Linear Stability Analysis and Dynamic Response of Shimmy Dampers for Main Landing Gears

Abstract

This paper presents the analysis and study of common shimmy dampers used today for main landing gears with the use of analytical and numerical tools. The shimmy phenomenon is studied by using the tire stretched string theory model and by developing linear approximations of the dynamics of a single tire landing gear. The dynamics of commonly used shimmy dampers are then incorporated into the model. The objectives of this paper are to study already developed shimmy damper designs and to develop tools to design a new innovative and better shimmy damper for main landing gears, those which have nonsteerable wheels. Two shimmy damper designs are studied in this paper, one developed by Boeing and another by UTC Aerospace Systems (UTAS). A linear approximation of the dynamics of these dampers is obtained, omitting the freeplay, saturation, and nonlinear dynamics. Stability plots are then created by changing the system's parameters, such as the velocity, caster length, and the shimmy damper stiffness and damping coefficients. These plots show the comparison of using a UTC two-arm design against the Boeing damper, for which the former spans larger zones of stability but requires higher damping coefficients due to the UTC damper's geometry which is very impractical. In addition, a multibody model is developed in MSC adams (from MSC Software Corporation) to study the dynamic response of these systems and to create a modeling tool that can be used to design a new and improved shimmy damper for main landing gears. The simulation results from the model show the disadvantages of using the UTC two-arm damper, which include an asymmetrical vibration response. Further recommendations are given to design an improved shimmy damper.