overdue at Sans Souci
Join Date: Nov 2007
Location: Midland Ontario
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Much thanks for the explain. But I did think apparent wind was also at play. In multielement wing sails for C-cats, they introduce twist in the top to prevent stallage that would otherwise occur. Apparent wind is higher at the top of the mast (as in all sails) and from a different direction because of wind gradient.But with rotating blades maybe this is less important than the lift loading issues.
My vector math was wrong. With the C-cat, increased wind velocity due to gradient or sheer at the masthead moves the apparent wind direction aft, thus requiring twist to avoid stalling. With the rotating blade, true wind speed is constant but forward motion changes from root to tip. That means with the blade moving faster at the tip than the root, the apparent wind direction moves forward, as has been noted.
I would think they manage loading by changing the lift area along the length of the foil. Lift force varies in direct proportion to the square of velocity, so you need very small blade area as you move toward the tip to produce the same amounts of lift that you do near the root.
That's enough aerodynamic theory of out me for a while.
C&C 27 Mk 1
Midland Bay Sailing Club
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