J. H. Stephenson, Tinney, C. E., Watts, M. E., and Greenwood, E., “Time frequency analysis of sound from a maneuvering rotorcraft,” Journal of Sound and Vibration, vol. 333, no. 21, pp. 5324-5339, 2014.
The acoustic signatures produced by a full-scale, Bell 430 helicopter during steady-level-flight and transient roll-right maneuvers are analyzed by way of time–frequency analysis. The roll-right maneuvers comprise both a medium and a fast roll rate. Data are acquired using a single ground based microphone that are analyzed by way of the Morlet wavelet transform to extract the spectral properties and sound pressure levels as functions of time. The findings show that during maneuvering operations of the helicopter, both the overall sound pressure level and the blade–vortex interaction sound pressure level are greatest when the roll rate of the vehicle is at its maximum. The reduced inflow in the region of the rotor disk where blade–vortex interaction noise originates is determined to be the cause of the increase in noise. A local decrease in inflow reduces the miss distance of the tip vortex and thereby increases the BVI noise signature. Blade loading and advance ratios are also investigated as possible mechanisms for increased sound production, but are shown to be fairly constant throughout the maneuvers.
DOI: 10.1016/j.jsv.2014.05.018: http://www.sciencedirect.com/science/article/pii/S0022460X14004088