How well polished does a glider need to be?

[Dan G]

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

[Bill Daniels]

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" > wrote in message
...
> 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

[Ralph Jones[_2_]]

On Wed, 9 Jul 2008 04:26:33 -0700 (PDT), 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 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.
>

There's a major difference between blunt bodies like a golf ball and
slender ones like an airfoil. Drag on the former is mostly pressure
drag, while drag on the latter is dominated by skin friction.

If the boundary layer on a golf ball never separated, with the flow
from all sides rejoining smoothly on the back side, there would be no
pressure drag at all...but in real life, on a smooth ball, it will
separate by the time it gets halfway around. Consequently, the back
side of the ball is in a low pressure area and there is lots of
pressure drag.

The dimples create small-scale turbulence on the surface, adding
energy to the airflow, which delays the separation of the boundary
layer to about the 3/4 point. That keeps high-pressure air on part of
the backside, reducing the pressure drag. The dimples also increase
the skin friction, but since that is small on a sphere, it's a
positive tradeoff.

Skin friction drag depends heavily on surface roughness: every time an
air molecule impacts the forward face of a bump, it extracts a little
bit of energy from the aircraft.

rj

[Ralph Jones[_2_]]

On Wed, 09 Jul 2008 08:13:04 -0600, Ralph Jones >
wrote:

>On Wed, 9 Jul 2008 04:26:33 -0700 (PDT), 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 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.
>>
>
>There's a major difference between blunt bodies like a golf ball and
>slender ones like an airfoil. Drag on the former is mostly pressure
>drag, while drag on the latter is dominated by skin friction.
>
[snip]

I forgot one other effect of the dimples: they make lift. A golf ball
is normally hit with backspin, and the curveball effect creates a
little bit of lift, making the ball a super low-performance
glider...;-)

The dimples magnify the effect somewhat, by generating a little extra
flow circulation around the ball in the proper direction.

If you've read up on the spinning mines that were used against the
Ruhr Dams, it's precisely the same effect: the backspin on the mine
kept it pressed against the surface of the dam as it sank.

rj

[Udo Rumpf[_2_]]

>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.
>Bill D

Hi Bill,
I have not seen any research data as it relates to sanding.
When sanding, it appears, mixing of air takes place sooner and laminar
flow is reduced.
Sanding with 360 to 400 grit works in some in some cases with the FX 67
and similar airfoils. It is just an other form of a turbulator in my
opinion. In the case of sanding, the run of laminar flow is reduced,
because of that a more energetic transition can takes place and attached
turbulent flow can be obtained in an otherwise unfavourable pressure
recovery as with the FX 67. The same can be achieved by placing
turbulator bumps at about 40% and 50% chord with a 1" inch spacing, the
bump is 3/16"diameter and .025" high, this allows one to keep the
polished surface, plus a positive flow trip. If there is a vestige of
laminar flow left in the region, it can move in-between the bumps. At
climb speed the wake spreads at 35 degree, give or take. When cruising,
the wake pattern is more parallel at first and then spreads.
I have used this technique to solve some short comings on my original
HP18. In either case it is not optimum I rather see
the airfoil fixed or at least corrected so it meets it intended
design criteria.
Similarly, HP18 owners complained about wing drops on there take off
rolls. I recommended to place a turbulator 50% chord on the top surface,
within the span of the aileron. which stop the wing drop problem.

Udo

[John Smith]

Do whatever you pilot wants you to do. A pilot wo is confident that his
glider is in the absolute best possible shape ist far more important
than half a percent of theoretical gain.

[Jonathon May[_2_]]

At 11:26 09 July 2008, 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 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 believe the difference between the wing and the golf ball is a result of
low reynolds numbers ,you get some strange affects when you increase the
size of aerofoils .
With regard to crewing in my competition flying I like to rig,water
ballast,grid and polish while grid squatting.It gives you something to do
and moves any finger marks .
Jon

[Andy[_1_]]

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

[jeplane]

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

Richard

[jb92563]

Dan,
How much you polish depends on several important things.

1) How old is the pilot?
If the pilot is old then you can just say you polished the wings and
he will not be able to tell the difference!

2) Is rain a possibility?
If you wax or polish a wing so that water beads on a laminar flow wing
the pilot will kill you....don't polish but wipe or sand it!

3) What nationality is the Pilot?
If the pilot is German you better start a couple days ahead of time
because perfection is the only acceptible job.
If the pilot is South West American, he will not recognise his own
glider if you polish it so don't.
If the pilot is Australian then you are probably mistaking your pilot
for someone else as Aussie pilots dont need crew.

4) Can you see the fiberglass weave on the wings?
If so then just mist the wing and dry if with a cloth and chase bugs
away from the glider as best you can.

5) What is for dinner?
If its a McDonalds burger then just clean the canopy.
If its a nice steak dinner with desert then polish for as long as it
would take to eat your dinner.

You will figure out what else is important after the first time
crewing ;-)

Ray



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 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

[01-- Zero One]

"Ralph Jones" > wrote in message
:

> On Wed, 09 Jul 2008 08:13:04 -0600, Ralph Jones >
> wrote:
>
> >On Wed, 9 Jul 2008 04:26:33 -0700 (PDT), 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 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.
> >>
> >
> >There's a major difference between blunt bodies like a golf ball and
> >slender ones like an airfoil. Drag on the former is mostly pressure
> >drag, while drag on the latter is dominated by skin friction.
> >
> [snip]
>
> I forgot one other effect of the dimples: they make lift. A golf ball
> is normally hit with backspin, and the curveball effect creates a
> little bit of lift, making the ball a super low-performance
> glider...;-)
>



Hmmm, I wonder if we should take a ball peen hammer to a 1-26? I might
be on to something here....

Larry
01 Zero One

[Michael Ash]

01-- Zero One > wrote:
>>
>> I forgot one other effect of the dimples: they make lift. A golf ball
>> is normally hit with backspin, and the curveball effect creates a
>> little bit of lift, making the ball a super low-performance
>> glider...;-)
>
> Hmmm, I wonder if we should take a ball peen hammer to a 1-26? I might
> be on to something here....

I think somebody must have had that idea for my club's 1-36, and I am not
at all convinced that it resulted in better performance....

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

[sisu1a]

On Jul 22, 7:19 am, Michael Ash > wrote:
> 01-- Zero One > wrote:
>
>
>
> >> I forgot one other effect of the dimples: they make lift. A golf ball
> >> is normally hit with backspin, and the curveball effect creates a
> >> little bit of lift, making the ball a super low-performance
> >> glider...;-)
>
> > Hmmm, I wonder if we should take a ball peen hammer to a 1-26? I might
> > be on to something here....
>
> I think somebody must have had that idea for my club's 1-36, and I am not
> at all convinced that it resulted in better performance....
>
> --
> Mike Ash
> Radio Free Earth
> Broadcasting from our climate-controlled studios deep inside the Moon

Actually there was a 1-26E on W&W last year or so that was nicknamed
"The Golfball" because it sat out a hailstorm on the tiedown line.
Apparently it DID actually increase it's performance slightly!
(established by many side by side glides with other 1-26's)

-Paul

[Bill Daniels]

"sisu1a" > wrote in message
...
> On Jul 22, 7:19 am, Michael Ash > wrote:
>> 01-- Zero One > wrote:
>>
>>
>>
>> >> I forgot one other effect of the dimples: they make lift. A golf ball
>> >> is normally hit with backspin, and the curveball effect creates a
>> >> little bit of lift, making the ball a super low-performance
>> >> glider...;-)
>>
>> > Hmmm, I wonder if we should take a ball peen hammer to a 1-26? I
>> > might
>> > be on to something here....
>>
>> I think somebody must have had that idea for my club's 1-36, and I am not
>> at all convinced that it resulted in better performance....
>>
>> --
>> Mike Ash
>> Radio Free Earth
>> Broadcasting from our climate-controlled studios deep inside the Moon
>
> Actually there was a 1-26E on W&W last year or so that was nicknamed
> "The Golfball" because it sat out a hailstorm on the tiedown line.
> Apparently it DID actually increase it's performance slightly!
> (established by many side by side glides with other 1-26's)
>
> -Paul

Northeastern Colorado is "hail alley" so there are many hail dimpled
airplanes around. People selling these often use the "golfball effect" in
their sales pitches...