Fabricated Tank Fuel Return Lines

I fabricated the tank fuel return lines.  Since I may put an engine on at some point in the future that requires a high flow fuel return line, I’m running the same size return line as the supply line (3/8″).

Unfortunately, I wasn’t thinking about the location of the capacitive fuel sender plates when I drilled the holes for the return lines, so I had to modify the plates to provide room for the lines to run through.  I kept the same clearance from the line as from the ribs/stiffeners.

The return line dumps fuel into the last bay so that hot fuel returned from the engine won’t immediately be picked up and returned to the engine.

Fabricated Tank Vent Lines

I fabricated the tank vent lines tonight.  These are the lines that will let air back into the tank as fuel is pumped out during flight.  First up was to flare one end of the 1/4″ tubing using the Parker Rolo-Flair flaring tool.  This will flare aluminum, copper, and stainless steel in sizes ranging from 1/4″ to 3/4″.

Here is what one of the flares looks like on the 1/4″ aluminum tube.  I’m using 5052 tubing instead of the 3003 tubing that comes with the kit.  It seems to be easier to work with and isn’t as prone to cracking when flared.

The holes that the vent line passes through the interior ribs doesn’t line up with the fitting in the end rib (I had to move it over to make room to mount the fitting.  The tube has to jog over 1 1/4″ in the inboard bay.  I needed a little bit of straight tubing at the fitting, so I determined that the jog could take up 5 5/8″ of the inboard bay.  A little quick trigonometry (arctan of 1 1/4 divided by 5 5/8″) yielded about 13º for the bends (I know I wrote arcsin below.  Give me a break; it’s been 20 years since I used this stuff.  I’m entitled to be a little rusty).

Here is the bending tool I’m using.  It’s an Imperial 470FH.  It can do 180º bends in 1/4″, 3/8″, and 1/2″ tubing.

And here is the resulting shape in tubing.

You can see how accurate the bend was here.  The flare lines up perfectly with the fitting without having to use any force to hold it there.

Here’s what the inboard bay looks like.  You can see that the bend goes almost all the way to the second rib.

Here is the outboard bay.  You can see how the clip I fabricated earlier supports the tubing.  I’ll give a slight upward bend to the end of the tube after installing this for the last time to get the end of the tube as high up in the tank as possible.  After spending about an hour fabricating this for the right tank, it only took about another 20 minutes to repeat the process for the left tank.

Started Riveting Tank Ribs

Andre stopped by tonight and helped me get started on the tank ribs.  We managed to get four done this evening.  With an earlier start tomorrow, we should easily finish up all of the internal ribs on both tanks by the end of the night.

Here you can see the inside of the tank and the next to last outboard rib (the outboard rib is removed in this picture.  I still need to form filets on both sides of the ribs and encapsulate each rivet head in sealant.

Flop Tubes

I modified the anti-rotation brackets to keep the flop tubes from rotating.

Here you can see that it holds the flop tubes roughly centered along the rib.  They can still flop freely to the top and bottom of the tank.

I also torqued and safety wired the flop tubes onto the fittings.

Riveted Fuel Tank Stiffeners

My buddy Andre stopped by today to help me work on the fuel tanks.  I had a few details to wrap up before starting to use the sealant.  First up is to debur and dimple the #8 screw holes on the perimeter of the tanks.

The rivets need to be cleaned by sloshing them around in some MEK.

I lined the area around the stiffeners with electrical tape to keep the tank sealant from making too big a mess.

Rivets are installed in the holes and held in place with rivet tape.

Then the stiffeners are coated with a thin coat of sealant on the mating surface.  Here is Andre in action.

They are then pressed over the rivets and back-riveted into place.

Then sealant is spread around the edge of the stiffener to create a filet and a dab of sealant is placed over each rivet.

The fuel filler neck is then riveted on.  We tried back-riveting these using a hand-held back-rivet bucking bar, but that didn’t work as well as expected and we had to drill out one of the rivets.  Either back-rivet these on the plate or shoot/buck them.

Here is the inside of the filler neck with the vent line retaining clip riveted in place.

We also riveted on the fuel drain mounts.  When you dab sealant over the rivets, be sure and leave some channels so that water can make its way to the drain.

Here is the outside of the fuel drain mount.  All of these rivets could be squeezed.

At the end of the day, we had both tank skins with stiffeners, drains, and filler necks in place.

More Tank Prep

No pictures today, but I got the other tank skin deburred and dimpled as well as deburred most of the ribs (did I mention how many holes are in the tanks?).  I’m hoping to have the tanks ready to start sealing by this weekend.

Trap Doors and Tank Prep

Both of the trap doors are finished and riveted to the second ribs.

I also deburred and dimpled the rivet holes on one of the skins.  There are a lot of holes in the tank skins since the rivets are placed closer together than on other skins.

Trap Doors

I also fabricated trap doors that assist in keeping fuel in the inboard bays during knife edge flight.

The trap doors swing open freely to allow fuel into the inboard bays, but swing closed to keep fuel in these bays as long as possible.