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  1. #1
    Ragz's Avatar
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    Help needed in debugging a fuel system fault.

    Recently I had a flame out on a jet just a few feet before touch down. Lucky that the engine flamed out where it did else it could have been bad.
    I am trying to debug the problem that could have caused it. Upon inspection I found that the two main tanks were half full, the hopper tank was completely full, the tube between the hopper and the bubble trap was half full, meaning half full of Air. However the tube from the bubble trap to the pump was full. Now the worst part. The bubble trap itself was almost empty. Needless to say there is a Big problem. I have checked all the tanks and tubes and found no leaks whatsoever. What can cause air to enter into a bubble trap? I have checked the bubble trap and it's totally sealed as well.
    The flame out occurred after several successful flights. So the fuel system did work flawlessly for those flights. Any advice about where to begin?
    Anurag

  2. #2

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    RE: Help needed in debugging a fuel system fault.

    Ragz,
    I know you said you checked the system, but you are sure that the clunk line have no issue? And did you check the fill line to make sure it wasn't leaking? If the bubble trap was half full I would be leaning towards a problem with the fill line or the trap itself.

    Jim

  3. #3
    mr_matt's Avatar
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    RE: Help needed in debugging a fuel system fault.

    Fill line good catch

    Airleak between hopper output nipple to the bubble trap (either end but usually on the hopper).

    This could be from the tubing getting less flexible with age. I have also seen cases where the leak was only under suction at high power.

    Just a thought.
    Matt

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    RE: Help needed in debugging a fuel system fault.

    Try a high flow UAT, this solved my problem.[8D]

  5. #5

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    RE: Help needed in debugging a fuel system fault.

    Ragz,

    I had a similar problem and it drove me crazy. The guys have offered some great suggestions. Once you think you have it solved, I recommend taking the supply line to the engine, placing it in a bucket and running your pump as max voltage for the entire fuel load of the plane. This will allow you to inspect the system without the motor running and at a worst-case scenario

    My problem turned out to be low pressure in the fuel causing air in the fuel to be off-gassed (kind of like opening a beer or soda; all the CO2 is in solution until the pressure drops, then it tries to escape solution). I solved my problem by ensuring better venting

    Dave

  6. #6
    mr_matt's Avatar
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    RE: Help needed in debugging a fuel system fault.

    Dave I have seen this as well. On my old RAM the line from the hopper to the UAT ended up looking like cafe au lait, the bubbles were that small.
    Matt

  7. #7
    CARS II's Avatar
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    RE: Help needed in debugging a fuel system fault.

    Looks like the problem secction is between the hopper tank and the UAT, the guys have given you good clues and suggestion you can fallow, if your lines are old you may want to change them or at least reattach them with a new cut end and the usual wire.

    Just reread your post and I would concentrate on the UAT, it is sucking air, try to do a UAT test under water.
    Carlos R.
    Go fly a Jet today.

  8. #8
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    RE: Help needed in debugging a fuel system fault.

    After many many flights with my JC Titan, i finally had a flame out on take off. Not good[]

    I tracked it down to the UAT's festo fitting that feeds the pump. Above 1/2 throttle it would suck in air. Im solving the problem by ording all new UATs for my planes that are high volume and that have BARB Fittings
    Club NO MAX! No boarder..No Limits..No POLITICS! NO "MAX"

  9. #9
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    RE: Help needed in debugging a fuel system fault.

    Hi Anurag

    I am sure you have a leak from the UAT somewhere which is exaggerated by the suction of your thirsty engine. It's either coming from one or both of the nipples or the cap.

    When I built a Olympus powered Ultra Bandit a couple of years ago I looked at the tank setup and decided to change it. I didn't think the hole in the hopper tank to UAT fitting was big enough and neither was the nipple on the UAT. What I did was fit 4mm ID nipples to the UAT along with a 4mm ID cap fitting. I drilled the hopper tank fitting out to take a 4mm ID fitting too and also made a clunk with again with a 4mm ID. This way I had 4mm ID all the way from the clunk in the hopper to the pump and the UAT never sucked in at all even when the Oly was at full power.

    As it happens I am just putting a Airworld F-100 together and I am doing the same thing for the UAT and here's the pic. The pic shows the two 4mm ID nipples next to a standard size one and a 4mm ID cap fitting.



    Jason
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  10. #10
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    RE: Help needed in debugging a fuel system fault.


    ORIGINAL: Vettster

    After many many flights with my JC Titan, i finally had a flame out on take off. Not good[]

    I tracked it down to the UAT's festo fitting that feeds the pump. Above 1/2 throttle it would suck in air. Im solving the problem by ording all new UATs for my planes that are high volume and that have BARB Fittings

    Oh yes, that urethane tube gets old, it changes shape and then the blue fitting gets loose, especially if there is a bend in the line as it leaves the UAT. I have seen this several times!
    Matt

  11. #11
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    RE: Help needed in debugging a fuel system fault.

    Since we are talking about fuel and UAT I have a question, How often do you change the UAT sack/filter?
    Carlos R.
    Go fly a Jet today.

  12. #12
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    RE: Help needed in debugging a fuel system fault.


    ORIGINAL: CARS II

    Since we are talking about fuel and UAT I have a question, How often do you change the UAT sack/filter?
    My biggest engines are P-120's, so it may be different for the larger motors, but I have UATs that have been in service for 4 or 5 years without changing the filter - which I usually do when I take the UAT out and re-build it for a new airplane. As long as your fuel is effectively filtered before going into the aircraft and nothing is "growing" inside your aircraft, I have yet to have a UAT filter get full enough to effect fuel draw on a 120 or smaller sized motor...

    For a P-200, it may be more of an issue - requiring more regular replacement...

    Bob

  13. #13
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    RE: Help needed in debugging a fuel system fault.

    I've never had one that needs changing but my fuel is filtered 3 times before it gets to the UAT.

    Jason

  14. #14
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    RE: Help needed in debugging a fuel system fault.

    Thanks for the suggestions guys.  The airplane is brand new.. Its an Ultra Bandit test flown last weekend.  Olympus HP for power.  All the fuel fittings are the extra large ones from JMP and the bubble trap is also a JMP accumulator.  I am using extra large ID tygon across the entire system.  I went over board on the vent and made it out of an 8mm ID copper tube shaped like a pitot for that extra pressure in the tanks.  I have checked and double checked and have found no leaks in the system anywhere.  My main tanks have no baffles and I am running 6mm anti static festo tubing with the large JMP clunk.  Its the black tube we get at dreamworks.  Everything is wire tightened.  Its a 20k bird and I went overboard in making everything close to perfect... but i guess I didnt do it well enough.  Perhaps I will look at my purging technique for the accu.  I simply fuelled the plane and flew it.  Didnt follow any purging technique.  I squeeze the filler line before removing the fueler so no air goes into the accumulator.  Other than that, I am not able to see any possibility of air going in.  Now that I have given specific description of the products used, maybe the advice will be more specific?

    According to Bennie at AMT, the pump has to go dry for 4 seconds before the ECU will shut it down with a low rpm error... I havent downloaded the engine logs yet, but when I refueled the system, I found the Accu was near empty... so its believable that the pump got dry for 4 sec or more. Needless to say this airplane wont fly until I know for sure what happened and that it does not repeat again.  


    Anurag

  15. #15
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    RE: Help needed in debugging a fuel system fault.

    Hi Anurag:

    That is scary!

    I am suspicious that you might have had trapped air in the ACCU sack from the start due to incomplete purging, and then with each flight you may have added a small amount of additional air until you reached the sucking air and not fuel limit. I have used Tom's ACCU before, but don't like the fact that you can't see what is going on inside. It is very similar to a BVM UAT with the semipermeable sack inside.....just has the non-collapsible alu housing. I no longer use them and am back to a BVM UAT in my UB.......I am well aware of all the posts about UAT and P-200, but in my case, I have had zero problems over about 57 flights. Zero flameouts with lots of full throttle hill climbs. I occasionally see a small amount of fine bubbles of cavitation forming at full throttle in hot weather over 90F, but the quantity of air at the end of the flight is no more than a few mm from the top of the UAT. I use all 6mm OD clear P.E tubing up to the pump. I only use Tygon on my fill line.

    I would first purge the ACCU. What I do is loosen all the fittings holding it in place. Then fire up the turbine, put on your ear protection and run up to at least 1/2 throttle....then shake and rotate the ACCU thru a full 360 degree, holding it top end up and then bottom end up. That way you don't have to shake and roll the whole airplane.

    When all bubble appear to be out, then run the turbine at 3/4 throttle until the mains are empty and about 3/4 fuel remains in the hopper, carefully checking for any bubbles......finally briefly run up to full throttle for 8-10 seconds before shutting down. Then refuel, carefully observing how much, if any air, escapes back to the hopper tank during the initial re-filling. Should be only a few bubble, no more......

    The only other way to get air into the ACCU and the line from the hopper to the ACCU would be a leak between the two, but it sounds like you have definitely ruled that out.

    Tom


  16. #16
    Ragz's Avatar
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    RE: Help needed in debugging a fuel system fault.

    Great info Tom. Thanks.  Will work towards this as a start.
    Anurag

  17. #17
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    RE: Help needed in debugging a fuel system fault.

    Hi Ragz. Have a look at this article on my blog. That might help you:

    http://www.ultimate-jets.net/blogs/f...considerations

    The second article explores setup options and testing procedures. The JMP accumulator is reviewed. In essence you get the same separator bag as the BVM UAT but with bigger inner bore on the nipples. It is still a Kuss filter based system and has its limitations. I am not too keen on that system because it is masking problems: you would not see the liquid level in the air trap, and how the level changes during a high flow test.
    Oli.

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    RE: Help needed in debugging a fuel system fault.

    What sort of fuel stopper have you got on the end of your fuel fill line. This is offten a problem area. I always use a ordinary bung type stopper and also a clamp on the pipe.

    I have seen some people that use a bolt and of course air will travel down the threads of the bolt and fill the UAT.

    Also large tygon that does not grip nipples and pipe well can be the source of air ingress

  19. #19
    Ragz's Avatar
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    RE: Help needed in debugging a fuel system fault.

    I use the large bung that comes with the jmp accumulator.  its very tight.
    Anurag

  20. #20
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    RE: Help needed in debugging a fuel system fault.

    I used this air trap on my lastest project and i plan on using this type from now on. The fuel flows beautifully thru this air trap and will not cave in under suction from the pump.
    Vin...

    http://dreamworksrc.com/catalog/gbrj...ml-p-1970.html


  21. #21
    CARS II's Avatar
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    RE: Help needed in debugging a fuel system fault.

    WOW!

    That is one awesome air trap .
    Carlos R.
    Go fly a Jet today.

  22. #22

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    RE: Help needed in debugging a fuel system fault.


    ORIGINAL: Vincent

    I used this air trap on my lastest project and i plan on using this type from now on. The fuel flows beautifully thru this air trap and will not cave in under suction from the pump.
    Vin...

    http://dreamworksrc.com/catalog/gbrj...ml-p-1970.html

    + 1. Got my first one the other day. Work of art. Can't wait to tryi out.
    Scratch it till it bleeds Convert it till it chokes

  23. #23
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    RE: Help needed in debugging a fuel system fault.


    ORIGINAL: hooker53


    ORIGINAL: Vincent

    I used this air trap on my lastest project and i plan on using this type from now on. The fuel flows beautifully thru this air trap and will not cave in under suction from the pump.
    Vin...

    http://dreamworksrc.com/catalog/gbrj...ml-p-1970.html

    + 1. Got my first one the other day. Work of art. Can't wait to tryi out.
    Yep, and it takes about twice as much vacuum as a BVM UAT before cavitating.
    Oli.

  24. #24

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    RE: Help needed in debugging a fuel system fault.

    Hi Oli, I was flying anurags UB when she flamed out. All the 3 tanks are sealed and no leaks found. I know it's proven, but I still doubt the 8mm tygon which is used all over, it seems soft since its new and I feel could collapse when the oly is at full thrust, thus drawing fuel from the accumulator momentarily. I personally still prefer the stiff PU tubing. I could be wrong of course.

    Chatty.

  25. #25
    Ragz's Avatar
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    RE: Help needed in debugging a fuel system fault.

    Chatty do you reckon a tygon tube could collapse in this kind of pressure? If so I think we may have nailed the problem. My tubes are indeed soft.
    Anurag


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