Alleviation of Buffet on the Twin-Tail Assemblies of High-Performance Aircraft (aasert)

Abstract : We carried out experiments on a structural dynamic model of the twin-tail assembly of the F-15 fighter built by Professor Sathya Hanagud of Georgia Tech. The model was placed on a 250 lb shaker and subjected to a principal parametric excitation. We fixed the excitation amplitude and varied the excitation frequency around 18 Hz. For the same excitation amplitude and frequency, we found five possible responses depending on the initial conditions: (a) very small-amplitude motions of both tails, (b) a large-amplitude motion of the right tail accompanied by a small-amplitude motion of the left tail, (c) a large-amplitude motion of the left tail accompanied by a small-amplitude motion of the right tail, (d) a large-amplitude motion involving both tails moving in phase, and (e) a large-amplitude motion involving both tails moving out-of-phase. The coexisting five responses are the result of the nonlinearities. These results point out some of the shortcomings of testing models with one rigid and one flexible tail or even testing only one tail counting on symmetry. We used nonlinear identification techniques to estimate the linear and nonlinear parameters in a mathematical model of the tail assembly. Then we devised a control methodology to suppress the vibrations of the structural model. Finally, we used the backpropagation-through-time neural controller to suppress its nonlinear responses.