Main Rotor Broadband Noise Study in the DNW

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Abstract
An acoustics test of a 40 percent scale model BO‐105 helicopter main rotor was conducted in the large aeroacoustic test section of the Duits‐Nederlandse Windtunnel (DNW). A range of operating conditions was tested from hover to moderately high flight speeds for various climb and descent rates at different thrust settings. Diagnostic tests including rotor speed and blade geometry changes were made to better isolate and study particular broadband self noise sources. Acoustic data in the form of acoustic pressure time histories and power spectra are used to demonstrate the regions of importance of the different broadband noise sources and their sensitivity to operating conditions. To help interpret the data, comparisons are made to predictions of rotor broadband noise. The predictions are based an self noise data previously obtained from isolated airfoil sections and the use of the NASA ROTONET program both to define rotor performance and to sum contributions of noise from individual blade segments. A key result of the paper is that an important rotor broadband noise source, related to blade interaction with rotor wake turbulence, is newly identified. This source is termed blade‐wake interaction (BWI) noise and it dominates the spectra in the mid‐frequencies for flight conditions where blade‐vortex interaction (BVI) noise is not intense.

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