rotary wing planforms
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 shown here is fully
Semi rigid types do not have a
vertical or horizontal hinge pin. Instead, the rotor is allowed to teeter or
flap by a trunnion bearing that connects the yoke to the mast:
The chord is the
longitudinal dimension of an airfoil section, measured from the leading edge
to the trailing edge.
The span is the length
of the rotor blade from the point of rotation to the tip of the blade.
The vertical hinge pin
(drag hinge) is the axis which permits fore and aft blade movement independent
of the other blades in the system.
The horizontal hinge pin
is the axis which permits up and down movement of the blade independent of the
other blades in the system.
The trunnion 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
The yoke 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
bearing is the bearing which permits rotation of the blade about its
spanwise axis so blade pitch can be changed (blade feathering)
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