The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 369
Fig. 9.21 The Dogship was Piasecki’s first tandem rotor helicopter and was widely demonstrated to attract
interest in the principle. It was the forerunner of the ‘flying bananas’ and the Chinook. (Boeing)
Fig. 9.22 The transmission system of the Chinook showing the synchronizing shaft between the rotors and
the twin engines.
Chapter 5 described the interaction between the main and tail rotor of a conventional
helicopter. In the hover it was seen to be highly variable with wind direction, whereas
in translational flight there was a beneficial weathercocking effect. In the tandem the
interference between the two rotors in the hover is beneficially almost independent of
wind direction, whereas in forward flight it is detrimental to stability. Consequently it
is simplistic to claim that one configuration is better than another. In some areas the
conventional approach is superior, but in other areas the tandem wins.
Figure 9.22 shows the transmission arrangement of the CH-47 (Chinook) in which
the two engines drive the synchronizing shaft via one-way clutches so that either engine
can drive both rotors. It is interesting to note that except for the use of piston engines,
the Florine III of 1935 had the same transmission layout. As the dominant masses and
the rotor forces are both concentrated at the ends of the hull, the structure needs to be
stiff against bending and torsion. The loss of disc area due to the overlap of the two