Navigational aid (NAVAID).  Any visual or electronic  device, airborne or on the surface, that provides point-to-point  guidance information, or position data, to aircraft in flight. Hunting.  Movement of a blade with respect to the other  blades in the plane of rotation, sometimes called leading or  lagging. In ground effect (IGE) hover.  Hovering close to the surface  (usually less than one rotor diameter distance above the  surface) under the influence of ground effect. Night.  The time between the end of evening civil twilight  and the beginning of morning civil twilight, as published in  the American Air Almanac. Normally aspirated engine.  An engine that does not  compensate for decreases in atmospheric pressure through  turbocharging or other means. Induced drag.  That part of the total drag that is created by  the production of lift. One-to-one vibration.  A low frequency vibration having  one beat per revolution of the rotor. This vibration can be  either lateral, vertical, or horizontal. Induced flow.  The component of air flowing vertically  through the rotor system resulting from the production of lift. Inertia.  The property of matter by which it will remain at rest  or in a state of uniform motion in the same direction unless  acted upon by some external force. Out of ground effect (OGE) hover.  Hovering a distance  greater than one disk diameter above the surface. Because  induced drag is greater while hovering out of ground effect,  it takes more power to achieve a hover out of ground effect. Isogonic line.  Lines on charts that connect points of equal  magnetic variation. Knot.  A unit of speed equal to one nautical mile per hour. Parasite drag.  The part of total drag created by the form or  shape of helicopter parts. Payload.  The term used for the combined weight of  passengers, baggage, and cargo. L$_{DMAX}$.  The maximum ratio between total lift (L) and total  drag (D). This point provides the best glide speed. Any  deviation from the best glide speed increases drag and reduces  the distance you can glide. Pendular action.  The lateral or longitudinal oscillation of  the fuselage due to its suspension from the rotor system. Lateral vibration.  A vibration in which the movement is  in a lateral direction, such as imbalance of the main rotor. Pitch angle.  The angle between the chord line of the rotor  blade and the reference plane of the main rotor hub or the  rotor plane of rotation. Lead and lag.  The fore (lead) and aft (lag) movement of the  rotor blade in the plane of rotation. Pressure altitude.  The height above the standard pressure  level of 29.92 "Hg. It is obtained by setting 29.92 in the  barometric pressure window and reading the altimeter. Profile drag.  Drag incurred from frictional or parasitic  resistance of the blades passing through the air. It does not  change significantly with the angle of attack of the airfoil  section, but it increases moderately as airspeed increases. Licensed empty weight.  Basic empty weight not including  full engine oil, just undrainable oil. Lift.  One of the four main forces acting on a helicopter. It  acts perpendicular to the relative wind. Load factor.  The ratio of a specified load weight to the total  weight of the aircraft. Resultant relative wind.  Airflow from rotation that is  modified by induced flow. Retreating blade.  Any blade, located in a semicircular part  of the rotor disk, in which the blade direction is opposite to  the direction of flight. Married needles.  A term used when two hands of an  instrument are superimposed over each other, as on the  engine/rotor tachometer. Mast.  The component that supports the main rotor. Mast bumping.  Action of the rotor head striking the mast,  occurring on underslung rotors only.  G-4