= Airfoil thickness convention
An \b[<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 satisfy
$$
(x_u,y_u)=(x-y_t\sin\theta,\ y_c+y_t\cos\theta),\qquad
(x_l,y_l)=(x+y_t\sin\theta,\ y_c-y_t\cos\theta).
$$
Their 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.<Applied Computational Aerodynamics by W. H. Mason>[Appendix A, p. A-1, equations (A-1)–(A-3)]{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>.
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