Talador, thanks for the info about ncrit, that is a parameter I have not fully understood yet. I've left it to the default value so far.
But your email also confuses me. 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. Maybe it's possible to avoid wing tip drag at the sea end of the wing/sail, if there is no gap between the boat and the wing/sail. But the top will get that vortex/drag. 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.
Did you look at the picture?
Did you look at the picture?
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?
http://www.hiswasymposium.com/assets/files/pdf/2006/Fossati@hiswasymposium-2006.pdf
On Sat, Nov 7, 2015 at 8:26 PM, taladorwood@yahoo.com.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
Bjorn - "That is in infinite span, and probably at about 80% of peak lift,"
No, it is 'cross sectional' lift determined by the Reynolds number (which is determined by the velocity, wing chord and fluid) and the Ncrit (turbulence or roughness of the surface.)The last part (turbulence and surface roughness) is more important than the AR (wing tip drag) and is where the Ncrit numbers come from. It is hard to get a nice clean airflow in a high speed wind tunnel and it has to be high speed to get higher Reynolds numbers with scale models.A wing above a sail boat is going to have high Ncrit numbers because the wing is smooth and the airflow is smooth.When you start adding flaps or jibs and masts, the intersections between the parts are going to not be smooth and disrupt the airflow dropping the Ncrit numbers way down (raising drag).You will see at Reynolds number 50,000, Ncrit 5 has a better L/D (23) than Ncrit 9 (14.9), this is because turbulent flow isn't as disrupted at this speed. It is why vortex generators work.If you drop down to Reynolds 1 million you will see that at Ncrit 9 the L/D is 86 and at Ncrit 5 it is 81.If you use the Reynolds number calculator (10m/2 wind, 3 m chord, 20˚ C,) you will get a Reynolds number of 2 million. and the trend indicates that the L/D will improve with higher Reynolds numbers.Like Rob mentioned L/D is primarily a surface smoothness (and angle rate of change) and fluid turbulence issue, not an AR issue like Rick suggested.The secret of a wing seems to be the constant changing of the line tangent to the surface, curves in other words.Straight sections cause separation.Talador
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