Wing Profiling Advice

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

Andy, see comments below:

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.

You're right. It's probably poor for profiling the whole wing. But it
worked reasonably well in picking up flaws that were bigger than the
wave gauge but substantially smaller than the chord. If I'd known I
had a significant problem when I started, I'd probably have tried to
make templates. But by the time I finally figured out what the problem
was, I felt like I could go ahead with the dial gauge.

Frankly, an experienced tuner like Hank Nixon would pick up that kind
of problem simply by running his hand over the wing!

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.

The main reason we sprayed to build up was that this was the second
time the glider was in for sanding and there simply wasn't enough gel
coat over the leading and trailing edges of the spar cap; I was
already seeing dark spots.

You're right about overall thickness. The later LS-3a wings (I had an
earlier "3") allegedly were as much as 2% thicker than spec in some
places. That's a half inch on, say, a 24" chord!!! From memory, Dick
Johnson found similar things on his PIK-20 and just extrapolated the
airfoil for thicker sections since he had the coordinates for both 15%
and 17% sections.

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

I sanded/contoured my LS-3 three times in the 12 years I owned it in
addition to extensive sealing and other tuning. The only thing I've
done to my ASW-24 over the past 12 years is add winglets. One reason
is that the '24 wing is amazingly stable over time (maybe one of the
best composite gliders ever). The other reason is that I was single
when I owned the '3 and with a 9 to 5 job. Now I'm married with two
children, and average 60 hour weeks!

If you're really worried about it, I recommend selling your '27 and
buying a PW-5.

Chip Bearden

Earlier, Andy Blackburn wrote:

Can anyone tell me if it really matters if the overall
thickness of the wing is off by a few thousandths...

Based on template checks of several top-ranked contest ships, I will
opine that it does not. For a few of the ships, you'd guess on the
basis of the template mismatch that the right and left wings weren't
even for the same aircraft. And yet these were all straight-flying,
go-like-stink gliders.

So you'll find me in the camp that says that waviness below .004" is
first priority, canopy and wing root sealing is second, and fidelity
to profile is a distant third.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24

In article ,
(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.

Chip, Please accept my apologies for dismissing this idea. After
taking a hard look at my dial indicator readings, comparing them to what
I see on the wings, and thinking more carefully about the method you
describe, I understand what you are saying and see how this could work.
It's actually a pretty clever idea. Sorry again.

Regards,

-Doug

Doug Hoffman wrote in message . com...
In article ,
(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.

Chip, Please accept my apologies for dismissing this idea. After
taking a hard look at my dial indicator readings, comparing them to what
I see on the wings, and thinking more carefully about the method you
describe, I understand what you are saying and see how this could work.
It's actually a pretty clever idea. Sorry again.

Regards,

-Doug

No apology necessary. In 1985 at the U.S. 15M Nationals, Walter
Schneider (of LS fame) took one look at my dial gauge as I tried to
explain what I'd done and had an even more dismissive reaction.

Regarding the idea, as I said earlier, an article in Technical Soaring
provided the basic theory. I just took it a little further and applied
it to profiling.

Also as I said earlier, using templates is a more accurate approach
and is the one I'd recommend. But if you've got the wing profile
coordinates and don't want to go to the trouble of making good
templates for multiple stations on the wing, the dial gauge approach
could be of help beyond just locating localized bumps and depressions.

Chip