Shimmy in aircraft landing gear

Abstract

Shimmy is an oscillation in aircraft landing gear that can occur both on landing and take-off, typically in a band of velocities. It causes excessive wear on components and can cause accidents. The nose wheel is roughly like a caster on a shopping trolley: the horizontal axle of the wheel is mounted in an assembly that is free to rotate about a vertical axis. Shimmy is (or at least includes) oscillation of the wheel assembly about this vertical axis. The current engineering approach has little understanding of the physical mechanisms causing shimmy, but relies on the use of shimmy dampers, and on systematic maintenance and replacement of landing gear components. Simulations are carried out with finite element models and multi-body systems, and there are theoretical models due to Stépán and Somieski. In fact shimmy can also involve lateral oscillation of the landing gear (as well as torsional) and can be coupled to and caused by flutter of the airframe. The phenomenon is multi-scale in nature, as it can be linked to normal mechanical wear of key components at one scale, and gross flexibility effects at the vehicle scale. Airbus wish to identify it earlier in order to address passenger comfort, pilot comfort, manage mechanical wear and avoid overfatiguing the system elements. Specifically, Airbus wish to identify key system elements that may cause shimmy, when given a particular configuration of an aircraft. At early stages of development the configuration may involve the shape and size of the fuselage and design of the landing gear, whilst at the other end of the development process, the configuration may also consist of detailed system elements such as actuators, etc. Airbus relies on systematic maintenance and replacement of landing gear components, thereby avoiding the occurrences of the abovementioned phenomena.