A New Experimental Approach to Study Helicopter Blade-Vortex Interaction Noise
Schmitz, Fredric H
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A unique and novel experimental approach has been developed to study the aerodynamics and acoustics of the helicopter Blade-Vortex Interaction in a controlled hover environment. This is achieved by having a non-lifting single-bladed rotor with a rigid hub interact with a carefully controlled gust disturbance that replicates the essential characteristics of the vortex velocity. This experimental approach termed the Blade-Controlled Disturbance-Interaction or the BCDI, decouples the rotor parameters from the charactersitics of the incident disturbance velocity, thus providing an ideal setup for studying the blade's aerodynamics and acoustic response in detail. Moreover, the angle of interaction between the disturbance field and the rotor blade can be controlled by orienting the gust, providing the ability to study both parallel and oblique interactions. The noise data was recorded at thirty different microphone locations. A series of experiments at various rotor tip Mach numbers and interaction angles, replicating many of the conditions of helicopter BVI, were performed. The results show that the the directionality of the BVI noise is strongly determined by the interaction angle. A small change in interaction angle results in the radiation of noise over a larger azimuthal area compared to the parallel interaction. Moreover, as the interaction becomes more oblique, the peak noise elevation angle approaches closer to the rotor plane. A linear unsteady lifting-line aerodynamic theory (corrected for chord-wise non-compactness )was used to estimate the blade aerodynamics during the interaction and hence the radiated noise. Although the theory under-predicted the noise levels for most of the cases, and did not replicate exactly the general pulse shape, the general directionality trends were predicted reasonably well. The theory was used to separate the contribution to the acoustics, from different spanwise blade sections, providing significant insights into the phasing mechanism of BVI noise.