NOISE-PROPAGATION THROUGH HELICOPTER GEARBOX SUPPORT STRUTS - AN EXPERIMENTAL-STUDY

A dominant source of noise in a helicopter cabin is the meshing of the gears in the main rotor gearbox. The main structural noise transmissi on path from this gearbox to the cabin is through the gearbox support struts, and this is the transmission path which is the subject of this paper. An experimental study is described which quantifies the way in which vibration propagates through one of these struts and the experi mental results are interpreted with the aid of some simple analytical models. The contribution of the various modes of vibration to the tran smission of the structure-borne noise is quantified by calculating the kinetic energy of the receiving structure from measured data. The res ults show that although the dominant mode of vibration is longitudinal , flexural resonances occur at some frequencies, and the contribution of the flexural vibration to the kinetic energy of the receiving struc ture at these frequencies can be comparable with that due to the longi tudinal motion. It is demonstrated that the lateral behavior of the st rut is dependent upon the static loading but the longitudinal behavior is relatively insensitive to this loading.