A wing is a surface used to produce lift for flight through the air or another gaseous or fluid medium. The cross-sectional shape of a wing is referred to as an airfoil. The word originally referred only to the foremost limbs of birds, but has been extended to include the wings of insects, bats, pterosaurs, and aircraft. The term is also applied to an inverted wing used to generate down-force in auto racing.
A wingâ€™s aerodynamic quality is expressed as a Lift-to-drag ratio. The lift generated by a wing at a given speed and angle of attack can be 1-2 orders of magnitude greater than the drag. This means that a significantly smaller thrust force can be applied to propel the wing through the air in order to obtain a specified lift.
The science of wings is one of the principal applications of the science of aerodynamics. In order for a wing to produce lift it has to be at a positive angle to the airflow. In that case a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed. On the underside of the wing a high pressure region forms accelerating the air there downwards out of the path of the oncoming wing. The pressure difference between these two regions produces an upwards force on the wing, called lift.
The pressure differences, the acceleration of the air and the lift on the wing are intrinsically one mechanism. It is therefore possible to derive the value of one by calculating another. For example lift can be calculated by reference to the pressure differences or by calculating the energy used to accelerate the air. Both approaches will result in the same answer if done correctly.
A common misconception is that it is the shape of the wing that is essential to generate lift by having a longer path on the top rather than the underside. While wings with this shape are always used in subsonic aircraft and sailing, symmetrically shaped wings can also generate lift by having a positive angle of attack and deflecting air downward. The symmetric approach is less efficient, lacking the lift provided by cambered wings at zero angle of attack.
The common aerofoil shape of wings is due to a large number of factors many of them not at all related to aerodynamic issues, for example wings need strength and thus need to be thick enough to contain structural members. They also need room to contain items such as fuel, control mechanisms and retracted undercarriage. The primary aerodynamic input to the wingâ€™s cross sectional shape is the need to keep the air flowing smoothly over the entire surface for the most efficient operation.