Bjorn
You will get more useful results using JavaFoil for multi-element foils than trying to fiddle a single element.
Any experienced sailor will know how to trim sails to get all telltales flowing. Sails are quite forgiving in terms of maintaining attached flow. It is reasonably apparent when a sail is luffing by the lack of tension in the luff while telltales will show when it is stalling and/or there is flow separation in the belly or the leech.
There is a good deal of detail in Javafoil about the boundary layer. Using this you can assess if there is separation before the trailing edge. If that is observed then the results will be questionable. A sail would need the leech cord to be excessively tight to induce enough trailing edge hook to get flow separation. I have seen this, particularly where a main has been trimmed for heavy wind then used in light wind. The leech can get a hook. There will be a slight improvement in speed once the leech cord is released.
In terms of sail efficiency, aspect ratio is king. A feature of the HPs is their unstayed mast. This give clean airflow over the sails. The Aerorig on a HP makes it difficult to get to the trailing end of the boom because it is inevitably hanging out over the water. That make adjusting the leech cord, for example, challenging. If anything gets stuck out at the end of the boom like the outhaul it is a tedious task to release it.
Also note the aspect ratio used in Javafoil is based on a span efficiency of 1. If you have a deck sweeping jib then the span efficiency approaches 2. In that case the entered span should be twice the calculated span. A recent development with A-class cats has been extending the sail to the bottom of mast:
This increases span efficiency. Even the HP gets span efficiency above 1 with the boom not far above the deck of the lw hull when sailing high on the wind. The aerorig boom also acts as a partial end plate. Any gap over the deck less than 20% of the rig span will increase the span efficiency. There is some data on the web for gaps versus span efficiency.
Rick
On 08/11/2015, at 12:09 AM, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
BjornNot sure how well that would compare to a true double element foil, because of the lack of gap. My reasoning is that the foil will stall earlier without the gap, so the results should be pessimistic compared to reality with a gap.I don't know how to simulate multi element foils in xflr5, so I've been "cheating". See this pic:But I've learned to love xflr5 because of the ability to get so much output, and how easy it is to make nice graphs of the data. I'm also a bit skeptical towards Javafoil because of the reasons you mention about flow separation. I remember being able to produce unbelievable numbers before I realized that. But maybe I would be better at judging that now, after experimenting with xfoil. The visual flow lines around the foil in xflr5 is a good tool at showing when flow starts to separate.Rick,I used Javafoil in the beginning, and I still use it to produce foils for xflr5. I like tools to modify the foils in Javafoil, and the text based coordinate list makes editing easy.
http://postimg.org/image/jc4x5lto9/
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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