Extracting Blade–Vortex Interactions Using Continuous Wavelet Transforms

Abstract

An extraction method is proposed to investigate blade vortex interaction noise emitted during helicopter transient maneuvering flight. The extraction method allows for the investigation of blade vortex interactions, independent of other sound sources. It is based on filtering the spectral representation of experimentally acquired full-scale helicopter acoustic data. The data is first transformed into time-frequency space through the wavelet transformation, with blade vortex interactions identified and filtered by their high amplitude, high frequency impulsive content. The filtered wavelet coefficients are then inverse transformed to create a pressure signature solely related to blade vortex interactions. Analysis on a synthetic data set is conducted, and it is shown that blade vortex interactions can be accurately extracted so long as the blade vortex interaction peak energy signal is greater or equal to the energy in the main rotor harmonic. A brief analysis shows that the extraction method performs admirably throughout a fast advancing side roll maneuver. Using this method, it was shown that peak blade vortex interaction noise levels are linked directly to the roll rate of the vehicle, and are directed towards the retreating side during the transient portion of the maneuver.