The Art of the Helicopter John Watkinson - Karatunov.net

358 The Art of the Helicopter
An alternative to the tilt rotor is the tilt wing in which the wing and rotors form a
single assembly that tilts. As the wing chord is vertical in the hover the obstruction of
the downwash will be reduced. In this attitude the wing forms a deep beam and will
naturally be very stiff. The wing needs no folding flap mechanism and therefore will
be less expensive. It may be possible to use the wing ailerons as a yaw control in the
hover, leading to an approach in which the rotors have no cyclic control at all, but
become variable pitch propellers. Hover roll control is then by differential collective,
but control of the pitch axis in the hover requires an additional rotor at the tail which
is only powered for the hover. The requirement for a horizontal tail rotor somewhat
offsets the saving in complexity due to eliminating the use of cyclic pitch on the main
rotors.
Whilst such an approach may be suitable for a civil machine, it is unlikely that the more
stringent requirements of the military could be met, specifically the hover performance
in a wind where the vertical wing is a drawback. The tilt wing also requires more caution
in the transition from aircraft to helicopter mode where the prop rotor thrust and wing
angle have to be carefully adjusted as a function of the airspeed.
9.5 Multi-rotor helicopters
Multi-rotor helicopters have advantages in some niches of the market, but are significantly
less common than the conventional type partly because of the added complexity
and partly because additional skills are needed to develop them. It is often the case that
one individual has perceived a particular advantage of a configuration and perfected it
for a particular niche. That niche having been filled, no other manufacturer then seeks
to compete. As a result each type of machine will be associated with a particular manufacturer.
Thus the tandem is associated with Vertol/Boeing, the coaxial with Kamov
and the synchropter with Kaman.
Figure 9.8(a) shows that there are a number of possibilities with two rotors. The
rotors may or may not overlap, and they may be placed in tandem or transversely to
the direction of flight. If they do not overlap, the rotors may turn in the same direction
or may contra-rotate. If the rotors turn the same way, they must be inclined away
from the vertical to counteract torque as shown in Figure 9.8(b). Nicolas Florine
successfully flew a machine of this kind in Belgium in 1933 as Figure 9.9 shows. In
those days helicopter dynamics were poorly understood and Florine chose same-way
rotation because the gyroscopic stability of the rotors would not cancel. Florine used
hingeless rotors and the same-way rotation would also have minimized the stress on
the hull. Test flights proved Florine right as in its day it was probably the most stable
helicopter in existence. The Cierva Air Horse shown in Figure 9.10 had three rotors
which all turned the same way. Torque cancellation was obtained using the Florine
principle.
9.6 The side-by-side configuration
The side-by-side arrangement of contra-rotating rotors has some advantages in forward
flight because it improves the aspect ratio of the ‘disc’ and reduces the induced power
needed. Heinrich Focke’s Fw 61 was a machine of this type and was probably the
first helicopter in the world to progress from the flying test bed into a truly flyable
machine. This machine achieved a degree of notoriety in 1938 when it was extensively
demonstrated as a Nazi success symbol flown by Hanna Reitsch. This should not