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Aviation History
1922
1922 - 0688.PDF
NOVEMBER 23, 1922- THEORY OF THE SLOTTED WING* LECTURE by A. BETZ, GOTTINGEN. THROUGH the intensive study of all technical aviation problems during the war, the most important aeroplane parts, especially the wing, were so thoroughly tested as to create the impression that no further substantial improvement was possible. The characteristics of the different wing sections were sufficiently known to enable one to select the most suitable section for almost any purpose. Then the discovery by Lachmann and Handley Page suddenly revealed entirely new possibilities and the wing section again became a rich field of problems. As probably you all know, this discovery consisted in making one or more slots m the wing section (Fig. 1). In this way it is possible to use the wing at higher angles of attack and thus considerably increase the lift. The lift-drag ratio,' however, seems to be no better 111 general than for ordinary wmg sections. The advantage lies principally in the ability to vary the coefficient of lift, and hence the speed, within considerably wider limits. Hereby, the difficulties of taking off and landing are diminished and greater flight speeds made possible. Our knowledge of the behaviour of such slotted wings under the most diverse conditions is, unfortunately, very limited, and there is still much work to do before we 20 1-6 i-e •8 vr 0 -4 A lift 71 j-<» 1 //~" y> -b /*\ X 0- 10 20 30 AQ Cf -c^e^ FIG. rendered more productive of results if we succeed in obtaining at least an approximate idea of what takes place. We are still, however, far from being able to give a complete theoretical explanation of the phenomena of slotted wings. Nevertheless, we can contribute something towards the explanation of the unusual increase of the lift-coefficient. I do not wish, however, to create the impression that what I am about to say is conclusive. I wish rather to bring the matter up for discussion, in the hope that still other view points may be presented which will help to clarify the problem. We must first consider the question as to how it happens that, with a given wing section, the lift-coefficient cannot be increased at will. In order to answer jrhis fundamental question, we must consider more carefully the process by which lift is generated. It is known that lift is produced by the greater velocity, and consequently smaller pressure, of the air on the upper side of the wing than on the lower (Fig. 2) . This difference must vanish at the trailing edge, around which the pressures can become equalised. The difficulty lies in the fact that a strong suction must be generated on the upper side, only to vanish again at the trailing edge. From the Fig. 1.—Polars of an ordinary wing and of two slotted wings. Fig. 2.—Flow around a wing section (a), and, (b), corresponding pressure distribution. c^C shall have carried our investigations so far as to be able to choose, from the many possible modifications, the one best adapted for any given purpose. The question, of the most practical importance is what must be done in order that with an aeroplane we can obtain the best possible lift-drag ratio if the lift-coefficient is low, and, in addition, be able to reach, by easily made changes, a considerably higher coefficient of lift where the lift-drag ratio does not need to be especially good. The former condition would be used in ordinary horizontal flight, and the latter in taking off or in landing. The purely experimental solution of all the problems connected with these new wing sections is rendered very difficult by the large number of possible modifications. The most diverse cross-sections may be given the component parts of the wing and their relative size may be varied, thus bringing the slot nearer either the leading or trailing edge. Furthermore, the relative position of the parts and the width of the intervening slot may be varied. Lastly, there is the possibility of varying the number of the component wing-parts by the introduction of one or more slots. Although, for structural reasons, many forms do not come into practical consideration, the number of possibilities is still very large. The experimental work will be considerably simplified and * rR^£ri?t fr0m Berichu und Abhandlungen cUr Wissenschaftlichen Gesellschaft jut Luflfahrt (supplement to Zeitschrift fur Flugtechnik und Motorluftschiffahrt), No. 6, January, 1922 (Technical Note No. ioa of the American N.A.C.A ). point of least pressure on, the kinetic energy of the air must therefore be transformed into pressure by a gradual increase in the cross-section of the tubes of flow. There accordingly takes place, on the rear portion of the upper side, a pheno menon very similar to the flow through a widening tube. Now, it is known that such a flow, in which kinetic energy is transformed into pressure, remains stable only for a very gradual increase in the size of the cross-section. If the diameter increases too rapidly, the air does not continue to flow smoothly along the wall, but separates from it and goes its own way as a free jet, and the increased pressure is not obtained. If we increase the angle of attack of an aeroplane, the cross-sections of the tubes of flow on the suction-side are increased ; and if a certain figure is exceeded, the air no longer flows along the upper surface of the wing, but is torn off, as it is expressed. This phenomenon is shown by Figs. 3 and 4. (The photographs were made by Dr. Heis and published in Prof. Prandtl's report on the Gottingen Aero dynamic Laboratory, in the Year-Book of the Air Traffic Association, 1912-1913.) The first picture shows a wing having a norma] angle of attack. The flow conforms quite well to the top of the wing, and is not seriously affected by the small vortices which cover the wing. With larger wings and greater velocities the vortices are probably still smaller. The second picture shows the same wing at a somewhat greater angle of attack, in which case the fluid no longer follows the top of the wing. 688
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