Awing is a structure which produces bothlift anddrag while moving through air. Wings are defined by two shape characteristics, anairfoil section and aplanform. Wing efficiency is expressed aslift-to-drag ratio, which compares the benefit of lift with the air resistance of a given wing shape, as it flies.Aerodynamics includes the study of wing performance in air.
The wordwing is a borrowing fromOld Norsevængr.[1] For many centuries, it referred mainly to the foremostlimbs ofbirds (in addition to the architectural aisle). But in recent centuries the word's meaning has extended to include lift producing appendages ofinsects,bats,pterosaurs,boomerangs,some sail boats andaircraft, or the airfoil on arace car.[2]
Condensation in the low-pressure region over the wing of anAirbus A340, passing through humid airFlaps (green) are used in various configurations to increase the wing area and to increase the lift. In conjunction withspoilers (red), flaps maximize drag and minimize lift during the landing roll.
The design and analysis of the wings of aircraft is one of the principal applications of the science ofaerodynamics, which is a branch offluid mechanics. The properties of the airflow around any moving object can be found by solving theNavier–Stokes equations offluid dynamics. Except for simple geometries, these equations are difficult to solve.[3] Simpler explanations can be given.
For a wing to produce "lift", it must be oriented at a suitableangle of attack relative to the flow of air past the wing. When this occurs, the wing deflects the airflow downwards, "turning" the air as it passes the wing. Since the wing exerts a force on the air to change its direction, the air must exert a force on the wing, equal in size but opposite in direction. This force arises from different air pressures that exist on the upper and lower surfaces of the wing.[4][5][6]
Lower-than-ambient air pressure is generated on the top surface of the wing, with a higher-than ambient-pressure on the bottom of the wing. (See:airfoil) These air pressure differences can be either measured using a pressure-measuring device, or can be calculated from the airspeed usingphysical principles –includingBernoulli's principle, which relates changes in air speed to changes in air pressure.
The lower air pressure on the top of the wing generates a smaller downward force on the top of the wing than the upward force generated by the higher air pressure on the bottom of the wing. This gives an upward force on the wing. This force is called the lift generated by the wing.
The different velocities of the air passing by the wing, the air pressure differences, the change in direction of the airflow, and the lift on the wing are different ways of describing how lift is produced so it is possible to calculate lift from any one of the other three. For example, the lift can be calculated from the pressure differences, or from different velocities of the air above and below the wing, or from the total momentum change of the deflected air. Fluid dynamics offers other approaches to solving these problems –all which methods produce the same answer if correctly calculated. Given a particular wing and its velocity through the air, debates over which mathematical approach is themost convenient[citation needed] to use can be mistaken by those not familiar with the study of aerodynamics as differences of opinion about the basic principles of flight.[7]
Wings with an asymmetrical cross-section are the norm insubsonic flight. Wings with a symmetrical cross-section can also generate lift by using a positiveangle of attack to deflect air downward. Symmetrical airfoils have higherstalling speeds thancambered airfoils of the same wing area[8] but are used inaerobatic aircraft as they provide the same flight characteristics whether the aircraft is upright or inverted.[9] Another example comes from sailboats, where the sail is a thin sheet.[10]
For flight speeds near the speed of sound (transonic flight), specific asymmetrical airfoil sections are used to minimize the very pronounced increase in drag associated with airflow near the speed of sound.[11] These airfoils, calledsupercritical airfoils, are flat on top and curved on the bottom.[12]
Trailing-edge devices such asflaps or flaperons (combination of flaps and ailerons)
Winglets to keep wingtip vortices from increasing drag and decreasing lift
Dihedral, or a positive wing angle to the horizontal, increasesspiral stability around the roll axis, whereasanhedral, 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 (usually near the wingtips) to roll the aircraft
Spoilers on the upper surface to increase drag for descent and to reduce lift for more weight on wheels during braking
Wings haveevolved multiple times in history: ininsects,dinosaurs (seebird wing), mammals (seebats), fish, reptiles (seepterosaurs), and plants. Wings of birds, bats, and pterosaurs all evolved from existing limbs, however insect wings evolved as a completely separate structure.[13] Wings facilitated increasedlocomotion, dispersal, and diversification.[14] Various species ofpenguins and other flighted orflightless water birds such asauks,cormorants,guillemots,shearwaters,eider and scoter ducks and divingpetrels are efficient underwater swimmers, and use their wings to propel through water.[15]
Wing forms in nature
Winged tree seeds that causeautorotation in descent
^"...the effect of the wing is to give the air stream a downward velocity component. The reaction force of the deflected air mass must then act on the wing to give it an equal and opposite upward component." In:Halliday, David; Resnick, Robert,Fundamentals of Physics 3rd Edition, John Wiley & Sons, p. 378
^"If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body""Lift from Flow Turning". NASA Glenn Research Center. Retrieved2011-06-29.
^"The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil..."Weltner, Klaus; Ingelman-Sundberg, Martin,Physics of Flight – reviewed, archived fromthe original on 2011-07-19
^"...consider a sail that is nothing but a vertical wing (generating side-force to propel a yacht). ...it is obvious that the distance between the stagnation point and the trailing edge is more or less the same on both sides. This becomes exactly true in the absence of a mast—and clearly the presence of the mast is of no consequence in the generation of lift.Thus, the generation of lift does not require different distances around the upper and lower surfaces." Holger BabinskyHow do Wings Work? Physics Education November 2003,PDF
^John D. Anderson, Jr.Introduction to Flight 4th ed page 271.
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