adam_one,

The chart is missing a curve labeled "thin airfoil". A properly selected thin airfoil, when compared to a plate of the same thickness, would have a higher maximum CL. I assume that phenomenon being represented in the chart is that, for low Reynold's numbers, a thick airfoil would require a pressure recovery that is not feasible, which would limit the maximum CL. An airfoil of the same thickness as the plate would probably not suffer from this problem. If this sounds wrong to you, try the following thought experiment. For a given thickness, optimize an airfoil to maximize maximum CL. Among your degrees of freedom is the chordwise thickness distribution. Do you think that a constant thickness (a flat plate) is the optimum solution? I don't think so. As I have mentioned before, there seems to often be some confusion between the thinness and flatness. If we properly separate those two characteristics of an airfoil, some of the confusion goes away.

banktoturn