A quasi-static acoustic mapping method has been developed to predict rotorcraft external noise. This method takes advantage of an expansion, to rst order, about a solution to the helicopter nondimensional steady-state trim equations to include the effects of acceleration parallel to the ight path and X-force control changes on the radiated noise. Application of the new method to predict helicopter blade-vortex interaction (BVI) noise has shown that choice of ight-path angle, X-force, and vehicle acceleration all have an important in uence on ground noise exposure during approach and landing. The effect of constant ight-path-angle approaches on BVI ground noise, with and without X-force control, is compared with decelerating approaches at the same ight-path angle. Two different quiet ight trajectories are suggested that use a combination of these controls to minimize BVI noise exposure on the ground during a helicopter approach to a landing.
NomenclatureA sph = radiation sphere total area A x = acceleration parallel to ight path a 0 = speed of sound, 1125 ft/s C T = thrust coef cient c = rotor blade chord D f = rotor drag d = blade-to-vortex separation distance F x = auxiliary X force f = equivalent at plate drag area of the helicopter f x = equivalent at plate area of X-force device g = gravity constant, 32.2 ft/s 2 M ht = hover-tip Mach number, ÄR=a 0 P av = average acoustic power on sphere R = rotor blade radius R obs = hub-to-observerdistance t = time t obs = observer time V = helicopter velocity v = rotor induced velocity W = helicopter weight x, y = ground plane coordinates x tpp ; y tpp ; z tpp = tip-path-plane coordinates ® tpp = tip-path-plane angle (positive nose up)°= ight-path angle (negative in descent) µ = radiation sphere elevation anglȩ = average rotor in ow (positive for upwash) ¹ = advance ratio, V =ÄR ½ = density of air, 0.002378 slugs/ft 3 ¿ source = source time à = radiation sphere azimuth angle Ä = angular rotation