Oleopneumatic shock strut dynamic analysis and its real-time simulation

An analog simulation representing dynamic shock absorption characteristics of a conventional oleopneumatic landing gear strut has been developed. This simulation includes both airplane and strut dynamics. The basic equations of motions are those used for a two-degree-of-freedom system. The hydraulic, pneumatic, and mechanical aspects of the shock strut dynamics have been analyzed. The metering orifice and snubber orifice discharge coefficients have been treated as a function of Reynolds number, orifice shape, and orifice orientation. The snubber valve hydraulic force during compression and rebound has been accounted for. Other factors considered in the system simulation are internal friction in the shock strut, wing lift, inclination of the landing gear, and the effects of wheel spinup and drag loads. A special circuit was devised on the analog simulation which allows the operator to vary the shape of the metering pin, observe the effects on shock strut load instantly, and optimize the load stroke curve(s). The simulation has the capability of high-speed, iterative operation. Good correlation has been established between flight/drop test data and the simulation predicted results. The simulation therefore provides an important design tool for optimizing pin profile, as well as evaluating existing shock strut designs.