Glossary Absolute altitude.  The actual distance an object is above  the ground. Antitorque rotor.  See tail rotor. Articulated rotor.  A rotor system in which each of the blades  is connected to the rotor hub in such a way that it is free to  change its pitch angle, and move up and down and fore and  aft in its plane of rotation. Advancing blade.  The blade moving in the same direction as  the helicopter. In helicopters that have counterclockwise main  rotor blade rotation as viewed from above, the advancing  blade is in the right half of the rotor disk area during forward  movement. Autopilot.  Those units and components that furnish a means  of automatically controlling the aircraft. Agonic Line.  An isogonic line along which there is no  magnetic variation. Autorotation.  The condition of flight during which the main  rotor is driven only by aerodynamic forces with no power  from the engine. Axis of rotation.  The imaginary line about which the rotor  rotates. It is represented by a line drawn through the center  of, and perpendicular to, the tip-path plane. Air density.  The density of the air in terms of mass per unit  volume. Dense air has more molecules per unit volume than  less dense air. The density of air decreases with altitude above  the surface of the earth and with increasing temperature. Basic empty weight.  The weight of the standard helicopter,  operational equipment, unusable fuel, and full operating  fluids, including full engine oil. Aircraft pitch.  The movement of the aircraft about its lateral,  or pitch, axis. Movement of the cyclic forward or aft causes  the nose of the helicopter to pitch up or down. Aircraft roll.  The movement of the aircraft about its  longitudinal axis. Movement of the cyclic right or left causes  the helicopter to tilt in that direction. Blade coning.  An upward sweep of rotor blades as a result  of lift and centrifugal force. Blade damper.  A device attached to the drag hinge to restrain  the fore and aft movement of the rotor blade. Airfoil.  Any surface designed to obtain a useful reaction of  lift, or negative lift, as it moves through the air. Blade feather or feathering.  The rotation of the blade around  the spanwise (pitch change) axis. Airworthiness Directive.  When an unsafe condition exists  with an aircraft, the FAA issues an Airworthiness Directive  to notify concerned parties of the condition and to describe  the appropriate corrective action. Altimeter.  An instrument that indicates flight altitude by  sensing pressure changes and displaying altitude in feet or  meters. Blade flap.  The ability of the rotor blade to move in a vertical  direction. Blades may flap independently or in unison. Blade grip.  The part of the hub assembly to which the rotor  blades are attached, sometimes referred to as blade forks. Blade lead or lag.  The fore and aft movement of the blade  in the plane of rotation. It is sometimes called “hunting” or  “dragging.” Angle of attack.  The angle between the airfoil’s chord line  and the relative wind. Antitorque pedal.  The pedal used to control the pitch of the  tail rotor or air diffuser in a NOTAR® system. G-1