How does spar protrusion affect performance

[Chris Davison]

On the top surface of the wings of a 17m Kestrel I
am looking at, the spar is clearly visible and you
can feel it (it stands out slightly proud) below the
surface of the wing itself. This is uniform down the
whole spar and on both wings.

Is this 'normal'? Will it affect performance??

Thanks
Chris

[tango4]

yes it's 'normal' and yes it will affect performance

it needs fairing, filling and refinishing.

Ian


"Chris Davison" > wrote in message
...
> On the top surface of the wings of a 17m Kestrel I
> am looking at, the spar is clearly visible and you
> can feel it (it stands out slightly proud) below the
> surface of the wing itself. This is uniform down the
> whole spar and on both wings.
>
> Is this 'normal'? Will it affect performance??
>
> Thanks
> Chris
>
>
>

[Doug Hoffman]

tango4 wrote:

> yes it's 'normal' and yes it will affect performance
>
> it needs fairing, filling and refinishing.
>
> Ian

Yes. According to Richard Johnson a laminar flow airfoil not only needs to
be accurate in shape , but it needs to have no "waves" larger than +/- .002"
in 2", in order to extract best performance. If you can see and feel the
bump of the spar you are violating both of these criteria. Fairing and
refinishing a wing, and making sure the airfoil is correct, is not a small
task, but doable. I am (finally) finishing up my RS-15 wings. I know the
shapw is accurate and I know the +/- .002" waviness limit is not exceeded
(this is best checked with a dial indicator).

Regards,

-Doug


>
> "Chris Davison" > wrote in message
> ...
>> On the top surface of the wings of a 17m Kestrel I
>> am looking at, the spar is clearly visible and you
>> can feel it (it stands out slightly proud) below the
>> surface of the wing itself. This is uniform down the
>> whole spar and on both wings.
>>
>> Is this 'normal'? Will it affect performance??
>>
>> Thanks
>> Chris
>>
>>
>>
>
>

[Derrick Steed]

Chris Davison wrote:
>On the top surface of the wings of a 17m Kestrel I
>am looking at, the spar is clearly visible and you
>can feel it (it stands out slightly proud) below the
>surface of the wing itself. This is uniform down the
>whole spar and on both wings.
>
>Is this 'normal'? Will it affect performance??
>
>Thanks
>Chris
>
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.

It will affect performance because it will trip the laminar flow into turbulent flow, this may or may not be beneficial depending on what part of the polar you are flying on - I think you can safely assume it will adversely affect it.

You will need to get the wing re-profiled to fix it.

Rgds,

Derrick Steed

[Marc Ramsey]

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

[Doug Hoffman]

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

[Marc Ramsey]

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

[Doug Hoffman]

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

[Eric Greenwell]

Doug Hoffman wrote:
> tango4 wrote:
>
>
>>yes it's 'normal' and yes it will affect performance
>>
>>it needs fairing, filling and refinishing.
>>
>>Ian
>
>
> Yes. According to Richard Johnson a laminar flow airfoil not only needs to
> be accurate in shape , but it needs to have no "waves" larger than +/- .002"
> in 2", in order to extract best performance. If you can see and feel the
> bump of the spar you are violating both of these criteria. Fairing and
> refinishing a wing, and making sure the airfoil is correct, is not a small
> task, but doable. I am (finally) finishing up my RS-15 wings. I know the
> shapw is accurate and I know the +/- .002" waviness limit is not exceeded
> (this is best checked with a dial indicator).

I thought Johnson's criteria was +/- .004".
--
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Eric Greenwell
Washington State
USA

[Udo Rumpf]

"Eric Greenwell" > wrote in message
...
> Doug Hoffman wrote:
> > tango4 wrote:
> >
> >
> >>yes it's 'normal' and yes it will affect performance
> >>
> >>it needs fairing, filling and refinishing.
> >>
> >>Ian
> >
> >
> > Yes. According to Richard Johnson a laminar flow airfoil not only needs
to
> > be accurate in shape , but it needs to have no "waves" larger than +/-
..002"
> > in 2", in order to extract best performance. If you can see and feel
the
> > bump of the spar you are violating both of these criteria. Fairing and
> > refinishing a wing, and making sure the airfoil is correct, is not a
small
> > task, but doable. I am (finally) finishing up my RS-15 wings. I know
the
> > shapw is accurate and I know the +/- .002" waviness limit is not
exceeded
> > (this is best checked with a dial indicator).
>
> I thought Johnson's criteria was +/- .004".
> --
> Change "netto" to "net" to email me directly
>
> Eric Greenwell
> Washington State
> USA


R.Johnson states .003" to .004" peak to peak in Soaring May 1998
The contact of the gage is held in the centre by a 2" cart.
The Johnson measuring cart picks up only limited deviations info.
Longer wave can not be picked up readily and longer
waves can be just as detrimental if they exceed certain limits
(That is where attention to details and craftsmanship is so important.)
I would like to refer Doug to an article by Vic Saudek
"Airfoil Smooth-Curve Measuring" of 1987 in Soaring for further reading.
Regards
Udo

[Doug Hoffman]

Eric Greenwell wrote:

> I thought Johnson's criteria was +/- .004".

Thanks Eric and Udo. I got the +/+.002" from a Soaring article *not*
written by Mr. Johnson. My mistake! I tried looking at some of the flight
tests from the SSA website and +/-.003" to .004" seems to be what Mr.
Johnson uses as his benchmark, as Udo says.

Now I'm feeling a lot better about my wing reprofiling work! ;-)

Regards,

-Doug

[Udo Rumpf]

Doug,
it is "not + or -" the total is max .004"
Udo

[Doug Hoffman]

Udo Rumpf wrote:

> Doug,
> it is "not + or -" the total is max .004"
> Udo
>

OK. Sorry. I read your post a bit too quickly. Then +/- .002" as I
originally stated wasn't really wrong? I guess.

Looking again at the 6 Richard Johnson articles I sampled he doesn't really
nail down a precise figure. For example, in a PW5 test Johnson states he
found: "an average wave of about .0065 inches (.165 mm) peak-to-peak over
the main spar region, which is somewhat high for a modern composite
sailplane".

-Doug

[Maule Driver]

"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...

[Eric Greenwell]

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

[Doug Hoffman]

Eric Greenwell wrote:

> Doug Hoffman wrote:
>> tango4 wrote:
>>
>>
>>> yes it's 'normal' and yes it will affect performance
>>>
>>> it needs fairing, filling and refinishing.
>>>
>>> Ian
>>
>>
>> Yes. According to Richard Johnson a laminar flow airfoil not only needs to
>> be accurate in shape , but it needs to have no "waves" larger than +/- .002"
>> in 2", in order to extract best performance. If you can see and feel the
>> bump of the spar you are violating both of these criteria. Fairing and
>> refinishing a wing, and making sure the airfoil is correct, is not a small
>> task, but doable. I am (finally) finishing up my RS-15 wings. I know the
>> shapw is accurate and I know the +/- .002" waviness limit is not exceeded
>> (this is best checked with a dial indicator).
>
> I thought Johnson's criteria was +/- .004".

Eric,

After reviewing the articles I could find with more care, and with Udo's
input, I think .008" peak-to-peak (or +/- .004") is too large a wave for
laminar flow.

Regards,

-Doug

[Derrick Steed]

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

*e +44 (0)7790 494589
+ >

Wavendon Tower
Wavendon
Milton Keynes
MK17 8LX

[Derrick Steed]

>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.
>
>--
>Change "netto" to "net" to email me directly
>
>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

[Eric Greenwell]

Doug Hoffman wrote:

>>I thought Johnson's criteria was +/- .004".
>
>
> Eric,
>
> After reviewing the articles I could find with more care, and with Udo's
> input, I think .008" peak-to-peak (or +/- .004") is too large a wave for
> laminar flow.

I'm sure you are right - I meant peak to peak.

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Eric Greenwell
Washington State
USA

[Eric Greenwell]

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.
>>
>> -- Change "netto" to "net" to email me directly
>>
>> 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