Mark is right.

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. Now if you
spline the upper and lower surfaces separately (preferably a with a
B-spline of some form) along with the vertical constraint, the curve
should be closer to the desired shape. It will, however, likely still
produce a suction spike at the leading edge due to a jump in curvature
(2nd derivative for the mathematician) at the leading edge. But if you
distribute points on this new curve more densely near the leading edge,
you have a better starting point than the coarse tabular data.

You could then do as Mark suggests to home in on an acceptable solution.
If you have a program like CATIA though, you could try one more
smoothing by using the first 5 or 10% of the upper and lower surfaces as
a smooth curve. If you examine the curvature (2nd derivative with
respect to the arclength of the curve), you can slightly move the points
near the leading edge to make a smooth transition in curvature between
the upper and lower surfaces. This should remove the any spike from the
pressure distribution at the leading edge.

You should, however, use a code like Mark's Xfoil to check for problems
with any of your airfoils.

Good Luck!

...... Neal

Mark Drela wrote:


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.