SCHLIEREN AND NOISE STUDIES OF ROTORS IN FORWARD FLIGHT

A wind-tunnel investigation of model rotors operating at full-scale Mach numbers in simulated partial power descent and high-speed flight is described. Schlieren flow visualization aided by full-scale blade pressure measurements helped identify seven blade/vortex intersections that occur during partial power descent. Two of the 1st-quadrant intersections have the potential of inducing bow shocks. Observation of the high-speed crescent-shaped shock which results from transonic flow over the advancing blade has provided new insight into its structure and directivity. Correlation of the flow visualization results with acoustic measurements gives clarification of the relationship between aerodynamic events that occur on the blade and the various components of the slap signature. Comparisons of rotors having different twist, airfoil profile, and thickness show qualitative differences that suggest means of alleviating this noise.