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Building Tips and technical articles. => 1/2 A building. => Topic started by: John KruziK on April 21, 2009, 04:06:47 PM
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Is it necessary to use presure on 1/2a fuel tank? Or could I use any 1oz. wedge fuel tank? I dont think I understand the reason for ballon type tanks. Just confused Thanks John
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I think the balloon tanks referred to are floppy balloons with no pressure on the fuel. This is in contrast to the surgical tubing tanks which are pressure tanks. With TD 049's I run backplate pressure to a uniflow metal tank with good results. I have not tried the floppy balloon tanks but they sound like they work well.
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Yes, the floppy balloon tanks are "relaxed envelopes", still trying to find a descriptive term that makes it clear...maybe "tank liner"? Anyhow, no pressure. No vents either, as the balloon simply collapses as the fuel is drawn out. Very even head pressure throughout the run. As I often say, it works better than it has any right to. Just don't inject air into it. Most all my 1/2As use this these days.
Any 1/2A should run fine on any kind of suction tank--metal, plastic, clunk, etc. You should keep the fuel tank close to the engine though, so the fuel line is short.
The only planes I use pressure tanks on (surgical tubing, inflated) are combat wings with their high-g violent maneuvers, a suction engine can get starved out.
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My most reliable 1/2A currently is a Tee Dee with the side pressure vent to a #8 Perfect tank. Screaming runs from beginning to end with no noticeable variation until the fuel supply becomes "iffy". The key is to make a nipple to go over the molded nipple so that you have reliable pressure tube retention. For the Tee Dee it is made from a 2x56 Blind Mounting Nut. For the Medallion, a 4x40 is needed.
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John. you can certainly use a wedge fuel tank but the reason many like the "relaxed envelope" tanks being described because they do one thing very well. They almost eliminate foaming which can be a real headache. Especially if you haven't figured out why an engine won't behave on a plane.
Some may not realize that it has this profound ability and like it for other reasons. They really are cheap, easy to manage and easy to work with.
Robert
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Thanks for all the info.
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Re: the no pressure on a "relaxed tank". There is pressure and that's the reason they work, it's called one atmosphere or about 14.7 psi. The fuel is in a sealed container surrounded by a bladder holding back the atmosphere. The only outlet is through the needle valve, simple fluid dynamics shows that that produces 14.7 psi as far as the fuel is concerned. Since fuel is bascially incompressible it is very important that any excess air is removed from the tank, otherwise pressure flucuations will happen as bubbles move from the tank through the fuel line.
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Well, you've got that same 14.7 psi at the other end trying to hold the fuel back, so effectively no pressure. that's why a water level works.
I believe the question was about induced pressure by an inflated balloon. Let's not muddy the waters.
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Uhmmm , not to create a contest but there is a difference in pressure. thats why the fuel outlet is in a venture. True when the engine is not running the pressue at the outlet is 14.7. once the engine starts the airflow creates a low pressure at the venturi and the fuel should run out. You adjust the needle valve to restrice how fast it pours out.
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...and that's the "suction" of a suction feed.
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Hmmmm I'm guessing your not an engineer ? How does a vacum machine(hoover) work...
It does Not suck dirt into it....
Dirt is Pushed into a vacum buy the higher pressure on the outside.
I also am not an engineer.
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Hmmmm I'm guessing your not an engineer ? How does a vacum machine(hoover) work...
It does Not suck dirt into it....
Dirt is Pushed into a vacum buy the higher pressure on the outside.
I also am not an engineer.
Yes but that WRRRRRRRRRRRRRRRR sound in an electric motor creating a vacuum inside the vac so the dirt can flow in. A balloon tank works because the engine sucks on the end of the fuel line -- That WRRRRRRRRRR sound again. The only difference between a balloon tank and a rigid tank is the balloon tank collaps as the fuel is sucked out preventing the build up of a vacuum inside the balloon that would stop fuel flow. A rigid tank has to have an air inlet tube to releave this vacuum. To me the big difference is the balloon tank has no air in it --- therefore no foaming --- therefore no air bubbles in the fuel line giving smoooooooooth engine runs to the end of the tank.
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Jim, this is getting a little silly. Note I put "suction" in quotes indicating an accommodative use of the word. Of course the pressure differential already mentioned is what pushes the fuel through the engine.
For what it's worth, I do in fact have an engineering background...not my field for many years now, though.
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Hey guys, it is really Gremlins with squirt guns shooting the fuel into the venturi! If you get them mad by hitting them with air bubbles, they stop.
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OK it is the differential pressure created by engine suction that causes the flow.
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Hey guys, it is really Gremlins with squirt guns shooting the fuel into the venturi! If you get them mad by hitting them with air bubbles, they stop.
Larry,
So, finally a scientifically accurate explanation!
Can we get the gremlins to tweek the needle valves while they're at it?
Stan
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Never heard it stated quite so succinctly, Larry.