Flight and Tunnel Test Research on Boundary-Layer Control

Abstract

Between 1941 and 1945 an experimental investigation was conducted a t the Lockheed Aircraft Corporation to determine whether drag reductions could be obtained a t level flight speeds by removing the boundary layer from the wing by means of suction slots. Prior to that time it was known that substantial lift increases could be achieved by this means under certain circumstances, and, as early as 1935, dataf were available showing large drag reductions at high lift coefficients with flaps deflected. Also, attention had been called to the growing importance of the possibility that the large drag increases caused by flow separation when shock waves occurred could be reduced or eliminated by suction slots. Nevertheless, the data available on the use of boundary-layer control at low lift coefficients did not indicate drag reductions, probably because the internal losses were large in the elementary slot-duct systems employed, Furthermore, it was felt that the early application of boundarylayer control hinged upon the success of this method in reducing the drag for high-speed and long-range flight. Otherwise, the performance gains were considered insufficient to justify the complication and developmental difficulties involved. Because of the recognition of the importance of the internal losses in the slot and ducts, an at tempt was made to develop satisfactory internal systems, after which a wing was tested in the Lockheed wind tunnel with suction applied. Finally, a series of tests was made at full scale and high Reynolds Numbers in flight when an opportunity arose to apply boundary-layer control to an envelope tha t had been employed in a low-drag-airfoil investigation.