Landing Gear

The study of the landing gear subsystem is important in simulation (and also in control analysis) of touchdown and ground roll. Landing gear dynamics play an important role during and after the touchdown and before takeoff. Also, the suspension geometry (cantilever or telescopic, lever or trailing link, semiarticulated), the braking system, and nosewheel steering and other related aspects should be kept in mind. Some important typical landing-gear elements to be modeled are described next [10].

Oleo-pneumatic strut: It contains two chambers telescoping into each other. It is partially filled with compressed air and oil. When the strut is extended or com­pressed, the oil flows between the chambers via a metering orifice. The damping is due to the viscosity of the fluid and the compressed air acts as a spring. The spring action is nonlinear and follows Boyle’s law: PVg = constant. Here, P is the chamber pressure of the gas, V is the chamber volume, and the exponent is the gas constant.

The damping effect is based on the orifice flow as expressed by Q0 = CdiA0 y/2DP.

Here, Q0 is the flow rate through the orifice, Cdi is the discharge coefficient, A0 is the area of the orifice, DP is the differential pressure across the orifice, and r is the density of the oil. The composite force across the strut is the sum of the spring and the damping forces and this model is equivalent to a spring and damper systems in parallel.

Tire Dynamics: The pneumatic tire acts as a spring in the vertical axis. As a first approximation the characteristics of an automobile tire can be used. The forward direction frictional force helps the aircraft to decelerate.