I'd thought about trying a bubbless tank but then I realised it can't be set up as a Uniflow which I think is more important.

Rod Claus used Tetra tanks for awhile, and liked the runs (engine runs, that is!) he got. I don't think he's used them in a few years, and I'm not sure why, but they are a bit of a hassle to fill, I thought. Maybe he'll check in on the subject.  Steve

Actually, the Bubbless tank is for all intents a form of uniflow,, it doesnt vary the head pressure at the needle as the tank empties. Thats why we use them in Pylon racing. When you run an engine at Peak, if it goes lean, you then have a paperweight! so its fairly important to have consitant runs start to finish.

   I don't see how that would work. I can see how it prevents any bubbles but I think it's going to be subject to the same forces as any other tank. In an RC plane it wouldn't necessarily be evident.

    Brett

Dan Banjok messed with a Tetra for a year. Used it on a vibrating profile to stop the bubbles. He wound up creating an inverse-tetra, fuel pick up on front of tank, exterior to the balloon. Fuel fill goes like this. All air is sucked from the balloon. Tube to balloon is capped or clipped. Tank is filled into the plastic, exterior to the deflated balloon. Air is vented out of the plastic interior as tank is filled. The aim is to eliminate all air from part of tank that holds fuel. Vent is capped. Balloon inlet is opened to the air or connected to muffler pressure. (I don't remember this step!) As fuel is used up during the flight the balloon expands allowing fuel to flow freely. Air does not mix with fuel. No air bubbles. Does it work? Yeah, kinda, Dan continues to use it on this plane. Engine does lean out slightly (and predictably) during the flight. The result is a much better engine run than possible with the original vibro bubble bubble machine. Worth the trouble? I have two of those $20 Tetra tanks laying around somewhere and am not inclined to putting them to use. Maybe if I build a profile with a horrendous vibration issue...

The Tetra tank can't be a uniflow.  There is no way to balance the fuel pressure head.  We used balloon tanks for a few years in slow combat.  The only way to make it work in CL is to make the spanwise width of the container less than 1.5 in.  1.25 or 1 is even better.  At that width there is very little change in fuel head.  The Tetra tank can be used with pressure on the outside of the bladder.  Then it becomes more uniform, since the fuel head pressure is such a small fraction of the total pressure.

Atta boy, Phil! I'm sure glad there are a FEW of us that understand why (non-uniflow) pressure fuel systems don't have as much transition as non-pressure systems!