Linear and nonlinear analysis of susceptibility of F/A-18 flight control laws to the falling leaf mode.

University of Minnesota M.S. thesis. May 2010. Major: Aerospace Engineering and Mechanics. Advisor: Prof. Gary J. Balas. 1 computer file (PDF); xv, 91 pages, appendix A.The F/A-18 Hornet aircraft with the original flight control law exhibited an out-of-control 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 and this led NAVAIR and Boeing to redesign 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 is 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 thesis compares the two flight control laws using standard linear robustness analyses, nonlinear region-of-attraction analyses and Monte Carlo simulations. The classical linear robustness analyses, i.e. gain and phase margin, does not indicate any significant improvement in robustness properties of the revised control law over the baseline design. However, advanced linear robustness analyses, i.e, the #22; and worst-case analysis, indicate that the revised design is better able to handle the cross-coupling and variations in the dynamics than the baseline design. However, it can be difficult to interpret these results since the falling leaf motion is a truly nonlinear dynamical phenomenon. Thus nonlinear analyses tools provide useful insight into the susceptibility of both control laws to the falling leaf motion. The results of the nonlinear analyses indicate that the revised flight control law has considerably better stability properties than the baseline design and less susceptible to the falling leaf motion