The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 5.5 A right-side wrong-direction tail rotor (a) can be made into a wrong-side right-direction tail rotor by
turning the gearbox over (b). The improvement due to the right direction exceeds the loss due to being on the
wrong side.
contracted slipstream which also has a higher relative airspeed. The thrust loss due to
blockage will be significantly higher. There is thus a ‘right’ side and a ‘wrong’ side to
mount a tail rotor; the blower installation will always be more efficient in the hover. In
an ab-initio design, the right side, right direction tail rotor is an automatic choice.
The existence of the right side was appreciated well before that of the right direction,
and many machines appeared with this configuration as shown in Figure 5.5(a). When
it was realized that the tail rotor should be reversed, cost considerations often dictated
the simple expedient of turning the tail rotor gearbox through 180 ◦ about its input shaft.
This had the desired effect on the rotation, but then put the tail rotor on the ‘wrong’
side as shown in Figure 5.5(b). The improvement obtained by turning the right way
exceeds the loss experienced by the increased blockage. As a result there are quite a few
right-direction wrong-side machines. The Westland Lynx and the Mil Mi-24 are both in
this category. In the Enstrom F-28A, the tail rotor originally went the ‘wrong’ way, but
by turning over the gearbox as described the blockage increase was negligible because
the tail rotor is mounted on a slim tube. The result was a significant improvement in
tail rotor authority.
Figure 3.24(b) showed that in forward flight the main rotor produces a trailing vortex
structure not unlike that of a wing. The streamlines converge above the plane of the
rotor and diverge below it. The tail rotor operates in this airflow and the yaw-stabilizing
action is affected by it. When the tail is mounted high, as in Figure 5.6(a) the amount
of inflow change for a given yaw is increased by the converging flow, increasing the
yaw-stabilizing action. However, a low mounted tail rotor may be operating in the
diverging flow (b) where the yaw-stabilizing action is actually reduced.
There are thus many good reasons to mount the tail rotor high on a cranked boom.
In addition to power saving in the hover, a high tail rotor is less likely to strike the
ground in a low level quick stop, and it enables a safe landing to be made in scrub. The
blades are less likely to suffer leading edge erosion from grit blown up by the downwash
during hovering. In forward flight a raised tail rotor will produce greater directional
stability, and encounters a cleaner airflow. The elimination of tail rotor roll as shown
in section 5.2 is an elegant but not an essential bonus.
Unfortunately to site the tail rotor on a cranked tail boom requires an extra gearbox
in the tail rotor shaft, and a stiffer tail boom. This adds expense that is unwelcome in
smaller machines. In this case the requirement for a little extra power is not a problem
because the turbine engine is usually heavily derated. In larger machines, the power
loss becomes significant and in very large machines a correspondingly large tail rotor
will be required and the issue of ground clearance also arises. Thus a cranked boom is
The tail 175