Nose-wheel landing gears can become laterally unstable during takeoff, taxiing, and landing, exhibiting divergent coupled lateral flexural and torsional oscillations called shimmy. The system stability is governed by the gear and tire dynamic characteristics, system nonlinearities, and vibratory modes of the vehicle, as well as by the degree of coupling that exists between these modes. Ground unevenness can produce significant lateral excitation on the landing gear and results in adversely impacting its lateral stability. To evaluate a nose-wheel landing gear system for its stability and response, it is necessary to model system components to capture the contributions of the landing gear structure, the tire, wheel configuration, system nonlinearities, and its interaction with the runway. This paper examines the lateral response of linear and nonlinear simplified nose-wheel landing gear models to ground-induced lateral excitation. Considering torsional free play as the source of nonlinearity and external excitation due to runway roughness, modeling and analysis of nose-wheel landing gear shimmy is presented. The spatial variation of the runway surface is modeled as a continuous profile, obtained as a combination of sinusoids to represent a randomlike ground-induced excitation with the specified power spectral density. Time-domain simulation studies on the linear landing gear system show that unacceptable levels of lateral accelerations may be caused even at subcritical velocities. Studies on nonlinear nose-wheel landing gear systems bring out the fact that the free play can result in a significant reduction of the critical shimmy velocity and the need for such simulations in the subcritical ranges of velocities.
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