A wing is a type of fin that produces lift, while moving through air or some other fluid. As such, wings have streamlinedcross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expressed as its lift-to-drag ratio. The lift a wing generates at a given speed and angle of attack can be one to two orders of magnitude greater than the total dragon the wing. A high lift-to-drag ratio requires a significantly smaller thrust to propel the wings through the air at sufficient lift. Lifting structures used in water, include various foils, such as hydrofoils. Hydrodynamics is the governing science, rather than aerodynamics. Applications of underwater foils occur in hydroplanes, sailboats and submarines.
Etymology and usage
For many centuries, the word "wing", from the Old Norse vængr, referred mainly to the foremost limbs of birds. But in recent centuries the word's meaning has extended to include lift producing appendages of insects, bats, pterosaurs, boomerangs, some sail boats and aircraft, or the inverted airfoil on a race car that generates a downward force to increase traction.
Aerodynamics
The design and analysis of the wings of aircraft is one of the principal applications of the science of aerodynamics, which is a branch of fluid mechanics. In principle, the properties of the airflow around any moving object can be found by solving the Navier-Stokes equations of fluid dynamics. However, except for simple geometries these equations are notoriously difficult to solve and simpler equations are used. For a wing to produce lift, it must be oriented at a suitable angle of attack. When this occurs, the wing deflects the airflow downwards as it passes the wing. Since the wing exerts a force on the air to change its direction, the air must also exert an equal and opposite force on the wing, resulting in different air pressures over the surface of the wing. A region of lower-than-normal air pressure is generated over the top surface of the wing, with a higher pressure on the bottom of the wing. These air pressure differences can be measured directly using instrumentation or can be calculated from the airspeed distribution using basic physical principles such as Bernoulli's principle, which relates changes in air speed to changes in air pressure. It is possible to calculate lift from: the pressure differences, the different velocities of the air above and below the wing, or from the total momentum change of the deflected air. Debates over which mathematical approach is the most convenient to use can be mistaken as differences of opinion about the basic principles of flight.
Cross-sectional shape
An airfoil or aerofoil is the shape of a wing, blade, or sail. Wings with an asymmetrical cross section are the norm in subsonic flight. Wings with a symmetrical cross section can also generate lift by using a positive angle of attack to deflect air downward. Symmetrical airfoils have higher stalling speeds than cambered airfoils of the same wing area but are used in aerobatic aircraft as they provide practical performance whether the aircraft is upright or inverted. Another example comes from sailboats, where the sail is a thin membrane with no path-length difference between one side and the other. For flight speeds near the speed of sound, airfoils with complex asymmetrical shapes are used to minimize the drastic increase in drag associated with airflow near the speed of sound. Such airfoils, called supercritical airfoils, are flat on top and curved on the bottom.
Leading-edge devices such as slats, slots, or extensions
Trailing-edge devices such as flaps or flaperons
Winglets to keep wingtip vortices from increasing drag and decreasing lift
Dihedral, or a positive wing angle to the horizontal, increases spiral stability around the roll axis, whereas anhedral, or a negative wing angle to the horizontal, decreases spiral stability.
Aircraft wings may have various devices, such as flaps or slats that the pilot uses to modify the shape and surface area of the wing to change its operating characteristics in flight.
Ailerons to roll the aircraft clockwise or counterclockwise about its long axis
Spoilers on the upper surface to disrupt the lift and to provide additional traction to an aircraft that has just landed but is still moving.
Hang gliders, which use wings ranging from fully flexible, flexible, to rigid
Kites, which use a variety of lifting surfaces
Flying model airplanes
Helicopters, which use a rotating wing with a variable pitch angle to provide directional forces
Propellers, whose blades generate lift for propulsion.
The NASASpace Shuttle, which uses its wings only to glide during its descent to a runway. These types of aircraft are called spaceplanes.
Some racing cars, especially Formula One cars, which use upside-down wings to provide greater traction at high speeds
Sailboats, which use sails as vertical wings with variable fullness and direction to move across water
In nature
In nature, wings have evolved in insects, pterosaurs, dinosaurs, and mammals as a means of locomotion. Various species of penguins and other flighted or flightlesswater birds such as auks, cormorants, guillemots, shearwaters, eider and scoter ducks and diving petrels are avid swimmers, and use their wings to propel through water.
;Wing forms in nature
Tensile structures
In 1948, Francis Rogallo invented a kite-like tensile wing supported by inflated or rigid struts, which ushered in new possibilities for aircraft. Near in time, Domina Jalbert invented flexible un-sparred ram-air airfoiled thick wings. These two new branches of wings have been since extensively studied and applied in new branches of aircraft, especially altering the personal recreational aviation landscape.
