How well polished does a glider need to be?

I'm crewing this season, and no doubt my pilot will be expecting me
out at 7am with Mer in hand.

My question is (and I'm sure it's been debated before, I did search,
but want fresh input) how worthwhile is polishing?

I can understand keeping the fuselage as slippery as possible to
reduce skin drag, but I've heard that glider wings would still work as
well as possible if their surface was 40 grit sandpaper! Also take a
look at a golfball -- it's dimpled specifically in order to reduce
drag, again implying that a smooth surface may not be perfect.

On the other hand I realise that bugs, chips in the gelcoat etc. are
bad news as they locally seperate the boundary layer, so you don't
want those around.


Dan

I'm crewing this season, and no doubt my pilot will be expecting me
out at 7am with Mer in hand.

My question is (and I'm sure it's been debated before, I did search,
but want fresh input) how worthwhile is polishing?

I can understand keeping the fuselage as slippery as possible to
reduce skin drag, but I've heard that glider wings would still work as
well as possible if their surface was 40 grit sandpaper! Also take a
look at a golfball -- it's dimpled specifically in order to reduce
drag, again implying that a smooth surface may not be perfect.

On the other hand I realise that bugs, chips in the gelcoat etc. are
bad news as they locally seperate the boundary layer, so you don't
want those around.


Dan

As a 'glider sander' and surface contourer I usually stop at 400 or 600 grit
and find the performance at least equal to a highly polished surface. There
is some wind tunnel data that suggests a slightly rough surface actually
keeps the boundary layer energized for greater laminar flow whereas a glossy
surface trips the flow to turbulent sooner.

One school of thought says that the big advantage of gelcoat is that it can
be resanded and polished each year to keep the airfoil contour perfect.
When it's sanded off, just recoat and start over.

The downside to a non-glossy surface is that it will pick up handprints very
easily. The only way to remove these is more sanding.

Bill D


"Dan G" <dangrey@> wrote in message
news:1857e124-54a7-4568-937b-b08a21e3aeff@c65g2000hsa..com...
> I'm crewing this season, and no doubt my pilot will be expecting me
> out at 7am with Mer in hand.
>
> My question is (and I'm sure it's been debated before, I did search,
> but want fresh input) how worthwhile is polishing?
>
> I can understand keeping the fuselage as slippery as possible to
> reduce skin drag, but I've heard that glider wings would still work as
> well as possible if their surface was 40 grit sandpaper! Also take a
> look at a golfball -- it's dimpled specifically in order to reduce
> drag, again implying that a smooth surface may not be perfect.
>
> On the other hand I realise that bugs, chips in the gelcoat etc. are
> bad news as they locally seperate the boundary layer, so you don't
> want those around.
>
>
> Dan

On Jul 9, 4:26 am, Dan G <...@> wrote:
> I'm crewing this season, and no doubt my pilot will be expecting me
> out at 7am with Mer in hand.

My advice is wipe off any water spots from dumping as soon after
landing as possible if flying in a hard water area, then knock off any
bugs on the leading edges, then have a few beers and sleep in late in
the morning. Final wing cleaning, at least in the dusty SW USU,
should be done on the grid shortly before takeoff. We all know if
make no difference to contest scores, it's just occupational therapy,
and best done by the pilot anyway.

Andy

Well, my ASW 19 had the same gelcoat for 33 years now, so talk about
sandpaper!
But hey, it still fly!...:-)

Richard