Be aware that the gauge alone will *not* detect errors in the shape of
the profile or thickness of the wing. For that you need accurate
templates.

Actually, it is possible to do *some* profiling of the wing using a
dial gauge. You need the airfoil coordinates and the dimensions of
your dial gauge (the distance between the contact points of the
"feet", and where between the feet is the dial gauge). This assumes
that the thickness is correct and that the very nose of the leading
edge is also correct, two big assumptions.

I believe it was Rudy Alleman who published a paper in Technical
Soaring (early 80s?) on comparing airfoils from one glider to the next
this way. I derived a slightly more elegant (but no more accurate)
solution and wrote a BASIC program to do the number crunching about 20
years ago. Basically for each chord length (spanwise station) in
question, you use three points on the segment of the airfoil spanned
by the dial gauge to calculate radius of curvature (any three points
lie on a circle). Then you can calculate what the dial guage should
read at that position for the correct curvature.

That's a potentially useful number but I found that the easier way is
to calculate how much the dial gauge should change moving from, say 3"
aft of the leading edge to 4" aft on a 28" chord. If it's supposed to
"unwind" (i.e., the curvature is getting flatter) .010" but it
actually drops more than that, then the curvature is getting too flat
too soon, and vice versa. It helps to print out a strip of paper with
the actual readings every inch or so and tape it to the chord line so
you can do the deltas from one point to the next as you slide the dial
gauge along the wing.

I had a little trouble at first visualizing what was wrong when the
actual numbers didn't agree with the calculations. It's especially
difficult when you come to a bad spot that spans more than the dial
gauge itself. Let's see, the needle went clockwise .005" too much
which means that the curvature is too sharp. So the back feet are
sitting in a depression. When the dial gauge slides into the
depression then...what?

I started putting small pieces of tape on the wing to build up the low
points, so the dial gauge feet could rest on the "reprofiled" wing.
Only then did it become obvious there was a large "flat" spot over the
spar cap on my old LS-3 caused by shrinkage. Moving from front to back
there'd be no tape, then .002" of tape, then .004" of tape, then
..006", then .008", then .006", then .004" and so forth back to zero.
Then the two sharp dial gauge needle reversals we had seen made sense
(one was the sharper point at the forward end of the flatter spot and
the other was at the after end of it).

It was too extensive to sand out so we sprayed gel goat over the flat
spot and built up that area, then used the dial gauge to get the
correct contour. The results were dramatic in improved performance.

Templates are a far more accurate way of profiling a wing, but you can
learn a lot, and even make some small adjustments, with a good dial
gauge. It helps tremendously if you have the airfoil coordinates, of
course. That was easier then than now.

Interestingly, when I first ran the program against the Wortmann
series on the LS-3, I found some strange discontinuities (i.e., the
deltas weren't a smooth curve in several spots) even though I had used
the corrections that Dr. Wortmann had published. It wasn't until I
cranked in Dan Somers' subsequent corrections that all the deltas
smoothed out.

Chip Bearden