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Feathering
Feathering is the controlled rotation about the longitudinal
axis of the blades that permits the pilot to
achieve directional control in either the horizontal or
vertical plane. Feathering is permitted by a pitchchange
assembly on some helicopters and by a sleeveand-
spindle assembly on other types of helicopters.
Leading and Lagging
Leading and lagging is permitted by the vertical pin,
which serves as a hinge or pivot point for the action.
Excessive leading and lagging is prevented by the use
of a two-way hydraulic damper in the system.
TAIL ROTOR HUBS
The tail rotor hub (antitorque rotor) is used as a
centering fixture to attach the tail rotor blades so that
they rotate about a common axis. It keeps the blocks
against centrifugal, bending and thrust forces. It
accepts the necessary pitch-change mechanism to
3-4
vancing and retreating blade, or equal and simultaneous
pitch change to counteract torque made by
the main rotor system. Hub design varies with the
manufacturer. Typical configurations are the hingemounted,
flex-beamed, and fully articulated types.
Hinge-Mounted Type
A single two-blade, controllable-pitch tail rotor is
located on the left side of the tail rotor gearbox
(Figure 3-3). It is composed of the blades and the
hub and is driven through the tail rotor gearbox.
Blades are of all-metal construction and attached by
bolts in blade grips, which are mounted through
bearings to spindles of the hub yoke. The tail rotor
hub is hinge-mounted to provide automatic equalization
of thrust on advancing and retreating blades.
Control links provide equal and simultaneous pitch
change to both blades. The tail rotor counteracts the
torque of the main rotor and provides directional
control.
Flex-Beamed Type
The tail rotor hub and blade assembly counteracts
torque of the main rotor and provides directional
control. It consists of the hub and two blades (Figure
3-4). The hub assembly has a preconed, flexbeamed-
type yoke and a two-piece trunnion connected
to the yoke by self-lubricating, spherical
flapping bearings. The trunnion, which is splined to
the tail rotor gearbox shaft, drives the blades and
FM 1-514
Fully Articulated Type
The articulated tail rotor system (Figure 3-5)
counterbalances disturbing forces in the same way
that the hinge-type rotor does. The major difference
is that the blades can lead and lag individually during
rotation.
MAIN ROTOR BLADES
The rotor blade is an airfoil designed to rotate about
a common axis to produce lift and provide directional
control for a helicopter. It is often referred to as a
rotary wing. The design and construction of a rotor
blade vary with the manufacturer, although they all
strive to manufacture the most efficient and
economical lifting device. The particular helicopter
design places certain requirements on the main rotor
blades, which influence their design and construction.
Most rotor blades are designed as symmetrical
airfoils to produce a stable aerodynamic pitching
characteristic. Aerodynamic stability is achieved
when the center of gravity, center of pressure, and
blade-feathering axis all act at the same point. The
blade is more stable in flight because these forces
continue to act at almost the same point as the blade
serves as a flapping stop for the tail rotor. The yoke changes pitch. At present only one Army helicopter
has two self-lubricating, spherical bearings as attach- is equipped with an unsymmetrical airfoil. This uning
points for each rotor blade. Rotor pitch change symmetrical airfoil blade is capable of producing
is accomplished at these bearings. greater lift than a symmetrical airfoil blade of similar
3-5
FM 1-514
dimensions. Aerodynamic stability is achieved by
building a 3° upward angle into the trailing edge section
of the blade. This prevents excessive center-ofpressure
travel when the rotor blade angle of attack
is changed. A variety of material is used in the construction
of rotor blades; aluminum, steel, brass, and
fiberglass are most common.
Types of Rotor Blades
Metal
A typical metal blade has a hollow, extruded
aluminum spar which forms the leading edge of the
blade (Figure 3-6). Aluminum pockets bonded to
the trailing edge of the spar assembly provide streamlining.
An aluminum tip cap is fastened with screws
to the spar and tip pocket. A steel cuff bolted to the
root end of the spar provides a means of attaching
the blade to the rotor head. A stainless steel abrasion
strip is adhesive-bonded to the leading edge.
　
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