Time Variation of Helicopter Rotor Broadband Noise

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Abstract
Rotor broadband noise is typically computed and analyzed as a spectrum over a time scale on the order of the rotor period. However, temporal variation of the broadband spectrum within a rotor revolution has been shown to be significant to noise levels and perception. The time variation of helicopter rotor broadband noise caused by aerodynamic interactions and the tail rotor was analyzed for a Bell 206 in level and descending flight. Spectrograms were constructed using both flight test noise measurements and computational predictions using the Brooks, Pope, and Marcolini (BPM) model; these are then compared to validate the predictions, provide insight into the important noise source physics, and inform how the prediction models could be improved. Modulation trends were well-predicted, especially modulation depths. In contrast, peak broadband noise levels were underpredicted, especially for the tail rotor and excess noise caused by aerodynamic interactions. The duty cycle of main rotor broadband noise modulation was overpredicted. Broadband noise modulation with the tail rotor blade passage frequency was found to be significant, especially for observers on the main rotor retreating side, even when blade-vortex interactions (BVI) are present. This is because tail rotor broadband noise had similar peak levels to main rotor broadband noise when BVI noise does not dominate. Tail rotor broadband noise modulation had a significant modulation depth (>10 dB), with a higher modulation frequency (equal to the blade passage frequency) than the main rotor. Furthermore, this modulation exhibited significant aperiodicity, causing substantially-different noise spectra between different blade passages.

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