Aventura 2 or Buc 2 airfoil shape

Hi all,

About to the place in my project where the airfoil shaping ribs are to be installed as inserts into pockets in sail envelope between the for and aft tube spars. No two ribs are exactly the same and the maximum cord width looks to be achieved closer to the 20% point rather than 33%. Anyone know best foil shape and max cord width point to use? These things have so much drag, even with a relatively high horse power engine for type I expect little improvement in top speed or cruise. More interested in as low a stall speed as possible with this structural wing design. Reported stall speeds in print with flaps from 30mph to 38mph, yet many owners report stalls with flaps in the 42mph to 46 mph range. That is a bit high of what I would expect. With a reported high cruise speed of about 70mph, flaps up could be as little as 20mph from max level flight to stall. Not much overhead if one leaves the ground in calm conditions, and must negotiate wind gusts returning. i have been told that the foil shape makes little difference. My years of building slow flying model planes tends to contradict this claim.

Anyone out there with real world experience with this sort of wing design, your thoughts, recommendations, informed opinion would be appreciated.

Thank you.