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An airfoil mean camber line is the reference curve midway between the upper and lower surfaces of an airfoil, using a specified surface-pairing convention. It separates the bending of the profile from its thickness. In a symmetric airfoil, the mean line lies on the airfoil chord.^[ref]

For the normal-offset construction used in a NACA four-digit airfoil, let the mean-line point be ${\mathbf{r}}_c=(x,y_c(x))$, let $\mathbf{n}$ be its unit normal vector, and let $y_t$ be the local half-thickness. The paired surface points satisfyAdding and subtracting gives ${\mathbf{r}}_c=({\mathbf{r}}_u+{\mathbf{r}}_l)/2$ and $2y_t=\Vert {\mathbf{r}}_u-{\mathbf{r}}_l\Vert$. The pair shares a construction parameter, not necessarily the same physical chordwise coordinate.^[ref]

For an original local example, take paired coordinates ${\mathbf{r}}_u=(0.30,0.08)\mathrm{m}$ and ${\mathbf{r}}_l=(0.32,0.02)\mathrm{m}$. Their midpoint is $(0.31,0.05)\mathrm{m}$, and the half-thickness is $\sqrt{0.01^2+0.03^2}\approx 0.031623\mathrm{m}$. The separation direction is proportional to $(-1,3)$, so the corresponding tangent direction is proportional to $(3,1)$. It would be incorrect to call either endpoint's horizontal coordinate the common construction station.

For surface data tabulated as two graphs at the same $x$, the vertical midpoint $(y_u(x)+y_l(x))/2$ defines a useful alternative convention. It need not exactly invert a normal-offset construction with a sloping mean line. Extracting a mean line from measured coordinates must therefore identify the pairing and interpolation rule. Airfoil camber summarizes its departure from the chord; directional measurement of trailing-edge thickness shows the related directional ambiguity at the rear endpoint.

Airfoil trailing-edge thickness measures the separation of the upper and lower surfaces at a specified terminal reference. A sharp ideal trailing edge has zero thickness. A finite-edge model needs both the endpoint convention and the measurement direction; these choices matter when comparing coordinates or manufacturing tolerances.

For directional measurement of trailing-edge thickness, choose terminal points ${\mathbf{r}}_u,{\mathbf{r}}_l$ and a unit normal vector $\mathbf{n}$. The reported projected gap isFor a NACA four-digit airfoil built with normal offsets, the full endpoint separation has length $2y_t(1)$. Its component normal to the airfoil chord is $2y_t(1)|\cos {\theta }_{\mathrm{TE}}|$, while their chordwise separation is $2y_t(1)|\sin {\theta }_{\mathrm{TE}}|$. Comparing a full normal gap to a projected gap silently changes the quantity being measured.^[ref]

For a symmetric section with $c=0.20\mathrm{m}$ and $\eta =0.12$, the standard polynomial gives $t_{\mathrm{TE}}=0.504\mathrm{mm}$ and $t_{\mathrm{TE}}/c=0.00252$. The same absolute $0.504\mathrm{mm}$ gap on a $0.40\mathrm{m}$ section would have only half that normalized thickness. Absolute and normalized tolerances answer different questions.

Measurement uncertainty also depends on shared coordinate errors. If signed endpoint heights give $t=y_u-y_l$, thenAn identical coordinate-origin shift cancels from the difference, whereas independent endpoint errors do not. Stipulated independent $10\mu \mathrm{m}$ standard uncertainties give $u_t=14.14\mu \mathrm{m}$. A common translation alone contributes zero. This is a covariance calculation, not evidence that every instrument error is common.

Directional measurement of trailing-edge thickness specifies which component of the gap between two terminal surface points is reported. A full endpoint separation, a chord-normal projection and a mean-line-normal thickness can be different quantities. The convention is part of the geometric measurement, not an adjustment to an aerodynamic result.

For selected endpoints ${\mathbf{r}}_u,{\mathbf{r}}_l$, write $\Delta \mathbf{r}={\mathbf{r}}_u-{\mathbf{r}}_l$ and choose a unit vector $\mathbf{n}$ in the measurement direction. ThenThe last inequality is the Cauchy-Schwarz inequality. Equality requires the measurement direction to align with the endpoint gap, apart from its sign. If that alignment is off by an angle $\varphi$, the reading is $g|\cos \varphi |$.

For an original coordinate example, let $\Delta \mathbf{r}=(0.30,0.40)\mathrm{mm}$. The full gap is $0.50\mathrm{mm}$; a vertical measurement gives $0.40\mathrm{mm}$ and a horizontal one gives $0.30\mathrm{mm}$. The direction $(3/5,4/5)$ recovers the full $0.50\mathrm{mm}$. A separate one-millimetre gap measured with $20{}^{\circ }$ misalignment gives $0.939693\mathrm{mm}$, about six percent low.

Changing coordinate frames is harmless when both the endpoints and direction are transformed consistently. For a rotation matrix $Q$, $(Q\Delta \mathbf{r})\cdot (Q\mathbf{n})=\Delta \mathbf{r}\cdot \mathbf{n}$. Using the page's vertical direction after rotating the profile, however, changes the measurement direction.

The NACA four-digit airfoil equations place the two terminal points on opposite sides of the airfoil mean camber line along its normal. When that mean line slopes at the endpoint, the gap has both chordwise and transverse components.^[ref] A profile-image or coordinate comparison should therefore identify the terminal-point pairing, the chord reference and the unit measurement direction. Interpolating both surfaces at the same horizontal coordinate is not automatically the same endpoint measurement.

A NACA four-digit airfoil encodes three quantities: maximum airfoil camber $m$ in hundredths of the airfoil chord, its position $p$ in tenths of the airfoil chord, and nominal maximum airfoil thickness-to-chord ratio $\eta$ in hundredths. Thus NACA 2412 means $m=0.02$, $p=0.4$ and $\eta =0.12$; NACA 0012 has zero airfoil camber and the same nominal thickness ratio.^[ref]

With $\xi =x/c$, the airfoil mean camber line isFor a symmetric airfoil, take $y_c=0$ directly; do not divide by the zero position parameter. The half-thickness $y_t$ follows the polynomial in trailing edge. If $\theta =\arctan (d{y}_c/dx)$, the upper/lower coordinates areThickness is placed along a normal vector to the mean line, rather than simply added vertically.^[ref]

For an original NACA 2412 calculation at $\xi =0.4$, the mean-line slope is zero, $y_c/c=0.02$ and the standard finite-edge polynomial gives $y_t/c\simeq 0.0580301$. Hence the surface points are approximately $(0.4,0.0780301)$ and $(0.4,-0.0380301)$ in chord-normalized coordinates. A 12-percent designation does not mean the section is 12 percent thick at every chordwise position.

The trailing-edge convention must also be specified. The usual coefficient $-0.1015$ leaves a small finite thickness; the modified $-0.1036$ closes the endpoint. With a sloping mean line, the upper and lower endpoint coordinates can differ in both components, so directional measurement of trailing-edge thickness must state which separation it reports. These constructions define geometry, not a universal aerodynamic performance curve.