DavidML

Hello pilots,

what's the reliable way to set-up a three-line fuel tank?

Lot of people in this forum suggest two clunks in the tank. Isn't there a risk of tangling up? Wouldn't it be better (instead of the second clunk) to use a brass tube pointing straight downwards (right behind the front wall of the tank) maybe with some extension silicone tubing reaching the bottom of the tank (no clunk)? It would fill-up and drain the tank ok and it seems safer than the two clunk system because the lines in the tank are wider apart. What do you think and mainly what's your experience with the three line system?

I'm using Irvine .72 and DuBro 16oz square tank on BTE Venture 60.

And since we are talking fuel I've got one more possibly stupid question: Consider a plane with a half-empty tank in a dive. The engine should die, shouldn't it? The fuel level is below the pick-up clunk, isn't it? Excuse a rookie for asking probably obvious thing. Thanks.

Cheers,
David.

Ed Smith

The two clunk system is based on an old concept called the 'Uniflow" tank. I believe the Control Line flyers were the first to use it. It is a very good system for an even engine run. Both clunks must be submerged in fuel at all times. The theory is that air can enter the tank only when fuel leaves it, and fuel can leave the tank only when air enters it. If the vent/pressure line was exposed air could enter the tank in an uncontrolled fashion. With the uniflow the fuel prevents uncontrolled air entering through the submerged end of the pressure line. As the engine sucks fuel, air bubbles up through the fuel to replace the exiting fuel. So, in effect the needle valve is controlling both the air going in and the fuel going out.

One has to pay attention to needle valve setting with this type of tank. What is set on the ground will pretty much stay like that in the air, Right or wrong! Also no warning is given when the tank is running low. The engine will just quit.

The big advantage with this setup is that with the correct needle setting the engine will not suffer from a lean run during the flight. This makes the system very worthwhile. I have used this setup in racing aircraft and it works very well.


Ed S

P.S.
I would guess that in a dive the aircraft is accellerating so the fuel is pushed to the back of the tank

Cogburn

Three line tank setups with one clunk work just fine for filling and draining cowled engine setups. I do use two clunks because if one clunk falls off i can simply switch to the other one to continue flying.

Chuckr53-RCU

My Feedback: (40)

In a 3 line setup, I aim a bare tube up to the top of the tank for the exhaust or pressure feed into the tank; I aim a bare tube down to the bottom of the tank as a fuel fill; and I insert a tube with silicon fuel line and a clunk on the end...which is connected to the needle valve feeding the carb. Only one clunk and only one flexible line flopping around inside the tank. It works fine.

Richard L.

My Feedback: (24)

Not if you offset the clunks with one clunk in front of the other. Plus it's not really a concern if they tangle up because they will still be resting at the bottom of the tank and fuel will still be drawn through them. They will untangle themselves once you pull up to do a loop.

In a vertical dive, you are most likely to chop the throttle. There is enough fuel in the engine and the fuel line to keep the engine running. The engine consumes very little fuel at idle. Even if you don't chop the throttle, the engine will accelerate the plane downward, pressing the fuel against the back of the tank. Fuel doesn't fall faster than the tank due to gravity (physics).

huey-pilot

My Feedback: (17)

You only need the 2 clunks if you want to be able to pump the fuel out of the tank at the end of the day. I have been using 2 clunks and 1 vent tube and it works great. The line that you use for fueling and defueling will have to be pluged when not fueling so the fuel doesn't go overboard when the engine is running. If you use a T fitting and fuel dot you will get air in the fuel line and this system helps prevent this since the fuel line to the carburator is never disconnected, this helps when restarting my gas engines. I am flying a CAP 232 with a ZDZ 50NG and I haven't had any problems with the fuel lines getting tangled or any other problems.

Azcat59

I was running the fill/unfill third line to the bottom front of the tank. Only problem, you had to stand the plane on its nose to defuel it. Then I read about the two klunk system, and I do a variation of that, by using 1/8" aluminum tubing, and bending it so that it runs along the lower left side of the tank, to the left bottom rear corner. Easy to defuel. Out of the way of the main klunk. So far I have it in at least two planes, and it is working well.

Clair

Ed Smith

To emphasise the purpose of the Uniflow tank.

All of the tanks described in the preceding threads work well. However the emphasis has been on ease of filling and emptying. The purpose of the Uniflow tank is to have a consistant engine run, far more important than being easy to fill, which it is anyway.

Ed S

DavidML

Thank you all for your thoughts and experience. I'll go with slightly modified Uniflow concept.
David.

jaka

Hello!
As Ed Smith says ..the Uniflow tank is the way to go .... sad thing so many of todays R/C flyers doesn't know basic CL facts..
I have used this 3 line tanksystem (2 clunks, one for pressure and one going to the carb, 1 overflow tubing going up at the top of the tank ..for 25 years now and it certainly has worked good.
It is good to have the pressure hose with its clunk a little shorter than the clunk hose going to the carb...that way you don't get air bubbles into the carb hose.
The two clunk hoses will not interfere with eachother!
Filling the tank on a airplane with a cowl were you can't get to the carb requiers a fourth fuel line and this just goes into the tank. This fourth line is plugged when your are running the engine..often this line is hanging under the cowl..plugged by a 3mm screw. The owerflow line is also hanging beside it ...it is also plugged by a 3mm screw when flying/running the engine
You fill the tank with the engine at idle ...that way no fuel is going to the carb...flooding the engine!
Forget T-fittings and fuel-dots...get rid of all the things that can make you crasch!!! Keep it Simple!!

Regards!
Jan K
Sweden

Richard L.

My Feedback: (24)

There is no need for a fourth fuel line when filling the tank on an airplane with a cowl. Keep it Simple!!!

All my planes have cowl, and for those planes using YS engines, I only have two lines coming out of the cowl, capped by two fuel dots. T-fittings on YS engines make life simple. On planes using other types of engines, I have a third line coming out of the tank for fueling and defueling. Again, no fourth line is needed.

JoeAirPort

My Feedback: (41)

Same here. I use what Richard uses for YS. I use the 2nd clunk to fuel dot for Saitos and 2-strokes.

jaka

Hi!
But are you using the two clunk set-up? Like in a Uniflow tank??
JK

pt19 flyer

My Feedback: (1)

hi

you could do it either way, I use two clunks with absolutely no problems and it does make de-fueling much easier and complete.

happy flying

pt19 flyer

pt19 flyer

My Feedback: (1)

hi

no the uniflow setup did not work for me. the two clunks were connected together in the tank of the uniflow which affected their movement with aircraft in position in any position but upright. I use two completly separate pickups and clunks make fueling defueling much easier, especially in cowled setup. I just let one of the clunk lines dangle from beneath the cowl.

or you could use one of the fuel dots.

good luck and happy flying

pt19 flyer

FrancisPerson

If you are planning to setup what is in my attached drawing then do not bother, it does not work.

Francis

JoeAirPort

My Feedback: (41)

If you connect that vent line to the muffler pressure tap it will.

jaka

Hi!
As Joe said!
Just attach the vent line to the pressure tap on the silencer..and presto! It will become a Uniflow tank!
But having the vent line brasstubing soldered to the fuel clunk perhaps is not so good (risk of sucking air). I always make this hose (the pressure line) and clunk a little shorter(1-1,5cm ) than the fuel pick-up clunk.
Regards!
Jan K
Sweden

pt19 flyer

My Feedback: (1)

hi

the setup as I described in a posting will work. have been using it on all my currect planes with absolutely no problem. do not connect the clunks together as described in the uniflow setup and use the pressure line as normally used.

doing it this way it works fine. the problem with the original was the clunks were connected together with affected their movement in the tank with the plane flying other that straight and level. try it you will like it, make fueling and defueling convenient.

good luck and happy flying

pt19 flyer

DUNNJR4

My Feedback: (1)

what do you use as an overflow? do you disconnect the line from the muffler every time you fuel

DUNNJR4

My Feedback: (1)

does this work?

flybyjohn

My Feedback: (2)

I have not seen the reason for using the uniflow system explained well in this thread yet so set me give it a shot. The whole Idea is getting the same pressure to the carb from full tank to empty tank. With out doing the math lets just imagine that with a full tank of fuel, you have the head (pressure) of the fuel pushing down to the bottom of the tank. If your exhaust line is also comming into the bottom of the tank, you have the fuel head also counter acting the pressure the exhaust is giving. So with full tank you have high head pressure from full tank to carb inlet and low pressure from muffler pressure. With empty tank you now have a higher exhaust pressure because the fuel is not longer counter acting the input pressure and you also have a lower pressure of fuel pressing on the carb inlet clunk. This leaves you with low pressure to inlet but high pressure from exhaust. If you do the math it turns out that the presure to the inlet is dead even from full tank to empty tank so the engine does not go lean during flight.

Now for reality. I have two 46 Thunder tigers and a chinese 46 engine running this set up and they work flawlessly. I tried a os 91fx two stroke on this setup and could not get a good run. The problem with the 91 is that the muffler pressure was to great in the tank and everytime I throttled down the fuel gushed into the muffler from the built up pressure in the tank and realy messed with the exhaust flow or something because It just would not run reliably. I set it up with the standard setup and all was well. So some engines will run great on the uniflow and some may not.

DUNNJR4

My Feedback: (1)

Will a 4 line system work?

flybyjohn

My Feedback: (2)

I don't know what you would do with 4 lines to the tank. I guess you need to let us know what you are trying to get out of a multi line tank. Are you trying to set it up for good even runs or for ease of fueling? If it is for constant runs then I would just set it up with 2 clunks, one a little longer than the other but both can touch the bottom of the tank at normal flying atittude. Then you would have a third tube that is bent up to the top of the tank. The tube with the long clunk goes to the carb, the tube with the short clunk goes to the exhaust, and the tube bent to the top just gets an extention on the outside of the tank and a plug put in it. When you fuel, you can put the fuel into any one of the two clunk lines and unplug the vent line, this will let air out of the tank. Then when full, plug the vent line and attach the two other lines to the engine, and then go run it. Don't forget to put the plug in or you will have a mess. When unfueling, just suck fuel out of the long clunk line.

DUNNJR4

My Feedback: (1)

I have a cowl on my plane. i can not get to the line on the exhaust without taking the cowl off or burning my hand. I would have one line for filling/ emptying (capped while flying), one for overflow(capped while flying),one to the engine and one to the exhaust. would this work or is there a different way i should set it up?