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Bladder tube diameters

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Keith Miller:
Hi All -
I'm about to dig in and build the Vintage Performance Airplanes Jr. Satan. They provide the option of metal or bladder tanks - I'm going bladder.

I fly for fun, not competition, and on my other ships (Barracuda, Wakkerman), I use the 1/4" latex tubing from Terminal Tackle. Unconstrained, it expands to 1.75" in diameter, but I don't know how big the bladder tubes are in those planes.

The tube provided by Vintage Performance is 1.25".

Here's my question:
Is it preferred to leave some room between the bladder and the tube so that the bladder can fully expand without being constrained by the tube?
Or is it safe to let the bladder expand fully and press against the walls of the tube?

If so, I've found some LT-316 tubing from Terminal Tackle that's 1/8" ID that expands to .95" fully filled. But that would make for a long skinny tank.

Thanks

pmackenzie:
I would say just let the bladder expand to fill the tube.
Mainly because pretty much every model I have ever had worked that way :)

Paul Smith:
If the tube is too small it will create "false pressure" when the fuel is forced into the bladder during filling.  Soon the fuel will be depleted to where the bladder is loose, and a drop in pressure will make the engine go lean.

So size the tube to where it is slightly snug, but not constrained.  If it takes extra effort to complete the fill, you are probably making false pressure.

Keith Miller:

--- Quote from: Paul Smith on September 20, 2020, 03:18:27 PM ---If the tube is too small it will create "false pressure" when the fuel is forced into the bladder during filling.  Soon the fuel will be depleted to where the bladder is loose, and a drop in pressure will make the engine go lean.

So size the tube to where it is slightly snug, but not constrained.  If it takes extra effort to complete the fill, you are probably making false pressure.

--- End quote ---

That's along the lines of what I was thinking. I imagine the most consistent pressure from the bladder would require it to be allowed to grow to it's full expanded shape.
So for this smaller wing (the Jr. Satan), I think I may go with latex tubing that will fit when fully filled.

Paul Smith:
As seen on many winning models, including World Champions, it's better to let the compartment stick out than to risk false pressure.

Also, don't ignore pacifiers or pipette tops.  They are easier to make than tubing bladders and they have lower pressure, which makes it easier to needle the engine.

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