Active Control of Structure-Borne Noise in Helicopter Cabin Transmitted Through Gearbox Support Strut

Abstract

A model based study on active control of multiple wave transmission in helicopter gearbox support struts is presented. The problems associated with transmission of structure-borne noise into helicopter cabin from its rotating components such as main rotor hub, gearbox, tail rotor etc. pose severe constraints in the service life of the structural components and on-board instrumentation. There is also a low frequency disturbance due to transient aerodynamics surrounding the rotor plane in addition to the steady state rotor harmonics. In this paper the analysis of active gearbox support strut mechanics is carried out and related complexities in modeling the closed-loop structure-control interaction is addressed. Unlike the previously available studies, finite cylindrical strut with distributed actuator dynamics and near-field effects on the sensors are considered. The computational strategy is based on a recently proposed model called Active Spectral Element Model (ASEM). This model, which is a displacement based, finite element type discretized model in the Fourier domain, can be used for developing “realistic” models of integrated active strut systems on which the gearbox is mounted. The development of a multi-input-multi-output feedback controller based on ASEM is also presented along with a discussion of the numerical implementation. Different cases illustrating control of axial wave, flexural wave, and axial-flexural coupled wave transmission through gearbox support struts are presented. The importance of considering actuator dynamics, coupling between axial, flexural and torsional waves, and actuator-sensor configurations are brought forth in this work. The efficiency of a closed-loop scheme based on velocity feedback is also demonstrated. This study shows that such accurately modeled mechanics integrated with control system parameters appears to be far better behaved when compared with the control system purely based on estimation of error signals.