 |
| Looking good! |
The final major task to complete the trim tab is to fit it to the left elevator and match drill the hinges. I started by clamping the hinge to the trim tab in its appropriate position, then testing the approximate fit by seeing where the forward part of the hinge would end up on the elevator skin.
Fitting the tab is difficult when the hinge is clamped on since the clamp can't fit in the space on the left between the elevator and the trim tab. I had to use some duct tape keep the left side of the hinge in place while I maneuvered the tab into place. Once I was happy with the location of everything I clamped the hinge onto the trim tab.
I match drilled the holes for the hinge with new clecos put into every new hole as I went down the line.
At this point I could disassemble the whole thing and start countersinking the top of the trim tab spar. Just like the trim spar on the left elevator, the spar is machine countersunk instead of being dimpled to allow the hinge to fit flush to the bottom side of the flange.
Normally this would be the part at which I'd prime the parts, but having decided to skip that part I jumped ahead to deburring and dimpling everything. At that point I could start riveting.
Riveting the spar to the bottom of the trim tab skin is one of the more frustrating things I've had to do in this build. First off, I had to use the 2.5" flange yoke which wouldn't fit around the clecos. I had to remove clecos, slide the squeezer into place along the spar, then cleco the skin back down before I could squeeze anything. Second, the yoke will push the rivet out of its hole if the yoke touches the bottom skin, so I had to prop the skin open with some wood blocks to make sure the rivet would stay flush when squeezed.
The most annoying rivets to set were those in the horns where the trim tab will connect to the servo. I could just barely reach the rivets at the front, but there was no way I could have reached the aftmost rivets with the squeezer. I contemplated trying to balance the bucking bar in this little space but I decided to take the easy way out and put some MK319-BS blind rivets here instead.
 |
| These probably look a lot better than any attempt at bucking them would have... |
With the bottom riveted on I could start the process of matching the hinge to the left elevator itself. This step is crucial to ensuring you have a trim tab that will have the travel it needs while staying in line with the trailing edge of the elevator itself. I grabbed the same piece of aluminum angle I used when I was sealing the rudder trailing edge and I fixed it to the elevator to keep everything in line.
With the tab in place I clamped the hinge to the elevator and checked the edge clearance on the holes to be drilled. I had drawn a line on the forward half of the hinge that showed approximately where I wanted the holes to go, and thankfully everything lined up perfectly.
Starting at the clamped end (since the other end was loose and couldn't be clamped) I kept everything as still as possible and match drilled the hinge, putting in clecos as I went down the line. With the hinge holes drilled and clecoed I could remove the angle and make sure the trailing edge was still straight.
 |
| Perfectly straight, good to go. |
Being very careful, I drilled and clecoed the holes for the inboard tabs.
At this point I took the trim tab off the elevator and riveted on the aft hinge.
There are no dimensions I could find for the holes on the outboard tabs, so I did my best to put in two holes that would adequately hold both of the tabs down while maintaining adequate edge distances.
I ran into an issue at this point. The plans state that CS4-4 countersunk rivets are to be used here, but the tab is now closed out and there's no way for me to dimple these holes. The skin is also way too thin to machine countersink enough for the rivet to sit flush without enlarging the hole. I briefly considered skipping the countersunk rivets and going for round head rivets instead (this would have worked for the holes on the inboard side), but I realized that this might cause clearance issues between the outboard side of the trim tab and the left elevator skin.
If I had had the foresight to read ahead I may have been able to dimple these before I riveted the tab closed, but I came up with a compromise. Since I couldn't fully machine countersink the hole, I decided to only do it a little to give the countersunk heads a good foundation. Instead of using the countersink cage I took my hex shank deburring bit and gave the hole ten good rotations with a small amount of pressure to give it a nice chamfer without enlarging it.
 |
| A small countersink is better than no countersink. |
It's not as pretty as a fully flush rivet, but it will keep things secured just fine. It's mostly an aesthetic issue as this point, one that won't be noticeable unless one looks closely.
At this point I took the opportunity to rivet on the forward part of the hinge to the left elevator...
...and finally I reattached the trim tab itself to the elevator. It's done!
Per the diagrams I made a bend in the lower skin of the trim tab to allow it to clear the trim spar and its rivets when enough nose up trim is applied.
 |
| Neutral trim... |
 |
| ...and nose up. |
Finishing this section, I drilled and riveted the tabs to closeout the left elevator cutout. The red marks show where the rivets in the trim tab are when the tab is at neutral, so I made sure to locate the round head rivets in a place that wouldn't cause interference.
Although it isn't necessary, I also made a 90-degree bend in the temporary hinge pin with the hand seamer. I'll have to make a couple of bends like this when I get the final hinge pin in my fuselage kit, so this was good practice.
