AUX Fuel Tanks

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I’ve completed the AUX fuel tanks since last updating this website. Let me first tell you that I am a heretic... I am not welding my tanks. I’ve decided to rivet and seal with Pro-Seal a la RV method. I’m using specs from a RV-8 as an example of acceptable methods.

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Here I’ve drawn out the rough cut of the end ribs of the AUX tanks.

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Last Update:

May 12, 2011

 

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Drill stress relief holes at the corners of the tabs where the bends come together.

I rough cut the rib blanks on my old band saw with a metal blade.

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Next I finished to final size and smoothed all of the edges.

To begin forming I used a brake to bend the bottom flange. This gave me a straight and consistent flange to register the form on for the rest of the forming process.

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Here I have clamped the rib form block onto the rib and bent the aft flange over to give me a solid vertical edge to register the form block against.

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Now to the vise to finish bending the fore and top flanges.

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Here are the four AUX tank ribs after bending all the flanges. As with the wing ribs, the top curved flange caused the rib to potato-chip in shape. I used a shrinker to flatten the top flanges and straighten the rib.

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Once I had the ribs finished, I used them to calculate the dimensions of the tank bottom and end bends to achieve a close fit of parts. You can see here that I made the bottom as a one-piece part to eliminate seams at the ends of the tanks.

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Here one tank is held together with clamps for further deliberations on how to rivet the parts together.

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I inspected the plans and the tanks of a RV-8 for guidance. The RV used -4 rivets at a 3/4 inch pitch on the tank end ribs. I used the 3/4 inch pitch but I staggered the line a bit. Since I formed the flanges 3/4 inches wide to increase the surface area for sealing, I thought staggering the rivets would help spread out the gripping forces a bit. I reduced the pitch a bit a the corners where I assumed the stresses were more concentrated by the straps holding the tanks in the wing.

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Here a tank is clecoed together before enlarging the holes to #30 and dimpling.

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The messy part! ProSeal is applied to both mating surfaces and clecoed together. Rivets are driven with wet ProSeal in the countersink. A fillet of ProSeal is formed at the edge of the two surfaces and the rivet shop heads are covered with ProSeal for extra measure.

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A few more shop heads to cover with sealant.

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The rivet line on the outside of the tank.

Here is a bung placed inside the AUX tank that will hold an optical fluid sensor which will drive an annuciator light on the panel to tell me when the AUX tank is empty.

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The fuel pickup line and the line exiting the AUX tank are facilitated with an AN bulkhead  fitting through the tank side. The AN fitting nut will be applied on the outside of the tank. To keep the fitting from turning, an anti-spin plate was fabricated to capture the fixed hex of the fitting. The complete assembly as will be installed is on the right.

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The anti-spin plate installed inside the tank.

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Here’s the fitting as installed from the outside. The two rivets are holding the anti-spin plate.

Here’s the inside. The fitting and the anti-spin plate are both sealed with ProSeal.

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The inside was completed by riveting in the fuel sensor bung and installing the fuel pickup tube. The pickup tube was attached to the bulkhead fitting with a B nut that had been drilled for safety wire. The nut was safetied to an angle riveted to the tank top flange. The pickup tube was bent to pick up the fuel at the position recommended by Bob and glued in place with a little ProSeal.

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The top was attached with ProSeal on all contact surfaces and secured with stainless closed-end pull rivets which were coated with ProSeal also.

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Final fitting of the AUX tank. Fuel transfer pump is accessible from the bottom inspection hole.

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