RV-3: Trial and Error

Lest anyone think that we don?t make our share of mistakes and errors, I present to you this simple fuel line that goes from the boost pump to the firewall of our RV-3 project. Remember that there are no drawings or even instructions for things like this on a -3 ? it is simply part of fabricating the fuel system with parts that you decide to use. Here is the simple finished line:



And here is the pile of tubing that gave its life in the development of the finished part!



This line that looks so simple exists in three dimensions in a very tiny space ? getting the bends in the right place is tricky, and leaving enough straight tubing to flare the end is even trickier. In fact, to make the winning piece, I had to put the flair on one end FIRST, then do a 90 degree bend. By actual count, this final piece was my sixth try at the part. Fortunately, a roll of tubing from ACS is pretty darn cheap (I am still using the roll I bought for the -8 six or seven years ago), so don?t be afraid to reject the parts that aren?t right ? get some more raw stock, and keep working at it until its right!

Paul

Perfect or close enough?

So, my question for you Paul is, does that horizontal part of the tube parallel the firewall bottom or is it close enough??

jthocker said:

So, my question for you Paul is, does that horizontal part of the tube parallel the firewall bottom or is it close enough??

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I could answer that two ways JT...

1) Trick of the camera!

2) Dang, I guess I gotta get out the big old roll of tubing again....

In truth, NOTHING in that tube is parallel or perpendicular - or even straight! It had to come back a ways to clear that firewall stiffener, then go back forward to pick up the angle fitting in the firewall (which is going to be replaced with steel BTW, for those wondering - the blue one is there casue it was in the tool box). It had to go UP a little to allow me the minimum bend radius - I crimped tubes three times by trying to bend it tighter.

Paul

Paul, I would like to swap your scrap tubing for a hidden oil door latch that didn't work. I have 25 hours into it, so you know its valuable.

Ironflight said:

snip
In truth, NOTHING in that tube is parallel or perpendicular - or even straight! It had to come back a ways to clear that firewall stiffener, then go back forward to pick up the angle fitting in the firewall (which is going to be replaced with steel BTW, for those wondering - the blue one is there casue it was in the tool box). It had to go UP a little to allow me the minimum bend radius - I crimped tubes three times by trying to bend it tighter.

Paul

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And that, gentlemen, is why flex hose was invented.

Maybe I give up too easily when such a problem jumps up a bites me, but that particular line (boost pump to firewall) is always a hose in any project I'm involved with. The firewall is likely to drum, and cause a failure of a hard line...eventually.

I would not want to be in any aircraft when that line gives up - had it happen last week in a 1/2" 1500PSI hydraulic line on the B25. A cloud of hyd fluid is not a fun entity to encounter, but lucky for us it gave out on the ground when we started the second engine. The dang 65 year old line gave up, and we replaced with like materials. I fully expect the new part to last another 65 years.

I'm thinking a hose should go here, Paul. Just sayin'...

Carry on!
Mark

Good post Mark, and something for folks to think about. I've always seen hard lines for this location, but certainly a flex line would make it easier to fit, and alleviate any concerns as you mention. Of course, it's something you'd have to replace with time, and I have never gotten a good definition of the "useful life" on flex hoses in aircraft use - even asked the parker guys at OSH last year, and they said "whatever the airframe manufacturer decides" ! (huh? Don't YOU guys make the hose...?)

There are folks substituting flex hoses throughout the fuel system, but I prefer solid lines in the airframe where they would be hard to change (because people won't change them when they should if they are hard) - this location is easy.

I figured that since I had a big roll of tubing, I'd make the line, but don't think I didn't picture a flex hose several times. And who knows....I have to have some custom hoses made for the FWF stuff anyway....


Paul

Paul,
I used aluminum antenna ground wire (Radio Shack) to work out my hard line geometry. Stick the end in the nipple fitting and do hard 90 degree bends as needed keeping in mind the minimum length need to do a flare, etc. Pull it out and work down the tubing, kind of eyeballing the bends. I found I could get spot on or pretty close on the first try. The fuselage fuel vent lines were one area where it worked very well.

Leaving enough straight line at each end of the tubing for the collar and flaring tool can be a problem. I think mine requires about 1 1/4".
You would think that someone would invent a "close quarters" flaring tool. It would make bending some of these fuel line runs so much easier.

fl-mike said:

Paul,
I used aluminum antenna ground wire (Radio Shack) to work out my hard line geometry. Stick the end in the nipple fitting and do hard 90 degree bends as needed keeping in mind the minimum length need to do a flare, etc. Pull it out and work down the tubing, kind of eyeballing the bends. I found I could get spot on or pretty close on the first try. The fuselage fuel vent lines were one area where it worked very well.

Click to expand...

Actually, I usually do the same thing with a piece of heavy gauge house wire, but am embarrassed to say that I couldn't find a chunk to do this with, so decided that since the tubing was short, I'd just see if I could do it by eye. See how well it worked?

The problem with this particular line is the shortness of the straight piece before the flair - you need a certain amount to fit the ferrule and still have enough room for the grip of the flaring tool. Like I said, everything on th e-3 is small.

But you're right - making a template from some wire will definitely save on waste!

Paul

benders

Bruce--I have 2 Imperial 180* benders, a 370, and a 470. I modified them to make bends closer to the flare, and also made a bracket so I can mount them in a vise, and free up my hands to work the tubing. I bought a Rigid RFT37 roll flare tool from Tom Brink at GAHCO, and it works great. Very consistant flares. Does take some practice, especially on stainless. The flare bar is 5/8 thick, so you can make a bend pretty close to the flare.
Tom

bruceh said:

Leaving enough straight line at each end of the tubing for the collar and flaring tool can be a problem. I think mine requires about 1 1/4".
You would think that someone would invent a "close quarters" flaring tool. It would make bending some of these fuel line runs so much easier.

Click to expand...

Ah...at least you didn't do like me and bend them all up and flare them, and then realize I didn't put the "B" nuts on!

Ironflight said:

I could answer that two ways JT...

1) Trick of the camera!

2) Dang, I guess I gotta get out the big old roll of tubing again....

In truth, NOTHING in that tube is parallel or perpendicular - or even straight! It had to come back a ways to clear that firewall stiffener, then go back forward to pick up the angle fitting in the firewall (which is going to be replaced with steel BTW, for those wondering - the blue one is there casue it was in the tool box). It had to go UP a little to allow me the minimum bend radius - I crimped tubes three times by trying to bend it tighter.

Paul

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Paul, I think the line looks great. I had just made a similar line on the "8" I'm building, it forms a square. Well, almost a square, the top of the square isn't quite horizontal. It fits well and I was quite happy that I got it on the second try! That is until I saw your line and your pile of tubing. Then I felt bad that I didn't try again for "perfection"!

Paul,

Only six tries?? I'm beginning to think you're human afterall. Thanks for sharing. One question though, from the angle of the picture, it looks like the line comes pretty close to the angle on the firewall. Is that an issue? In my 8, I have a line running through the left gear tower that comes pretty close to the edge of one of the holes in the bulkhead of the tower. The hole has a plastic grommet in it, but I'm wondering if over time, vibration or a hard landing or two might cause some rubbing.
Thanks again.
Jim Baker
RV8-Finishing Kit

rockbottom said:

Paul,

Only six tries?? I'm beginning to think you're human afterall. Thanks for sharing. One question though, from the angle of the picture, it looks like the line comes pretty close to the angle on the firewall. Is that an issue? In my 8, I have a line running through the left gear tower that comes pretty close to the edge of one of the holes in the bulkhead of the tower. The hole has a plastic grommet in it, but I'm wondering if over time, vibration or a hard landing or two might cause some rubbing.
Thanks again.
Jim Baker
RV8-Finishing Kit

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Keeping the line from contacting that brace was what made it so tough to form Jim. It now has plenty of clearance that isn't obvious from the angle of the picture, but that was the challenge. Like Mark mentioned, a flex line would have been a lot less hassle - once it was made of course....

Paul