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determine the coning angle mathemically and build
a precone angle into the rotor hub that is similar to
the coning effect in normal flight. The preconed hub
lets the blades operate at normal coning angles
2-3
FM 1-514
without bending, which reduces stress. It is not
necessary to precone the articulated rotor hub because
the blade can flap up on horizontal hinges to
the correct coning angle.
Lead and Lag of Blades
The horizontal movement of the blades around a
vertical pin is called leading and lagging (hunting)
(Figure 2-10). This is found only on fully articulated
rotor heads. During starting the blades will resist
rotational movement and will lag behind their (true
radial) position. As centrifugal force reacts on the
blade, the blade will gain momentum and find its own
position of rotation. The blade will hunt about the
vertical hinge close to a 5° range during normal
operation. The movement of the blades about the
vertical hinge is restricted by a hydraulic damper.
Feathering Axis
The spanwise axis about which a rotor blade
rotates to change pitch is known as the feathering
axis (Figure 2-11). Feathering action varies according
to the position of the cyclic control in forward
flight, the dissymmetry of lift, and the collective pitch
control when the helicopter hovers.
Hover
The versatility of a helicopter is due to its ability to
hover at a point above the ground. This lets the
helicopter vertically rise from and descend to small,
unimproved landing areas. When main rotor angle
of attack and engine power are adjusted so that lift
equals weight, the helicopter will hover. Hover is
considered an element of vertical flight. Assuming a
no-wind condition exists during hover, the tip path
plane of the rotor will remain horizontal with the
earth. When the angle of attack of both blades is
increased equally while blade speed remains constant,
more thrust will result and the helicopter will
rise. By upsetting the lift-gravity balance, the
helicopter will rise or come down depending on
which force is greater. Hovering takes a great deal
of power because a large mass of air must be drawn
through the rotor blades at high speeds.
Ground Effect
When hovering near ground or water surfaces at a
height no more than one-half of the rotor diameter,
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FM 1-514
the helicopter encounters a condition referred to as the rotor tip vortex and the flattening out of the rotor
ground effect. This condition is more pronounced downwash. The benefit of ground effect is lower
nearer the ground. Helicopter operations within blade angle of attack, which results in a reduction of
ground effect are more efficient due to reduction of power requirements for a given load.
2-5
FM 1-514
CHAPTER 3
ROTOR SYSTEM OPERATION
An understanding of the rotor system is necessary to
be able to troubleshoot it analogical manner. It is
important to know and understand the operation of
rotor heads and how rotor blades are driven.
Remember that if the components of the rotor system
are not properly maintained, a malfunction may
occur while in flight causing possible loss of life and
equipment. For a complete detailed description of a
specific helicopter rotor system, refer to the applicable
aircraft multipart maintenance manual.
SINGLE AND TANDEM ROTORS
Helicopter configurations are classified as single,
tandem, coaxial, and side by side. The single- and
tandem-rotor configurations are the only ones used
in Army helicopters.
Single Rotor
Helicopters designed to use a main and tail rotor
system are referred to as single-rotor helicopters.
The main rotor provides lift and thrust while the tail
rotor counteracts the torque made by the main rotor.
This keeps the aircraft from rotating in the opposite
direction of the main rotor. The tail rotor also
provides the directional control for the helicopter
during hovering and engine power changes. Power
to operate the main and tail rotors is supplied by the
power train system. The single-rotor configuration
has the advantage of being simpler and lighter than
the tandem-rotor system, and it requires less maintenance.
Since the tail rotor uses a portion of the
available power, the single-rotor system has a smaller
center-of-gravity range.
Tandem Rotor
Normally used on large cargo helicopters, the
tandem-rotor configuration has two main rotor
systems, one mounted on each end of the
fuselage. Each rotor operates the same as the main
rotor on the single-rotor helicopter, except for the
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