The Art of the Helicopter John Watkinson - Karatunov.net

otates the rod in the screw thread. Alternatively the drum may be replaced with a
sprocket engaging a length of roller chain inserted in the wire.
An elegant improvement is to couple the collective pitch setting into the tail rotor
pitch controls so that to some extent torque variations are automatically compensated.
The Westland Lynx has such a system.
In larger machines a power assistance system may be necessary and this will be
considered in Chapter 7.
5.2 Balancing the torque
The torque reaction of the main rotor is a pure couple, which is to say it has only a
turning effect, and no resultant force in any direction. The tail rotor is mounted pointing
sideways at the tail, and so the thrust it produces results in a moment about the centre
of gravity of the machine. Unfortunately a moment cannot cancel a couple. The thrust
of the tail rotor can be adjusted completely to cancel the rotation of the machine, but
if the main rotor thrust is vertical, the tail rotor thrust will cause the machine to move
sideways, a phenomenon called tail rotor drift. Tail rotor drift is prevented in the hover
by tilting the main rotor disc in the opposite direction. Figure 5.2(a) shows that the
main rotor thrust can be resolved into a vertical component opposing the weight of
the machine, and a horizontal component. When the latter is equal to the tail rotor
thrust, a perfect couple has been produced that opposes the main rotor torque reaction
completely. The pitch of the tail rotor blades is controlled using the foot pedals. By
disturbing the balance of the couples, yaw control is obtained.
A very slight increase in rotor thrust is necessary so that the vertical component still
balances the weight. The tilted attitude of the disc during hover can readily be seen.
The hull may or may not tilt due to the disc tilt, depending on the design of the rotor
head, the way the transmission is installed and the location of the tail rotor. If the
hull tilts, the CM of the helicopter, being some way below the rotor hub, will develop
a rolling couple that can only be zero when the CM is directly below the rotor head.
The final attitude of the hull will be at whatever angle this restoring couple balances
the tilting couples from the rotor head and the tail rotor.
Figure 5.2(b) shows that if the tail rotor thrust is not in the plane of the main rotor,
there will be a couple about an axis through the rotor head running in a fore-and-aft
direction. The figure shows the situation viewed from the rear of a helicopter having
a clockwise-from-the-top rotor and the tail rotor below the plane of the disc. In this
case, the side force from the main rotor is above the side force from the tail rotor and
this causes a clockwise roll couple called tail rotor roll.
If the tail rotor is above the plane of the rotor (seldom seen in practice), the side force
results in an anticlockwise couple. Clearly it is only when the tail rotor shaft is in the
plane of the main rotor hub that this couple will be zero, and helicopters which mount
the tail rotor high on the top of the fin can achieve this elegant result, although there
are more important reasons for such a location, which will be explored in section 5.5.
The reader is cautioned that some texts assert that the tail rotor must be mounted at
the same height as the centre of mass to eliminate tail rotor roll, but this is incorrect.
In fact if the tail rotor is mounted at the vertical centre of mass of the hull, the couple
due to the different heights of the main and tail rotor side thrusts makes the hull tilt
until it is balanced by the couple due to the laterally displaced CM. This is shown in
Figure 5.2(c). The result is that the shaft axis and the tip path axis align and so there
is no lateral flapping and the type of rotor head is irrelevant because, with no flapping,
The tail 169