Saturday, November 14, 2009

Taking a stab at the spar

Or sparring with the stab. I can't decide. In either case, I spent a beautiful Native American Summer day down in the Schmetterling shop working on the spar for the horizontal stab. This spar is different from those in the vertical stab and rudder in that it is built up from pieces to form a spar box.

As you may recall, there has been a bit of a delay in building the spar box as I waited for the 120 degree countersink bit to be delivered. It finally showed up a few days ago, but I've been spending some time working on the broken RV-6 cowl. Now that I'm waiting for parts for the cowl, I can get back to the RV-12.

Given the near miss on drilling the countersink holes with the wrong bit, I was very cautious with marking the holes to be countersunk and to what degree, so to speak. The drawings on the plans are a little out of the ordinary in that they "flatten" the spar pieces and show the flanges next to the webs:

Once I figured out the code, I marked the holes:

I also marked the new 120 degree bit since (other than a coat of rust) it looks nearly identical to the 100 degree bit:

Then it was drill baby drill:

With that finally done, I went on to prepping some of the other parts that I would be using in building up the spar box. This is the work I would have done while waiting for the 120 degree bit if it hadn't been for the repair work needed on the RV-6. Some of the parts were to be separated from an aluminum sheet that was far too wide to cut with snips. Out came the trusty hacksaw:

Not so trusty, as it turns out. It's a relatively harsh tool, and at times it can be a little obtuse when it comes to delicate parts. Snips can be like that too, but to a lesser degree. In this case, it was a simple matter to just bend the part back to where it belonged:

Here are all of the newly separated parts all lined up and ready for deburring:

The lightening holes in the bigger pieces needed to be deburred by hand:

NOTE: I have returned from the future with a warning. When it came time to test fit the horizontal stab to the aft bulkhead, I found that the AN4-12A bolts wouldn't fit through the holes in the HS-1213A brackets. It appears that they need to be final drilled to 1/4". What you should do RIGHT NOW is go find the AN4-12A bolts and ensure that they will fit through the holes on the HS-1213A brackets. Only the threads of the bolts go through the HS-1213B brackets, so those should be just fine as-is.

Because all of the parts on the relatively complex spar box have to be positioned perfectly, I laid everything out before I even thought about starting to cleco things together. The nice thing about having the holes countersunk in advance was that it made it easy to catch the fact that I had one of the little doubles in the wrong place. After all, it wouldn't make much sense to wait a week for a special countersink bit just to cover the countersunk hole with another part, would it? I'm rapidly learning that with this kit, if it doesn't look right, it isn't right. After re-checking the drawing, I got the little doubler in the right position:

With all of the parts having been verified to be in the correct position, I started clecoing parts together. The plans are very specific about the orientation of the in-spar ribs. They use two holes in each rib to determine the correct positioning. This is the rib on the left side:

This is the right side:

If you get them oriented correctly, the large lightening holes in the ribs will all be in alignment:

You can look down through it like a telescope to make sure they all line up:

What you can't do, apparently, is take a good picture of it.

Just before starting to rivet it all together, I realized that I had made a pretty serious mistake in positioning two of the ribs. See if you can see the difference.

First, the wrong way:

Here's the correct way:

What tipped me off that I had it wrong? Same thing as before: countersunk holes that didn't appear to have a purpose.

I got it all straightened out and put back together, then marked the holes for the correct rivet size since two different lengths were to be used. I also marked the holes that were to be left open with masking tape:

The rivets were all painfully easy to squeeze. Why painfully? Because the only reason I could come up with to use the flush blind rivets that caused such a delay was if it would be too hard to reach the rivets to squeeze them. I think I could have reached them easily. The flush blind rivet is the one in the middle:

It's possible that it would have required a bigger yoke on the rivet squeezer to reach it, though, and those cost about $100. Maybe it was just to moot the need for another expensive tool.

Here is the spar box with the top and bottom pieces riveted into place:

Then it's just a simple matter of clecoing on the forward and aft spar pieces.

Easy, that is, except for one thing: the holes in the ribs didn't line up with the holes in the spars:

That made No. Sense. At. All. With the precision with which this kit was designed, it was inconceivable to me that these parts could be that far out of alignment. After all, if it doesn't look right, it isn't right. I went back through every step of building up the spar box and could not find anything I had done wrong. I thought I was going to have to post a plea for help on the Vans Air Force forum and wait for an answer. That's when I noticed that the positioning holes in the in-spar ribs weren't where they were supposed to be; I had somehow managed to get the inner part upside down. All I had to do was de-cleco the fore and aft spars, turn the inner part over, and cleco it all back together:

Voila! If you put it together correctly, everything lines up! Again, I am very, very impressed with the thought and expertise that went into the design of this kit.

By the time I had the spar box clecoed together, I had spent the entire day in the shop. Being a Fall Saturday in Central Ohio, it was time to call it a day and watch some Ohio $tate football!

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