The European Union has set itself the goal of making aviation climate neutral by 2050. The planned use of hybrid-electric aircraft is a first step in the field of propulsion towards this goal, but structural design must also be rethought by considering the design of flexible and very flexible aircraft.
Highly flexible aircraft with a high aspect ratio and lightweight structure increase aerodynamic efficiency and reduce weight, which translates into reduced fuel consumption and thus carbon emissions. The industry has already moved in this direction with aircraft such as the Airbus A350 and Boeing 787, but the potential for savings has not yet been exhausted. Previous obstacles, such as nonlinear structural behavior and the coupling of inherent elastic modes of motion with flight dynamics, must be overcome through extensive research. This is where the planned research project comes in.
The modeling of the dynamics of a flexible aircraft depends on its degree of flexibility. For flexible aircraft, with wing tip deformations up to 10% of half of the wingspan, methods based on a linear structural formulation are used. When the deformations exceed 10%, the aircraft reaches a new level of flexibility, and is classified as a very flexible aircraft. These aircraft require the use of nonlinear structural dynamics and nonlinear aerodynamics. Although the theory of these methods is partially available to the commercial aircraft industry, they have not yet been validated, nor have they been fully developed with respect to the overall aircraft subsystems.
In particular, flight control systems (FCS) (which are responsible for much of the flight control in today's commercial aircraft) are heavily influenced by flexibility. Traditionally, autopilot design has been based on the rigid-body approximation of aircraft flight dynamics. To avoid dangerous interactions between FCS and structural dynamics, the industry uses notch filters. However, these reduce the achievable control quality and thus also the safe control orientation of the aircraft, especially in the case of strong coupling between flight and structural dynamics.
To ensure the safety of future aircraft (which will have a higher level of flexibility) new and, above all, validated methods are needed. The objective of this project is to generate a multidisciplinary aircraft design process in which aeroelastic control and structural load control are directly considered in the design phase to overcome current limitations and contribute to neutral flight.
In this joint project, the Department of Flight Mechanics, Flight Control and Aeroelasticity at the Technical University of Berlin (which has its expertise in the field of highly flexible aircraft dynamics and control), and the Institute of Air Systems Engineering at the Technical University of Hamburg (which has expertise in the field of load control) join forces to provide the missing tools for the multidisciplinary aircraft design process, including development and validation by flight testing.
TU-Hamburg's Super Dimona flying platforms and the TU-Berlin's TU-Flex platform (currently being jointly developed by TU Berlin and DLR) will be used to validate the models and controls developed during the project. TU Hamburg will use the Dimona to contribute to the design of future wings, whose loads can be specifically mitigated by control engineering approaches. This will enable significant weight reductions and further increase the aspect ratio. The TU-FLEX was designed as a flexible testbed with a configuration that allows to draw conclusions applicable to both transport and commercial aircraft. The aircraft was designed with exchangeable wings. This will allow to increase the level of flexibility of the platform as shown in Fig. 1. The aircraft carries an integrated flight controller that considers both the nonlinear effects of structural dynamics and flight dynamics. This platform allows validation of model formulations and control implementations in flight tests. As a result, the validated methods will be available to industry for use in the development of more aerodynamically efficient, lighter and therefore more climate-neutral aircraft.