Vortex Ring Regime
During helicopter vertical descent the air flow accelerated downward by the rotor increases its velocity from V_^ to 2V^ at a distance from the main rotor equal to about 2R. With further distance from the main rotor the flow decreases its velocity to as a result of "friction" with the opposing air (Figure 58a).
If the flow velocity V3 equals the helicopter vertical descent velocity
V, , the velocity of the flow accelerated by the rotor V„ = 0, i. e., the /86
rotor essentially "catches" the air which it has accelerated. As a result of inflow above the rotor there will be an "interface" where the rotor essentially "runs away" from the inflowing air with the same velocity. This means that two interfaces are formed: below and above the rotor. At these
surfaces the flow leaving the rotor turns and forms closed vortices, which have nearly no effect on thrust formation, since they are far from the rotor.
As the vertical descent velocity is increased, the interfaces where Vg = approach the main rotor. The vortices become more intense and
unstable. The rotor expends the power obtained from the engine on rotation of these vortices. The main rotor thrust decreases sharply, since air is not ejected from the closed vortex system (Figure 58b). The vertical descent velocity increases still further. The helicopter begins to toss from side to side, control of the helicopter becomes difficult, and heavy buffeting appears. This flight state corresponds to the developed vortex ring regime.
The vortex ring regime occurs with vertical descent at a velocity more than 2-3 m/sec with the engine operating.
The most effective technique for recovery from the vortex ring state is to transition the main rotor into the autorotation regime along an inclined trajectory. But this requires considerable altitude and the absence of obstacles, therefore, the vortex ring regime is dangerous and must be avoided.