Susceptibility of F/A-18 Flight Controllers to the Falling-Leaf Mode: Nonlinear Analysis

The F/A-18 Hornet aircraft with the original flight control law exhibited a nonlinear out-of-control phenomenon knownas the falling-leafmode. This unstablemodewas suppressed bymodifying the original control law. This paper employs the falling-leaf phenomenon as an example to demonstrate the utility of recently developed nonlinear analysis methods to flight control validation tools. The flight control law is usually 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 significantly improved nonlinear robustness properties as compared with baseline design.