Compressible dynamic stall control using a variable droop leading edge airfoil

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.