TABLE OF CONTENTS The demands of high-performance characteristics such as high maneuverability of modern flight vehicles have led to aerodynamically unstable aircraft configurations. Advancements in modern control theory, microelectronics, and computer techniques make such basically unstable aircraft flyable. As depicted in Fig. 9.1, there are two possible approaches to unstable aircraft identification, namely closed-loop and open-loop identification.1 Both the approaches pose certain problems; however, the nature of the difficulties encountered is characteristically different in the two cases. The classical closed loop identification approach to model the command to response behavior does not pose any numerical problems, because the overall system is stable and any standard parameter estimation method like for example output error can be applied without encountering any serious difficulty. The main limitation of this approach is that it yields an equivalent model, giving overall system representation.
Although highly unstable aircraft can be flown only with the aid of a flight controller, that is, in closed loop, determination of aerodynamic characteristics of the basic unstable aircraft, that is, of the open-loop plant, is of primary interest in several instances: 1) for validation of wind-tunnel and theoretical estimates, and if necessary, updating of these predictions with those from flight data, which is essential for new conceptual designs in particular, 2) for flight envelope expansions, and 3) to support flight evaluation of the control systems and provide more accurate models for design modifications of the stability and control augmentation system. Open-loop parameter estimation...
