Ajinkya A. Gharapurkar

Abstract: 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.

Abstract: Shimmy vibration of aircraft nose landing gear is damped and controlled using a nonlinear control which is optimal and robust against parametric uncertainties and external disturbances. Shimmy vibration is the lateral and torsional vibrations in the wheel of the aircraft that is self-excited and causes instability in high speed performances which can damage the landing gear of the aircraft, its fuselage and even may result in hurting the passengers. Thus, control and damping of this vibration are extremely important. In this paper a robust optimal controller is designed by integrating sliding mode control (SMC) together with State-Dependent Riccati Equation (SDRE) to prevent the shimmy vibrations in aircraft nose landing gear. The SDRE compensator controls the nonlinear system in an optimal way while the sliding mode controller guarantees its stability against uncertainties and disturbances. The proposed controller can effectively suppress the shimmy vibration of the landing gear with variable taxiing velocity and wheel caster length. To verify the optimal performance and robustness of the proposed controller, vibration response of the system is simulated by MATLAB software and its performance and efficiency are verified using comparative analysis. Considerable improvement can be seen in the performance of the closed loop system since not only the vibrations are effectively damped but also the consumption of energy is minimized.

Abstract: The magnetorheological (MR) damper is the newest approach to replace the traditional passive damper which cannot change their dynamics in response to different operating conditions of the aircraft landing gear. This paper presents the simulation study of a semi-active controller for a landing gear equipped MR damper. Furthermore, a new method combined skyhook control with force control, called hybrid control, is developed to improve the performance of the MR damper landing gear. Finally, the numerical simulation result of the landing gear using SIMSCAPE-Simulink is discussed.

Abstract: Several uncertainties in the landing environment of an aircraft are not considered, such as the falling speed, ambient temperature, and sensor noise. These uncertainties negatively affect the performance of the controller applied to a landing gear. The sliding mode control (SMC) method, which maintains the optimal performance of a controller under uncertainties, is used in this study. The landing gear is equipped with a magnetorheological damper that changes the yield shear stress according to the applied magnetic field. The applied controller employs a hybrid control combining Skyhook control and force control. The SMC maintains the optimal performance of the hybrid control by minimizing the tracking error of the damper force, even in various landing environments where parameter uncertainties are applied. The effect of SMC is verified through co-simulation results from Simscape and Simulink.

Abstract: The shimmy phenomenon is a significant concern in aircraft landing gear dynamics. The prediction of the shimmy instability is an essential issue in landing gear design to develop a passive or active suppression method. This work investigates the application of the nonlinear energy sink (NES) concept to mitigate the effects of shimmy in landing gears. The NES concept has been used in recent research on mechanical vibrations. It comprises a passive target energy transfer method that refers to a one-way energy transfer from a primary to a nonlinear subsystem. The landing gear model is based on torsional displacement coupled with the tyre classical elastic string analogy model. The NES device connects to the wheel shaft, and it comprises a mass, a linear damper, and a pure cubic spring. The numerical integration in time was used to assess the shimmy onset speed and the post-shimmy limit cycle oscillations. A parametric analysis of the landing gear nonlinear dynamics without the NES is presented. The design space of possible NES parameters is given, obeying design constraints, and inspected to assess adequate NES designs. The best NES samples are included in the landing gear dynamics to study their influence. Results have shown that the NES can adequately expand the operational speed range for no shimmy and lead to lower LCO amplitudes in the post-shimmy for a reasonable range of speeds. The NES concept’s successful employment for the landing gear dynamics suggests an enormous potential form of passive shimmy control.