How does spar protrusion affect performance

"Derrick Steed
It happens to some extent to all GRP gliders as they age (some say PIK's
are immune - hmmm...), and it's due to shrinkage of the GRP.


I reprofiled some PIK wings some years ago - they are different but they
aren't immune. Interestingly, I think the PIK was Dick Johnson's first
major profile job.

Of course if you just splash some water on that airfoil the waviness can be
ignored...

Maule Driver wrote:
"Derrick Steed

It happens to some extent to all GRP gliders as they age (some say PIK's

are immune - hmmm...), and it's due to shrinkage of the GRP.


I reprofiled some PIK wings some years ago - they are different but they
aren't immune. Interestingly, I think the PIK was Dick Johnson's first
major profile job.

The later PIKs suffered primarily from an improper profile due to mold
problems, not the usual shrinkage from a correct profile. It was so
severe, Dick had to increase the chord to get the profile thickness
percentage right.

It's possible they had less shrinkage once out of the mold, due to the
high temperature curing.

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Eric Greenwell
Washington State
USA

At 16:30 10 July 2004, Chris Davison wrote:
Is this 'normal'? Will it affect performance??

Yes and yes. As others have said, you'll need to
have the wings reprofiled to fix it. However, you

should also ask yourself whether it is worth fixing.

You are looking at a few hundred hours of work
by someone who knows what they're doing (or
several hundred by someone who doesn't).
How much time and/or money are you willing to
expend, given that there is no guarantee that
you'll end up with a noticeable increase in
performance?

A related anecdote: my partner and I have a
LAK-17A which has quite obvious spar bumps on
the top and bottom wing surfaces. We're perfectly
happy with the performance, so we haven't done
anything about it. A top US contest pilot had flown
with us a number of times in his new ASW-27B
(when we were using 15M tips), and noticed that
he couldn't keep up in higher speed glides. So, he
spent several thousand dollars getting the bottom
of his wings reprofiled. He is now much happier
with his ship. Of course, he hasn't asked for a
comparison glide since the reprofile job 8^)

Marc

Marc Ramsey wrote:

A related anecdote: my partner and I have a
LAK-17A which has quite obvious spar bumps on
the top and bottom wing surfaces. We're perfectly
happy with the performance, so we haven't done
anything about it. A top US contest pilot had flown
with us a number of times in his new ASW-27B
(when we were using 15M tips), and noticed that
he couldn't keep up in higher speed glides.

So you are going to make impressions of your "unique" airfoil in case LAK
fixes their process? That way you could get them to build you another
glider (should you need one) with assurances you will still have the magic
bumps. ;-)

Regards,

-Doug

Doug Hoffman wrote:
So you are going to make impressions of your "unique" airfoil in case LAK
fixes their process? That way you could get them to build you another
glider (should you need one) with assurances you will still have the magic
bumps. ;-)

Not to worry, I don't think LAK is any more capable of fixing their
processes than Schleicher 8^)

Marc

Marc Ramsey wrote:

Doug Hoffman wrote:
So you are going to make impressions of your "unique" airfoil in case LAK
fixes their process? That way you could get them to build you another
glider (should you need one) with assurances you will still have the magic
bumps. ;-)

Not to worry, I don't think LAK is any more capable of fixing their
processes than Schleicher 8^)

Of course. How naive of me. :-)

Btw, here at Ionia we have a LAK-17A driver whose ship also exhibited some
problems at the surface over the spar. Although I can't recall if it was
protrusion or depression. Regardless, the local repair guru, David Nelson,
did a repair job on it and it now looks flawless. The LAK-17A driver seems
to really like his ship as well.

Regards,

-Doug

Maule Driver wrote:
"Derrick Steed
It happens to some extent to all GRP gliders as they age (some say PIK's
are immune - hmmm...), and it's due to shrinkage of the GRP.


I reprofiled some PIK wings some years ago - they are different but they
aren't immune. Interestingly, I think the PIK was Dick Johnson's first
major profile job.

Of course if you just splash some water on that airfoil the waviness can be
ignored...


Tell me about it! I have a PIK 20B. The reason I said "some say" is because
I've noticed some shrinkage in my wings (not a lot, but it's there).

I don't think it's the water so much that ruins the working of the wing,
it's how it's applied - a smooth even sheet of it would be just fine aside
from the fact that it might destroy the no-slip condition (now there's a
thought...)

Rgds,

Derrick Steed
Aon Transformation Programme
EDS Leveraged Delivery

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Wavendon Tower
Wavendon
Milton Keynes
MK17 8LX

Maule Driver wrote:
"Derrick Steed

It happens to some extent to all GRP gliders as they age (some say PIK's

are immune - hmmm...), and it's due to shrinkage of the GRP.


I reprofiled some PIK wings some years ago - they are different but they
aren't immune. Interestingly, I think the PIK was Dick Johnson's first
major profile job.

The later PIKs suffered primarily from an improper profile due to mold
problems, not the usual shrinkage from a correct profile. It was so
severe, Dick had to increase the chord to get the profile thickness
percentage right.

It's possible they had less shrinkage once out of the mold, due to the
high temperature curing.

--
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Eric Greenwell
Washington State
USA

I recall that Dick Johnson found the profile near the leading edge to be too blunt and some friend of his (at JPL, I think) also checked the published airfoil figures and found them to be slightly off and corrected them, he re-profiled a 'B to this and tested it at higher Reynolds numbers (add water, inside rather than outside the wing) - he reported maximum 45:1 and average 43:1. The repairer who works on my glider always cures the repairs he does at elevated temperature as recommended by the resin manufacturer - I believe PIK did just the same.

Rgds,

Derrick Steed

Derrick Steed wrote:
The later PIKs suffered primarily from an improper profile due to
mold problems, not the usual shrinkage from a correct profile. It
was so severe, Dick had to increase the chord to get the profile
thickness percentage right.

It's possible they had less shrinkage once out of the mold, due to
the high temperature curing.

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Eric Greenwell Washington State USA


I recall that Dick Johnson found the profile near the leading edge to
be too blunt

It was more than just a too blunt leading edge: the airfoil was
significantly thicker at it's maximum than it was supposed be ,
requiring a larger chord to bring the thickness percentage to the
correct value.

I believe the problem was due to molds that slowly warped over time,
perhaps because of the higher than normal curing temperatures required
of the resin used on the glider.

and some friend of his (at JPL, I think) also checked
the published airfoil figures and found them to be slightly off and
corrected them, he re-profiled a 'B to this and tested it at higher
Reynolds numbers (add water, inside rather than outside the wing) -
he reported maximum 45:1 and average 43:1. The repairer who works on
my glider always cures the repairs he does at elevated temperature as
recommended by the resin manufacturer - I believe PIK did just the
same.

The PIK used a resin that cured at a substantially higher temperature
than almost all other gliders; in fact, they could be painted colors
other than white. There was an entirely yellow one in California/Nevada
many years ago that I flew with several times.
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Eric Greenwell
Washington State
USA