Time to start building the trim tab and closing it up. With the initial hinge fitting sorted out, now we can close up the bottom side of the trim tab. First off you need to attach the little 717/718 control horns that attach the electric trim pushrod to the trim tab itself. Simple enough right? WRONG! Lord this little task took forever and was not easy. First I decided not to use the aft most hole as it was too close to the folded edge. I just match drilled another hole a little ahead of this one and was able to use that instead. The problem with a few of these holes is that they are too close to the edge to get any sort of dimple die in there, there just isn't enough space. You could grind down (a rather expensive) dimple die to make one fit in there but that's a waste IMO. The new aft most hole I was able to get a pop rivet dimple die in there to make that dimple but the second one wouldn't even allow for that. On top of that trying to buck or squeeze these two rivets was going to be next to impossible. So pull rivets it was. The Mk319 BS rivets I just used on the other portion of the skin will suffice here. So only issue now is that the other hole, the one closest to the edge, again couldn't be dimpled. What I ended up doing was to use the deburring tool to actually countersink the receiving hole and the hole on the control horn. Not as accurate as using a cs bit but it worked ok. With that accomplished I could finish off those two holes with the pull rivets.

One thing I unknowingly did in this process is bent the aft most part of the trailing edge of the control horn for the longer one, so now it's sticking up just a tad. I was opening up the bend on the trim tab to get in and out of there and had the new hole cleco'd in place and didn't realize I was bending that little tab. I haven't decided yet what to do with it. Structurally it's fine, it's more of an appearance thing. I may just leave it or I may trim it off very carefully and clean it up. TBD.

So that little task took over an hour to sort out. The two forward most holes that attach those control horns get riveted through the little Z spar that holds the bottom of the skin in place so don't get in a hurry and miss that step. Now I could move on to riveting the bottom of the skin to the spar finally. Again it seems easy enough with a squeezer but after a few minutes of studying it, I realized there was no way to get the squeezer yokes I have in that tight space. Maybe there is a yoke squeezer shape that I don't posses that will work but my only option was going to be to buck all of them which I wasn't thrilled about. Very limited space to work with here and it's blind riveting. Fortunately you can rest a rectangular bucking bar up against the top of the spar if you have one that's the right shape. I have the little tungsten bar that I use on almost everything that worked in this space. To get the rivets going, the first one I had to use a shorter bar, more of a cube shape to set the first one, the adjacent clecos protruding through the piece make it a bit tight to get in there and again you can't see how the rivet is setting as you are hitting it. I did my best to set the first one and it came out ok. The -3 rivets are so easy to mess up due to their small size but fortunately the first one went in ok. From there I could remove a few clecos to the side I was riveting down and use the tungsten bar for everything else. I went slow and tried to take my time but by the end my hand was pretty fatigued trying to hold the bucking bar in place and put pressure against the action of the gun. I only messed up two of the rivets which I thought was pretty good given the circumstances. I was able to drill each one out without any problems and reset new ones properly.

So a lot of little tedious work on this section but the bottom side is done. Next is to move on to the ends and the top side.

At this point I'm tempted to order new parts and build another trim tab like a lot of builders apparently have. But if perfection is the enemy of progress, my work is acceptable to my standards and the accepted building standards so I will keep moving on. It's not the prettiest work but it will do.

Here's what I would do differently if I built another trim tab:
I would wait to bend the piece to final shape to do a few things first.
I would locate and match drill the holes for the two small control horns BEFORE I bent the end tabs down.
I would dimple those holes in the skin BEFORE I bent the end tabs down.
I would try to then rivet those control horns onto the bottom skin BEFORE I bent the opposite side (top skin) end tab into final position.

Doing the above would make riveting the control horns on much easier and faster.

Last thing I worked on was test fitting the top skin to the spar. The top side of the spar is the one that got countersunk last time around. Again the big gotcha is that you can get carried away with the countersinking on this thin material and remove too much. So I was very conservative. Turns out in multiple places I was too conservative and the skin with its dimples, even cleco'd in place, wouldn't nest inside the countersunk holes. So out came the deburring tool again and I began to remove more material in those holes with the deburring tool. I would just do one semi-forceful turn in each hole to remove material and then adjust the next hole, cleco everything back together and see how it fit. It took about 3 or 4 rounds of this to get all the holes to nest adequately inside their partner hole. I think if I was to do this part over, I would make the skin dimples as shallow as I could get away with. That would help to keep the countersunk holes from having to be as deep. In the end, it's all in order but again more work to get there. Next session we will close the top and ends up and we'll have a completed trim tab ready to be hung to the elevator.