How does spar protrusion affect performance

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

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

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

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

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

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

"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