Hi Again; I could have put this in the tube and rag section, but it is about design principles, so I chose the Design section. Forum chiefs, move it if you prefer. So, I would like to know if it's acceptable practice to design a welded 4130 steel tube fuselage with different sized "cells" (as viewed from the side). What I mean by "cells" is the open areas between the vertical side tubes. Of course, what I'm really talking about is spacing the verticals differently at different locations from nose to tail. Around the wing area, where it attaches to the fuse, you've got a lot going on. There might be 2 or 3 attach points (indicated by arrows), and I would never have them bolt onto the side of a tube wthout having a vertical or diagonal meet at that location. This is why we might need more vertical tubes added into certain areas of the truss structure. The best way I can explain what I mean is to show a simple picture. It's a side view of a 3/4 scale Mustang replica, but it could be any other similar aircraft. Notice that there are simply more tubes in the area that's circled. So, is this acceptable practice, or do the stresses and forces not "flow" properly with differently spaced tubes like you see in the picture? Thanks, Tom.