RE: Tanks
And, Jim (Jayseas, that is) the reason the uniflow venting does what it does is...
- The fuel's effective "gravity head"... Picture a town water tank, a big bubble supported fifty feet off the ground... The weight of the water provides a pressure into the pipes down at ground level, right?
When we fly CL, centrifugal force and gravity combine into what could be a single force pointed out and slightly down, with the effect of 3 or more g's of "weight." Like the town water tank, the effective gravity head is from the surface of the liquid open to the air around it, to the pipes which carry the liquid away to where it will be used.
The traditional "over and under venting tank" in the diagram has its fuel head at the surface of the fuel inside the tank, since the vent tubes are at the inside wall of the tank, and uncover to being both open shortly after takeoff. As the fuel burns off, the fuel surface moves nearer to the outer wall of the tank. The "height" determining 'gravity head' gets smaller. There's less pressure as the surface falls toward the pickup. That's why they run rich at first and lean out over the course of the flight.
The uniflow venting, on the other hand, sets the vent height that releases air into the tank interior very nearly at the same "height" (in the slanted direction of that equivalent force I mentioned) as the pickup tube. That "height" makes the fuel surface location irrelevant, since the tank is sealed, except for the uniflow vent end and the fuel pickup. NO outside air affects the fuel, except what enters the tank through the uniflow tube, and then, only at THAT height 'above' the pickup. (Along that slanted equivalent force direction.)
Uniflow tanks are not all sweetness and light, though. It may take some effort to place them where they work best. If they are too high, the act like an enema bag, pouring fuel downhill to the spraybar. If they are too low, the engine must draw fuel 'uphill' - like with a leaner needle setting. Once the uniflow tank is the right vertical height for the engine, not only is the same setting held through almost all of the flight, but it is the same upright and inverted, and in inside and outside maneuvers.