Nonlinear fluid-structure interaction in propeller aircraft cabins

A procedure for studying the acoustic superharmonic response in propeller aircraft cabins subject to stationary single frequency load excitation is proposed. The harmonic balance method is used to solve the nonlinear fluid-structure interaction multi-degree-of-freedom problem at hand. In the problem studied the structure is nonlinear while the fluid remains linear. In the solution method proposed, generalised coordinates of the assumed series expansion for the displacements are used as unknowns. Two examples, simulating an aircraft structure with a fluid cavity, are examined The present calculations show that in a lightly damped one-dimensional system with cubic stiffness, the noise levels from the superharmonic resonance may be slightly lower than those resulting from the fundamental frequency. For a typical model of a cross-section of an aircraft cabin, it is shown that nonlinear damping in spacing material will result in a considerable influence of the response in the third tone. For the one-dimensional system, good agreement is obtained with results from parallel nonlinear analyses where the discretized system of inertia equations is solved employing explicit time integration. For the multi-degree-of-freedom system modelling the aircraft cabin, a comparison of results between the harmonic balance method and the explicit time integration of a corresponding FE model indicated a partial agreement of the two. However, several different solutions may exist to a nonlinear equation, which make the comparison uncertain.