Firewalkerd1

My Feedback: (1)

Read an article in Fly RC on a fuel tank setup called, uniflow system. Has anyone used this system? Please let me hear pros and cons. The article mention a check valve to keep the fuel from interring the muffler. Where can this type of valve be found that the fuel won't affect it? Maybe at the local auto parts store. Hope I have this in the right form.

redfox435cat

yea I'd like to hear about this. how is it phsycally possible?

50%plane

My Feedback: (5)

Muffler pressure.

redfox435cat

Oh I get it. air from muuffler but no fuel to muffler. then how do you fuel up the tank. their is no were for the air to go when ya fill it up?

RCKen

My Feedback: (9)

Firewalkerd1,
Well, I've never heard of a "uniflow" fuel system but I'm pretty sure that it's similar to what get's used with a YS motor. On YS motors pressure for the fuel tank it provided by the motor instead of from exhaust, and a check valve in the pressure line is used to maintain postive pressure in the tank. Whenever you remove the fill line to fill with fuel you will here the "hiss" as the pressure is released.

The same thing can be accomplished with exhaust pressure too, just use a check valve in the pressure line. You can get these check valves at your LHS or you can order them from Tower Hobbies.
[link]http://www2.towerhobbies.com/cgi-bin/WTI0095P?FVSEARCH=check+valve&FVPROFIL=++[/link]

Hope this helps

Ken

Ed Smith

With the Uniflow system there are three lines. The feed line and the pressure line both have a clunk and got to within 1/4 to 3/8 of the back of the tank. The third line is the vent and goes to the top of the tank. As the pressure line is usually the most accessible it is used for filling the tank however either line can be used for filling. Some fuel will run up the feed or the pressure line but very little as the escaping air is easier to push out of the vent. when the tank is full fuel will come out of the vent. the vent will have to be plugged for flying. A check valve is not necessary.

In very basic terms, because both lines are submerged the only time that air can enter is when fuel exits, so there is a better balance of pressure/fuel in the tank. That is the very short answer. One of the disadvanteges is that the engine will quit at the end of the tank with no warning splutters.

Ed S

RCKen

My Feedback: (9)

Ed,
Thanks for clearing that up. Sorry if I confused anybody. No matter how hard I fight it, I wind up learning something new every day!!!

Ken

JoeAirPort

My Feedback: (41)

What I read in a magazine recently was that because both the vent clunk and fuel supply cluck are right next to one another at the bottom of the tank, the mixture of the engine stays the same througout the entire tank. With a vent at the top of the tank, the mixture is rich at the start of the flight and lean at the end. The thing is that while the Uniflow this is better, it does not replace a pump or fuel regulator (the tank cannot be on CG).

Mad Mad Max

In control line flying, espically racing, mufflers are not allowed. A uniflow tank is used as it allows a very steady engine run and uses almost every drop of fuel. One tube, the uniflow, points into the air steam for pressure and ends very close to the pick up tube. The pick up tube does not have a clunk but ends about 1/3 into the tank. The tank has a shape to it that allows this method to work. There is a third tube is used during filling. Fuel is added thru the uniflow tube and the third tube is the vent. This vent line generally uses a piece of surgical tubing and an alligator clip to open the line for venting and closes the line to run the engine. Go over to the control line forum and you'll get the whole story plus pictures on how it works

jrpav1

My Feedback: (17)

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

N1EDM

My Feedback: (2)

Hi John,
I read your article. There was a lot of info in it.

I hate to sound like a skeptic, but perhaps you can clear something up for me... From what I can tell from reading you (and other) articles on Uniflow, it seems like just the 'head' pressure of the fuel in the tank is enough to create the difference between this and a conventional two-line system?

I am considering using this for my Cermark Javelin II with OS 55AX, but I just can't seem to wrap my brain around the concept that this small difference in pressure is enough to make a difference. I'm not trying to argue with you, just wondering if my concepts are correct..

Thanks,

Bob

iron eagel

Bob,
Don't feel bad I have a new OS 70 Fl and can not figure this 3 line setup out myself with the manual in front of me. They have a coupe of T's check valves, stoppers, two clunks, a vent and two nipples to the muffler. I kid you not there are 9 sections of line running between the motor tank and muffler. This is what they suggest for the "aerobatic" set up.
Although after reading this post it makes a bit more sense.

N1EDM

My Feedback: (2)

Paul,
Do you get Fly R/C? I was wondering if you had had a chance to read John's article? Not sure which edition it was in, but I might have it here.

Bob

iron eagel

Bob,
No I have not seen that ....
Although what was said here does make some sense out of what I was looking at. From what I can see this should make it a constant flow of fuel to the engine at all times makes some sense.

Sport_Pilot

It must be speed, because at the stunt events I have attended the muffler is required. Most of the time muffler pressure is not used, however somtimes the uniflow vent is attached to the muffler tap.

Sport_Pilot

The problem with the check valve is that it keeps the tank pressure high till enough fuel is burned off and then he pressure starts to drop. That means that the idle will start leaning out and the engine will quit. Typically this is a problem when the tank is full but not when it is near empty because when full there is little compressed exhaust gas to take up the slack. Fuel getting into the muffler is not a real problem, other than the mess.

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

jrpav1

My Feedback: (17)

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

pt19 flyer

My Feedback: (1)

hi jrpav1

just installed and started using the uniflow as described in article, engine is air bleed carb. osla46 and I get air bubbles in line to carb . could the air coming in to the non pickup line being picked up by the carb feed line? since they are so close together. did not connect the non pick up line to muffler. overflow line is plugged.

other than air bubbles in line to carb I seem to run ok for about 1/2 tank then have to richen mixture about 1/4 turn<rough estimate>. what would you advise on this?

thanks in advance and happy flying

pt19 flyer

Sport_Pilot

You should make the clunk to the muffler pressure about a half inch or so shorter than the fuel pickup clunk to prevent the pickup from picking up the muffler exhaust. Otherwise you may have a hole in your line somewhere.

iron eagel

John,
I got my hands on a copy of your article from Fly RC today and just finished reading it. I now understand why this is a far superior method of supplying fuel to the engine. Thanks for all of your advice regarding the setup of my four stroke inverted.

ag4ever

How does the fuel not aerate from the exhaust bubbling through it?

Sport_Pilot

Because there is not that much exhaust that go's through it. It only comes in as fast as it goes out.

scratchonly

Can one of you artistic guys send us a picture? Thanks.

Sport_Pilot

Check out the link in post #17