To repeat, If you do not plug the overflow, you have an open vented tank, not a uniflow. You can run muffler pressure to the uniflow, or not, as you prefer. Incidentally, I fill through the uniflow line.
To back this up:
The function of a uniflow tank is to provide an engine run whose pressure at the needle valve (and hence fuel flow) is not a function of the level of fuel in the tank (although it is still a function of the attitude of the airplane). The way this is done is to make a tank arrangement where there is only one functioning vent, and this vent is submerged in the fuel for most of the engine run. Because the vent is submerged, and is very close to the fuel pick-up, the pressure at the fuel pick up is essentially atmospheric pressure. You only want the vent to surface when you have one or two laps of fuel left, so that the engine will lean out to give you warning of the end of the flight.
Contrast this with an open vented tank. In that tank, the pressure at the fuel pick up is atmospheric pressure
plus the head from the weight of fuel above the pick up. This means that as your engine runs it leans out -- and because our "stunt runs" depend so much on the fuel mixture to regulate the engine speed the engine run will undergo considerable variation from the start of the flight to the end.
If you don't block the overflow tube in your uniflow tank, you then have what is known as a "uniflow tank with an air leak", which, as Jim T. points out, acts just like an open vented tank. You'll find that -- depending on your engine set up -- if you get the mixture right at the beginning of the run it'll be too lean at the end (if it gets that far). If you get the mixture right at the
end of the run, you'll have to make it too rich (perhaps blubbery rich, depending on your engine) at the start. I even had this experience recently, with a plastic tank that I made up from some really old parts, including a rubber plug that had apparently passed it's "sell by" date while still in the bag. Once I figured out that the plug was leaking, the odd engine run problem became clear.
Note: You can adjust planes to fly just fine on open vented tanks, as long as you aren't trying for a "stunt run". In that case you adjust the needle valve for a run that's close to the fastest that you can get, only a bit richer, and you don't get the engine speed regulation that you would with a "stunt run".
Topic: uniflow tank (Read 6146 times)