In article , Martin Gregorie writes:

sucked into autocad and then joined using a cubic spline starting and
ending at the TE should give a reasonable result if you use, say, 10
line segments per point. The critical points a

- use enough co-ordinate points
- use at least 10 line segments per point
- make sure you're using a CUBIC spline, not a circular spline


None of this will solve the problem. The real problem
is that like with most early Wortmann sections, the original
FX67K150 coordinates are grossly too coarse at the leading edge.
A cubic spline will produce bad glitches just above and just
below the 0,0 leading edge point. There are many different
types of cubic spline parameterizations possible, but they
all produce shape glitches with various degree of severity.

What I usually do in such situations is to add points
near the LE point, and then smooth the local LE shape
by smoothing the local Cp(s) distribution in Xfoil at high
and low angles of attack, like +15 and -10 degrees.
Whether or not this produces the "correct" shape is a moot point,
because the correct shape cannot be determined from the
official coordinates. At least it produces a shape with
a well-behaved Cp spike, which is really what matters.