DISC AREA—The area swept by the blades of the rotor. It is a circle with its center at the hub and has a radius of one blade length.
DISC LOADING—The total helicopter weight divided by the rotor disc area.
DISSYMMETRY OF LIFT—The unequal lift across the rotor disc resulting from the difference in the velocity of air over the advancing blade half and retreating blade half of the rotor disc area.
DRAG—An aerodynamic force on a body acting parallel and opposite to relative wind.
DUAL ROTOR—A rotor system utilizing two main rotors.
DYNAMIC ROLLOVER—The tendency of a helicopter to continue rolling when the critical angle is exceeded, if one gear is on the ground, and the helicopter is pivoting around that point.
FEATHERING—The action that changes the pitch angle of the rotor blades by rotating them around their feathering (spanwise) axis.
FEATHERING AXIS—The axis about which the pitch angle of a rotor blade is varied. Sometimes referred to as the spanwise axis.
FEEDBACK—The transmittal of forces, which are initiated by aerodynamic action on rotor blades, to the cockpit controls.
FLAPPING HINGE—The hinge that permits the rotor blade to flap and thus balance the lift generated by the advancing and retreating blades.
FLAPPING—The vertical movement of a blade about a flapping hinge.