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

The F/A-18 Hornet aircraft with the original flight control law exhibited an out-ofcontrol phenomenon known as the falling leaf mode. 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. This paper applies a variety of linear analysis methods to investigate the robustness properties of the original (baseline) and the revised 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. ∗Graduate Research Assistant: [email protected]. †Senior Research Associate: [email protected]. ‡Professor: [email protected].