| Abstract | Following the program to test a hybrid actuation system for high-agility aircraft buffeting load alleviation on the full-scale F/A-18 vertical fin structure, an investigation has been performed to understand the aerodynamic effects of high-speed vortical flows on the dynamic characteristics of vertical fin structures. Extensive wind-tunnel tests have been conducted on a scaled model fin integrated with piezoelectric actuators and accelerometers to measure the afttip vibration responses under various freestream and vortical airflow conditions. Test results demonstrated that the airflow induced considerable damping to the fin structure, which generally increased with airflow speed as well as the vertical fin angle of attack relative to the airflow direction. Moreover, it was observed that at the angle of attack of 10 deg, the high-speed airflow introduced large deflection to the smart fin structure and caused significant frequency shift to the vibration modes due to nonlinear geometrical coupling of bending and torsional modes. These aerodynamic effects may adversely affect the performance and robustness of the closed-loop control laws developed based on vertical fin dynamic model identified without considering the varying aerodynamic effects. To explore this problem, the structured singular values synthesis technique was adopted to develop robust control law using smart fin model identified without aerodynamic excitations, and the aerodynamic effects on the fin structure were assumed as smart fin parametric and dynamic uncertainties. The effectiveness and robust performance of the control law was demonstrated through extensive closed-loop wind-tunnel tests using various airflow conditions. This provided a verified control law design strategy for future flight tests of the full-scale aircraft buffeting load alleviation system. |
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