ORIGINAL: jrpav1

Guys,
I wrote the article in Fly RC so maybe I can clear things up a bit (I hope): the check valve's main purpose is to keep fuel from entering the muffler when you go from full throttle to idle. In a Uniflow system this can happen rather easily because the muffler pressure line is always submerged in the fuel. With a conventional 2-line system this can't happen when the plane is upright. Now, think about this: a conventional 2-line system actually becomes Uniflow when you're flying inverted! Most of the time you don't fly inverted at idle so you'll never notice the condition I described. I've installed the Uniflow system in quite a few airplanes and I have used the check valve, but only with motors that have 2-needle carbs. It does create a lot of pressure - similar to a YS system. You end up closing the idle needle a bit but other than that it tunes just like a 2-line system except that you can run a little leaner on the top end since you don't have to worry about the mixture changing as the fuel level drops. You CAN run a Uniflow without the check valve but you run the risk of filling the muffler with fuel if you close the throttle really fast.
As far as filling the tank with a cowled motor, I've started using an external feed line between the carb. and the pickup in the tank. This works really well and it allows you to pinch the fuel line to set the top-end mixture. If the motor is exposed, just drain and fill through the carb. feed line. Give it a try.

John Pavlick

ORIGINAL: Sport_Pilot


With a two line system the vent is at the top and muffler pressure is supplied to that, the pressure in the tank is the same as the muffler pressure, and pressure to the carb is the differance in the height ot the fuel level to the carb plus the muffler pressure. For the following examples assume the tank is 4'' tall and the centerline is mounted level with the spraybar. So if you have 10'' of fuel head on the muffler pressure, and the tank is full and 2'' above the carb the pressure is 12 inches of fuel to the carb, if the tank is empty then you have 8'' of pressure to the carb. If you put a clunk on the vent or muffler pressure line so that it is at the same level as the pick up then the tank pressure is reduced by the head of fuel. So when the tank is full the muffler pressure is restricted by 4'' of fuel so the tank pressure is 10'' - 4'' or 6'', but the pressure to the carb is the muffler pressure plus the head on the carb or 8''. When almost empty the muffler pressure is not restricted so the tank pressure is the same so it is 10'' or the same as the muffler pressure, but the fuel level is 2'' below the carb so the carb pressure is 8'' which is the same as when it was full.

Perhaps the following link can explain it better.

http://www.fraserker.com/heli/uniflo...flow_works.htm

ORIGINAL: jrpav1

Yeah that's pretty much the idea. Basically, in a Uniflow the fuel mass does not affect the fuel pressure seen at the carb. the same way that it does in a conventional 2-line system. As someone pointed out, the net pressure in a Uniflow is sowewhat less than what you see in a 2-line system because the fuel mass does not ADD to the net pressure. Because of this, the higher pressure cused by running the check valve is a welcome side effect. I really need to get a good gauge and measure all of these parameters so that we have some numbers to compare. The only thing I can tell everyone at this point is that it DOES give you very consistant runs. Almost scary. You really need to keep trrack of your fuel usage (set a timer) because you have no indication that you're about to run out of fuel. If you're at all skeptical about this, don't take my word for it. Try it for yourself. Don't put it in your favorite plane and hover 3 feet off the ground until you're comfortable with it though.

John Pavlick