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Many control problems are based on control objectives easily quantified and consequently realisable by standard control synthesis methods. When an unmanned aircraft navigates, it moves inside a complex environment due to interactions with its surrounding and time varying environmental conditions. Several causes of perturbations have been identified as for example gusts and corrupted information of position. The characteristics of possible missions carried out by the un manned aircraft leads to the desire to construct navigation control systems which when operated in perturbed environments combine the advantages of smooth control with accurate navigation. Rule based, and adaptive controllers have favourable properties for such systems. This thesis investigates the use of a rule based navigation controller for a particular unmanned aircraft, the XRAEl aircraft. To achieve this objective several different types of fuzzy logic controllers are analysed as for example conventional and direct and indirect adaptive fuzzy controllers. They are designed by employing simple control engineering knowledge and subsequently validated using a stability method. For this purpose diverse stability methods are described and a new technique presented, the fuzzy root locus method, which is also based on the introduction of a new concept for fuzzy logic controllers, the fuzzy cell. The realisation of this work has been achieved by a series of simulation tests employing different processes and a simulation model of the XRAEl aircraft. The conclusions drawn from the results of the experiments consider in general that a rule based controller can improve the quality of navigation when compared to conventional controllers.

The introduction of fly-by-wire electronic flight control systems into transport aircraft has given the flying qualities engineer the opportunity to optimise the flying qualities of these aircraft for their specific tasks. With this technology has come the opportunity to introduce new technologies into the cockpit, such as non-linked or backfed sidesticks and non-backfed throttle levers. A comparative survey of airline pilots flying such a very high technology unconventional aircraft and a high technology but conventional aircraft suggests that these technologies may reduce the available channels of communication to the pilot in the very high technology aircraft, resulting in the possibility of reduced situational awareness. A closed loop piloted simulation survey of ten transport aircraft in current operation was undertaken which demonstrated that they all suffered from flying qualities deficiencies, limiting the performance that the pilot could achieve. In particular poor dynamics precluded the pilot adopting tight closed loop, or compensatory, control. Instead it was necessary to adopt a more open loop, precognitive, technique with medium frequency modulation, resulting in a degradation in landing performance. Through appropriate flight control system design it should be possible to produce aircraft that can be flown using the full range of control inputs from open to closed loop. The major study of this thesis assessed, through piloted simulation evaluations, the suitability of a wide range of longitudinal commanded response types for the approach and landing tasks. It was concluded that a response type that closely resembles that of angle of attack is optimum for these tasks due to its conventional characteristics of speed stability on the approach and monotonic stick forces in the flare. Such a system, appropriately implemented, should allow the transport aircraft pilot the full range of piloted control inputs, from open loop, precognitive, to closed loop, compensatory, resulting in improved landing performance.

The development of a microprocessor based digital flight control system for a particular R.P.V. is described. The tasks required of this system are defined, and thereafter, the hardware circuits and the software structure necessary to implement a prototype are presented. The autopilot control laws are inferred from z-plane root loci, and then confirmed using digital simulations of the de-coupled roll and pitch attitude loops. The problems of the finite wordlength implementation of the control laws are discussed, and then both hybrid simulation and actual flight results are used to prove the performance of the prototype. To exploit the adaptive capabilities of a software based system, a sliding mode variable structure control law is developed for the roll attitude loop. Digital simulations are used to show that significant improvements in sensitivity reduction can be achieved under some conditions. These improvements are lost if a realistic servo-actuator model is employed. Another objective, namely the reduction of the disturbance error induced by trim imbalance, is maintained provided a reduced order switching function is used.

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