Common terms used to describe the helicopter rotor system are shown here. Although there is some variation in systems between different aircraft, the terms shown are generally accepted by most manufacturers.
The system below is an example of a Fully Articulated rotor system:
Semirigid Rotor Systems do not have vertical / horizontal hinge pins. Instead, the entire rotor is allowed to teeter or flap by a trunnion bearing that connects the yoke to the mast (this method iscommonly used on two blades rotor systems):
The Chord (1) is the longitudinal dimension of an airfoil section, measured from the leading edge to the trailing edge.
The Span (2) is the length of the rotor blade from the point of rotation to the tip of the blade.
The Vertical Hinge Pin (3) (drag hinge) is the axis which permits fore and aft blade movement independent of the other blades in the system.
The Horizontal Hinge Pin (4) is the axis which permits up and down movement of the blade independent of the other blades in the system.
The Trunnion (5) is splined to the mast and has two bearings through which it is secured to the yoke. The blades are mounted to the yoke and are free to teeter (flap)around the trunnion bearings.
The Yoke (6) is the structural member to which the blades are attached and which fastens the rotor blades to the mast through the trunnion and trunnion bearings.
The Blade Grip Retainer Bearings (7) is the bearing which permits rotation of the blade about its spanwise axis so blade pitch can be changed (blade feathering).
Blade Twist is a characteristic built into the rotor blade so angle of incidence is less near the tip than at the root. Blade twist helps distribute the lift evenly along the blade by an increased angle of incidence near the root where blade speed is slower. Outboard portions of the blade that travel faster normally have lower angles of incidence, so less lift is concentrated near the blade tip.