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
The effect of steady headwinds and tailwinds on helicopter Blade Vortex Interaction (BVI) ground-noise exposure levels is analytically investigated using a Quasi-Static Acoustic Mapping (Q-SAM) method. The analysis combines a quasi-static point performance model of an AH-1 helicopter with dipole acoustic modeling of BVI noise radiation to develop ground noise exposure trends during landing approaches under steady uniform longitudinal wind conditions. Two distinct landing approach procedures have been formulated, and their impact on ground noise exposure assessed. For the AH-1 helicopter, shallow, upwind and steep, downwind approaches can significantly reduce radiated BVI noise. The relative requirements of the resulting noise abatement landing trajectories are discussed. γ g inertial flight path angle (negative in descent) µ advance ratio, V / R µ a aerodynamic advance ratio, V a / R µ g "inertial" advance ratio, V g / R angular rotation rate of the main rotor
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