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Aviation History
1943
1943 - 0435.PDF
FEBRUARY I8TH, 1943 Topics of the Day FLIGHT 175 A Question of Stability A Few Remarks on the Subject of "Qround Loops" : Have We Yet Produced the Right Kind of Landing Qear? : The Incidental Importance of Brakes : Unnoticed Engine Failure /ANYONE who is prepared to waste an hour or two /-\ watching the landings and take-offs at any advanced •i- •*• training unit is certain, sooner or later (and par- ticularly if there is anything of a cross-wind) to see some very startling and sometimes amusing gyrations. The C.F.I.s and other experts rarely have any sympathy, and the unfortunate pupil is too often on the mat for a per- formance which can only be blamed on the designers of long ago—the designers who thought that the " back-to- front tricycle'' was a good and satisfactory idea. Only those who, for one reason or another, have felt an aircraft taking charge can have any real, idea of what it is all about. The fact is that nearly every "conven- tional" aircraft is directionally unstable except when it is being pulled along with a nice fat slipstream on the tail surfaces, or when it is going fast enough to have the necessary air control. Aircraft are poor miserable duck- lings when they are travelling slowly on the ground. The other day the fact of this astounding directional instability was brought very firmly home to me when I was in a fast twin-engined machine with no available brake pressure. And I realised how much one does depend (without thought—the action is quite reflex) on the brakes for the correction, after landing, of just those little initial movements from the straight which are the beginning of the howling, uncontrollable plunges into the rough. As it happened, there was a slight crosswind, and the land- ing was not as good as it might have been. The devia- tion started in a small way, but because the undercarriage is well ahead of the centre of gravity in this particular type, it was automatically built up until it felt as if the entire machine would either go over on its back or pull its undercarriage off. One had then the strange and appalling case of a heavy aircraft trying to continue in its original course down the runway, while the e.g. was pulling the tail farther and farther round. The poor old thing made the best of a bad job; it turned, with the port tyre screaming and the port oleo leg rammed up solid, until it reached the haven of mud off the runway, when it slid bodily sideways in the - direction in which it had originally wanted to go. Fortunately there were no obstructions, and by this time the starboard engine had decided to answer propejrly-'.to a burst of throttle. We sailed through the rest of the mud like a destroyer going at full speed in shallow water, decks awash with mud and stones. Low-speed Control From now- on I shall have much more sympathy for the tyro who, even with adequate braking—-and conse- quent steering—arrangements, lets the thing go just too far. For when it- has gone too far, the amount of braking effort needed to bring the tail round may easily be enough to put the aircraft on its back. Even if there is that much braking power available. It is all a question of e.g. and the amount of aerody- namic control at comparatively low speeds. The best of aircraft, when incorrectly loaded, can do the most extra- ordinary things. I remember once, in the Battle of Britain days, being one of fourteen passengers in a docile taxi aircraft. It flew quite well but it was impossible for the load of bodies to be distributed evenly at or neartfa e e.g. Consequently, almost as soon as the wheels touched the ground—and certainly as soon as the tail was down—the entire outfit swung sharply round and faced the opposite way while still proceeding quite quickly in its original direction. Fortunately it was a grass surface and no damage was done except to the nerves of the occupants. Of course, the machine should never have been flying with such a load, but in those days aircraft were badly needed, and any reasonable risk was taken to ensure that the maximum possible number of Hurricanes and Spitfires was delivered to the squadrons in the mini- mum time. Quite obviously, any vehicle with the two main wheels in front arid a castoring wheel at the back is going to behave queerly when it once goes off the straight. Par- ticularly with the load too far aft and with no positive steering system. The undercarriage as we know it to-day was designed for use with quite different kinds of aircraft— and even some of those swept round in ground loops from time to time if the designer had decided that the dangers of such ground-looping were less than those of going over on the back, and had placed the undercarriage well ahead of the e.g. (Incidentally, the word "ground-looping" is not a really expressive one. It should mean the process of going over on one's back, when, in fact, it means executing a smart semi-circle in the horizontal plane. Or so I believe.) Two Nose-wheels I don't feel that the tricycle is .the final answer, either. It is a directionally stable arrangement, but far too much depends 011 the nose-wheel and its shimmy dampers. Furthermore, any "level-landing" system means that there is, practically speaking, nothing but the brakes to stop the aircraft once it is on the ground. The effect of the area presented by the wings and the flaps, when the machine is in a tail-down attitude, is very considerable at high and even medium speeds. Such a loss ought, at least until wheel brakes are more reliable, to be compen- sated for by the use of some air-brake system. Generally speaking, however, the level-landing idea, with the e.g. ahead of the main wheels, seems to be the best one at the moment. But is it necessary to have only one small castering wheel in the nose? Why not a sort of four-wheel system ? I can foresee the most enormous diffi- culties in the way of retraction, and it seems to be a 'mechanical impossibility to design any sort of direct-steer- ing system. Apart from the reaction effects, it would in- volve another control for the pilot. The only steerable tricycle type I've flown was a very small one, and the nose-wheel kicked the rudder pedals all over the place. Through suitable damping devices the steerable wheel or wheels might reasonably be connected to the control wheel. When on the ground the pilot would have to forget the original flying uses for the wheel—and that might be psychologically impossible or dangerous. Once upon a time I flew briefly in the Hordern-Richmond twin, which had the rudder and the ailerons controlled through a single column ; turning the wheel produced rudder effect, and rocking the whole column sideways operated the ailerons. That was difficult enough, though pilots became duly accustomed to it—as they would, o£ course, if all aircraft were designed that way. I don't know what the final answer may be. I certainly haven't got it, though I'm quite certain that the " back-to-front " arrangement is all wrong, and that something must be done about it. People have talked quite a lot about the shimmying and other dangers of nose-wheels. They are, I think, liable
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