AEROELASTICITY & STRUCTURAL DYNAMICS 2024

16:00 Flutter control
Chair: Mohammadreza Amoozgar

16:00 30 mins	Control design for active flutter suppression flight testing Bálint Patartics, Béla Takarics, Bálint Vanek, Ramesh Konatala, Matthias Wüstenhagen, Özge Süleözgen, Manuel Pusch, Thiemo Kier Abstract: Aeroelastic flutter is an adverse interaction between the structural dynamics and the aerodynamics of an aircraft, that manifests as undamped oscillations of the wings. It occurs at lower airspeeds for lightweight aircraft equipped with high aspect ratio wings, which would be desirable for fuel efficiency. To retain efficiency and ensure safe operations, active control solutions to mitigate this problem are widely explored in the literature. This paper presents two flutter suppression control design methods for the P-Flex flexible unmanned aerial vehicle developed within the FliPASED H2020 project. This aircraft has a wing bending-torsion dominated flutter mechanism that first appears at 56 m/s at roughly 8.65 Hz. Another flutter mode is present in the attainable fight domain at 69.9 m/s. The aeroelastic model of the P-Flex aircraft includes rigid body, structural, and unsteady aerodynamics which amounts to over 1000 states [1]. This model undergoes significant reduction to arrive to a control-oriented model applicable for control synthesis [2]. A custom-made actuator, called direct drive, is used to provide the sufficient bandwidth required by the high flutter frequency. Two control approaches are detailed in the paper. The first is an H∞ optimal solution that minimizes the sensitivity function of the closed-loop which in turn maximizes robust stability [3]. Structured H∞ synthesis is employed to obtain two controllers stabilizing the two flutter modes. Disk margin analysis verifies the achieved increase in the closed-loop flutter speed. The second approach finds two optimal blending vectors of the actuating signals and sensor measurements allowing for the isolated control of the unstable modes with minimal influence on the overall flight performance [4]. Both this and the H∞ method expand the safe flight envelope by 15% and are shown not to degrade the performance of the baseline controller which governs the rigid body motion of the aircraft. Also, the refinement of both control laws based on preliminary (below flutter speed) flight test results is presented. The final controllers are flight tested above flutter speed as elaborated in a different paper submitted to this conference [5]. References [1] Wuestenhagen M., et. al. “Aeroservoelastic modeling and analysis of a highly flexible flutter demonstrator”. In 2018 Atmospheric Flight Mechanics Conference, p. 3150. [2] Takarics, B., et. al. “Flight control oriented bottom-up nonlinear modeling of aeroelastic vehicles”. In 2018 IEEE Aerospace Conference, pp. 1-10. [3] Patartics, B., et. al. “Application of structured robust synthesis for flexible aircraft flutter suppression”. IEEE Transactions on Control Systems Technology, 30(1), pp. 311-325. 2021. [4] Luspay, T., et. al. “Flight control design for a highly flexible flutter demonstrator”. In AIAA Scitech 2019 Forum, p. 1817. [5] Bartasevicius J., et. al. “Lessons learnt from flight testing active flutter suppression technologies” Submitted to the 2024 IFASD.
16:30 30 mins	Design of flutter suppression controllers for a wing in compressible flow based on high-fidelity aerodynamics Boris Micheli, Jens Neumann, Jürgen Arnold Abstract: Since active flutter suppression technologies could lead to more efficient aircraft, they are acquiring increasingly importance and research activities relying on wind tunnel demonstrators are flourishing to gain experience and knowledge. Aeroelastic models employed for control activities are traditionally based on Generalized Aerodynamic Forces (GAFs) computed through DLM in reduced frequency domain. The main limitation of this approach is that compressible flow exhibits nonlinearities which are not captured by DLM. To overcome this shortcoming, this paper solves the governing equations of motion in time domain coupling a structural dynamic solver and CFD Euler aerodynamics. The aeroelastic system is excited to identify the CFD GAFs which are coupled with the structural matrices yielding a more accurate state space realization of the aeroelastic system suited for control design activities. The state space realizations are then exploited to design $\mathcal{H}_\infty$-based flutter suppression controllers, which are implemented in the fully coupled computational fluid/structural dynamics solver to demonstrate the damping augmentation capabilities of the compensators. The approach is demonstrated on a realistic aeroelastic system and comprehensive nonlinear computations using controllers synthesized based on GAFs either computed via Euler CFD or uncorrected DLM are presented. Differences in the results, even at subsonic Mach numbers, will be explained based on comparative analyses of the different pressure fields, highlighting the benefits of using high-fidelity aerodynamics.
17:00 30 mins	Aeroservoelastic models for design, testing, flight test clearance and validation of active flutter suppression control laws Thiemo Kier, Matthias Wüstenhagen, Özge Süelözgen, Ramesh Konatala, Yasser Meddaikar, Keith Soal, Nicolas Guerin, Julius Bartasevicius, Bela Takarics, Daniel Balogh, Balint Vanek Abstract: 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.
17:30 30 mins	Aeroelastic aircraft design and flight control methods applicable for flutter suppression: The flipased perspective Balint Vanek, Virag Bodor, Bela Takarics, Thiemo Kier, Keith Soal, Charles Poussot-Vassal, Nicolas Guerin, Mirko Hornung, Christian Roessler Abstract: The European FliPASED H2020 consortium developed multidisciplinary design capabilities for commercial aircraft that increases competitiveness in terms of aircraft development costs and environmental impact via a closer coupling of wing aeroelasticity and flight control systems. The team developed not only advanced design tools but also flight tested several key enabling technologies to mature the technology readiness level of previously unviable designs: common models, methods and tools across disciplines could provide a way to rapidly adapt existing designs into derivative aircraft, at a reduced technological risk (e.g. using control to solve load alleviation and flutter problems discovered during development). The paper describes the fundamental hurdles tackled to successfully conduct active flutter control experiments on a 7 m wingspan conventional configuration demonstrator. Motivation, driven by industry needs, of active flutter control, is mostly established within the prior project FLEXOP H2020 and the basic concept of the demonstrator aircraft built to perform research on active flutter control methods. To be able to conduct successful active flutter control experiments the team had to mature experimental capabilities as well as theoretical mathematical modelling and control design tools hand in hand. Key components for success with dedicated chapters in the paper were: • Establishing multidisciplinary workflows for understanding the benefits of flutter control as a key enabler of designing high aspect ratio wing aircraft, • Development of a scaled demonstrator exhibiting many of the required aeroelastic phenomena applicable for commercial aircraft industry, including wing bending torsion coupled flutter at an attainable flight speed and acceptable flutter frequency, • Mature the ground and flight test procedures and infrastructure to conduct tests and iterate the results within a short timeframe, • Build control oriented aeroservoelastic models, including flutter prediction methods, what can be tuned with experimental results including high fidelity CFD, GVT and flight tests, • Solve the control problem of flutter, including sensor selection, highly complex dynamics and coupling of control loops, • Mature operation modal analysis methods to help conducting safe flight tests, with special emphasis on running the algorithms onboard the demonstrator real-time, • Develop custom actuators, sensors and flight control computers, with hardware-in-the-loop testing capabilities working across teams. Within an iterative design cycle theoretical results and analytical predictions were improved and validated by independent flight tests by a highly motivated team, leading to the success of the ground-breaking flutter control experiments. Several of the key building blocks to achieve successful flight tests, including major research findings are discussed within the paper, with reference to other papers diving deeper into specific topics presented at the same conference.

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