The Art of the Helicopter John Watkinson - Karatunov.net

and autostabilization, it also provides power-assisted cyclic control, with the penalty
of an increase in drag due to the paddles.
The Hiller control system incorporates another unusual feature in that the swashplate
works in reverse to that of a conventional helicopter. Figure 7.46(a) shows that in
order to simplify the control runs, Hiller took an arm from the non-rotating part of
the swashplate straight down to the pilot. The pilot would move the end of this arm
in the same way as he would a conventional floor-mounted cyclic stick. In order, for
example, to apply aft cyclic, the pilot would pull the end of the arm towards himself.
This, however, had the effect of tipping the swashplate down at the front. Hiller solved
this problem by connecting the rotating part of the swashplate to the control rotor with
a scissors linkage that reverses the action.
Figure 7.46(b) shows that when the swashplate tilts forwards, the control axis of the
control rotor tilts backwards as required. When the overhead control was replaced by
a conventional floor-mounted cyclic in later machines, the reversing swashplate was
retained and the cyclic pushrods were simply rigged to make the swashplate move in
the correct sense.
It is interesting that many examples of the Bell 47 and the Hiller Raven are still flying,
although the Bell 47 first flew in 1943 and the Hiller in 1948. The longevity of these
designs is testimony to the original concept. The flybar was also adopted in the UH-1
Iroquois, better known as the ‘Huey’ which itself achieved legendary status.
7.26 The Lockheed systems
The Bell and Hiller systems are ideal for two-bladed teetering rotors, but it is not easy
to apply them to rotors with a larger number of blades. Irven Culver at Lockheed
developed stabilizing systems to work with rotors having any number of blades. Culver
had rightly concluded that flapping and dragging hinges were not necessary and set
out to build hingeless rotors. Without flapping hinges, the cyclic response of a rotor
can be extremely rapid and exert huge forces on the hull. Gyroscopic stabilization was
essential to contain control response.
The Lockheed CL-475 was a flying test bed that sported a variety of rotors and
stabilization systems. Initially a two-bladed rotor was used having no flapping hinges
and no collective pitch control. Lift was controlled by rotor speed. Cyclic pitch was
provided by a single span-wise hinge. A flybar was mounted transversely to the rotor.
This superficially resembled a Bell flybar, but only because of the streamlining of the
tip weights. The attitude of the flybar with respect to the mast controlled the cyclic
pitch of the rotor and the pilot’s cyclic stick applied forces to the flybar via springs and
the swashplate to make it precess. As the main rotor changed its attitude, pitch link
loads would make the gyro bar follow, serving the same function as the dampers in the
Bell system. A second gyro stabilized the yaw axis by controlling the tail rotor pitch.
In the Bell system some of the travel of the flybar is lost in the mechanical mixer
needed to add in the cyclic control from the swashplate. The direct control of the main
rotor by the gyro in the Lockheed system allowed greater loop gain and was thus
more stable. However, the two-bladed hingeless rotor suffered from excessive 2P hop in
forward flight and work concentrated on hingeless rotors with more than two blades.
In 1960 a new three-bladed rotor was designed, again having no flapping hinges.
Initially, each blade had its own single-ended flybar known as a gyrobar. This is shown
in Figure 7.47. In the gyrobar system, the feathering axis is not parallel to the blade,
but instead the feathering axis is swept back with respect to the blade. As was shown in
section 4.7, this causes coupling between the flapping and feathering axes. If the rotor
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