Wing tip ready for fitting, brazing practice

Some good hours in the shop this past week.

The wing tip inner liners came out really well:

Next up for the wingtip is final fitment and pilot drilling the rivet lines top and bottom of the wing and into the spar tip support L and rear channel L.

I decided to try my hand at brazing some aluminum in preparation to assemble the wing fuel tanks.  I started with a couple of scraps of 025, the same aluminum the tank walls and body are made from.  The goaI was to simulate a welded seam:

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To accomplish a good brazed joint on any metal, the surfaces of both pieces need to be cleaned completely to take off any oxidation.  I used the Dremel with a stainless steel wheel brush to clean both:

The shorter strip will be on top of the longer one and offset to simulate a lap joint, so I cleaned the centre of the longer one:

My choice for flame heat is MAPP gas - from what I'm told it is hotter and cleaner than propane:

Gently heat up the aluminum joint focusing on the seam where the brazing will be.  It needs to be hot enough to flow the brazing rod into the joint.  Anyone who has done soldering should be familiar with this.  The brazing rod melts at a much lower temperature than the aluminum.  Unfortunately it is difficult to capture the brazing rod melting into the joint.

My first attempt wasn't very good.  I found it hard to flow the brazing rod into the seam evenly.  The bending of the aluminum wasn't from the heat, but from tapping with hammer after it cooled.  The "weld" held but not very well (it broke much easier than it should have) and appears to be full of pinholes - not good for sealing a fuel tank seam!

Another attempt with two circular scraps was even worse....

I did manage to get a very strong bead along the edge furthest from the vice.  I think a big part of the issue is that the cast steel vise is acting as a very large heat sink, drawing the heat built up in the aluminum from the torch away and into the vice quicker than it should.  I'll need to consider this when I have the tank pieces clamped together, but I clearly need more practice before I can trust this as a leak proof fuel tank.

I've decided I also need more time to test the pressure sensors before committing to using them in the tanks, so I'm going to put the tank assembly off until later and focus on finishing this wing and the other one.

​I've added the root skin to the right wing:

The root skin goes from perfectly flat at the rear channel to very rounded over at the spar.  The root nose skin closes up the front corner of the nose.  It's going to be fun!

One issue I ran into is the upper root skin in proper position, won't sit flat on the spar root doubler.  Forcing this convex skin flat to meet the doubler only serves to crinkle the skin, so I had to reach out to some fellow builders to see if they had this issue and how they dealt with it.  I have an simple answer, more on that in my next blog.

This picture shows how much the upper root skin tapers down at the root.  You can imagine the root nose skin shaped somewhat like a cone as it tapers both inboard from the nose rib to the root nose rib:

Getting very close and happy with my decision to put off making the wing tanks.  I definately need more time with the torch and brazing rod in hand to make good consistent brazed joints.  Like everything else it will come with practice so I'm not worried.  This will also give me time to get some serious testing done on the pressure sensor set up which I still believe will work as I designed.

Thanks for reading along.  Onwards....