RE: OV-10 Bronco 81"
Flite:
I have not seen a lower surface door on slotted flaps before. I wonder what that's about. Even on airliners where cruise drag is king, they just have the bottom skins faired to the flaps in general, I think.
Dmyers, the flap pivot should be underneath where the flap surface meets the slot-lip (extended upper skin) or just slightly forward. That makes the circular geometry so the flap slides backward away from the skin, then down. What you get (look at the picture) is a converging passage between the wing and the flap. large on bottom, small on top. Air thru a converging passage accelerates. This jet of high speed air over the top of the flap is what makes the airflow stay attached and the high lift. Seems weird to let 'high-pressure' air thru the wing and get more lift, but the net result is more lift on the top skin. As long as you place the pivot so you get a converging passage at the flap settings you want to use, it should work very well.
I've used slots on a few planes, and they do work well. I'd love to see how yours does with the flaps walled off between fuse and nacelles and immersed in the prop blast. I've seen wind tunnel CL values reported around 5 for those sections - while a high CL for a plain airfoil is around 1.5
At some point of deflection, 40, 50, 60 deg or more, there is no longer a converging passage and the lift advantage is completely lost. This can work to advantage as well, if you want to make super steep high drag approaches.