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Aviation History
1937
1937 - 1414.PDF
530 FLIGHT. MAY 27, 1937. PRIVATE Ste-?s FLYINC Topics of the Day Imaginary Effects E VERY now and then the old business of wind effect crops up again in some argument between flying people. Feeling certain of the course of logical reasoning, I have always claimed that any aircraft, once loose from the ground, can be treated as moving in an absolutely stationary medium, and have listened with growing suspicion to any statement, for instance, that down-wind turns are dangerous. The gliding people are insistent about that last fact, and claim that pilots always find that, whereas a turn made into wind can be made carelessly and may, in fact, provide one with reserve speed, turns down-wind must always be made with care and an adequate supply of speed. As it stands, that is pure mediaeval nonsense, in my opinion, though I take their word for it that pilots do find such difficulties. A few days ago I had a long argument with somebody who found it extremely difficult to dissociate his ideas of ground and air speed. The immediate result of my en deavours to clarify the situation for him was that it was cleared for me as well, and I am now a Philistine again. You and your aeroplane or glider can move about as satis factorily in a hundred-mile-an-hour wind as you can in a flat calm—so long as the wind speed remains constant. One might imagine a racing car on a steeply banked track, the latter being moved bodily at a constant speed ; the drivers would not notice the slightest difference unless they happened to see the rest of the landscape as it went by. Probably the best simile to use is that of an express train, in which the train is equivalent to the wind mass and the grip of one's feet on the corridor floor to the grip of the wings on the air. So long as the train's move ment, fast or slow, is steady, one can run in each direction or turn circles with equal ease and even play cricket in the corridor without making any allowances. Hang it all, the earth itself and the solar system are hurtling through space at some incredible velocity, but this does not make the slightest difference to our movements. To the gliding people I say: Go up above the clouds on a day of really high wind, and I defy you to tell me, without reference to the compass, whether you are turning up-wind or down-wind. The effect, in so far as it is real, may be the result of changing air currents near the ground—affecting the lift in different ways according to the attitude of the machine. Psychological BUT the great worker of all such apparent magic is nothing more or less than applied psychology, and the effects are so much more noticeable to the glider pilot because his air speed is so comparatively low. After moving over the ground at a mere 10 m.p.h., it is some what disconcerting to find oneself suddenly moving at 50 m.p.h.—quite apart from the strange sensations obtained during the turn itself. The impression of extra lift after a turn in the other direction is the equally natural result of the sudden change in ground speed. If the gain or loss of height is very real on occasion, it is surely the result of the instinctive application of various incorrect conclusions to the actual control of the machine, either during or after the turn itself. Somebody else can work that idea out thoroughly. It is certainly a fact that some pupils will never learn to do correct gliding turns near the ground though they may be able to do them very well indeed at a safe height. The swift movement over and the proximity of the ground upsets them, and their turns are flat to the point of being actively dangerous. Hence the amount of spinning-in off gliding turns which is still done year after year. There is another point, too, and that is that the wind velocity near the ground is very much lower than it is higher up, and a down-wind gliding turn made into a lower- speed layer naturally causes a sudden drop in air speed. Wind Changes AS far as that is concerned it might appear that I have left a large loophole in the argument. Whereas no movements of an aircraft in a constantly moving mass of air can have any possible effects other than those which are actually made by the controls, a sudden change in wind velocity is quite a different matter. In that case the machine must either accelerate or decelerate instantan eously if its speed through the air is to remain constant. The undoubted fact that a flat calm hold-off is very much more prolonged than one carried out into a fair breeze has always struck me as being "agin nature." The only reasonable reason which I can produce is that the wind speed at ground level is quite a lot lower than it is, say, at 10 feet. If the machine is brought in at an air speed of 60 m.p.h. against a 20 m.p.h wind, and this wind drops to 10 m.p.h., then the machine's air speed is only 50 m.p.h. In any case, the additional roughness of the air near the ground on a windy day would account for quite a rapid loss of speed. Most of us know of aerodromes where, when the wind is in a certain direction, the approach must be made quite fast if control is to be retained for a reasonably accurate hold-off. When approaching over hangars, for instance, the air may be "piled up" on the windward side so that one drops into almost still air once they have been crossed. An aerodrome with a pronounced camber may have a dead air region near the boundary on the leeward side, since the wind will not necessarily follow the contour of the ground. INDICATOR.
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