BLADE FLAPPING

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 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. 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 helicopter’s 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. 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 rotor.” CONING 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. 10-3