That's pretty impressive Chip.

I would think that using rate of change of curvature
(or absolute curvature) to estimate the thickness of
the overall airfoil would be really challenging. It's
kind of like using an accelerometer (or variometer)
to estimate your altitude - it's possible if you're
really good at integrating the values, but you need
very accurate measurements to keep the integrated sum
from drifting pretty significantly.

Since these dial guages are generally only good to
001' or so, it seems like they're best at estimating
surface waviness. I'm not disputing what you did -
I'm just amazed that you were able to do it.

Can anyone tell me if it really matters if the overall
thickness of the wing is off by a few thousandths.
This is the difference between sanding down to the
spar wave and building up gelcoat in the spar wave.
I think most people sand down if they can and build
up if they have to.

Even if I make my wing .005' thinner overall from sanding,
that's around 1/10 of 1% of the total thickness - I'm
not sure if the design/manufacturing tolerances are
that good to start with and I don't know if the factory
allows for some shrinkage in the initial construction.
Of course if my spar wave is .05' instead of .005'
than maybe the answer changes. At what point does it
start to matter?

I thought the main objective of reprofiling these days
was to remove surface waviness - but this adds a new
level of complexity - should I even worry about it?

9B

At 17:54 18 September 2004, Chip Bearden wrote:
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