DYNAMIC STALL OF SINUSOIDALLY OSCILLATING THREE-DIMENSIONAL SWEPT AND UNSWEPT WINGS IN COMPRESSIBLE FLOW

Measurements have been made of the unsteady aerodynamic response of a three dimensional wing to sinusoidal pitching oscillations. The model scale and test conditions were selected to simulate the Reynolds number, airfoil section, Mach number, reduced frequency, angular range, and sweep angles of a full scale helicopter main rotor blade. Surface pressure measurements at five spanwise locations are used to identify the physical processes and characterize the resulting airloads. Qualitative agreement is found with data for constant pitch rate ramps, implying that previous observations regarding the details of the stall process remain valid for sinusoidal motions. The effects of the primary test parameters on the airloads during large amplitude sinusoids are described. Study of a series of moderate amplitude motions at varied mean angle of attack suggest that it is not feasible to acheive significant unsteady lift enhancements and stall delays without generating a strong dynamic stall vortex and thus an undesirable unsteady pitching moment.