Acoustic Directivity of Maneuvering Helicopters Using Wave Tracing Concepts

The principal directivity of the acoustic field generated by a maneuvering rotor was analyzed using trace Mach number and wave-tracing concepts. A time-accurate, viscous corrected free-vortex wake method was used to predict the aperiodic rotor wake and the resulting unsteady airloads over the rotor disk during maneuvering flight. The principles of wave tracing from the blade vortex interaction (BVI) locations that were identified as having acoustically significant supersonic trace Mach numbers were then used to determine the principal directivity of the rotor noise. Maneuvers such as starboard and port rolls, as well as roll reversals were studied. The results demonstrate the viability of a wave-tracing method coupled with the free-vortex wake methodology as an effective numerical tool for understanding the principle directions of sound propagation produced from Bvi source points on a helicopter rotor during maneuvering flight. The ability of the method to trace back the sound field to the acoustically significiant BVI source points on the rotor is a first step toward effective BVI mitigation. The results suggest a strong dependence of the rotor acoustics on the temporal wake dynamics, and show the need to simulate rotor aerodynamics with models using very high spatial and temporal resolutions if credible quantitative predictions of the noise directivity from maneuvering helicopters are to be achieved.