How do you account for engine breaking? Among other variables. If the uniflo hitch up created a constant pressure that accurately countered the momentum shift of the burnt up fuel, fuel head perceived by the engine would be constant even when model nose was pointed up or down. A well setup uniflo does not preclude tuning in an engine break.
What the heck are you talking about? It sets a constant pressure WRT fuel burn at the point of the open end of the vent. It certainly doesn't ensure that the fuel delivery pressure is constant at the needle end in all conditions. The exact same principle applies to the rest of the system, so, on the ground, point the nose up, and the fuel delivery at the needle is atmospheric pressure at the vent minus the pressure head difference related to the vertical distance from the end of the vent to the needle. Point it nose down, same thing, except it adds to the pressure.
It makes the fuel pressure independent of the fuel remaining, and nothing else. All the other pressure variation effects still exist more-or-less unchanged.
Moreover, the "break" has less to do with fuel delivery pressure than it does with load on the engine. Compare from ground to air. Start a Fox, and you set it just barely into a constant 2-stroke. With a suction tank, when you take off, the fuel deliver pressure goes up. With a uniflow tank, it goes down (again from the same physical principle about pressure head and depth). In either case, after about a lap or so, its running in a 4-stroke. That's because the *load* went down, not because the pressure went up. Same with maneuvering - when you first start a corner, the pressure goes up, dramatically, as the airplane rapidly decelerates. You might think it would go dead rich because, effectively, it's the same as pointing the nose down. But in fact, it breaks into a 2-stroke. Plus all the other counter-examples (like 45 degree circling flight). Sometimes the pressure matters and the engine response follows the pressure, but many times, it goes in the opposite direction to the pressure. You can even model it on the bench, by changing prop load without moving the needle. It also shows, dramatically, how much unload there is, when you have to change your 12" 3-blade into an 8-4 2-blade to get the in-flight RPM.
In fact, many problems with engine runways can also be attributed to the same issue, when the fuel draw is minimal. With suction, once it takes off, the pressure is higher than static on the ground for the first part of the run, and everything is OK. As the fuel runs out, the pressure drops, leading to the well-known speeding up as the flight goes on. How much it changes depends on the width of the tank. Up to a point, the system is predictable, but if the tank it too wide, the pressure drops excessively towards the end and the system goes, effectively, unstable, as the engine can no longer suck the fuel that far "uphill", the speed increased, the pressure drop increases, etc, in a wild runaway that only stops when the engine goes so lean it starts to sag.
Convert the exact same tank to uniflow, change nothing else - and it happens almost immediately at launch, because the fuel pressure in a uniflow system from start to finish is the same as it is in a suction system at the end of the flight. There is a theory amongst nitwits that "uniflow causes runaways", because they don't notice that their suction tank does exactly the same thing at the end of the run. The real cause is a combination of inadequate fuel draw and an excessively wide tank.
That's the only issue I have with Terry's drawing. I would put the "bottom" of the tank up against the fuselage, instead of the "side" to get less pressure head drop on suction and an overall higher fuel pressure on uniflow.
Brett
Topic: Standard vent vs Uni-flo (Read 8610 times)