Study On Analyses Of Nonlinear Land Gear Damping On Helicopter Ground Resonance Instability

The helicopter is a prime example of a nonlinear multi-body dynamic system that is subjected to numerous forces and motions to which the system must react. When a helicopter, with a conventionally articulated rotor head, is resting on the ground with its rotor spinning, a condition called ground resonance can develop. Ground resonance is a specific self-excited oscillation of the helicopter and is caused by the interaction between the main rotor blades and the fuselage structure. Inertia forces of the blades perform an out-of-phase lagging motion, which reacts with the elastic landing gear of the helicopter. For certain values of the main rotor angular velocity, the frequency of these inertia forces coincides with a natural vibration frequency of the fuselage structure. If this occurs, the inertia forces of the lagging blades produce oscillations of the fuselage, which then further excite the lagging motion of the blades. This interaction is responsible for an instability of conventionally articulated main rotor helicopters, which is called ground resonance.A helicopter may be damaged severely once the ground resonance happens. It must be solved during the design of a helicopter, so the problem we study is of great significance in engineering. In the paper, the author, beginning with the linear ground resonance studying, completely analyzes the mechanism and influence factors of the linear ground resonance, describes the analytical model of the ground resonance and its analyzing methods, investigates the possibility of occurrence of ground resonance to a certain type of aircraft based on its parameters, and calculates the critical stability curves. When the landing gear is of non-linear damping, however, the method used to analyze linear systems can not be employed. This paper, based on Lyapunov stability criterion and Hurwitz law studies the stability of the system and obtains the critical conditions of limit cycles. The gist of this article lies in the fact that it analyzes the ground resonance innovatively with the incremental harmonic balance method (IHB method), Due to coulomb friction damping and square damping, the system is no longer continuous. Consequently, numerical simulation applying the Runge-Kutta method is fairly time-consuming, and has difficulties in analyzing the problem correctly, On the contrary, the IHB method faces no obstacle in numerical simulation. What's more, it can be applied to calculate the strongly nonlinear problems, and can derive the frequency of self-excited vibration directly, which is quite conducive to further work, such as vibration suppression and so on.