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The large amount of camber your wing has allows it to fly slowly. At very slow speeds, a highly cambered wing might well have lower drag than a less cambered wing. At higher speed, the less cambered wing will have less drag. A flat-bottomed wing is generally also a pretty highly cambered wing. If you want less drag at high speed, you want lower camber than a flat bottom. If you want to change the camber of the wing, the cleanest way is to build a new wing with a different airfoil section. Camber is determined by the shape of the 'mean line', which is the curve that runs through the center of the airfoil from the leading edge to the trailing edge. For a symmetric airfoil, the mean is straight, and for a cambered airfoil, it is curved ( cambered ). The shape of the bottom surface is not what determines camber. If the bottom surface is concave, it usually means that the airfoil has some camber and that the airfoil is thin enough to make the camber show up on the bottom. Most wings with moderate camber and moderate thickness remain convex on the bottom. The terms 'undercamber' and 'semi-symmetrical' kind of confuse the issues of camber and thickness. The only advantage of a flat-bottomed wing is that it is easier to build. You can't determine anything about camber from the shape of the bottom surface.

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