 Leading-edge radius estimation from sampled coordinates

Leading-edge radius estimation from sampled coordinates infers local nose curvature from a finite set of profile measurements. Unlike reading a radius from a drawing, it must account for the coordinate scale, the fitted nose location and the range of points treated as a local model.

Choose coordinates with a smooth nose at the origin and a vertical tangent. A local model iswhere $r_0$ is the true airfoil leading-edge radius. Ignoring the quartic term gives the quotient estimate $\hat{r}=y^2/(2x)$. Even exact coordinates then givefor the displayed quadratic-plus-quartic model. Moving closer to the nose reduces this model bias, but also makes the horizontal displacement small relative to a fixed coordinate error.

For an original numerical example, set $r_0=3\mathrm{mm}$ and $B=10^7{\mathrm{m}}^{-3}$. At $y=0.5\mathrm{mm}$, the exact horizontal displacement is $42.2917\mu \mathrm{m}$ and the quotient gives $2.9557\mathrm{mm}$. If $y$ is treated as exact while $x$ has an absolute error bounded by $10\mu \mathrm{m}$, the inferred radius ranges from about $2.3904$ to $3.8710\mathrm{mm}$. At $y=1.5\mathrm{mm}$, the coordinate-error range narrows to about $2.5825$–$2.7068\mathrm{mm}$, but the exact-coordinate quotient is already biased down to $2.6432\mathrm{mm}$.

A least squares fit can combine several points. If the origin and $y_i$ are known and errors are assigned to $x_i$, minimizing ${\sum }_i{w}_i(x_i-A{y}_i^2{)}^2$ with positive weights givesprovided the denominator and fitted coefficient are positive. This fit does not remove an omitted quartic term; the leading-edge radius fitting window controls the tradeoff between local-model error and coordinate noise. An uncertain nose origin or appreciable errors in both coordinates require a correspondingly expanded fitting model.

A practical geometry check uses calibrated profile coordinates, tests several fitting windows and reports the variation alongside measurement uncertainty. Analytic profiles provide an implementation check: the four-digit family has the known relation $r_{\mathrm{LE}}/c\simeq 1.1019{\eta }^2$.^[ref] That family relation validates a calculation on a known profile; it does not determine the radius of an unrelated measured nose.