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

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.

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

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.

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.

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.

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.

OK it is the differential pressure created by engine suction that causes the flow. 

Never heard it stated quite so succinctly, Larry.