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?

RE: How does a fuel tank work? help?
 

looking at that photo(original set up). there's a coupler on the intake..which is a NO..NO
in my book. You're taking a chance on air entering the fuel system more
than you have to. Plus i don't see any type of foam wrap around the fuel tank.

As someone mentioned, inspect the clunk line for small cuts

I uaually wrap soft foam around my fuel tank to prevent foaming due to
vibriations. Air bubbles in the fuel line will make the engine run lean at the
very least. You know.... what happens when you run too lean.lol

when the model in the air it's always going to run leaner. So tune it rich on the ground.
or hold the nose of the model up and tune it.

You should be able to see a smoke trail

The engine will dy due to over heating after a while, running too lean. if you're lucky
you're not going through glow plugs.

plus give your engine for time to break in
plus you have to tune pipe on it, so you'll have to richen it too.

Not sure how you use your throttle. Sometimes you need to clear out the pipe.

Another way to get more fuel in the intake line is to use a fuel filter..it's acts like a mini
hoper tank or install a slightly longer intake line.

RE: How does a fuel tank work? help?
 

No, lowering the tank has nothing to do with it.

If it did planes would not be able to fly at all of their different attitudes.

The tank pulls fuel because of the lower pressure at the carb coupled with the increased pressure provided by the muffler to the vent line.

As long as the clunk is submerged in fuel, the fuel will continue to flow. That is why a plane can fly nose straight up in the air. The pressure differential causes fuel to move UP to the carb w/o problems.


You say that this only occurs at idle... but you didn't say if the tank is full or almost empty when this occurs.

It would help us if you answered the questions that are asked of you.


So when the plane is at idle with 1/2 a tank or MORE, have you performed a "pinch" test?

e.g. when you pinch the fuel line for a second or so, does the engine speed up slightly?

When you put the nose down about 15 degrees, do you see air getting into the line?


Can the clunk "swing" down a bit as indicated in the above example pictures?

A degree of "swinging" down is very necessary to keep it submerged as the tank empties, and helps forstall the problems I mentioned in my post about clunks becoming uncovered.

RE: How does a fuel tank work? help?
 

lol This is getting repetitve, and that picture was not my plane. I have yet to post a picture. THIS ONLY OCCURS AT 1/4 TANK OR LESS, NOSE DOWN, AT IDLE ONLY!!!. The fuel is not able to reach the clunk and it starts to suck air at a nose down attitude. I'm thinking a headr tank might do the trick. Anyway, here is a picture of my plane just for arguments sake, so before you say I'm not answering the questions asked of me, pelase read this post along with the many others on this site referring to my plane.

RE: How does a fuel tank work? help?
 

usafloadc17,
I understand your frustration at this point, but as I said before, I have this same exact plane. There is not issues with the location of the fuel tank, and I've got countless hours of flight on this plane with no issues whatsoever. Reconfiguring the airframe to accomodate a header tank is simply making more work for yourself before you have explored all of the problems. First of all, you don't need the plane to stay at an idle for more than a minute. It's next to impossble to have the plane in a nose down situation without hitting the ground first. Second, look at the situation you are describing. If this happens at ONLY a certain level in the fuel tank then there is something wrong in the tank. If it was because of fuel flowing to the front of the tank and leaving the clunk exposed it would happen at other fuel levels as well. My gut instinct there is to tell you to replace all of you fuel lines, both inside and outside of the fuel tank. I've got a sneaking suspicion that you have a pinhole in your fuel intake line inside the tank, and then then the tank is down below that level it starts sucking air. Pinholes can sometimes be next to impossible to see, so it's easier to just replace all of your fuel tubing.

Ken

RE: How does a fuel tank work? help?
 

I have. I've replaced ALL lines and tanks Twice! Even an engine!

RE: How does a fuel tank work? help?
 

I stand corrected on the fuel lines. But something is going on. As I said, I've had zero problems with this plane and the fuel tank setup.

Ken

RE: How does a fuel tank work? help?
 

I'm still kind of confused. If your tank is 1/4 full and you have a nose down attitude (45 degrees), will you're engine run?

RE: How does a fuel tank work? help?
 

Absolutely. Will it run like that forever?? Not a chance. Eventually the engine will use all the fuel in the fuel line and will die. But in normal operations of a RC plane the time it is in a down line isn't long enough to drain the fuel completely out of the fuel line. If you're having real world issues with running out of fuel in a down line try adding a loop of fuel line between your carb and the fuel tank. The extra line will keep it running longer.

Ken

RE: How does a fuel tank work? help?
 

so just off the top of you're head, how long will it stay running before it dies at a nose down attitude? Is a minute normal then?

RE: How does a fuel tank work? help?
 

A minute is an eternity in airplanes. Consider this. If you are at an altitude of 200' and you point the nose down to the ground. I'm going to use 60 mph has a nice round figure for an example here, and it is a realistic number too. At 60 mph the plane will travel 88 feet per second. So from 200 feet to altitude to the ground will take just a little bit over 3 seconds. See what I mean about the minute being an unrealistic number. There is almost no way you are ever going to be in a nose down position for a minute or longer while the plane is in the air. You'll smash into the ground long before you hit a minute's time.

Ken