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Fuel line flexibility in tank

Old 05-18-2005, 11:30 PM
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Chevelleflyer
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Default Fuel line flexibility in tank

My low wing trainer ARF has a fuel tank that measures 5" long X 2" wide X 2.25" high. The fuel line I'm using is DuBro medium. When I tilt the tank down toward the front (as if the plane was diving) the clunk does not move to the front of the tank because the fuel line is too stiff. Seems like this will be a problem once the fuel gets slightly less than full. In a dive (or just nose down attitude), the engine won't get any fuel. I don't think the hard tubing going through the rubber stopper is excessively long either.

Is there a more flexible fuel line that I should look for? Does the clunk need to be able to move to the front of the tank when tilted that way?
Old 05-18-2005, 11:45 PM
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carrellh
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Default RE: Fuel line flexibility in tank

The clunk should not move to the front of the tank.
There should be enough fuel in the line to keep the engine running for the brief times the clunk is not submerged.
Unless the tank is almost empty, the only time it's not submerged is in a very steep dive.
I've read tips where people slip a section of a plastic drinking straw over the clunk line so there's no way it can flex enough to go to the front of the tank.
One of our club members uses a hard line, soldered to the clunk, with a 1" section of silicone fuel line to connect it to the stopper.
It can move up/down/left/right but not forward.
Old 05-19-2005, 05:30 AM
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Kaos Rulz
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Default RE: Fuel line flexibility in tank

You want the fuel line and clunk to be flexable enough to go up-down-and side to side without falling to the front. In a dive (even straight down) most of the fuel will be at the rear of the tank due to g-forces. The plane would hit ground way before the fuel reaches a syncronized velocity with the plane causing the engine to starve for fuel.
Fly the plane and don't worry about where the fuel is. Just try to land before you run the tank dry.
Old 05-19-2005, 06:32 AM
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joeb102072
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Default RE: Fuel line flexibility in tank

Ditto on ED STEFAN's answer. If you chance getting the tubing to flex even though it doesnt need to you stand the chance of it getting the clunk stuck up front or worse manuvering itself into a knot. Both would not be good when the engine calls for fuel.
Old 05-19-2005, 06:42 AM
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LSP972
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Default RE: Fuel line flexibility in tank

Ditto to the above.

A typical beginner's aggravation is, during a hard landing, the clunk gets bounced up to the front of the tank and stays there. Next flight, the engine dies after just a few minutes. A lot of needle-tweaking follows, making things worse... well, you get the picture.

I put a short section of tubing in the clunk line, directly between the clunk and the stopper tubing. This allows the clunk free range of movement around the back of the tank, but does not allow it to go forward. I've found that a length of "hard" tubing equal to one third of the total length of the clunk line does the trick.

Most ARFs are made in Asia, and they generally come with crap fuel line that is very flexible. IMHO, one should always replace this stuff with some good, thick-walled Aero-Trend or Prather line. I use the Prather pink line, made for high-nitro boat applications. That stuff lasts almost forever.
Old 05-19-2005, 09:50 AM
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raideron
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Default RE: Fuel line flexibility in tank

I did the same as Steve on my last two planes and from here on out..
I add a short piece of brass tubing between the silicone tubing that's
connected to the fuel outlet of the tank and the clunk.. It still moves
all around about an 1/8" at the back of the tank, But can't get stuck
at the front of the tank.... I also use wire ties, or tubing clamps on
the inner tank connections...
Old 05-19-2005, 12:17 PM
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Chevelleflyer
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Default RE: Fuel line flexibility in tank

Thanks for all the help.

The clunk will move all around the back of the tank, so it sounds like that's O.K. I won't worry about it now.

I never thought about the fuel staying toward the back of the tank in a dive - I just figured that gravity would make the fuel go to the lowest point.

It's a good feeling to learn something new.

Thanks again!!


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