Control of Reverse Flow over a Cantilevered Blade Using Passive Camber Morphing

Abstract

In high-speed rotorcraft applications, large sections of a blade undergo reverse flow due to a high advance ratio. Flow separation at the sharp aerodynamic leading edge during reverse flow leads to negative lift, pitching moment, and drag penalties. The kinematics of a rotor blade leads to a dynamic stall in reverse flow, which further accentuates the problem by causing unsteady loading. The present experimental study shows that these problems can be mitigated by passively morphing the camber near the trailing edge of the blade. A finite-span cantilevered NACA 63-218 blade was examined at a chord-based Reynolds number of with and without trailing-edge morphing. The morphing was implemented by deflecting the last quarter-chord near the trailing edge by 5, 10, or 15 deg. Experiments included global measurements of the forces and moments using a load cell as well as detailed flow measurements using stereoscopic particle image velocimetry. A couple of scenarios were explored: static pitch and dynamic pitch at different pitch amplitudes and frequencies. Cambering of the blade near its trailing edge led to a significant reduction in the size of the separation region over the model, and of the wake. For the static pitch case, it led to a large reduction in drag and pitching moment and a minor reduction in negative lift. For the dynamic pitch cases, the hysteresis loop was significantly reduced, leading to a large reduction in unsteady loading.