A mathematical model of the helicopter carrying an external slung load is built based on the multiple-mass-point hypothesis. The equilibrium states along variable configurations are sought through numerical optimization, while the eigenvalue distribution reveals the open-loop stability characteristics. To alleviate the degradation of flight control efficiency caused by load oscillations, a movable hook beneath the fuselage is introduced to provide auxiliary inputs for the underactuated system and stabilize the load motion. A hierarchic control is designed utilizing the state feedback-based nonlinear dynamic inversion (NDI) incorporated with the augmentation device. Considering that the acting point of the sling force on the helicopter is variable and the deviation from the original model appears, the inversion system for each control loop is constructed and combined with the sliding mode control (SMC) to compensate for inversion model errors. Simulation results indicate that the load pendulum motions are rapidly suppressed during forward and circular flights under automatic control. Meanwhile, the helicopter maintains high-fidelity command tracking. Furthermore, the system robustness is validated through the simulations in external disturbances.
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