= Airfoil chord
{title2=$c$}
An \b[airfoil chord] is the straight segment joining an <airfoil leading edge> to its <airfoil trailing edge>; its <length> is conventionally $c$. 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.<Wing Geometry by NASA Glenn Research Center>[Airfoil Geometry: leading/trailing edges, chord and mean camber line]{ref}
If the edge positions in one section plane are $\mathbf r_L$ and $\mathbf r_T$, then
$$
\mathbf d=\mathbf r_T-\mathbf r_L,\qquad c=|\mathbf d|,\qquad
\mathbf e_c=\frac{\mathbf d}{c}.
$$
The chordwise coordinate of a point $\mathbf r$ is its projection $x_c=(\mathbf r-\mathbf r_L)\cdot\mathbf e_c$. Thus a point a quarter of the way along the <airfoil chord> has $x_c/c=1/4$, 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 $(0.02,0.01)$ and $(0.42,-0.02)$ in metres. Then
$$
c=\sqrt{0.40^2+(-0.03)^2}=0.40112\,\mathrm m.
$$
The horizontal projection is $0.40\,\mathrm m$, while the <airfoil chord> is slightly longer. Its quarter point is $(0.12,0.0025)\,\mathrm m$. Using a ruler along the page's horizontal axis rather than along the segment would measure a projection, not $c$.
A finite wing generally has a local <airfoil chord> $c(y)$ varying along its span. For a stipulated unswept trapezoidal planform of span $b=10\,\mathrm m$, root chord $2\,\mathrm m$ and tip chord $1\,\mathrm m$, integrating the linear chord distribution gives <area>
$$
S=\int_{-b/2}^{b/2}c(y)\,dy=\frac{b(2+1)}2=15\,\mathrm{m^2}.
$$
Its span-average chord is $S/b=1.5\,\mathrm m$; 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.<Wing Geometry by NASA Glenn Research Center>[Wing Geometry: span, planform and variable chord]{ref}
For a scaled <wind tunnel> model, a specified pressure-tap position $x_c/c=0.20$ therefore remains a comparable geometric location as $c$ 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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