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

The F/A-18 Hornet aircraft with the original flight control law exhibited a nonlinear out-of-control phenomenon known as the falling-leaf mode. This unstable mode was suppressed by modifying the control law. This paper employs the falling-leaf phenomenon as an example to investigate the applicability of linear analysis tools for detecting inherently nonlinear phenomenon.Ahigh-fidelity nonlinearmodel of theF/A-18 is developed for controller analysis using F/A-18 High-Alpha Research Vehicle aerodynamic data in the open literature. A variety of linear analysis methods are used to investigate the robustness properties of the original (baseline) and the revised F/A-18 flight control lawat different trimpoints. Classical analyses, e.g., gain andphasemargins, donot indicate a significant improvement in robustness properties of the revised control law over the baseline design. However, advanced robustness analyses, e.g., analysis, indicate that the revised control law is better able to handle the cross-coupling and variations in the dynamics than the baseline design.