NONLINEAR RESPONSE OF AN AIRCRAFT TO PULSE EXCITATION

A typical aircraft landing gear consists of a tyre and a shock absorber. Shock absorbers use a quadratic type of damping. Fluid friction is created by making air or some other fluid pass through an orifice. Relative motion across the absorber forces hydraulic fluid through an orifice which absorbs some of the energy of the system. Most of the hydraulic fluids have a limited usable temperature range. The damping characteristics alter with large temperature changes. Shock absorbers lose efficiency necessitating large strokes or higher load factors, either of which add weight to the aircraft. The performance of the landing gear system is closely linked with the energy absorption characteristics of the shock absorber/tyre combination. In practice the tyre and the shock absorber have nonlinear characteristics. This paper presents the investigations on the application of nonlinear damping and stiffness to an aircraft landing gear. An aircraft and its landing gear are idealized as two degree-of-freedom system incorporating nonlinear damping and stiffness. The input excitation considered for the analysis is the 'rounded step' pulse. Nonlinear responses have been obtained for both aircraft and landing gear on a digital computing system using the Runge-Kutta algorithm. The digital computer provides a rapid means of studying the effect of nonlinearities. The effects of nonlinear damping and stiffness on the response of both aircraft and landing gear have been plotted. By increasing the nonlinear stiffness parameter, the response will be increased and as will the frequencies. The maximum nonlinear response will occur earlier than the linear maximum response. By increasing the nonlinear damping parameter, the maximum response will be reduced, resulting in a decrease of frequencies. The maximum response for the linear case occurs earlier than the nonlinear maximum response.