ORIGINAL: Rodney
The flat portion at the bottom of a Clark Y airfoil is at a negative angle of attack if you are comparing it to the actual angle of attack of the wing. Thus, if the stab is parallel to it, you have approximately a negative angle of attack built into the stab.
BINGO! ! !
The airfoil's center line is NOT the flat bottom. It is the line that passes from the most extreme point of the leading edge to the point of the trailing edge.