The main spar of the Bede BD-4 is a 6 inch diameter aluminum tube. This is, literally, the piece from which the entire airplane hangs as the wings fly through the air. As you can imagine, it is kind of vital that the fuselage be well attached to the spar.
Here is a photo of the spar attached to the main braces. You can click on this photo, or any other, to see a larger version.
That structure gets beefed up even more with a .063 inch thick aluminum gusset. There will be 31 screws attaching this gusset to the underlying angles. I have a drawing which shows where the screw holes should be but it got too confusing to keep referring to the drawing, measuring something on the gusset, referring back to the drawing, measuring again on the gusset, etc. etc. etc. I solved that problem by sketching the positions of the angles and the holes directly on the gusset with Sharpie. When I got done drilling all of the holes, it was kind of amusing to look and see how close (or far away) my sketch was from where the actual holes ended up being.
It is a very good thing that acetone neatly cleans Sharpie off of aluminum!
