In this paper, a mixed-norm H_2/H_∞ fault tolerant controller is developed for a flexible aircraft subject to gust disturbances and loss of control effectiveness fault in control surfaces. First, a flexible aircraft model that captures the coupling between rigid-body motions and flexible modes is presented. For each control surface, an adaptive observer is designed to estimate on-line its effectiveness factor. Then a controller that gain schedules with the estimated effectiveness factors is synthesized to accommodate loss of control effectiveness faults. By solving a mixed H_2/H_∞ problem, the gain-scheduled controller can achieve rigid-body motion regulation and gust load alleviation on the flexible wing in both nominal and faulty cases. Finally, the effectiveness of the proposed design is demonstrated in a numerical study on an aircraft model with high aspect-ratio wings.
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