Flexible Fuel Lines to fuel selector

[Nev]

Has anyone installed flexible fuel lines from the joiner in front of the doors to the fuel selector ? I’ve made up hard lines (twice ;/ ) and now installing the front floor panel. It’s getting to be a busy area, running the fuel lines around hydraulic lines etc. I’m seriously considering running a short length of flexible fuel line to the selector that would make the fitting a lot easier and give a continuous arc with no tight corners, and allow much easier fitting of the floor panel. Any thoughts ?

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[Mark Goldberg]

You could do that Nev. It does add weight though. Mark

[whee]

Totally doable. My concerns would be minor flow restriction from the flexible line, ensuring there is no low spot where the line sags, the need for additional anchor points to prevent line movement while in service.

IMO, it is better to use solid fuel lines and run them where they need to be. Then use flexible lines for the brakes.

I completely understand where you are coming from. We have the supply lines, return lines, a fuel pump, and brake lines all under the front floor pan. Several of those lines had to be made several times.

[svyolo]

I believe it is always OK to use flexible fuel line. Hard lines have limitations on their use. But hard lines are a lot lighter and cheaper, as are their fittings. I think in general you use hard lines when you can, flexible when you have to. Movement, vibration, and those combined with high pressure, at some point require the use of flexible lines.

I have soft line connections at the wing root, and upstream of the header tank, all the way to the EFI injectors.

[Nev]

Thanks chaps. I’m currently getting my flexible brake lines made by a local hydraulic supplier - they have sourced some aluminium 37* AN4 fittings which are significantly lighter than the steel ones. Discovered another local racing supplier that were extremely helpful and can also make AN6 fittings in aluminium.

[zkelley2]

There's airplanes out there with flex lines from the tank to the carb. They do weigh more, and unless you get the right ones, not ethanol resistant if that's a thing that matters to you.

There is a tool you can buy to make your own aeroquip lines. It's a royal pain to use, but it's a lot cheaper than premade ones on spruce.

[jaredyates]

I think the flex lines will have a much larger bend radius, and as you have found there is not a lot of extra room there. Something I tell myself is that I don't want to select a method or process because a lack of skill or proficiency boxed me into it. Rather, I want to keep working to make the best final product, or seek help. I made some of those lines 4-5 times. If flex lines are the best product for the mission, use them. But if flex lines are the second best, because the first choice is difficult, don't let the fuel lines bully you into a compromise.

[spinningwrench]

If you use flexible lines you need to make sure you constrain their motion both to avoid low spots and potential abrasion. Overall better to do with hard lines. I am using only one flex line at the connection to the firewall. It vibrates and has been a point of failure on some aircraft when made rigid. A lot of home builders use flex for less critical brake lines. Summit Racing is an excellent source for custom AN lines and far cheaper than Aircraft Spruce. All lines are high pressure tested as well.

[PaulSA]

When I was building my RV-10, I had a weekly repeat order in for 12' lengths of tube

Certainly forming pipes is not the easiest job but you get the hang of it after about (say) 20 attempts ......

[svyolo]

I have to give Whee a shout out for a great quote. "Light as possible, heavy as necessary". My opinion of fuel lines is "hard lines when able, flexible when necessary". Hard lines are lighter and cheaper (not corrected for having to make a hard line 3 times).

[James]

Hi Nev,

You better slow down on that build mate, you're making the rest of us southerners look bad!

1) I just did this job, with 3/8" 5052. I got both lines perfect first time. But it did take me two whole days just to make those two lines. By far the most troublesome lines of the build so far, because it goes from "hard point to hard point", so getting the length right after flaring is critical.

2) I realise NZ has different (often more sensible) airworthiness rules - but my feeling is that CASA has a mandatory 10 year replacement interval on all fuel flex lines used on aircraft, might be something you want to ask around about?

3) looks like you've supported the selector valve with a bracket - other people rely partly on the rigid lines to hold the selector up through the floor. I found an old aluminium flange from ACS in the bottom of a box, not sure what it was meant to be for originally, but I used it anyway to support my SPRL valve above the floor. I think PN = 10350-8, or find the size that fits :-)

James

[Mark Goldberg]

The bracket you see in Nev's picture is what Bob designed for the SPRL fuel valve. The gascolator attaches directly to the fuel valve bottom outlet with a "pipe nipple" with 1/4" NPT on both sides. Mark

[James]

Thanks for the info Mark - I didn't realise this was the solution recommended by Bob - do you think there's any problem? My installation is pretty solid! FWIW, if anyone else is reading this, please go with Bob's suggestion.

I close-coupled my SPRL selector valve to the gascolator, but was worried it would foul the flap handle, especially if I extended it down the track as per Battson's recommendation. I mounted the gascolator directly to the plate that's welded in front of the tube cluster on my factory kits, with the inlet and outlet North/South. I had to remove part of the forward lip to allow it to clear the outlet. The aft outlet just clears the weld cluster. I did this because on other forum threads people are worried about the gascolator sitting too low, and having to modify the aft tunnel to clear the gascolator. I close-coupled the valve to the gascolator, but realised it was too close to the flap handle. I used two 45 degree pipe fittings, and so the valve is close-coupled, but off-set 1" to the left of the centreline. French really helpfully told me to think ahead to the engine - if your carby has the inlet on the LHS, it makes sense to run the fuel line up the LHS of the floor.

[Nev]

Here’s a couple of pics to expand on what Mark is explaining above. I’m still waiting for my rivet squeezer to arrive (the tax department intercepted it at the border !!) so I’ve tapped the steel tabs and used #6 machine screws to attach the bracket.

There is one minor issue the way I’ve done it with clearance - when you attach the fuel lines the AN NPT fittings don’t have clearance below the bracket to screw in. I fixed the issue with a Dremel. I suspect when my gascolator arrives I may have to get the Dremel out again. I’ll take a photo of the finished plate late (after Dremeling ) and attach it for reference.


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[James]

Ah yeah Nev - I see a bit more now. My kit (an older one) doesn't have any of those tabs in your photo, just a large 0.060" plate in front of the cluster.
I'd say the valve ends up in pretty much the same place, just 1" to the left (I think it'll clear my feet - or else I'll just fly from the other seat).

James