Saturday, July 3, 2010

A fuelish mistake

You might as well get used to awful puns around the word "fuel"and other fuel system terms for two reasons: it looks like we're going to be here for awhile, and because I think they're a gas! Hopefully I'll get it out of my system before I make a fuel of myself.

What? Too late? I've already gone around the bend? I just don't have a flare for this kind of thing? The words just don't flow right? I need to filter my.... okay, I'm done. Tanks for bearing with me while I got that out of my system.

The few days of delay while waiting for the tube bender to arrive courtesy of the emergency Visa card were well spent enjoying the gorgeous weather we're having. Sure, there was mowing to be done, but there was also some pleasant evening flying in the RV-6. I was even introduced to 12 year old Scotch by the guy that hangars his Cherokee across from my hangar. He said it has a "bold, oak texture." Yeah, so does a good rocking chair. I suppose it's an acquired taste, but I thought it tasted a lot like mouthwash with the cool minty flavoring removed. Still, if I'm ever to be a man-about-town in the William Powell sense, I will have to learn to like it. 

Ironically, once I had the tube bender in hand, the first step of building a segment of fuel line was to unbend the tube. Van's, quite understandably, didn't want to have to ship an eleven foot long box just to enclose the aluminum tube, so they coiled it up. Being aluminum, it uncoils pretty easily.

Of course, being aluminum it also springs back a bit when you stop rolling it along the flat surface you're using to uncoil it against. Trying to get it truly flat required a little hand bending, which resulted in an almost flat, kind of wavy tube. The fuel won't care about that, though.

The particular segment that I started with (after practicing on a much shorter segment, just in case) will run between the fuel shut-off valve and the little electronic fuel flow measuring gizmo. The plans give a specific length of tube to start with, so the first step after flattening out a length of tube is to measure the length and cut the tube. The tube cutter is a low cost Harbor Freight variant that works just fine. It's a simple thing to do, as you can see if you watch the video that I linked to in the Video Tutorial sidebar.

The next step is of critical importance, most of the time. The tube is useless without any means of threading it onto a fitting somewhere, so a "sleeve" and "nut" need to be slid onto the tube. The flare that we're going to make next will do two things: it will hold the sleeve and nut onto the tube, and it will preclude the sliding on of same if you forgot to do it first. On the first side of a straight tube, it's okay to forget because the other side is not yet flared and there are no bends in the tube to stop the sleeve fro sliding along, but it's a very good habit to make sure you always get the sleeve and nut in place before making the flare. And, as it turns out, it's a good idea to make sure they stay on the tube and don't slide off the other end, fall to the floor, and roll under the tool bench.

Yes, really, I have personal experience on this, so trust me.

The flaring tool is a wickedly expensive (well, it's about $90 if you can't find anyone to borrow one from, although even that should cost you at least a nice breakfast or lunch to show your appreciation, or even a twelve year old Scotch if you've acquired the taste for that kind of thing) and complex looking affair, but again, if you watch the video I so thoughtfully provided you will see that it really isn't all that difficult. Actually, it's somewhat fun once you've acquired the taste. Like machine countersinking, it's a fun tool to operate and it results in a professional looking piece of metal.

Viola, as they play in France. What? Oh. Sorry. VoilĂ , as they say in France.

Now, the fun and scary part. The bends. "Bends," a word that strikes fear into the hearts of novice airplane builders and seasoned scuba divers alike. The novice builder gets an assist from the fine folks at Van's, though. They have provided full scale drawings to work from.

And at long last, the tube bender is in the game!

I just bent the tube until it matched the picture. I suspect that this is one of those things that you could over-think if you read the instructions that came with the bender. There are measurements to make and index marks all over the bender to use for various types and degrees of bends, but I just figured matching the drawing would get me in the neighborhood. 

And it did. 

To do the bend in the opposite direction, I just flipped the tube over. I made sure that it was going to bend in the same plane by visually ensuring that the already bent portion was perpendicular to the horizontal plane of the tool. Sound complicated? It's not; it's purely intuitive.

This tube passes through one of the plastic bushings in a bulkhead, so the flare at the other end will have to be done with the tube already in the plane. Otherwise, the flare would make it impossible to pass the tube through the bushing. Even without the flare, it takes a little maneuvering to get it through the bushing. Good thing that big hole behind the valve was there!

I wanted to get the tube threaded onto the valve before checking the other end for fit. Finger tight is good enough as long as the flared part of the tube is hard against the matching part of the fitting.

For the next picture, I was going to suggest that we indulge in a little game of "what's wrong with this picture," but it wouldn't be fair. The obvious answer would be "the tube is too short," but that's the photographer's fault. Once the tube was attached to the valve, if anything it appeared that the tube was actually too long. It appeared that way because it was too long, but that wasn't the real problem. The real problem was that the fittings didn't line up at all and the fuel flow transducer itself was sitting on only half of the little shelf provided for it. That was happening because the little shelf was installed the wrong way around. Its rivets had to be drilled out and it then had to be reinstalled the right way around.

With the transducer properly orientated the alignment was much better, albeit still not perfect. Tube is bendable for a reason, though. It would just take a simple S-bend to fix that. Alas, the tube would have to be removed from the plane to do that. You can see that the tube is too long:

With the tube shortened and the S-bend made, it all aligns much better.

The S-bend required was not shown in the plans, so I had to ad lib a little. I needed the tube to go a half inch higher and a half inch to the left. Again, I'm sure that there are ways to do all of that mathematically but it was just as easy to eyeball it. I also had to shorten the tube a little bit. I just put the tube up against the fitting, determined where the flare should be, added a titch to that because some of the length of the tube will be lost in the flaring, and cut the tube. Easy!  More difficult was to again resist the urge to flare the other end of the tube before getting it back into the plane, but my unassailable self-discipline was up to the challenge.

Flaring the tube in the plane? A royal pain. Not a lot of room to work. On the plus side, that sleeve and nut ain't gonna be hitting the floor again!

All said and done, the whole operation took about an hour.

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