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angle. The decreased angle reduces drag, and a
reduction in throttle is needed to prevent rotor overspeed.
The throttle is mounted on the collective
pitch grip and is operated by rotating the grip, as on
a motorcycle throttle. The collective pitch stick is
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synchronized with the control of the carburetor so
that changes of collective pitch will automatically
make small increases or decreases in throttle settings.
On turbine engine helicopters, the collective pitch
stick is synchronized with the fuel control unit, which
controls the power and rotor RPM automatically.
Torque Control
In tandem-rotor and coaxial helicopter designs the
main rotors turn in opposite directions and thereby
neutralize or eliminate torque effect. In single-rotor
helicopters torque is counteracted by an antitorque
rotor called the tail rotor. It is driven by a power
takeoff from the main transmission. The antitorque
rotor runs at a speed in direct ratio to the speed of
the main rotor. For this reason, the amount of thrust
developed by the antitorque rotor must be changed
as the power is increased or decreased. This is done
by the two directional control pedals (antitorque
pedals), which are connected to a pitch-changing
device on the antitorque rotor. Pushing the left pedal
increases the thrust of the tail rotor blades, swinging
the nose of the helicopter to the left. The right pedal
decreases the thrust, allowing the main rotor torque
to swing the nose to the right.
MAIN ROTOR HEAD ASSEMBLIES
The main rotor head assembly is attached to and
supported by the main gearbox shaft. This assembly
supports the main rotor blades and is rotated by
torque from the main gearbox. It provides the means
of transmitting the movements of the flight controls
to the blades. Two types of rotor heads used on Army
helicopters are semirigid and fully articulated.
Semirigid
The semirigid rotor head gets its name from the fact
that the two blades are rigidly interconnected and
pivoted about a point slightly above their center (Figure
3-1). There are no flapping or drag hinges like
those on the articulating head. Since the blades are
interconnected, when one blade moves upward the
other moves downward a corresponding distance.
The main rotor hub is of a semirigid, underslung
design consisting basically of the —
Yoke (1).
Trunnion (2).
Elastomeric bearing (3).
Yoke extensions.
Pitch horns (4).
FM 1-514
• Drag braces (5). each acting as a single unit and capable of flapping,
• Grips (6).
The yoke is mounted to the trunnion by elastomeric
bearings which permit rotor flapping. Cyclic and
collective pitch-change inputs are received through
pitch horns mounted on the trailing edge of the grips.
The grips in turn are permitted to rotate about the
yoke extensions on Teflon-impregnated fabric friction
bearings, resulting in the desired blade pitch.
Adjustable drag braces are attached to the grips and
main rotor blades to maintain alignment. Blade
centrifugal loads are transferred from the blade grips
to the extensions by wire-wound, urethane-coated,
tension-torsion straps.
feathering, and leading and lagging. The assembly is
made up primarily of —
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An internally splined hub.
Horizontal and vertical hinge pins.
Extension links.
Pitch shafts.
Pitch housing.
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Dampers.
Pitch arms.
• Bearing surfaces.
• Connecting parts.
Fully Articulated The extension links are attached to the hub by the
A folly articulated rotor head gets its name from the horizontal pins and to the forked end of the extension
fact that it is jointed (Figure 3-2). Jointing is made link. The pitch shafts are attached by the vertical
with vertical and horizontal pins. The fully articu- pins. The pitch housing is fitted over and fastened to
lated rotor head assembly has three or more blades, the pitch shaft by the tension-torsion straps, which
are pinned at the inboard end of the pitch shaft and
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FM 1-514
the outboard end of the pitch-varying housing. One provide automatic equalization of thrust on the adend
of the dampers is attached to a bracket on the
horizontal pins; the other end is fastened to the pitch
housing.
Flapping
Flapping of the rotor blades is permitted by the
horizontal pin, which is the hinge or pivot point.
Centrifugal force on the blades and stops on the head
prevent excessive flapping.
　
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