Reliable computation of the robustness margin for a flexible aircraft

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Reliable computation of the robustness margin for a flexible aircraft

Ferreres, G. / Biannic, J.M.

For economical reasons, the size of civilian airplanes has in recent years tended to increase significantly. As a consequence, flexible modes get very close to the control bandwidth, so that classical techniques, which consider these bending modes as high frequency unmodelled dynamics, can no longer be applied. It becomes necessary to introduce these flexible modes in the design model, so as to actively control them. The aim in this paper is to analyze the robustness properties of such an active flight control system, with respect to uncertainties in the bending modes. An efficient method for directly computing a reliable estimate of the maximal structured singular value mu is proposed. A method, which directly computes a mu upper bound over a frequency interval, is first extracted from Ferreres and Biannic. The problem is then to evaluate the conservatism of this upper bound by computing an accurate mu lower bound which is the central issue of the paper. The proposed algorithm is successfully applied-with a reasonable computational effort-to a challenging aeronautical application, namely the evaluation of the robustness properties of the flight control system of a flexible transport aircraft.