wojtek

My Feedback: (73)

I have been having issues with my engine flaming out when pulling hard Gs ( 20 – 30 or so ) … I had narrowed this down to bubbles getting in the lines. I actually saw air in the system. I swapped out all fittings and lines, and made sure all connections were tight and safety wired every line. The problem kept occurring. The last time I flamed out pulling hard vertical, I found almost all air between the AUT, and through the pump, up and out to the engine … I think it is the UAT that under high Gs can not deliver fuel up to the nipple. There is always going to be some air in the UAT, and when pulling hard Gs, the fuel gets pulled down, as the air pushes up to the top( through the bag) , it enters the fuel tubing. This I suspect is only a problem if the UAT is placed vertically. Im hoping to solve this problem by placing the UAT in its side. This way even if there is a little air in the system, it will not be forced out to where the fuel nipple is … I think if the UAT had a pick up line running to the center of the tank, this would solve the issue completely.

Just something for you guys who like to fly hard to consider. I will be switching the orientation of my UAT around, and see if this resolves the issues ..

Anyone have any other thoughts on this ?

Voy

obilaser

I am very interested in this problem.
Not the BVM UAT but high G fuel feed issues.
I suspect problems like cavitation at the fuel shutoff valve and other fittings and it is very likely that the fuel pump might stall due to fuel inertia under high G loads.
Even if the fuel pump can sustain the high head pressure induced, the ECU probably cannot compensate fast enough to ride out the disturbance safely.
Any input regarding this issue would be appreciated

Jetkopter

My Feedback: (1)

Wojy, you flew the plane hard for a long time before having these problems. I would look at the pick up line in the hopper tank or main tanks as your source of air. If you think about it you have done some changes and reinstalls on your equipment also a possible source of a pinhole or cut line. I know you said you swapped the lines, but did you do all of them ?

Don

wojtek

My Feedback: (73)

Yea, I know a lot has changed … I had these issues originally on the P120 after reinstalling the fuel system ( some minor repair and maintenance work ) .. that ended up being traced to a festo fitting that was not as tight as it could be …

Now I have the Rhino installed, and with the brand new install I started getting flame outs , but only at very high G loads … I have had the plane at full throttle for over 2 min straight on one flight, and without pulling hard Gs, it ran fine …as I yank it hard after a pass, it dies … every time ( besides when I ran it out of fuel ) it quit, I had air coming from the UAT , out to the filter, and through the pump .. I am now 100% positive my air system is as air tight as can be, and the UAT is what it seems to narrow down to. The only air im getting through the hopper into the UAT is from things like hard snaps, inverted flight, and maybe some knife edge stuff, but that’s minimal. The UAT never has more than ¼” or less of air in it at worse, and the UAT is supposed to work bubble free even if its less than ¼ full of fuel.

I do also have around 150 flights on this UAT, so it might be getting a bit dirty ( no I have not changed the sock out yet … ) , and I wonder if this might have something to do with it not being able to draw the fuel up … the way the UAT operates, it makes sense that the fuel would be drawn away from the top as its pressed to the bottom under a heavy G load.…


I am replacing the Rhino on my Predator cat back with the p120, and the UAT will stay vertical for now .. I will see if the problem continues … the Rhino is going on a 2nd bobcat I picked up. Will mount the UAT sideways in that one .. I will soon enough find out what happens ..

Voy

mick15

why don't you bypass the uat just for one flight?

wojtek

My Feedback: (73)

you know, thats a good idea .. especially early into the flight where there wont be much if any air in the hopper ..


Voy

vinnyjet

My Feedback: (39)

woj
fly the cat with the 120
i think obilaser is right on
the g forces with the rhino
are higher and the pump
is stalling
vinny

just my 2 cents

wojtek

My Feedback: (73)

i don't think thats the issue .. i have put more Gs on the plane with the 120 than i have yet come clost to pulling with the Rhino on it .. When i pulled over 50Gs on the plane and actually failed the wing, the engine never quit that time, and i did not have issues with air in the UAT ...I htink the air right out of the uat, and out through to the pump also would exclude the pump being able to hold up as an issue ( the pump not holding up would not cause air in the lines ... )

i think some trial and error will help find the culprit ( which i think is the UAT, and its positioning) .

You coming to Maine Vinny ?

Voy

Vincent

My Feedback: (61)

Are you running the stiff clear tubing supplied by BVM from the UAT to the pump. ?? Also is there lots of air in the UAT at the end of the flight. V..

wojtek

My Feedback: (73)

all my lines are the hard type ( like the stuff that comes with the UAT) .. I do get some air in the UAT, but its minnimal and typical to what i have always seen with these ... the air in the lines is always sourced from the UAT nipple .. the rest of my lines up to the UAT have always had fuel in them.


Voy

rcjet lynch

Wojtek

Had the same issue in Indiana with my KingCat an a RTI engine. I would do a standard pattern style turn around and would puff a long smoke trail. Thought it was a pump issue but it turned out to be my UAT was dirty. This KingCat has at least 175+ flights on it changed my UAT to a brand new one and had no more problems the rest of the week I was there and still have had no more flame out issues from fuel draw.

Oh ya my line coming from the UAT to the pump was stretched even though the nut was tight it might had been leaking from there as well. My bag in my UAT had a brown coating all over it.

wojtek

My Feedback: (73)

Rob,

I have not checked, but i bet my UAT is dirty by now also ... maybe they lose efficiency in their ability to draw up fuel as dirt builds up in the bags ?? I actually have several spare pick up socks/bags, and can replace the one in my UAT if it in fact is dirty .. Glad to hear from someone who experienced similar issues ...


regards,


Voy

mr_matt

My Feedback: (10)

Voy,

One thing you might want to check is the oulet line out of the UAT (the big one with the screw on ferrule).

We have found that the tubing can get old and start to leak with even just a bit of side pressure on this line. Does not matter how tight that ferrule is. It could be that something is moving (like the UAT itself) at hi "G" and ends up putting a load on that nipple and causing it to draw in air. Just a thought.

Good luck finding it.

Regards,

wojtek

My Feedback: (73)

Thanks Matt,

another area to check .. i believe this is still old original fuel tubing i have over that fitting ...

by the way I had just pulled the UAT out, and the bag is actually not too dirty at all ,,, just slightly yellowed up, so im going to keep it in there for now.


regards,

Voy

Robrow

Could be pump cavitation, caused by the pressure differential accross the UAT and accentuated by the high +g loading. As you know higher power engines need higher capacity pumps which need to receive an 'easy' high volume fuel flow.

Assuming all your lines and connections are airtight even under high g then probably the UAT orientation is contributing. You could try inverting it (if possible) after filling your tanks, that way during flight the pump will receive a positive head of fuel which will only increase with more +g

Rob.

wojtek

My Feedback: (73)

you bring up a good point , which i why i am also considering placing the UAT higher, and above the elevation of the pump in the plane. I am also thinking of mounting the pump on the floor of the fuselage, so any Gs on the fuel load will exert positive fuel pressure into the pump ..

glad i started this thread, because a lot of good points are coming up .


regards,


Voy

wojtek

My Feedback: (73)

i figured i'd update this thread .... on my new cat, i installed the UAT sideways so the top of the tank is always submersed when the plane is upright... i yanked it around pretty hard where at other times the engine would flame out .. no more problems ... I think the UAT ends up passing air through the sock under load as the fuel is pulled to the bottom ( the surface tension of the bag is no longer enough )


later ...


Voy

joeflyer

My Feedback: (48)

Do you have an in line filter after the pump? They can trap quite a bit of air. It seemed that every time I opened up my fuel system to do any maintenance I would get a flame out on the next flight.

Finally I mounted the filter vertical with fuel flowing bottom to top. Even then I can get quite a lot of air bubbles traped in it. Prime the line with fuel past the filter then flick the filter a number of times with your finger until you can't get any more air bubbles out of it. Then after starting flick it a few more times to be sure. Once the air is out of the filter it's no problem. I had been blaming my UAT but it was my in line filter that was the culprit. It's the one that came with my Artes engine.

Joe

wojtek

My Feedback: (73)

all my filters are vertical .. the new UAT position is what it took to get rid of flameouts ... It just can't suply fuel upwards through the sock under high Gs ..


Voy

Vincent

My Feedback: (61)

Voy, that does make sense to me, i guess no one else pulls quite as many G`s as you do. I was going to mention that Tom Cook sells a all metal type UAT that may be better suited for your type of hard flying.

V..

wojtek

My Feedback: (73)

yea, i know, its the JMP acumulator .. I have one that came with my JMP Firebird. talk about a serious piece for the fuel system !!

Voy

CARBUNCLE

JMP acumulator = no problems forever [sm=thumbup.gif]