I know that, for fixed winglets, there are a couple of (equivalent) ways of looking at them. The first way to look at them is that they resist the tendency of h

I know that, for fixed winglets, there are a couple of (equivalent) ways of looking at them. The first way to look at them is that they resist the tendency of higher pressure air under the wing to leak around the wing tip to the lower pressure air on the top. The second way to look at them is that the winglets are flying in a circulating flow, and a portion of their lift is in the direction of travel of the airplane. (That one is trickier to see, for some.) My educated guess is that tip propellers would operate counter to the vortex direction, so that their vorticity would tend to cancel the trailing vortices. Notionally, you could think of them as throwing air back under the wing, though that's just a heuristic I made up to help imagine the direction they'd spin. Tl;dr: It's complicated!