Rotor system types.
There are three types of rotors: rigid, semirigid, and fully articulated. In a rigid rotor, the blades are free to change pitch but other-wise are made as rigid as possible. The blades of rigid rotor have no hinges that allow them to flap or drag. In a semirigid system, the blades are fixed to the hub, generally by a teetering or see-saw hinges, allowing the blades to flap and feather but not lead-lag or drag. A fully articulated rotor system is one in which the individual blades are free to flap (i.e., move up and down), drag (i.e., move back and forth in the plane of rotation), and change their pitch angle. The advantages and disadvantages of each type are shown in the illustration.