Quote:
Originally Posted by sblack
A friend bought a fuel tank for his bushby mustang II homebuilt airplane from the designer. It was 6061t6, probably .040 or 0.050. It was made in the traditional way with the ends pounded over a form block and the skins wrapped around and flange welded. On that airplane the tank sits on the fwd fuselage behind the panel, so basically in your lap. He finished the airplane and was doing initial engine runs. He had the tank full. After a few minutes of running the tank seams opened up completely, essentially the tank burst and 30 gal of av gas was dumped on his lap, with the engine running and electric system powered. The fuel started draining out the drain holes in the belly, right by the hot exhaust. He pulled the mixture and jumped out, waiting for 12 yrs of work to go up in flames. Luckily it did not. It was a huge mess.
He had a new tank locally made and the welder said that flange welds are no good for tanks and did butt joints instead. I have read all of Kent’s articles on tank construction and I know that flange welds have been widely used for decades on all sorts of containers. So I suspect that there was a problem with the welding or the tank design i.e. not stiff enough etc. Strange as the bushby mustang has been around for eons so you would think the design was proven.
So this us a bit off topic but an interesting story and it reminds us that you really want a fuel tank to be well built.
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Hi Scott,
The problem with that tank is not the design. That design is proven by the tens of thousands of airplanes carrying fuel inside it, for decades, in rough environments. Had there been an issue with that design it would have been subject to an Airworthiness Directive, and the entire industry would have changed that design.
The problem is in the execution of that design, by the craftsman building that tank.
Example from real world aviation:
I have two wing tanks here that were made by a "pro fab/welder" shop.
.040 5052 with flanged ends that were fusion welded with a tig/TIG.
Made to "copy" an original 1940's existing pair of tanks.
The cracks in these end seams are not longer than 14 inches, and the fuel also splashed all over a hot engine and pilot, as he got down as fast as possible, and got it all shut off in time to exit safely and the plane was saved.
Problem: no baffles to support the long distances between end plates, so under positive and negative G's the seams failed, opposite each other and at the same end panel.
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It does help to have some familiarity with the various aviation tank designs so as to make clear and accurate conclusions.
