I posted this on the Yahoo Foildesign group, but thought some folks here might find this of interest.

This thread on the Paragliding Forum offers some great insights into
the relationship between washin/washout, arc, and how the control bar
affects AOA: http://www.paraglidingforum.com/viewtopic.php?t=11147

Olivier Caldara's posts are particularly informative. You have to register to see his pics, but it's worth it.

Many hang gliders and rigid wing aircraft use tip washout (either thru
geometric or aerodynamic twist)to insure tips don't stall before the
root. A lot of foil kites are designed with washin (tip AOA greater
than center section AOA)to keep tips from luffing before the center.

krumly

It sounds interesting but, only understood bits & peices of it as it applys to kites... Im always of the mind though as I just mentioned in another post, that Im all for modifications to a kite as long as they benifit, the flyers ability to better handle/understand the kite, or become a real upgrade as far as "Performance"...

The basic gist from the posts is that for a paraglider (or foil kite) with a simple speedbar and riser system, similar to many foil kites with depower bars, is that:

with a FLATTER arch, any change in angle of attack of the wing by raising or lowering the bar will create a GREATER change in AOA at the tips than at the kite center. Tips are set at higher AOA than center so that when the kite is sheeted out, the tips won't luff.

with a DEEPER arch, any change any change in angle of attack of the wing by raising or lowering the bar will create a A MORE CLOSELY MATCHED change in AOA at the tips than at the kite center. Tips can be set with AOA closer to center section because they will be at about the same AOA as the center section when kite is depowered.

And of course this all depends on the ratio of chord length at the tips to the center chord, and on exactly how the bridle lines are rigged and distributed to the ribs. But Caldara's hand-drawn sketch shows it pretty clearly.

krumly