Nonlinear Predictive Control of Semi-Active Landing Gear

When airplane touches down and taxis on uneven runways with high speed, there is heavy ground impact and huge vertical load to the airframe. To improve safety and make passengers comfortable during landing, an effective landing gear capable of absorbing impact energy as much as possible is indispensable for modern airplane. Besides the basic function of reducing impact loads, the landing gear must also allow sufficient maneuverability during ground operation, which leads to conflicting requirements in terms of the suspension system (Krüger, 2000). Traditional landing gear consists of tires and passive shock absorbers, which can only be optimized before leaving factory to ensure the landing gear having a fairly good performance in particular design operational conditions, typically hard landings. However, due to its fixed structure, the passive shock absorber cannot always work well on various ground conditions and operational conditions. A heavy landing or a coarse runway may lead to significant deterioration of its performance, which is harmful to the fatigue life of the landing gear and of the airframe. Active control and semi-active control are widely used approach in the field of construction vibration control and vehicle suspension control. Compared with passive control, active and semi-active control has excellent tunable ability due to their flexible structure. Active control needs an external hydraulic source to supply energy for the system. The main drawback of active control approach is that its structure is very complex and the external energy may lead to instability of the system. The semi-active approach (Fig.1) modifies the damping characteristics by changing the size of the orifice area and does not introduce any external energy. Studies by Karnopp (Karnopp, 1983) for automotive applications also suggest that the efficiency of semi-active dampers is only marginally lower than of a fully active system, provided that a suitable control concept is used. In consideration of its simple structure and high reliability, semi-active control approach could be a better choice for landing gear systems. The main component of semi-active landing gear system is a tunable oleo-pneumatic shock absorber, which contains multidisciplinary and highly nonlinear dynamics. It is not an easy task to design an effective controller for such complex system. Krüger (Krüger, 2000) focuses his studies on optimization of taxiing performance of a semi-active landing gear. SIMPACK software is used to run simulation with a complete aircraft FEA model. Ghiringhelli builds a 13