Abstract: A simplified analytical method for calculating the limit cycle behav ior of a helicopter model with nonlinear dampers in both the landing gear and blades is presented. It has provided a new way to protect effectively against the ground resonance instability for helicopter designers. The chaotic oscillations induced by both the geometrical nonlinearities of the blade lag motion and nonlinear dampers and the effect of initial disturbances on the instability response are also discussed; An experiment has been carried out using a helicopter scale model. The experimental results agree with those of the theoretical analysis.