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.