One more thing: Slotted flaps don't necessarily increase wing area.
Fowler flaps do that. A slotted flap may actually decrease wing area
(by shortening the chord line) as it deploys. It depends on how far
below the chord line the hinge point is. Fowlers, by definition, move
back considerably before their angle increases much. They run on
rollers in a track rather than on a fixed hinge point.

Dan