thanks willow and thedaemon for your patient explanations. And thanks zeid for your sketch.
I'll have to dig for more videos of flying bat, but i have to disagree with your proposal of "full force down" reverse-flapped wings. I agree that a sheet of paper would do that but unlike a wing, a sheet of paper don't make me fly. Afaik, what makes something fly is either
- speed (like with a plane) and difference of pressure between the two sides of a wing
- wing movement "catching" the air within the wing and "pushing the air down" to lift up.
If you get a look at
this video of goose taking off, you'll notice that only the very end of the wings (that is just a feather's length where feathers are separated for direction control, not for lifting up) bends like a sheet of paper. The rest of the wing follows the angle of the bone, and much like i cannot scratch my back with my hand because of how my elbow is made, a goose cannot really appear as the frame #2 of thedaemon's picture.
Depending on which video i watch, it seems that bats do even smaller (but much faster) flaps than those goose, but they still won't show as on frame #2, which i see completely "non-batty".