Subsonic wind-tunnel tests on a crescent-winged aircraft, including tests of leading-edge droop designs and several tailplane heights

Abstract

The longitudinal characteristics of the aircraft (Aspect Ratio 4.0) were investigated on a half-model in the Royal Aircraft Establishment 10 ft × 7It Wind Tunnel at a Reynolds number of 1.3 million up to a Mach number of 0.93, and at higher Reynolds numbers at low speed. The tests showed that the high-speed characteristics at low lift were satisfactory, but that at all Mach numbers a severe instability occurred at moderate lift coefficients (C~ = 0.45 to 0.6). Examination of tuft and off-flow patterns established that for Mach numbers up to M = 0.8, the tip stall began as a leading-edge separation at the outer kink; at the highest Mach numbers the flow separation was initially shock-induced. Further tests were therefore made with leading-edge droop applied over the outer wing and with wing fences near the outer kink. The 'pitch-up' at low Mach number was delayed by these means to CL = 0.8 (ΔCL = 0.25), but the stability at high Mach number was not improved. A further modification, in which the plan-form was changed to eliminate the outer kink, brought a small improvement at high Mach number, the pitch-up being delayed to CL = 0.65 at M = 0.9. Lowering the tailplane had a much more powerful effect on the aircraft's stability than these wing modifications, and with the tailplane below the wing chord plane (where wake surveys showed the downwash to be greatly reduced), the nose-up instability.was almost eliminated throughout the test range of Mach number.