Main Fuel Tanks

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Last Update:

May 12, 2011

 

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The main tanks were formed and dry fitted with the same methods as the AUX tanks. Again, a one piece bottom , front and back was used to eliminate joints.

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Final assembly with rivets and sealant began with the baffles first.

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To facilitate riveting the bungs to the main tanks I turned the flanges off of the bungs to have enough space for the rivets. Here is a before and after picture of the modified bung.

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Fitting the fuel bungs on the main tank. I decided to put the bungs on the inside of the tanks to give me more room in the tank bay for fitting the external fuel lines. Space is at a premium in the bay and this change should buy me at least 3/4 of an inch of extra space to work with.

To make this work I had to modify the threads of the bungs since these were bought commercial bungs threaded from the end opposite the flange. Remember that they are threaded with a tapered NPT thread. First I cleaned the threads well and filled the threads opposite the flange with JB weld and let it cure. I then tapped the threads from the flange side to reverse the taper in the bung. I’m not sure the JB weld was necessary, but it should increase the thread engagement when a fitting is screwed into the bung.

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A finger strainer applied from outside the tank. You can see that I also trimmed the flanges on the sides and bottom to get the fuel line as close to the bottom of the tank as possible.

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I applied another bung on the outboard end of the main tank to accept the AUX tank transfer fuel line. This bung was also mounted inside the tank.

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Here’s the AN fitting screwed into the bung that will receive the AUX tank transfer fuel line.

I decided to use fuel gauges with float type senders instead of the sight gauges at the wing roots. I made up reinforcing rings for mounting the sending units from .063 aluminum plate. These will stiffen the mounting hole and provide a base to mount nutplates for securing the sending unit to the tank.

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Here the sender mounting ring is fitted to the tank at the calculated position. Float type sending units come in left hand and right hand, but I’ve never understood why.

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The sending unit is fitted to the tank. The float wire comes straight and is designed to work when the unit is mounted on the top of a tank. To make it work when the unit is mounted on the end of the tank the wire must be bent so that the float uses the full range of motion between the bottom and the top of the tank. The height that the sending unit is mounted from the bottom of the tank must also be calculated. After digging out my old trig tables for angles and arcs, I managed to get the lengths and dimensions figured out. To check out my calculations, I hooked the sender to a fuel level gauge and tested the levels indicated against different float heights/fuel levels. I must say I was pleased with the results. I forced most of the error on the top end when the tanks are full. 

Looking at the finger strainers I bought long ago, I figured I could gain another 1/2” to work with in the bay by making my own. Instead of having a finger strainer with another AN fitting for the tube flare, I made a single piece strainer with the correct fitting for the bung and the 3/8” fuel line flare. I made the strainer longer which provides 40% more straining surface than the commercial fitting.

All 4 finger strainers needed. The wire mesh is stainless steel of the same screen size and wire diameter as the commercial fittings.

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