A New Experimental Approach to Study Helicopter Blade-Vortex Interaction Noise

A new experimental method to study helicopter Blade-Vortex Interaction (BVI) noise has been developed. Called the Blade-Controlled Disturbance-Interaction (B-CD-I), the method nearly replicates a rotor BVI by having a single-bladed rotor pass through a stationary gust field specially designed to simulate the highly impulsive induced velocity field of a vortex. This approach can be used to evaluate the effectiveness of blade design changes to noise radiation during a BVI event, as well as to study the effect of different interaction geometries on the resulting noise. The first experiments in this facility show that the directionality of BVI noise is radiation very sensitive to the interaction angle. Moreover, oblique interactions spread the acoustics energy over wider azimuth angles than parallel interactions, along with bringing the peak noise location closer to the rotor plane. Linear two-dimensional unsteady aerodynamic theory predicts the overall directionality trends reasonably well. However the actual pulse shapes do not match and noise levels are underpredicted, particularly for oblique interactions.