Many crashing accidents of helicopters and multicopters are attributable to the propellers plunging into a vortex ring state. Therefore, safe operation of a helicopter or a multicopter needs a thorough study on the vortex ring of a propeller and the vortex ring state.
Though many studies have been conducted on a vortex ring and its state, few of them deal with the cause of a vortex ring and determination of the critical descent velocity.
Myong Song Nam, an institute head at the Faculty of Mechanical Science and Technology, has determined the cause of the vortex ring of a descending propeller and the critical descent velocity by CFD method.
He used the MRF(Moving Reference Frame) method of Fluent for adequate prediction of the wake flows of a propeller.
The simulation showed that the origin of the vortex ring is not the vorticity line, but the interaction between the wake stream tube and the surrounding air, and that as the position of the vortex ring core varies with the increase in the descent velocity, the vortex ring approaches the propeller and consequently, the propeller enters into a vortex ring state and the vortex ring sharply moves above the propeller. Then, he predicted the boundary value of the descent velocity at which the propeller enters into a vortex ring state. The vortex ring state relates closely to the position of a vortex ring core or the thrust, which starts to vary suddenly at about 4.2m/s of descent velocity.
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