Stability and Robustness of a Tandem Tilt-Wing VTOL With Dynamic Inversion Control Laws

Tilt-wing aircraft are a promising concept that combines the advantages of vertical take-off and landing (VTOL) capabilities with efficient cruise flight. However, the control design for tilt-wings poses significant challenges, particularly due to the different flight phases and the transition between them. Nonlinear dynamic inversion-based (NDI) control approaches have been demonstrated to offer an effective solution as they decouple the dynamics throughout the envelope. The susceptibility to uncertainties and disturbances arising from complex aerodynamic interactions raises questions about the stability and robustness of control systems designed for such aircraft. This study proposes methodologies for assessing the stability and robustness of control systems for transformational VTOLs. To illustrate, the stability and robustness of a hybrid NDI control system integrated within an optimization-based control allocation scheme will be examined across the different flight phases of a tandem tilt-wing VTOL. Precisely, we assess the robustness against uncertainties using worst-case disk and stability margins, as determined by the structured singular value μ. The findings indicate that the controller can stabilize the vehicle throughout all flight phases while ensuring sufficient margins to uncertainties. However, transitioning from thrust-borne to aerodynamic flight decreases the margins as the influence of aerodynamic uncertainties increases. Additionally, the backward or decelerating transition has to be performed carefully to avoid unstable flight phases.