Thanks for the summary of the theory, which I'm sure is sound. I think the best L/D would give the best angle upwind ?
I also find it hard to explain several practical competition results, such as why C class catamaran have proven fastest with solid wing sails - despite their weight. Also why double surface hang glidiers wiped out single surface in competition , see
I suspect response to varying angles of attack is as important as coefficent of lift and L/D. Actually our current 'flexy' sails seem more tuned for wind range than max lift coeffiecnt ? Could a wing sail be so user friendly ?
I agree downwind looks like an area where experimentation might still produce new concepts. Back on the phantom 380 thread I posted
"No arguing that drag will increase with slots, which is why I've only really imagined them in the downwind dash scenario. There we are operating at as high as angle of attack as we can , proabaly well past most efficient. I wonder whether by running deeper, if a little slower, with slots (ie closer to the spin equipped boats) it would be possible to make more VMG. For general aviation take landing drag is probably pretty irrelevant to the designer. However if the goal is different then the placement and sizing of the slots would differ to meet the changed target. The only windtunnel work I know of in this area has been done for paragliders, and is shown on
According to independant tests (in the table 2/3 the way down) they got virtually 20% decrease in stall speed (ie high angle of attack) for 2% increase in drag at minimum angle of attack (equivalent to upwind). Certainly those things sell well enough. I've spoken to owners and they say the only disadvantage is a narrow 'poor performance window' at medium angle of attack - might be a problem for a sail on beam reach, but not up/downwind. Just makes me wonder - but we don't see so much experimentation nowdays with rigs, that all seems to be on the kites."