Aeroservoelastic models for design, testing, flight test clearance and validation of active flutter suppression control laws

ifasd2024 Tracking Number 103

Improving the aerodynamic efficiency of aircraft with high aspect ratio wings is a current trend for future designs. However, the slender wing structures are prone to suffer from an adverse interaction between aerodynamics and structural dynamics causing a destructive instability called flutter. Active Flutter Suppression (AFS) is a key technology to enable high aspect ratio wings by stabilizing the flutter behavior without weight penalties using an active control system. The EU funded projects FLEXOP and FliPASED addressed this topic and matured the technology up to a successful demonstration of the AFS control laws in flight tests with the P-FLEX UAV. Accurate simulation models of the closed loop flexible airframe including sensor and actuator dynamics are essential for the design and successful demonstration of such an AFS system. This paper addresses the development of such mathematical models, their validation and enrichment with available data along the project progress timeline. The mathematical models are employed for various different design activities. Linear low order models are required for model-based control law design. This applies to the baseline controller and autopilot functions, as well as the flutter suppression control laws. Their validation is then performed with an integrated simulation containing nonlinear flight dynamics, flexible structure and unsteady aerodynamics. During the development various design analyses, e.g. flutter analysis with and without the AFS active need to be performed. Realtime capable models are required for hardware in the loop tests to validate controller implementations with adequate sensor and actuator characteristics including system delays. Finally, flight test clearance of the control laws must be performed to ensure safe operation during flight tests. As the project progressed, available test data was used to improve the simulation models. A Ground Vibration Test (GVT) was performed to update the modal frequencies, shapes and damping. To match the flight dynamics behavior of the simulation models, new updating techniques were applied using flight test data of identification manoeuvres. Online and post flight modal identification was used to confirm the open loop flutter speed predictions during subcritical flight tests. These test results served as final validation of the simulation model to clear the AFS control laws for the successful demonstration beyond the open loop flutter speed.