Then your air trap was deficient. The aluminium core needs to be set into a vice if you want to remove it from the bottle. If it is not the case, you need to wrap one or two turns of teflon bands into the oring groove.

Once again, the plastic cap is not meant to seal the bottle.
I have forgotten it once after a fuel maintenance job on the Mystere. The flight was unevetful and I only realized my mistake because the core popped out during the subsequent high pressure refueling.

There is no possibility of air leaking through the aluminium top. All the nipples are screwed into the aluminium for a thickness of 1/3" and sealed with a fuel resistant glue.

Carsten,

If you want a good advice, try the PST air trap instead of the BVM UAT.

Yes, typical of the UAT operation on large engines.

Cavitation through the filter bag.

I explain it on the first post:
"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 a bubble generator..."

This type of cavitation can occur very badly in case of UAT collapsing ( your first test runs ) or be more subtle ( subsequent tests with the appropriate tubing ).
The fine bubbles are generated through the bag mesh due to the pressure differential and local speed velocity increase in the mesh.

Already ordered

Let me know when you get it. I will give you some hint to get it setup correctly for the P-200SX.
In the mean time you can do some engine tests with the current setup.
As long as the micro bubbles don't collate you will not flameout the P-200SX. However I would not fly it like that...

Will do Oliver, thanks!

I will do some testing on the ground, depending on this, I will see what to do next.
The PST UAT should arrive shortly, but I would like to experiment a little more with the UAT mounted, to see what impact it has on the engine running.

I will be limiting the engine probably a little anyway, it's in a Skymaster Viperjet, a P160SX would have been more than enough, so might as well dial the 200 down to a6..18 Kg

I'll be back with more results, but I'm glad you started this thread Oliver!!

Well, if it saved you from a flameout on the maiden, I can say that I didn't work for nothing...

Oli, I think it's fair to say that your posts (and articles) are among the most well thought out and helpful in the entire community.

Without contributions from you, KC, and other similarly experienced and unselfish folks, RCU would be a FAR lesser place.

So on behalf of myself and others, I want to (again) thank you all.

Right on Don!
The first articles I read in the Jet magazines are always Olivers [8D]

Anyone every tried the sack that is used in medical drips to make their own UAT? I was in hospital for my wife and i couldn't help looking at the "rig" they use off the bottom of a drip bag and it's doing exactly what a UAT does. I wonder if it's fuel resistant??

The nurse told me they just throw them away but didn't offer it to me[&o] Never mind, my sister is a nurse, I'll see what she can get for some testing. I'm sure someone would have tried this before.

Thanks

dave

Hospital IV tubing is quite small, about 4mm. It wouldn't be suitable for our large engine installs. And that's not to mention if they are compatibale with fuel.

I changed over to JMP's accumulator on my Ultra B with a P200. I also added a larger header tank with JMP's monster fittings through out as well as drilled out fuel fittings to 5/32. After running my pump at 6V for 5:30 minutes I get small "burps" throught my pump as well.

Since JMP's accumulator is impossible to see through (machined aluminum) I can't see if any cavitation is happening, but I suppose cavitation is probable in any install if the appropriate conditions could exist.

At full throttle my P200 was at 5.0V and I've yet to try to burn through my tanks at 5.0V to see if I get those "burps", but Tom Cook (JMP) has tons of experience with these big engines...

Raf

Ah, but i don't play with the big ones[&o].. yet


I was thinking about cutting up the hospital's offering to get to the sack and then installing that to a "UAT" sized bottle. I'm only running a little Wren 54 that uses 4mm plumbing anyways.

Thanks

dave

This is an outstanding thread! thanks to all...

So, if the problem seems to be cavitation of the fuel flowing through the porous bag due to too much flow impedance making the fuel "boil" from the low pressure on the suction side, perhaps what's worth a try is two UAT's in parallel each with half the flow. So long as both stay full, the only thing that's really being given up is twice as much fuel "tied up" in the UATs. At least worth a try on the test stand, I'd say...

Dave McQueeney

Now THAT is what I would call thinking outside of the box ! I'm just happy that I don't have a 200 and thus have no need to worry about boiling the trap.

Hello Guys.

A few more words about the PST air trap.

The aluminium assembly top has an o'ring that seals the bottle. The plastic cap has no sealing function.
On the following picture, you can clearly see the difference in diameter between the O'ring and the bottle opening ( red arrows ).
This is what creates the sealing by exerting a pressure on the bottle neck walls ( green arrows ).



This pressure is high enough to make the aluminium assembly hardly impossible to remove by hand.
I recommend to put the aluminium top in a vice and gently pull the bottle with some side motion to remove it.

There is no possibility of leak from the nipples. They are oversize, fastened into the 1/3" aluminium top and glued in position ( red arrow in the next picture ).





The filter assembly consists of a large falter membrane which surface is about 5 times larger than the UAT filter bags. This membrane is folded around the fuel pickup and offers a much higher flow capability than anything else I've seen so far.



The pickup is a large brass tube that is tapered onto the main nipple. Is has an additional filtering clunk at its end.



Tips to get the UAT working correctly for the 200N+ size engines

The pickup clunk shall be positioned far away from the filter end to get a proper filtering effect ( red dot in the previous picture ).
The filtering element is sometimes pushed too much onto the aluminium cap and the clunk is too close from the filter end ( red arrow below ) thus offering a limited air trap effect.



Also the filter might itself need a bit of tweaking to improve the air trapping effect. Just add tow safety wires at the extreme ends of the filter after having centered the clunk correctly ( the red dot shows the position of the clunk and the red arrows the additional safety wires )



Here is the end result: A tight wrapping of the membrane that does not show the brass clunk...



This system is to me the more appropriate for the 200N+ engines available on the market. This is because of three essential items:

1.The 250 ml bottle ( large, rigid and transparent ) that provides a good buffering affect
2. The size of the nipples and the pickup
3. The very large air membrane surface that allows a large flow through and thus less possibility of cavitation.

Hello Harley,

In reply to you post here:
http://www.rcuniverse.com/forum/fb.asp?m=9377896

Thank you for the feedback. I have added that in the beginning of this post.

The BVM UAT was a fantastic upgrade from previous header tank with a filter placed at the center of the the tank volume.
The great innovation was the buffering effect offered by the membrane bag.
The UAT has a usable volume of 3 oz ( ie the volume on the supply side of the membrane ) or 88 ml.

For a P-160SX @ 540 ml/min, this volumes equals to a theoretical buffering effect of 10 seconds ( ie if the UAT is disconnected form the supply line, the engine will shut down after 10 seconds ).
For the P-200SX @ 730 ml/min the UAT offers a buffering of 7 seconds. I found out that on Jetcat engines, a buffer effect of 6 seconds is a minimum.

The problem is that if the suction side does offer too much resistance, the UAT will start to get vacuumed because of its soft nature.
The P-160SX will deflate the tank moderately by about 30% and the buffer effect will drop to 7 seconds in the worst cases. This remains acceptable.
However, on a non perfect installation, a P-200SX could vacuum the UAT to less that 50% of its capacity. The bag remaining mostly unaffected by the vacuum effect, the usable buffer volume will shrink so much that the buffering could drop to less than 3 seconds.

In that case, the membrane itself will start to cavitate.

One solution that BVM could offer, could be to include a slightly larger glass fiber ( ie transparent enough material to clearly see the fuel level ) rigid UAT tank along with the plastic one in their UAT kit, thus offering the possibility for the customer to choose between a low flow and a high flow application. The term "Universal" of the UAT name will be reinforced.

Oliver

Have you done any experiments not using a UAT, it seems the whole problems stems from the UAT bag looking at the posts here, I have been using an Orbit membrane clunk in the main tank without a UAT on a 120SX admittedly not the same fuel demand as the 160 or 200, but I don't get any air in the lines at all, 6mm Tygon from the tank too the pump and 4mm everywhere else, again admittedly a single tank installation so slightly different application, but probably the most often used.

Mike

Hi Mike
Is the clunk hard mounted in the middle of your tank ?

Oli,

Have you any experience of the JMP large bore UAT, had one supplied for a Hawk I'm building that will run a P200?

Cheers
Marc

Marc, thinking about one of these for the T Hawk. They look stunning but the only downside is you can't see what's going on inside. Sometimes it's easy to see bubbles etc forming in the UAT giving an indication that all is not well before it's too late. I may go for the PST option mentioned above in the thread.

Oli, if it were your $20k jet, and it was being provided free of charge, which UAT option would you fit

Cheers,
Mark

Oliver

No its a clunk the same as any normal clunk on the end of a piece of fuel tubing, but its an Orbit fabric one, quite large and if my memory serves me well its felt internally.

Mike

Tom Cook has been ahead of the game since first running an Olympus NL some 7 years ago..........The BVM UAT was originally actually a Jeff Seymour(SWB Turbines) design developed right here in Wisconsin. The semi-permeable sac concept was developed for his UAV projects and was eventually scaled down to the 4 oz size that was compatible with smaller R/C jets using KJ-66 and smaller turbines.

BV has always been conservative in his turbine selections for his birds, and even suggested a P-160 as a suitable motor for the UB....no problem with the UAT....but when guys started going big blocks, especially P-200 and Oly HP in the UB, the standard UAT was incriminated in some fuel starvation flameouts which led to his AD on dual inlets on the center hopper tank.....but there still are tight spots in his recommended setup for the UB.

All goes back to tubing ID as flow is related to the 4th power of the radius, so doubling the radius allows 16X more flow at the same pressure, viscosity, and tubing length.

Tom Cook's unit for big blocks is set up for 3/16"(approximately 4.75mm) ID tubing throughout, in fact he machines a fitting for AMT fuel pumps that converts the input nipple to one that perfectly fits 3/16" Tygon. In addition, he put a perfect shoulder on his nipples so they don't leak without safety wires. I also ran 6mm tubing from my pump output nipple to just before the turbine fuel inlet, saving 4mm tubing for only the last inch, using a Festo reducer.

I did testing on my UB before I lost it to rudder flutter, powered by Oly HPES NL at full throttle and never could demonstrate any bubble in the effluent. Another nice feature of the solid case is that repeated bulging during inflation and collapse during hi-throttle settings on the PE 4 oz UAT Nalgene bottle has led to loosening of the cap and fittings over time and subsequent air leaks......I had started out with a standard UAT with the standard 6mm/4mm tubing setup, but knew that the severe UAT collapse at full throttle, with the sides almost touching was inevitably going to cause problems.......Oli did a nice job describing the results of that collapse....

I do agree it would be nice to have a glass bottle, rather than aluminum, but I have really found that unnecessary in my applications.

Reading this very informative thread and from my past experience, it seems clear to me these problems manifest from the pump, in that pumps are designed to pump as opposed to suck. This is why motor manufacturers and others fit their pumps in the tanks to get the pump as close to the origin of the fuel as possible, so.

Has anyone considered pressurising the fuel system from the engine, this could be regulated to around 3to4 psi, a very simple regulator could be manufactured to fulfil this requirement, it is something I am considering trying.

m

TC does do mild pressurizing on his Firebird with the nozzles on the leading edges of the wings, which are in fact, the fuel vents, rather than venting on the relatively low pressure bottom of the fuse as we most often see.

Photo shows a set of tank fittings my friend Paul Jacques machines on the left, which accepts 3/16" ID Tygon, similar to what TC uses vs standard BVM fittings on the right.

The precision machined gear pumps attached to relatively crappy motors that most turbine manufacturers use actually perform quite well without pressurization, as long as you don't restrict inflow.