= Leading edge
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= Airfoil leading edge
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The \b[leading edge] is the front of an <airfoil> in its intended direction of motion through the surrounding <fluid>. Together with the <airfoil trailing edge>, it defines the reference <airfoil chord>. It is a feature of the geometry; the <stagnation point> of a particular flow need not stay at that exact location as the <angle of attack> changes.<Wing Geometry by NASA Glenn Research Center>[Airfoil Geometry, leading edge and chord reference]{ref}
The <airfoil leading-edge radius> quantifies local nose shape. For a smooth nose represented near its foremost point by $x=y^2/(2r)$, the <curvature> there is $1/r$. The same overall <airfoil chord> and maximum thickness do not by themselves fix that local shape.
For a <NACA four-digit airfoil> with thickness-to-chord ratio $\eta$, the standard nose-radius relation is $r_{\rm LE}/c=1.1019\eta^2$.<Applied Computational Aerodynamics by W. H. Mason>[Appendix A, p. A-4, equation (A-5)]{ref} For an original scaled example, $c=0.20\,\mathrm m$ and $\eta=0.12$ give $r_{\rm LE}=3.1735\,\mathrm{mm}$. Doubling $c$ with the same normalized profile doubles this <radius>, while doubling $\eta$ in that family multiplies $r_{\rm LE}/c$ by four.
An informative <wind tunnel> comparison preserves the intended <airfoil leading edge> geometry while measuring surface <pressure> and total <lift> over several <angles of attack>. Matching <Reynolds number> and reporting surface condition helps distinguish a shape effect from a change in the test conditions. A nose drawing alone does not determine the onset of <flow separation> or <aerodynamic stall>.
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