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 Rumpf wrote:
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.

In my limited experience, you are generally correct. But the PIK20b in
my experience is *particularly* sensitive to water.

I flew the PIK in a number of eastern US contests. We didn't fly in
much rain but one particular encounter in the mid-80s captured my
attention. I pulled up into a thermal just as a light sprinkle of rain
hit. I stalled out of the pull up! This is the only time I can
remember stalling accidently in any conditions.

It didn't at first occur to me that the rain and the stall were related
but it soon became apparent that I wasn't flying the same sailplane I
was flying before the rain.

I didn't make it home that day and landed at an airport. The anemic
Citabria was sent from Dansville to pick me up at Grand Canyon airport
(NY State). We started the tow just as another light sprinkle fell.
The Citabria lifted off in its normally anemic way but I couldn't get
the unballasted PIK off the ground. I rolled off into the grass at the
end. Anyone familiar with Grand Canyon (in the mid-80s) knows the
reason for the name. Fortunately the ground falls away quickly and I
became airborne.

I would suggest that the PIK is a notable exception to your conclusion.

"Maule Driver" wrote in message
news
Udo Rumpf wrote:
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.

In my limited experience, you are generally correct. But the PIK20b in my
experience is *particularly* sensitive to water.

I flew the PIK in a number of eastern US contests. We didn't fly in much
rain but one particular encounter in the mid-80s captured my attention.
I pulled up into a thermal just as a light sprinkle of rain hit. I
stalled out of the pull up! This is the only time I can remember stalling
accidently in any conditions.

It didn't at first occur to me that the rain and the stall were related
but it soon became apparent that I wasn't flying the same sailplane I was
flying before the rain.

I didn't make it home that day and landed at an airport. The anemic
Citabria was sent from Dansville to pick me up at Grand Canyon airport (NY
State). We started the tow just as another light sprinkle fell. The
Citabria lifted off in its normally anemic way but I couldn't get the
unballasted PIK off the ground. I rolled off into the grass at the end.
Anyone familiar with Grand Canyon (in the mid-80s) knows the reason for
the name. Fortunately the ground falls away quickly and I became
airborne.

I would suggest that the PIK is a notable exception to your conclusion.


On hind sight I should have realized how bad "moisture" is on the FX 67
airfoil

I had done experiments with this airfoil under the assumption
that it was only related the way it was constructed, as in the HP
technique,
I used turbulators successfully on the top surface to counter some of the
ills of that airfoil.
Wing dropping on take off for one, as well as in landing mode when the
glider with this airfoil
showed stall behaviour well before the stall speed was reached.

One other experiment I conducted, installing a .040" wire, tape down along
the span
about 5% from the leading edge. My surprise was It climbed much better but
the cruise was no better then a K6 which made sense to me.

It seems everything has be just dead on with this airfoil shape for it to
perform to it potential.
One glider I know of is the LS3, which has done just that I never heard of
any complains.
Regards
Udo

Udo Rumpf wrote:
"Maule Driver" wrote in message
(snips)
I would suggest that the PIK is a notable exception to your conclusion.

It seems everything has be just dead on with this airfoil shape for it
to perform to it potential.

That seems to be what Dick Johnson confirmed with his PIK improvement
work back in the 70s or early 80s.

One glider I know of is the LS3, which has done just that I never heard
of any complains.

I didn't know it had the same airfoil but I never heard complaints either.

Thanks
Bill Watson (MauleDriver)

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 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

My experience from 6 years racing the PIK agrees with Bill's. Sand
with 600 at 45 degrees back to high point of airfoil. Clean with water
with generous helping of dish soap. Wipe dry, don't rinse.
Other very important factor in climb of PIK is flap setting. It wants
to fly at a constant AOA with additional lift for tighter turns added
by flap setting. As I recall, I used 6 deg for 30 deg of bank, 8 deg at
45 and changes flap setting whenever bank changed, even recentering. A
lot of work, but it did help performance.
Good Luck UH