Ref + 10
Mad Scientist, I've never heard that winglets create thrust! I thought that they created more efficient lift characteristics over the wing area by preventing air "spilling" over the end of wing. Aren't lift and thrust different forces? Increasing lift results in a smaller demand for thrust which then translates into lower fuel flows but they are secondary effects of winglets not primary ones.
Winglet Design
specifically:
"Unlike the other methods mentioned above, the winglet does not strive to reduce induced drag so much as it uses it to create an offsetting thrust."
Essentially the winglet is designed such that as the wingtip vortex strikes the airfoil surface of the winglet, the "lift" created by the winglet is directed both inboards and SLIGHTLY forward. By this means some of the wasted energy in the wingtip vortex is extracted and used to provide thrust. This energy extraction has a secondary, but lesser effect, in that the less powerful tip vortex does create less downwash (and hence induced drag).
Because this is a three dimensional flow problem, the winglet must be carefully designed and is optimised for a design point. Winglets are therefore best suited for aircraft with well defined single (or limited) design points, such that the off-design point penalties are minimized.
(For that reason alone, I am not surprised to note that the Yak-140 soon lost its winglets; although I believe that the god of aircraft structures took a helping hand in that case
)
The effect that you are describing is more properly that of an "endplate"; see the following for a discussion:
Winglets and Endplates
Essentially an endplate is a plate added to the entire end of the airfoil section; the design of the rear wings on an F-1 car, and the plates on the ends of the front wings too, are examples of endplates. As the article notes, endplates are a simpler, cruder approach to the same desire - a more efficient airfoil.