Saturday, January 9, 2010

Strings and things

It's been almost a full week since I last did any work on the RV-12. It's been a week of fighting the elements (both Honda and natural) to and from work, only to head right back out into the winter weather to follow the snow blower around clearing the driveway. It's not that we've had a lot of snow as measured by inches on the ground as much as it is that we've had some form of frozen precipitation or another each and every day. Between that and returning to the sturm und drang of the paying job after a two week holiday vacation, there hasn't been a lot of time or enthusiasm for a trip out to the Schmetterling remote production facility.

Even today got off to a slow start, mostly due to the need for a refill of the fuel canister that provides the energy required to generate the Cone of Comfort. That necessitated a trip to Lowes, and no trip to Lowes is complete without the frustration of trying to take care of other to-do list items. Today's extra tasking was to replace a broken set of window blinds in Co-pilot Egg's room. The frustration arose when the machine used to cut the blinds to the correct width turned out to be broken. A blessing in disguise, that, for when I returned home empty handed I found that I had been prepared to irrevocably and without hope of return purchase blinds in the color of Walnut. That would never have done as the blinds to be replaced were in Oak. This is the big city, though, and what the local Lowes does not have can always be found by making a drive to a different Lowes. Well, "always" isn't really the word I'm looking for; "sometimes" or "rarely" seem to be more appropriate. It seems that all Lowes stores have replaced their stock of Oak hued blinds with Walnut.

I am now faced with the two most dreaded words in home improvement: Special and Order.

They did have propane, though, and that ain't nothing. Not when it's 20 degrees (aka "Minnesota heat wave") out.

Having spent almost a thousand words on the weather already, it's somewhat redundant to provide the picture that would have rendered the need for them moot, but here it is anyway:

Fortunately, that area is well out of the Cone of Comfort. Or, as suggested last week, the new Cone of Mild Discomfort that I'm using to save fuel. Even at the lowest setting on the heater, it's not bad at all:

There will come a day in July when that looks pretty appealing!

With the environment as comfortable as it can realistically be, I picked up where I left off last week. The first step was an easy one. If all of the steps were this easy, it would actually be true that absolutely anyone can build an airplane. All I had to do was run a length of string back through a couple of holes in the aft bulkhead:

I'm just guessing here, but I'd bet that this string will eventually be used to pull the horizontal stabilator control cables from the control stick back to the rear of the plane, where they will be attached to the stab control horns. On the topic of "Things that Would be Greatly Annoying," I would say that "having that string fall out before I'm ready to use it" is somewhere near the top. Van's agrees, and they suggest that we tape the ends of the strings down to avoid that potential irritation:

That was too easy, wasn't it? Well, it must have been, because the very next step is one of those clever little traps Van's throws in now and then to see if you're paying attention. It sounds easy enough: do the same thing you just did string, but now do it with the WH-P30 wires. These are the wires that will carry electrical current back to the electric trim motor. It sounds easy, but it actually gets added to the ever-growing list of things that end up being harder than they sound.

What could be hard about it? Well, note that the ends of the wires are not identical; one end has little brass pins, the other has partially stripped wires:

Note too that the drawing does not tell you which end goes to the front or which end goes to the rear. Either one of those details provided individually would have been adequate; the process of elimination works particularly well in binary decision situations. I sometimes wonder if Van's does things like this to teach you to read ahead in the plans. It's not like there hasn't been a revision to this page that would have provided a golden opportunity to disambiguate this step; a revision was released just a few months ago. The only difference between the original page and the revised page is the addition of an 's' to improve an uncomfortable grammatical situation and the replacement of a specific part number on a snap bushing with the less maintenance prone text "snap bushing." Seriously, I'm not making that up. Those changes were worthy of a revision, while telling the builder which way to run the wire was not. It's a very strange decision making process they're using, at least from where I'm sitting.

All of that said, the answer is only a few pages into the future.

In section 11, the ends of the wires coming through the aft bulkhead get crimped with connectors and thence crimped onto the wires coming from the trim motor:

Using the best-case process of elimination described above, I was able to intuit that the brass pins go towards the front of the airplane. All that remained was to actually do it, and that didn't present any difficulties. The wire gets pulled through the snap bushing, then it rides along inside the comfortable trough created by the J stiffeners. Wires are never left to their own devices in an airplane, though. They must always be constrained in some way to avoid having them interfere with other moving parts and to ensure that they don't rub against any edges that might cause them to be cut. Which is why I found this so perplexing:

Once the slack is taken up on that wire, it is virtually guaranteed to chafe against the edge of that J stiffener. If you consider where the wire is and how utterly inaccessible it is going to be when the airplane is done, it certainly makes you wonder why it would be positioned where it is. I'm going to poke around on some other builder blogs and see what others have done to protect the chastity of the wire, but for now I positioned it such that the part number tag bears the brunt of whatever chafing may occur in the future:

To do that I had to leave just a little more than the 6" extending from the back of the aft bulkhead that the plans call for - I hope I don't find out some time next year that my wires are an inch too short to reach the trim switch in the panel!

As I said before, wires have to be restrained somehow, and in the case of this wire that job is performed by tie wraps. In case you aren't familiar with tie wraps, here's what one looks like:

Tie wraps are very convenient and easy to use, but it is important to note that they are a one-time use item. They tighten in a ratcheting manner, which is to say once you tighten one, you can't loosen it. What that means is that if you are going to tighten it, you had better be sure that everything that you want it to hold for you is already in it. That's okay here in the back of the plane, but there is a bold font warning to leave them loose once you get forward of the furtherest forward fuselage frame. There will be other things that need to be installed before the tie wraps are tightened.

Installing the tie wraps was a simple matter of feeding the nose of the strap through a hole conveniently pre-drilled in the J stiffener, making sure that the trim motor wire was captured in the loop formed as the nose of the tie wrap was fed into its ratcheting mechanism, tightening the tie wrap, and cutting off the excess tie wrap with a pair of side cutters:

Just like the string before it, the remaining length of wire gets secured to the front edge of what Van's refers to as "the existing airframe."

Continuing to work through the list of "stuff that you had better put in now because it is soon going to be very difficult to get at," the plans refer us to the instructions included in the separate static line kit. More specifically, this separate kit provides the static ports and lines that will feed outside ambient air pressure to the flight instruments. For those of you that have no idea what I just said, I refer you to Wikipedia's description of the Pitot-Static System here. And for those that can't be bothered to follow the link (you know who you are), here's the bare bones gist of it:

Static pressure
The static pressure is obtained through a static port. The static port is most often a flush-mounted hole on the fuselage of an aircraft, and is located where it can access the air flow in a relatively undisturbed area. Some aircraft may have a single static port, while others may have more than one. In situations where an aircraft has more than one static port, there is usually one located on each side of the fuselage. With this positioning, an average pressure can be taken, which allows for more accurate readings in specific flight situations.

The RV-12 has two static ports.

The ports themselves are essentially glorified blind rivets and are, in fact, installed exactly the same way as any other blind rivet:

When I read ahead to this step, I was curious as to why more information wasn't provided in the drawing to provide a specific location for the hole that would need to be drilled for the rivet. Well, the answer to that question turned out to be "because the hole was already drilled for you":

While I said above that the static port rivets were pulled just like any other blind rivet, I wasn't being entirely honest. Yes, the method is the same, but there is a lot more pressure to get them perfectly flush to the skin the first time - two are needed, and only two are provided.

Good news! They went in just fine!

That wasn't the full extent of my lie of omission, though. Normally a blind rivet retains a piece of the mandrel (the "nail" that the rivet puller pulls out to form the shop head of the rivet) after the rivet is installed. That's a good thing - it increases the strength of the rivet. It would be a bad thing with these rivets, though, because it would seal the static port. A sealed static port is really nothing but a decoration on the side of the plane (planes have actually crashed from having static ports blocked), so the remaining mandrel has to be driven out of these particular rivets. Remember that "things that sound like they should be easy but aren't" list?

Yeah, this goes on it.

It's assumed in the instructions that you emerged from your mother's womb knowing how to remove the mandrel from a static port rivet. I wasn't so lucky. I had to figure it out for myself.

My first attempt involved using one of the hundreds of blind rivet nails that I have captured from building the existing airframe by inserting it into the static port and whacking it with my dead fall mallet. That approach had a couple of problems. First, the sharp pointy end of the nail was digging holes in my mallet. Sure, it's a Harbor Freight tool and therefore easily and cheaply replaced, but what kind of attitude is that?? Second, the tail cone skin is thin and whacking it with a mallet seemed like a good way to create a dimple where the static port was supposed to be.

Solving the second problem first, I drilled a hole into a block of wood that the inside part of the static port rivet would fit into and clamped the wood into place on the side of the tail cone. That gave me a little more rigidity in the vicinity of whacking:

The problem of the sharp nail was solved by a tool that is nearly as ubiquitous in problem solving as duct tape: the vise grip. I clamped the vise grips down hard on the nail, then used the mallet to whack the vise grips:

Even with all of that, it takes nerves of steel to pound a mallet against the side of your airplane. Me, I have nerves of highly malleable lead, so it was a very, uh, puckering experience. But finally, success!

The ports are connected with a length of clear rubber hose that resembles the type of hose used in aquariums. Remember that problem with blocked static ports? Well, it doesn't have to be the port itself being blocked to cause a problem; anything blocking the rubber hose would have the same effect. That's why I was very concerned with the kink that got creased into the provided hose as a result of pressure from a metal part being applied to the hose during shipping:

Fortunately the kink is near one of the ends of the hose and it's possible that more hose than is actually needed is provided in the kit. Unfortunately, I won't know that until the hose is installed. The problem here is that the hose gets installed with tie wraps, and as we learned before, tie wraps can only be used once.

What to do, what to do.


Here's a nice ball of string! And what is a short piece of string if not basically a cleco for a tie wrap?

Yep, that worked nicely!

It looks at first glance as if there will be plenty of hose to allow the damaged part to just be cut off and thrown away.

The next step is to push the ends of the hose onto the inner nipples of the static ports and seal that in place with clear silicone RTV. I don't have any, though, and for some reason I'm enormously reluctant to go to Lowes to get it.

But I will. It's going to take awhile, though. I have a special order to place.

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