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Fuel line different size tubing problem or..

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opcod

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Aug 28, 2010
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Hi

I would like to know any problem may appear if using different fuel tube size. If we go for an engine with carburator running usually with 1/4 fuel tubing and got 2 wing tank :

System layout as follow
wing tank fuel pickup : on 1/4 tubing, goes into a 3/8 fuel filter
go into a 3/8 fuel pump
go into a 1/4 fuel selector
an 1/4 line up to the engine.

Basically, is a mix of 3/8 tubing and 1/4 can cause any issues ?

Thanks
 

Brian Clayton

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Not on low hp stuff. The fuel system will only pass what will go thru the smallest orifice in the system. As long as 1/4 line will supply your fuel flow needs, it doesn't really matter.
 

rv6ejguy

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MDRA often likes to see 3/8 line plumbing for the fuel system but this is at the discretion of the inspector quite often. Maybe get a ruling before you do all the work and then have it rejected.
 

Pops

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I have 1/4" dia fuel lines on my little single seat Super Cub. The fuel flow at 18 deg nose up is 15 GPH from the wing tanks 3 times what my 60 HP, VW will use at WOT.
The fuel lines in my Bearhawk is 3/8" lines and its not finished, so I have not done the fuel flow test. In other Lyc powered homebuilts, I used 3/8" fuel lines. Dan
 

PTAirco

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Just make sure you do as fuel flow test with the airplane propped up in a nose high attitude. I'd consider this mandatory, but few builders seem to bother with this. Fuel related first-flight crashes abound....
 

Pops

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Just make sure you do as fuel flow test with the airplane propped up in a nose high attitude. I'd consider this mandatory, but few builders seem to bother with this. Fuel related first-flight crashes abound....
Yes -- I would NOT test fly any aircraft without a fuel flow test. Dan
 

Dan Thomas

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Sep 17, 2008
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One thing to remember: some fuel-related failures are caused by poor tank venting. The fuel will flow fine during the nose-high ground test, but in flight the airflow changes things.

For example, an Aerosport Quail had a failure when the header tank ran dry. It turned out that the builder had separate vents for the main tanks and the header, and the header's vent generated a bit more pressure than the main vents did, holding back the flow of fuel from the mains to the header. That's why FAR 23.975 includes this requirement for certified aircraft: "(4) Airspaces of tanks with interconnected outlets must be interconnected..."

Similarly, even a system without a header can fail if the tank vent generates suction in flight.

Dan
 

opcod

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Aug 28, 2010
Messages
77
Location
Canada
Thanks for the input. Indeed, it's mandatory to perform fuel flow check with the MDRA. But safety aside, it's the best way ot confirm the fuel go without problem and at the right gph up to the engine. Let say you have a line too high up and down.. and the pump is under size/rated..

But about the venting, I think i will go with the same setting as some RV plane used.
As a tube coil in double loop. as this one :

Vent RV rig.jpg
It might be better than using a fuel check valve.. As for the sizing, is a 1/4 again will be ok ? If a bug cover the end, the size can be enought for...
 

Dan Thomas

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Sep 17, 2008
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What is the purpose of the double loop???
Probably to discourage overflow out the vent. But it also provides a low spot for water to collect and freeze and shut off the venting and hold back the fuel and kill the engine.

So many ways to die...

Dan
 
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