I believe pre-twist is a remnant from windsurfing sails. I remember when pre-twist sails first hit the market as the big thing. Basically the head of the sail was designed to rotate a bit farther downwind vs. the angle of the boom. This was intentional in order to spill some wind especially in gusts.
If you were looking straight down on the sail from above the boom would be at one angle from the mast, while the head was at a different angle (pointing farther downwind). Kind of like people when they do the old dance called The Twist.
This should more correctly be called washout and in theory will result in a wing where the tips stall after the center section with a resulting more gentle change in lift. Likely better flagging stability.
Select to expand quotepatronus said..
Some wings claim pre twist in wing tips, anyone know what's twisted and why?
Windsurfing sails need twist as the wind speed is slower close to the surface than up at the top of the sail.
The the resulting wind direction ("real" wind + the relative wind created by your speed, coming always from the front of the board) changes between the sail foot and the sail top.
Here is a figure for a big sailboat:
I cannot see how it can be used for Wings, however. As wing tips act as a mast foot or a mast top depending on the tack, the wing would have to be pre-twisted in opposed directions on tack change. Windsurfing and sailboat sails do not exchange mast tip & foot on a jibe.
So wings naturally twist a ton between the strut and the wing tip because the structure just isn't great. I think "pre twist" means that unloaded they are twisted in the opposite direction(towards the rider) so that when they are loaded up they end up in a more neutral position
A few years back, I rode Ensis. It had no tip washout. At the time, Maui Clouds also had no tip twist built in.
Then I owned the 2021 Ocean Rodeo. It was the first wing I owned with a lot of washout in the tips. It rode butter smooth. Not punchy power like my old Ensis with no washout.
Duotones have a good amount of tip washout built in too.
It's a pro and con designer choice.
To my knowledge, no designer has cracked the code to free twisting dynamic tips like sails. So it's built in to the right compromise.
Pre-twist in a wingsail is the aero performance equivalent of mainsail on a sailboat designed with a vang and/or mainsheet system that can never be pulled on all the way. Combined with no adjustment of luff or foot tension. And building the mainsail out of stretchy cloth and giving it a hollow leech profile and tiny leech battens. Then marketing pre-twist as a desirable feature providing "gentle control" and "easy de-powering". Where the target customers would be those want a forgiving feeling in their hands, and a sail that behaves well when drifting the front handle or leash. But definitely not for those people who know the basics of how to operate a sail or who care about the performance and power of a sail for its size and weight.
