This trim tab....

Where to start? With the hinges all sorted out, it was time to work on the trim tab riveting and shaping. Despite my best efforts to prevent it, this trim tab, Version 2, is also a bit warped. Started off ok but after riveting and bending the skin in various directions to access the inside, it now seems to be off again. Giant sigh...

Let's talk about this trim tab design and why it's less than optimal. Even Van himself has said in the past that he didn't think they could get away with selling pre-punched kits for the very reason I'm about to explain. On a smaller part like the trim tab, with no internal bracing, structure or stiffeners, the skin is free to deviate from the desired shape with any little bit of extra stress or pulling in a given direction. Where does this stress come from you might ask? It comes from riveting the skin to the underlying spar. After you bend the trailing edge to shape, and then go and drill and align the hinges, dimple the holes, etc. the chances for you to introduce a change to your initial bend are quite high. Just the act of cleco'ing the skin to the underlying spar can stretch the skin slightly. Just enough so that when you finally rivet it all together, there is enough tension now on the skin to pull it in various directions. Think about it, the cleco's themselves don't hold the skin exactly in place, there's some wiggle room available. When you finally rivet the skin in place, that's it, no more wiggle room and hence, if the skin isn't perfectly aligned with the holes in the spar, it will start to pull the skin in that direction towards the front of the piece and away from the trailing edge. This results in what has happened to trim tab version 2. The top of the skin is pulling a little tighter than the bottom forcing the bottom to be ever so misshaped. The top is very flat front to back but has a noticeable bulge in the middle. The bottom is noticeably not flat from front to back and has a concave section through the middle. So sigh.... The design itself could be vastly improved. Maybe it is on the 10, 12 and 14 and soon to be 15 models. The 8 was designed in the golden days of home building where things were done at low cost to save money I suppose.

So what to do about it? That's my dellimma right now. Leave it and build on or try to squeeze it and bang it back to shape as Van's says you can do via their instructions in Section 5. I build the squeeze tool today and I have a block of wood to bang on the trailing edge. As I mentioned in the previous update I need to do this to the elevator trailing edges anyhow. Unfortunately so far, my efforts to straighten the trim tab have not produced any significant changes. Do I just need to squeeze a bit harder? Do I need to hammer a bit harder? We'll see. But for now, it's not terribly misshapen so for now I'm going to leave it alone and press on. In the end, it's not a huge process to build a new one (again) down the road if the airplane handling isn't quite right.

So last night I decided to rivet the skin together to the spar and move on. The bottom side went a bit better as I was able to set those rivets with my pneumatic squeezer which I hardly ever use. Only issue was getting the head of the yoke behind the skin and spar. Those all set fairly easily. So feeling good about my progress (it was getting late) I decided to go ahead and set the rivets on the top side of the skin. I could already tell this wouldn't be as easy due to the lack of space at the outboard end of the trim tab spar. I had read from other builders to rivet the hinge in place with the whole thing attached to give it more rigidity and keep the lower half from flexing or bending which is what I did.

WARNING: THE RIVET CALLOUT FOR THE TOP SIDE RIVETS IS WRONG. You need to go through the skin, the spar and the hinge piece. Those three pieces combined mean that the 3-4 rivet called for in the plans is not long enough! It was only after I set the first rivet that I realized this as it was way too shallow once it was set. So another sigh.... Fortunately I have some 3-4.5 rivets laying around and those were the correct size. So with that sorted out I was already off to a bad start. I could set the very end rivet on the outside with a no hole yoke and even the next one in but the next few I don't have anything that can get in there. Even a bucking bar is going to be tight. So I finished the rest of the rivets, the ones that I could set with a hand squeezer. Unfortunately I messed up a few of them during this process and spent a bit of time drilling them out. The inboard most rivet I tipped over. When I drilled it out, I came out the back side off center and enlarged the hole. Crap. Time to go up to a -4 rivet. Then as I was working my way back towards the outside, I somehow squeezed the skin and structure instead of pinching the rivet. I immediately saw a rather nasty bend in the skin, spar and hinge. The hinge pin itself was now no longer smoothly able to be removed, in fact it was stuck!!! Crap! So I decided to at least set the rivet next to it and then see if it could be unbent. I started to squeeze that rivet and did nearly the same thing!!! @#$% !!!!! What in the world was going on here?? Basically I let my guard down and didn't make sure the squeezer was contacting both sides of the rivet properly. Honestly I don't think I've ever done this before when riveting a skin on. Holy cow this was bad as now the skin, spar and hinge were bent in not one but two places!!! I kinda freaked out thinking I had just destroyed yet another trim tab after all this work.

Hang on, take a deep breath and let's look at it. My first task was the hinge pin. I knew since it was stuck that the alignment of the hinge loops was now off enough so that the hinge pin was stuck. Sure enough the little dents I had put into the skin, spar and hinge structure where enough to misalign the hinge loops to the point where the pin would not move. So I carefully took a set of needle nose plyers and began to unbend my mistakes. Slowly but surely I was able to get everything almost back to normal. I was able to get the hinge pin to move freely again and then was able to reset those two rivets safely this time around. This left me just two rivets which you can see in the pictures. I may have to buck those or I may just elect to go with blind rivets once again. We'll see.

So all in all, a very frustrating night even though I made some progress. This stupid little trim tab is gonna be the end of me I swear. And just think, there's probably many of these instances to come. Good times! Build on!