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surface designed to develop a desired force by reaction with a fluid, especially air, that is flowing across the surface. For example, the fixed wing surfaces of an airplane produce lift, which opposes gravity. Airfoils that are manipulated to produce variable forces are called control surfaces. Ailerons, control surfaces hinged to the trailing edges of wings, can produce rolling, which is rotational motion of the aircraft about a line running through its fuselage, or yawing, which is rotational motion about a line running from the top to the bottom of an aircraft. Modern aircraft have fairly complex arrays of control surfaces, including elevators, a rudder, and flaps. Elevators, which are hinged to the rear of the horizontal airfoil of the tail assembly, are used to produce pitching, which occurs when an airplane in level flight points its nose upward or downward. The rudder, which is hinged to the rear of the vertical airfoil of the tail assembly, is used to produce yawing. Flaps are located near the ailerons to increase lift for takeoff and landing. Spoilers, which can be made to protrude from lifting surfaces to give controlled reduction of lift, often replace ailerons and elevators. In aircraft of the swing-wing type, in which the sweep of the wings is variable, the entire wing can be considered a control surface. Other airfoils include propeller blades and the blades utilized in turbojet engines.


The cross section of a body that is placed in an airstream in order to produce a useful aerodynamic force in the most efficient manner possible. The cross sections of wings, propeller blades, windmill blades, compressor and turbine blades in a jet engine, and hydrofoils on a high-speed ship are examples of airfoils.

The mean camber line of an airfoil (see illustration) is the locus of points halfway between the upper and lower surfaces as measured perpendicular to the mean camber line itself. The most forward and rearward points of the mean camber line are the leading and trailing edges, respectively. The straight line connecting the leading and trailing edges is the chord line of the airfoil, and the distance from the leading to the trailing edge measured along the chord line is simply designated the chord of the airfoil, represented by c. The thickness of the airfoil is the distance from the upper to the lower surface, measured perpendicular to the chord line, and varies with distance along the chord. The maximum thickness, and where it occurs along the chord, is an important design feature of the airfoil. The camber is the maximum distance between the mean camber line and the chord line, measured perpendicular to the chord line. Both the maximum thickness and the camber are usually expressed in terms of a percentage of the chord length; for example, a 12% thick airfoil has a maximum thickness equal to 0.12c.

Airfoil nomenclatureenlarge picture
Airfoil nomenclature

The airfoil may be imagined as part of a wing which projects into and out of the page, stretching to plus and minus infinity. Such a wing, with an infinite span perpendicular to the page, is called an infinite wing. The aerodynamic force on the airfoil, by definition, is the force exerted on a unit span of the infinite wing. For this reason, airfoil data are frequently identified as infinite wing data.

The flow of air (or any fluid) over the airfoil results in an aerodynamic force (per unit span) on the airfoil, denoted by R. The relative wind is the magnitude and direction of the free-stream velocity far ahead of the airfoil. The angle between the chord line and relative wind is defined as the angle of attack of the airfoil, denoted by α. By definition, the component of R perpendicular to the relative wind is the lift, L; similarly, the component of R parallel to the relative wind is the drag, D.

The airfoil may be visualized as being supported by an axis perpendicular to the airfoil, and taken through any point on the airfoil. The airfoil has a tendency to twist about this axis; that is, there is an aerodynamic moment exerted on the airfoil. By definition, the moment is positive or negative if it tends to increase or decrease respectively the angle of attack (that is, if it tends to pitch the airfoil up or down, respectively).


(aerospace engineering)
A body of such shape that the force exerted on it by its motion through a fluid has a larger component normal to the direction of motion than along the direction of motion; examples are the wing of an airplane and the blade of a propeller. Also known as aerofoil.


airfoilclick for a larger image
Relative air flow passing over an airfoil produces lift and drag.
A body of such shape that the force exerted on it by its motion through a fluid has a larger component normal, or perpendicular, to the direction of the motion than along the direction of the motion. This force is in the form of lift or thrust. A wing is a typical airfoil. The shape of an airfoil varies depending upon its use. Wings, control surfaces, and turbine blades are all examples of airfoils. An airfoil section is parallel to an aircraft's longitudinal axis and, in the case of aeroengines and rotors or propellers, perpendicular to a blade's major axis.


(US and Canadian), airfoil
a cross section of an aileron, wing, tailplane, or rotor blade
References in periodicals archive ?
Fowler flap: Improved flap performance could be concluded when the flap produces a Fowler action, where the Fowler action is defined as "the measure of change in position of the leading-edge of the flap in the plane of the chord of the fore element." Also the extended chord could be defined as the cruise airfoil chord plus the Fowler action; this increase in airfoil area increases the achievable lift without significant increase in drag.
Figure 6 shows the 4-digit NACA airfoil developed with the NACA 4-digit airfoil generator (Divahar Jayaraman, 2009).
The geometry of each element was further determined using discrete variable selection, from which the optimization solver could choose from a database of known airfoil geometries.
The structural parameters of the airfoil are chosen to be [[bar.x].sub.[alpha]] = 1.8, [[bar.r].sup.2.sub.[alpha]] = 3.48, [bar.a] = -2.0, [[omega].sub.h] = 100 rad/sec, [[omega].sub.[alpha]] = 100 rad/sec, and [mu] = 60, respectively.
The effect of circulation was explored by comparing results for two angles of attack with the same symmetrical airfoil, low speed, and separation distance in order to eliminate any large effects due to thickness or compressibility.
The airfoils configuration for blades is the key factor for high efficiency of the turbine to convert from kinetic to mechanical energy.
However, the detailed mechanism of the rod-airfoil noise generation is still seldom investigated, such as the configuration of the rod and the distance between the rod and the airfoil, which are very important to the vortex-structure interaction noise reduction.
In the NACA 0012 airfoil and WINPhase 10 wind turbine cases (described later), the flow field around the airfoil or the small wind turbine is simulated in the CFD component.
Thus, turbine airfoils subjected to the hottest gas flows take the form of elaborate superalloy investment castings to accommodate the intricate internal passages and surface hole patterns necessary to channel and direct cooling air (bled from the compressor) within and over exterior surfaces of the superalloy airfoil structure.
Windmill rotor blades today have airfoil cross-sections which reduce drag and increase the performance.
LOS ANGELES (CyHAN)- In partnership with Aerion Corporation of Reno, Nevada, NASA's Dryden Flight Research Center's tested supersonic airflow over a small experimental airfoil design on its F-15B Test Bed aircraft during the spring of 2013.