Trajectory Control of Very Flexible Aircraft

In the pursuit of a reduced carbon footprint and more sustainable transportation methods, commercial aircraft are evolving toward more efficient designs. New wing concepts such as those found in the Boeing 787 and Airbus A350 incorporate composite materials that allow for much lighter wings that are capable deforming significantly. These large deformations require that aeroelastic - coupled structural and aerodynamic - models are integrated into the aircraft design process to predict the behaviour and interaction between large wing deformations, unsteady aerodynamic forces and the aircraft's flight dynamics. It is paramount to have a clear understanding of these interactions since they can have catastrophic consequences, as seen through past mishaps like that of the NASA Helios aircraft in 2003.
In addition, developments in powerful and lightweight electronic components are giving rise to the concept of high-altitude pseudo-satellites (HAPS), used to provide data communication, imaging etc. services worldwide. Compared to their orbital counterparts, HAPS have the added benefit of a more controllable flight trajectory and good maintainability prospects. These platforms, like the Airbus Zephyr, aim at staying airborne for months at a time at high altitudes, well above weather systems, by using solar power alone. To such extent, only designs with extreme aerodynamic efficiencies that are achieved by very high aspect ratio wings with lightweight structures can be considered. Such wings are very slender and in cruise conditions have deformations that are comparable to the wing span, introducing the possibility of hazardous interactions between aerodynamics and structural dynamics as experienced by Helios.
Controlling these aircraft is the challenge at hand since the large changes in the geometry of the vehicle during the flight mission often require the use of model predictive control strategies capable of updating their internal model as the vehicle deforms. However, models that describe the aircraft's aerodynamics, structural deformations and flight dynamics with an adequate level of fidelity are typically prohibitive in terms of size and cannot be used directly for the purposes of control. Therefore, one must turn to model reduction techniques to reduce the size of such systems to enable controller operation in modern hardware. The model reduction process itself is not trivial since the most important dynamics must be retained and the size of the underlying system is large enough to pose a computational challenge. Consequently, the concept of creating a database of pre-computed reduced order models for the aircraft in different flight conditions for the controller to interpolate between them "online" will be investigated. This will allow for all the heavy computations to be performed "offline". The controller then will guarantee stability of the aircraft within its flight envelope while optimising the resource-constrained flight trajectory given the limited use of solar energy that can be extracted during a 24 hour cycle.
This project will use and develop the open-source, in-house tool, SHARPy (Simulation of High Aspect Ratio airplanes in Python1) with the capabilities necessary to gather the complex and large aeroelastic models describing these new high-altitude platforms, perform model reduction, interpolation between models and finally incorporate flight control methods. It will require adopting the latest state-of-the-art techniques from control theory and model reduction to apply them to the aeroelastic problem at hand, all while keeping computational cost to within the reach of modern hardware. This will hopefully benefit the design of disruptive high-altitude platforms envisaged to provide services ranging from internet connection in remote areas to imaging the evolution of wildfires and that we could see entering service in a short to medium time frame.