The limitation of the closed-loop combination method

Modern aircraft include a variety of automatic control systems that aid the flight in navigation, flight management, and to augmenting the stability characteristics of airplane. In order to reduce cost and design cycling time and improve performance, Liu proposed closed-loop combination approaches in 2001. In this paper, we discuss the method again and apply the close-loop combination technique to the longitudinal control channels of the nonlinear F-16 fighter. We conclude that the closed-loop combination method cannot be applied in nonlinear F-16 system since the design of closed-loop combination controller does fully depend on the closed-loop transfer function and its minimal pole-zero cancellation form of the system, and in nonlinear system there are some unstable states that cannot be accurately cancelled due to the differences between linear model and nonlinear model. The simulation results are presented to show this limitation of the closed-loop combination method