I, a newwbee to jet airplanes .. I've flown my P-60 a couple of times today...Noticed that starting the flight I would have an air bubble about the size of a pea in my Header tank / UAT After about 5 mins. of flying I find that bubble has expanded to abou the size smaller than a ping pong ball.... I used Teflon plumbing tape on theads of tank... Looks like good fittings on fuel lines and festo.....Any suggestions ?? I assuming this tank should remain unchanged after a flight

Thanks

BIll

Bill,

In all likelyhood the problem is not with your UAT but with your fuel system picking up air somewhere upstream of the UAT. The UAT is doing its job by preventing that air from getting to your turbine and causing a flameout. However, that much air in 5 minutes indicates a leak somewhere.

Check all your fittings upstream of the UAT. Safety wire where the tubing goes over fittings and avoid Festo fittings. Fuel caps are often a source of leaks so check they are tight, seal them with silicone if in doubt. It's also possible you have a hole in a pickup line inside a fuel tank which can be difficult to detect.

As for the UAT itself, make sure the cap is very tight and the knurlled fitting in the center of the cap is tight as well. The intake and fill lines connecting to the UAT should also be safety wired and make sure your fill line plug is air tight.

And yes, keep fuel in your UAT at all times. This helps keep the filter bag saturated and free of air.

Keep your flights short and keep looking until you fix the problem!

Craig

Correct.

You MUST have an air leak at the UAT nipples to fuel/ fill lines. Using Tie-wraps to the nipples will not allow a perfect seal... use stainless steel tie-wire.

... Another overlooked problem for very few people is the use of a common screw to plug the fill line after filling... this will 'leak' as outside air will slowly follow the threads of the screw to the UAT tank.

The UAT air bubble should not change in flight unless you have a leak.

I use a felt clunk in the fuel tank of a Wren 44 which I run without a UAT. I have found that the clunk itself generates cavitation in the fuel at high throttle settings with a high fuel flow, causing a line of tiny bubles in the fuel line. The engine can handle these SMALL bubbles without any issues.

Are you using a filter type of clunk in the main fuel tank? If you have searched for leaks everywhere & have found nothing perhaps the clunk is generating these tiny bubbles which gather in the UAT. Check the fuel line from the main tank to the UAT for these tiny bubbles. - John.

Here are my two cents.

Check your fuel filter, if you are using the one that comes with the JetCat engines it needs to be very tight or it will leak and then after flying for some time check it again ( this came from bob him self ) [8D]

You can isolate the fuel lines by using a pair of forceps and your inline shutoff valve.

Clean the outside of the UAT and make sure it is dry.

With the lines isolated and your UAT full, squeeze the UAT and look for leaks around all of the fittings and cap.

After squeezing the UAT and then releasing it (assuming that it collapsed) if there is a leak you will see tiny bubbles where the leak is along with a wet spot of fuel.

Wow, had to go through thsi thread today.

For the forst time in my R/C Turbine experience I am having problems with bubbles in the UAT. When I fill up the tanks, the bubbles are virtually non existing. After about 30 seconds, I notice that the UAT is about 1/8th empty, that'a a lot of air. The lines coming in from the main tanks Tee is pushing virtually pockets of air.

Anyway, I always thought that the UAT would fix issues like this.

Question, I have my UAT in a horizontal position with the fill and the vent nipple on the side facing 45 degrees up. It is a Hanson trap. Is the sack inside the tank supposed to be horizontal or vertical (as in when you rotate it with the tank sitting on it's side). Does it matter?

I'm also skeptical about using large size tygon tubing. I'm changing to a smaller size and will be safety wiring all connections.
I missed a really good day of flying today.

David

vertical which should also be the widest part of the tank, this gives the fuel more chance of keeping the bag wet.

You said from the T on the main tanks... Make sure your feed lines and vent lines are the same length, and your clunk lines and clunks sit about the same place in the tanks if they are parallel. sometimes saddle tanks just dont transfer evently, but you must watch out for that because of one dries up faster than the other, you can induce air and flame out. I recentlly ran my hanson trap to see how long it would keep fueling. It kept the turbine running all the way down below 1/2 full... good confidence builder..

keep troubleshooting. If the tanks are plumbed in serial, its easier to trouble shoot,, but for saddle tanks, cant do that... some guys put a small header tank below saddle tanks to feed them too... I see that around a bit instead of parallel tanks feeding directly into a uat.

I plan to do that on my Flash because the main tank doesn't feed the best below 1/3. I also plan to do that on my F-16 which has saddle tanks..

Yeah, I think I'm gonna stick a 14 oz "header" tank in there.

My F-16 has 2 saddle tanks plumbed in paralell and one 16 oz. header tank behind the UAT. During construction, I was very careful in keeping clunk lines, vent lines and fuel lines of the two saddle tanks, of equal lenght.

No problem until now......because it has not yet flying.....

My lines are 100% equal. I will try applying adhesion promoter and then RTV to the fuel tank caps (problem is they will never come out). Before that I will make sure that the clunks and the lines are in good shape.

Remember that as we make different maneuvers in the sky, even our normal working tanks can have a clunk that might suck in air as it falls back into where the fuel might be.

True, if one flight you have no bubbles and then a different similar flight you have bubbles, its definately time to check your fuel system.

Here is a good way.

1. Unhook the festo fitting from your P-60 and put that end in something that can drain and hold your entire fuel volume in your airplane

2. Plug in your GSU

3. Go to test menu and go to Prime fuel tubing

4. Press the start button and then increase (with the "+" button) the pump voltage to atleast 4V (I would recommend 0.5 volts ABOVE max pump voltage at full throttle, for a Titan max voltage is 3.5-3.75 V, for a P-200 I understand it can be as high as 6v, I'm not sure about a P-60, but I think 4 volts would be enough)

5. Watch for bubbles in your UAT. If you get some on the ground, you got a leak. Don't fly until you find it.

Raf

Raf, not a P-60 it's a Rabbit.
I did just that before heading out to the field and it seemed ok. It maybe that the bubbles happen at high velicity and I can't see them.