olnico

Hello guys.
I am starting this thread following the discussion on the FEJ F-16 thread where several guys had flame outs with their P-200 and a normal fuel plumbing.
I think that this discussion might be of a general interest.

https://www.ultimate-jets.net/blogs/...considerations

https://www.ultimate-jets.net/blogs/...ing-cavitation

https://www.ultimate-jets.net/blogs/...considerations

I would first like to mention that I have written an article that partly covers this matter, to publish in the December/January RCJI issue.

The big engine available on the market ( 160 N and above ) have a high flow requirement. This means that the fluid velocity in a standard tygon tube is very high.
The flow Q is given by: Q=SxV. If the section S of the tube decreases, the the velocity V will increase at a constant flow ( non compressible fluid ). What happens is that passed a certain velocity, the fluid will start to cavitate: bubbles of gaseous kerosene will form in a perfectly leak free system. So any restriction in the lines could create cavitation at a certain point ( restriction= S decrease = V increase )...
Additionally the flexible UATs like the BVM or Hansen products will collapse to such an extend ( on very big engines ) that the walls of the tank will touch each other. This will restrict the flow around the air trap bag and might induce cavitation downstream it !
So in that case the air trap system becomes the bubble generator...

I have done extensive tests with all kinds of UATs and all kinds of engines.
I have a very simple rule: no flexible UAT for engines above 120N.

1) I use the biggest Tygon size for my vents ( 8 mm ) and two 6 mm brass tubes for the vent pickup ( big bore ).
2) All the cluncks are re-drilled
3) 6 mm fuel tubes everywhere ( big bore ), no filter: the air trap does this job.
4) the 6mm tube slides on the pump nipple with a small piece of 4 mm tubing and 2 turns of safety wire.
5) I use the rigid PST air trap that is fitted with 8 mm nipples and ultra large air trap foil folded about 8 times ( very large air retention area )
6) Keep the fuel lines downstream the air trap as short as possible

The PST air trap is very big and offers a significant buffer time in case of intermittent bubbles. It does not cavitate at all because of the large retention area. It is also correctly priced given its technical specifications ( 50 USD ).
The other option is to use the JMP accumulator (air trap) that is made of aluminium and is of course very rigid.

I prefer the PST version for the several reasons:
1) The transparent bottle makes you see what is happening while doing the test runs
2) It has a very high volume
3) It is much cheaper





When I have finished setting up the system, I do a stress test in real conditions without the engine running. I just loop the fuel line that is supposed to feed the engine back to the fuel vent ( possibly with a Tee in a 3 tank configuration ). This way the system runs on a closed loop.
I then enter the engine test menu and rum the pump at whatever voltage is needed ( 6v in the P-200 SX case ). I can then have a good look at what the system is doing at a simulated full load.
While doing this test, I move the plane in all the possible positions ( ie: nose up, nose down, both sides, inverted ). I always keep a good look at the air trap behaviour and any air bubble entering/exiting it, as well as at the engine fuel feed ( which is looping back to the tank and not connected to the engine on this test ).
I also do the same test while feeding the fuel back to the my Jersey modeller field tank ( open loop ). This way I can see the behaviour of the fuel clunks when the tanks empties.

I have not had any problems with this setup on my two P-200 for many test and flight hours...

Edgar Perez

My Feedback: (13)

Thanks for the very detailed writeup.

ww2birds

My Feedback: (14)

Very interesting reading, thanks for posting!

Vincent

My Feedback: (61)

Oliver wrote:>>

That is a bold statement considering the many thousands of flights out there with 120-160n motors using a flexible type UAT.

V..

olnico

Yes Vincent, most of the setup will work on a flexible UAT if done properly ( limited restrictions, large vent ) up to 160N class engines.

However I have seen my F-84G getting 4 flameouts with a brand new P-160SX. That was only due to the BVM UAT getting vacuumed and generating cavitation.
This same UAT has been used since then with a P-60 and gives complete satisfaction.

I have extensively checked the fuel plumbing and could not see any restriction. It's only when I did a very careful test with the closed loop that I realized what was going on.
I changed the UAT with a PST air trap and never got any problem any more.
Same story with the P-180 and P-200.

This happened to me over a year ago and I have never had any problems since then with my new setups.

So to be on the safe side, I apply this rule...

avicom

My Feedback: (14)

That's great to know. My friend is experience same UAT bagging in his Euro with Titan engine. I check the fuel sys and all the lines are 6mm. It never happen before but the UAT had lots of flights there. Same thing with this Rhino powered L-39. Wish only flew one time for the maiden at time of delivery.

Oliver, what about the old center pick up tanks? There are few guys that never an issue using them. Rounded bottles are less susceptible to collapse and can be find in more rigid plastic.

Alex

Hammbone

My Feedback: (309)

Does anyone know if there is a source for the PST Bubble Suckers in the US?

Thanks, Jim

KC36330

I have to agree with Vin, it's what i like to call 'Troubleshooting Symptoms Not Problems', the likely cause of your problem was restrictions in the fuel system that caused problems with your UAT and if the P-60 system saw a proportional restriction it would give the same cavitation.

avicom

My Feedback: (14)

Yes Jim, this guys:

BTE Engineering, Bruce Tharpe: http://www.btemodels.com ; [email protected]
RCI RC International: http://www.airworld-usa.com/contact_page.htm ; [email protected]
PST California: Ehab: http://www.pstjets.com ; [email protected]
PST Washington: Chuck Bower: [email protected]
PST Texas/Florida: Barry Raborn: [email protected]
PST New York: Len McIntosh: [email protected]

Alex

Hammbone

My Feedback: (309)

Thanks Alex, I did a search, but nothing came up.

Jim

Vincent

My Feedback: (61)

BTW i do like the looks of the pst air trap and would be interested in getting one, who would have one in stock??
V..

HarryC

My Feedback: (1)

But is that solving a symptom rather than solving the problem? Whatever low pressure existed to cause the BVM UAT to collapse, still exists. If the UAT collapsed then there is a restriction somewhere in the fuel feed to the UAT or the air vents to the tanks, the fuel pump will have to be straining against that restriction, and changing to a hard bottle UAT doesn't change that.
The new Wren 160 uses 3mm tube on the pressurised side from the solenoid valve to the engine, I haven't been hearing any reports of it suffering flame-outs. I agree with what you say Olnico, about the need to look at using larger diameter tubes to slow the flow rate and the drag of the fuel that the pump has to work against, but how does the Wren 160 experience fit in with that?
Harry

Seinfeld

I once got caught up with the whole air bubble, cavitation issue when I got my first turbine and was setting it up.
Wondering how to avoid it and what the best setup was.

After thinking about it I realised its just so simple.

I used 6mm Festo line as much as possible. My pump has 6mm inlet and 4mm outlet

My fuel lines are extremely short especially from UAT (which is flexible) to pump.

I dont use any filters and I set things up so that I don't have any sharp bends.

I paid attention to keeping it simple and tidy and all my runs have been without any issue and the Merlin sucks around 450ml/min at full throttle and zero air bubbles. UAT is always full.

To be honest I found setting up my turbine easier than my electrics.

Just don't go complicating things.

mr_matt

My Feedback: (10)

Great tips! I like that PST design, I have not seen it before

For me, I try to keep the suction side from the UAT to the pump as short and straight as possible. I have noticed that certain types of fuel line, when used with a relatively tight bend from the UAT feed line can (over time) develop a suction leak at the UAT feed nipple.

Also, I do not use any festos on the suction side, and use large fittings made for garden sprinkler systems. And make sure the vents are huge and do not constrict at the single vent. It would also be a good idea to have any parallel tanks that feed into a header to each feed directly into the single header as opposed to going to a "T" fitting then going to the header.

avicom

My Feedback: (14)

Welcome Jim, best choice is Bruce. We order 4 Reaction-Arf with him and all four arrived in less than a week directly from Thailand.

Chris P. Bacon

My Feedback: (4)

Great post, Olnico. Do you mind sharing your source for those brass bulkhead fittings? I'm in need of some.

JackD

My Feedback: (4)

Olnico

I think all your ideas are good, however, I'm not sure if a rigid UAT alone will solve it


Cavitation happens when the pressure is so low, that the fluid evaporates. A UAT collapsing is an indication that the pressure is really low in the uat (either because of fluid velocity or by restrictions in the system). So, having the UAT be rigid instead of flexible is not changing the fact that the pressure is too low, which could still create cavitation. I understand that making the cross section smaller by collapsing it will make the fluid faster and make the pressure even lower, but my point is that the low pressure still exists (although maybe not low enough for cavitation) if the UAT is rigid.


However, it is still good to keep the UAT from collapsing. If you suddenly cut the power when the UAT is collapsed, the uat will also suddenly expand, momentarily starving the system, and since you are at idle, it might cause a flame out.

Interesting discussion....

Jack G

Joe C

My Feedback: (23)

Tom Cook of JMP Machined Air Accumulator, there is nothing better, my opinion. Joe C!!

Kmarks

My Feedback: (36)

This Thread is excellent!

I tested my system today by letting the fuel system run into the fuel can, no closed loop. I am using all stock copper tubes supplied with FEJ F16. My set up is like this: the overflow vent runs into one of the top fuel tanks, these top fuel tanks each bleed into the top copper tube in the bottom fuel tanks and the two bottom fuel tanks are T by a festo blue T to the BVM UAT. The second top vent is connected to the UAT for fueling. The BVM UAT is 5 inches to the Jet Cat fuel pump. No filters on this side of system, and standard yellow fuel tubing used. I am not sure of size. I set Jet Cat GSU to 6 volts and let her run for 1 minute.

The result was half my UAT was filled with air when I shut her down. The sides did infact cave in a small bit, but the amount of cavitaion is way out of line.

I will go to the Hobby shop and get the larger fuel line, larger copper tubing and drill out all the clunks and stoppers. The Tom Cook air trap is back ordered, so I will have to wait on that.

I hope this solves this cavitation problem, I dont think I'd be as lucky if I had another flame out to walk away with as little damage as I did with the first.

Kevin

nraheja

My Feedback: (15)

This is a great post but like others have stated ...the root cause is a narrow fuel line. With that said I have used the PST UAT but with very poor results. No matter how much the cap is tightened, it still let air leak ..guess thru the cap. I have let the filters soak overnight but it still gave out bubbles. A friend had the same experience so I decided to pull it out after several attempts to get it to seal. The cap fitting to the bottle just has room for improvements and the inside of cap seem to lack good smooth finishing.

Not saying it does not work but just that my experience was bad.

I pulled out the PST UAT and replaced it with the Tom Cook accumulator and it has been rock solid....have over 100 flights with it. It is just not possible to get an air leak with all machined aluminum. As a standard it comes with large nipple / tygon tubing. Tom addressed this issue by proving large tubing with all his fuel system products long time back and like all Tom Cook products, his design and workmanship is impeccable.

olnico

Hello Guys,
Thanks everyone for the great tips, please keep the information coming.

olnico

nraheja,

Thanks for the feedback. That is a very interesting fact that I have not heard before. How long ago did you get your PST air trap ?

I have had 6 PST air trap so far and no problem at all.
I'll have a look at the 2 spare ones I have at the workshop to see if I can pinpoint your problem.

However I am sure about one thing: on my air traps the aluminium assembly end is so tight ( o ring seal clearance minimal ) that it needs large pliers to get removed from the bottle. No possibility of an air leak on that side.
By the way, if you remove the aluminium assembly, I strongly advise to fit it back with a good load of silicon grease on the o'ring.
Just to confirm as well, the plastic cap is not here to seal the bottle, but just to hold the aluminium assembly in place.



On this picture you can see the large black o'ring. That's the one that does the sealing job.
If the o'ring is not tight enough to get a proper sealing effect, just wrap one or two turns of Teflon plumbing tape in the ring groove...

olnico

Dude, I make them myself with my welding equipment...

olnico

You're 100% correct Jack. That is why I have stated 5 points to look at in the first thread for a proper fuel system setup ( including a 3 lines sentence about fuel restriction ).
The UAT is only one of the 5 points.
However I wanted to start this discussion to point out this specific UAT suction problem which is not very well known among jet modellers and talk about specific fuel system test routines...

Robrow

Fuel pumps are good at pushing but lousy at pulling so keep the feed line as short as possible and big bore (remember big bore is not the same as big o/d).

Rigid UAT's do not function any better than flexible, what matters is the pressure drop on the feed side and pressure drop is caused by restriction. Chances are if a rigid UAT has cured a flexible UAT problem it is because there is less restriction.

The flexible UAT's I use on Merlin/Rhino 170's do depress slightly at full power but still maintain good fuel feed without cavitation.

Hope that helps.

Rob.