winglets

At 04:12 07 January 2006, Bagmaker wrote:

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

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

Consider this..
The HIGH pressure area is UNDER the wing, we want to
keep this fro
migrating to the LOW pressure area ABOVE the wing.
Our current wingti
style will restrict the LOW pressure from bleeding
sideways (and wh
would it anyway) yet the majority (?) of our lift is
generated from th
HIGH pressure BELOW the wing.

This high pressure is what we want to retain and utilise,
yet it i
still allowed to bleed out sideways. Sure the current
shape of wingti
will reduce the vortices caused by the two pressures
meeting, and thu
the drag, but it wont increase the lift available by
retaining the HIG
pressure UNDER the wing, where it is most useful.

I also note the return of the 'plate' style winglet
on lates
jetliners, half above, half below the wingtip, instead
of a larg
winglet.

Enlighten me, O knowledgable ones!



It's not so much about blocking the flow since it's
all going to mix once it gets aft of the wing anyway.
The winglet actually reduces the vortex by creating
circulation off the vortex itself - this is why a winglet
is an airfoil and not just a plate. It flies in the
local flow field at the tip and creates a lift vector
that is inward and slightly forward (that's right forward
lift - thrust!).

I honestly don't know if pointing up versus down is
for practical or aerodynamic reasons - I can't think
of a reason why it would matter, you'd have to reverse
the airfoil and reoptimize for the underside flow field.

9B