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A wing is a surface used to produce lift and therefore flight, for travel in the air or another gaseous medium. The wing shape is usually an airfoil. The first use of the word was for the foremost limbs of birds, but has been extended to include the wings of insects, bats and pterosaurs and also aircraft.
A wing is a device for generating lift. Its 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.
[edit] Design features
Aeroplane wings may feature some of the following:
- A rounded leading edge cross-section
- A sharp trailing edge cross-section
- Leading-edge devices such as slats, slots, or extensions
- Trailing-edge devices such as flaps
- Ailerons (usually near the wingtips) to provide roll control
- Spoilers on the upper surface to disrupt lift and additional roll control
- Vortex generators to help prevent flow separation in transonic flow
- Wing fences to keep flow attached to the wing by stopping boundary layer separation from spreading
- Dihedral, or a positive wing angle to the horizontal. This gives inherent stability in roll. Anhedral, or a negative wing angle to the horizontal, has a destabilising effect
- Folding wings allow more aircraft to be carried in the confined space of the hangar of an aircraft carrier.
[edit] Science of wings
The science of wings is one of the principal applications of the science of aerodynamics.
Internal mechanical construction of a generic mono spar wing. Black=solid, red=tube used for the spar, green=foam or wood or honeycomb or sheet metal used for the ribs. The leading edge gives torsional stiffness. The trailing can either have a flexible skin, which does not break under wing bending (bird-like) or have a stiff skin (made of carbon fiber or aluminum, jet-like) which is prevented from bucking by span-wise stringers.
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. Debates over which mathematical approach is the more convenient can be wrongly perceived as differences of opinion about the principles of flight and often create unnecessary confusion in the mind of the layman.
For a more detailed coverage see lift (force).
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. This is not the case, thin flat wings can produce lift efficiently and aircraft with cambered wings can fly inverted as long as the nose of the aircraft is pointed high enough so as to present the wing at a positive angle of attack to the airflow.
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 wing’s cross sectional shape is the need to keep the air flowing smoothly over the entire surface for the most efficient operation. In particular, there is a requirement to prevent the low-pressure gradient that accelerates the air down the back of the wing becoming too great and effectively “sucking” the air off the surface of the wing. If this happens the wing surface from that point backwards becomes substantially ineffective.
The shape chosen by the designer is a compromise dependent upon the intended operational ranges of airspeed, angles of attack and wing loadings. Usually aircraft wings have devices, such as flaps, which allow the pilot to modify shape and surface area of the wing to be able to change its operating characteristics in flight.
The science of wings applies in other areas beyond conventional fixed-wing aircraft, including:
- Helicopters which use a rotating wing with a variable pitch or angle to provide a directional force
- The space shuttle which uses its wings only for lift during its descent
- Sailing boats which use sails as vertical wings with variable fullness and direction to move across water.
Structures with the same purpose as wings, but designed to operate in liquid media, are generally called fins or hydroplanes, with hydrodynamics as the governing science. Applications arise in craft such as hydrofoils and submarines. Sailing boats use both fins and wings.
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[edit] External links
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