28 January 2014

Wings: Fuel tanks. Doing them over.

My fuel tanks tested leak free, both left and right.  But there was one thing that bothered me so much, that I decided to scrap these completed tanks and make news ones:  The skin-to-baffle countersinks.

First issue was that those countersinks were too shallow.  They were just on the hairy edge of being at the 0.006" proud spec for shaving.

I went ahead and shaved them all with my shaver back in October of 2013, figuring I'd probably do the tanks over in the future anyway.

The bigger concern was that the holes in the skin were excessively elongated (they went down to a "knife-edge" but that may be unavoidable with a skin thickness of 0.032") as I wasn't very careful with keeping the drill speed slow.  I hadn't noticed the issue until after I was done sealing the baffle.  So, though I don't know if the corresponding baffle holes also got elongated, I had to assume they were.

Now, the plans initially have you skip countersinking every 10th hole to help keep things aligned.  The holes that you skip get countersunk after the baffle is sealed in.  This meant that I could rummage through my image library and possibly find illustrative images with the non-countersunk holes to see how they compare to the countersunk ones.  Two such images are below.  You can see which holes were skipped (highlighted in red) by their smaller radius.  That's the radius appropriate for an AD3 rivet.  The larger holes were too big. 

The two images of the baffle holes I found (not shown) didn't show evidence of elongation.  Under the assumption that the baffle holes weren't elongated, this is what the squeezed, set rivets would probably look like (pardon my poor computer art skills).

I also wasn't confident that the AD-42H pop rivets holding the tank attach zees were properly pulled.  Here's an image from inside the tank (looking through the filler flange).  The heads didn't look consistent to me.  I may not have been pushing down hard enough on the rivet puller when setting these rivets.

Perhaps I'm being too conservative, however I just wasn't comfortable with these outcomes.  So I shelled out the $1223.90 for replacement tanks.  For the second build, I will:
  1. Dimple, rather than countersink, the skin-to-baffle holes (being deliberate with my sealant use to ensure the baffle seals).  There is successful precedent for this (here is the reason for using the countersink approach).  It would be a stronger joint and avoid the pitfalls of dirty countersinking.  Apparently it's important to use reduced diameter dies on the baffle to avoid distorting the flange.
  2. Rivet the flange bearings last to ensure access to all rivets on the tank/rib assembly.
  3. Avoid missing a dimple.
  4. Avoid needing an AD5 rivet.
  5. Seat the AD6 rivet filling the outboard rib tooling hole so that the adjacent AD4 rivet can be fully set.
  6. This time, I will not use deeper dimple dies.  There's an interesting thread about this
  7. Do a better job routing the capacitive probe wires.  I thought they were too loose.
  8. Do a better job shaping the fuel float wires.  I had to buy a second set on the first run and they ended up with slightly different deflection extrema on each side.
  9. Generally, apply the sealant more cleanly (example of poor aesthetics can be found on the third picture above). A sealant gun should help here (purchased 3-Feb-14).
  10. Continue using the fay seal technique as described here
  11. Update 26-Feb-15:  If I were ever to do a third set of tanks, after sealing the mating surfaces, I would not shoot the rivets wet. I would remove the sealant from the dimple, buck/squeeze the rivet then dollop the shop head.  Apparently even Van's doesn't shoot their rivets wet and doesn't seal in the shop heads in QB kits (read whole page 15).  At least, that's how I construe the conversations in those two links.
Click here to follow my fuel tanks redo in reverse chronological order.

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