Originally Posted by
blackmail
2) there are models (i don't recall which ones) that have both, upwards winglets and smaller downward winglets mounted just aft of the upwards ones, on the same wingtips.
The MD-11 springs to mind, though in its case the small downward winglet seems just slight forward of the large upward one for leading edge, so trailing edge is far forward.
see:
http://www.airlinercafe.com/Walkarou...let%20Stbd.JPG