Enlarging wing tiedown holes

[Watzlavick]

The holes for the wing tiedowns in my QB RV-10 kit were too small to use a jam nut. Using a jam nut is important to keep the threads in tension to avoid cyclic loading on them. I needed to open up the skin hole a bit but I wanted to keep it as concentric as possible and also not damage the threaded fitting inside. I ended up drilling out a 3/8-16 hex bolt and then turning down the hex portion on the lathe so it would fit inside a 5/8 piloted hole saw. When threaded into the tiedown, it provided an arbor for the hole saw. I went slow and stopped as it broke through the skin. I opened it a bit more with a deburring tool and used a dremel with a small scotchbrite end to get the edges completely smooth. The jam nut still sat a bit deep so I added 2 washers so I can get a wrench on it.

I’m sure there are other ways to do this but just wanted to pass on what worked for me.

 

[wirejock]

For others, there's another option.
Buy two 3/8 hex socket head bolts from McMaster.
Run it in deep enough so the tie down hits it exactly where you want it. Remove it counting turns as you go. Apply some blue Loctite and reinstall.

The other option for builders is to install the tie down brackets flush to the skin. Assuming edge distances will allow.

 

[David Paule]

What analysis did you do to decide that avoiding low-cycle cyclic loading on that part was bad? And how do you reconcile that with the applied tension plus bending on the rest of tie down ring?

Thank you!

Dave

 

[Watzlavick]

What analysis did you do to decide that avoiding low-cycle cyclic loading on that part was bad? And how do you reconcile that with the applied tension plus bending on the rest of tie down ring?

Thank you!

Dave

I didn't perform any analysis, just leaning on my understanding of bolted joints where it's best when possible to avoid tension/compression cycling.. I wanted the option to leave the eyebolts attached during flight if needed and without the jam nut, you can't guarantee the orientation will be lined up with the freestream, well at least without Larry's idea of the socket head screw, didn't think about that! I don't think you'd want to just thread the eyebolt in partway and leave it loose, that would either back out or gall up the threads. For a tiedown only application, I agree there would be less bending without the jam nut and washers.

 

[Watzlavick]

For others, there's another option.
Buy two 3/8 hex socket head bolts from McMaster.
Run it in deep enough so the tie down hits it exactly where you want it. Remove it counting turns as you go. Apply some blue Loctite and reinstall.

That's a good idea, I did something similar for the Planearound door latch to get the angles on the pins to line up at the right place.

The other option for builders is to install the tie down brackets flush to the skin. Assuming edge distances will allow.

That is the best solution since then you can just use a washer between the eyebolt and skin, plus you don't have to enlarge the hole. I was trying to figure out how to get a spacer in there somehow but there wasn't any convenient access on a QB wing.