I have a tower hobbies 40 trainer and i can fly for about 5 min and then it will die. i can run it on the ground and it will stay running through a whole tank of gas. today i started the plane gave it half throttle and picked it up so I could tip it and turn it put it upside down basicly tried to mimic the movements in the air. after i had it pointed down for a few seconds it died and i noticed air bubble in the fuel line. all my lines are tight no cracks tank is good. Is there something wrong with the pickup line in the tank?

Sound like the line inside the tank has a split or pin hole in it. replace all fuel lines and see how it works. Leave the clunk, the weight on the end of the fuel line inside the tank about 1/4 to 3/8 of an inch for the back of the tank.


Buzz.

Also when you pointed it "down", how much fuel was in the tank?

If less than 1/2, pointing the plane down tends to uncover the clunk, leading to bubbles.

Normally our engines hold operation through the bubbles and the air flowing through the prop tends to prevent the engine from dying out, so we hardly notice this.

However if the tank was 3/4 or fairly full I'll echo blvdbuzzard's suspicions.

Test it on the ground at the fuel level it has when it quits,, if you see a lot of bubbles then,, that may be the issue

assuming the tank is fine,
Another cause could be as simple as you have too small a prop and when it unloads in the air your engine is going lean, gets hot and quits.

so hard to tell without seeing it in person, good luck

What engine are you using?
When tunning an engine you only need to point the engine straight up, if the engine picks up slightly she is tuned well, a larger increase means it's rich, if the engine decreases any at all no matter how slight it's too lean.
I always peak for max rpm and back the needle off 3 clicks.

For anymore tunning instructions I'll need to know what engine you have.

Superdo9:

If you're using a 40 size engine and the tank that came with the TT40 (which is a 9 oz), you should be ok, BUT...

I have noticed that on most of the trainers, it is very difficult to get the tank high enough in the airplane. Ideally, the centerline of the tank (top to bottom) should be lined up with the spray bar in the carburetor. If it is significantly lower, then the engine will tend to starve for fuel during the second half of the tank.

You might have better luck getting the Hayes slimline 8 oz tank, as it is very short (top to bottom) and can be got higher up into the fuse. Alternatively, try remounting the engine so that it is sideways in the airplane. This cures all tank alignment problems.

Hmmm...

I have an 16oz tank on my Big Stik 60, that lets me fly for 18 minutes non-stop.

It came with the plane.

I'd like it to be a bit bigger.


On most of my ARF's I'd LIKE a bigger tank.

I use the 3/4 part of the tank to line up with my spray bar, much less siphoning

I agree with Buzz. Replace ALL fuel lines inside and outside the tank. Don't trust an inspection because pin holes are very difficult to see.

I had two airplanes whose engines would quit in the air after a few minutes. Both had an annoying problem with bubbles in the line for different reasons.

First, for my KAOS, I wanted to seal off the engine compartment from tank compartment. I made sure the hole in the firewall, through which I ran the fuel line, was a snug fit. Bad decision. Vibrations, transmitted to the line, were causing bubbles. After opening up a larger hole to allow the fuel line to pass through the firewall without makng contact, the problem was solved.

Second, for my Sr. Falcon, I stuffed a rather large tank, surrounded with latex foam rubber, into the compartment. Pressure was required to hold down the hatch so I could secure it to the body. As a result, I was squeezing the foam down to the point where it no longer damped the vibration. After making a new (bulging) hatch to allow the foam freedom to expand, the problem was solved.

So I bought a new tank and same problem. When the plane is running level its good but as soon as I point the nose down I can see air bubles in the fuel line then it dies. This is with a 1/3 of a tank fuel. When its full its fine. Basicly I can fly for half a tank and I better land or if the tank get to 1/3 any pointing down with my plane it will starve

Sounds like the fuel is moving forward and the clunk is high and dry. You could try some softer fuel line inside the tank to allow the clunk to move more. A little shorter clunk line will move the clunk forward.
\
Buzz.

Well of course with only 1/3 a tank that will happen,,

In the air, as you "Dive" typically you'll back off the throttle and run off the fuel in the line, also positive "G"s help hold the fuel toward the rear of the tank in most maneuvers. So unless you're taking it up a mile high and doing some full throttle dives with it only having 1/3 tank shouldn't be an issue.

What exactly "In the Air" are you doing when it quits?

That's a myth. We have videos to prove it too!!


However the first statement of "of course with only 1/3 a tank" is correct.

Point the nose down, and the clunk becomes uncovered.... and air gets into the line. There is no avoiding that unless you go to a bladder tank.

You have video that proves fuel isn't affected by G forces?

I knew someone would say this but this actually wrong. It will work fine if the centerline of the tank is even with the spray bar but the actual ideal location is with the centerline of the tank about 1/4 to 1/2 inch below the spraybar. This is because when inverted the muffler pressure vent is now below the fuel which reduces the tank pressure, the correct location when inverted is the same distance above the centerline to make up for the reduced pressure. Most ARF''s including trainers are fine, but sometimes when oversizing the enginethe carb is too high.

This is normal, you could find more flexable fuel line, allowing the clunk to move foward but this can cause problems with the clunk knoting around the pickup line. opjjose was correct as this is not normally an issue when flying. If it is an issue you could try a header tank. If it is a problem then a rucher mixture will keep the engine running while it sucks out the bubble. Normally if the RPM is peaked with the nose straignt up and the tank less than 1/4 full, then it is rich enough to run through the bubble. Possibly not on the ground though.

It proves that "fuel does NOT stay in the back of the tank" when the nose of the plane is pointed down.


In other words, to keep the fuel at the back of the tank, you need at LEAST a constant 1.5G+ forward acceleration.

If you do the rather simple math, you'll see that our planes cannot possibly achieve this for more than some fractions of a second..


During a hard pull the video shows the fuel sloshing immediately forward as the plane looses speed.


It also shows that no matter how you fly the plane, once the nose points down, fuel moves forward... and if you have less than 1/2 of a tank of fuel, this will uncover the clunk.

So yes positive 'G's DO NOT hold the fuel at the back of the tank, because such a force is only momentary.

Right, and because liquids are not compressible but gasses are, clearing the bubble from the carb occurs so quickly that we hardly notice unless there is a sustained length of air in the lines.

The "slightly rich" setting does a lot of good things for the engine, and keeping the engine running during downlines, etc... is yet another.

I never said 1.5 G or less, Hard Pulls or when the plane points down,
I said positive "G"s help hold the fuel toward the rear of the tank in most maneuvers,,

The maneuvers you describe produce negative G and decelatration conditions
I just love it when guys misinterpret something said and then correct based on Their Misinterpretation.


Problem with most of these Hypothetical replies is the OP isn't accurately describing what he is doing when the engine quits "In the Air"

So far this is the only in flight info we have
We don't know what engine, Fuel nitro%, prop size, if he's pointed up down, inverted,, nothing

Sounds like he's taking it up lean to me, but becuse he mentioned bubbles in the fuel line when he points the plane down, (while on the ground), the tank is the focus???
Of course you'll get bubbles if you point he plane down and hold it there,,,

Nope it doesn't... that's the myth.

Our planes cannot sustain sufficient "positive G's" for any length of time no matter the move.

So the fuel does NOT "hold toward the rear of the tank in most maneuvers" as you've stated.

I also love it when people repeat an often heard fallicy without testing.

So let see the video you're talking about

There's a whole thread where this was discussed over a year ago.

Check the "fuel stays in the back of the tank" thread for the original link.


If need be I have a copy of that video I could put up.

It settled all of this fairly well and convinced quite a few to rethink.

Ok,, I'll go search for what you want to use to prove one part of the point I was trying to make wrong,,

and BTW you misquoted me again

I agree but it is the differance in viscosity not compressability that allows the air bubble flow through so quickly.