The Art of the Helicopter John Watkinson - Karatunov.net

184 The Art of the Helicopter
5.9The tail boom
The tail boom must provide structural support for the tail rotor, the fin and the tail
plane, as well as having some aerodynamic characteristics. Figure 5.13(a) shows that a
tail boom which is a smooth continuation of the hull will have lower drag in forward
flight than the ‘pod and boom’ construction shown at (b). However, a wide tail boom
will suffer a greater download in the hover. Figure 5.13(c) shows that the tail boom often
has a rounded rectangular shape to allow reasonable depth without excessive hover
download.
A streamlined tail boom may be practical on an executive transport, but for other
purposes rear loading ramps or clamshell doors may be needed and these always result
in a hull having higher drag. The boom has to be high set and slim to give clearance
for rear loading.
The tail boom effectively couples two masses together; the main rotor and the tail
rotor. These will each experience different forces, some static and some alternating. The
main rotor will apply vibrations to the hull, but the tail rotor will tend to lag behind
because of its mass and the result will be stress on the tail boom. Figure 5.14(a) shows
that when starting, whirling forces from the main rotor will rock the hull from side
to side. A tail rotor mounted atop the tail fin will resist the rocking and place the tail
boom in torsion. The mass of the tail and the torsional stiffness of the boom will create
a resonant system and if the resonant frequency coincides with an exciting frequency
the tail assembly will oscillate in torsion.
If torsional oscillation results from a flight frequency, the resonant frequency will
have to be changed. Intuitively, stiffening the tail boom would do this, but weakening
it would also change the resonant frequency to a lower value. A lengthwise slot in the
Fig. 5.13 (a) A hull in which the tail boom is smoothly faired into the cabin will cause less drag in forward
flight. (b) A pod-and-boom structure suffers from turbulence at the rear of the pod. (c) Tail boom cross-section
which is deep but narrow gives strength without excessive download in the hover.