Rolls and tricks aside, would you fly a plane with the wings fixed to the craft upside down?  Take off? Land?  Not being a pilot I'd imagine one would curse such a plane and say it flies like a brick (so my use was metaphoric).  I didn't say they can't fly upside down.  Rather they don't as a means of getting from A to B.  Boats should fly upside down (sail on port tack) as efficiently as they do right side up (starboard).  The point of what I wrote was to explain why the devil sail wings have flaps.