I recall a French fabric two-popper had no bottom of its main airfoil - if I can find a pic of the thing, I'll post it.
The Osprey has sheeting on the underside of all its airfoils.
Lots of old aircraft had no "bottom" to the airfoil, among them I believe the Wright Flyer.
Obviously, there are exciting photos of the Osprey on this thread, and posting this pic would be like pitting a pedal car to a Formula One racer, so I'll let a link suffice...;) http://www.ctie.monash.edu.au/hargrave/images/nieuport_1_1909_01_1000.jpg
