Mike,
I have just gone through a very extensive search on this subject to deal with a run away situation. What I found was that basically the uniflow vent is the pressure reference point relative to the fuel pickup line in the tank. You can effectively equalize the upright and inverted lap times by moving the end of this tube. With muffler pressure has more than atmospheric pressure available to the tank but it also works the same (just as effective as shimming the tank) without the pressure.

The fuel head seems to be the driving force when it comes to controlling the amount of air let into the tank with the uniflow plumbing. Muffler pressure seems to allow pressure to equalize faster and may reduce upwind/downwind rpm swings a bit but still the fuel head/liquid viscosity holds back the incoming air until the engine pulls out some fuel. Some have even used it with crankcase pressure, works the same just a little higher pressure. The bubbles don't get sucked into the fuel pickup as long as there is as little as 1/4" separations, Brett has explained this several times.

Tank shape also plays a part in this. Short narrow tanks (like the coffin tank used in suction speed ships) set up for  uniflow could have a higher negative pressure in the tank and go rich at the end of the flight once the pressure hold back of the liquid is clear of the vent. But the wider 2" uniflow tanks we use in stunt  tend to go lean since the fuel head is much higher at the start of the flight than the small negitive pressure at the vent. Some (Bob Gieseke for one) have use conventional venting with muffler pressure to one vent and capped the other. This will run like a conventional tank rich to lean but be a little less drastic change since the muffler pressure helps push fuel rather than just having suction.

It seems each engine/tank/plane may run its best with a particular set up. I have used open uniflow on most of my ships but have recently used muffler pressure to the uniflow vent. Upright/inverted lap times were the same with either approach.  I liked the pressure engine run a little better than the open uniflow, seemed steadier. I guess you pays your money and takes your pick.

Best,   DennisT

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

I think that the attached drawing can help to explain the concept graphically.

Thanks Massimo.  A picture's worth a thousand words -- and often takes longer to draw.

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...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?

Correctomundo, sir.

I'm pretty sure that the muffler pressure goes up with engine speed, and that this would provide some active regulation, in addition to just swamping out the effect of the head change on the tank.

I don't know for sure, though.

       Not always, which leads to unconditional instability.

     And there are significant dynamics, almost all in the form of lag which could make it go conditionally unstable. The best part is that part of the dynamics involves the tank ullage volume (more ullage = more low-pass-filter = more phase lag) so it also changes during the flight.   Also, with conventional uniflow tank arrangements (and particularly on profiles) the fuel pressure at the spraybar tends to go down as the centrifugal force goes up, which is unstable, too, if that was the only thing going on.

     Brett