by horizontal flight or by the wind when the
helicopter is hovering. When hovering in a no-wind
condition, the speed of the relative wind in relation to
the rotor is the same. However, the speed reduces at
points closer to the rotor hub, as shown in figure 10-3.
When the helicopter moves into forward flight, the
relative wind moving over each blade becomes a
combination of the rotor speed and the forward
The advancing blade is then the
combined speed of the blade speed and helicopter
speed. While on the opposite side, the retreating
blade speed is the blade speed minus the speed of the
helicopter. For example, figure 10-4 shows a
helicopter moving forward at 100 mph. The
advancing blade has a tip speed of 350 mph plus the
helicopter speed of 100 mph, or 450 mph. The
retreating blade has a tip speed of 350 mph minus the
helicopters speed of 100 mph, or 250 mph. Hovering
over one spot in a 20 mph headwind is the same as
flying forward at a speed of 20 mph.
During forward flight or hovering in a wind, the
lift over the advancing blade half of the rotor disc is
greater than the retreating half. This greater lift
would cause the helicopter to roll unless something
equalized the lift. One method of equalizing the lift is
through blade flapping.
Blades attached to the rotor hub by horizontal
hinges permit the blade to move vertically. The
blades actually flap up and down as they rotate. The
hinge permits an advancing blade to rise, thus
reducing its effective lift area. It also allows a
retreating blade to settle, which increases its effective
lift area. Decreasing lift on the advancing blade and
increasing lift on the retreating blade equalizes the lift
over the rotor disc halves.
Blade flapping creates an unbalanced condition
resulting in vibration.
To prevent this vibration, a
drag hinge allows the blades to move back and forth
in a horizontal plane.
A main rotor that permits
individual movement of the blades in both a vertical
and horizontal plane is known as an articulated
Coning is the upward bending of the blades
caused by the combined forces of lift and centrifugal
force. Before takeoff, centrifugal force causes the
blades to rotate in a plane nearly perpendicular to the
Figure 10-3.Symmetry of lift.
rotor hub. During a vertical liftoff, the blades assume
a conical path as a result of centrifugal force acting
outward and lift acting upward. Coning causes rotor
blades to bend up in a semirigid rotor. In an
articulated rotor, the blades move to an upward angle
through movement about the flapping hinges.
Figure 10-4.Dissymmentry of lift.