Sunday, June 13, 2010

Sealing the steps

The aerodynamic shape of the steps in the portion that sticks out into the airstream is nice, but carrying that shape all the way up into the fuselage would make the bolting of the steps onto the supporting bulkhead require a much longer bolt. As a result, the streamlining stops just before the part of the step that goes through the fuselage skin, leaving an opening at the top. Van's asks that this hole be sealed using fuel tank sealant. That is the extent of their advice on the topic. Left unanswered are the questions regarding whether the entire void should be filled or just enough sealant put in to form a sort of lid, how one goes about keeping the sealant in place if one chooses the "lid" option, or how much sealant would be required to completely fill the void.

The choices are 1 quart (way too much), 1 ounce (probably enough for the "lid"), and 3.5 ounces. I went with the 3.5 ounce option - once you're paying shipping, it's better to err on the side of buying too much than not enough. Besides which, the 3.5 ounce size comes in a nice dispenser while the other two options come in cans. The 3.5 option also has a downside: since you mix it in the dispenser, you have to use all of it at once. It turns out that there is another problem with the 3.5 ounce option: the instructions are real head scratchers.

Here's what it all looks like right out of the bag:

Here are the directions provided:

Between all the dashers, rams, and vexation, I didn't know if I was learning how to load a cannon or if I was reading a Christmas story. "On Dasher, on Vexation...." Doesn't it go something like that? As I figured out how the thing worked, I grabbed a Sharpie and labelled the pieces:

Follow along as I go through the steps and translate into English.

Step 1: "Pull dasher rod towards the neck of the cartridge so that the mixing head is located at the nozzle end of the cartridge." Translation: pull the white knob out as far as it will go.

Step 2: "Insert the ram rod into the hole of the dasher rod. Push the ram rod until approximately 10% of the material contained in the dasher rod has been expelled into the cartridge." Translation: put the black stick in the hole in the white knob. Push like hell against tremendous resistance until the seal breaks and shoots 90% of the material contained in the dasher rod into the cartridge. Swear a blue streak at the 80% over-expellation of material.

Step 3: Read it yourself. That's too much typing. Translation: if by some remote chance you haven't already expelled 90% of the material contained in the dasher rod into the cartridge, continue to push the black stick in while simultaneously pushing in the white knob. Step 3: (cont.): "DO NOT expel a large quantity of the material in the dasher rod into the top or bottom of the cartridge."  Translation: ha ha ha ha ha ha! Now you gotta problem, Bub!

Step 4: Well, does it really matter what Step 4 says? You're already in a pickle. Just hope for the best and pull white knob back out and push it back in, twisting it clockwise all the while. Repeat 75 times or until you're arms are so tired that you just can't do it anymore.

After a couple of cycles

After approximately 75 cycles

It's pretty easy from here. Just unscrew the white knob (the dasher rod will follow along) and screw in the nozzle. Stuff the whole mess into a caulk dispenser.

Start squeezing it into the step.

So, here's the deal. This stuff stays where you put it (which is a good thing, but it also means it stays where you put it whether you put it someplace you wanted it or not - wear rubber gloves!!) so the seal is going to be somewhere between the "filling the void" method and the "lid" method. It will only go as far down into the step as the nozzle will reach. I filled it to the top, scraped off the excess with a scrap of wood, and put duct tape on it to hold the "lid" in place just in case the sealant actually moves at a pace indiscernible to the eye.

Be careful when setting aside the dispenser - it has some kind of momentum behind it and will continue to ooze.

As I mentioned, I taped the ends with duct tape and put the steps in a vise to keep the sealant from sinking away from the opening, assuming that's even possible.

I had a lot left over.

If I was doing this again, I'd either order the 1 ounce size and just spoon it into the hole to for a "lid" using a popsicle stick or I'd just wait and do it at the same time that I was building the fuel tank or sealing the edges of the firewall. This stuff has a shelf life (and it's not particularly long) so I couldn't just order a quart and leave it waiting around, but someone building at a much faster pace than me could get away with that.

Here are the finished steps:

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