Bjorn
Talador is a character from a book and is prone to fairy tales. Nothing he/she writes can be taken seriously.
Xfoil used with correct input will give good results for single element. I am not certain how well it handles multi-element foils. I use Javafoil for foil analysis and care needs to be taken when setting up the multi foils that the total area is 100%. The results are wrong if that is not the case. Xfoil handles flow separation better than Javafoil. The Javafoil results are not reliable if a good portion of the foil is experiencing flow separation.
Multi-element rigs give high lift at the expense of higher drag. The aspect ratio is the main determinant of efficiency whether it is a multi-element fabric sail or rigid wing. The very best gliders achieve an L/D up around 40.
The lift coefficient of a sloop rig could be up around 1.5 - maybe higher with high camber sails.
Rick
On 07/11/2015, at 1:09 AM, "Björn bjornmail@gmail.com [harryproa]" <harryproa@yahoogroups.com.au> wrote:
I've learned to use softwares like xflr5. When playing around with it, I have not been able to find anything special with a single element wing compared to a multi element wing / wing with flap. I have found that a double element wing with about 50:50 chord ratio has a very similar lift/drag curve to a single element wing of similar shape. The difference in drag (I assume) comes from the roughness around the gap between the elements, and higher wetted area. But it's minor.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?
I have not found much difference in peak L/D between a single element wing, double element wing, or "sail" (very thin wing). Something tells me you are using different Reynolds numbers when comparing. Which would maybe make sense, since a craft like the Greenbird would sail fast, and a keel boat slow, in the same wind. But I don't think that is a fair comparison.
__._,_.___
Posted by: Rick Willoughby <rickwill@bigpond.net.au>
| Reply via web post | • | Reply to sender | • | Reply to group | • | Start a new topic | • | Messages in this topic (19) |
.