do335a, for the short span I use 5/16" steel rods 36" long. For the long span 1/4" steel rods 48" long. On the long span I use a support block to support the middle. You can just make out the support block in the picture after the 5th rib. I sand the rods clean then light coat with primer. I waxed the rods so ribs slide on easy. I use PAM cooking oil in the workshop. Good for lots of things. I'm going to try it on the rods. I forgot about PAM and used wax this time.
Skyhawk, two wheels; tail dragger. That is all I have been building.