involuntarily pulls back an imaginary joy-stick, the Pilot moves the real one and places the machine at its best climbing angle. Like a living thing it responds, and instantly leaves the ground, clearing the hedge like a—well, like an Aeroplane with an excellent margin of lift. Upwards it climbs with even and powerful lift, and the familiar scenes below again gladden the eyes of the Pilot. Smaller and more and more squat grow the houses and hills; more and more doll-like appear the fields which are clearly outlined by the hedges; and soon the country below is easily identified with the map. Now they can see the river before them and a bay of the sea which must be crossed or skirted. The fog still lingers along the course of the river and between the hills, but is fast rolling away in grey, ghost-like masses.Out to sea it obscures the horizon, making it difficult to be sure where water ends and fog begins, and creating a strange, rather weird, effect by which ships at a certain distance appear to be floating in space.
Now the Aeroplane is almost over the river, and the next instant it suddenly drops into a "hole in the air." With great suddenness it happens, and for some two hundred feet it drops nose-down and tilted over sideways; but the Pilot is prepared and has put his craft on an even keel in less time than it takes to tell you about it; for well he knows that he must expect such conditions when passing over a shore or, indeed, any well-defined change in the composition of the earth's surface. Especially is this so on a hot and sunny day, for then the warm surface of the earth creates columns of ascending air, the speed of the ascent depending upon the composition of the surface. Sandy soil, for instance, such as borders this river produces a quickly ascending column of air, whereas water and forests have not such a marked effect. Thus, when our Aeroplane passed over the shore of the river, it suddenly lost the lift due to the ascending air produced by the warm sandy soil, and it consequently dropped just as if it had fallen into a hole.
Now the Aeroplane is over the bay and, the sea being calm, the Pilot looks down, down through the water, and clearly sees the bottom, hundreds of feet below the surface. Down through the reflection of the blue sky and clouds, and one might think that is all, but it isn't. Only those who fly know the beauties of the sea as viewed from above; its dappled pearly tints; its soft dark blue shadows; the beautiful contrasts of unusual shades of colour which are always differing and shifting with the changing sunshine and the ever moving position of the aerial observer. Ah! for some better pen than mine to describe these things! One with glowing words and a magic rhythm to express the wonders of the air and the beauty of the garden beneath—the immensity of the sea—the sense of space and of one's littleness there—the realization of the Power moving the multitudes below—the exaltation of spirit altitude produces—the joy of speed. A new world of sensation!
Now the bay is almost crossed and the Aerodrome at B. can be distinguished....
On the Aerodrome is a little crowd waiting and watching for the arrival of the Aeroplane, for it is of a new and improved type and its first 'cross-country performance is of keen interest to these men; men who really know something about flight.
There is the Squadron Commander who has done some real flying in his time; several well-seasoned Flight-Commanders; a dozen or more Flight-Lieutenants; a knowledgeable Flight-Sergeant; a number of Air Mechanics, and, a little on one side and almost unnoticed, the Designer.
"I hope they are all right," says someone, "and that they haven't had difficulties with the fog. It rolled up very quickly, you know."
"Never fear," remarks a Flight-Commander. "I know the Pilot well and he's a good 'un; far too good to carry on into a fog."
"They say the machine is really something out of the ordinary," says another, "and that, for once, the Designer has been allowed full play; that he hasn't been forced to unduly standardize ribs, spars, struts, etc., and has more or less had his own way. I wonder who he is. It seems strange we hear so little of him."
"Ah! my boy. You do a bit more flying and you'll discover that things are not always as they appear from a distance!"
"There she is, sir!" cries the Flight-Sergeant. "Just a speck over the silvery corner of that cloud."
A tiny speck it looks, some six miles distant and three thousand feet high; but, racing along, it rapidly appears larger and soon its outlines can be traced and the sunlight be seen playing upon the whirling propeller.
Now the distant drone of the engine can be heard, but not for long, for suddenly it ceases and, the nose of the Aeroplane sinking, the craft commences gliding downwards.
"Surely too far away," says a subaltern. "It will bea wonderful machine if, from that distance and height, it can glide into the Aerodrome." And more than one express the opinion that it cannot be done; but the Designer smiles to himself, yet with a little anxiety, for his reputation is at stake, and Efficiency, the main reward he desires, is perhaps, or perhaps not, at last within his grasp!
Swiftly the machine glides downwards towards them, and it can now be seen how surprisingly little it is affected by the rough weather and gusts; so much so that a little chorus of approval is heard.
"Jolly good gliding angle," says someone; and another, "Beautifully quick controls, what?" and from yet another, "By Jove! The Pilot must be sure of the machine. Look, he's stopped the engine entirely."
Then the Aeroplane with noiseless engine glides over the boundary of the Aerodrome, and, with just a soft soughing sound from the air it cleaves, lands gently not fifty yards from the onlookers.
"Glad to see you," says the Squadron Commander to the Pilot. "How do you like the machine?" And the Pilot replies:
"I never want a better one, sir. It almost flies itself!"
And the Designer turns his face homewards and towards his beloved drawing-office; well satisfied, but still dreaming dreams of the future and ... looking far ahead who should he see but Efficiency at last coming towards him! And to him she is all things. In her hair is the morning sunshine; her eyes hold the blue of the sky, and on her cheeks is the pearly tint of the clouds as seen from above. The passion of speed, the lure of space, the sense of power, and the wonder of the future ... all these things she holds for him.
"Ah!" he cries. "You'll never leave me now, when at last there is no one between us?"
And Efficiency, smiling and blushing, but practical as ever, says:
"And you will never throw those Compromises in my face?"
"My dear, I love you for them! Haven't they been my life ever since I began striving for you ten long years ago?"
And so they walk off very happily, arm-in-arm together; and if this hasn't bored you and you'd like some more of the same sort of thing, I'd just love to tell you some day of the wonderful things they accomplish together, and of what they dream the future holds in store.
And that's the end of the Prologue.
Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum. It therefore obeys Newton's laws14and resists movement. It is that resistance or reaction which makes flight possible.
Flight is secured by driving through the air a surface15inclined upwards and towards the direction of motion.
S = Side view of surface. M = Direction of motion.
S = Side view of surface.
M = Direction of motion.
Chord.—The Chord is, for practical purposes, taken to be a straight line from the leading edge of the surface to its trailing edge.
N = A line through the surface starting from its trailing edge. The position of this line, which I call theNeutral 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 andabovethe line of motion. If it is coincident with M, there is no lift. If it makes an angle with M andbelowit, 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 theneutral lift line makes with the direction of motion relative to the air. You will, however, find that in nearly all rigging specifications the angle of incidence is taken to mean the angle the chord makes with a line parallel to the propeller thrust. This is necessary from the point of view of the practical mechanic who has to rig the aeroplane, for he could not find the neutral lift line, whereas he can easily find the chord. Again, he would certainly be in doubt as to "the direction of motion relative to the air," whereas he can easily find a line parallel to the propeller thrust. It is a pity, however, that these practical considerations have resulted in a bad definition of the angle of incidence becoming prevalent, a consequence of which has been the widespread fallacy that flight may be secured with a negative inclination of the surface. Flight may conceivably be secured with a negative angle of chord, but never with a negative inclination of the surface, if, as seems reasonable, we regard the surface from the point of view of the neutral lift line. All this is only applicable to cambered surfaces. In the case of flat surfaces the neutral lift line coincides with the chord and the definition I have criticized adversely is then applicable. Flat lifting surfaces are, however, never used.
The surface acts upon the air in the following manner:
As the bottom of the surface meets the air, it compresses it and accelerates itdownwards. As a result of this definite action there is, of course, an equal and opposite reactionupwards.
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 ofair on the top of the surface is decreased, thus assisting the reaction below to lift the surfaceupwards.
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 V2.
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, at efficient angles of incidence, approximately at right-angles to the neutral lift line of the surface, as illustrated above; and it is, in considering flight, convenient to divide it into two component parts or values, thus:
1. The vertical component of the reaction,i.e., Lift, which is opposed to Gravity,i.e., the weight of the aeroplane.
2. The horizontal component,i.e., Drift (sometimes called Resistance), to which is opposed the thrust of the propeller.
The direction of the reaction is, of course, the resultant of the forces Lift and Drift. The Lift is the useful part of the reaction, for it lifts the weight of the aeroplane.
The Drift is the villain of the piece, and must be overcome by the Thrust in order to secure the necessary velocity to produce the requisite lift for flight.
Drift.—The drift of the whole aeroplane (we have considered only the lifting surface heretofore) may be conveniently divided into three parts, as follows:
Active Drift, which, is the drift produced by the lifting surfaces.
Passive Drift, which is the drift produced by all the rest of the aeroplane—the struts, wires, fuselage, under-carriage, etc., all of which is known as "detrimental surface."
Skin Friction, which is the drift produced by the friction of the air with roughness of surface. The latter is practically negligible having regard to the smooth surface of the modern aeroplane, and its comparatively slow velocity compared with, for instance, the velocity of a propeller blade.
Lift-Drift Ratio.—The proportion of lift to drift is known as the lift-drift ratio, and is of paramount importance, for it expressesthe 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 necessityto 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 increasingvelocity, 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-resistanceis 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, thusdecreasingDD.
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 suchpressure, then, the better is head-resistance D able to get its work in, and the more thrust will be required to overcome it.
The "fineness" of the stream-line shape,i.e., the proportion of length to width, is determined by the velocity—the greater the velocity, the greater the fineness. The best degree of fineness for any given velocity is found by means of wind-tunnel research.
The practical application of all this is, from a rigging point of view, the importance of adjusting all stream-line parts to be dead-on in the line of flight, but more of that later on.
2.Angle of Incidence.—The most efficient angle of incidence varies with the thrust at the disposal of the designer, the weight to be carried, and the climb-velocity ratio desired.
The best angles of incidence for these varying factors are found by means of wind-tunnel research and practical trial and error. Generally speaking, the greater the velocity the smaller should be the angle of incidence, in order to preserve a clean, stream-line shape of rarefied area and freedom from eddies. Should the angle be too great for the velocity, then the rarefied area over the top of the surface becomes of irregular shape with attendant turbulent eddies. Such eddies possess no lift value, and since it has taken power to produce them, they represent drift and adversely affect the lift-drift ratio. Also, too great an angle for the velocity will result in the underside of the surface tending to compress the air against which it is driven rather than accelerate itdownwards, and that will tend to produce drift rather than theupwardsreaction, or lift.
From a rigging point of view, one must presume that every standard aeroplane has its lifting surface set at the most efficient angle, and the practical application of all this is in taking the greatest possible care to rig the surface at the correct angle and to maintain it at such angle.Any deviation will adversely affect the lift-drift ratio,i.e., the efficiency.
3.Camber.—(Refer to the second illustration in this chapter.) The lifting surfaces are cambered,i.e., curved, in order to decrease the horizontal component of the reaction,i.e., the drift.
The bottom camber: If the bottom of the surface was flat, every particle of air meeting it would do so with a shock, and such shock would produce a very considerable horizontal reaction or drift. By curving it such shock is diminished, and the curve should be such as to produce a uniform (not necessarily constant) acceleration and compression of the air from the leading edge to the trailing edge. Any unevenness in the acceleration and compression of the air produces drift.
The top camber: If this was flat it would produce a rarefied area of irregular shape. I have already explained the bad effect this has upon the lift-drift ratio. The top surface is then curved to produce a rarefied area the shape of which shall be as stream-line and free from attendant eddies as possible.
The camber varies with the angle of incidence, the velocity, and the thickness of the surface. Generally speaking, the greater the velocity, the less the camber and angle of incidence. With infinite velocity the surface would be set at no angle of incidence (the neutral lift line coincident with the direction of motion relative to the air), and would be, top and bottom, of pure stream-line form—i.e., of infinite fineness. This is, of course, carrying theory to absurdity as the surface would then cease to exist.
The best cambers for varying velocities, angles of incidence, and thickness of surface, are found by means of wind-tunnel research. The practical application of all this is in taking the greatest care to prevent the surface from becoming distorted and thus spoiling the camber and consequently the lift-drift ratio.
4.Aspect Ratio.—This is the proportion of span to chord. Thus, if the span is, for instance, 50 feet and the chord 5 feet, the surface would be said to have an aspect ratio of 10 to 1.
Fora given velocityanda given areaof surface, the higher the aspect ratio, the greater the reaction. It is obvious, I think, that the greater the span, the greater the mass of undisturbed air engaged, and, as already explained, the reaction is partly the result of the mass of air engaged. I say "undisturbed" advisedly, for otherwise it might be argued that, whatever the shape of the surface, the same mass of air would be engaged. The word "undisturbed" makes all the difference, for it must be remembered that the rear part of the underside of the surface engages air most of which has been deflected downwards by the surface in front of it. That being so, the rear part of the surface has not the same opportunity of forcing; the air downwards (since it is already flowing downwards) and securing there from an upwards, reaction as has the surface in front of it. It is therefore of less value for its area than the front part of the surface, since it does less work and secures less reaction—i.e., lift. Again, the rarefied area over the top of the surface is most rare towards the front of it, as, owing to eddies, the rear of such area tends to become denser.
Thus, you see, the front part of the surface is the most valuable from the point of view of securing an upwards reaction from the air; and so, by increasing the proportion of front, or "span," to chord, we increase the amount of reaction for agiven velocity and area of surface. That means a better proportion of reaction to weight of surface, though the designer must not forget the drift of struts and wires necessary to brace up a surface of high aspect ratio.
Not only that, but,providedthe chord is not decreased to an extent making it impossible to secure the best camber owing to the thickness of the surface, the higher the aspect ratio, the better the lift-drift ratio. The reason of this is rather obscure. It is sometimes advanced that it is owing to the "spill" of air from under the wing-tips. With a high aspect ratio the chord is less than would otherwise be the case. Less chord results in smaller wing-tips and consequently less "spill." This, however, appears to be a rather inadequate reason for the high aspect ratio producing the high lift-drift ratio. Other reasons are also advanced, but they are of such a contentious nature I do not think it well to go into them here. They are of interest to designers, but this is written for the practical pilot and rigger.
5.Stagger.—This is the advancement of the top surface relative to the bottom surface, and is not, of course, applicable to a single surface,i.e., a monoplane. In the case of a biplane having no stagger, there will be "interference" and consequent loss of efficiency unless the gap between the top and bottom surfaces is equal to not less than about 1-1/2 times the chord. If less than that, the air engaged by the bottom of the top surface will have a tendency to be drawn into the rarefied area over the top of the bottom surface, with the result that the surfaces will not secure as good a reaction as would otherwise be the case.
It is not practicable to have a gap of much more than a distance equal to the chord, owing to the drift produced by the great length of struts and wires such a large gap would necessitate. By staggering the top surface forward, however, it is removed from the action of the lower surfaceand 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.
H.E., Horizontal equivalent. D., Dihedral angle.H.E., Horizontal equivalent.D., Dihedral angle.
6.Horizontal Equivalent.-The vertical component of the reaction,i.e., lift, varies as the horizontal equivalent (H.E.) of the surface, but the drift remains the same. Then it follows that if H.E. grows less, the ratio of lift to drift must do the same.
A, B, and C are front views of three surfaces.
A has its full H.E., and therefore, from the point of view from which we are at the moment considering efficiency, it has its best lift-drift ratio.
B and C both possess the same surface as A, but one is inclined upwards from its centre and the other is straight but tilted. For these reasons their H.E.'s are, as illustrated, less than in the case of A, That means less vertical lift, and, the drift remaining the same (for there is thesame amount of surface as in A to produce it), the lift-drift ratio falls.
The Margin of Poweris the power available above that necessary to maintain horizontal flight.
The Margin of Liftis 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 Incidenceis the smallest angle at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained.
The Maximum Angle of Incidenceis the greatest angle at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained.
The Optimum Angle of Incidenceis 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 Angleis 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 surfacewill be necessary in order to engage the necessary mass of air to secure the requisite lift.