I agree with Mr. Danjo and Mr Wescott except for the second bend in the uni flow pipe and soldering the pipes together. One bend towards the tank wall about half way into the tank works and without soldering you can adjust the pipe by turning it up or down. On a clunk tank the clunk can get caught on the uniflow pipe if it's too long.
Regarding a 6 oz. plastic uniflow clunk tank, which I need 4.5 or so ounces of fuel on my la46 sized models. I have always thought the reason they tend to lean out beginning around the start of the overheads is the uniflow tube becoming uncovered. Is this correct thinking on my part?
Yes that's highly likely! The farther forward the uniflo line is in the tank the sooner this will occur! Some engines are more sensitive to this change than others!
Randy Cuberly
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I finally just wire-wrapped and soldered the two vent tubes at the very top, touching but chamfered to avoid sucking up to the top wall.
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This arrangement gives me steady runs all the way up to one or two laps before shutoff when it leans and speeds up before cutting off clean with no fuel in the tank.
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Maybe I was using the wrong type fuel tube when I was doing that, I never could get it to fall into all the corners with the two tubes connected together.
Heres another way to do the feed and uniflow tubes together. I ground a groove in side of clunk (with edge of thin cutoff wheel on an angle grinder) to make a recess to solder in the brass tube. I install this assy with brass tube outboard and the hole in side of brass tube is engine feed, and the clunk is the uniflow. The end of brass tube is crimped and soldered shut. I am using this in a profile with 6 oz slant oval tank so there really is no vertical movement of assy to speak of. Getting a cleaner shut off than before with more conventional setup. There is a shorter time between uniflow and feed becoming uncovered.
Part of the uniflow magic is that the vent tube stays in one place. A uniflow with two flexible tubes is going to behave differently. I'm not sure differently how, but it'll behave differently.Yea that's the theory , but I have read where others put the uniflow attached to feed clunk and reported it works great. This in a full fuse using rectangle tank with normal clunk vertical movements. Planning to validate this myself in next full fuse build, but will be using the Sullivan round and/or cylinder tanks.
Allen,No, with engine feed and uniflow that close together the engine would be sucking air bubbles from the uniflow. You need at least 3/8" separation between them..
Wouldn't it work the same without that side hole and not soldering the tube end shut?
CB
Yea that's the theory , but I have read where others put the uniflow attached to feed clunk and reported it works great.
Heres another way to do the feed and uniflow tubes together. I ground a groove in side of clunk (with edge of thin cutoff wheel on an angle grinder) to make a recess to solder in the brass tube. I install this assy with brass tube outboard and the hole in side of brass tube is engine feed, and the clunk is the uniflow. The end of brass tube is crimped and soldered shut. I am using this in a profile with 6 oz slant oval tank so there really is no vertical movement of assy to speak of. Getting a cleaner shut off than before with more conventional setup. There is a shorter time between uniflow and feed becoming uncovered.
Correction, just took closer look at tank and my clunk assy is about 1/4" from rear tank wall and this does allow clunk to move vertically, looks like plus minus 1/2" relative to tank centerline. And this setup is working good, don't understand why, but it does..
No, with engine feed and uniflow that close together the engine would be sucking air bubbles from the uniflow. You need at least 3/8" separation between them..
It works because it is in the same relative position to mother earth inverted and up right. I have tried the same setup but used the clunk as the fuel feed. I think what you have done is a better way to do it. I am guessing the purists are using rigid tubing. ala the nut to twist the tube as posted above. y1
Part of the uniflow magic is that the vent tube stays in one place. A uniflow with two flexible tubes is going to behave differently. I'm not sure differently how, but it'll behave differently.
Interesting.
I find, and physics backs up the empirical evidence, that a uniflow tank goes rich once the vent is uncovered, since the pressure in the empty part of the tank increases once the vent is open to the ambient pressure.
I think a lot of people mis-understand exactly what happens in a uniflow tank. It automagically adjusts the pressure of the empty part of the tank in order to keep a constant pressure and thus constant suction head at the pickup as the tank empties.
Perhaps the most elegant engineering solution ever put on a model aircraft, originally done on FF.
But how would you make it stay put from flight to flight, or even start>finish during one flight?
Steve
This is my setup for 4oz tank.
Aki
Heres another way to do the feed and uniflow tubes together. I ground a groove in side of clunk (with edge of thin cutoff wheel on an angle grinder) to make a recess to solder in the brass tube. I install this assy with brass tube outboard and the hole in side of brass tube is engine feed, and the clunk is the uniflow. The end of brass tube is crimped and soldered shut. I am using this in a profile with 6 oz slant oval tank so there really is no vertical movement of assy to speak of. Getting a cleaner shut off than before with more conventional setup. There is a shorter time between uniflow and feed becoming uncovered.
Correction, just took closer look at tank and my clunk assy is about 1/4" from rear tank wall and this does allow clunk to move vertically, looks like plus minus 1/2" relative to tank centerline. And this setup is working good, don't understand why, but it does..
You're probably using muffler pressure. That's not a uniflow tank at all, but with the LA engines, a standard-vent tank on muffler pressure can give a very steady run.
Finally - one other person who understands that a pressure tank is NOT really uniflow, even if the pressure input is submerged.
Thank you for your very useful post. I directly followed your advice on preparing the uniflow pipe and soldering it to the clunk. The output tube was connected to the central tank neck tube facing outboard. The inflow tube was concocted to the lower neck tube facing inboard. The tank was located in its usual position on the model (a little above the centre line of my OS FP40 powered profile model). The result was an excellent even run throughout the flight. The only minor snag is there is almost no warning of engine cut out. Not a problem when flying the schedule on a known volume of fuel, but I do keep an eye on the clock when free flying to practise specific manoeuvres.
You want the "no warning cutoff", ...
Hello Thanks Kiwibrit, Shug's video was interesting watching, the fuel was all over the place depending on the attitude the model was in. Looks like the engine must take in some air as it sloshes about, particularly at the end of the run.
But surely that's the point of having the 'clunk' on the feed line - it goes where the fuel goes. It was interesting that the fuel seemed to vary more top to bottom via G forces but didn't seem to be thrown to back of the tank through forward momentum?
If you figure that the needle valve needs to be the governing restriction in the fuel line, it can be opened up to compensate for other fuel line restrictions. But when the total losses (lines, bends, fittings, filters, and needle valve) become great enough to reduce flow below the engine needs in the most critical orientation, then its going to burp lean. If the critical condition is longer duration, it won't just be a burp, it will last until the conditions change back to something more favorable.
I would think that muffler pressure would have the positive stabilizing effect of increasing the fuel flow as rpm increases since the venturi size is fixed.
Would also be interesting to scrounge up a low mass TC and insert it into the tank outlet and see if the temp rise is significant. That would back up any calculations. My gut feel is that the heat content of one tank volume of displacement air will not be greatly significant--but there are neither calcs or tests to support that yet.