Vecchio A., Janssens K., Gallina A., Cenedese F.

In the quest for quieter, faster and more efficient transportation means, the helicopter industry is currently investing in research and development programs aimed at improving NVH performances of helicopters. The development of a quieter helicopter requires a systematic study of NVH behaviour, with special attention to operating conditions. Several experimental techniques such as Transfer Path Analysis are in use in automotive industry that can help identify relevant noise sources and provide hints for design improvements. TPA is a hybrid modelling technique based on experimental data that allows identifying the energy transmission paths from sources to target locations. The availability of a reliable TPA model paves the way to the prediction of achievable comfort improvements induced by design modifications at sources level or on the transmission paths. An important requirement of TPA modelling is to quantify vibroacoustic sources and rank them by relative strength. In helicopters the main noise sources are the jet engines the rotors and the gearbox. The latter is commonly recognised as the most important source affecting cabin comfort. Experimental techniques such as Acoustic Intensity measurements can be used to characterize a helicopter gearbox, but this would require controlled testing conditions, ideally in anechoic room. Unfortunately test-benches capable of operating a helicopter gearbox are not always compatible with noise testing requirements therefore sub-optimal test conditions apply that may endanger data quality. In such case, hybrid modelling can help improve accuracy in the estimation of the source strength. In this paper a hybrid modelling technique is presented that allows using test data (both acoustic and vibrations) measured in an industrial test-bench to characterize the helicopter gearbox behaviour in operating conditions. The proposed approach uses inverse technique to compute the gearbox surface velocities and propagates back the pressures to finally compute the gearbox acoustic power for different operating conditions.