The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 83
Fig. 3.16 When the helicopter descends vertically, it can catch up with its own wake and the rotor vortices
are not swept away, but result in recirculation. If allowed to develop fully, the result is the vortex ring condition.
Fig. 3.17 The relationship between induced velocity and vertical climb/descent velocity. Both have been
normalized with respect to induced velocity in the hover. Note there is a discontinuity between powered
descent and autorotation due to the vortex ring condition.
and so had low induced velocity. Some suffered from power settling where above a
certain rate of vertical descent the descent could not be arrested except by moving into
forward flight.
Figure 3.17 shows the relationship between the induced velocity needed and the
climb/descent velocity. As the effects are proportional to the hover induced velocity,
Figure 3.17 has been normalized by dividing both parameters by that factor. As a
result the curve passes through unity in the stationary hover. Note that as the rate of
descent increases, following the curve to the left, there is a discontinuity where a fully
developed vortex ring condition occurs. The recirculation has now enveloped the rotor
whose ability to produce lift is considerably reduced. The effectiveness of the cyclic
pitch will also be reduced. Increasing collective makes it worse. The machine drops like
a stone, pitching and rolling randomly. If this condition is entered close to the ground,
recovery may be impossible.
To create lift a rotor must have mass flow whose momentum it can increase. In hover
or climb, the inflow is downward and the rotor puts momentum into the inflow by
accelerating it. In a vertical autorotation the inflow is upward (effectively making the
momentum negative) and the rotor puts momentum into the air by slowing it down.
Both of these conditions are stable and the results are predictable from actuator disc