(AIAA Paper 2003-0048), Journal of Aircraft, Vol. 41, No. 4, pp. 862-969, Jul. - Aug. 2004.The control of compressible dynamic stall using a variable droop leading edge airfoil is described. The leading
25% of a VR-12 airfoil is drooped as it executes sinusoidal pitch oscillations such that the leading portion of the
airfoil is always at a low effective incidence to the flow. Airfoil performance data determined for freestream Mach
numbers ranging from 0.2 to 0.4, at reduced frequencies from 0 to 0.1, and using unsteady pressure transducer
measurements, show that droop reduces the tendency of the airfoil to enter the dynamic stall state. Even when it
does, the strength of the dynamic stall vortex is significantly reduced, which is reflected in the 40 to 50% smaller
negative peak pitching-moment values, with positive damping of the airfoil. Also, the airfoil drag when the droop
is dynamically varied is reduced by up to 75% relative to a nondrooped airfoil, making a strong case for the use of
this concept for dynamic stall control.U.S. Army Research Offic
