The Art of the Helicopter John Watkinson - Karatunov.net

84 The Art of the Helicopter
Fig. 3.18 Avoiding the vortex ring. Autorotation is entered in forward flight, and then the forward speed is
brought to zero. Prior to resuming powered flight, forward speed must be regained.
theory. The problem is that in vertical flight to get from downward inflow to upward
inflow the state of zero inflow must be traversed.
Zero inflow makes nonsense of actuator disc theory, and the real helicopter can’t
fly in that condition either, hence the discontinuity in the curve of Figure 3.17. The
position of the discontinuity is affected by blade twist. A twisted blade produces more
thrust at the inner parts of the disc and excites the vortex ring less, so it is possible to
descend faster.
It must be accepted that there are certain combinations of vertical rate of descent and
induced velocity in which a helicopter will be uncontrollable. Thus an important part
of pilot training is to impart a practical knowledge of how to fly outside that region,
how to recognize the onset of a vortex ring and how to recover from it.
Using forward speed to guarantee an inflow allows the vortex ring condition to be
avoided. Figure 3.18 shows that to enter autorotation from forward flight the collective
is simply lowered until the inflow reverses. Once in autorotation the forward speed can
be brought to zero for a vertical descent. In order to terminate the vertical autorotation
forward speed is regained before raising collective pitch again to obtain normal
downward inflow.
3.17Ground effect
When hovering close to the ground, the air passing downward through the rotor cannot
escape as freely as shown in Figure 3.19(a). Air pressure below the machine builds up,
and reduces the induced velocity. The RAFis closer to the horizontal and the blade
resultant is closer to the vertical. The same lift can be obtained from the rotor with less
rotor drag. As Figure 3.19(b) shows, considerably less power is needed to hover close to
the ground. As the figure shows, ground effect is noticeable up to a height approximately
equal to the rotor radius. In still air, a helicopter will reach equilibrium in ground effect
at a height where the lift just balances the weight without any input to the collective pitch