Landing is the most critical operational phase of an aircraft since it directly affects the passenger safety and comfort. The factors such as the undesirable wind and ground effects, runway unevenness, excessive sink speeds and approach speeds and pilot errors can deteriorate the landing performance of an aircraft several times during its entire lifetime. When an aircraft lands, large amplitude vibrations get transmitted to the fuselage from the runway thereby causing safety and comfort problems and hence need to be suppressed quickly. 
Landing gear is an essential assembly that prevents the aircraft fuselage from the ground loads. A shock absorber which is considered as the heart of the landing gear assembly plays an important role in this process by absorbing the vibrations during landing. The existing Oleo-pneumatic shock absorbers are the most efficient in absorbing the vibrations during each aircraft operation. However, they are unable to provide the continuously variable damping required during the landing phase which might reduce their efficiency. Moreover, to account for the uncertainties during landing, a damper capable of providing the variable damping effect can play a vital role in increasing the passenger safety. 
A semi-active control system of a landing gear suspension can solve the problem of excessive vibrations effectively by providing a variable damping during each operational phase. Magnetorheological (MR) dampers are one of the most efficient and attractive solutions that can provide the continuously variable damping required depending on a control command.
This thesis focuses on the concept of the semi-active aircraft suspension system using the MR damper with the implementation of robust control strategy. Initially, the dynamic behavior of the MR damper is studied using the parametric modeling approach. Spencer dynamic model is adopted for simulating the dynamic behavior of the MR damper. This is followed by the analysis of the energy dissipation patterns of the MR damper for different excitation inputs.
A semi-active suspension system is developed for a three degree-of-freedom (3 DOF) aircraft model considering a tri-cycle landing gear configuration. A switching technique is developed in the simulation of the landing procedure which enables the system to switch from the single degree of freedom to three degrees of freedom system in order to simulate the sequential touching of the two wheels of the main landing gears and the nose landing gear wheel with the ground. For developing the semi-active MR suspension system, two different controller approaches, namely, the Linear Quadratic Regulator (LQR) and the H∞ control are adopted. The results of the designed controllers are compared for a particular landing scenario for studying the performance of the controllers in reducing the overshoot of the bounce response as well as the bounce rate response. The simulation results confirmed the improved performance of the robust controller compared to the optimal control strategy when the aircraft is subjected to the disturbances during landing. Finally, implementing the robust control approach, the landing performance of an aircraft embedded with the semi-active suspension system is simulated and analyzed for different sink velocities considering the disturbances.

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Robust Semi-active Control of Aircraft Landing Gear System Equipped with Magnetorheological Dampers

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H. Hall

Papers

Some theoretical studies concerning oleo damping characteristics