HELICOPTER INTERIOR NOISE REDUCTION BY ACTIVE GEARBOX STRUTS

This paper presents the development, layout, and validation of an active gearbox strut for the reduction of helicopter interior noise. Measurements of vibration, transfer functions and interior noise in the Eurocopter BK117 showed that the structure-borne noise path which is characterised by the gear meshing frequencies is dominant. Helicopter measurement tests in the laboratory supported the development of the active system. The first tests on a single-axis test rig for the evaluation of the basic features have been followed by tests on an EC135 prototype structure. Moreover, an effective control system is evident for the active noise reduction system. To ensure this an adaptive feedforward system has been further optimized and adapted to the framework of the active strut in the helicopter environment. An additional feedback control system will be tested in the next evaluation phase. The results of the laboratory tests show that the active gearbox strut has the potential for a major reduction in the interior cabin noise. In order to demonstrate the high noise reduction capability of the system the active strut will be installed on an Eurocopter BK117 helicopter. Ground and flight trials will show the reduction of interior noise due to the active strut system.