Autorotation - The process of producing lift with freely-rotating airfoils (rotor blades) by means of the aerodynamic forces resulting from an upward flow of air.

Airfoil - Any surface designed to obtain a useful lift reaction when moving through the air at specified velocity.

Angle of Attack -  The acute angle between the chordline of an airfoil adn teh relative wind.

Center of Gravity - An imaginary point where the resultant of all weight forces in a body can be considered to be concentrated for a position of the body.

Drag - The component of the resultant of the aerodynamic forces acting on a body that acts paralllel to the relative airflow; the force that tends to resist movement of an airefoil through the air.

Ground Effect - The cushion of denser air confined beneath the rotor sytem of a hovering helicopter that gives additional lift and thus decreases the power required to hover.

Helicopter - A helicopter (informally known as a "chopper") is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally.

Hovering - Maintaining a fixed position over a spot on the surface at some altitude above the surface at which no additional lift is obtained from ground effect.

Hovering in Ground Effect - Maintaining a fixed position over a spot on the surface that compresses a cushion of high-density air between the main rotor and the surface and thus increases the lift produced by the main rotor.  Normally the main rotor must be within one-half rotor diameter to the surface in order to produce and efficient ground effect.

Hovering out of Ground Effect - Maintaining a fixed position over a spot on the surface at some altitude above the surface at which no additional lift is obtained from ground effect.

Nautical Mile - One minute of arc on a meridian (one minute of latitude); one minute of arc on the earth's equator. One nautical mile equals approximately 1.15 statue miles or 1.85 kilometers; therefore, to convert knots to miles per hour, double knots for a rough approximation, or multiply knots to kilometers by 1.85, to be more precise.

Newton's Third Law -  - To every action there is an equal and opposite reaction.

Rigid Rotor - A rotor system with blades fixed to the hub in such a way that they can feather but cannot flap or drag.

Rotor - A system of rotating airfoils.