Aux Fuel tank
From the Archives, a fuel tank that fits in the baggage compartment that contains 19.144 US Gallons of fuel, of which 18 gallons is usable. 18 gallons of fuel in the baggage compartment that is limited to 100 pounds of weight is a pretty significant weight shift,( I estimate the installation at about 122 lbs.when the tank is full) so if you do this, consider the effects of all the extra weight. I have no 337 approval for this installation, anyone who has or gets a 337, please fwd to me or email to the webmaster. A fuel cap that fits the filler neck of our airplanes is an Ideal #46146, or from any Napa auto parts dealer.
The aux. tank is fabricated from 5052 .032, the baffles are .032 and the end plates are .040.
The tank is assembled with 1/8” self-plugging Aluminum “Pop” rivets, and then welded, all joints and rivet heads. Welding such thin metal requires a lot of skill and should be done by a competent welder, or the tank will buckle and not lie flat. Fuel Cap and Filler neck, use style No. 2 0513-0018, (all part numbers refer to the Aircraft spruce Catalogue.).
Weld a ¼” Hose nipple into the side of the weldable filler neck, this is for a vent line. 6 or 8 feet of 3/8” O.D. x ¼” I.D. Vinyl tubing is run from the tank in an overhead loop, then out the bottom of the fuse. The fuel pick up is One Alum. AN833 Bulkhead elbow size –6D. Weld fitting and a 2” Alum. Washer to the top of the tank as shown below.
No drawings are to scale. Recommend you cut patterns out of stiff cardboard, then Aluminum.
The Pickup tube is fitted to the elbow after the washer is attached, then the assy. Is placed in the tank and the washer welded to the tank and the elbow fitting. The Fuel lines are installed as shown below,
Install an indicator light or a low-pressure gauge (0-10#) in the Aux. Fuel line to give a warning when the fuel pump is starting to cavitate, allowing the electric pump to be shut off before any damage can occur. Replace the 2 position Gas Gauge switch with a 3 position switch that will show Left, Aux., Right. Note that the pump is located at a lower level than the bottom of the tank. A 3 way valve is fitted into the line from the left engine. Same type of valve as the main fuel selector can be used, HD 108-6 will work fine.
The Filler is centered 4” back from the radius line and 2 ½” from the edge of the tank. The fuel supply line is centered, 1 ½” from the edge, 2 ½” from the break line. AC sender is centered 4 ½” back from radius line, no height given below top edge. (I think 1 ½” below would work.) Sender is located on Left side of tank. Radius for front roll is 3 1/4”, = 7 ”inches for radius.
Make 2 baffles with fan tails around the edge and break to fit inside the tank . No figure is given for the flange on the baffle or the end plates. I would go with 1”, Use as few rivets as possible on the end plates, if you feel confident about your ability to weld this unit together, a good place to start would be to get the baffles tacked, then welded in place with ¼” runs every inch or two, that way the whole tank is more likely to stay straight. I intentionally did not show the entire radiused end with fan tails as I figure you can see what is needed. The end plates are made up the same as the baffles but without the drain and surge holes. Break to fit into each end of the tank, drill de-burr, Cleco and rivet in place. Before installing the tank be sure to leak check, and get a 337. If you are contemplating an Aux. Tank discuss this with your mechanic/AI before the project, it helps to talk to the local GADO folks as well. This can solve a lot of problems that could occur down the road.