The production of lift independently of incidence--the Thwaites Flap, Parts I and II

Abstract

In Part I of this paper, the possibility of obtaining lift on a body in a uniform stream independently of the incidence is discussed, and a practical method which obtains this effect is given. It is shown that a small thin 'flap' which may be moved about a well-rounded trailing edge through which, for example, continuous suction is applied will produce circulation about the aerofoil. A necessary feature of this method is tile prevention of separation of flow by boundary-layer suction, which is also used to reduce substantially the width of the wake. The method uses principles quite different from those which have been proposed in the past for obtaining increased lift on aerofoils. The practical applications of the device are briefly discussed, and some interesting consequences pointed out. It will, for instance, be possible to fly with an aerofoil always at zero incidence. Again, the stall in which the flow separates from near tile leading edge may be completely avoided, for as the circulation and lift increase, the incidence may be decreased so that severe adverse velocity gradients occur nowhere but near the trailing edge. In Part II of the paper, a report is given of a preliminary experiment which was set up to investigate whether the theoretical predictions made about the efficacy of the Flap were largely confirmed. A wholly porous circular cylinder was fitted with the Flap and measurements were made of the pressure distribution round the cylinder for various positions of the Flap. These observations shewed that for angular deflection of the Flap of less than 20 deg, about 85 per cent of the theoretical value of CL was realised : a maximum CL of about 5-6 was obtained. These results are taken to shew that tile physical principles of Part I are sound and that the Thwaites Flap does, in fact, enable lift to be generated independently of incidence.