Force
Resultant
Blade
Angle
Figure 3-4. Rotor blade coning occurs as the rotor blades
begin to lift the weight of the helicopter. In a semirigid and
rigid rotor system, coning results in blade bending. In an
articulated rotor system, the blades assume an upward angle
through movement about the flapping hinges.
Centrifugal Force—The apparent
force that an object moving along
a circular path exerts on the body
constraining the obect and that
acts outwardy away from the center
of rotation.
To counteract this drift, one or more of the following
features may be used:
• The main transmission is mounted so that the rotor
mast is rigged for the tip-path plane to have a builtin
tilt opposite tail thrust, thus producing a small
sideward thrust.
• Flight control rigging is designed so that the rotor
disc is tilted slightly opposite tail rotor thrust when
the cyclic is centered.
• The cyclic pitch control system is designed so that
the rotor disc tilts slightly opposite tail rotor thrust
when in a hover.
Counteracting translating tendency, in a helicopter with a
counterclockwise main rotor system, causes the left skid
to hang lower while hovering. The opposite is true for
rotor systems turning clockwise when viewed from above.
PENDULAR ACTION
Since the fuselage of the helicopter, with a single main
rotor, is suspended from a single point and has considerable
mass, it is free to oscillate either longitudinally or
laterally in the same way as a pendulum. This pendular
action can be exaggerated by over controlling; therefore,
control movements should be smooth and not exaggerated.
[Figure 3-3]
CONING
In order for a helicopter to generate lift, the rotor blades
must be turning. This creates a relative wind that is
opposite the direction of rotor system rotation. The
rotation of the rotor system creates centrifugal force
(inertia), which tends to pull the blades straight outward
from the main rotor hub. The faster the rotation, the
Hover
Rearward
Flight
Forward
Flight
Figure 3-3. Because the helicopter’s body has mass and is
suspended from a single point (the rotor mast head), it tends
to act much like a pendulum.
3-3
rotation and blade deceleration takes place. [Figure 3-5]
Keep in mind that due to coning, a rotor blade will not
flap below a plane passing through the rotor hub and
perpendicular to the axis of rotation. The acceleration
and deceleration actions of the rotor blades are absorbed
by either dampers or the blade structure itself, depending
upon the design of the rotor system.
Two-bladed rotor systems are normally subject to
Coriolis Effect to a much lesser degree than are articulated
rotor systems since the blades are generally
“underslung” with respect to the rotor hub, and the
change in the distance of the center of mass from the
axis of rotation is small. [Figure 3-6] The hunting
action is absorbed by the blades through bending. If a
two-bladed rotor system is not “underslung,” it will be
subject to Coriolis Effect comparable to that of a fully
articulated system.
GROUND EFFECT
When hovering near the ground, a phenomenon known
as ground effect takes place. [Figure 3-7] This effect
usually occurs less than one rotor diameter above the
surface. As the induced airflow through the rotor disc is
reduced by the surface friction, the lift vector increases.
This allows a lower rotor blade angle for the same
amount of lift, which reduces induced drag. Ground
effect also restricts the generation of blade tip vortices
due to the downward and outward airflow making a
larger portion of the blade produce lift. When the helicopter
gains altitude vertically, with no forward airspeed,
induced airflow is no longer restricted, and the
blade tip vortices increase with the decrease in outward
airflow. As a result, drag increases which means a
Axis of
Rotation
Blade
Flapping
Center of Mass
Figure 3-5. The tendency of a rotor blade to increase or
decrease its velocity in its plane of rotation due to mass
movement is known as Coriolis Effect, named for the mathematician
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