Optimum Gurney flap height determination for “lost-lift” recovery in compressible dynamic stall control

A variable droop leading edge (VDLE) airfoil was successfully used to control compressible dynamic stall and its associated adverse pitching moment variations. But, the price for this success was a 10% loss of lift. A Gurney flap was then attached normally to the airfoil pressure surface at its trailing edge to recover this “lost-lift”. Gurney flaps are seldom used in compressible flow environment, especially when large angles of attack changes are involved. So, it became necessary to find the optimum flap height that worked satisfactorily for all helicopter retreating blade flow conditions of interest. Parametric experimental studies of airfoil relative performance data with three separate Gurney flaps ( 0.01 c – 0.03 c height) indicated that a 1%-chord height flap was the most satisfactory. Even though the appropriate Gurney flap height was thus established, a post-experiment analysis of the ensemble averaged airfoil unsteady canonical pressure distributions offered an explanation for the observed success of the 1% Gurney flap. Based on the findings, it is suggested that canonical pressures could serve as a new parameter that can be used for the determination of the Gurney flap height and minimize the experimental effort.