Susceptibility of F/A-18 Flight Control Laws to the Falling Leaf Mode Part II: Nonlinear Analysis

The F/A-18 Hornet aircraft with the original flight control law exhibited an out-ofcontrol phenomenon known as the falling leaf mode. The falling leaf mode went undetected during the validation and verification stage of the flight control law. Several F/A-18 Hornet aircraft were lost due to the falling leaf mode which led to the redesign of the lateral-directional axis of the flight control law. The revised flight control law exhibited successful suppression of the falling leaf mode during flight tests with aggressive maneuvers. Prior to performing expensive flight tests, the flight control law was extensively validated and verified by performing linear robustness analysis at different trim points and running many Monte-Carlo simulations. Additional insight can be gained by using nonlinear analyses. This paper compares the two flight control laws using nonlinear region-of-attraction analyses and Monte Carlo simulations. The results of these nonlinear analyses indicate that the revised flight control law has better significantly improved nonlinear robustness properties as compared with baseline design.