Fine scratches on wing gel coat

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

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.

"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

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 :-))