I figgered the same - - filling the void made sense - but the other thing that bugged me is that it looked like nature - busily reducing unbalances -- moved the load away from the tips - (more work out there -)
So -If I were to try to prove any of this - I guess I would mount a prop on a long rod - in an open field, then spin the rod from a remote location and see where the the incoming low pressure load came from and where the outgoing - compressed air went -then I would make streamlined shapes (and not so streamlined) in front and then behind the prop to see what it did to the airlow ---and more importantly how it reduced or increased the load on the driving force.
(Country boy engineering)