"bagmaker" wrote in message
...

OK, I can get the theory (OR can I?) with winglets to reduce wingtip
vortices.

Surely the best direction for the tip is DOWN!, I understand this may
be difficult for actually making, especially with gliders and
groundloops, aesthetics, etc, but can someone explain why they are
pointing up?

Consider this..
The HIGH pressure area is UNDER the wing, we want to keep this from
migrating to the LOW pressure area ABOVE the wing.

:-)

Look at it the other way: 2/3 of the lift is generated by the low presure
over the wing, and 1/3 by the high pressure under it ... so what you want to
do is to "seal" in the partial vacuum above the wing.

Anyways, according to the 2/3 1/3 explanation you'd expect a winglet to have
about 2/3 of its area above the chord line, and 1/3 below, which is rougly
what you see on modern winglets on commercial aircraft.

MH.