Sunday, October 25, 2009

There may be more than one way to skin a cat,

...but I'm not sure there's more than one way to skin a vertical stabilizer. And, just like skinning an unsedated cat (or so I would imagine, anyway), it's not particularly easy to skin a vertical stab.

The final steps actually began last night when I found myself back down in the shop just before midnight pulling blind rivets on the two stiffeners in the vertical stab rear spar. As Rick and I were riveting in the ribs yesterday, we found it odd that the plans didn't have us take care of those dozens of rivets at the same time. Rick suggested reading further forward into the plans to see if maybe it was taken care of later, but in previous forays into the depths of pages not yet reached I had seen nothing of the sort. Later in the evening it was still nagging at me so I went down to look again; still nothing. It wouldn't make any sense to drill all of those holes and not fill them, so I knew I must be missing something.

I was.

There it was, hiding right there in plain sight:

See in the upper right corner where it says LP4-3 TYP and points at one rivet hole? 'TYP' is short for 'typical,' and 'typical' is apparently Oregonian (home of Van's Aircraft) for "do this to every hole that we haven't told you to so something else with." This is not the only case of the Oregon taciturnity that makes reading the plans such a challenge, as we'll see in a minute.

I lost count of how many rivets there were to pull, but during the grueling process I re-learned something I had learned years ago when I was new to kart racing. I had overheard a couple of guys talking about how sore they were after races, and I thought it odd that I never seemed to be sore at all. Then, after a few weeks of experience, I realized that I wasn't sore because I wasn't doing it right. Once I get more competitive and started going home with aching muscles and big bruises on my back from racing so much harder, I realized that when I had heard those guys before, I simply had not yet learned how to do it right.

Fast forward to pulling blind rivets, a topic upon more than one person has suggested using a pneumatic rivet puller. I have one of those, but it's in the hangar along with the air compressor needed to feed it compressed air. "Too much hassle to bring home," I thought, "especially when it's not all that hard to pull them with a hand puller." Well, that was before I had to do a couple of dozen of them by hand. I didn't think I was going to be able to finish the last few, but I toughed it out.

And I'm sore from it.

The weather this morning had been forecast to be very nice for flying so I had planned an 0900 meeting with Co-pilot Rick. I wanted to fly down south to Portsmouth to have breakfast partially because it's a nice change of pace from the usual Urbana breakfast and partially because the fall colors are nicer down that way than they are up here in the Pool Table Flats region of Central Ohio. The problem with Portsmouth, though, is morning fog. Which they had plenty of, but then again, so did we. With a departure delay in effect, I went down to the shop to do a little work. This morning it was to be dimpling.

The front of the vertical stab has a fairing that screws onto it. The screws that will hold that fairing in place will use nut plates to attach to rather than nuts. The nutplates are attached to the inside of the skin that will cover the vertical stab frame (the Xmas tree). The skin is too thin to countersink to make room for the flush rivets that hold the nutplates, so it has to be dimpled instead. The hole where the screw will go in also has to be dimpled so the screw will sit flush against the skin. This is accomplished by using a dimple die on the rivet squeezer. There is a nipple side and a receiving side to the dimple dies:

Sounds easy enough, it in all actuality, it is. But I had another run-in with Oregon taciturnity. Here are the directions in the plans:


Final drill is easy, and the concept of dimpling is well within my grasp. But "flush outer surface??" I hadn't a clue. Near as I can figure, it's Oregonian for "dimple this hole so that the attachment part going through it will be flush with the outer surface." Which, because that's so patently obvious anyway, seems to cause more confusion than it abates. Once I got over that mental hump, the actual dimpling was quite easy. Here are some dimpled holes:

Too bad it's not quite as easy as that. You see, the nutplates have to be dimpled too so that they will fit snugly against the skin. That's a bit tricky because the receiving end of the dimple die is too thick to sit close to the hole without getting caught on the threaded part of the nutplate. Fortunately, this problem wasn't just invented when they put together my tail kit; others have dealt with it before. Rick provided a cut-down/ground-down receiving piece for just this purpose:

You can see the difference between the two of them in the 3/32" column. Here's what it looks like in action:

Man, am I ever glad to have his help! If I was trying to do this completely on my own, I don't think I'd ever get it done.

With the dimples done, it was back to squeezing rivets to attach nutplates, something I'm familiar with.

 Here they are all riveted in:


With that done, I was just about ready to put the skin on the Xmas tree. All that was left to do was deburr all of the rivet holes and the edges of the skin. But before I could do that, I had to do the most onerous part of working with aluminum: remove the blue vinyl protective layer. It's not that hard to do on the small parts, but on large sheets like the skins it's an enormous pain. I had already pulled off the layer on the outside of the skin, but had put off doing the inside because it was an even bigger pain. I think it took at least an hour to get it all off of there:

Finally! Time to cleco the skin to the Xmas tree. It started out well enough as I was able to easily get the first few holes to line up, but as I worked my way across the skin it got harder and harder. Until the skin gets put on, the skeleton (the Xmas tree in this case) of an airplane is surprisingly fragile and floppy. It isn't until the skin is put on that the frame develops any rigidity. This is called a "stressed skin" construction, and I'm here to tell you that today is the day that I put the stress into stressed skin. The last corner of the first side that I clecoed put up quite a fight. You can see where I had to amass the most troops in the battle:

Once I got it that far, I mistakenly assumed that the battle was won. Just when I thought it was all over but the ticker tape parade, I ran into a couple of pockets of resistance:

Hey look! No hole!! Where is it??? Well, the tip of that rib was reluctant to move up into the fold of the skin, and I had no way to persuade it, or so I thought. I eventually realized that the other side wasn't clecoed yet so I could simply lift up the skin and reach up inside there to push it into place. That turned out to be harder than it sounds (mostly because I couldn't be on both sides of the skin at the same time) but I got it done. There was another hole in the same situation, but even when I'd get it lined up with the hole in the skin, I still couldn't get a cleco in. Eventually my attempts just caused the tab inside the skin to simply bend down. I had to reach in there and straighten it back out, and as I was doing so I'd swear that I noticed that the hole in the tab was only drilled to a #40 size, not a #30. It was the same on the other side too. I just lined them up real nice and ran a #30 bit through them and all was fine. Thus was the battle finally won:

Does that look like a triumphant warrior with a splitting headache? Well, it should - that's exactly what it is!

The stab was ready to be riveted, but that would have to wait until I could haul it up and out to the hangar in order to use the pneumatic rivet puller. That, and until the headache went away. I wish I had gotten my morning coffee sooner... stupid fog:

1 comment:

Matt said...

Looks good Dave... but before you complain too much about the RV-12 plans, check out the "plans" for the other models sometime! I've been on the step that reads "now might be a good time to install an electrical system, if you want one" for about a year now. :)

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