How does spar protrusion affect performance

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

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

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

"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.

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

Eric Greenwell
Washington State
USA

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

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
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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

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.

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

Eric Greenwell
Washington State
USA

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

Eric Greenwell
Washington State
USA