The classification of main rotor systems is based on how the blades move relative to the main rotor hub. The principal classifications are known as fully articulated, semi-rigid, and rigid.
In the fully articulated rotor system, the blades are attached to the hub with multiple hinges. The blades are hinged in a way that allows them to move up and down and fore and aft, and bearings provide for motion around the pitch change axis. Rotor systems using this type of arrangement typically have three or more blades. The hinge that allows the blades to move up and down is called the flap hinge, and movement around this hinge is called flap. The hinge that allows the blades to move fore and aft is called a drag or lag hinge. Movement around this hinge is called dragging, lead/lag, or hunting. These hinges and their associated movement are shown in Figure 3-82. The main rotor head of a Eurocopter model 725 is shown in Figure 3-83, with the drag hinge and pitch change rods visible.
The semi-rigid rotor system is used with a two blade main rotor. The blades are rigidly attached to the hub, with the hub and blades able to teeter like a seesaw. The teetering action allows the blades to flap, with one blade dropping down while the other blade rises. The blades are able to change pitch independently of each other. Figure 3-84 shows a Bell Jet Ranger helicopter in flight. This helicopter uses a semi-rigid rotor system, which is evident because of the way the rotor is tilted forward when the helicopter is in forward flight.
With a rigid rotor system, the blades are not hinged for movement up and down (flapping) or for movement fore and aft (drag). The blades are able to move around the pitch change axis, with each blade being able to independently change its blade angle. The rigid rotor system uses blades that are very strong and yet flexible. They are flexible enough to bend when they need to, without the use of hinges or a teetering rotor, to compensate for the uneven lift that occurs in forward flight. The Eurocopter model 135 uses a rigid rotor system. [Figure 3-85]