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Aeroacoustic analysis of main rotor-tail rotor interaction

Fletcher, Timothy M. and Duraisamy, Karthikeyan and Brown, Richard (2008) Aeroacoustic analysis of main rotor-tail rotor interaction. In: 34th European Rotorcraft Forum, 2008-09-16 - 2008-09-19.

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The increased restrictions placed on helicopter noise levels over recent decades have encouraged manufacturers to better understand tail rotor noise and its aerodynamic sources. A generic single main rotor and tail rotor helicopter has been simulated in high speed forward, and quartering, flight using the Vorticity Transport Model. The unsteady loads developed on the tail rotor blades and the resulting acoustic noise propagation have been computed. The sound propagation from isolated tail rotors with top-aft and top-forward senses of rotation in high speed forward flight results in impulsive sound being directed downward from the former and upward from the latter. The principal source of tail rotor noise in high speed forward flight is a periodic blade-vortex interaction between the tail rotor blades. The effect of aerodynamic interaction on tail rotor noise is highly dependent on the flight speed and trajectory, such that the noise produced as a result of interaction is, for the particular helicopter geometry simulated here, greater in quartering flight than in high speed forward flight. The sound pressure produced by periodic impulsive loads in high speed forward flight and the high frequency sound generated in quartering flight is sensitive to the scales to which the vortical features within the wake, and the radial and azimuthal distributions of blade loading, are resolved.