 Leading edge

The 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.^[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 $r_{\mathrm{LE}}/c=1.1019{\eta }^2$.^[ref] For an original scaled example, $c=0.20\mathrm{m}$ and $\eta =0.12$ give $r_{\mathrm{LE}}=3.1735\mathrm{mm}$. Doubling $c$ with the same normalized profile doubles this radius, while doubling $\eta$ in that family multiplies $r_{\mathrm{LE}}/c$ 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.