= Trailing edge
{wiki}
= Airfoil trailing edge
{synonym}
The \b[trailing edge] is the rear region of an <airfoil> where the upper and lower surfaces approach or meet. The <airfoil chord> connects its chosen section reference to the <airfoil leading edge>. A mathematical profile may end at a sharp point, but a specified family can retain a small finite <airfoil trailing-edge thickness>.<Wing Geometry by NASA Glenn Research Center>[Airfoil Geometry, trailing edge and chord]{ref}
For the symmetric thickness envelope of a <NACA four-digit airfoil>, let $\xi=x/c$ and let $\eta$ be the maximum thickness-to-chord ratio. One standard form is
$$
\frac{y_t}{c}=5\eta\left(0.2969\sqrt\xi-0.1260\xi-0.3516\xi^2+0.2843\xi^3-0.1015\xi^4\right).
$$
Here $y_t$ is half the thickness, so the full terminal thickness is $2y_t(1)$. This polynomial leaves a finite rear gap.<Applied Computational Aerodynamics by W. H. Mason>[Appendix A, pp. A-3–A-4, equation (A-4) and the finite-thickness observation]{ref} Substitution gives
$$
\frac{t_{\rm TE}}c=10\eta(0.0021)=0.021\eta.
$$
Thus the original example $c=0.20\,\mathrm m$, $\eta=0.12$ has \b[$t_{\rm TE}=0.504\,\mathrm{mm}$]. Replacing the final coefficient $0.1015$ by $0.1036$ makes the endpoint thickness zero. That is a different geometric specification, not a numerical rounding of the same profile.
\Image[/airfoil-edge-geometry.png]
{title=Symmetric four-digit airfoil geometry and the difference between finite and closed trailing edges. Original calculation from the stated thickness polynomial.}
For a sharp edge, the <Kutta condition> selects an attached-flow solution with smooth departure and finite rear-edge <velocity>, helping determine <circulation> and <lift>. It does not require fluid parcels passing above and below the <airfoil> to arrive simultaneously. For a finite-thickness edge, a model must also represent the rear region and its <wake>; simply imposing a sharp-edge condition does not remove that geometric difference.
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