Nice find!! P.S. Love the engine mount photo!
Perhaps "structural" isn't the right word for the ribs, however I feel that an explosive device ripping into them would cause a lot of the wooden ribbing to also be affected around them. The metal? Not unless the metal itself was hit. Maybe. Although the force of an explosion inside the metal could deform it outwards along with the force of the blast, causing all kinds of harm.
On a personal note I've played with steel tube chain link fence posts a number of times and had to uproot some steel clothes lines cemented into the ground. Depending on the thickness of it, the steel (by itself) isn't all that strong. It can bend and flex if you apply the right twist or torque to it. If a 20mm round hit this (granted "this small target in the tail") it would no doubt shred it, steel or not.
Makes me think of the 20mm fired from Glacier Girl at a 50gal oil drum. These things are super thick. They can hold toxic waste, hundreds of pounds of liquid, can take a beating, but this one round shredded it like a 1-ply tissue in cold/flu season!
http://www.youtube.com/watch?v=-p8h43TRXwkQuestion is would it blow large chunks off the wood, possibly destabilising the frame, maybe adding stress where it ought not be, causing a failure, etc?
Or would it hit the frame, maybe cause a total failure in half the tail?
Interesting thoughts to ponder. I guess I should leave off with "If the pilot could aim, he'd bring the plane down regardless of what it as made with" but sometimes it's fun to get into the details.
