How does a fuel tank work? help?
 

When a fuel tank is 1/4 full and the nose is down, how does the clunk get to the fuel if it is in the abnk of the tank? My engine always stalls when the nose is down b ecause fuel will not go into the engine?

RE: How does a fuel tank work? help?
 

I've heard alot about issues with fuel tanks and to be honest i don't know the answer. But mabye someone else does and will read this.

RE: How does a fuel tank work? help?
 

Presuming the tank is correctly set up, the clunk (the metal fitting on the end of the fuel supply line) will drop to the front/bottom of the tank, and remain in the fuel.

Essentially, again if the tank is set up correctly, the clunk is "free"...that is to say, it can rattle around anywhere in the tank it needs to. As a result, no matter how the tank is tipped, the clunk falls to the lowest point...exactly where the fuel is.

One of the common problems with tank setup is cutting the supply line too long. As a result, the clunk either gets wedged against the back of the tank, or the line is too long to allow the clunk to "bend" it, and fall to the front. This sounds like a likely suspect for the issue you describe.

When setting up a tank, cut the line so that, if held "upright"...that is, with the plug at the top...the clunk will dangle about half an inch off the "bottom" (the back if the tank is mounted flat) of the tank.

Here in a moment, i'll make up a couple drawings to indicate what I mean.

RE: How does a fuel tank work? help?
 

Here's what I'm talking about...sorry about the quick and dirty drawing. :)

http://img.photobucket.com/albums/v1...nes/clunks.jpg

RE: How does a fuel tank work? help?
 

so the clunk should be able to do a 180 and move to the front?! My clunk will ony stay in the back of the tank.

RE: How does a fuel tank work? help?
 

well don't forget there is pressure in the tank and centrifiugal force... my cluck is off the "back of the tank" but the line is too thick to allow the cluck to fall forward. I don't have a problem with flameouts either..I guess cause you don't have full power on when you're vertical down for long. if you're engine is dying when your vertical down then you have a prob with the lines or with the mixture.

RE: How does a fuel tank work? help?
 

well its not even a vertical problem, its when I come in for a landing. When the engine is slightly down for the approach, it dead sticks. Air gets in the line and shuts off the motor. I am using a pitts muffler. Maybe a backpressure prob?

RE: How does a fuel tank work? help?
 

No the clunk should not be able to go to the front of the tank. If the clunk line bends and sticks in the front it will give you more problems. Went I set my tanks up I put a piece of brass line inside the clunk line so it can't bend forward ever.

I'd check to see if it isn't stuck forward all ready. Can you even see your tank?

I think you have some other problem besides the cluck, even if you only had small amount of fuel in the tank and you nosed down there's still fuel in the line, you couldn't nose down long enough to run out. If your getting air bubbles in your line its probably because theres a small pin hole in a line or the tank itself or a problem with the stopper cap.

What kind of plane is it?
How is the tank mounted?
Is it high or low in relation to the needle vavle?

-Steve

RE: How does a fuel tank work? help?
 

Hey sthone--how far do you run that brass line in the fuel pick-up line?

RE: How does a fuel tank work? help?
 

You don't need much, just enough to make the line too stiff to fall to the front of the tank.

Ken

RE: How does a fuel tank work? help?
 

Who says a picture is worth a thousand words. Amen. Thanks

RE: How does a fuel tank work? help?
 

its a .40 great planes cub. The tank is actually about 2 inches below the need valve which may be a problem, however, the plane is designed like that or the cowl wont fit. It's not an air leak because I bought al new lines and replaced the tank twice. The pitts muffler has 2 gaping holes, maybe its a pressue problem? Whenever I point the nose down, the engine runs for about a minute then dies. Maybe this is normal?

RE: How does a fuel tank work? help?
 

did you change the lines inside the tank that go to the clunk?

RE: How does a fuel tank work? help?
 

I think your problem is the tank being too low and creating a fuel draw problem.It is just coincedence the engine dies when the nose is pointed down.at full power you are getting muffler pressure plus a full tank (head pressure)to help feed fuel to the engine at idle the pressure in the tank will slowly bleed of to 0 and a tank with low fuel has less head pressure to feed the engine.I have built 3 cubs and there has always been alot of room behind the firewall what is preventing you from raising the tank up?formers can be cut away to allow the tank to be raised

RE: How does a fuel tank work? help?
 

Can you reproduce these results on the ground?
Hold the plane tip the nose down with the same amount of fuel your flying with.

-Steve

RE: How does a fuel tank work? help?
 

Exactly which Cub do you have?

Hogflyer

RE: How does a fuel tank work? help?
 

I want to know how high you're flying if you can fly nose-down for a minute without hitting the ground.

RE: How does a fuel tank work? help?
 

When you say a minute do you mean simply momentarily or actually a minute? I am by no means an expert but have had simular problems.As stated above you dont want the clunk to fall forward because it will pinch the line and you will starve it anyway.When in a dive you will not be applying power(I hope) therefore the engine will continue to run even though the clunk may not be covered with fuel.If you are acually talking about a minute or close to it nose down under power it will starve itself in my experiece.

RE: How does a fuel tank work? help?
 

Image test. Please ignore

RE: How does a fuel tank work? help?
 

on the ground it will stay running for 30 secs. My avg approach lasts 45 secs or so and the engine is stalled by the time it hits the ground. Yes, the formers are preventing me from raising the tank. Should I cut away the top former and raise the tank? How should I go about it? Please hep get this thing flying!!!.

RE: How does a fuel tank work? help?
 

Didn't you say that this was the Great Planes Cub??? If so, I've got that same plane. I installed the tank in the stock postion and haven't had a bit of problems with it. And yes, I've used a Pitts muffler on it. I really think that the problem would lie with something other than the tank position.

Ken

RE: How does a fuel tank work? help?
 

I have a Dura Plane (brick) that has the same problem.I removed the tank and replaced the clunk line(looks just like the drawing earlier in this post) but when the plane is nose dow,it sucks air and makes the plane die.I can see it when I hold the plane,and tip the front end down,it idles right down and will stall unless I tip it back level or upright in the front. I am so used to dead stick landings its not funny,maybe 1 out of 12 time I can keep it running. What about a small header tank? I also have a Hanger 9 tribute I am leary of flying because I'm afraid it will dead stick and not being used to it might crash.I am running a 40LA engine.I know what you are going through!!!!!

RE: How does a fuel tank work? help?
 

It is possible that the clunk HAS moved to the front of the tank. this can happen in a hard landing or nose over . grasp your plane FIRMLY by the landing gear and snap it backward sharply now gently shake it to see if you can hear the clunck bouncing in the tank. you should be able to hear it move. I under stand this can happen to any one but it NEVER happens to me...LOL;);)

RE: How does a fuel tank work? help?
 

Raising the tank won't help.

You said, "Whenever I point the nose down, the engine runs for about a minute then dies."

Does it do this on the ground with a full tank? If it does, my guess is that you are rich on the low end. The added pressure of the fuel (when pointing the nose) down is flooding it.

RE: How does a fuel tank work? help?
 

If you think its the tank hieght try turning the plane up side down and see if it still dies. Upside down the tank would then be higher now.....Just a thought.

Does this only happen at say idle when your coming it for landing to would it happen also if you were to say dive at half or full throttle?

I still think you have some other problem.

-Steve

RE: How does a fuel tank work? help?
 

it only happens at idle. It's perfectly fine at half or even full throttle. I can see whats causing it. AIr gets into the line because the clunk is not sucking the fuel that moves to the foward of the tank. It's not a rich problem, in fact, the idle screw has little to no effect on the engine at idle. Yes it is a great planes cub. ***************I know for sure its a fuel sucking problem, at half tank, the fuel moves to the front of the tank and the clunk at the back cannot pick it up************* What would cause this?

RE: How does a fuel tank work? help?
 

If it doesn't happen at half or full throttle it can't be the clunk as you describe because it still wouldn't be in the fuel.

Does the engone transition from idle to full if you advance the throttle quickly or does it hesatate then go to full?


If its turely what your saying happens then the only thing I can think of is to run a header tank.


-Steve

RE: How does a fuel tank work? help?
 

Yes, a header tank should do the trick.

Another option is to cut the clunk line a little shorter, or use a bigger tank, or keep you flight times shorter.

The only way a clunk can suck air at half-tank is for the nose to be down at more than a 45 degree angle.

One other thing you could try - you mentioned raising the tank - that won't do any good, but if you can raise just the front of the tank, that would help.

RE: How does a fuel tank work? help?
 

45 seconds of approach time! How far away are you flying in from? Why don't you just get in the "pattern" and do a quick turn in from your downwind leg and land? But even if you like long approaches your engine should not be flaming out. I agree with Minnflyer that you might be too rich on the low end idle.

RE: How does a fuel tank work? help?
 

I would start with MiniFlyer's suggestion first about checking for an overly rich mixture on nose down with an almost full tank...

Then:


Unfortunately the "Centrifugal force" and "fuel stays in the back of the tank" is a misconception.

Posted videos back this up.


On even lightly angled downlines the clunk can and will become uncovered once the tank begins to empty.

As the tank approaches 1/2 or less the problem gets more and more dramatic as fuel is consumed.


Uncovering the clunk creates a large air bubble that can starve the engine of fuel.

Increasing the fuel tubing length doesn't help much, nor does wrapping the fuel line around the tank ( verified with videos we took btw... air still gets sucked into the line. ).

An engine that is able to run a bit richer when the nose is pointed down, and the clunk uncovered for a few seconds can sustain operation because it has two things going for it...

1 - the engine is unloaded and the airflow helps keep the engine running or idling
2 - the nose down attitude pushes a bit more fuel into the carb helping it to get through those "bubbles".


In an ideal situation the clunk WOULD travel forward and stay submerged in the fuel.

The problem with this is it can indeed get "stuck" up front.

Kinking, where the fuel is actually cut off, is really more problematic in the smaller tanks where the angle of the bend exceeds the tubing's strength/ability to keep the wall diameter somewhat consistent. In this case the wall collapses or kinks and the fuel flow is cut off.

The larger tanks ( assuming that the internal tank piping is set up to stay out of the clunk's way ) with medium tubing the wall thickness and strength, coupled with the unencumbered travel, permit you to set up an arrangement where the clunk may be permitted to travel forward.


MiniFlyer states: "The only way a clunk can suck air at half-tank is for the nose to be down at more than a 45 degree angle."

As you go beyond half a tank things get much worst rather quickly.

As fuel is consumed the required angle becomes less and less.


To solve all of this on several planes with 16oz+ fuel tanks I've set things up so that the clunk is permitted to travel forward.

To do this I have to be VERY careful with the vent tubing so that it stays close to the top wall of the tank. The intent being to eliminate any areas where the clunk and line may lodge or be caught into.

This has worked rather well for me, particularly in those cylindrical tanks with curved backs.

RE: How does a fuel tank work? help?
 

what engine is this?does it have a plastic remote needlevalve?

RE: How does a fuel tank work? help?
 

im about just ready to sell all of this crap. I cant figure it out. It's not the mixture

RE: How does a fuel tank work? help?
 

usafloadc17, do you ever time your flights?

The best method to fix your problem has already been mentioned (Header tank) We're just trying to find a simplier solution - or rather, we're trying to find out why it's happening to you, and not most of us.

RE: How does a fuel tank work? help?
 

Don't give up... it's always something small.

First, is this only happening with 1/4 of a tank?

If so it is as I described above, and you may want to set your timer so as to not let the engine run so low.

Another option is a bigger tank or as others have stated a helicopter "header" tank cures the fuel flow problem.


If this happens at all tank levels, could there be something in the carb itself.

For instance some of the Evolution engines have rather weak springs that are supposed to keep the low speed needle pushed away from the high speed needle receptacle.

The springs used are much too weak sometimes causing the mixtures to go off when the plane is sideways in flight, etc.

Changing the carb or putting on heaver springs frequently cures this.


When you see air in the line, do you see on large bubble or many small ones?

If the latter try one of those sintered ( sp? ) Dubro clunks that help eliminate fuel foaming.


Under what situations does the engine run correctly?

RE: How does a fuel tank work? help?
 

what is a header tank? and this is the 2nd engine its been doing it to.

RE: How does a fuel tank work? help?
 

A header tank is a small tank that is connected to the main tank. As you draw fuel from the header, IT draws fuel from the Main. This way, if the header sucks some air, it will still be mostly full

RE: How does a fuel tank work? help?
 

Also:

First, is this only happening with 1/4 of a tank or so?

Under what situations does the engine run correctly?

RE: How does a fuel tank work? help?
 

Here's a crude picture of a header tank setup.
-Steve



Edit- oh well Mike beat me to it while I was drawing......either way this is what you may need to do.

RE: How does a fuel tank work? help?
 

any chance that mounting the tank with its back end lower would help?

RE: How does a fuel tank work? help?
 

the formers allow no room for the tank to be lowered. Tight fit. And it only occurs at idle, every single time, with different engines. It's a fuel sucking problem. I thought it as the pitts muffler not providing enough back pressure when the nose is down. BTW, hoes the the tank pull fuel from the front when the nose is down? If the clunk stays at the back, back pressure is not enough to feed more fuel to the back is it?