Effects of crosswind flight on rotor harmonic noise radiation

To develop recommendations for procedures for helicopter source noise characterization, the effects of crosswinds on main rotor harmonic noise radiation are assessed using a semi-empirical model of the Bell 430 helicopter. Crosswinds are found to have a significant effect on blade-vortex interaction (BVI) noise radiation when the helicopter is trimmed with the fuselage oriented along the inertial flight path, resulting in a "slipped" flight condition. However, the magnitude of BVI noise remains unchanged when the pilot orients the fuselage along the aerodynamic velocity vector, "crabbing" for zero aerodynamic sideslip angle. The effects of wind gradients on BVI noise are also investigated and found to be smaller in the crosswind direction than in the headwind direction. The effects of crosswinds on lower harmonic noise sources at higher flight speeds are also assessed. In all cases, the directivity of radiated noise is somewhat changed by the crosswind. The model predictions agree well with a limited set of flight test data for the Bell 430 helicopter for cruise and approach flight conditions, both measured with and without crosswinds present. The results of this investigation would suggest that flight paths for future acoustic flight testing are best aligned across the prevailing wind direction to minimize the effects of winds on noise measurements when wind cannot otherwise be avoided.