Tests of Pneumatic Means for Raising Airfoil Lift and Critical Angle 270028
DISCOVERY of a satisfactory method of increasing the maximum lift of an airplane wing that should have structural simplicity, high wing-loading, low landing-speed and reasonably low drag, was the object of experiments and wind-tunnel tests made by the engineering division of the War Department Air Service at McCook Field. Previous study of the “burbling” characteristics, or discontinuity of air-flow, of airfoils at McCook Field indicated that the attainment of high lift depends upon an extension of the burble angle, that the angle at which burbling occurs can be controlled, within a range of about 5 deg., by changes in velocity or in turbulence of the wind, and that if burbling can be deferred artificially still further than the 5-deg. range the lift will increase in the same proportion.
Studies abroad with rotating cylinders and the magnus effect confirmed the boundary-layer theory and principles enunciated by Dr. Prandtl in Göttingen and showed that discontinuity of air-flow, which usually occurs first at mid-span of the wing, need not be regarded as incurable. Some success was attained at McCook Field in regulating the boundary-layer effects by use of a jet of air made to flow tangent to the upper surface of an airfoil to blow away the turbulent skin-frictional layer of air and draw it downward from its burbled position into contact with the airfoil. These experiments are described and the results given.
Greater success in deferring the burble angle was attained recently in Göttingen by inducting the boundary-layer air into the wing through perforations in the airfoil by suction, thereby increasing the angle of maximum lift and the maximum lift itself threefold. Details of tests by both the jet aspiration and the boundary-layer induction method are given in the paper. The tests were made with a 6x18-in. wooden airfoil-model in a wind-tunnel provided with glass sides for observation. The surface of the airfoil was coated with a mixture of lampblack and kerosene in which the air currents created flow-lines that were photographed. In the induction tests the number and location of the induction perforations were varied and the angle of the airfoil was varied.
Results obtained show that with the aspirator jet the lift is increased throughout the range of angles tested, that the burble angle can be deferred 2 deg. by the jet and the maximum lift increased 35 per cent, and that the drag is also increased although in a smaller proportion. With the induction method it is concluded from the evidence of the photographs, that the burble angle can be raised 25 deg., or 10 deg. above normal, and that its benefits are noticeable for angles as high as 35 deg., or 20 deg. above normal. At the same time the drag is decreased. Power required for operation of the suction-pump is estimated at approximately 4 per cent of the power needed for flight.