Towards Noise Abatement Flight Procedure Design: DLR Rotorcraft Noise Ground Footprints Model and its Validation

Kurzfassung

As a first step towards noise abatement flight procedure design, this report presents a numerical technique for rotorcraft noise ground footprint prediction using either purely theoretical computations or measured sound fields. The first methodology adopted for the ground footprints is based on the purely theoretical computations. It consists of a three-step procedure which calculates first the unsteady aerodynamics of the simulataneously turning main- and tail rotor. The aerodynamic code, UPM-Mantic 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 (FW-H)-equation based code, APSIM to define the acoustic pressure on a hemispherical "noise source" surface beneath the aircraft. The hemispherical sound field in narrow band spectrum together with weather profile, flight trajectory and conditions are provided to a flyover noise prediction code, HEMISPHERE (propagation procedure) to fulfill footprints prediction. 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 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 Doppler effect correction, spherical spreading, 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 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 footprint. The second methodology of generating acoustic ground footprints is to use measured acoustic data which were collected from e.g. a microphone array as input into the HEMISPHERE code to enable an estimation of acoustic footprints in a large area which were not directly measured. The ground to hemisphere transformation or reverse propagation procedure embedded in HEMISPHERE code is first used to transform ground measured acoustic data to a hemisphere surface beneath rotorcraft. The transformations are fulfilled by correcting for spherical spreading, atmosphere absorbing and ground reflection. The measured data including weather profile data, flight trajectory data and acoustic spectrum data are used in the reverse propagation procedure. After obtaining a free field lower hemisphere sound field, the propagation procedure in HEMISPHERE code can then be used for a footprint on a user-defined grid of observer locations.