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Trim Tab Spar

skelrad

Well Known Member
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I just countersunk the top holes of the left elevator trim tab spar, and it has me wondering if I need to take a second attempt. While the holes turned out relatively round without a lot of the wandering chatter that I'd read about with such thin material, the holes are definitely larger than I was expecting. Maybe I went overkill, but by the time I countersunk to a point that the skin dimple would nest flush, the hole seemed huge! I checked, and sure enough, it's about 4/32" vs 3/32". I don't know how much of that is due to the thin material, or if maybe I got more wandering than I thought.

This spar gets sandwiched between the skin and trim tab hinge, so I don't know if the larger hole is expected and okay, or if I need to get a new part and dial it back a bit.
 

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The oversize holes here are expected and OK. This flange is sandwiched between two other pieces with good support for the rivet, and the dimpled skin above serves to locate on these oversize countersinks. Section 5.5 has an illustration showing this.

The other thing you touch on is getting the holes out of round. That happens when the countersink pilot loses center as the cutter breaks through the back of the metal. A good answer is to take a piece of extra angle/strap/etc and match-drill it to the part beforehand, then cleco it on and it's your pilot guide for countersinking. But if what you've already done is consistently round and lines up well with the dimples, it's probably fine. Sort of a judgment call you have to make though.
 
You should have added a strip behind or even a piece of wood, match drill into the extra material, then countersink. This allows the pilot to have something to guide the cutter. Keep this in mind when you get to putting the nutplates in the wing spars!
 
Keep this in mind when you get to putting the nutplates in the wing spars!

Somewhere in the instructions it says its perfectly fine to rivet the nutplate to the spar and then use the nutplate as the guide for the countersink pilot while you drill out the screw hole c'sink. That was how I did it and turned out fine.
 
How much deeper did you go for this?
Section 5 specifies to countersink a maximum of .007" deeper than would be required for a rivet to fit flush.

I did have the countersink set to .007" deep of flush, but after doing some more experimenting, it looks like I did let the countersink bit wander more than I thought it did initially. I'm pretty sure that's what has given me the unacceptably large holes even though the countersink is the right depth. Lesson learned about how to back up the bit with a guide block. I'm going to order a new part and move on.

Thanks for all of the feedback.
 
Dimple...

This is where I deviated from the plans on the -7 and -14 (applicable to the -9 and -10 also).

Instead of countersinking the trim tab rear spar, dimple it. Dress the dimples to remove the microscopic "bulge" on the dimple conical section with the small, single hole/flute deburr bit. You will need to dimple the hinge material (AN257 or MS20001) also. It's a bit on the thick side, but it can be done.

I like the dimpled material interface over countersunk. Probably because I've partially funded Scott's retirement through all of the bad countersink attempts over the years. :)
 
Somewhere in the instructions it says its perfectly fine to rivet the nutplate to the spar and then use the nutplate as the guide for the countersink pilot while you drill out the screw hole c'sink. That was how I did it and turned out fine.

Exactly! I did the same thing and followed the plans.. the point I was making is that the cutter needs something to guide the pilot!
 
I decided to order a new trim tab spar and take a second attempt at this. I was expecting to get a note from Van's that it would likely be a while before the part would be available, but 4 days later, I already have the part (and a few other odds and ends) in hand! Pleasant surprise for sure!
 
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