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 nonlinear model of the F/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 law at different trim points. Classical analyses, e.g., gain and phase margins, do not indicate a significant improvement in robustness properties of the revised control law over the baseline design. However, advanced robustness analyses, e.g., μ analysis, indicate that the revised control law is better able to handle the cross-coupling and variations in the dynamics than the baseline design.
Bibliographical noteFunding Information:
This research was partially supported under the NASA Langley NASA Research Announcement contract NNH077ZEA001N entitled “Analytical Validation Tools for Safety Critical Systems.” The technical contract monitor is Christine Belcastro. We would like to thank John V. Foster at NASA Langley for providing insight into the simulation modeling of the F/A-18 aircraft. We would also like to thank Ufuk Topcu at California Institute of Technology and Andrew Packard at University of California at Berkeley for useful discussions. We would also like to thank the reviewers and the editors for their insightful comments, which have strengthened the paper.