Helicopter Acoustic Characterization by Simulation: Comparison with Flight Test Results and Predicted Impact of New Design Solutions

摘要

The proposed paper describes the application of the "AGUSTA Operational Aeroacoustic Simulator" to a real flying helicopter. Differently from the development phase, in which wind tunnel data were mainly exploited for validation activity, the simulation tool has been tested here through direct comparisons with flight test results coming from an acoustic campaign of a light turbine engined helicopter. The aircraft was flown over a ground mounted linear microphone array and covered a wide range of flight operations; acoustic measurements on the ground were synchronized to accurate flight trajectory data recorded by means of a DGPS system. Selected test results are presented and compared to numerical simulations in terms of noise footprints emitted in stabilized flight conditions. From the simulation technique point of view, particular emphasis has been given to the aerodynamic calculation accuracy: a tool has been exploited that is supposed to offer a realistic simulation of rotor-wake interaction, by coupling a vortex lattice code for wake system prediction with an Euler code for flow field calculation around blades. Acoustic behaviour of both rotors has been evaluated through a Linear Fofwcs Williams-Hawkings formulation and also fuselage scattering effects have been included in the calculations. Good correlation has been achieved with experimental results, especially considering the complications induced by the heavy directivity characteristics of radiated noise as well as the repeatability problems mainly due to the high variability of meteorological parameters, together with the difficulty in maintaining perfectly stabilized flight conditions during the tests. These are on the other hand the main reasons that make the flying helicopter acoustic characterization through mere prediction an extremely challenging task: most of the available computational techniques are indeed limited at interpolating experimentally acquired database for determination of the far field sound radiation coming from the main and tail rotors throughout the flight regime, rather than directly simulating noise emission at sound sources. The final part of the paper deals with a further practical application of the prediction methodology, aimed at assessing the acoustic impact of a new experimental tail rotor which has been developed as the result of a research activity, having the acoustic performance improvement as one of the main design constraints. The experimental tests to be conducted on the new configuration tail rotor are expected to provide a useful database for validation activity of blind calculations.