Experimental Evaluation of an Advanced Buffet Suppression System on Full Scale F/A-18 Fin

Abstract

Buffeting is an aeroelastic phenomenon that plagues high-performance aircraft, especially with those with twin vertical tails such as the F/A-18. At high angles of attack, unsteady vortices that emanate from wing leading-edge extensions interact with the vertical fin, resulting in premature fatigue failure. An advanced hybrid buffet suppression system was experimentally evaluated on a full-scale F/A-18 empennage as an international technical collaboration activity among Australia, Canada, and the United States under the auspices of The Technical Cooperative Program. The advanced hybrid buffet suppression system incorporated two distinct actuation systems to control the bending and torsion modes independently. The vertical fin bending mode was counteracted using the inertial loading from the rudder structure while the first torsion model was controlled using surface-mounted macrofiber composite conformable actuators optimally located on the fin. The buffet excitation input generated by the shakers matched the buffet modes and magnitude expected under aerodynamic loading, but it was difficult to match the aerodynamic damping. Therefore, closed-loop tests were conducted under two damping conditions that were lower and higher than the expected aerodynamic damping level. The lower damping condition was produced by imposing free vibration condition on the active fin, and the higher damping condition was tested using forced vibration. The real-time adaptive controllers were able to effectively suppress both the bending and torsion vibration modes under representative buffet load spectra. The performance of the hybrid actuation system was significantly greater at the low damping condition, which led to higher reduction in vibration. Results showed that the conventional rudder and macrofiber composite actuator systems were capable of simultaneously reducing both the bending and torsion modes sufficiently to double the fatigue life of the fin. The full-scale closed-loop tests demonstrated that the advanced hybrid buffet suppression system is a feasible solution to alleviate vertical tail fatigue due to buffeting in fighter aircraft.