Post-flight system identification and aeroservoelastic model updating for prediction and validation of the onset of flutter

ifasd2024 Tracking Number 128

A validated mathematical aircraft model allows, among other things, the extensive study of system performance and characteristics, the verification of wind-tunnel and analytical predictions, the support of flight envelope expansion during prototype testing, and the design of flight control laws. The aeroservoelastic (ASE) stability analysis is another crucial component of the configuration optimization and certification process over the intended operational envelope. In this context, the flutter phenomenon is a well-known example of a selfexcited aeroelastic instability resulting from the interaction between unsteady aerodynamic forces and structural vibrations. Significant amplitudes of vibration can be induced, eventually resulting in the structure's catastrophic failure. With the guidance of accurate aeroservoelastic models, a reliable prediction of an aircraft's susceptibility to flutter across its intended flight envelope is possible. Through developments in control systems theory and hardware, as well as the development of high-bandwidth actuators, it became feasible to suppress aircraft flutter instabilities through the actively controlled closed-loop action of control surfaces. Active flutter suppression (AFS) presents a robust and effective solution when passive approaches, such as modifying mass distribution or structural stiffening, are impracticable for eliminating flutter. The EU-funded projects FLEXOP and FliPASED investigated this topic and made technological advancements to the point where the AFS control laws were successfully demonstrated by the P-FLEX UAV during flight tests. Using data from flight tests of the fixed-wing P-FLEX UAV with a 6m wing span, this paper will demonstrate post-flight system identification and, by extension, ASE model updating outcomes. The predictions provided by the updated model regarding flutter boundary will be thoroughly assessed. An additional significant topic is the post-flight verification of the openloop flutter speed obtained from Operational Modal Analysis during flight flutter testing. In this context, the onset of the flutter during flight testing is demonstrated theoretically through the flight modal identification from the simulated flight flutter test. This is achieved through a qualitative comparison of the stability diagrams of the system's poles, which were gathered from flight test data and simulated flight flutter testing. Finally, flutter boundary expansion enabled by the AFS controller of the closed-loop system will be verified via post-flight modal identification.