Influence of oscillation amplitude and Mach number on the unsteady transition on a pitching rotor blade airfoil

The unsteady flow around the pitching helicopter main rotor blade airfoil EDI-M109 was experimentally investigated at conditions similar to those existing on a retreating rotor blade in forward flight. High speed pressure measurements and hot film anemometry were used to investigate the unsteady transition characteristics of the airfoil. Results are presented for dynamic test points with attached flow, light dynamic stall and deep dynamic stall at M = 0.3 - 0.5 and Re/M = 6x10e6. The transition process of the pitching airfoil is analyzed and the influences of the oscillation amplitude, the mean and the minimum angles of attack are discussed for the time-dependent transition characteristics and the transition hysteresis. The effect of the Mach number on the flow field and on the unsteady transition behavior is presented and a significant change in the transition characteristics is revealed at high Mach number