Multilayered radial isolator for helicopter interior noise reduction

Multilayered radial isolators are considered for attenuation of vibration from a helicopter gearbox to the cabin. A detailed finite-element model and an augmented assumedmodes model were developed to predict the dynamic behavior of such isolators at the location where the input shaft enters the gearbox housing. The two models are in good agreement when predicting the natural frequencies, mode shapes and transmissibility of three-and five-layered isolators. The augmented assumed-modes model is an excellent design tool since it yields results of acceptable accuracy while running almost 2000 times faster than the finite-element model. Two experimental prototypes, one with three layers and one with five layers, were developed, built and tested. Transmissibilities on the order of 20% were measured over the frequency range from 900 to 2500 Hz. Experimental results validate the augmented-assumed modes model and show that multilayered 2-D isolators can exhibit "stop-band" behavior similar to that of 1-D multilayered isolators.