PhD Oral Exam - Muftah Saleh, Mechanical Engineering
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Providing safer environment and minimizing the fatalities during helicopter harsh impacts and crashes have been a concern to scientists, engineers, and the regulating agencies since the dawn of helicopter industry. Tremendous efforts have been devoted to enhance the crashworthiness capabilities of helicopter’s skid landing gear (SLG) system. These efforts have been aiming to improve the energy absorption capabilities of the conventional SLG designs while maintaining minimum weight and adequate strength to comply with the airworthiness requirements. The emerging of smart materials and the advances in control engineering have provided a new horizon to design lightweight landing gears with enhanced energy absorption capacity.
In this context, the main objectives of the present dissertation research is to fundamentally investigate the crashworthiness performance of the conventional SLG; to formulate a design optimization strategy to design light-weight SLG with enhanced energy absorption capacity and finally to propose an adaptive SLG utilizing bi-fold magnetorheological dampers in an attempt to exceed the requirements of the crashworthiness specifications for skid landing gear systems while minimizing the level of sudden acceleration experienced by the aircraft occupants in the event of impact. The present research consists of four synergistically related phases. In the first phase, the dynamic response analysis has been conducted on the baseline conventional skid landing gear system in order to assess its capabilities and to establish a reference benchmark for the subsequent work. In the second phase, a design optimization strategy has been formulated to identify the optimal cross sectional dimensions of the round shaped crosstubes in the SLG system to maximize their energy absorption at sink rate of 2.44 m/s. The optimization results showed that the specific energy absorption of the design optimized SLG could be substantially increased compared to that of baseline design. Deign curves and guidelines have also been presented to directly determine the required effective mass of the helicopter and the corresponding desired maximum deflection of the helicopter under given sink rate and different values of rotor lift factor.
In the third phase, governing equations to predict the damping force and dynamic range of a bi-fold magnetorheological energy absorber (MREA) under impact have been presented. To predict the behavior of the MREA more accurately under high impact velocities, the Bingham plastic model with minor loss factors (BPM) has been incorporated in the problem formulation. The optimal geometrical parameters of the candidate bi-fold magnetorheological energy absorber (MREA) under volume constraint to maximize the damping force at piston velocity of 5 m/s have then been presented. Results showed a dynamic range of about two has been attained at this design speed. The proposed optimization problem has been solved using combined stochastic based (Genetic Algorithm) and nonlinear mathematical programming (Sequential Quadratic Programming Algorithm) techniques.
In the fourth phase, the MREA device model has been incorporated in a single degree of freedom helicopter model to assess the performance of the adaptive SLG system. The comparison of the responses revealed that the proposed adaptive SLG equipped with the MREA could minimize the induced acceleration while utilizing the full energy absorption stroke without encountering end-stop impact. The MREA performance was evaluated in terms of Bingham numbers for compression and rebound strokes. New optimum Bingham numbers-rotor lift factor chart has been introduced to control the generation of the required damping force based on the activated rotor lift force. Finally, to investigate the closed-loop performance of the tuneable MREA in the SLG system, a simple semi-active control strategy has been presented. The semi-active controller was designed based on the optimum Bingham numbers identified previously. Using the values of the velocity at the impact instant and the mass of the helicopter, the controller evaluates the required current for the MREA to generate desired yield force.