An airfoil chord is the straight segment joining an airfoil leading edge to its airfoil trailing edge; its length is conventionally . The airfoil chord is a geometric reference, not the curved distance along either surface or the curved mean camber line. The chord direction supplies the reference for angle of attack, while its length supplies a scale for reporting an airfoil's thickness and positions.[ref]
Wing Geometry by NASA Glenn Research Center. Airfoil Geometry: leading/trailing edges, chord and mean camber line
If the edge positions in one section plane are and , thenThe chordwise coordinate of a point is its projection . Thus a point a quarter of the way along the airfoil chord has , regardless of how the profile is rotated in a drawing. The aerodynamic center's approximate quarter-chord location refers to this straight geometric reference.
For an original tilted section, choose edge positions and in metres. ThenThe horizontal projection is , while the airfoil chord is slightly longer. Its quarter point is . Using a ruler along the page's horizontal axis rather than along the segment would measure a projection, not .
A finite wing generally has a local airfoil chord varying along its span. For a stipulated unswept trapezoidal planform of span , root chord and tip chord , integrating the linear chord distribution gives areaIts span-average chord is ; that average is not the chord at every section. The mean aerodynamic chord is a different wing reference and should be identified explicitly when used. NASA's definitions distinguish section geometry from this spanwise wing geometry.[ref]
For a scaled wind tunnel model, a specified pressure-tap position therefore remains a comparable geometric location as changes. Equal normalized geometry alone does not establish equal flow: comparisons must also consider Reynolds number, Mach number and the relevant boundary conditions.
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