http://www.mandhsoaring.com/winglets.html

Here is a link to the M & H web site with a bunch of tech lit on the subject. Great reading for insomniacs.

My understanding is the winglet is an airfoil creating lift directed inboard to counteract the span wise flow that exists due the fact that the airfoil is finite in length. An infinite span would have no span wise flow and therefore no tip vortices (induced drag). It is not just fence to stop the span wise flow as that fence would have to be much larger and create as much or more drag than it reduces. As one of these papers indicates it is much easier to design a bad winglet and get a reduction in performance than it is to design one that actually works. Dr. Mark Maughmer (sp) at Penn State is the wizard (and the author of some of the tech lit referenced above). His designs are still evolving even after much study and experimentation so the idea that the average pilot can slap some board on his wing tip and actually get a performance advantage is a pipe dream.

Fly safely.

BC