A vortex-ring-state-avoiding descending control strategy for multi-rotor UAVs

The vortex ring state (VRS) is a dangerous phenomenon that may typically arise in rotorcraft descent process. In VRS, the rotor will lose lift and get into a wobbling state. As a kind of rotorcraft, multi-rotor UAV(Unmanned Aerial Vehicle) also encounters the same problems in descending flight. So in this paper, we propose a VRS-avoiding control strategy for multi-rotor UAVs. The real phenomenon of multi-rotor in vortex ring state is described through experiments. Based on test data, a VRS alarm and recovery strategy is proposed for vertical landing. To meet the demand of rapid descending, a spiraling down control strategy is devised to make the multi-rotor avoid entering into the VRS based on Gao and Xin's criterion. Using the developed simple model, this control strategy has been proved effective in hardware-in-loop simulation test, where larger than 4 m/s stable descending is realized.