Flight-Path Management/Control Methodology to Reduce Helicopter Blade-Vortex Interaction Noise

Abstract

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

Closed-loop controlled vortex-airfoil interactions

“…Typical examples include modified blade tip shapes, 6 the use of spoilers or vane wings, 7 and flight path management. 8,9 Requiring no external energy input, these methods change the dynamic behavior of the fluid-structure system via modifying its geometrical or physical parameters.…”

Quasi-Static Flight Path Control of Helicopter Blade Vortex Interaction Noise in a Steady Wind

“…This is done in a quasi-static manner by treating acceleration as another flight path control through the Quasi-Static Acoustic Modeling (Q-SAM) method, introduced in Ref. 10. Conceptually, Q-SAM interpolates a measured or predicted set of acoustic data taken in trimmed level steady-state flight to equivalent quasi-static conditions, and then maps the resulting acoustics at these equivalent conditions to positions on the ground.…”

“…One-third octave band BVI noise levels on the sphere are then projected to the ground plane observer using the spherical spreading law, corrections for atmospheric absorption and A-weighting (Ref. 10).…”

“…16 G. GOPALAN JOURNAL OF THE AMERICAN HELICOPTER SOCIETY These trends have also been generalized through mathematically modeling the dominant features of BVI noise radiation (Ref. 10). Using a Quasi-Static Acoustic Mapping (Q-SAM) method, and assuming ideal operating and environmental conditions, flight path angle, X-force, and acceleration/deceleration were chosen to develop trajectories that reduce BVI ground noise exposure of the AH-1 helicopter.…”

“…[10][11][12] to explore the effect of steady headwinds and tailwinds on the radiated BVI noise and the resulting ground noise exposure trends of the AH-1 helicopter during landing approach. It is still assumed that the pilot is the perfect controller and that ideal atmospheric conditions exist-except for the effects of steady headwinds and tailwinds.…”