The angular movement of a rotor blade above or below the plane of the hub. Blades ride higher and lower during their rotation, but they can do so only if they are allowed to move freely about a horizontal hinge or flexible root element. Design features allow for the blades to flap vertically about the flapping hinge. Blade flapping compensates for dissymmetry of lift as the retreating side flaps down resulting in increased angle of attack with consequent increase in lift. The advancing blade flaps up and the angle of attack reduces resulting in decreased lift.