Effects of freeplay on aircraft main landing gear stability

A study is made into the occurrence of shimmy oscillations in a dual-wheel main landing gear system. Nonlinear equations of motion are developed for the system and various nonlinear effects are considered including gyroscopic coupling, nonlinear tyre properties, geometric nonlinearities and fluid damping. Of particular interest in this study is the presence of freeplay - this is introduced as a lateral play at the apex of the torque link joints. Using bifurcation analysis methods, the dynamics of this system are explored as the forward velocity and loading force acting on the gear are varied. For the zero freeplay case the system is found to be stable over its realistic operating range with shimmy oscillations appearing only for extreme loading forces and speed. However, with the introduction of freeplay, shimmy may be observed over more typical operating conditions and the resulting oscillations are found to scale linearly with freeplay magnitude. Using numerical continuation, the (V; Fz) parameter plane is further subdivided into areas of different shimmy dynamics. With the inclusion of freeplay the variety of observed behaviour is shown to increase with the appearance of low and high frequency shimmy, bi-stable behaviour and stationary solutions of non-zero yaw.