BRITE-EURAM II project RHINO: Reduction of helicopter interior noise

The objective of this work is to develop measurement methods for determination of the vibratory source strength of the main rotor gearbox, which is considered to be the dominant source of structureborne noise in helicopters.
Two novel measurement techniques have been developed for quantifying the vibro-acoustic source strength of leightweight helicopter gearboxes. The accuracy, robustness and implementation of these methods have been examined by a comprehensive investigation, comprising theoretical studies of simple multi-modal beam systems and extensive experiments with more realistic three-dimensional small scale models, and finally with large, detailed 3/4-scale test structures of a medium size helicopter. In addition partial verification tests of the methods have been conducted by flight trials and ground tests with a Eurocopter Deutschland BK117 helicopter in a joint effort by the three task partners; a full verification, however, which requires measurements in a gearbox test bench and measurements of the airframe dynamic properties with the gearbox removed, was beyond the budget of this task. Nevertheless, it is concluded that the equivalent source power methods have been developed to a stage where they can be implemented at the manufacturer. Moreover, the scale model predictions have revealed that the power transmitted to the fuselage is dominated by contributions in the axial direction of struts and tie-rods. Such information is valuable, for example, in the selection of control measures for reducing structureborne noise transmission to the fuselage, whether this is attempted by using passive dynamic neutralizers or by suitable actuators driven by an active control system.