[Structured singular value] analysis tools for the flight clearance of highly augmented aircraft

This study describes the development and application of new analysis techniques using the structured singular value m for the clearance of flight control laws for highly augmented aircraft. Motivated by the limitations of the classical methods currently used by industry, these new techniques have been developed to provide a more rigours and efficient analysis of worst-case aircraft stability and performance characteristics. The classical approaches currently used in industry for the clearance of flight control laws are explained and evaluated. New m-tools are developed to directly address specific clearance criteria currently used by industry. Different approaches to represent uncertain linear and nonlinear systems as Linear Fractional Transformation (LFT)-based uncertainty models are described: a fast numerical approach that can easily generate LFT-based uncertainty models for nonlinear aircraft models at the expense of some conservation, a more complex symbolic approach that requires detailed information about the uncertain parameters in the aircraft dynamic equations, and a physical modelling approach which can generate LFT-based uncertainty models in a straightforward manner assuming the availability of the aircraft model in a block diagram representation. Two new algorithms for computing tight bounds on real m are introduced. Both methods are shown to be capable of generating good lower bounds on m, even for high-order uncertainty models. The application of the new m-analysis tools developed in this study to the flight control law clearance process is illustrated for a detailed fighter aircraft model called the HIRM+, for a VSTOL fighter aircraft model called the HWEM and for a civil transport aircraft model. Comparisons between the newly developed analysis techniques and the classical approaches demonstrate that m-analysis tools can significantly improve both the reliability and efficiency of the flight control law clearance process.