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Air Bubbles

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Air Bubbles

Old 01-14-2016, 07:36 AM
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Default Air Bubbles

Has anyone ever done any comprehensive testing on how big of an air bubble a turbine could handle and keep running? I'm sure turbine size and power setting would be a big factor.
Old 01-14-2016, 08:07 AM
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Its the software not the turbine. Bubbles going through the pump and resistance changing is what triggers things. Everything about our turbines is fuel flow, its why its so important to have a free flowing, solid fuel supply.
Old 01-14-2016, 09:16 AM
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Originally Posted by jofunk View Post
Has anyone ever done any comprehensive testing on how big of an air bubble a turbine could handle and keep running? I'm sure turbine size and power setting would be a big factor.
Small cavitation bubbles would go undetected through the pump. However, because the fuel stick capillaries are small, even tiny bubbles could stop the supply and shut down the flame temporarily. If only one capillary is shut down, the flame from the combustion chamber will re-light it. However if too many capillaries shut down, the combustion chamber will shut down...

Have a look at this:

And this:
Old 01-14-2016, 09:28 AM
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I have seen some engines handle "air" quite well, while some will not tolerate any. It seems the newer engines tend to handle it better than older ones, and that may be due to as Dave suggest.
Old 01-14-2016, 11:22 AM
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Great question!

It would be REALLY useful information. Imagine a list of engines tested with say a known quantity bubble (say 1cc) and see in which engines are the least tolerant to this sort of thing.

And for the engines that don't flame out, keep increasing the bubble until a flame out is seen.

Old 01-14-2016, 11:42 AM
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I did a test on my turbine test stand, no air trap, most turbines still ran fairly when even when the fuel started to get very low and you could clearly see large gaps in fuel supply, a small bubble here or there didn't seem to make a difference. I think modern turbines and there software is fairly fault tolerant.
Old 01-14-2016, 12:05 PM
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JetCat added a bubble counter to the later software-it can't really count bubbles! but it does give an indication of fuel supply quality. I see turbines flame out with a stream of smoke (fuel supply issue, rather than initiated electronic shut down) and looking on the ground the bubble has gone...they rule out an air bubble because they cannot see one.
If it smokes there is a good chance of fuel supply problems. (If you can't find something on the field, take the model home and search, its less painful than 'bagging the bits')
Its such a complicated situation with many factors, for sure some software copes better than others and some fuel manifold/combustion chamber designs cope with interruptions better (you can get leaks inside the front cover and now valves and pumps are buried, its not so easy to spot)
But there are too many variables to say one is better than another-ultimately a perfect fuel supply is what you need and kidding yourself that your chosen brand will deal with a sub standard system, is a sure fire way of having a bad day and not enjoying your hobby. One day lost flying while you fix it or a month lost while you build a new model-do not accept anything other than perfect...

Old 01-14-2016, 12:21 PM
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According to the manufacturer, Hawke Turbines are bubble tolerant and the owners of the company state their turbines are not affected by bubbles in the fuel supply as the design is completely different to all the other turbine makes out there, naturally I guess there is a limit!

Old 01-14-2016, 02:35 PM
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It's a nice idea, but I think it would be almost impossible to develop any reliable data for flame-out resistance because there are an infinitely variable set of operating conditions that can affect this. You can dump a bunch of air into the line and run it at various throttle settings on the ground, and it won't tell you anything about what the motor will do when it ingests a bubble at full power at the top of a loop at 1000AGL on a humid day because a weather front moved in the night before.

I have had my P-120 start and run without a hitch even though the fuel line was full of air ( I got lazy and didn't feel like purging it after tracking down a leaking Festo fitting), but I would never dare fly my plane with any unresolved air pockets.

There seems to be some consensus that among conventional model turbines, the Kingtechs generally keep running after digesting big bubbles, but I would not want to bet my model on it.

Should be a moot point anyway with all of today's great fuel system products and fittings. The is absolutely no reason to get bubbles into your engine if your fuel system is set up correctly and is properly maintained.

Last edited by TTRotary; 01-14-2016 at 02:45 PM.

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