In many (most?) models equiped with wings with Clark Y airfoils, the horizontal stabilizer (assume a flat plate) is usually parallel with the bottom of the wing airfoil. Shouldn't the stab be positive by 3 - 4 degrees?

Paul

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.

It should be remembered that there may be other reasons for having some positive in the stab. A Clark-Y foil is more often on a high wing plane and it is often the location of the wing that determines giving a stab some corrective incidence because of the wings drag and the resulting pitch force that it introduces because of location. Often the drag factor of the wing is countered with engine thrust but it wouldn't be improper to counter it with stab incidence but I doubt that numbers like 3-4 degrees would be seen. Probably less than a half degree.

A stab on a high wing plane might get a touch of positive stab to counter the drag. My most recent build a Spacewalker having a long and thick low wing, required a touch of negative to counter the wing drag force. These trims are so subtle that it is probably better to build the plane 0-0 and then trim them to correct flight than to try to build a certain amount of incidence variation.

And... it doesn't normally make much difference which of the wing or stab that gets trimmed. Choose the easier of the two. If the stab is on a tube and the wing in a saddle, adjust the stab, or if the stab is glued and fixed, adjust the wing, only remember that the wing and stab will get opposite incidence adjustments.

What makes you think so, Paul?

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.