The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 5.12 (a) A top-mounted fin is efficient in climb, especially if it has some sweepback. A fin under the tail
boom is in turbulent air in the climb. (b) In autorotation the fin under the boom becomes more efficient.
A centrally mounted fin is not in an ideal location as it will be in the turbulent
wake of the rotor head and mast. The result may be tail shake as vortices pass the fin.
Mounting the fin area at the ends of the tail plane will beneficially move the fins into
cleaner airflow.
The helicopter differs from fixed-wing aircraft in that it doesn’t have to move the
way it is pointing. A helicopter with a near-level hull may be flying level, climbing or
autorotating. Figure 5.12 shows that the relative airflow can approach the fin from
a wide range of directions. At (a) in a climb, the RAF is angled downwards, and a
top-mounted swept-back fin will present a high aspect ratio and be efficient whereas a
bottom-mounted fin is at a disadvantage and may also induce tail shake or ‘squirreling’
because of turbulence around the tail boom.
Figure 5.12(b) shows that in autorotation the position is reversed. The swept topmounted
fin now has a low aspect ratio and will be inefficient. Thus in practice fins are
frequently found with upper and lower sections both having sweepback as shown so
that good performance can be obtained both in climb and autorotation. The fin of the
Bell 206 JetRanger is a good example. The same sweep configuration may be found on
end-plate fins such as the BK-117.
A large fin area is beneficial in forward flight and if the tail rotor fails, but it will be a
drawback when hovering crosswind as it will create a large source of drag which must
be overcome by the tail rotor. As a result fin area must be a compromise. In civilian
machines larger fins may be found than those used in military machines because the
crosswind hovering requirement is not so important.
Fins are generally, but not always, fixed. In some cases the fin may be cambered
and/or fitted at a small angle of attack to offload the tail rotor in cruise. Sometimes
the fin may be fitted with a rudder or tab connected either to the pedals or to a yawstabilizing
system. Pedal operated rudders are more common on coaxial helicopters
and synchropters.
Unlike the fins of fixed-wing aircraft, the helicopter fin has to behave predictably
over a wide range of possible horizontal RAF directions because of the crosswind
hover requirement. One possibility is the adoption of the blunt trailing edge, as if
the rear of a conventional airfoil had simply been cut off. This technique allows the
fin to operate up to a larger angle of attack without stalling. The penalty in forward
flight is remarkably small because the increase in turbulence is balanced by a reduction
in wetted area. Blunt trailing edges were adopted on all Chinooks after the CH-47A,
and can also be used as a neat way of finishing off the rather fat fin needed to contain
a fenestron.
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