Robust Fly-by-Wire under Horizontal Tail Damage

[摘要] ENGLISH ABSTRACT: Aircraft damage modelling was conducted on a Boeing 747 to examine the effects ofasymmetric 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 tointuition, 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 characteristiceigenvalues move as a result of damage, and comparison to the non-FBW aircraftis made. Furthermore, the extent of stabiliser loss that the system can successfully handle,without loss of acceptable performance, is identified. A handling qualities evaluationis presented to provide an understanding of how the pilot would perceive the damagedaircraft. The results of the study show that a generic FBW system improves robustnesssuch 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 itsmission task.The damaged FBW aircraft is formulated into an H2 control problem. Convex optimisationtechniques are employed to represent the problem as a linear matrix inequalityand a solution is synthesised through the interior point method. An analysis of the statefeedback 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 feedbackprovides good disturbance rejection in the low frequency range, however, it attenuates thecontrol signal at higher frequencies thus resulting in loss of robustness. By comparisonwith a different class of aircraft it is shown that pitch angle feedback is only advantageousfor aircraft with slow closed-loop longitudinal poles. The generic C* fly-by-wire systemis 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 disadvantagespresented. For the case of 50% damage, the phugoid poles of the system are stable whilstthe 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 controlstrategy that improvement of short period robustness can be achieved by increasing thesystem 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 androbust control. An analysis of change in dynamics due to horizontal tail damage is carriedout in a method that provides visibility to changes in trim and manoeuvrability of theaircraft after damage. An evaluation of FBW robustness against this kind of damageis presented as well as change in handling qualities. A novel approach of analysingdisturbance rejection capabilities of an aircraft with available actuators through a morerobust combination of feedback states is discussed. From this analysis a new FBWcontrol law is developed and its robustness evaluated. Through a comparison with anideal system the limiting factors to improving the robustness of the B747 class of aircraftare identified.

[发布机构] Stellenbosch University