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 rotor."

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.

Figure 10-4.—Dissymmentry of lift.

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