winglets

True.
But I`m teaching aerodynamics in my club and it is extremely hard to
explain this to people. I don`t want to confuse people with vorticity
fields so the explanation "pushing air down will result in an
underpressure on top and an overpressure on the lower side of the wing;
that`s lift" seems to be sufficient for the understanding of lift as
far as gliding is concerned.

http://www.navier-stokes.net/

Not too smart to post when you`re just woken up. For those who wanna
know why, just take a look at my last post ;-)

Jarno Nieuwenhuize.
The Netherlands

"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.

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