Convert standard tank to Uniflow

[Phil Goldberg]

I have a standard vent tank 4oz and want to convert it to uniflow.  Am I right in thinking that I need to remove tank end, pull out the fill tube and replace it with a tube that ends up (how much distance) from the pick up tube.  I can make the exposed end of the tube turn over the fuselage and then turn forward on the inboard side of the plane.  A replace one tube situation.   

[Dwayne]

Here's a pic of a uniflow tank, hope it helps. As you can see it still has fill and overflow tubes. But if you want to remove the fill tube you can.

[Phil Goldberg]

Thanks, I can use this diagram and it gives me a good idea of where to place the inside end of the uni flow tube.  Looking forward to re purposing this standard tank.   

[Randy Cuberly]

The end of the uniflo tube shoud be about 3/8 to 1/2 inch from the end of the fuel pickup tube.  much closer than that and it's possible to pick up air bubbles from the uniflo tube that make for erratic running.  Much further than that and the engine can go a little rich before the end of the tank.  The engine tends to richen a bit as the uniflo tube uncovers with fuel.

Randy Cuberly

[Jared Hays]

I think you have that backwards Randy...The engine leans out when the uni flow tube becomes uncovered.  Thus giving you a heads up your about out of fuel and knock off any maneuvering and get ready to land.

[Jared Hays]

Here is a tank my dad and I are working on right now you can use for example.  We make a fill and overflow tube for ease of filling, that way you don't have to disconnect your pressure line to fill from and you can leave the plane sitting level on its wheels to fill.  We then use a piece of fuel tubing to connect the two when the tank is full, sealing them off by doing so.  When we are filling we just pinch the uniflow line off.  Makes fueling a snap.  And motors run consistent the whole flight right up until the end...lean out for a lap or 2 and quit.  Just make sure you check for leaks in any solder joints before you fly.  We fill a cooking pot with water and then cap all lines but one.  Take an empty syringe and pressure test the tank with air.  If its holds a couple oz. worth of air pressure and you see no bubbles while holding the tank under water in the pot your good to go.






 

[EddyR]

Your brass fank will siphon out the uniflow as you have built it . if you run the uniflow to the top then out the bottom it will not leak siting full before takeoff. Most stunt planes get around this problem by venting wih tubing above the tank top. When uniflow first came out most were built with the uniflow running from top to bottom before exiting the tank. Built that way engine run problems go away

[Will Davis]

Another thing to consider when making your own fuel tank, if possible, always keep the tubes that exit the tank  as short as possible , this will eliminate the issue of the long tube working itself loose at the solder area at the entrance to tank. Vibration of engine, g forces, heavy handed when removing filler and vent lines will over stress the area , and cause a leak, usually at a contest .   

 On a built up model, Just make a exit tube in the model and connect it to the tank with a short piece of  fuel tubing . On a profile, just keep the  fill, pickup and overflows tubes as short as you can.

[Randy Cuberly]

I think you have that backwards Randy...The engine leans out when the uni flow tube becomes uncovered.  Thus giving you a heads up your about out of fuel and knock off any maneuvering and get ready to land.

No Jared...you have it backwards.  A uniflo system works by providing a restriction on the amount of fuel the engine can draw. This restriction mitigates the fuel head on the fuel pickup line as the fuel is reduced in the tank there by keeping the pressure more constant.
When the uniflo line becomes uncovered the restriction is eliminated and the fuel mixture will richen.

If this happens very near the end of the tank the fuel pressure from centrifugal force is so low that it is hardly noticible but if it happens earlier it definitely will be noticed and the mixture to the engine will richen!

Simple physics! 

Randy Cuberly

[Randy Cuberly]

Jared,
Also on the fuel tank you've made with the long unsupported tubing external to the tank, I would also mention that vibration of those lines will tend to crack the solder joints around the tubes and cause leaks.

I've found over many years and many tanks to always use short tubes external to the tank or bundle them together and solder them at several points to make them stiff enough to move the fundamental frequency of the tubes high enough that the vibration of the engine cannot excite the tubes!

Randy Cuberly

[Bill Johnson]

Randy's right, of course. People get confused because when using a uniflow set-up, the mixture leans suddenly at the end of the run when the uniflow line is uncovered. What is missed is the fact that without the uniflow line venting the tank, the engine would have been leaning constantly from the start. The uniflow vent airflow is slightly opposed by the fuel head pressure seen at the end of the uniflow line. That tends to keep head pressure constant throughout the run until the uniflow is uncovered.

It took me awhile to get my head wrapped around the principle.

[Jared Hays]

Randy's right, of course. People get confused because when using a uniflow set-up, the mixture leans suddenly at the end of the run when the uniflow line is uncovered. What is missed is the fact that without the uniflow line venting the tank, the engine would have been leaning constantly from the start. The uniflow vent airflow is slightly opposed by the fuel head pressure seen at the end of the uniflow line. That tends to keep head pressure constant throughout the run until the uniflow is uncovered.

It took me awhile to get my head wrapped around the principle.

This is what I meant.  A standard tank makes the engine run leaner and leaner for the duration of the flight as soon as the vent becomes uncovered.   And when a uniflow vent becomes uncovered at the end of the run the engine leans out.  Then your out of fuel anyway.

My dad and I have made our tanks this way for years and have no trouble with them.  We simply clip the uniflow line closed or pinch the fuel tubing line that is going to the muffler pressure fitting closed while filing and when we are ready to start unclip it.  Siphoning hasnt been a problem though even the few times I've forgotten to pinch the line.  As far as the tubes go they are long enough to just get out the bottom of the fuse and we will cut them to the desired length.  Which wont be to much as you can see from the pics below.  We haven't seemed to have any problems with our solder joints cracking due to vibration but if a guy wanted to solder a support strap it wouldn't be hard to do so.  We have had the sheet of brass or body of the tank itself develop a crack but that's on tanks from 30+ years of flying too.

[Paul Smith]

The use of two fill tubes is good.  That's how I always do it.  You can cap them with a single connecting tube.   And if you happen to have a hard or soft tube on your fueler, you're covered either way.

But I don't trust having the uniflow vent below the fuel mass.  It could let fuel drain out, replaced air through the needle valve.  This could be mitigated by plugging the uniflow until you're ready to start.  I prefer to play it safe and always have the uniflow above and inboard of the fuel mass.  That way it can't dump fuel on the ground or in the air.