Bearhawk fuel tank construction illustrations

above: Image 1

I build my main 20 gallon tanks using a standing seam and a baffle plate.

This results in 3 ends being exposed so that they can be welded together.

above: Image 2

The three thicknesses that make up the standing seam at the tank baffle plate has been tacked together in a number of places. I tack the entire tank, all 3 sections, 2 baffles and 2 end caps before I go back to finish the welds. When filling in between the tack welds, I start by heating the existing tacks and start welding from there to the next tack. The tacked spots are already bonded so that the heat flows equally to all 3 of the parts making the welding easier since all 3 sections are pretty much equally heated.

above: Image 3

This small (3 inch?)  modified locking pliers are used to hold parts together for tacking and then completing the welds

I bought 10 of these in two different sizes from harbor freight for next to nothing.

above: Image 4

This illustrates one problem that occurs when you are trying to tack the 3 pieces of aluminum

together at the tank baffle

The tank baffle is a plate of aluminum that will absorb a lot of heat before it can be welded.

If you concentrate the flame in one spot you run the risk of two things happening.

First, the two tank pieces that surround the tank baffle will melt and fall out long before the

baffle plate becomes hot enough to weld.

Second, aluminum expands like mad when it is heated. If you heat one of the three parts more

than the other two it will end up getting longer because of expansion and cause it to buckle and

leave a gap.

The solution to this is to work the flame over the welding site evenly over about 2 -3 inch area, gradually bringing

all 3 parts up to welding temperature. Once you have tacked the seam together, the rest of the welding goes easier if you start by heating

up the tacked spots where the 3 pieces of aluminum are already bonded, allowing the heat to flow evenly into all 3 parts.

above: Image 5

A section of the tank showing the tank baffle with its holes to allow, but restrict, the rate of fuel flowing

from section to section of the tank.

above: Image 6

This is a section of one of the ends of the tank showing the end cap that is welded into place.

Working on the end caps you will be welding 2 pieces of aluminum, not 3 pieces like at the

tank baffles. Since there are only two pieces to be joined you need less heat so you need to be

careful not to linger too long in one spot or you'll cause a drop-out.

above: Image 7

The end caps are made from a single piece of aluminum that is cut and folded.

When welding these in the ends of the tank make sure to weld where the metal

was removed to allow the end cap to be folded.

above: Image 8

The main fuel tank in the Bearhawk is designed to hold ~20 gallons of fuel.

The tank, as designed by Bob Barrows, is made from two large pieces of aluminum

that form the top&back of the tank and the bottom&front of the tank.

Two other pieces are made for the end pieces. These are riveted in place.

Internally there are two baffles that are also riveted in place.

Later, the rivets are welded "fuel tight".

I built one of these tanks in this manner an showed it to my EAA tech consultant.

He was wondering why I didn't use a standing seam to construct the tank.

I stubbornly continued trying to construct the tank according to specs with rivets... and I'm

sure I'd have been successful if I were welding things up with a MIG welder.

Turns out, as I see it, welding aluminum with an oxy/acetylene torch requires

that you apply a lot more heat in general. This causes a lot more expansion and

resulted in a tank that is really warped.

I decided to design a tank using standing seams. As it turns out, the standing seam,

because you bend the metal into a lip, provides some structural rigidity that holds

things in place far better than simply heating the edge of a large flat sheet.

To make this work I built the tank in 3 sections, with 2 baffles and 2 endcaps.

Each section was built out of one narrow, long piece of aluminum that I bent

to form the top, back, bottom and front. The outer tank sections would have one

end that is closed up with an end cap and one side that is bent into a flange that

matches the size of one of the tank baffles. The center section of the tank has two

flanges that match the size of the tank baffles.

To assemble this tank, start with the center section. Tack one tank baffle in two spots

at the back of the tank. Then tack the bottom of the tank in about 4 - 5 spots. Next tack the

front of the tank in 2 - 3 spots. Finally, working from the back of the tank, tack the top

to the baffle but only about half way, leaving the rest of the tank flapping in the breeze.

Now follow the same procedure for the tank baffle on the other side: back, bottom, front,

and half of the top.

At this point you can make the rest of the top fit. Mine took some reshaping of the flange on

the top so that it matched up  to the flange on the front. Once I got a pretty good match, I

added a few spot welds working from the back to the front, first one side and then the other.

Finally, I tacked the front top flange to the front front flange in 3 places and then completed

the entire front flange weld.

Having tacked the center section of the tank to its baffles, I then fitted the left and then the

right tank sections to the center section with a series of tack welds to hold everything together.

Next comes the end caps. Start with the back of the tank, then the bottom then the front and

then finally, the top. Then tack the front flange of each side section and you are ready to complete

all those tack welds.

I find it better to start from an existing tack weld to get the metal hot enough to weld. Since it is tacked

together, the heat flows more evenly into all 3 parts (at the baffles) and once the tack is hot enough to

start adding more welding rod, proceed to weld up to  the next tack weld. Continue this process until

the welding is done.

above: Image 9

This is just another illustration that shows the endcap and the section line that relates to the previous section illustrations

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jimrowanisdr.jimrowan@gmail.com