created by horizontal flight or by wind in a hover.
You should be aware that hovering in a 20 mph
headwind is the same as flying forward at 20 mph.
When hovering in a no-wind condition, the speed
of the relative wind is the effective speed of the
rotor. However, the speed is lower at points closer
to the rotor hub, as shown in figure 7-2. When
the helicopter moves forward, relative wind over
each blade becomes a combination of the rotor
speed and forward movement. 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. Figure 7-3
shows dissymmetry of lift at 100 mph forward
flight.
During forward flight, lift over the advanc-
ing blade half of the rotor disk is greater then 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.
Figure 7-3.-Dissymmetry of lift.
Blade Flapping
Blades attached to the rotor hubby horizontal
hinges permit the blade to move vertically. The
effective lift area. The combination of decreasing
lift on the advancing blade and increasing lift on
blades actually flap up and down as they rotate.
The hinge permits an advancing blade to rise, thus
the retreating blade equalizes the lift.
reducing its effective lift area. It also allows a
Blade flapping creates an unbalanced condi-
retreating blade to settle, thus increasing its
tion, 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 blades in a
vertical and horizontal plane is known as an
articulated rotor.
Coning
Coning is the upward bending of the
blades caused by the combined forces of lift
and centrifugal force. Before takeoff, due to
centrifugal force, the blades rotate in a plane
nearly perpendicular to the 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.
Gyroscopic Precession
The spinning main rotor of a helicopter acts
like a gyroscope. It has the properties of
gyroscopic action, one of which is precession.
Figure 7-2.-Symmetry of lift.
Gyroscopic precession is resulting action occurring
7-3
