Actively-enhanced periodically-layered mount for helicopter gearbox isolation

Helicopter gearbox-generated vibrations contribute to interior cabin noise in the frequency range of 500 to 2000 Hz. Drive train alignment dictates that gearbox mounting struts have high static stiffness, precluding conventional soft-mount isolation approaches. A combination of passive and active techniques are pursued to reduce noise transmission into the cabin. The core of the passive mount comprises a periodically-layered metal-elastomer composite. Due to static stiffness and mass constraints, however, this purely passive approach is not effective over the entire frequency range. To reduce the dynamic stiffness and expand the operational frequency range, fluidic motion amplification elements are introduced. Experimental results show that this passive fluid-elastic mount reduces vibration transmission by about 40 dB over the range of interest. Because the vibration disturbance has strong, irritating tonal components, additional attenuation is desired. An adaptive feedforward approach was developed to reduce vibration transmission through the last cell of the passive isolator. A piezoelectric stack provides the small active forces needed to counteract the residual vibration, and tracks variations in tonal frequency and amplitude. In experiments, this yields about 40 dB of additional active reduction for each of three simultaneous tonal disturbance. These results demonstrate the potential effectiveness of an actively-enhanced, periodically-layered gearbox mount for rotorcraft cabin noise reduction.