Sunday, June 24, 2012

Over the Edge

So this morning, we set out to Sears to return our smoking 33 gallon air compressor. Ben decided maybe that one was too small anyway, and it was a noisy little bastard being a single-stage, so we just got our money back and decided to upgrade to a Kobalt 60 gallon, 3.7 hp two stage. It's a big mean-looking sucker but it fits nicely in the corner of the garage near the door and is actually much quieter than the little one, and it runs less, so it should turn out to be the best fit for this suburban paradise we're in.

By 5:30 and after several trips to Ace hardware for fittings & such, including a comical conversation with the hardware guy on the sexual nature of plumbing terms, I was finally able to get to work on the plane. I had laid out my first line of rivets and the first stiffener on the right side of the rudder last night before the compressor blew, so I was ready to go. I had relished the thought of doing this all day... after all, my little backriveting test on the scrap piece turned out spectacular, so I couldn't wait to have a finished skin all nice and pretty. Backriveting is different from regular riveting in that you hit the shop head of the rivet directly with the gun, instead of using a "bucking bar" on the shop head. It works well with countersunk/dimpled flush rivets. You put your rivets in place in the rivet holes, lay a piece of tape over the rivet heads to hold them there, then turn the skin over and lay it on top of the backriveting plate. The backrivet set is springloaded with a plastic ring that pushes down the metal around the rivet, making it easy to hold both pieces down against each other and keep them flat to the plate while you set the rivet.

We put two layers of thin carpet on the work bench and I cut around the plate to form a padded, flush surface around the plate. Ben drew extended edge lines on the carpet to help mark the edges of the backrivet plate. He did not have to tell me that attempting to drive a rivet off the plate would lead to complete disaster.

The first line went great. He gave me a pointer on how to hold the plastic part of the backrivet set in place on the surface so it wouldn't slide, and then I was off like a herd of turtles. I finished the line and turned the skin over, carefully peeled the clear tape away and was relieved to see a pretty row of really nice, flush rivets. Every two holes I checked to make sure the rivets were safely within the boundary lines of the backrivet plate. Soon the right side was done.

I began working my way up the left side of the rudder in the same way. I thought about how easy backriveting is. You really have to try hard to screw the pooch doing this.. I mean, you don't have to hold any bucking bar. The rivet gun feels solid against the plate, which is firmly planted on the workbench and completely & tightly surrounded by carpet so it can't go anywhere. Yes, this is the life. I wish every rivet could be back riveted. And then it happened.

It is not a matter of "if" it will happen, but "when." It's a phrase I've read on the internet, namely Van's Air Force and a few blogs about building. It's the same phrase people use to describe landing a retractable-gear airplane with the landing gear UP, even though no pilot ever believes it will happen to them. Until it DOES. I placed the rivet gun on the last hole on the fourth stiffener from the bottom on the left side. It was over the backrivet plate, but the right side of the skin was in my way so I gave it a nudge. Being the cocky riveter that I was, I neglected to resituate the rivet above the plate. "BLAM-O," I hit it with the typical burst. It made not the strong sharp RAP as the rivet smashed against the plate, but a sickening tinny THUD as the rivet gun smashed the rivet and the skin into the carpet. "MOTHER F.....!!!!"

Ben came running over. He knew what I had done probably before I did. There was indeed a dent. We turned the skin over and looked at the outside. The tape was smashed. He peeled it away like a first responder tears a shirt off a gunshot victim and examined it closely as I sat there on my bar stool, wanting to barf. "Will this thing be polished?" he asked.

"No." He was thinking we could just smoosh the dent down and put some filler on it.  "But it's the last hole, so it will crack eventually," I said. As RV-3s and RV-4s age, they typically develop cracks in the rudder and elevator skins at the trailing edge of the stiffeners. The skin is very thin, only .016", and this is a delicate area that doesn't handle vibration very well. You do everything you can as a builder to avoid any undue stress, scratches, or bad rivet holes in this area.

He looked at me. "It's already cracked. I'm sorry... there's nothing we can do."

It was like looking down on the operating table at a dead... thing. Not a friend, and not really a pet. Nevertheless, I was really pissed that I had just wasted a couple dozen hours creating this thing, and now it was just a useless pile of crap. "GAAHHHH!!"  I wailed. He gave me a beer and a cookie. He's a good man.

I guess the good thing about this is that now I can actually test the hinged wood contraption I need to build to make the final trailing edge bend on my rudder, elevators and ailerons. This is something I have been dreading. What if you oversqueeze it and screw it up? The skin is intentionally under-bent at Van's so that we have room to get in there and rivet the stiffeners. Once they are all in, you squeeze it down to form a perfectly round radius at the folded trailing edge. That edge has to be perfectly formed in order for the plane to fly straight. If it's lumpy on one side or the other, it will induce a turn. If it's creased, it will induce a turn. If you overbend it, you're screwed. You live with it or you start over. Just thinking about it makes me nervous-- but now, I can try it first on my bum rudder.

Time to order some new parts from Van's. At least since we moved to Yakima, Van's is close enough (Oregon) that I won't have to wait a week for stuff to get here.

1 comment:

  1. OK, if you're with me here...I have a question. I'm considering two options for control surface skins, both with an eye toward getting away from the cracking problem -

    1) Use top and bottom skins, with a machined TE piece (kind of like the Midget Mustang uses). I have no problems with a row of rivets at the TE, and can live with the drag.

    2) The Cool Option...corrugated control surface skins, again top and bottom, joined at the TE. It would mean building a bead roller, which isn't a problem.

    What do you think? Am I going overkill to address what is a minor issue?