Help! Help in building a very light balsa slow flyer with a 13ft wingspan

[Keaton Hare]

Hi all

I'm building a solar long endurance platform - target speed is 14 mph or so and target airframe weight is about 3 or so lbs for just balsa and covering - what construction should I use - and do you have any tips for construction? My plan at the moment is to use 5 ribs on each side with a carbon spar connecting. I'd like to know what the best thickness for the ribs would be - total airframe weight is ~12lbs. Trying to get the minimum weight but have only built foam before so interested in any expertise you can offer.

 

[Piotrsko]

The ribs do two things: transfer weight and aero forces to the spar AND keep the upper surface and lower surface from touching. 10 ribs on a 13ft span won't hack the stresses involved even with being carbon fiber and a stressed skin, imho. Might if the spar is thick enough to be glued to the skins to act like a rib.