i just came home from flying my own design funflyer with an 18 % airfoil and could not se the difrence from flying it with a 12 % airfoil ! my quistion is does it relly make that much diffrense?
at low speed were it relly shoud shine the reynolds numbers is low ... am i totally wrong?? some guys even fly a flat wing and the plane does not seme to mind.

you are seeing things as they really are .
Here is the short course:
On these small slow planes -- the thick wing is easier to build and get best strength.
It also is LESS efficient- but less critical to angles of attack.
So -if you could build it at say 5%- yet mantain rigidity -it would fly just fine .
We have tested this to a fare thee well.
radical angle of attack changes with no funny tipping /snapping etc., is easy
The secret (no secret ) is make it as light and strong as possible lots of control throw -lots of reserve power .
and make it at least as long from prop to tail as wing tip to wing tip -
All rules about "why do things fly "and CG nature does apply BUT the very nature of being small and light gives you a huge window of design to play in .

thank you fore your quik reply.. i only build foam wings so it would be just as easy to build a 12% ving as i would not have to make it hollow.

by making it hollow- how many oz's would you save vs the many hours of trying to make the center hollow? also extra internal bracing activity? forget it and plunge ahead. dick

foam weighs one pound per cubic foot.
You are lucky to scoop out an ounce - if you leave in torsional resistance.
It is the least effective weight reduction method.
The strength drops faster than the weight!
Thick stuff - yes - double cut the wing -one outer cut -one inner cut.
here is the tricky part
make the inner cut with a little bit of draft -such that you can re-insert the center and it will slide in tightly
Now - slice off a few 1/2" wide ribs -from the cente cut - wet them with a mix of ProBond and water and slide em into place
This is effective weight reduction as the strength is still good .
but under a 12% airfoil - fer get it

making a hollow foam wing is not mutch trouble .. i make them in to pieces and glue them together with white glue , and it will save me 1/2 the weigth off the foam. i just made a 10% wing 1100 mm x 350mm weigth 290 grams (with brown paper skin) the hollow 18% was 380 grams..

I'm not going to beat the airfoil thickness into the ground because I believe Dick's analysis is pretty much on the mark. I will however, add this: Generally speaking you see real thick airfoils on 3D and fun fly type aircraft which are flown slowly and close in. A thick wing is draggy and will help keep the plane from accelerating too much on down lines.