The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 81
Fig. 3.14 In ideal autorotation, the total blade reaction must be vertical as shown. For a given pitch angle θ,
the rotor operates at the intersection of the α − θ line with the drag bucket. See text.
speed may be irrevocably lost. In this region rotor speed may be changed by alteration
of the balance of forces. Reducing the angle of attack causes the blade reaction to move
forward from the vertical. Negative induced power exceeds the profile power requirement
until the rotor speed has increased. On the other hand, increasing the angle of
attack causes the reaction to move behind the vertical, and the rotor speed falls.
3.16 Tip loss and the vortex ring
The pressure differential between the upper and lower surface of the rotor disc causes
air to try to flow transversely around the tip in order to equalize the pressure. This
rotary flow combines with the blade velocity to leave a corkscrew-like trail shown in
Figure 3.15(a) and known as a vortex. The loss of pressure differential due to this vortex
generation cancels out the lift generated at the blade tip; an effect known as tip loss.
Nearly 10% of the blade length is useless. The power needed to overcome induced drag
at the tip is wasted generating the vortex. There must also be a vortex at the inboard end
of the blade but it is less powerful because the lift gradient is much lower. In the steady
hover and in forward flight, the tip vortices are swept away downwards by inflow.