^^^^ That curvature is what the Wright brothers discovered that enabled them to generate enough "thrust" to stay in the air for more than just a few seconds. Accord to a book I recently read on the subject they applied the same principles to the propeller as had been applied to the wing. BTW, that's one of the limiting factors to the speed of a propeller driven aircraft. The propeller tips get to traveling so fast that they break the sound barrier which creates a lot of "buffeting" and interruptions in the air flow. That buffeting is why supersonic aircraft utilize the swept wing design rather than straight wings. Swept wings minimize that effect.