Results from a joint DARPA/Boeing/NASA/Army wind tunnel test demonstrated the ability to reduce in-plane, low-frequency noise of the full-scale Boeing-SMART (Smart Material Actuated Rotor Technology) rotor with active flaps. Test data reported in this paper illustrated that near-field acoustic energy in the first six blade-passing harmonics could be reduced by up to 6 dB at a moderate-airspeed, level flight condition at an advance ratio of 0.30. Reduced noise levels were attributed to selective active flap schedules that modified in-plane blade airloads on the advancing side of the rotor, generating counteracting acoustic pulses that partially offset the negative pressure peaks associated with in-plane, steady thickness noise. These favorable reduced-noise operating states are a strong function of the active flap actuation amplitude, frequency, and phase. The reduced noise levels resulted in reduction of predicted aural detection distance, but incurred vibratory load penalties due to increased hub shear forces.

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

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The geometric factors that govern the radiation of helicopter blade-vortex interaction (BVI) noise are explored using simplified aerodynamics and wake modeling. Formulation of the BVI problem is restricted to a constant strength, epicycloid tip-vortex system that is initially confined to a plane parallel to the tip-path-plane at a fixed distance below the rotor. The resulting equations, when solved, illustrate the connection between the trace Mach number profile of each interaction and the phased summation of acoustic sources for each individual BVI. Areas of peak acoustic radiation are identified on a radiation sphere surrounding the helicopter as a function of advance ratio and hover tip Mach number for a two-bladed rotor. BVI noise radiation patterns show that the peak noise of each distinct BVI is very directional and is primarily a function of the advance ratio of the helicopter. The directivity of the peak level of each interaction changes in a continuous manner as advance ratio is increased-first radiating predominantly in the direction of the flight path near the plane of the rotor for oblique BVI, and then increasing in level and radiating more downward to the advancing side of the rotor for near parallel BVI. When the miss-distances of the tip-vortices are allowed to vary, the closeness of the younger shed vortices associated with oblique BVI tend to make the intensity levels of the oblique and near parallel interactions similar for many operating conditions. For flight along constant angles of descent at varying advance ratios, the direction of peak helicopter BVI becomes discontinuous with advance ratio-suddenly switching when different BVI become dominant. Nomenclature