Isolation of helicopter gearbox vibrations using periodic support struts

A new class of support struts, called Periodic Struts, is introduced in this dissertation as a viable means for isolating the vibration transmitted from the gearbox of a helicopter to its airframe. The struts exhibit unique dynamic characteristics that make them act as mechanical filters for wave propagation. As a result, waves can propagate along the periodic struts only within specific frequency bands called the “Pass Bands” and wave propagation is completely blocked within other frequency bands called the “Stop Bands”. The effect of such unique characteristics of this class of struts in controlling the wave propagation from the gearbox to the airframe is presented both in the spectral and spatial domains. Emphasis is placed on studying and demonstrating the unique filtering and localization characteristics of one-dimensional struts in their passive and active modes of operation.; When designed properly, the struts can stop the propagation of vibration from the gearbox to the airframe within critical frequency bands and consequently minimize the effects of transmission of undesirable vibration and sound radiation to the helicopter cabin. Tuning the spectral width and location of the pass and stop bands in response to the structural vibration is achieved when the struts are operated in their active control mode.; The theory governing the operation of this class of periodic struts is introduced using the transfer matrix method. The basic characteristics of the transfer matrices of periodic struts are presented and related to the physics of wave propagation along these struts. The unique filtering and localization characteristics of both the passive and active periodic struts are demonstrated theoretically and experimentally as function of their design parameters for struts with geometrical and/or material discontinuities. Close agreement is found between the theoretical and experimental characteristics.; The strain distribution over the struts, in general, and at the discontinuity surfaces, in particular, are predicted theoretically using the theory of finite element modeling in order to ascertain the effect of the discontinuities on the structural integrity of the struts. The predictions of the finite element models are also validated experimentally. The predictions are found to be in close agreement with the experiments.; The presented concept of the passive and active periodic struts can be easily employed in many critical applications where the control of the wave propagation and the force transmission both in the spectral and spatial domains is essential to stopping/confining the propagation of undesirable disturbances.