ahhhh-
OK lets look at one more "defination"
- If---- flaps add more lift -- why not leave them deployed for maximum cruise efficiency?
The obvious reason is that they are ONLY better at lower speeds.
And each increased angle is better ONLY at a lower speed
Now there is one interesting plain flap for fully symmetrical foils
That is a flat plate flap added to trailing edge the curved shape - such that there is a reflex top and bottom at the juncture of the flap and the wing
when the flat plate is lowered (or raised)
one surface becoes a smooth path from leading edge to end of flap the other side becomes an undercambered shape.