8% thick or so.
Figure on enough depth on the center ribs to accomodate your servos, battery, RX. The classic DD has the fuel tank semi exposed.
The only rib you need to lay out is the one at mid span. On your full sized drawing, figure out how long it will be, then make the high point about 8% of the rib's length. The high point should be about 30 to 40% back from the leading edge.
Little 1/8" thick plywood corner braces wrapped with thread and glue help tie the structure together. The fiberglass kite tubes really take CA glue well after they have been hit with fine sandpaper.
I make the mid span ribs out of 1/4 thick balsa. The center ribs can be made from 1/8" lite ply or "door skin" that you can get at a lumber yard.
There was a guy here known as "Da Screw" who was posting a new DD every week. He must have posted dozens. Try looking him up for more details. I'm pretty sure he did a build thread that shows his methods. He has some very cool looking covering schemes to look at too.
Last edited by combatpigg; 05-29-2014 at 11:14 AM.