Bjorn - "The figure of L/D=140 sounds like the performance of a wing section in infinite span. A peak L/D of 30 with infinite span sounds very low? a sail is that bad? why?"
Admittedly the L/D of 140 is the best that I found, but the reasons sails are so bad is due to a couple of things and I was being optimistic with the sail at 30/1. First off, the triangle shape is horrible, the mast instantly causes separation, twist is a mixed blessing, stays and shrouds are big issues, but mostly getting the sail to hold the optimum shape is the biggest problem.
With a wing none of those are issues. The drag on a wing is essentially equal to the cross section of the trailing edge, about equal to to a single stay. . . So just count all the stays and shrouds and count the mast as a dozen and you can see the hole a sail starts with.
I am probably going to use an Eppler 479 so that I can get the width I want for the bearings, chord and wall thickness.
EPPLER 479 AIRFOIL (e479-il)
It probably has a realistic L/D of around 90, it is one of the best that I have found that seems to meet all of my criteria.
Bjorn - "Are you sure that a sail would have a hard time exceeding Cl=.8?"
No not at all, it is easy for the lift on a sail to exceed .8 but the drag goes up faster than the lift. Same with a wing with flaps, the flaps increase both the lift and the drag.
I don't know if you fly but putting the flaps down is like riding up an elevator and putting on the brakes at the same time. It all depends on what you want to do.
Like I mentioned before, a displacement hull boat probably wouldn't (won't) perform as well with wings as with sails because the the drag of the boat kind of wastes all the drag savings of the wing. It is impractical to have a wing with the same surface area as is possible with sails. . . I am trying though^^
Talador
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Posted by: taladorwood@yahoo.com.au
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