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Aviation History
1910
1910 - 0465.PDF
JUNE I8, 1910. J^Hf] HOW AN AEROPLANE LIFTS. (Concluded from page 446.) Now at the very outset it is a purely arbitrary assump tion to say that the plane brings the air-stream to rest in the manner stated. If the air is composed of molecules of matter, why should they not bounce off on striking the plane ? To suppose that they do bounce off is just as reasonable as to suppose that they are brought abso- V- v + (A). v- PELTOh bUCKET A- KREA OF JET V+" = MEL OF WPACT ft/sec" V V--VEU OF DISCHARGE. - V /'DENSITY MP&S/SEC -/>AV , CHfNPtqE 0fVEL-jv-(-v) =+2V CHP.NQE OF MOMENTUM =Q/'AV2 PRESSURE UK (ktftlR) «. i£AV2 = |5^ ' ^(Wmf>h^ lutely to rest, but the resultant formula for pressure would be very different; in fact, if the velocity with which they rebounded were the same as that with which they struck— as it would be under conditions of perfect elasticity—the resultant pressure would be twice as great as that given by the above formula. This principle is put into practical application in the Pelton water-wheel (Fig. 9), where the buckets are so shaped that the stream is reversed on impact. It is evident in the first place, therefore, that the fiat plane is not the most efficient sort of obstruction con sidered from the point of view of receiving maximum wind pressure; in fact even the formula first mentioned, which only supposes that the air is brought to rest, is well known to give results about 50 per cent, higher than its true value. The reason for this is because the air when it strikes the normal plate is not actually brought to rest except possibly at the very centre of the stream, the remainder spews out over the edges and is carried on again by the main current in a manner which we have already had occasion to describe in another article. While this fact explains in what respect the hypothesis of the theoretical air-pressure formula is inadequate in practice, it does not directly suggest any reason to suppose that an inclined aeroplane should be more effi cient in respect to air pressure than a normal plate. As a matter of fact, the inclined flat plane at small angles of incidence is only subjected to a fraction of the pressure experienced by the normal plane. The real point containing the solution to Mr. Hickman's question lies in the fact that all practical aeroplane flyers and gliders have cambered sections, instead of being perfectly flat. The advantages of the cambered section have long been appreciated, and were well known to Lilienthal, Pilcher, and other early pioneers, who were more or less successful with practical machines. The cambered section is, moreover, a distinctive feature of a bird's wing, and in this connection it may not be without value to abstract the following information relating to the subject from a chapter of Mr. F. W. Lanchester's " Aerodynamics " : — " One of the most remarkable, and it may almost be said unexpected peculiarities of wing form, is the dipping front edge or arched section. This is a characteristic in the wing form of all birds capable of sustained flight, but it is only within comparatively the last few years that this feature has been the subject of observation. It is scarcely credible that so marked a peculiarity should have escaped observation for centuries, but it would seem that such is the case. Historically, so far as the author has been able to ascertain, the credit of the discovery of the dipping edge is due to Horatio Frederick Phillips, whose publication is to be found in the specification of patent 13768 of 1884. The discovery appears to have been made as a matter of practical experience, and, as often takes place under these circumstances, the theory given by the inventor in his specification is erroneous. The advantages of the arched form of wing section were known to the late Herr Lilienthal at the time of his experiments in flight, 1890-94, and the discovery has been attributed to him by some writers. About the same time as Lilienthal was at work, the author succeeded in evolving the arched form or dipping front edge purely from theoretical considerations, at that time having no knowledge of the previous work of Phillips, or of the experiments then being conducted by Lilienthal." The theory which Mr. Lanchester associates with the advantages of the cambered section is based on the spewing action of the air when striking a flat plate. This spewing action con stitutes a kind of cyclic disturbance around the edge of the plate, and that disturbance which occurs round the leading edge being superimposed upon the horizontal motion of flight or horizontal wind as the case may be, produces a resultant relative wind with an upward trend. In order to take advantage of this relative direction the leading edge of the aeroplane is dipped downwards, and in order to avoid the sharp corner which would thus occur at the junction between the fore and aft parts of the bent aeroplane, the very natural result is a cambered section, which is suitably designed to receive the air and change its momentum without shock. Dealing with the air in this manner (Fig. 10) is somewhat analogous to dealing with a water jet by means of a bucket of a Pelton water-wheel, at any rate the lift efficiency is very materially enhanced thereby, apart from other advantages which are of equal importance. From what has now been said it should be fairly clear that the relationship between the cambered aerofoil of practice and the normal plane is not altogether as simple as it appears at first sight, and that there is no reason why the actual reaction on the decks of a well-designed NEWTONS THIRD LAW (Cambered Pievofoil) Lift R* Force • Weight W H<3 »0 . 463
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