Shimmy of an Aircraft Main Landing Gear With Geometric Coupling and Mechanical Freeplay

The self-sustained oscillation of aircraft landing gear is an inherently nonlinear and dynamically complex phenomenon. Although such oscillations are ultimately driven from the interaction between the tyres and the ground, other effects, such as mechanical freeplay and geometric nonlinearity, may influence stability and add to the complexity of observed behaviour. This paper presents a bifurcation study of an aircraft main landing gear, which includes both mechanical freeplay, and significant geometric coupling, the latter achieved via consideration of a typical side-stay orientation. These aspects combine to produce complex oscillatory behaviour within the operating regime of the landing gear, including longitudinal and quasiperiodic shimmy. Moreover, asymmetric forces arising from the geometric orientation produce bifurcation results that are extremely sensitive to the properties at the freeplay/contact boundary. However this sensitivity is confined to the small amplitude dynamics of the system. This affects the interpretation of the bifurcation results; in particular bifurcations from high amplitude behaviour are found to form boundaries of greater confidence between the regions of different behaviour given uncertainty in the freeplay characteristics.

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