Investigation of Effect of Dynamic Stall and Its Alleviation on Helicopter Performance and Loads

The static and dynamic stall characteristics of VR-7 baseline and two modified airfoils were computed and compared with available experimental data. The unsteady, compressible Reynolds-averaged Navier-Stokes equations based on an unstructured-grid approach with the one-equation Spalart-Allmaras turbulence model has been used to investigate flow over these airfoils in stationary and oscillating conditions. The baseline VR-7 results correlate well with static test data; the computed dynamic results of the VR-7 show a large negative pitching moment and drag observed in the hysteresis curves and agree fairly well with dynamic test data. An optimization technique was used to modify the upper surface of VR-7 airfoil with the cost function of minimized drag while maintaining a specified lift. The computed static and dynamic characteristics of the modified airfoils at low Mach numbers show improvement in the static characteristics and a large reduction of the negative pitching moment in the dynamic case. The effect on helicopter performance and loads are analyzed using a comprehensive analysis code with the computed static and dynamic characteristics of VR-7 and modified VR-7 airfoils for aerodynamics.