We have an ongoing, sometimes heated, discussion about fuel tank plumbing when using muffler pressure on a uniflow tank...As I understand it, the uniflow tank was developed as a way to equalize the atmospheric pressure inside the tank at the fuel inlet tube during the engine run no matter the fuel level remaining in the tank..
It keeps the fuel feed pressure *at the tank* equal to the pressure at the uniflow vent entrance regardless of the fuel level. It can be atmospheric pressure, muffler pressure, atmospheric pressure+Q, etc.
I maintain that if muffler pressure is applied to a uniflow tank, the pressure line should be applied to the the vent tube rather than the uniflow tube. The whole reason for using a uniflow tank in negated when muffler pressure is applied and there is no longer actual "atmospheric pressure" to deal with...The tank will have a positive pressure and the fuel will be "forced" to the venturi...The proof of that is when the pressure is lost (pressure line comes off etc) during the flight, the engine will go very lean as the the fuel is no longer being force fed. The question is: Does it matter where the pressure is applied? I maintain it should be applied to the vent tube rather than the uniflow tube which amounts to pumping bubbles into the tank 1/4" away from the fuel pick-up. What say you?
The "vent" tube on a uniflow tank is the uniflow tube. The third tube is the overflow that you are supposed to cap off to achieve uniflow.
If you feed pressure into the uniflow tube and cap the overflow, and the pressure is constant, the pressure at the fuel outlet should remain at the pressure you feed it regardless of the fuel level.
If you feed pressure into the overflow tube and cap the uniflow, the pressure at the fuel outlet will be the feed pressure+the density of the fuel*the depth of the fuel. That means as the fuel runs out, the pressure at the fuel outlet will drop as the fuel runs out.
There are some complicating factors, but the one you probably care about here is that adding pressure to the overflow has the effect of raising the pressure overall, meaning that the change in pressure from the fuel depth change is reduced. You raise the fuel feed pressure overall, you close the needle to get the setting back (compared to no pressure), and then the change from the fuel depth has less effect, since the overall pressure change is less, percentage-wise.
It's relatively easy to calculate what the pressure does over a flight with different variations, I am traveling again so I don't have the time to walk through it right now, but it's simple.
Brett