Kelly:

The angle bracket on the firewall is actually per the design of the Falconar
F-12. A lot of Jodel engine mounts are this as well. The plans are unclear
as to material it only indicates 1&½" x1/8 thick angle. I tried finding this
in 4130 but was unable to so I settled for mild steel. The triangular end
caps are a great idea and I think I will use it. The design does call, for a
strap to be welded so that it bridges the tube to angle iron on the back
side as well. Here is an example as seen on Hans Teijgeler's Jodel DR1050
with a Subaru in Holland.

The design is not what is commonly seen for that kind of junction. It leaves the angles in bending along the fold lines. However, if the loads are not enough to cycle the joint in bending, then it's good enough. The mild steel to 4130 connection is OK for light loads.

 

The angle inserts in 4130 would help a bit. Double the part of the angle against the firewall with a piece of 1/8 4130 as a big washer but don't load the bend (or weld area).

 

Paint the bend and the tube to angle joints with just a bit of white lacquer. Grit blast the joints with rice hulls and paint without any primer. Just enough to prevent rusting.

 

Should a problem develop, the paint will crack along the failure and be obvious. Enamels and powder coats will stretch and conceal the crack. 

 

Lynn E. Hanover