Source: {"pile_set_name": "USPTO Backgrounds"}

Rotorcraft, such as helicopters, tiltrotors, and autogyros, have at least one rotor for providing lift force. These rotors typically have an engine that rotates a mast coupled to a hub, and at least two airfoil rotor blades coupled to the hub. Rotor blades may be detachably coupled, allowing a rotor blade to be removed for service and storage.
A helicopter typically has a main rotor, rotating substantially horizontally, that provides lift and thrust. The torque created by the rotation of the main rotor causes the fuselage of the helicopter to turn in the opposite direction. The most common mechanism to control undesired fuselage rotation is a tail rotor that provides sideways thrust to counteract the torque of the main rotor.
Rotor assemblies come in multiple varieties including fully-articulated, semi-rigid, and rigid types. These types differ in which blade motions may be controlled by the pilot of the rotorcraft. Blade flap motion is deflection perpendicular to the plane of rotation. Blade lag motion (also called lead-lag motion, drag motion and hunting) is deflection within the plane of rotation. Blade feather motion is a change of blade pitch with respect to the plane of rotation (blades may rotate about their longitudinal axis). Fully-articulated rotor assemblies allow each blade to independently flap, lag, and feather. Semi-rigid rotor assemblies (also called semi-articulated rotors) lack independent flap and lag hinges, allowing only coordinated flap motion and independent feathering motion. In a semi-rigid rotor assembly, the blades are rigidly attached to each other. The assembly flaps as a unit; when one blade flaps up, another flaps down. A two-bladed semi-rigid rotor may be referred to as a teetering rotor or a see-saw rotor. If a semi-rigid rotor has three or more blades, it may be referred to as a gimbaled rotor. Rigid rotor assemblies lack any flap or lag hinges (and are sometimes called hingeless rotors). The blades, hub, and mast are rigidly attached to each other. Though other components are rigidly connected, rigid rotors may include feathering control of the rotor blades.
Feathering occurs by changing the pitch of the blades relative to the plane of rotation. A pitch bearing is typically incorporated into the rotor assembly to ease the blade feather motion. Feather may be collective (all blades set to the same pitch), allowing modulation of the magnitude of the rotor thrust, or may be cyclic (blade pitch depends on azimuth position), allowing modulation of the direction of the rotor thrust. A pilot can control the pitch and roll of a rotorcraft by appropriate cyclic feathering of the main rotor blades. When a tail rotor is employed, a pilot can control the yaw of a rotorcraft by appropriate collective feathering of the tail rotor blades.
Rotor blades experience substantial dynamic forces. Therefore, the blades and any hinges, bearings or couplings must be precisely engineered to provide acceptable stability, control, and performance.