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General control line discussion => Open Forum => Topic started by: Shawn Kuntz on March 23, 2007, 09:45:33 PM
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Okay guys...I got a question. Can anyone tell me the advantages of running a uniflow setup for profile models? Also, what are all the details for doing it properly? I am running uniflow vent tanks from RSM with the fill tube pointing forward into the airstream. I plug the overflow tube when I am flying. I believe this is correct. Also when I run standard tanks with an upright fill tube, I attach a short piece of fuel line cut at an angle with the opening facing into the airstream. I also plug the overflow tube on this one when flying. I believe I am doing this correctly. To the best of my knowledge I think I am getting more uniform runs using these methods but sometimes I doubt it would matter if I plugged the overflow tubes or not. I do not run muffler pressure. Any tips, advice, etc.? I fly a 50th Anniversary Fox 35 and an old McCoy 35 blackhead.
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If you want to run uniflow you have to plug the overflow, by definition it employs only one vent...which should terminate at the outboard side of the tank, close to the fuel pickup, the idea being it stays submerged and "bubbles" the vent air into the tank. Supposed to give more consistent runs without the typical leaning out as fuel is burnt off. It does work.
With a standard two-vent tank, the vent tubes are usually on the inboard side, clear of fuel, plugging one is unnecessary but probably doesn't make any difference. You will likely notice it leaning out as the flight progresses.
I'm using unifow tanks with my 1/2As and getting much more consistent runs throughout. I built one with the vent tube bent into the slipstream but it richened up too much in flight so I went back to jutting straight out. The only other fuel system that approaches the performance of a uniflow setup for me is, surprisingly (at least to me), the floppy-balloon tanks I sometimes use. They are unvented and collapse as fuel is consumed. I suppose there's a similar principle at work but I can't envision it clearly.
That's all I know about tanks. Well, except pressure, that's another story.
--Ray
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Ray, can you tell me more about your balloon tanks? Sounds interesting.
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Ray, can you tell me more about your balloon tanks? Sounds interesting.
Very simple, just standard round party balloon tied with a twist-tie onto a piece of fuel line that's perforated a few times at one end...held in some kind of container (I use 35MM film cannisters, free at any place that develops film). Called "floppy" because you DO NOT fill them to the stretching point, these are not pressure tanks. Fill them to their relaxed shape, 1/2 to 1 oz. depending on balloon size of course.
Pics below are a tutorial I prepared sometime back for another guy. Self-explanatory I think.
When filling, you must first suck all the air out. Just draw back on the syringe until the balloon is completely collapsed. Or if you're using a fuel bulb, partially squeeze the bulb, attach the fuel line, release the bulb, proceed to fuel.
--Ray
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I have not had very much luck with uniflow. Lately, I put a uniflow tank on a model and it did precisly what it is supposed to do. The mixture was exactly the same from start of the flight to the finish. My problem is that the mixture changes so much from launch to flight. Whatever mixture I get after 1 lap is where it stays for the entire flight, so it's managable.
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Thanks, Ray. How long do the balloons last? Do you pitch them after each flying day?
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The balloons will last up to a month sometimes;depends--probably on how much nitro in the fuel; how much fuel is left ih the balloon as it hangs on the wall, etc. I use 'em until I notice a little degradation starting, usually it starts getting a little sticky. If you're not sure, toss it; balloons are cheap.
Garf, if the mixture goes too lean after launch, bend the vent into the slipstream; if it's going rich, bend it the other way. You can sort of tune it by the angle of the vent.
--Ray
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Garf, if the mixture goes too lean after launch, bend the vent into the slipstream; if it's going rich, bend it the other way. You can sort of tune it by the angle of the vent.
--Ray
I use a fitting like this on my profiles.
(http://home.att.net/~philbrown36641s5/fitting1.jpg)
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When I tried one pointed forward like that, it had to be launched so lean it was sagging or it would flood out in a lap or so. Also the wind seemed to affect it more (richer with a headwind). Leaving the vent "neutral" (straight out, 90 d. from thrustline) seems to work best on my setups. Course I'm talking 1/2As, my usual disclaimer.
--Ray
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I have to set it almost too rich to take off, then it leans out when airborne.
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Used this uniflow tank on a profile. Vent was facing into the air flow. ST 46. Set rpm on ground within 10 feet of launch engine was setteled into it's 4 stroke mode. Ran that way for the rest of the flight. Loved this tank and motor combo, it was always dependable. Settled on 15% nitro. I could always tell by the pull of the plane during the first 2 level laps if the needle was set right or not. Slight change in RPM 1 lap before quiting and always quit clean.
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Hi Guys,
Here is some food for thought, with most racing events where a suction type fueling system is required uniflow is the only way to go.
After reading the threads on this I would add only one thing and that is the tube in the airstream should be at least 1/2" away from the fuselage to insure a consistent air flow across the air inlet tube. The size of the inlet area of the tube will also affect the consistency of the engine run right up to the last drop of fuel and give a predictable indication of when the engine will cut out.
One more thing they do not work well, or in some cases at all, with needle valves not connected directly to the venturi this is not the setup to use with a remote needle valve as the amount of vacuum created to pull the fuel is substantially less with a remote needle.
Scott
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I use a one way valve in non pressurized uniflows. I have a very small hoarded supply of the old KK valves. I understand Jett Engineering has a similar valve.