An Integrated Approach to Aircraft Modelling and Flight Control Law Design

Abstract

The design of flight control laws (FCLs) for automatic and manual (augmented) control of aircraft is a complicated task. FCLs have to fulfil large amounts of performance criteria and must work reliably in all flight conditions, for all aircraft configurations, and in adverse weather conditions. Consequently, a large part of the FCL design process involves extensive simulation analyses, hardware-in-the-loop testing, and, eventually, flight testing. Multi-disciplinary aspects hereby play an important role. For example, control laws heavily influence (aerodynamic) loads on the airframe during manoeuvring and in turbulence, as well as flutter stability of the structure. These aspects are extensively addressed, but only -after- the actual design phase. As a consequence, problems that arise with other disciplines usually give rise to re-design of control law functions. This thesis proposes an FCL design process that allows multi-disciplinary aspects to be addressed from the beginning. In the first place, this requires multi-disciplinary aspects to be present in the aircraft dynamics models used for FCL design. To this end, the use of object-oriented modelling techniques is proposed, which, in contrast to contemporary techniques, inherently supports the development of models consisting of components from various engineering areas. As a specific application, its use for development of integrated flight mechanics and aeroelastic aircraft models is discussed. In the second place, multi-disciplinary FCL design requires a means to automate tuning of design parameters, since consideration of the many additional criteria make manual parameter synthesis very elaborate. For this reason, the use of multi-objective optimisation is proposed. This technique allows parameters to be optimised with respect to many, possibly conflicting, design criteria via a so-called min-max approach. The process is demonstrated on the design of control laws for automatic landing (autoland) of a passenger aircraft. The certification of autoland systems requires extensive Monte Carlo (MC) analyses to be performed in order to show that landing mishaps in all sorts of extreme conditions are very unlikely. The proposed design process allows the MC analyses to be directly addressed in the synthesis of control law parameters, so that MC analyses for certification can be passed in one shot. The proposed multi-disciplinary design process further allows the control design department to increase participation in aircraft preliminary design, by providing a means for rapid control law prototyping. This methodology allows nonlinear control laws for a specific aircraft design status to be automatically generated from an object-oriented implementation of a current flight dynamics model, using the technique of feedback linearisation and the possibility of automatic model inversion from object-oriented model implementations. For the control department, rapid prototyping allows for quick experimentation with controller structures, the selection of command variables, etc. For other engineering departments, the methodology results in early availability of representative control laws to analyse dynamic flight characteristics of the current aircraft design status.