The objective of this study is to investigate the effects of time-varying, compressible flow on rotorcraft dynamic stall. The problem was studied in an unsteady transonic wind tunnel and partitioned into three segments to facilitate understanding: pitch oscillations in a steady freestream, investigating unsteady effects (reduced frequency) and compressibility effects (Mach number); fixed pitch in a modulated freestream, studying various pre- and post-stall angles with fluctuating Mach number; and oscillations of both angle of attack and Mach number, studying the coupled effects and the importance of phasing between the two. The impact of compressibility on the traditional dynamic stall process was evaluated, with higher Mach numbers (and corresponding increased compressibility effects) changing the character of dynamic stall on the SSC-A09 from leading-edge to a trailing-edge phenomenon. Mach oscillations on a fixed-angle airfoil were found to have significant hysteresis at pre-stall angles, and substantial unsteadiness (periodic pressure fluctuations) in the post-stall regime. Coupled pitch and Mach oscillations showed that periods of flow acceleration have a stabilizing effect on the dynamic stall event.

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