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.