Nonlinear blended aerodynamic modeling and anti-harmonic disturbance robust control of fixed-wing tilt-rotor UAV during transition

Abstract

For designs like the tiltrotor unmanned aerial vehicles (UAVs) and the aerodynamically similar tailsitter and tiltwing UAVs that can convert between helicopter flight and fixed-wing modes and thus feature both efficient and fast cruise flight, as well as vertical take-off and landing (VTOL) capabilities, the dynamics during the transition flight are often dominated by strongly nonlinear aerodynamic effects mainly caused by high angles of attack on the wing with stalled flow states and the external disturbances, including the rotor-wing aerodynamic interference and the environmental wind. This study proposed a novel nonlinear blended aerodynamic model used to aggregate the flat-plate mode and the linear mode of the aerodynamic coefficients, whose aerodynamic coefficient predictions in the stalled region cause the aerodynamic coefficient variations compared with the wind tunnel data. A super-twisting sliding mode controller (STSMC), which is robust to internal disturbances and capable of attenuating chattering, is employed to address the aerodynamic coefficient variations in the stalled region. Additionally, a harmonic disturbance observer (HDO) is introduced to provide an accurate estimation of the external harmonic disturbances, which will further be compensated for with STSMC. The comparative results with the wind tunnel data verified the feasibility and superiority of the proposed HDO-STSMC with the nonlinear blending aerodynamic model due to the insensitivity to aerodynamic coefficient variations of the robust STSMC and the accurate disturbance estimation of the HDO. Two performance indices revealed the robust stability and rapid convergence of the proposed transition blended aerodynamic model with the HDO-STSMC control scheme.