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Keeping UATs completely full in aerobatic flights

Old 06-11-2023, 07:55 PM
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acw
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Default Keeping UATs completely full in aerobatic flights

I'm trying to understand what I could do to make sure that my UAT stays completely full during an "agitated" flight and refill itself after brief fuel starvation events.

Consider a typical setup with say a CMJets tank and a Digitech UAT, the issue I have is that when the tank is less than half full, it is possible to move the plane from one position to another such as the clunk is briefly no longer in some fluid and sucking a little bit of air. The issue is that the air accumulates to the UAT but the UAT never refills itself. If enough air accumulate a flameout is possible (happened to me a few times). I'm wondering if there would be a way to improve the setup so there is more pressure or less resistance in the main tank such as the flow always refills the UAT once the clunk is back in the liquid.

I know some of you will suggest, there is an air leak somewhere. There is none. My setups are tight and tested to several PSIs. I'm also not sure I want to revive the felt clunk versus [other type] of clunk debate. At the end of the day, it is always possible to cause a fuel starvation with enough aerobatics or simply on the ground. The clunk can't always be physically in some liquid.

I realize I could always install a small hopper tank to spare the UAT. But I'm wondering if there is a better solution.

Thanks for any information you can share.

Old 06-11-2023, 08:06 PM
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You’re 100% sure the clunk isn’t hanging up on the back of the tank?
Old 06-11-2023, 08:17 PM
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acw
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Yes. Actually I can recreate with all the jets I have. On some models it is worse than others.

To recreate on the ground fill your jet to say 40 percent full
Start the turbine and accelerate it to about 30 percent power
Move the plane from one knife ede to the other and abruptly move to the nose down
You will see some air going to the uat and staying there

I'm trying to create a pressure difference or something like that to have the uat refill itself from the main tank after these fuel starvation


Old 06-11-2023, 09:19 PM
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Originally Posted by acw
Yes. Actually I can recreate with all the jets I have. On some models it is worse than others.

To recreate on the ground fill your jet to say 40 percent full
Start the turbine and accelerate it to about 30 percent power
Move the plane from one knife ede to the other and abruptly move to the nose down
You will see some air going to the uat and staying there

I'm trying to create a pressure difference or something like that to have the uat refill itself from the main tank after these fuel starvation
You have something setup wrong. This pic is of my completely full UAT after an 8 min very aerobatic flight. The tank is 1/3 full now where it was after I landed and there is no air in the UAT.

So are you sure the clunk line isnít catching on something inside the tank? Did you install the clunk line and you know itís right or is it in a PNP plane that someone else installed?





Old 06-12-2023, 12:42 AM
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This is the system I have been using for over 4 years on aerobatic jets, set up for F3S competition. It will go against the grain of many jet pilots....However, the main question that should be asked of any "naysayer" is: "Do you actually fly competition?"

So here goes.... My CMJ tank is set up with a Digitech clunk (it is not felt, but is filtered) and NO brass tube to restrict the clunk's movement during flight. The UAT is also Digitech (NO I'm not sponsored by Digitech) and the photo below shows one small bubble after a rigorous 8 minute aerobatic flight, with many vertical up/down lines, 45 degree up/inverted, outside pushes and numerous point rolls etc. This is a genuine photo and not doctored to indicate anything else but after a complete practice flight.

I'm not trying to sell you anything, and I'm just answering your initial question with what I and several F3S pilots have been using.
Hope that helps.
Wayne



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Old 06-12-2023, 03:49 AM
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I always thought that was the purpose of an air trap. As you fly acro/aggressive maneuvers the main tank clunk will have periods that it doesn't pick up fuel. That air is "trapped" by the UAT instead of it being sucked into the motor and cause a flame out. A good UAT will have a filter system to allow fuel pickup all the time, even with air in the UAT. I know many guys hate the BVM UAT but I have never had a BV UAT cause a flameout and use them exclusively. If you have air in your trap.then it is doing its job.
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Old 06-12-2023, 04:01 AM
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I have my UAT in the Rebel mounted on the top deck. When I fill it, I have to lift the nose to force the air into the main tank. When I disconnect the fill line, I use forceps on the fill line to prevent air getting into the UAT. After a flight, the UAT never has air in it. On the T-1, the UAT is below the Main Tank inlet. Any air in the UAT goes right to the Main Tank. I never see any air in that UAT after a flight either. Also, the only time I ever see air inside the UAT's element is if I drain it fully. Then I may have to lift a tail or nose, depending on which direction I have the UAT's fittings positioned relative to the Main Tank, to force the air out when fueling. If you are getting air in the system during a flight, there is a leak on the suction side of the system. If you are getting air during fueling, either the pump is set too fast or there is an air leak in the fueling lines, etc.
Old 06-12-2023, 04:55 AM
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Originally Posted by tp777fo
I always thought that was the purpose of an air trap. As you fly acro/aggressive maneuvers the main tank clunk will have periods that it doesn't pick up fuel. That air is "trapped" by the UAT instead of it being sucked into the motor and cause a flame out. A good UAT will have a filter system to allow fuel pickup all the time, even with air in the UAT. I know many guys hate the BVM UAT but I have never had a BV UAT cause a flameout and use them exclusively. If you have air in your trap.then it is doing its job.
I'm with you. To me, if you never have any air in your UAT, you don't need a UAT.
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Old 06-12-2023, 05:15 AM
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acw
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Thanks for all the very interesting replies. I totally agree that there will always be some fuel starvation events at pickup and some air the UAT as a result, and that's one of the key reasons we use an UAT in the first place.
The key question remains: is there any way to create a pressure differential such as the UAT always refills itself after the clunk has been sucking a little bit of air.

Old 06-12-2023, 08:53 AM
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Default Yes and NoÖ.

One of the things that I do is use a beveled over flow tube facing into the relative air. In normal forward flight, the relative air pressure is putting a positive head pressure into the main tanks. The picture below is on a CARF Lightning with which I also perform inverted flat spins. I also use this method on my warbird jets with 220 engines and only 4mm tube overflow. Full scale use this technique. I do use BVM UATís with Teflon tape on the screw cap.

I donít believe that the beveled tubing and positive air pressure is enough to fill the UAT after getting air into it. But in my mind, it doesnít hurt, and my UAT are always full.



The copper over flow tube facing forward into the relative air flow.



I repaired a warbird jet for a client after he had a flame out from a pinched line and too heavy of a clunk. The original builder used too thin walled Tygon. The clunk went forward and pinched the line causing fuel starvation from a pinched line. Of course, the UAT emptied the the center stone pick up.

The tank was a long tank, no baffles, and the metal tube insert would be able to turn around inside of the tank.

To me, this is a very heavy clunk.



My solution was to install heavier tubing. I used 8mm clear tubing from the cap to the 6mm copper tube.

Then used a very heavy, thick walled Tygon to prevent the tube from going forward and pinching the fuel off. He hasnít had any problems since the repair.

Old 06-12-2023, 10:01 AM
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Originally Posted by acw
Thanks for all the very interesting replies. I totally agree that there will always be some fuel starvation events at pickup and some air the UAT as a result, and that's one of the key reasons we use an UAT in the first place.
The key question remains: is there any way to create a pressure differential such as the UAT always refills itself after the clunk has been sucking a little bit of air.
Where would the air go? There is no vent on your UAT. When you fill the plane through the UAT the air goes backwards to the tank and then out the vent in the tank.
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Old 06-12-2023, 12:19 PM
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That's a fair point... I accept that as a proof this can't be done! Thank you.

Originally Posted by why_fly_high
Where would the air go? There is no vent on your UAT. When you fill the plane through the UAT the air goes backwards to the tank and then out the vent in the tank.
Old 06-12-2023, 12:41 PM
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Only ever add a hard brass tube when there is a fibre glass baffle. Tygon 1/8 I’d clunk line.
In Europe and especially in the U.K. a felt clunk is almost std fitting.
I have never owned a model without a felt clunk. Mig, Mephisto, Bolt etc etc etc.



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Old 06-12-2023, 05:30 PM
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You're always going to get a little air in your uat.
Old 06-12-2023, 06:31 PM
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Originally Posted by acw
Thanks for all the very interesting replies. I totally agree that there will always be some fuel starvation events at pickup and some air the UAT as a result, and that's one of the key reasons we use an UAT in the first place.
The key question remains: is there any way to create a pressure differential such as the UAT always refills itself after the clunk has been sucking a little bit of air.
What are the fuel starving events?

There are two pictures of UATís post flight with no air in them in this thread..

What does your tank look like, what does your clunk line look like, what are you using for clunk line, and what are you using for a clunk?
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Old 06-12-2023, 07:09 PM
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2 pictures of UATs being full after a flight doesn't mean much. How was the plane flown? how much fuel was left after shutdown? These are important variables. The point is that with our clunk systems, there is always a possibility for the clunk to skip fuel pickup for a brief instant creating these fuel starvation events and bubbles in the UAT. Some tanks and clunks setup mitigate the issue but none solve it completely. Next time you go fly, try these steps with your setup. You will see what I'm talking about.

To recreate on the ground fill your jet to say 40 percent full
Start the turbine and accelerate it to about 30 percent power
Move the plane from one knife edge to the other and abruptly move to the nose down
You will see some air going to the uat and staying there

I'm going to try new clunks to see if they make any difference. Thanks for all the information on this thread.

Old 06-12-2023, 08:07 PM
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Originally Posted by acw
2 pictures of UATs being full after a flight doesn't mean much. How was the plane flown? how much fuel was left after shutdown? These are important variables. The point is that with our clunk systems, there is always a possibility for the clunk to skip fuel pickup for a brief instant creating these fuel starvation events and bubbles in the UAT. Some tanks and clunks setup mitigate the issue but none solve it completely. Next time you go fly, try these steps with your setup. You will see what I'm talking about.

To recreate on the ground fill your jet to say 40 percent full
Start the turbine and accelerate it to about 30 percent power
Move the plane from one knife edge to the other and abruptly move to the nose down
You will see some air going to the uat and staying there

I'm going to try new clunks to see if they make any difference. Thanks for all the information on this thread.
I answered your questions in my original post.
Running the plane on the ground and simply turning it doesnít recreate anything, there are no g-forces as there are when youíre flying.
Keep chasing your problem then, if you canít answer the questions I asked you then we canít help you.
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Old 06-12-2023, 09:06 PM
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IMHO, much ado about nothing. Bubbles in the trap mean nothing more that it's doing it's job......
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Old 06-13-2023, 02:55 AM
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f for any unsolvable reason someone is still getting a lot of air inside UAT (and I repeat, not to get attacked by some police here, if he can NOT solve it any other way and is concerned about it a lot)

there is a way to solve it with small smoke pump (30$) via return line from UAT.

I draw schematic to clarify, ,
and activate that pump durin level flight between acro moves...it would suck content from UAT and return it back to the main tank,
regardless is it fuel or air, it would be replaced with new fuel from main tank outlet....
(one way check valve should be fitted on thet purge line to prevent passive feeding UAT bypassing the main clunk because UAT is @ negative pressure during turbine running)







someone mentioned felt clunks are a norm here in Eu well it seems so,

I replace all metal ones that have a simple hole in them with big felt ones, even use winged walbro ones for smaller suckers like 45N and gas engines,
they can provide clean fuel flow even if taken out of the fluid for a moment (gassers even for 10 seconds!)

and doing that I have no air bubbles in my UAT at all, but I do use somehow small turbines 45,80 and 100N,

bigger ones could suffer from fuel starvation if felt clunks could not filter through required ammount of fuel and produce a lot of suction resistance causing fuel to cavitate somewhere downstream the clunk....

in my Mig21 tank has a hole for a wing spar through it , and that limits the movement of the clunk,
but felt 'wings' collect and keep some fuel to keep the center clunk felt wet and prevent suctioning of the air, firstly it sucks all fuel from wings and then itself before air can pass through, which means it can be out of the fuel and still provide air-free output to the engine
(same as waterboarding technique prevents air to pass a rug as long as it is kept wet)




and in the end, let's not forget bag tanks, where they are legal to use, and can fit inside,
they eliminate need for UAT completely at first place
(I do not use bag tanks)

Last edited by dr.tom; 06-13-2023 at 03:16 AM.
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Old 06-13-2023, 03:24 AM
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Originally Posted by acw
Thanks for all the very interesting replies. I totally agree that there will always be some fuel starvation events at pickup and some air the UAT as a result, and that's one of the key reasons we use an UAT in the first place.
The key question remains: is there any way to create a pressure differential such as the UAT always refills itself after the clunk has been sucking a little bit of air.
Once you get your problem solved you won't need the UAT.
Old 06-13-2023, 04:45 AM
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Wouldn't a bladder tank solve this issue?
Old 06-13-2023, 05:17 AM
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Originally Posted by dr.tom
f for any unsolvable reason someone is still getting a lot of air inside UAT (and I repeat, not to get attacked by some police here, if he can NOT solve it any other way and is concerned about it a lot)

there is a way to solve it with small smoke pump (30$) via return line from UAT.

I draw schematic to clarify, ,
and activate that pump durin level flight between acro moves...it would suck content from UAT and return it back to the main tank,
regardless is it fuel or air, it would be replaced with new fuel from main tank outlet....
(one way check valve should be fitted on thet purge line to prevent passive feeding UAT bypassing the main clunk because UAT is @ negative pressure during turbine running)







someone mentioned felt clunks are a norm here in Eu well it seems so,

I replace all metal ones that have a simple hole in them with big felt ones, even use winged walbro ones for smaller suckers like 45N and gas engines,
they can provide clean fuel flow even if taken out of the fluid for a moment (gassers even for 10 seconds!)

and doing that I have no air bubbles in my UAT at all, but I do use somehow small turbines 45,80 and 100N,

bigger ones could suffer from fuel starvation if felt clunks could not filter through required ammount of fuel and produce a lot of suction resistance causing fuel to cavitate somewhere downstream the clunk....

in my Mig21 tank has a hole for a wing spar through it , and that limits the movement of the clunk,
but felt 'wings' collect and keep some fuel to keep the center clunk felt wet and prevent suctioning of the air, firstly it sucks all fuel from wings and then itself before air can pass through, which means it can be out of the fuel and still provide air-free output to the engine
(same as waterboarding technique prevents air to pass a rug as long as it is kept wet)




and in the end, let's not forget bag tanks, where they are legal to use, and can fit inside,
they eliminate need for UAT completely at first place
(I do not use bag tanks)
pipes on that tank bung are too small for any turbine over 45N!
lock wiring large (3.2mm bore) Tygon on 3mm pipes will never seal completely
Old 06-13-2023, 06:22 AM
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Originally Posted by Dave Wilshere
pipes on that tank bung are too small for any turbine over 45N!
lock wiring large (3.2mm bore) Tygon on 3mm pipes will never seal completely
Keep calm,
It is 45N and rarely sees over 50% since model AUW fueled up is less than that, only 3.6 kg and it is not acrobatic plane.





and pipes have round 'solder balls' on ends to prevent slippage and make great seal to 3.2mm tubing.





Flying w/o problem, no air in uat after flying,
on other plane same setip I even did not have UAT at all and all worked ok, winged felt clunk makes a big difference on those small setups.



but really, here we are talking about UAT and getting air put of it whem someone can't get around it getting in the first place.

Last edited by dr.tom; 06-13-2023 at 06:56 AM.
Old 06-13-2023, 07:49 AM
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Use a BVM UAT !!


Jack
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Old 06-13-2023, 09:19 AM
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What Diaz said and be done with it 😵‍💫😵‍💫

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