A prediction methodology for rotorcraft noise ground footprints

Abstract

This paper presents a numerical technique for the noise ground footprint prediction for given helicopter flight trajectory and conditions. The methodology adopted is a two-step procedure which calculates first the unsteady aerodynamics of the simultaneously turning main- and tail rotor. The aerodynamic code is based on a 3-D unsteady panel method which simulates all motions of an articulated rotor and the relative motion between main- and tail rotor blades. The fuselage is not simulated. The unsteady pressure distribution on the main- and tail rotor blades serves as input to a Ffowcs Williams Hawkings (FWH)-equation based code to define the acoustic pressure on a hemispherical "noise source" surface beneath the aircraft. A flight trajectory may consist of a number of flight segments such as take-off, level flight and descent. For each of these flight sequences, a separate noise source surface is generated. Knowing the flight trajectory and the flight condition, the relevant noise source surface is moved along the flight segment with the desired speed. The propagation of the noise on to the ground is treated as "free" propagation with consideration of atmospheric absorption and ground reflection. To generate the noise footprints, the overflown area is overlaid with a lattice of discrete points. With a step by step movement of the noise source surface along the flight trajectory, the perceived noise on the ground is obtained as a sound pressure level time history at each ground point which is processed to obtain the desired noise metric. A contour plot routine uses the data to map the foot print.