Aeroacoustics research in Europe: The CEAS-ASC report on 2004 highlights (edited by J.A. Fitzpatrick): 7.1. Flight procedure design for helicopter noise abatement

Abstract

As a first step towards noise abatement flight procedure design, this paper presents techniques for rotorcraft noise ground footprint prediction developed/used at DLR, using either purely numerical computations, measured sound fields or measured blade pressures. The first method adopted for the ground footprint prediction, based on the purely numerical computations consists of a three-step procedure which calculates first the unsteady aerodynamics of the simultaneously 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 unsteady pressure distribution on the main- and tail rotor blades serves as input to FW-H equation based code, APSIM to define the acoustic far field pressure on a hemispherical 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 (re-propagation procedure) to perform footprints prediction. The propagation of the noise on to the ground is treated as “free” propagation with consideration of atmospheric absorption and ground reflection. The second method of generating acoustic ground footprints consists in using measured acoustic data on limited microphones. 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 the rotorcraft. The transformations are performed by correcting back spherical spreading, atmosphere absorbing and ground reflection. In general, the measured data, including weather profile, flight trajectory and acoustic spectrum are used in the reverse propagation procedure. After obtaining a free field lower hemisphere sound field, the re-propagation procedure in Hemisphere code is then used for a footprint on a user-defined grid of observer locations. The special interpolation schemes are introduced to allow using total angular coverage of measured sound field. The comparison of the 2 methods are carried out for a BO105 helicopter, using also measured noise of the RONAP flight test conducted on Cochstedt airport, Germany in 2001. The noise ground footprints for the flight conditions, such as 6° descent and 12° takeoff are first simulated using the first method (in a MR and TR configuration, without the fuselage). The other method is then used for these flight conditions and for additional descent flight conditions. The methods are compared and cross-check validations are performed. Finally, a simple flight procedure with combination of different segment of flight trajectories is tested to demonstrate the capability of DLR Hemisphere code.