Nonlinear Flexibility Effects on Flight Dynamics and Control of Next-Generation Aircraft

This project will develop a systematic approach to flight control system (FCS) design for very flexible or very large aircraft, of the type being considered for low-environmental-impact air transport and for long-endurance unmanned operations. It will create a virtual flight test environment that will support the design of advanced nonlinear FCS that fully account for the vehicle structural flexibility. To model the flight dynamics of flexible aircraft, it is necessary to develop analytical methods for generating Reduced Order Models (ROMs) via reduction of the full-order nonlinear equations of motion, and to do this in such a way that the essential nonlinear behaviour is preserved. The key issues addressed by our approach are that:1. The usual separation of flight dynamics and aeroelasticity is not appropriate for flight control when very low structural frequencies (which are also often associated with large amplitude motions) are present. Modelling and design methods based on a fully coupled system analysis are therefore necessary.2. Large wing deformations bring nonlinear dynamic behaviour, but current model reduction methods assume linearity. The development of nonlinear ROMs is an area that urgently needs advances, in general, and is necessary for control applications of flexible aircraft, in particular.3. Standard linear control design methods are inadequate for highly flexible aircraft, since their dynamic behaviour is intrinsically nonlinear. Fresh approaches to nonlinear FCS design are then required to control these systems in a provably robust way.The technical and scientific challenges to be overcome then include the simulation of significant aerodynamic and structural nonlinearities in full aircraft dynamics through the systematic development of a hierarchy of fully coupled large-order models, the reduction of these models to small-order nonlinear systems suitable for control development, and the development of robust control laws based on these reduced nonlinear models for gust load alleviation, trajectory control and stability augmentation. These methods will be exemplified in next-generation aircraft concepts that will be defined in discussion with end users. In fact, the project will benefit from a strong collaboration with major UK industrial partners, which will provide substantial technical inputs and support to the planned research activities.

The development of tightly coupled simulation methodology is an important facilitating step in the development very large transport aircraft and very light long endurance aircraft. The tight coupling of flight dynamics and control, aerodynamics and structural dynamics arises for these cases because of the presence of low frequency structural modes. Of particular interest is the nonlinear response to gusts. This motivates the development of nonlinear reduced order models which are suitable for load control development. The project involves interaction with Airbus, QinetiQ, BAE SYSTEMS, Stirling Dynamics and Dstl. All of these organisations are potential users of the methodology to be developed for the design of control laws. Airbus and Dstl will host the PhD students to facilitate transfer of the advances. All organisations will attend project meetings, and have agreed to supply a test case to evaluate the outcomes. Advances in the first part of the project will be presented to regular meetings of the Garteur action group that includes several of the main European aircraft manufacturers. The code framework developed, together with some test cases, will be made open to the community on the project website to allow widespread take-up. A workshop will be held to publicise the outcomes of the project to the three academic communities identified above. Finally, the project will produce 4 qualified researchers who have experience working in a multi-disciplinary environment, combining the disciplines in a manner likely to become increasingly important.