The amount of well-known-information on winglets is only
exceeded by the amount of spam I receive, and by the
well-known-misinformation about winglets.

Most of what I do know about winglets was learned from
Dr. Mark Maughmer (Penn State), Dr, Mark Drela (MIT),
and others that take drag reduction very seriously. They are the
experts. I am not an expert, but I can repeat some of the
things that they have taught...

The first thing to realize is that adding a winglet to a good
design won't help much. In order to achieve much benefit
(drag reduction) from winglets, the entire aircraft must be
working efficiently with the winglets. Also, a perfect winglet
installed at an imperfect angle will degrade the preformance
considerably.

So... why use winglets?
To reduce induced drag, by reducing the amount of energy
shed by spanwise flow into the tip vortex. Winglets increase the
effective wingspan by about half of their height.

An added meter of span on each wingtip, with appropriate
twist, airfoil, and planform adjustments is worth about as much
as adding two meter tall winglets and making the appropriate
changes in planform, airfoil, and twist.

Why would you want two meters of winglet instead of 1 meter
of wing?
1) because you can't change the size of the parking spaces (737).
2) because the competition rules limit the wingspan (15m sailplanes).
3) because the resulting lift distribution causes less bending moment
on the center spar, allowing a lighter structure
4) it provides a place to put the fuel tank vent that is above the fuel level
5) it looks cool.

Since Greg Cole did a pretty good job designing recent Lancair wings,
only reasons 5 and 6 carry much weight.

In the case of the recent test showing that winglets degraded
the performance of a Lancair 320, it is likely that the winglet
airfoil and toe-out angles were less than ideal. While a
winglet can do a little to improve a good design, it can also
ruin a good design if improperly applied. Here's an example
where someone went to a little more trouble to understand
what was happening... the main point is that it is much
easier to make things worse than it is to make it better!
http://www.standardcirrus.org/Winglets.html
The oil patterns in the photos indicate the extents
of laminar flow, and any separation bubbles.

In the case of the 737, the winglet is not simply bolted on
and the fuel gains magically appear. Along with the winglet,
the flaps are adjusted downward by 4 degrees, and the ailerons
by 1 degree, increasing the average camber of the wing.
These changes work with the winglets to provide a more
efficient total flow field around the wing. Without the
accompanying changes, the winglet alone might be detrimental.

An added benfit of the increased lift, and reduced drag
is that the engines can operate at lower power settings
allowing quieter takeoffs from some sensitive airports.


For more background on how winglets work, see Mark Maughmer's papers:
http://www.mandhsoaring.com/articles/WL-Soaring.pdf
http://www.mandhsoaring.com/articles/ostiv97.pdf
http://www.mandhsoaring.com/articles/Winglet_Design.pdf
http://www.mandhsoaring.com/articles/Winglet_Testing.pdf

-bob mackey