Robust Stability

An aircraft is said to be operated in a trim condition when the body axis accelerations are zero. In other words, no resultant forces or torque act on the aircraft. The purpose of the longitudinal controller of an aircraft is to help pilots to move the aircraft from one trim condition to another, or to stabilize the aircraft in an operating trim condition while the aircraft is subjected to disturbances. The dynamics of the aircraft is highly nonlinearly depending on the trim condition which the aircraft is operated Etk82]. For simplifying the controller design task, the scheduling control methodology is usually adopted. By scheduling control, a set of controllers are designed to stabilize the linearized aircraft models around each trim conditions. When the aircraft moves from one trim condition to another, the controllers switch correspondingly. Therefore, instead of a complex nonlinear control design problem, one only has to design controllers for a set of linear time invariant systems and the control design task is dramatically simpliied. However, in exchange, the stability analysis of the control system becomes important issue, because there is a need to guarantee stability when the system is switching between trims conditions. Figure 1 shows a longitudinal control 1