Although it may not appear like it, the C-172 does have dihedral. The Cessna 172S Pilot's Information Manual states that the wings have a 3.5 degree dihedral. For this model, you will have to add extra dihedral to increase the roll stability of the glider. Bend both wings upward until the wingtips are even with the top of the vertical stabilizer. Make sure both wings have the same dihedral and that the camber was applied evenly. In addition, ensure even up-elevator on the horizontal stabilizer and that the fuselage and the vertical stabilizer are straight and without warps.