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airfoil
Definição1
Any surface designed to obtain a useful reaction of lift or negative lift as it moves through the air. Rotor blades and stabilizers are the most commom airfoils on helicopters.
Fonte1
KUMAR, Bharat (ed.). An illustrated dictionary of aviation. New York: McGraw-Hill, c2005. 752 p.
Definição2
An airfoil is any surface producing more lift than drag when passing through the air at a suitable angle.
Fonte2
FEDERAL AVIATION ADMINISTRATION. Helicopter flying handbook (FAA-H8083-21A). Washington, DC: FAA, 2012.
Nota adicional1
Helicopters are able to fly due to aerodynamic forces produced when air passes around the airfoil.
Nota adicional2
Airfoils are most often associated with production of lift. Airfoils are also used for stability (fin), control (elevator), and thrust or propulsion (propeller or rotor). Certain airfoils, such as rotor blades, combine some of these functions.
Contexto
There are four forces acting on a helicopter in flight. They are lift, weight, thrust, and drag. Lift is the upward force created by the effect of airflow as it passes around an airfoil. Weight opposes lift and is caused by the downward pull of gravity. Thrust is the force that propels the helicopter through the air. Opposing lift and thrust is drag, which is the retarding force created by development of lift and the movement of an object through the air.
Subárea1
Aerodynamics
Variante
aerofoil
Imagem

 An airfoil section is displayed at the tip of this Denney Kitfox aircraft, built in 1991.

An airfoil section is displayed at the tip of this Denney Kitfox aircraft, built in 1991.

Fonte: http://en.wikipedia.org/wiki/File:Denney.kitfox.g-foxc.arp.jpg