Helicopter Landing Gear Shimmy Analysis

The design of helicopter nose landing gears could largely benefit from improved shimmy predictive methodologies. Shimmy is a dynamic instability that causes the landing gear to have both lateral and torsional oscillations. This phenomenon typically occurs at high velocities during helicopter takeoffs, landings, or taxi [1]. Traditionally, shimmy analysis has used linear dynamic models such as the method used by Moreland [2]. However, these methods do not capture the non-linear behavior of the coupled landing gear / fuselage system and often lead to shimmy damper solutions experimentally derived from trial and error. This process is not ideal and causes test delays after the aircraft has completed the design phase. A standalone nose landing gear and full aircraft CH-53K shimmy model were developed by Sikorsky and the CH-53K landing gear supplier: Heroux Devtek. The standalone nose landing gear model was initially used to evaluate the dynamics of the nose landing gear. However, this model did not adequately capture the flexibility of the helicopter fuselage and the complicated dynamic system excitations that occur during operation. Therefore, a full aircraft model was developed which enabled better insight into the non-linear shimmy phenomena. Sikorsky has had prior experience with shimmy occurring during landing due to the natural excitation created by the tire impact on the ground. Based on this prior experience, Sikorsky used the full aircraft model to evaluate the shimmy phenomena for several aircraft configurations during landing. The results of the analysis showed several frequencies present in the tire yaw angle response. The cause of several frequencies being present in the yaw angle time history is due to the excitation being coupled with the compression and extension of the landing gears and with the flexible fuselage response. To capture the damping of the various coupled aircraft / landing gear modes excited by the landing, a generic wavelet tool was developed based on prior work by Nguyen Phong Dien [3]. The intent of this paper is to describe the development of a full aircraft CH-53K shimmy model and show correlation of the model with past helicopter test data trends.