New Aluminum Gas Tank

Back in the '70s we built a larger,18 gallon, aluminum gas tank so Dad would have more reserve flying in the AC Flight Rally to Oshkosh.  In the end it had 3 problems, there was very little room for your feet, The filler neck sat below the cowl surface so water could get in when it rained, the angle of the bottom allowed water to get trapped when setting on level ground.  After finding a pint of water in the tank one day I decided a new tank design was needed.

I decided a tank of about 14 gallons would be plenty for my flying.  As a result I could afford to use the space for my feet.  The bottom also had to be round so water could not collect.

In 2008 I started working on this.  I made a corrugated and poster board mock-up of the tank to work out a shape which would satisfy my goals.  To get foot room I strapped my biggest pair of boots to the rudder pedals and built my mock-up around them, lots of room, YES.

The old tank had a cutout to fit the radio, and the instruments were very close to the back of the tank.  I made the tank length short enough to leave room for the instruments and for the radio I plan to add some day.  The radio will go at the bottom of the panel where there is the most room.

I tapered the top to follow the cowling so I could use the extra space for gas.  It also allowed me to move the filler neck up so the cowl would be below the gas cap.  The filler will be located toward the aft end of the tank.

If you look close you can see 4 square holes in the end of the mock-up.  When I started making the mock-up it did not have a wrap, just the end plates with square tubes taped to hold them together.  That way I was free to shape the end plates to best use the space.  The wrap was made after I was happy with the shape.  The wrap was only needed to make a pattern.

The problem then was how to mount the tank.  We had removed the original mounts when we built the larger tank.  In the end I decided to reshape the sides so that the tank would basically overhang and set on the upper fuselage longerons.

It doesn't actually touch the longerons.  It sets on 2 supports made of 3/4" plywood curved on top to set in the tank corners.

These support set on the mounts we added to the sides of the fuselage where a piece of plywood sat to hold the larger tank.

The support holds the tank about 3/8" above the longeron.

The supports have blocks on the back to engage the old tank supports and to allow them to be bolted to the fuselage sides.  The bolts for holding them in place are the bolts for the straps which hold the tank down.  I may run some straps under the tank as well.

One advantage to these supports was that it provided a convenient place to attach the engine control cables and hoses.  The gas shutoff valve is operated by a lever on the throttle quadrant.

The cables from the quadrant have stops to prevent fore and aft motion but can wiggle to align with the quadrant arms as they move.

The tank is made from 0.040" 3003-H14 aluminum.  Originally I wanted to use 5052-H32 aluminum but I have never been successful welding it.  I made all the pieces from 5052 but when I could not consistently weld the sample pieces so I threw it all away and made it over again with 3003.

The process started with plywood forms for the end plates, which have a 1/16" radius on the bend corner and a 5 degree spring back angle on the edge.  The fir plywood pieces are backing blocks to hold the aluminum tight to the form while bending the edge.

I found a polishing wheel for the Dremel tool worked well as wheel to draw a cut line around the form block on the aluminum sheet.

The aluminum was sandwiched between the blocks and clamped tight.

The edge was hammered over with a plastic or a rubber mallet to form the lip.

I don't plan to weld at the edge of the lip most of it will get trimmed off before welding.  It's a convenient way to hold the parts together, we'll get back to this.

The warp was drawn from the revised poster board wrap pattern, I lost the photos of some of this.  The large oval is for an access panel where the filler neck is located.  I wanted good access for future repairs and for cleaning after welding.

The support tubes of the Shop Smith were used to form the bends in the corners of the tank.  The bottom has no taper to make forming these easier.

I have a piece of oak with a 7 degree spring back angle and 1/16" radius on one corner which I use as a break for odd pieces like the welding flange on the top of the tank.

The tank was strapped together with ratchet straps to hold it tight while punching holes to cleco it all together.  I'm not going to rivet the tank together.  It's just to hold it all while I weld.  I'll nibble away most of the flange between the clecos and weld those areas.  Then I'll cut off the tabs where the clecos are and weld those areas.

First we have some more parts to make.

I made a form block and hammered a more defined sump in the bottom of the tank wrap.

I need a flanged hole to weld in the outlet fitting.

I discovered that one of my hammers was the same outside diameter as the outlet fitting, 1 9/16".  I sawed a 1 5/8" hole in a piece of particle board.  That gives clearance for the aluminum.  I used the router to put a 1/16" radius on the corner.

I sawed a 1 3/8" hole in a scrap of the aluminum.

It was just a matter of pounding the hammer into the hole to form the flanged hole.

It worked and fit perfect.  The hole is a snug fit on the fitting.

The next trick was how to locate the hole in the wrap so it was aligned with the hole in the board.

I marked a piece of soft foam rubber with 2 concentric circles.  The inner circle is the size of the hole in the wrap.  I trimmed the foam to the outer circle and plugged it into the hole in the board.

You align the hole in the wrap with the inner circle and pound the hammer in.  The foam just squeezes out of the way as the hammer enters.

The fit is perfect and the fitting sets low enough that the water will drain to the gascolator.  One of the reason the old tank held water was that the top of this fitting was higher than the bottom of the tank.  If the flat bottom of the tank was setting exactly level water could collect to the depth of the fitting.  When you're taking off over power lines and houses, where everyone is home on a Sunday morning, it just gets too exciting to hear the engine sputtering because water is getting in the carburetor.

I used the access cover and filler neck from the old tank. The top of the old tank did bot taper like this one so I hammered a curve in the cover.  This lets the Cub type fuel gauge be vertical when flying level.  The nut plates for the screws are on the bottom of the cover and there is a reinforcing ring to weld on the top of the tank.

When the weather improves I'll weld up the tank.  I weld in a tent so nice weather makes welding more fun.  Especially since this will take quite a while to weld.