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(aerospace engineering) A major airfoil. An airfoil on the side of an airplane's fuselage or cockpit, paired off by one on the other side, the two providing the principal lift for the airplane.
(geology) vesicle
(zoology) Any of the paired appendages serving as organs of flight on many animals.

A lifting surface of a heavier-than-air object, either bird or airplane. Lift is created by a pressure difference between the upper and lower surfaces of the wing, the average pressure on the upper surface being lower. The average velocity on the upper surface is larger than on the lower surface, resulting in the lifting pressure difference in accordance with Bernoulli's theorem. The velocity difference is caused by having a greater curvature on the wing upper surface, or a positive wing angle of attack (that is, leading edge up), or both. The amount of lift is proportional to the angle of attack, the wing area, the air density, and the square of the velocity. See also Aerodynamic force; Bernoulli's theorem; Subsonic flight.

The important physical characteristics of a wing are wing area, measured in the plan or top view, the span or distance from the left wing tip to the right wing tip, the aspect ratio, the taper ratio, and the thickness ratios of the airfoils. The aspect ratio is the ratio of the span to the average chord. The chord of a wing is the distance from the leading edge to the trailing edge. In all but the simplest airplanes, the chord varies along the span, being largest at the root. The taper ratio is the ratio of the tip chord to the root chord. Airfoils are the cross-sectional shapes of wings as defined by the intersections with planes parallel to the oncoming airstream and perpendicular to the plane of the wing surface. The thickness ratio is the ratio of the maximum thickness of an airfoil to the chord and often varies between the root and tip. If an airfoil has greater curvature on the upper surface than on the lower surface, the mean line midway between the upper and lower surfaces is curved. The amount of this curvature is called camber. All of these wing characteristics affect flight efficiency and must be carefully chosen. See also Aircraft design; Airfoil.

There is a particular angle of attack of a wing that provides the necessary lift with the least drag. Wing area selection attempts to have the airplane fly at this angle of attack at the desired speed and within the range of desirable altitudes. Of course, takeoff and landing fields are important in area selection. A larger wing area permits slower flight, which is associated with shorter takeoff acceleration distances and shorter stopping distances after landing.

Wings must be designed to stall safely. Above the maximum angle of attack at which the flow will remain smoothly attached to the wing surface, there is a sharp loss of lift and a large increase in drag. This is known as the stall, a condition that is normally avoided. Wings are designed to stall near the root first so that the tendency to roll sharply is minimized and the ailerons on the outer wing remain effective. This is done by varying the airfoil sections and thickness ratios across the span in a careful manner.

The flight of airplanes is controlled primarily by varying the magnitude and direction of the wing lift and by varying the thrust or power contributed by the engines. An important aspect of flight is the speed, which is controlled by adjusting the wing angle of attack with respect to the oncoming airstream. The angle of attack is adjusted by varying the angle of the elevator, a control surface usually located on the horizontal tail. After adjusting the flight speed by using the elevators, the angle of the flight path, zero for level flight, is controlled by setting the engine thrust. See also Elevator (aircraft).

The direction of flight is basically controlled by the angle of bank of the wing. When the wing is level and the resultant force, or lift, is vertical, the airplane flies in a straight line. Ailerons are trailing-edge flaps on the outer part of the wing that deflect in opposite directions on the left and right sides of the airplane. When the airplane banks or rolls because of the deflection of ailerons, the lift force is tilted toward the side since it remains perpendicular to the banked wing. This provides a sidewise force which accelerates the airplane in a direction perpendicular to the flight path and thereby curves the flight path. Application of the rudder keeps the airplane pointed into the wind during the turn, although the vertical tail will do much of that job even without rudder deflection See also Aileron; Elevon.

High-speed aircraft also use spoilers, essentially plates ahead of the flaps, to lose lift on only one side to roll the airplane. These spoilers are also used symmetrically to slow down an airplane and increase the rate of descent. Spoilers are also used after touchdown to quickly reduce lift and dump the weight on the braked wheels, thereby greatly improving the stopping effectiveness.

Trailing-edge flaps and leading-edge slats in (a) cruise, (b) takeoff, and (c) landing settings.

Wings also carry moving elements that serve lift-increase functions. Trailing-edge flaps (see illustration) inboard of the ailerons increase the lift that can be carried before the stall. Thus the minimum flight speed can be decreased. Leading-edge flaps and slats (see illustration) are used to increase the angle of attack for stall and further reduce the minimum flight speed. The primary purpose of increasing the lift capability and obtaining the lowest flight speed is to reduce the required field lengths for takeoff and landing or to reduce the necessary wing area. See also Aileron; Elevator (aircraft); Elevon; Flight controls.

Wings also serve as fuel tanks, a function that sometimes sets the minimum wing area—especially on small aircraft such as executive jets. Wing thickness ratio is important in determining the volume available for fuel within the wing. Wings often house all or part of the landing gear. Engines are mounted on the wing of many aircraft. See also Aircraft engine; Airplane; Landing gear.

(WINdows Games) A programming interface (API) that lets Windows application developers access the video frame buffer directly. It allows game programs to be written to run as fast in Windows as they do under DOS.

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