The Art of the Helicopter John Watkinson - Karatunov.net

172 The Art of the Helicopter
longer a dominant blade-passing frequency. Additionally with a carefully chosen skew
angle and spacing between the two rotors a small gain in aerodynamic efficiency can
be obtained because the axially spaced pairs of blades act to some extent like staggered
biplane wings.
As with the main rotor, airspeed alternately adds to and subtracts from the blade
speed due to rotation. The resulting roll couple is subject to precession and the result
will be flapback and dragging. As the tail rotor tip path axis is tilted backwards due
to the flapping, the thrust has a rearward component acting as a drag which the main
rotor has to overcome. However, the inflow has a small component along the tip path
axis in forward flight reducing the angle of attack and the shaft power needed. The
power saved is then available to the main rotor and is precisely the correct amount the
main rotor needs to overcome the drag. As a result tail rotor flapping is not detrimental
to efficiency but it could lead to stress and/or wear.
In very large helicopters the tail rotor will need both flapping and dragging articulation
to contain the stresses and the dragging axis will need damping. In smaller
machines it is enough to have flapping hinges. The hinge will often incorporate some
delta-three effect (see section 4.7). The delta-three hinge has the effect that as the rotor
flaps, some cyclic pitch change is applied. The rotor finds equilibrium with a smaller
amount of flapping.
The tail rotor counters the torque of the main rotor in the hover, but it also aids
the directional stability of the machine in forward flight by acting as a fin. Figure 5.3
shows how this happens. If the tail swings to one side or the other, a component of
the airspeed will change the inflow and with it the angle of attack. The result will be
a change of thrust in such a sense as to return the tail to its original position. This
is a highly desirable characteristic, except in rearward flight where the effect makes
the machine unstable in yaw. An attempt to fly backwards at speed results in the tail
swinging round, a phenomenon known as weathercocking.
Fig. 5.3 The tail rotor acts like a fin in forward flight because if the machine yaws, the angle of attack of the
tail rotor blades will change in the sense that opposes the yaw.