Curve fitting to airfoil coordinates

If you have a CAD program that will allow you to impose tangency
constraints as well as point location (like CATIA or UniGraphics), you
can force the curve to be vertical at the leading edge.

In Xfoil you can effectively do this by placing a point just above
the 0,0 LE point, and placing another point just below. For example,
change the three points at the LE...

0.001070 0.004620
0.000000 0.000000
0.001070 -0.001450

to...

0.001070 0.004620
0.0 0.00001
0.000000 0.000000
0.0 -0.00001
0.001070 -0.001450

Xfoil's arc-length spline parameterization doesn't care
about the resulting very non-uniform point spacing,
so these new coordinates spline OK without any difficulty.

But in the case of the original FX67-150 coordinates,
this still produces overshoots, with a concavity below
the LE point (top looks better, but still wavy).
The real problem is that the necessary geometric
information is simply not present in the coarse
coordinates. An adequately-smooth interpolated shape
has to be literally "made up" in one way or another.