spar holes and part 5 of Fuel tanks

Finished laying out the outboard nose skin.  This area has a double row of rivets along the spar line into the wing tip end of the spar:

Also drilled the main spar out to A5.  The amount of aluminum swarf created by this project never ceases to amaze me - I can't imagine how much debris is created by the match drilled factory kits!

Ron and I went to visit with Henri, a local builder and master welder that Ron knows and who is also an aviation guy.  His shop is next level - he is building a six seat CompAir 6 which will go on floats.  He too is doing an "auto conversion engine" which is an 8 cylinder twin turbo diesel with 6 bladed propeller.  Unlike my Corvair which has a proven method, he's doing all this design and testing himself.  His work is amazing and has to be seen to be appreciated.

After much tongue wagging over his shop and project, we had a look at my fuel tanks.  Although Henri says welding my fuel sender assemblies would be no issue, he wasn't comfortable attempting to weld my fuel tanks.  The concern is the thin 025 aluminum will most likely warp beyond easy correcting with the heat of welding.  Based on this, his suggestion was to seal and rivet the tanks and after a bit of research, that process is equally effective without the risk of warpage.  Kit builders get their tanks from the factory pre-welded but they are expensive to order as individual parts especially the long range versions I want.

When I was forming the tank sides, I followed the same method as the wing ribs which included relief holes at the corners.  I originally thought these wouldn't be a issue as welding the tanks would fill in these spaces easily.  Sealant isn't capable of covering this big of a gap between parts, so I needed to come up with a way to "fill" the holes that wouldn't also change or interfere with the assembly of the tank.

A piece of 025 "L" can be formed to back the outward facing corners of the tank side panels.  A 45 degree notch in one leg of the L allows the angle to be bent to the same angle as the corner, and some light brazing welds the angle into a corner:

This picture shows the concept better.  This is the same angle in the vice prior to bending the second corner:

The completed angle backer, brazed closed and sitting in place.  This will be treated with sealing compound before being riveted with the tank skins.  This will effectively close the corners of the tank sides:

The front corners of the tank side.  By placing on the outer side of the the tank wall, it doesn't change the geometry of the corners and the tank skin will still fit correctly.

When sitting in place, I marked where the ends of the corner backers would be, then transfered that mark to the outside of the tank skin so I could lay out the rivet lines:

On the bottom side of the fuel tank, there is one flute in the side wall and several on the top side.  I made a mark to remind me that I won't be able to rivet here:

Laid out the rivet line.  A3 rivets on 2omm spacing worked well and avoided the aforementioned flute mark.  I shortened the rivet spacing at the corners to account for the corner backers.

Cleco clamps hold the skin in place on the side wall.  Line up the wall and drill A3 holes along the rivet line:

Same process for the front of the tank:

I didn't drill near the tank outlets.  I'm going to drill and tap the outlet fittings for stainless screws.  With sealant between, the screws will tighten the fittings against the skin for a leak proof outlets: 

Repeat the process for the inboard lower side (shown on the right):

Very pleased how the bottoms and front turned out.  They will be the first to be sealed once I get the sealant from Aircraft Spruce in a couple of weeks.

Flip the tank over, and repeat the process, starting at the rear of the tank (3 rivets on 15mm spacing) and working forward, bringing the top tank skin down tight.  There are 5 flutes to avoid here, but the spacing at 20mm continued to work out well:

Then the inboard side:

Cleco clamps are very handy here!

The front edge was originally going to be welded, so edge size wasn't a concern.  Now however, I'll need to bend a 025 L and install it.

I've read many concerns from kit builders that their factory made tanks are slightly out of square, but this rivet/seal method seems to keep the tank much more square and straight.

Next up, adding the front edge angle, fit the corner doublers and drill/tap/fit the outlet fittings. Very happy this complicated assembly (welded or otherwise) is coming together.  I also heard back from the fuel cap manufacturer, I need to order the threaded flange for the tank that interfaces with the threaded tank neck.  Not expensive, but wish there were better instructions or explanations as I would have ordered them with the caps and picked them up at the same time from Aircraft Spruce.

As always, thanks for following along.