Theory of Rotary Wing Flight Practice Test

In a semi-rigid rotor system, the hub provides a single flapping hinge, which allows the rotor disk to tilt as a unit. The mast is the fixed reference to the airframe, so the observable tilt of the rotor disk occurs relative to the mast. This arrangement lets the disk rotate its plane to balance lift differences between blades without requiring multiple hinges at the hub. The tilting is not described relative to the hub, blade root, or the aircraft’s centerline, but specifically relative to the mast.