PREDICTING ROTORCRAFT TRANSMISSION NOISE

A methodology for predicting the acoustic emissions of high-speed transmissions has been evaluated and is presented here. It employs the use of finite element methods to determine the dynamic response of the gear box casing. It is this excitation of the casing, due to the gear meshing forces, which results in radiated noise. This information is then used for the acoustic calculations, which are accomplished by two distinctly different procedures. A deterministic approach, based on the boundary element method, was used for determining the case-radiated noise at the lowest gear mesh frequencies. Given the limitations of the computer resources available, it was necessary to supplement this method with a stochastic approach, based on statistical energy analysis (SEA), for evaluation of the higher frequencies. SEA could not be used exclusively because of its limited precision at the lower frequencies where the boundary element method performed best. The overall noise prediction methodology, which incorporates a combination of both approaches, was implemented by McDonnell Douglas Helicopter Company to evaluate the noise emissions of a transmission currently used in the AH-64 Apache helicopter. The predicted results were compared with an acoustic data base obtained during a comprehensive sound intensity survey of a production transmission operating on a regenerative test stand. The computed sound power levels at the gear mesh frequencies. and associated harmonics were within 2 to 5 dB of the measured values. The methodology described here is intended to allow for "design-to-noise" capability. Details of the computational methods are presented. The experimental test program is also discussed.