I notice however, that in some cases, small gliders in particular, a flat plate airfoil works wonders. It would seem that Bernoulli effect is not present in any way. Am I correct here, or not ??

From : Don Stackhouse

Not.

The problem here is that you're looking at the airfoil the way we see it, which is definitely not the same as the air molecules see it. In particular, what we see as the leading edge is not the same as what the air sees as the leading edge, at least if the angle of attack is not zero.

Some of the air approaching the wing goes over the top, and some goes underneath. There is a point somewhere near the leading edge called the stagnation point. All air above the stagnation point goes over the top, and all air below the stagnation point goes underneath. As the air approaches the leading edge of a wing that has a positive angle of attack, it can feel the approaching influence of the high pressure under the wing and the lower pressure above. Obviously it would prefer to head for the low pressure area, and so some of the air just below the leading edge manages to sneak over the top. This has the effect of shifting the stagnation point to a spot a little below what we consider to be the leading edge. The air hooks its way back around the leading edge and over the top, rounding out the shape as it goes, so that the upper surface flow now has a long, curved path from the stagnation point to the trailing edge.

Meanwhile, the flow path for the lower surface flow is shortened a little by the downward and aftward shift of the stagnation point, and so sees a short, straight, relatively high pressure flow path.

The net result is that to the air molecules, the flat plate airfoil with a positive angle of attack looks very similar to a cambered airfoil, and sees Bernoulli effects in much the same manner. The same is true of symmetrical airfoils with a positive angle of attack, of which the flat plate airfoil is only one particular example.

In the specific case of those small gliders, you're also observing the effects of very low Reynolds numbers (abbreviated "Re", it's the numerical measure of what modelers often call "scale effect").

For an airfoil in sea level standard-day air:

Re = 778 x (speed in MPH) x (chord in inches)

At very low Re's, the air doesn't like to be bullied aside by a thick airfoil. I recall a Japanese study not too long ago that found that at an Re of about 20,000 (about like some R/C HLGs and Mosquito class models, and many indoor R/C models), a flat plate actually generated more max lift than a Clark Y airfoil. However, a properly designed airfoil optimized for that operating condition would probably outperform both of those.

In my experience in designing airfoils for this realm (which is considerable), I've found that as the Re gets smaller, the necessary thickness gets smaller, but there are also penalties for being too thin. The max thickness point needs to move forward, and the distribution of thickness along the airfoil becomes increasingly critical as well. The amount and distribution of camber also becomes an issue. The amount and distribution of the slope of the upper surface aft of the high point is particularly important. If you want an airfoil for very low Re's that also has a wide speed range, the job gets really tricky! You also need to be extremely familiar with how to interpret the results of whatever design software and/or wind tunnel data you;re using, since both of those become increasingly unreliable at very low Re's. In general, I've found that designing really good airfoils for extremely low Re's is some of the most difficult and challenging work in all of model aerodynamics.

Don Stackhouse
DJ Aerotech