I’m relatively new to control line as the last time I flew CL was around 1972. Fox and maybe OS Max 35 on Magician or Flitestreak. I do recall inconsistencies in runs particularly inverted.
Have been flying RC and have alway used exhaust pressure to fuel tank. Two lines from tank, one to carb and one to exhaust. Worked pretty well. Getting back to CL, I hear about Uniflow. Researching on Stunthangar and it sounds like different ideas on where the Uniflow tube should go in the tank. Or maybe I’m just not reading correctly.
My question is this—can exhaust pressure be used instead of a normal vent to atmospheric? Or can I simply use two lines like I did with RC, one to Venturi and one to exhaust tap?
If this has been previously covered, my apologies. I’ve looked but haven’t been able to locate anything.
The end of the uniflow vent should terminate at the far outboard edge of the tank - on the wedge side of the tank, same as the pickup. There is no other place that it should go, contrary information is incorrect. And to start, right in the middle, vertically. Same position as the fuel pickup line. Fore/aft, you need some distance from the end of the fuel pickup. Mine are about 1/16" apart, but most people use much more, like 1/2".
The effect of uniflow is to hold the pressure at the outboard edge of the tank constant as the fuel runs out, it doesn't change through the flight as the result of the fuel level. The pressure it holds constant is the pressure at the air inlet end. This can be ram air, "static" air (vent with no net pressure), or muffler pressure.
You want a third line coming from the tank, going to the front/top/inboard edge of the tank. This is the overflow. For uniflow operation, you open the overflow, fill through the uniflow vent, the overflow releases the air from the tank until it is full, then fuel comes out. You then plug the overflow. With the uniflow and pickup tubes right next to each other, you will have all sorts of problems filling through the uniflow tube, because the fuel will want to come out the pickup, same if you remove the fuel line to the engine to fill it (NOT RECOMMENDED!) for every flight.
To function as an overflow, the tube only has to be at the top of the tank. But you want it inboard and the front, because if it is, you can switch from uniflow to suction and back with the same tank To run it as a suction tank, you can fill normally, but plug the uniflow and leave the overflow open. Then the tank will not hold the pressure constant, but instead lean out as the fuel runs down.
That gives you 4 basic configurations to try:
Suction and no pressure
Suction with pressure
Uniflow with no pressure
Uniflow with pressure
Of those, the first three are very reliably predictable. Suction with no pressure leans out a lot, suction with pressure leans out a little bit, uniflow with no pressure runs at a constant setting. Uniflow with no pressure nominally runs at a constant setting, just at a higher feed pressure than an open vent. But sometimes, it has some other effect, usually, leaning out about halfway through the flight, but sometimes richening up (as the fuel gets heated and thus thinner). It's great when it works, and I have some configurations where that worked best, but it is also prone to the most unexpected events.
I almost always run uniflow with an open vent, either ram air or "neutral" pressure. I don't want my engine speeding up throughout the flight. But I do have one system with suction with muffler pressure (muffler pressure line to overflow, capping the uniflow).
Brett
p.s. Interesting point about ram air pressure. At stunt speeds (say, 80 feet/second or 55 mph), the ram air pressure is only about .05 psi. That doesn't sound like much, but it's definitely detectable. But the pressure change, beginning to end, from running suction on a 2" tank is only .07 psi. We are talking about tiny ranges of pressures, and they clearly matter.
Topic: Uniflow and pressure (Read 1488 times)
