Bjorn - "I don't see how a wing (smooth or rough, in turbulent or steady air) will avoid the wing tip drag if it has a finite length."
It doesn't, and the vortex drag is very hard to compute^^ But it isn't a big factor for a wing compared to a V topped sail, that is why the trend to square topped mains, not to increase lift but to decrease drag.
On a wing the vortex doesn't increase drag rather it is a loss of lift for 1/2 to 1 chord length (not strictly true but close enough).
Bjorn - "So a sail might have half the induced drag of an airplane wing with the same aspect ratio. Not sure though if the endplate will remove the drag entirely, or just decrease it."
No, a sail has 10 times or more induced drag especially vortex drag. (That is kind of the point of the wing ^^).
On a wing an endplate increases the lift on the outboard section at the cost of increased drag. The same at the root unless the wing is carefully joined to the fuselage. Endplates on wings are a bad idea, unless more lift is needed, but they have a drag penalty, just like flaps.
Bjorn - "The polars of the foil section shows Cd=0.006 at Cl=1 (L/D=~160). The induced drag is 0.06 at Cl=1 with AR=5. This should mean that the L/D on the real 3d-wing is about 1/10 of the foil section, at Cl=1, right?"
It is probably accurate for a chord length at the wing tip, not for the middle section of the wing.
Bjorn - "In this paper i linked in a previous email, page 23, figure 21, you can see an induced drag of 0.2 at Cl=1.6, while the profile drag is much less. This should mean L/D is 8. It wouldn't matter much if the rig is a wing section with Cd=0.005 or a sail with 0.02, since that drag is so small compared to the induced drag."
The drag you see in the polars like a cd of .02 includes the induced drag, that is the whole point of the polars. It is also the miracle of the wing, it basically eliminates induced drag. I am an atheist and don't use 'miracle' lightly^^
Bjorn - "Low section drag will help at low lift coefficients, since there is less induced drag. My simulations show that a sail will have higher drag than a wing at low lift. So the wing is better in that case."
It doesn't matter what the angle of attack is, the wing will have less drag than a sail.
Bjorn - "That is how I see it. And I hope this feeling of confusion is not a sign of me being wrong."
I think I know what the problem is. At high angles of attack (10˚ to 15˚) just past maximum lift, drag starts going off the charts while lift declines. I think that is what you are seeing in your simulations. When the wing stalls, drag goes way up and I think that is what you have been looking at.
Also it depends on the wing shape you are looking at. . . Some wings have lots of lift and lots of drag. I am referencing a specific low drag wing shape that gives up some lift, because I need symmetry and a nice Cm too.
Talador
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Posted by: taladorwood@yahoo.com.au
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