MECHANISMS OF NOISE TRANSMISSION THROUGH HELICOPTER GEARBOX SUPPORT STRUTS

Helicopter gearbox support struts are structural members designed to t ake large compressive and tensile mechanical loads. They also act as v ibration transmission paths connecting the major noise and vibration g enerators of the main rotor and gearbox to the fuselage and cabin. In this paper the mechanisms of vibration transmission through these stru ts are examined. The differences between the low frequency behavior th at is important in the transmission of the rotor vibrations, and the h igh frequency behavior that is important in the transmission of gear n oise are discussed. A flexible element is introduced in series with th e strut, and the limitations of this simple passive measure in attenua ting longitudinal and flexural vibrations are highlighted. The analysi s shows that there is a difference in the mechanisms of vibration tran smission between low and high frequencies, and that there are difficul ties in applying effective passive vibration isolation measures to thi s system.