Lateral Stability of Aircraft Nose-Wheel Landing Gear with Closed-Loop Shimmy Damper

Abstract

Shimmy of aircraft nose-wheel landing gear is a coupled lateral and torsional dynamic instability that may occur during take off, taxi, and landing. The potential for shimmy is often discovered when the aircraft undergoes taxi (runway) tests. At this stage the options available to the designer are limited, often to introducing a damper in the steering degree of freedom. This paper presents modeling and analysis of shimmy of a 3 degrees-of-freedom linear nose-wheel landing gear model with a closed-loop shimmy damper mechanism attached to its hydraulic steering. A typical control law is considered for representing the closed-loop hydraulic control in the steering rack and pinion system. The stability of the system is studied using root locus plots. The results for critical velocity obtained for different values of gain in the closed-loop shimmy damper show that it can be simulated by an equivalent open-loop shimmy damper with appropriate value of damping. This suggests that an equivalent open-loop shimmy damper model can be used in place of the closed-loop shimmy damper to simplify the modeling of integrated airframe— landing gear dynamics.