European Congress on Computational Methods in Applied Sciences and Engineering ECCOMAS 2004 P. Neittaanmäki, T. Rossi, S. Korotov, E. Oñate, J. Périaux, and D. Knörzer (eds.) Jyväskylä, 24—28 July 2004 1 FLYING HELICOPTERS ACOUSTIC IMPACT ASSESSMENT: TOWARDS A MORE REALISTIC SIMULATION METHODOLOGY R. Ponza * * Aerodynamics Department of AGUSTA S.P.A., Via G. Agusta 520, 21017, Cascina Costa (VA), Italy e-mail: r.ponza@it.agusta.com , web page: www.agusta.com Key words: Rotorcraft Aeroacoustics, Vorticity Confinement, Noise Footprint Prediction. Abstract. The environmental friendliness of helicopters is nowadays a major design requirement and manufacturers are making remarkable efforts to reduce the noise emission of rotorcraft, being the acoustic performance one of the critical aspects that can influence their penetration and diffusion into the civil market. Since the beginning of any new project, the abatement of sound radiation emerges as one of the driving design parameters, especially considering the rising concern for environmental issues, together with the more and more stringent rules that govern acoustic pollution. A simulation methodology was developed within AGUSTA to fulfil the need for an integrated calculation tool, aimed at assessing the acoustic impact of helicopters in wind tunnel testing as well as in real flight operations, and is currently in use since the very early stages of the design process. Particular care has been taken of the aerodynamic calculation: a tool is exploited that couples a vortex lattice code for wake system prediction with an Euler code for flowfield calculation around blades. Acoustic behaviour of both main and tail rotors is evaluated through a Linear Ffowcs Williams-Hawkings formulation and also fuselage scattering effects are included in the calculations. The basic methodology has already been validated through direct comparisons with flight test results of a light turbine engined helicopter. Good correlation has been achieved with experiment, in spite of some complications induced by the heavy directivity characteristics of radiated noise, as well as the strong dependence of the really perceived acoustic perturbation on some difficult-to-evaluate environmental factors, such as the presence of reflecting terrain and the atmospheric propagation mechanisms, influenced in turn by highly variable meteorological gradients. This paper is focused on some recent developments of the simulation process: first of all the effects of Vorticity Confinement on the aerodynamic calculations have been evaluated; secondarily, a in-house developed code, basically built on a FEM approach, has been exploited for the aeroelastic analysis of rotors. Finally, particular emphasis has been given to the evaluation of environmental effects on the radiated noise: a post- processing tool has been implemented that, given the sound emission of rotorcraft in free space, allows the introduction of terrain reflection and atmospheric absorption effects, that are known to significantly influence the perceived acoustic impact on a generic observer. These improvements will be shown to represent a step forward towards the acoustic characterisation of helicopters in a more and more close-to-reality environment, which can ultimately help both designing low-impact new rotorcrafts and significantly reduce annoyance of already flying helicopters via definition of low noise flight procedures in the vicinity of sensitive areas.