May be they reduce drag, but in my opinion they can=B4t produce thrust. A=
n
airfoil can produce nothing but lift and drag. A wing produces "thrust"
because it=B4s lift is partially directed "forward".
I=B4ve made two graphics, I think they are more clearly than my bad engli=
sh:

http://www.ich-habs-doch-gleich-gesagt.de/fluegel.gif
http://www.ich-habs-doch-gleich-gesagt.de/winglet.gif

The black vectors are the real airforces.

Bernhard