## Glossary 

Absolute altitude.  The actual distance an object is above the ground. 

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. 

Agonic Line.  An isogonic line along which there is no magnetic variation. 

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. 

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. 

Airfoil.  Any surface designed to obtain a useful reaction of lift, or negative lift, as it moves through the air. 

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. 

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. 

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. 

Autopilot.  Those units and components that furnish a means of automatically controlling the aircraft. 

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. 

Basic empty weight.  The weight of the standard helicopter, operational equipment, unusable fuel, and full operating fluids, including full engine oil. 

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. 

Blade feather or feathering.  The rotation of the blade around the spanwise (pitch change) axis. 

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.” 

