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An airfoil thickness convention specifies which two surface points are paired, the direction used to measure their separation and how the result is normalized. An airfoil thickness-to-chord ratio is reproducible only after these choices are fixed. A vertical gap at common chordwise coordinate is not generally the same quantity as thickness constructed normal to a sloping airfoil mean camber line.

For the NACA four-digit airfoil construction, let a mean-line station be $(x,y_c)$, its tangent make angle $\theta$ with the airfoil chord, and the prescribed half-thickness be $y_t$. The paired upper and lower points satisfyTheir separation vector is $2y_t(-\sin \theta ,\cos \theta )$, with Euclidean norm $2y_t$. Their chordwise coordinates differ when the mean line slopes. These normal offsets are the construction specified in the originating aerodynamic geometry notes.^[ref]

Two different vertical measurements must also be distinguished. The vertical component of that paired separation is $2y_t\cos \theta$. A vertical line cutting two locally parallel straight surfaces instead spans $2y_t/\cos \theta$. For an original local model with true normal thickness 10 mm and $\theta =30{}^{\circ }$, these values are 8.660 mm and 11.547 mm, respectively. The first projects a selected segment; the second intersects surfaces at a common horizontal coordinate. For curved surfaces, the actual intersection must be calculated rather than assuming the local straight-line approximation is exact.

At a zero-slope station the three quantities coincide. This explains why a maximum-camber example can conceal a convention difference that becomes visible elsewhere. Taking the maximum over a profile also requires stating which thickness function is being maximized; a family designation is not a complete recipe for every possible projected maximum.

A coordinate-based geometry check should therefore retain the airfoil chord, mean-line rule, surface pairing and measurement direction. A photograph adds a further projection problem unless its viewing direction and scale are controlled. These are geometric qualifications; choosing a thickness convention does not by itself predict lift, drag or aerodynamic stall.

The airfoil thickness-to-chord ratio is the dimensionless maximum section thickness divided by the airfoil chord:Its percentage form is $100\eta$. The airfoil thickness convention must specify how upper and lower surface points are paired and in which direction their separation is measured. For the standard NACA four-digit airfoil construction, the prescribed thickness envelope is placed normal to the airfoil mean camber line, not simply added vertically everywhere.^[ref]

For an original scale example, $t_{\max }=24\mathrm{mm}$ and $c=200\mathrm{mm}$ give $\eta =0.12$, or twelve percent. Scaling every coordinate by three gives $72\mathrm{mm}$ thickness and $600\mathrm{mm}$ chord, with the same ratio. Holding the chord fixed while adding $0.2\mathrm{mm}$ thickness instead gives $\eta =0.121$: the percentage thickness rises by $0.1$ percentage point.

The differential relationshows why an identical fractional scale error in both lengths cancels to first order, while an independent thickness error survives. Exact common scaling cancels in the quotient itself. This is the geometric counterpart of a ratiometric measurement.

The scalar $\eta$ does not specify where the thickest station lies, the airfoil leading-edge radius, or the airfoil camber. Two sections can therefore share $\eta$ while having different pressure distributions and structural envelopes. The early NACA investigation varied thickness and mean-line form independently in a variable-density wind tunnel, making those distinctions experimentally useful.^[ref]

A geometric check of a manufactured section should report the chord reference, the thickness direction and the coordinate scale. Measuring a projected thickness in a tilted image without correcting its direction can change the reported ratio even when the physical section is unchanged.

NACA 2412 is a NACA four-digit airfoil with maximum airfoil camber two percent of the airfoil chord, located at forty percent chord, and nominal maximum airfoil thickness-to-chord ratio twelve percent. It combines a nonzero airfoil mean camber line with the same nominal thickness parameter as NACA 0012.^[ref]

The construction is developed in NACA four-digit airfoil. A new calculation away from the maximum-camber station illustrates its normal offsets. At $x/c=0.2$, the mean line gives $y_c/c=0.015$ and $d{y}_c/dx=0.05$. The finite-edge thickness polynomial gives $y_t/c\approx 0.05737543$. With $\theta =\arctan (0.05)$, the normalized surface coordinates areTheir midpoint is exactly the construction point $(0.2,0.015)$ before numerical rounding. Their horizontal coordinates differ because thickness is placed along the local normal vector. Simply adding and subtracting $y_t$ vertically would produce a different geometry at this sloping station.

For a $0.50\mathrm{m}$ chord, these paired points are separated by $2y_t\approx 57.3754\mathrm{mm}$. This local normal separation is smaller than the nominal $60\mathrm{mm}$ maximum thickness. Airfoil thickness convention explains why a projected or same-horizontal-station measurement must be distinguished from this paired normal distance.

The two-percent camber is not a two-degree incidence or a prescribed lift coefficient. Its aerodynamic effect depends on the mean-line slopes and the flow. A wind tunnel comparison with NACA 0012 can hold nominal thickness and Reynolds number fixed while observing the change due to airfoil camber. Such comparisons follow the systematic separation of thickness and mean-line form in the original NACA investigation.^[ref]

NACA 4412 is a NACA four-digit airfoil with maximum airfoil camber $m=0.04$ of the airfoil chord, its maximum at $p=0.4$ chord, and nominal maximum airfoil thickness-to-chord ratio $\eta =0.12$. Thus the first two digits specify a mean-line shape, while the last two specify a thickness-envelope parameter. The designation is geometric rather than a universal measured lift coefficient.^[ref]

For an original $c=0.50\mathrm{m}$ section, the maximum mean-line displacement is 20 mm at a station 200 mm aft of the mean-line origin. The nominal twelve-percent thickness is 60 mm. That thickness does not occur at every station and is not the same quantity as maximum camber.

A calculation away from the maximum-camber station makes the airfoil thickness convention visible. At the mean-line parameter $\xi =x/c=0.2$, the standard construction givesUsing the finite-trailing-edge thickness polynomial and normal offsets, the upper and lower points areTheir different horizontal coordinates are part of the construction. Adding and subtracting the same half-thickness vertically would create a different profile at this station.

Compared with NACA 2412, the mean-line amplitude is doubled while $p$ and the prescribed thickness envelope are unchanged. Thin airfoil theory therefore doubles the camber contribution to its modeled zero-lift angle of attack; direct evaluation gives about $-4.1545{}^{\circ }$ for this ideal mean line. That number follows from the ideal model and is not a wind-tunnel measurement. It does not establish a universal aerodynamic stall angle or drag curve.

The physical motivation for a related family was to vary thickness and mean-line form systematically, then measure the resulting aerodynamic behavior under controlled conditions. The original NACA report describes such a comparison in a variable-density wind tunnel at large Reynolds number.^[ref] A performance statement about NACA 4412 must still identify flow conditions, surface condition and the measured quantity.