Fine scratches on wing gel coat

[Discus 44]

I have noticed some fine scratches on the top of my wing surface that
are diagonal across the chord in opposing pattern. Can anyone explain
if there is an aerodynamic reason for having these scratches remaining
on the surface?

[Papa3]

Discus 44 wrote:
> I have noticed some fine scratches on the top of my wing surface that
> are diagonal across the chord in opposing pattern. Can anyone explain
> if there is an aerodynamic reason for having these scratches remaining
> on the surface?

Do you mean "is there a good reason for them to stay there in order to
enhance performance?" Uhhh, no. Will the fine scratches degrade
your performance? Uhhh, no.

These come from the finish sanding process. Standard procedure is to
sand on a 45 degree bias to the chord line, alternating orientation
with each succeeding grade of paper. So, what you're seeing is the
remaining scratches from two passes of sanding (e.g. 600 and 800 grit).
If you look at them with a 4x loop magnifier, you can usually see if
one bias is slightly different from the other (ie. if the line sloping
up to the left is 600 and the line sloping up to the right is 800).
If you look at your ailerons, you won't see this, since all of the
scratch marks will be aligned spanwise, as that's the way these are
sanded.

If you're really anal, you can go back to something like 600 grit and
resand the whole wing, carefully working your way up through the grits
to either 1000 or even 1200 grit (a pretty boring and labor intensive
procedure that requires someone to tutor you for a while). Or, you
can ignore it and fly. I would choose the latter :-))

P3

[Discus 44]

Papa3 wrote:
> Discus 44 wrote:
> > I have noticed some fine scratches on the top of my wing surface that
> > are diagonal across the chord in opposing pattern. Can anyone explain
> > if there is an aerodynamic reason for having these scratches remaining
> > on the surface?
>
> Do you mean "is there a good reason for them to stay there in order to
> enhance performance?" Uhhh, no. Will the fine scratches degrade
> your performance? Uhhh, no.
>
> These come from the finish sanding process. Standard procedure is to
> sand on a 45 degree bias to the chord line, alternating orientation
> with each succeeding grade of paper. So, what you're seeing is the
> remaining scratches from two passes of sanding (e.g. 600 and 800 grit).
> If you look at them with a 4x loop magnifier, you can usually see if
> one bias is slightly different from the other (ie. if the line sloping
> up to the left is 600 and the line sloping up to the right is 800).
> If you look at your ailerons, you won't see this, since all of the
> scratch marks will be aligned spanwise, as that's the way these are
> sanded.
>
> If you're really anal, you can go back to something like 600 grit and
> resand the whole wing, carefully working your way up through the grits
> to either 1000 or even 1200 grit (a pretty boring and labor intensive
> procedure that requires someone to tutor you for a while). Or, you
> can ignore it and fly. I would choose the latter :-))
>
> P3

Thanks for your reply. Is the reason for this factory sanding to
remove high spots or to blend the whole surface. Given the high
quality of the tooling, there should be little reason to sand the
surface. This is how I would perceive the process to be if the tools
are good.

Anyway I was just curious if there was a secret competition reason for
roughing the surface to keep the airflow attached longer than a shiny
smooth surface would give.

[Bill Daniels]

"Discus 44" > wrote in message
ups.com...

>
> Anyway I was just curious if there was a secret competition reason for
> roughing the surface to keep the airflow attached longer than a shiny
> smooth surface would give.
>

There is research to suggest that a slightly rough surface will keep airflow
attached a bit longer than a mirror smooth surface. I've flown wings sanded
with various grits from 400 to 2000 and find no difference in performance.
Stopping at 400 grit is a LOT less work than continuing to 2000. The REAL
reason for a mirror surface is that it's a lot easier to keep clean.

That said, I've always suspect that the "sand your wings" suggestion came
from a well known competitor trying to mess with another competitors head.
80 grit anyone?

Bill Daniels

[Papa3]

Bill Daniels wrote:
> "Discus 44" > wrote in message
> ups.com...
>
> >
> > Anyway I was just curious if there was a secret competition reason for
> > roughing the surface to keep the airflow attached longer than a shiny
> > smooth surface would give.
> >
>
> There is research to suggest that a slightly rough surface will keep airflow
> attached a bit longer than a mirror smooth surface. I've flown wings sanded
> with various grits from 400 to 2000 and find no difference in performance.
> Stopping at 400 grit is a LOT less work than continuing to 2000. The REAL
> reason for a mirror surface is that it's a lot easier to keep clean.
>
> That said, I've always suspect that the "sand your wings" suggestion came
> from a well known competitor trying to mess with another competitors head.
> 80 grit anyone?
>
> Bill Daniels

IIRC, one reason that was given for not waxing wings was to avoid
beading of raindrops if you flew through a shower. I've also heard
the other suggestion that a slightly rough surface reduces drag. I
imagine one of the problems is that measuring the delta between these
two conditions would be outweighed by the measurement errors
themselves. I'm sure someone lurking here has thought more about
this?

P3

ps. I always encourage my competition to use a coarse file on the
leading edge; removes bugs very effectively :-))

[Don Johnstone]

At 19:24 27 September 2006, Discus 44 wrote:
>Thanks for your reply. Is the reason for this factory
>sanding to
>remove high spots or to blend the whole surface. Given
>the high
>quality of the tooling, there should be little reason
>to sand the
>surface. This is how I would perceive the process to
>be if the tools
>are good.
>
>Anyway I was just curious if there was a secret competition
>reason for
>roughing the surface to keep the airflow attached longer
>than a shiny
>smooth surface would give.

Contrary to popular belief gel coat when it is removed
from the mould does not have a shiny mirror surface
that is why it has to be polished (sanded). Over time
the gel coat shrinks and you can often see the pattern
of the underlying glass layup.
>
>

[Bob Caldwell]

The bigger issue is with flying in rain. The highly polished, waxed finish will bead up with water droplets. The rougher surface will shed water without forming the really draggy (is that a word?) droplets. Same as if you waxed your windshield in your car, water beads up instead of sheeting off. So if and when you have to blast through that virga line the drop in performance will not be as severe.

Bob

[Jack[_1_]]

Bob Caldwell wrote:
> The bigger issue is with flying in rain. The highly polished, waxed
> finish will bead up with water droplets. The rougher surface will
> shed water without forming the really draggy (is that a word?)
> droplets. Same as if you waxed your windshield in your car, water
> beads up instead of sheeting off. So if and when you have to blast
> through that virga line the drop in performance will not be as
> severe.

So, should we be using RainEx on our wings?


Jack

[Tim Mara]

absolutely NOT........RainX causes rain to form tiny droplets which are then
blown off by wind......you want no droplets at all...ever..
tim

"Jack" > wrote in message
et...
> Bob Caldwell wrote:
>> The bigger issue is with flying in rain. The highly polished, waxed
>> finish will bead up with water droplets. The rougher surface will
>> shed water without forming the really draggy (is that a word?)
>> droplets. Same as if you waxed your windshield in your car, water
>> beads up instead of sheeting off. So if and when you have to blast
>> through that virga line the drop in performance will not be as
>> severe.
>
> So, should we be using RainEx on our wings?
>
>
> Jack

[Jack[_1_]]

Tim Mara wrote:
> absolutely NOT........RainX causes rain to form tiny droplets which are then
> blown off by wind......you want no droplets at all...ever..
> tim
>
> "Jack" > wrote in message
> et...
>> Bob Caldwell wrote:
>>> The bigger issue is with flying in rain. The highly polished, waxed
>>> finish will bead up with water droplets. The rougher surface will
>>> shed water without forming the really draggy (is that a word?)
>>> droplets. Same as if you waxed your windshield in your car, water
>>> beads up instead of sheeting off. So if and when you have to blast
>>> through that virga line the drop in performance will not be as
>>> severe.
>> So, should we be using RainEx on our wings?


But many do wax the wings -- and so get BIG drops -- do they not.

Is the only answer to stay out of the rain and/or simply clean your
wings with pure water, but never apply any protective finish?


Jack

[[email protected]]

Jack wrote:
> Tim Mara wrote:
> > absolutely NOT........RainX causes rain to form tiny droplets which are then
> > blown off by wind......you want no droplets at all...ever..
> > tim
> >
> > "Jack" > wrote in message
> > et...
> >> Bob Caldwell wrote:
> >>> The bigger issue is with flying in rain. The highly polished, waxed
> >>> finish will bead up with water droplets. The rougher surface will
> >>> shed water without forming the really draggy (is that a word?)
> >>> droplets. Same as if you waxed your windshield in your car, water
> >>> beads up instead of sheeting off. So if and when you have to blast
> >>> through that virga line the drop in performance will not be as
> >>> severe.
> >> So, should we be using RainEx on our wings?
>
>
> But many do wax the wings -- and so get BIG drops -- do they not.
>
> Is the only answer to stay out of the rain and/or simply clean your
> wings with pure water, but never apply any protective finish?
>
>
> Jack

Re Wax- Some airfoils don't want to be polished and waxed. The Wortman
FX 67-150/170 series is an example of this. These like a little(400grit
at 45 degrees) roughness. When my PIK climb started to drop off, I'd
give it a bit of a scuff. I washed with dish soap and wiped dry
without a lot of rinsing. Soap wetted out rain and it tolarated rain
much better. Otherwise, when the rain drop hits the canopy- put the
gear down.
hard to keep looking nice when doing this though.
UH

[Eric Greenwell]

wrote:

> Re Wax- Some airfoils don't want to be polished and waxed. The Wortman
> FX 67-150/170 series is an example of this. These like a little(400grit
> at 45 degrees) roughness. When my PIK climb started to drop off, I'd
> give it a bit of a scuff. I washed with dish soap and wiped dry
> without a lot of rinsing. Soap wetted out rain and it tolarated rain
> much better. Otherwise, when the rain drop hits the canopy- put the
> gear down.
> hard to keep looking nice when doing this though.

And it might be hard to preserve them, too, unless they are painted like
the PIK's wings (urethane?). Gel coat on wings lasts longer when it's
kept waxed. I haven't heard that periodic light sanding protects the gel
coat against cracking, but I haven't heard of any testing done, either.

--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[[email protected]]

Hank's experience echoes mine. I kept my LS-3 (same airfoil as PIK-20B;
also Mosquito, Nimbus 2, etc.) at 400 grit most of the time and it did
seem to make a difference in the rain. Keeping it clean was no problem.
Water worked for most things. And for stubborn bugs, just sand them
off. :)

Chip Bearden
ASW 24 "JB"

[Eric Greenwell]

wrote:
> Hank's experience echoes mine. I kept my LS-3 (same airfoil as PIK-20B;
> also Mosquito, Nimbus 2, etc.) at 400 grit most of the time and it did
> seem to make a difference in the rain. Keeping it clean was no problem.
> Water worked for most things. And for stubborn bugs, just sand them
> off. :)

Cheez, all this talk about rain. Chip, you need to come out West and fly
in the sun! It's more fun, and it eliminates sanding time. Waxing time
might increase, but you are rewarded with a shiny glider.

And one more thing: if you do fly in the rain out West, that's because
there is good lift there. Saved myself quite a few times by thermalling
on the edge of a rain shaft.


--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org