The Art of the Helicopter John Watkinson - Karatunov.net

180 The Art of the Helicopter
Fig. 5.9 In rotor heads having flapping stiffness, a couple can come from the main rotor to help balance the
hull drag.
This is another example of requirements in hover and translation being at odds. In
practice it is not the hover download on the tail plane that is the major problem. Of
more concern is the serious fore-and-aft trim change that results should the downwash
move clear of or back onto the tail plane. In a teetering or zero-offset machine with
limited cyclic power this could cause control problems. In machines having higher
cyclic authority, a greater tail plane load in climb and autorotation can be handled. In
this case the trim shifts can be handled, but may present an excessive pilot workload,
especially if instrument flight is contemplated.
There are three possible solutions to avoiding trim shifts of this kind. One is to
ensure that the tail plane is always in the downwash, another to ensure it is never in the
downwash and a third is to use a variable incidence tail plane or stabilator. Stabilators
are considered in the next section.
Helicopters often go through a surprising number of tail plane modifications and
locations as development proceeds and designers try to find the right combination of
characteristics. Figure 5.10 shows some possible locations for the tail plane. At (a) the
tail plane is mounted well forward so that it is in the downwash at all but the highest
forward speeds. This is the approach used on, for example, the Bell 206 (JetRanger)
which has a teetering head. At (b) the tail plane is mounted high on the fin so that it is
never in the downwash. This gives the least down force in the hover and a long moment
arm for stability at speed, but it does require a stiff and strong fin and tail boom. The
MD-500 has a tail plane of this type with the tail rotor below and mounted in-line with
the tail boom to simplify the transmission.
Where the tail rotor shaft is mounted in the plane of the main rotor, an out-ofdownwash
location can be obtained with an asymmetrical tail plane fitted on the
opposite side of the fin to the tail rotor. Many Sikorsky machines use this approach,
the S-65 being a good example, shown in Figure 5.10(c).
The tail plane can also be put below the tail rotor as in (d). This is almost out
of downwash in the hover, but as the machine moves forward from the hover the
downwash impinges on the tail plane from above causing a strong trim shift. In this
location, however, the tail plane does at least prevent ground personnel approaching
the tail rotor.
The section used on the tail plane is subject to some variation. In machines with
zero-offset heads, there is no couple from the rotor to prevent hull blowback in forward
flight and this must all be balanced by tail plane downthrust. Such machines have
limited cyclic control authority and in autorotation the tail plane would produce an
undesirable upthrust. One solution is to use an upside-down cambered section that