The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 365
a lower blade working in the downwash of an upper blade is offset by swirl recovery.
The lateral separation of the rotors means that the disc area is greater than the area of a
single rotor and this reduces the disc loading and the induced power needed, although
some of the advantage is lost because the rotor thrusts do not align. The horizontal
components of the rotor thrusts are in opposition and some power is wasted. In forward
flight the rotor separation gives a better disc aspect ratio.
From a performance standpoint, the twin rotor heads cause a drag penalty and the
synchropter is not appropriate for high speed. However, as the synchropter is practically
limited to two-blade rotors, it naturally suggests low solidity and disc loading which is
the ideal for low speed, high altitude work or heavy lifting.
The synchropter is controlled in a similar way to the coaxial helicopter in that differential
collective pitch is used as a yaw control. This will be subject to reversal in
autorotation as for the coaxial helicopter. Some yaw control is also possible by the use
of differential fore-and-aft cyclic, although the short distance between the rotor heads
makes this ineffective. The usual fore-and-aft cyclic control affects both rotors equally,
but the lateral cyclic may be adapted so that the discs tilt outwards more than they tilt
inwards in order to preserve clearance between the heads and the blades. Generally
synchropters have poor yaw control and require a large amount of fin area.
The outward tilt of the rotors means that the torque cancellation is not perfect. It
can be seen from Figure 9.16 that although the torque cancels in the vertical axis, there
is a component of rotor torque in the horizontal axis which is the same for both rotors
and therefore adds. If the synchropter is arranged to have the advancing blades on the
inside, the torque reaction will tend to pitch the hull nose up until the CM is far enough
ahead of the masts to counter the torque. In the hover this will require the application
of forward cyclic so that the rotor thrust remains vertical. In forward flight the forward
CM is a useful stability aid. As forward speed builds up, the drag on the hull will act
below the rotor heads and produce a couple tending to tilt the hull nose down. The
rotor torque will counter this. Consequently there is a correct way for synchropter
rotors to turn, i.e. with the advancing blades on the inside.
The Flettner 282 Kolibri (Figure 1.4) was not only the world’s first synchropter, it was
also the world’s first production helicopter, beating Sikorsky’s R-4 by several months
as well as being technically superior. The Kolibri had very closely meshed two-bladed
rotors. Figure 9.17 shows that these were fully articulated with friction disc dragging
dampers. There was substantial flapping hinge offset so that the rotor blades would not
flap into contact with the opposite head. This allowed space for the feathering hinge
to be mounted inboard. The swashplates and pitch links were a compact and elegant
arrangement. A 160 hp BMW radial aircraft engine mounted in the conventional
aircraft attitude drove the transmission through an inclined shaft. The foot pedals
Fig. 9.16 In the synchropter there is a component of torque along a horizontal axis.