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Aviation History
1955
1955 - 0012.PDF
12 FLIGHT HIGH-LIFT GENERATION PART 1 By A. R. WEYL, A.F.R.AcS. THE application of high specific-lift forces which aero-foils may produce is still in its infancy, and for thisour obsession with speed is responsible. However, the rising demand for imperfect, uneconomical and complicated machinery, such as is represented by the modern helicopter, is proof of the existence of vast fields which require development. Before we go into the wide and important subject of aerofoil lift regardless of drag, certain conceptions need clarifying. Lift Coefficients.—Aerodynamically, lift (or cross-force) isnormal to the direction of motion. Under certain conditions, however, aerofoil lift can be generated without forward motion.In such cases, lift can be any air force which opposes gravity. In these instances, also, the usual definition of "lift coefficient" losesits sense, and a different basis must be adopted. Lift coefficients are often victims of fallacy or deception. Inpractical flying, the maximum lift coefficient is derived from a stalling speed and referred to a standard wing area, regardless ofwhether chord-extending flaps or other slow-flying devices actu- ally increase the effective wing area. So far as achieved valuesof the maximum lift coefficient are concerned, these associations cause confusion. Aircraft manufacturers, eager to sell their products as miraclesof low-speed performance, are alive to the fact that accurate determination of a stalling speed is beyond the scope of theordinary customer; they prefer to quote "landing" speeds and the indication of airspeed indicators. A landing speed is a measuremade of rubber. It allows the production of synthetically enormous Ci,max values, aided and abetted by a shrewd choice inpositioning the pitot head of the A.S.I. Thus a plain wing which, at its best, could have a maximum lift coefficient of 1.6, has beencredited with one of not less than 6 (a value hitherto never achieved in practical flight). On the other hand, many modern aircraft do not even achieve,during take-off and landing, the modest CLIIUX values with whichthe labours of aerodynamicists have provided them. This is simply by reason of imperfect design features, such as lack ofcontrol power in the low-speed range, or because of deficient stalling qualities. We are still in the unfortunate stage where theincidence of the maximum lift and that of incipient stall are close together, and often so close that they practically coincide. Thisdoes not apply only to certain wing shapes and notorious aerofoil sections; most commonly employed high-lift devices suffer from it. High Lift.—The generation of very high lift is linked with theoccurrence of high drag forces. Even if it were possible to keep the profile drag low (and there is little prospect of this) the induceddrag increases with the square of the lift coefficient. Since the finite span of a wing would also decrease the lift, because ofpressure equalization over the tips, design features such as end discs, fences, curtains, etc., have to be applied in order to utilizefully the benefits of very high lift coefficients; these features add to the drag. A.s the drag force depends upon the square of the flying speed,a high drag coefficient means little at low speeds, and during vertical take-off and landing nothing at all. At a lifting aerofoil, the air assumes velocities of flow whichdiffer from that of the forward speed of the aircraft. Along the dorsal surface the air has greater velocity, along the ventral surfacelower velocity. This is the result of the superimposition of the undisturbed free-stream flow of air by a circulatory flow aroundthe aerofoil section (Fig. 1). This "circulation" causes velocity and pressure differences at the aerofoil surface (Fig. 2) and hence isresponsible for the lift. To set up a lift-producing circulation, the aerofoil must eitherbe asymmetrical in its section or have an incidence against the undisturbed direction of flow. Asymmetry is expressed as camberof the aerofoil centre-line, and camber without incidence is capable of producing lift. With a symmetrical body, such as a circularcylinder, circulation can be induced and/or sustained by rotating it in a fluid stream (Fig. 3). This is genuine "rotor" lift (Magnuseffect). For high lift, wings normally make use of camber and incidence combined. In conjunction with symmetrical aerofoilsections, a deflection of flaps gives the effect of camber lift. For most of us "circulation" implies something connected with PARALLEL FLOW ; .: :;: ::r ;v CIRCULATION >' . • ' POTENTIAL FLOW WITH CIRCULATION ( lifting aerofoil ) Fig. 1. Generation of lift in potential flow, from the superimposition of a parallel flow with a circulation. , LIFT FORCE VENTRAL SURFACEVENTRAL SURFACE oc= -6" DORSAL SURFACE Fig. 2. Distribution of pressure around an aerofoil section. Fig. 3. Rotating cylinder in inviscid flow.
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