Nonlinear Analysis And Control Of Landing Gear Shimmy

The unwanted oscillations called shimmy can occur on both nose and main landing gears duringaircraft's take-off or landing. The paper has studied landing gear modeling, influence of landing gearparameters, active and semi-active control of shimmy, and bifurcation control. Numerical simulationmethod based on bifurcation theory and time domain analyses are used in the research.A nonlinear nose landing gear shimmy model is built. The model including strut lateral bending ofcharacterizes the motion of the system in terms of the torsional mode and lateral mode. Bifurcationanalysis based on pseudo arclength continuation method is used to study the stability of the systemabout landing gear structure and geometry parameters. The sensitivity of the system is studied tochanges of rake angle, torsional damping coefficient, torsional stiffness coefficient, lateral dampingcoefficient and lateral stiffness coefficient. The amplitude of shimmy changes with the forwardvelocity. The results have an important meaning to landing gear design.A nonlinear shimmy damper model is constructed including coulomb damping,linear viscousdamping and square damping. The bifurcation diagram from nonlinear shimmy damper model is moreaccurate than bifurcation diagram from linear shimmy damper model. An example shows the accuracyof the nonlinear shimmy damper model. Another example shows more accurate stability region usingpseudo arclength continuation method. The torsional mode and lateral mode are distinguished byamplitude and frequency of limit cycles. Time domain analysis and frequency analysis verify theresults.The numerical continuation method is proposed to analyze the effect of active shimmy control. Theresults show that active control can increase stable area in bifurcation diagram comparing withpassive control. The numerical continuation method can be used to determine the control law. Thetime domain analyses verify the effectiveness of the method. Direct fuzzy adaptive control isproposed to suppress shimmy. Simulation results show direct fuzzy adaptive control can removeshimmy vibrations caused by parameter perturbations.Nose Landing gear with magnetorheological shimmy damper is modeled and a semi-active controllaw is designed. The bifurcation diagrams are studied about the stable area, torsional shimmy area andlateral shimmy area. The results from semi-active control are compared with passive control and showthat semi-active control of shimmy can effectively decrease torsional shimmy region. Square and cubic nonlinear state feedback controllers are used on nonlinear shimmy model toinvestigate the controllability of shimmy amplitude, shimmy frequency, stable area and shimmy area.The results show that the controllers can control both shimmy amplitude and frequency, but have noeffects on either stable area or shimmy area.