DESIGN AND WIND TUNNEL TEST RESULTS OF A FLAPPED LAMINAR FLOW AIRFOIL FOR HIGH-PERFORMANCE SAILPLANE APPLICATIONS

Abstract

A laminar flow airfoil with camber changing flap, named DU89-1.34 / 74, has been designed and windtunnel tested for application in the high-performance sailplanes ASH-26E and ASW-27 produced by Alexander Schleicher Segelflugzeugbau, Germany. The ASH-26E is an 18m span selflaunching sailplane with retractable propellor and the ASW-27 is a 15m span FAI competition sailplane. Primary objectives were: low drag at a specified range of lift coefficients and Reynolds numbers, no abrupt loss of lift beyond the upper boundary of the low drag bucket at high lift conditions - to avoid bad handling and climbing qualities in thermal flight conditions, gradual stalling characteristics, and a maximum lift coefficient insensitive to leading edge contamination. These requirements have been met, as verified experimentally, by the design of long laminar flow regions on the upper and lower surface and, at increasing angle of attack, a controlled growth of the turbulent separated area while transition moves forward to the leading edge. Flap deflections and artificial transition were integrated from the start into the design. Flexible slot sealings save drag and, at the high speed flap settings, the sealing on the lower surface enables the boundary layer to remain laminar up to 95% chord, where pneumatic turbulators cause transition. In comparison with the well-known Wortmann sailplane airfoil FX62-K-737/77,the new airfoil shows superior performance.