FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1926
1926 - 0126.PDF
20 SUPPLEMENT TO FLIGHT FEBRUARY 25, 1926 THE AIRCRAFT ENGINEER leaves^the ground, which arrangement gives increased travel and provides for considerable energy absorption before the undercarriage again reaches unit load. Many tests were made to find the range of movement in service, and strangely enough the greatest deflections have been recorded in high-speed taxying over rough ground. Landings, made purposely severe, on the same ground have not produced so great a deflection. Fig. 3 shows a representative load/deflection curve for an undercarriage suitable for a machine having a total all-up weight of about 3,200 lbs. The total work of com- pression for the two legs is equal to 3,250 ft./lbs., which figure is ample when allowance is made for the energy absorption of the tyres and the strain energy present in the axles and struts under load. STALLED FLIGHT AND CONTROL. The Practical Aspect. By FRANK T. COURTNEY. The fundamental factor in the question of the safety or otherwise of an aeroplane is the stall. The vast majority of aeroplane accidents, including landing accidents, can be directly attributed, in some way or another, to stalling. It follows, therefore, that the study of stalling is not merely the study of one of the very many interesting problems of aviation; it goes right to the root of aviation. Conse- quently, it is a matter on which the most accurate conclusions should be drawn. In my humble opinion the great amount of talking, writing and experiment that has been done recently on this subject has been done from a hopelessly unpractical standpoint, is very largely inaccurate, and is directly harmful as Well as wasteful. I have for a very long time closely studied this question, and I give here, for what they may be worth, my practical conclusions on the matter. In the first place, what is the stall ? The lift on a cambered aerofoil is due to a phenomenon of air flow around the cambered surface, which has little or no resemblance to the lift that would be given by the vertical component of the air pressure on an inclined flat plate. When the cam- bered surface exceeds a certain angle of incidence (which, in actual practice, corresponds generally to a certain minimum or '•stalling" speed), this flow quite suddenly breaks down. It is true, in theory, that this does not mean the complete loss of lift, since that form of lift is replaced by a much lower degree of lift, corresponding to the flat-plate lift. But, in practice, and in any normal reasonable aeroplane, this loss of lift is so great and so sudden as to amount to the complete loss of all lift on the ivings. Moreover, even if the remaining lift us taken into account, it is accompanied by so high a drag that the corresponding gliding angle, is steep enough to correspond with the dire of a liftless aeroplane. The forces then are too low to provide any useful parachute effect, so that I adhere to my contention that, for all practical purposes, the stalling of an aeroplane corresponds to complete loss of lift. This is where, in my view, recent research in the matter has parted company with practical politics, since it has concerned itself with the pointless problem of '" What shall we do when we have stalled ? " It has spent much time and money on the useless study of stalled flight, and inci- dentally evolved a useless mechanism to deal with it, which is based on arguments as plausible as they are inaccurate. Before dealing with the arguments of the Aeronautical Research Committee, I should point out that the theoretical and aerodynamical facts concerned with stalling are reasonably simple and accurately known. The rest, therefore, is purely a flying problem, so that the flyer who has closely examined the facts as they occur is not in too weak a position to dispute with the scientists. The arguments on which the Aeronautical Research Committee have based their conclusions are approximately as follows :— (1) It is admitted that the vast majority of aeroplane accidents are due to stalling near the ground, and this occurs when a pilot in difficulties near the ground, accidentally stalls his machine in trying to extricate himself from the difficulty. (2) Such stalling being almost invariably accompanied by the dropping of one wing or the other preparatory to spinning, the use of the ordinary aileron for raising this wing is applied. This, after stalling, has very little effect for raising the wing, but assists the spinning tendency owing to aileron drag. (3) If an aileron were to be designed so as to maintain powerful lateral control, the pilot, though stalled, would be able to raise that wing, thus avoiding a spin, so that— (4) The machine would then drop horizontally into the ground with less serious harm to its occupants than if it had spun (which at a low height generally means dived) into the ground. At first sight these argiiments seem very reasonable. A closer study shows, however, that they not only have little or nothing to do with the real circumstances involved, but are even technically inaccurate. Let us take argument Xo. 1. The whole fallacy of this rests in the idea that the pilot accidentally stalls. He doesn't. The vast majority of stalling accidents are due to the deliberate attempt of the pilot to hold the machine in the air—to reach a field, to clear a tree, or what not—for a greater distance or period than is physically possible for the machine. He does not, of course, deliberately stall, but he does deliberately carry out those actions leading to the stall, and the mental attitude which makes him try and hold the machine up in defiance of his air-speed is really a subconscious defiance of the stall. Thus the arguments of the Aeronautical Research Committee have already broken down, since the problem has already almost ceased to be a mechanical one and has become a psychological one. The state of mind of the pilot immediately preceding such a crash is one of a greater or less degree of mental panic. Hence I might say in passing that I am convinced that those anti-stalling devices which give the pilot a warning forward kick on the control would in practice probably be worse than useless. For if a pilot's stick were kicked forward at the moment when his governing idea is to keep out of the ground he would pull it back still harder. Argument No. 2 similarly leaves out most of the practical factors. In the first place, after stalling the impression given to the pilot by the dropping wing is really that of the nose diving away in a sideways direction. His governing idea, then, is to get the nose up, with such aileron control as he may manage to use when the stick is hard back. I have watched many such crashes, and have only once noticed any large amount of aileron movement. I consider that there is no question that a pilot who has let himself get into those circumstances has no other idea than " stick back." The ailerons, then, whether he uses them or not, have little to do with the main fact that the wings have lost their effective lift, the nose drops, and whether the wing drops or does not drop has no importance for the ailerons. For if the pilot had sufficient reason left to use his ailerons he would also use corresponding rudder, which, under the circumstances, would be a far more effective control. If a pilot under those crash conditions cannot raise his wing, he dives into the ground somewhat sideways. If he does raise his wing he still dives into the ground forwards. For, as regards argument No. 4, there can be nothing what- ever to show that, even if under complete control, a stalled glide will differ much from a dive. Moreover, it has the additional disadvantage that, whilst a dive is something on the way to normal flight, a stalled glide under control would tend to remain a stalled glide, for, in any attempt to regain normal flight, the pilot would have to put himself through the dive stage. Nor would the machine maintain anything like a horizontal attitude fore-and-aft, unless an elevator control of preposterous dimensions were employed, or all normal stability systems upset. 110/?
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
