A New Experimental Approach to Study Helicopter Blade-Vortex Interaction Noise
A New Experimental Approach to Study Helicopter Blade-Vortex Interaction Noise
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Date
2007-10-01
Authors
Koushik, Sudarshan
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Abstract
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 con-
trolled gust disturbance that replicates the essential characteristics of the vortex velocity. This ex-
perimental 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, replicat-
ing 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.