Doctoral Degrees (Electrical and Electronic Engineering)
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Browsing Doctoral Degrees (Electrical and Electronic Engineering) by Subject "Aircraft accidents"
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- ItemRobust Fly-by-Wire under Horizontal Tail Damage(Stellenbosch : Stellenbosch University, 2016-12) Dlamini, Zinhle; Jones, T.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Aircraft damage modelling was conducted on a Boeing 747 to examine the effects of asymmetric horizontal stabiliser loss on the flight dynamics of a commercial fly-by-Wire (FBW) aircraft. Change in static stability was investigated by analysing how the static margin is reduced as a function of percentage tail loss. It is proven that contrary to intuition, the aircraft is longitudinally stable with 40% horizontal tail removed. The short period mode is significantly changed and to a lesser extent the Dutch roll mode is affected through lateral coupling. Longitudinal and lateral trimmability of the damaged aircraft are analysed by comparing the tail-loss-induced roll, pitch, and yaw moments to available actuator force from control surfaces. It is presented that the aircraft is completely trimmable with 50% tail loss. Robustness of a generic C* FBW control system is investigated by analysing how characteristic eigenvalues move as a result of damage, and comparison to the non-FBW aircraft is made. Furthermore, the extent of stabiliser loss that the system can successfully handle, without loss of acceptable performance, is identified. A handling qualities evaluation is presented to provide an understanding of how the pilot would perceive the damaged aircraft. The results of the study show that a generic FBW system improves robustness such that the aircraft is stable with 50% horizontal stabiliser loss. With 50% damage, the aircraft is controllable but unsafe to fly and may be unable to effectively complete its mission task. The damaged FBW aircraft is formulated into an H2 control problem. Convex optimisation techniques are employed to represent the problem as a linear matrix inequality and a solution is synthesised through the interior point method. An analysis of the state feedback gains is carried out to ascertain a suitable control strategy to minimise the in fluence of disturbance on longitudinal dynamics. It is proven that pitch angle feedback provides good disturbance rejection in the low frequency range, however, it attenuates the control signal at higher frequencies thus resulting in loss of robustness. By comparison with a different class of aircraft it is shown that pitch angle feedback is only advantageous for aircraft with slow closed-loop longitudinal poles. The generic C* fly-by-wire system is augmented to include pitch angle feedback and thus creates a novel system, the C* FBW. This system is compared to the original C* and its advantages and disadvantages presented. For the case of 50% damage, the phugoid poles of the system are stable whilst the short period poles are within level 2 handling qualities. A small loss in robustness is, however, observed for the short period poles. It is shown through an alternative control strategy that improvement of short period robustness can be achieved by increasing the system gain, however, this destabilises the marginally stable phugoid poles of the aircraft. The original contributions presented in this thesis are in the field of flight dynamics and robust control. An analysis of change in dynamics due to horizontal tail damage is carried out in a method that provides visibility to changes in trim and manoeuvrability of the aircraft after damage. An evaluation of FBW robustness against this kind of damage is presented as well as change in handling qualities. A novel approach of analysing disturbance rejection capabilities of an aircraft with available actuators through a more robust combination of feedback states is discussed. From this analysis a new FBW control law is developed and its robustness evaluated. Through a comparison with an ideal system the limiting factors to improving the robustness of the B747 class of aircraft are identified.