Fully FEM-based simulation approach for advanced helicopter interior noise design using noise sources extracted from flight test data

Abstract

Apart from exterior noise emissions which are subject of the type certification of a helicopter, also the noise perception inside the cabin poses major design challenges in rotorcraft development. In particular, rotating components such as rotors, drive shafts, engine components and gears are commonly known sources of sound. Of particular relevance are those sources whose frequency range interferes with that of human voice, for which the Speech Interference Level (SIL) is a widely used metric. In the recent past, availability of computational resources as well as development of efficient and robust numerical solution methods have experienced a steep gradient. This paves the way for the use of Fine Element Methods (FEM) to simulate acoustic wave propagation for successively higher frequencies in ever larger volumes such as helicopter cabins. With the aim of designing effective measures to optimize the SIL4 noise level of a helicopter cabin we present the applicability of a full FEM-based acoustics simulation approach. The presented model is capable to cover the full frequency range bounded by the SIL4 range. To describe location and strength of the acoustic sources, we exploit measurement data recorded during regular flight test campaigns allowing to compute equivalent accelerations for the major sources of sound. Using the AW09 prototype helicopter as a practical example, the acoustic performance in terms of SIL4 reduction is investigated for two treated configurations: One with a carpet and a second with carpet and a ceiling panel. The materials are acoustically described by frequency-dependent absorption coefficients from literature. As a result, the reduction potential of the carpet is quantified to range between ? 1.2 and 1.8 SIL4dB and up to approx. 11.5 SIL4dB for the case with carpet and ceiling panel. The great potential of model lies in its implementation in a multiphysics simulation environment. This allows for example to include more complex interaction effects such as acoustic-structure coupling as well as consideration of structures like acoustic metamaterials.