Basic Study on Vortex Ring State Detection of Multi-rotor UAV Using Motor Counter Torque Observer

The descent phase of the multi-rotor Unmanned Aerial Vehicles (UAVs) is one of the situations when UAVs tend to lose control because of the Vortex Ring State (VRS). The VRS is one of the states of turbulence. The VRS is a well-known phenomenon in manned helicopters and can also occur during the descent of a UAV. It is difficult to escape from the VRS without taking the VRS recovery motion. Some previous studies have proposed to avoid the VRS condition. However, those methods do not consider the effect of the wind disturbance. It is important to know the real-time turbulence information and detect the VRS to be robust to unexpected wind. This paper proposes a novel VRS detection method using the counter torque of the propeller motor to obtain real-time turbulence information. The method is verified with the bench experiments in a wind tunnel. The results show the possibility of the proposed method to detect the airflow flows into a propeller from dangerous airflow angles.