Full Envelope Longitudinal Flight Control Law Using a Nonlinear Controller Combined With Quantitative Feedback Theory

This paper presents two approaches to designing a flight control law for the unstable, nonlinear, short-period dynamics of an F-16 aircraft. The first approach is a direct application of nonlinear quantitative feedback theory (QFT), while the second design incorporates dynamic inversion to linearize the pitch-rate dynamics before applying QFT. The objective in the second case is to reduce the conservatism of the design by canceling known but nonlinear dynamics rather than treating the nonlinearity as additional uncertainty. The system uncertainties are quantified by fitting linear transfer functions between matched pairs of control inputs and the resulting outputs generated by a series of closed-loop, nonlinear simulations. QFT templates quantifying uncertainties are formed by computing magnitude and phase values at selected frequencies for every transfer function in the simulation set. The resulting controllers demonstrate considerable robustness, successfully following large angle of attack commands at flight path velocities ranging from 53 to 150 meters/sec while subjected to ±20% variations in pitching moment coefficient magnitudes. The inclusion of dynamic inversion further enhances the performance of the control laws.