RE: Aerodynamic of undercambered vs. flat-bottom airfoil?
Think of an airfoil this way - it is a streamlined profile wrapped around a curved line, called the mean camber line.
If the line is straight, then of course the airfoil is what we modellers call a symmetric airfoil, ie. it is just the streamlined profile. This is a non-cambered airfoil.
If the line is curved then effectively the streamlined profile is bent like a banana and we have what modellers call a semi-symmetric airfoil. This is a cambered airfoil.
If the line is bent enough, the bottom of the airfoil will be bent upwards and this is what modellers call an undercambered airfoil. This is another cambered airfoil.
It is clear that an undercambered airfoil is merely a cambered airfoil with a lot of camber and not much thickness. A flat bottomed airfoil is one where the thickness and camber are matched so that the bottom of the airfoil is flat.
To see the effect of thickness and camber, get a copy of FoilSim 11 (do a web search). You should even be a able to use this program to do a smart airfoil selection for your biplane.