Reduced-order controllers for active flutter suppression of a two-dimensional airfoil are studied using two design approaches. One is based on the Generalized Hessenberg Representation (GHR) in the time domain, and the other called the Nyquist frequency approximation (NFA) is a method in the frequency domain. In the NFA method, the reduced-order controllers are designed so that the stability margin of the Nyquist plot may be increased over a specific frequency range. To illustrate and to make a comparison between the two methods, numerical simulations are carried out using a thirteenth-order controlled plant. It is to be noted that the GHR method can yield quasi-optimal controllers in the sense of minimizing quadratic performance indices. The designed controllers, however, do not have enough stability margin, and the order reduction resulting from full state controllers may not be satisfactory. On the other hand, reduced-order controllers in the NFA method can be designed with increased stability margin at the expense of the performance index. For all simulation cases the NFA method yields second-order controllers with better stability margin than those by the GHR method. Thus, the NFA method gives us one of the effective methods for synthesizing robust reduced-order controllers.
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