Fuel line sizing

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Bede
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Fuel line sizing

#1 Post by Bede » Sat Jun 11, 2016 7:36 am

I am installing a fuel line in an ultralight. I know that you should have sufficient flow for 150% of the max flow. Is there a formula for calculating fuel line sizing based deltaP, line length, etc?
Alternatively, if I know a typical pipe flow velocity for aircraft I can work backwards using common plumbing/hydronic formula to size the diameter. What is a typical pipe flow velocity in a fuel line?
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photofly
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Re: Fuel line sizing

#2 Post by photofly » Sun Jun 12, 2016 3:58 am

Average flow velocity must be flow rate divided by cross sectional area.

Aircraft Spruce sells fuel hose with inside diameters of 1/4", 3/8", and 5/16".

An aircraft engine needs no more than 1gph per 10 hp at maximum (full rich takeoff power) fuel flow.

Lots of aircraft have between 150 and 300HP engines.

Does that help?

Edit: Doubtless, you've also considered that the putative fuel line before installation, a bathroom scale, a bucket and a stopwatch will give peace of mind regarding any calculations.
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Re: Fuel line sizing

#3 Post by Bede » Mon Jun 13, 2016 6:38 am

I'm familiar with that stuff. In most fluid flow calculations (water pipes) you would use the Hagen-Poiseulle equation. I was curious if it's similar for aircraft. I have a feeling fluid velocities in aircraft fuel lines are much lower than the upper bound of the laminar flows used in other industrial fluid flow applications (1-2 m/s).
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ahramin
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Re: Fuel line sizing

#4 Post by ahramin » Mon Jun 13, 2016 10:04 am

With 5 l of fuel in the tanks and an exaggerated nose high attitude my 3/8 fuel lines gravity feeding gave me 1 l / minute flow at the carb end Bede. For an ultralight I would think 1/4" would be enough but without specific guidance from the kit manufacturer maybe you want to go with 3/8. Don't want to get the thing built, do the fuel flow check and then find it inadequate :).
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Re: Fuel line sizing

#5 Post by Jungle Jim » Tue Jun 14, 2016 11:59 am

Guys,

It's probably best to use 3/8" minimum because water will bridge in a 1/4" line. If it's cold it will freeze and create a plug. Hence the requirement for 3/8" minimum in Chapter 549.

Jim
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