Lateral response of nonlinear nose-wheel landing gear models with torsional free play

Nose-wheel landing gears can, under certain conditions, exhibit instability in lateral dynamics, causing divergent coupled lateral flexural and torsional oscillations called shimmy. The stability of the system depends on the dynamic characteristics of the gear and tires, nonlinearities in the system, and vibratory modes of the vehicle as a whole, as well as the degree of coupling that exists between them. Shimmy may be caused by a number of conditions such as low torsional stiffness of the strut, free play in the gear, wheel imbalance, or worn parts. Free play in the steering degree of freedom has the potential to significantly reduce the divergent shimmy velocity. Nonlinear behavior of landing gear makes the evaluation of the shimmy phenomenon more complex and its prediction more difficult. This work presents a study of the shimmy instability of a three-degree-of-freedom simplified nose-wheel landing gear model with linear flexible tire model and nonlinearities arising out of torsional free play. Response results are presented for a typical range of values of various problem parameters. Numerical studies bring out several interesting features of the shimmy and its dependence on free play