RE: PL-12 Airtruck
I had thought about planking them but they are 1" in diameter at the tail end and 1 3/8" at the wing end. It seems to me that planking would have been more efort than it is worth and would not be as strong as if it is wraped. I ended up covering each side of the boom with one piece per side.
The balsa I used is the thinest I could get at the hobby store, 1/16". I tried with 3/32" but it kept spliting. Once it is glassed it should be pretty strong with the I beam down it's center.