Modelling and Analysis of a Dual-Wheel Nosegear: Shimmy Instability and Impact Motions

This paper studies landing gear stability and shimmy behaviour of a cantilevered dual-independent-wheels nosegear through modelling, analysis and numerical simulation. The landing gear model includes key features of nonlinearities such as freeplay and nonlinear damping in the steering system, dry friction between the piston and cylinder, as well as spring hardening effects of the bending and torsional stiffness. The shock strut is treated as a flexible beam described by a lateral displacement and a rotation angle. The trail arm and axle are considered as rigid bodies. Interaction between the tire and the runway surface is considered as a single point at any instant, and thus the cornering force is treated as linearly proportional to the tire slip angle and its time derivative. For fixed system parameters, the gear may become unstable and subsequently develop shimmy as the aircraft taxiing velocity is increased beyond a critical point. Thereafter, due to the existence of torsional freeplay, the amplitude of the resulting oscillatory motion will keep growing until the collar starts impacting with the steering damper. The stability properties of the system are also conducted in parameter space by varying a few key parameters individually. Time curves are used to verify the analytical results and to illustrate the nature of motion at different parameter values.