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
s e t t l e , w h i c h i n c r e a s e s i t s e ff e c t ive l i f t a r e a .
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 rotor."
Figure 17-3.--Symmetry of lift.
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 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.
The spinning main rotor of a helicopter acts like a
gyroscope. It has the properties of gyroscopic action,
one of which is precession. Gyroscopic precession is
the resulting action occurring 90 degrees from the
applied force. A downward force to the right of the
disc area will cause the rotor to tilt down in front. This
action is true for a right-to-left (counterclockwise)
turning rotor. The cyclic control applies force to the
Figure 17-4.--Dissymmetry of lift.
main rotor through the swashplate.
retreating blade has a tip speed of 350 mph minus the
To simplify directional control, helicopters use a
helicopter's speed of 100 mph, or 250 mph. Hovering
mechanical linkage that places cyclic pitch change 90
over one spot in a 20-mph headwind is the same as
degrees ahead of the applied force. Moving the cyclic
flying forward at a speed of 20 mph.
control forward will cause high pitch on the blades to
the pilot's left. At the same time, low pitch occurs on
During forward flight or hovering in a wind, the lift
the blades to his/her right. This combination of forces
over the advancing blade half of the rotor disc is greater
results in the rotor tilting down in front.
than the retreating half. This greater lift would cause
the helicopter to roll unless something equalized the
If not for this offset linkage, the pilot would have to
lift. One method of equalizing the lift is through blade
move the cyclic stick 90 degrees out of phase. In other
words, the pilot would have to move the stick to the