There are several different main rotor system designs that are used on modern helicopters. The three basic designs that have traditionally been taught to students are semi-rigid, fully articulated, and rigid. Today there are versions that make extensive use of composite materials and are known as hinge less systems.
A fully articulated system normally has more than two blades. In this design each blade is attached to a hub with hinges that allow it to move independently of the others. A feathering hinge is used to change the pitch of each blade. A flapping hinge allows each blade to move up and down to compensate for dissymmetry of lift. Blades are able to move fore and aft or lead-lag, (called hunting) by use of a drag hinge. Normally a damper is attached to the blade and hub to restrict excessive movement. The drag hinge is used because, when a rotor blade flaps up, its center of mass moves closer to the axis of rotation. This causes the rotor system to spin faster, much like a spinning ice skater speeds up when pulling her arms in closer. Allowing the blades to lead-lag reduces this tendency.
A semi-rigid system refers to a two-blade system where each blade is mounted to a hub that has a center teetering hinge. In this configuration, when one blade flaps up the other one flaps down – like a see saw. As with the fully articulated system, each blade has a feathering hinge. The two blades are mounted in an under-slung position, that is where the teetering hinge is mounted above the plane of rotation. The geometry of this arrangement minimizes the change in distance between the center of mass and the axis of rotation during flapping. This allows a semi-rigid system to not need a drag hinge.
In a slight departure from the traditional semi-rigid design, Frank Robinson used a coning hinge on each blade (some refer to this as a flapping hinge, but it is used for blade coning). When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). This places a high stress load at the blade’s root, so in order to relieve this stress Robinson’s design allows the blade root to cone about a hinge. This reduced the amount of reinforcing required at the blade root making for a lighter easier to manufacture rotor blade.
Rigid rotor systems do not use hinges and limited movement is absorbed through the hub and rotor blades. Many of the modern composite rotor systems also do not use traditional hinges, but have elastomeric and specially designed composites structures (flextures) that allow the blades to flap, feather, and hunt. Manufactures do not use the term rigid rotor system, opting instead to describe these systems as a fully articulated hinge less rotor system. These systems do not require lubrication and are less maintenance intensive. The extensive use of composite materials also increases reliability and helps absorb vibration.