Fuel tank fabrication issues

[MarkM]

Hi All!

After a long hiatus, I'm finally back!
I have a build log website started on EAA's website. I'll link to it here once I get it updated.

My top wings are finished and in the hangar, while my bottom wings and center section are in my workshop. With the bottom wings, I only have the wing walks and leading edge plywood to go. Not too many issues there, fortunately.

I'm also working on the center section. I decided to outsource the fuel tank to a professional, but even he is a bit stumped on the top skin. When trying to weld the .090 top skin to the rest of the tank, the welding distortion left the first effort looking like a large, very expensive metal potato chip.

I realize most people are going rogue when it comes to the tank, using rivets and pro-seal, or fiberglass, and just about anything other than the welding the tank per the Makelan plans. Most everyone who does have a stock to-the-plans tank were those fortunate enough to buy one from Makelan when Jeff was still in business. I know I'm grasping at straws here: have any of you here actually welded your tank per the plans? If not, does anyone have some insight as to just what secrets Jeff Shoemake had to get his tanks coming out so nicely? I've seen photos of the Makelan tanks, and it's clear to me that the weld bead does not penetrate completely through the skin.

[painless]

Hi Mark. Welding aluminum is an art form. I became comfortable with welding 4130, but like you, I farmed out aluminum welds.

I had a pro from Enstrom Helicopters, located in Menominee Mi close to where I live do mine. I prosealed and riveted my tanks, using my experience building tanks for my RV6A, so I just had fittings welded for me. Craig made sure he had some moist towels near where he was welding to suck up any excess heat, and did each weld in increments, letting heat dissipate in between welding runs. Still had some distortion. One thing you can try is prior to closing up the top skin, rivet and proseal aluminum angle as stiffeners. Can’t be too close to the weld line due to heat possibly degrading the proseal, but even being within a couple inches should help. Just my two cents worth, and that’s probably all it’s worth…..

[MarkM]

Thanks for the input, Jeff!

What I eventually had my fabricator do was cut off the top 1/2" off the ribs and just weld a .063 top on, ugly plug welds on the baffles and all. It will be installed in my center section with straps, a' la CB-1. I will glue wood cap strips to the top of this and cover the whole shebang with fabric, also per the CB-1 plans. It will probably fit somewhere in between the 18 gallons for the stock CB-1 tank and the real-world 22 gallons I've been hearing for the Classic tank. (Picture the end ribs and baffles like the Classic, only 1/2" shorter, not the straight shape of the stock CB-1 design.) Unfortunately, this leaves me a little short-legged for an O-320. Still, considering the mission of the airplane (boring holes in the sky and short cross countries for pancakes and $100 hamburgers), this shouldn't be a problem.

BTW, I took the SportAir TIG welding workshop in Oshkosh last weekend to brush up my welding skills to start the fuselage this summer. They've greatly improved this workshop since I last took it about 10 years ago: they've even included an introduction to welding aluminum and stainless steel. You're right, welding aluminum is a bit of an art, and will take lots of practice, but it is not beyond us mere mortals. I did present my tank story to the instructor. He agreed with my assessment that the per-plans Classic tank is NOT a good design. In order to keep that .090 top from distorting during welding, those welds must necessarily be kept very small (to keep it from fully penetrating the top) and probably pretty cold (to keep it from distorting). Such welds are unlikely to hold up well in service, especially since that top is also acting as a mounting flange, and those welds are supporting the entire weight of the tank and 132# of fuel. With that in mind, I would advise the folks with the Makelan tanks to keep a close watch on those welds in service.

I think the best approach with this tank is to weld up a lower tub, with the bottoms, ends, baffles, and fittings welded, and rivet & pro-seal the top on to that... which sounds like pretty much what you have done, Jeff.