Advanced uncertainty modeling and robustness analysis for the basic flight control system of a modern jet trainer

Due to the continuously increasing design requirements on modern aircraft the robustness properties of their flight control systems against multiple model uncertainty becomes a more and more important issue for flight control system certification and flight clearance, particularly in the case of an unstable aerodynamic basic design. Most classical approaches (like stability margins in the Nichols Chart) only provide incomplete information about the qualitative and quantitative degree of robustness. Therefore, an innovative approach for robustness analysis of the primary control laws of a modern jet trainer will be presented here which uses the structured singular value (SSV) μ and a physically motivated parametric uncertainty model. The methodology is applied to both the longitudinal and lateral basic controller and shows a direct and exact way from the definition of parametric uncertainties to an uncertain system model suitable for μ-analysis. Particularly, the introduction of a trim point uncertainty emphasizes the advanced character of this analysis method by incorporating the dependence of the control loop parameters (plant and controller gains) on the current trim point into the uncertainty model. Consequently the analysis of the controller can be accomplished for entire regions of the flight envelope in a single robustness test. Therefore, the proposed approach demonstrates the possibility of substituting the conventional procedures for proving controller robustness by a more efficient methodology.