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Drilling the dreaded fuel tank z-bracket to baffle holes

I’ve got a couple of days off so I managed to get into the shed this afternoon. It’s been a few weeks since I was last in there so I spent a few minutes working out where I was up to. I managed to figure out that I had to drill the aft flange of the fuel tank fibs to size. This also match drills the z-brackets and tank baffles. This requires you to drill towards the main spar but not actually touch the main spar. Many other builder’s have made mistakes here and drilled too far resulting in them having to completely replace their main spars. Obviously, a rather expensive exercise so be very, very careful and make sure you’re completely comfortable with what needs to be done before you even think about reaching for the drill.

In order for me to reach the outer fuel tank ribs, I had to remove the outer wing skins. I just couldn’t see any other way around doing this. So I removed them and made sure all the fuel tank ribs were clecoed correctly.

I then got my 12″ #30 bit and set a drill stop on it so that there was JUST enough length to drill through all three layers (rib, baffle and bracket) without being able to touch the main spar rivets. This picture gives you an idea of how long I set this too. Be conservative and make sure you have plenty of air gap between the drill and the inner main spar rivets. When you drill you compress the spring on the drill stop and push the drill a little lower so if anything, err on being too shallow.

I then drilled all the rib,baffle,bracket holes to the correct size. The drill stop almost guarantees that the drill flutes get bogged up with swath. This stops the drill from biting into the metal and results in the drill taking forever to drill through the layers. I ended up cleaning out the gunk with my finger nails after every hole. I also checked the drill stop to ensure that the depth was still set where I wanted it to be.

After all the holes in the baffles were drilled I then clecoed the stiffeners to the right fuel tank skin and match drilled them. I had only done the left tank to date. This then brought both wings up to the same step in the plans.

With that done, it was time to put all the skins back on (again). I had enough clecoes to put both of the outer wing skins back on. This was a tight fit and it was difficult to get the skin holes to line up with the pre-drilled holes on the main spar. I had to twist the frame a little to get things to line up before I could get some clecoes in. As more clecoes were inserted, everything straightened up and it got easier the further I went. It was time to go pick the kids up from school so I had to call it quits before I could get the fuel tank skins on.

Build time – 2 hours.


About gstrack

Husband and father of 2. Control & Instrumentation Engineer. Flying nut. Gadget geek.


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