The Art of the Helicopter John Watkinson - Karatunov.net

368 The Art of the Helicopter
K-Max: basically the mechanics of a Husky with a minimalist single-seat hull optimized
for underslung load working. The K-Max is the only synchropter currently in
production. As the K-Max is a single-seater, a number of Huskys have been refurbished
to act as trainers.
9.9 The tandem rotor
Once the stress-reducing benefits of blade articulation became known, contra-rotation
could be adopted in the tandem. This largely cancels torque reaction. Claims will often
be seen that because there is no tail rotor to waste power, the tandem must be more
efficient than the conventional helicopter. This is not necessarily true because the two
rotor discs may overlap reducing the total disc area. In fact the real advantages of the
tandem are that the position of the centre of mass is much less critical than in single
rotor machines and that hovering is much less sensitive to wind direction. In military
operations the ability to hover on any heading, load in a hurry and simply fly off saves
valuable time. Later Chinooks have three load hooks in line beneath the hull so that
supplies can be taken to three locations in one flight. This would be impossible with
a single rotor machine because it could not handle the CM travel as the loads were
dropped off one by one. Figure 9.20 shows that if the CM position is not in the centre
the trim will be operated so that the rotor at the ‘heavy’ end produces more thrust and
the other rotor produces less. A secondary effect of this condition will be that there is
not a perfect torque balance and some yaw control will be required.
It is quite possible to construct a tandem rotor machine in which the two rotor discs
are separated. The Florine machines and the Piasecki ‘Dogship’ (Figure 9.21) had this
configuration. This approach results in a long and possibly heavy hull and a physically
large machine requiring a lot of hangar space: a drawback in naval operations. As a
result in modern tandems the two discs will be significantly overlapped such that the
distance between the rotor heads is less than the rotor diameter. The rotors must be synchronized
by a shaft joining the front and rear gearboxes so that, like the synchropter,
the two rotors can share the same airspace without clashing.
Fig. 9.20 The tandem configuration can deal with longitudinal CM offsets using a different amount of thrust
on each rotor.