Sanding or Waxing - PIK 20 Question

Sanding rather than waxing a PIK 20 Question
Has anyone have info on this I was told that when this persons PIK did not
clime well he would sand the wings with 400 grit .

The Wortmann FX-67-K-170 airfoil that the PIK-20 and Nimbus 2 use is
very sensitive to bugs, and even a very small amount of rain. With a
waxed wing surface the rain drops tend to stand tall, severely
separation the top surface air flow. Leaving the wings sanded and
unwaxed allows the rain drops to flow more smoothly on the wing
surface; significantly reducing drag.
However, only a bug wiper system appear to help the bug problem.

I am always surprised when I read comments about sensitivity to rain
or water and I agree water that pearls on the wing surface will have
a negative effect on "all airfoil shapes". When was the last time you flew
through rain and where it mattered in regards to getting home or not.
Either one is cut off and one has to land in any case or one escapes
a light and short duration shower and waits it out. I have no interest
in how my airfoil performs when exposed to water.

As to the question, sanding or waxing,

I have some empirical experience, and I can make a deduction.
If any one has a better insight please let us know.

The reason to sand an airfoil shape, that has shown to be
sensitive to being polished, could be as follows.

The laminar flow on a polished surface, at the transition point, turns into
a large transition bubble due to an unfavourable pressure gradient.
This bubble is large enough to cause a noticeable drag increase.
By sanding one maintains most of the designed laminar flow over that
region but the transition is forced sooner and it has enough energy to
keep the bubble small. Hence less laminar flow but overall reduced drag.

I would not be surprised if the same could be achieved with a thin
turbulator ahead of the transition as is done on many, but not all airfoils
on the bottom surface. This would keep the cleaning shores to a
minimum, as the polished surface can be maintained

The advantage of the sanding would be that the transition could take place
anywhere in the critical range, automatically, since the transition
position
will change with speed. A turbulator will give you one position only and
one
speed, hence the placement of the turbulator strip has to be conservative
to cover all ranges.
Regards
Udo



wrote in message
oups.com...

The Wortmann FX-67-K-170 airfoil that the PIK-20 and Nimbus 2 use is
very sensitive to bugs, and even a very small amount of rain. With a
waxed wing surface the rain drops tend to stand tall, severely
separation the top surface air flow. Leaving the wings sanded and
unwaxed allows the rain drops to flow more smoothly on the wing
surface; significantly reducing drag.
However, only a bug wiper system appear to help the bug problem.

Udo R. wrote 11.9.05: [snip] The reason to sand an airfoil shape, that
has shown to be

sensitive to being polished, could be as follows.



The laminar flow on a polished surface, at the transition point, turns
into a large transition bubble due to an unfavourable pressure gradient.

This bubble is large enough to cause a noticeable drag increase.

By sanding one maintains most of the designed laminar flow over that
region but the transition is forced sooner and it has enough energy to
keep the bubble small. Hence less laminar flow but overall reduced drag.

[snip]

Sorry to be ignorant, but I don't understand the technicalities of this.
What is the " . . . unfavourable pressure gradient . ." ? One in the
wrong direction, or too large, or what?

And in what sense does a transition have energy? Particularly "enough
energy to keep the bubble small" ? If it had energy, the idiot layman's
thinking is that more means bigger.

Hoping for education, not flames or sarcasm.

Chris N.

=======================




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"culverflyer" wrote in
:

Sanding rather than waxing a PIK 20 Question
Has anyone have info on this I was told that when this persons PIK did
not clime well he would sand the wings with 400 grit .



I had a PIK-20d and the performance loss with bugs or rain was memorable.
The drag increase could be heard and the sink felt before rain was wisible
on the canopy.

I never waxed the glider, hoping that might help the droplets to lay flat
on the wing. The urethane finish also didn't need to be waxed to protect
it. I always wondered if a thin layer of some sort of surfactant (like
soap, maybe?) might help the problem. Never tried it, though...

I finally solved the problem by selling the PIK and co-owning a LAK-17a.
It has a urethane finish for easy care and flies just fine in the rain. It
also does't drop wings, handles great, has automatic hookups, and has
outstanding performance. Yes, it does cost a bit more.
-Bob Korves

wrote in message
oups.com...

The Wortmann FX-67-K-170 airfoil that the PIK-20 and Nimbus 2 use is
very sensitive to bugs, and even a very small amount of rain. With a
waxed wing surface the rain drops tend to stand tall, severely
separation the top surface air flow. Leaving the wings sanded and
unwaxed allows the rain drops to flow more smoothly on the wing
surface; significantly reducing drag.
However, only a bug wiper system appear to help the bug problem.


I sand the wings on my Nimbus 2 and they seem to deliver the advertised L/D.
I have tried surfactant on a few gliders and, by appearance only, it seems
to work. A capful or two of dishwasher "rinse clear" or "spot free" or
something like that in a bucket of water seems to be enough.

As for the bugs, as slow as I fly, they have time to get out of the way.

Bill Daniels

What grit do you use on the Nimbus 2 do you sand top and bottom and how far
back??


"Bill Daniels" wrote in message
...

wrote in message
oups.com...

The Wortmann FX-67-K-170 airfoil that the PIK-20 and Nimbus 2 use is
very sensitive to bugs, and even a very small amount of rain. With a
waxed wing surface the rain drops tend to stand tall, severely
separation the top surface air flow. Leaving the wings sanded and
unwaxed allows the rain drops to flow more smoothly on the wing
surface; significantly reducing drag.
However, only a bug wiper system appear to help the bug problem.


I sand the wings on my Nimbus 2 and they seem to deliver the advertised
L/D.
I have tried surfactant on a few gliders and, by appearance only, it seems
to work. A capful or two of dishwasher "rinse clear" or "spot free" or
something like that in a bucket of water seems to be enough.

As for the bugs, as slow as I fly, they have time to get out of the way.

Bill Daniels

I owned the Pik 20B for three years now. I usually never fly in rain since
there is never some around. The horror stories narated will only mean that
my glider would descend like a Cessna 150 in rain. No big deal. I have had
quite a bit of bugs on the leading edge as we tend to manufacture them in
Canada. I pratically did not see any traumatic difference in the flight
characteristics.

Finally, I would not worry one bit about this wounderfull glider. In fact, I
wax it every spring. It gives it a great look and is much easier to clean
after a long day. I would not spend any time sanding the wings unless you
have a serious bump bear the spar.

I see that a fortunate fellow flew his Pik on the Ridge in april for 1 000
km. Can't be that bad, would you not agree?

Best regards, keep the blue side up!

Marc Arsenault
"74"

I use 400 grit up through 1200 grit and sand all the way to the trailing
edge top and bottom using a 20mm thick bar of super flat Plexiglas as a
sanding block.

I'm battling aging and cracked gelcoat that must be replaced someday. I've
used matching white lacquer primer to stabilize the old gelcoat and provide
a good sanding base. The Lacquer primer also replaces the gelcoat lost to
sanding. The whole prime and sand operation takes about a day and I only
have to do it once a year in the spring. So far, it seems to be a good
solution for an old glider.

Maybe it's the condition of the wing but I don't see much degradation in
rain or with modest bug buildups. If you are interested, I uploaded a
flight to the OLC for September 2nd - you can download the IGC file and
analyze the performance for yourself. The wing got wet a couple of times in
that flight.

Bill Daniels


"culverflyer" wrote in message
...
What grit do you use on the Nimbus 2 do you sand top and bottom and how
far
back??


"Bill Daniels" wrote in message
...

wrote in message
oups.com...

The Wortmann FX-67-K-170 airfoil that the PIK-20 and Nimbus 2 use is
very sensitive to bugs, and even a very small amount of rain. With a
waxed wing surface the rain drops tend to stand tall, severely
separation the top surface air flow. Leaving the wings sanded and
unwaxed allows the rain drops to flow more smoothly on the wing
surface; significantly reducing drag.
However, only a bug wiper system appear to help the bug problem.


I sand the wings on my Nimbus 2 and they seem to deliver the advertised
L/D.
I have tried surfactant on a few gliders and, by appearance only, it
seems
to work. A capful or two of dishwasher "rinse clear" or "spot free" or
something like that in a bucket of water seems to be enough.

As for the bugs, as slow as I fly, they have time to get out of the way.

Bill Daniels

I take it from your question that your PIK-20 is not climbing well.

When I first began flying this type, I was disappointed with the climb until
I discovered top rudder. If your yaw string is out on the forward canopy and
while circling is in line with the longitudinal axis, you are in a slight
skid. There is some controversy about the magnitude of the skid and this
subject is covered very well by Richard Johnson in the October 2004 issue of
Soaring Magazine.

The climb performance of my PIK-20B improves a lot when I apply top rudder.
When the climb is optimum, the yaw string is displaced about 5-10 degrees
toward the outside of the turn. This seems to be a trait of most gliders
that I have flown but my PIK-20 seemed especially sensitive in this regard.

So before you begin sanding or polishing, you might try this first...if you
haven't already.

Paul
ZZ



"culverflyer" wrote in message
...
Sanding rather than waxing a PIK 20 Question
Has anyone have info on this I was told that when this persons PIK did not
clime well he would sand the wings with 400 grit .