adam_one,

This relationship between Reynold's number and the 'best' thickness for maximum lift is a pretty fundamental one, and worth understanding. As air flows from the leading edge over a typical airfoil, the pressure goes down, roughly until the thickest point of the airfoil, and then starts going back up, or 'recovering', as the airfoil guys would say. During the pressure recovery part of the flow, the air is flowing from a lower pressure to a higher pressure, which makes it much more likely to separate. The higher the Reynold's number, the more likely the air is to acheive pressure recovery without separating. Thicker airfoils have a bigger variation in pressure over the chord, and so the pressure recovery is more extreme. For this reason, wings operating at lower Reynold's numbers cannot be as thick as wings operating at higher Reynold's numbers without separating. This is why 'good' low Reynold's number airfoils are thinner, and tend to resemble flat plates, and also why flat plates compare more and more favorably to traditional airfoils as the Reynold's number gets small. Having said that, it is the thickness that relates to Reynold's number, not the flatness. For a given thickness, the optimum airfoil shape will not be a plate with constant thickness (i.e. a flat plate), at least in terms of lift and drag. If aerobatic requirements dictate that a wing be stallable on command, then a flat plate may have some favorable characteristics, although those could be acheived by reducing the leading edge radius as well. If, in addition to the ability to stall on command, you want high drag, and don't need high lift, then the flat plate starts to look really good, especially since it is so simple to make. We shouldn't overstate this by saying that flat plates are just as good as traditional airfoils for low Reynold's numbers, even though they are obviously perfectly good choices for lots of planes.

banktoturn