Folks,
Need some help please. At the end of a 9 minute flight my BVM UAT is half empty (or half full!) I'm going to guess this is caused by air leaking into the main tank pickup line? Open to any and all suggestions as to the cause of this. Help!

Dave Rigotti

Dave, sometimes the fill line over time needs to be chopped where you plug it. If it is sucking air, that is your problem.

The plug does not fit perfect forever.

Other than that, check the cap to the uat make sure it is tight. Also are all your lines safety wire??

That's about it, other than a pinhole in fuel line???

Good Luck

Sean

Sean,
All good ideas. Yes the lines are safety wired. I'll check the UAT cap and fill line. The little alum plug "seems" to fit well. I'll report back this evening.

Dave Rigotti

dave

any connection on the entire fuel system has to be suspect including stoppers..........saftey wire all of them ( twice around )...it could even be a clunk that is too long and when doing manuevers it could get caught momentarily out of the fule and into the air in the tank thus draining your uat...sometimes a little here and little there ( ie...you got away with it this time but upon extented manuevers....it would show up ) .... i have even seen clunk lines get old and " trained" stiff and hang if they are too long.i have even seen people use festo check values for filling fuel...works great for 20 lb turbines as they dont pull enough vaccumm....then they use them on a 30 lb turbine and the vacumm pulls the festo open and pewwwwwwwwwww engine flame out

we always use the uat as a fuel system intregrity check....where as when setting max rpm we always look at it and expect it to be full to the top....if not ....its a no go

hornetdriver

Hi Dave,

If at anytime during the flight, your main tanks pickup(s) is being exposed to air, then that air is going into the UAT. This can be particularly troublesome in a twin main tank install on a Y connector going to the header or the UAT.

If the main tank clunk is not being exposed, then there is a leak under suction somewhere. Sometimes this only leaking only occurs at high power. You can usually see the leaks occuring at some kind of junction (timy bubbles or foam going upstream, either at the tank fitting or at a T fitting or something.

Good luck

Matt

These are all good points to watch out for. I had a UAT cap that would not seal properly under full power and was sucking in a small amount of air. Found two problems - the feed line to the fuel pump was too small (upgraded to medium Tygon, be sure to wire wrap or wire tie these connections!) as the UAT was collapsing, and the cap just would not seal tightly. I used a couple wraps of teflon tape around the neck and it sealed perfectly afterwards.

Barry

>>>>I used a couple wraps of teflon tape around the neck and it sealed perfectly afterwards.

as per barry.....ALWAYS use teflon tape.....IT MUST be 2 to 3 wraps with the uat facing you going clockwise...NEVER counter clockwise

hornetdriver

I always use a on/off festo valve on the fill line, this is a good way to keep air from sucking into the uat. Joe.C

Are there any festo fittings or filters between the UAT and the main tank?

When it is running on the ground, can you see any airbubbles traveling in any of the fuel lines?

Any kinks or bends in the fuel lines anywhere?

Is the main tank vent clear??

Half of a UAT is a lot of air... If it were me, and none of these suggestions could not find the leak visually, I would be pulling the entire fuel system out and giving it a very close exam, and likely as not, I'd change all the lines....gives a second chance to make the system perfect and leakproof.

Entire fuel system is out of the aircraft. I'll be inspecting all components this evening or next. No filter or festo is used between the tank and UAT. I'll test the system before I replace the tubing and lines. It is either in the tank pickup tubing/line or the UAT itself is leaking air from a fitting or the fill plug. "I gotta know!" ALL great suggestions! Thank You all for your input. I'll post my findings.

Dave Rigotti

Pardon my ignorance, but what is a UAT? [sm=confused.gif]

no problem, we sometimes forget that not everyone understands the abreviations....

UAT stands for Ultimate Air Trap..it is a special hopper tank marketed by Bob Violett to minimize bubbles in the fuel going to the turbine. A tiny bit of air in the fuel is enough to put the fire out....which is a bad thing!

uat, universal air trap, this unit is designed to keep air from reaching turbine.


i also had a leak causing the same problem, i changed all the fuel lines, tied them all off, and the fill plug, i use a zip tie on it after every fuel fill

mark

Dave:

I had the same condition a while back with the engine shutting down casue the UAT was empty and interestingly the Main tank feeding the UAT half full !! Was quite puzzling.

What it was is the UAT itself was sucking air AROUND THE CAP because it was one of the early ones that did NOT have the Teflon tape on the threads !!!! This was pointed out to me during the investigation by none other than SWB themselves who developed this technology and make these things.

SO, it may be several other things as previously mentioned but it is worth spinning the cap off the UAT and replace the Teflon tape with a generous amount if you have any to start with.

BR

I have noticed that the cap can back off a little with repeated fueling and defueling. It's best to check it every once in a while.

zip ties>>>> thay can cause a "pinch" in the line at the "gripper" thus i saftey wire ( 2 wraps ) and then 1 zip tie ...personally the saftey wire is enough especially on ferralled items like the uat has... but i sometimes dont use a brass sodered ferral on the outside of my stoppers ( but always on the inside where you cant see ) because its another step in stopper service and can cause stopper damage while sodering )

also it has been my experience that some fuel tubing can swell over time and get bigger where as others dont....

saftey wire should pinch the tubing but not cut into it...so there is a common sense window of tightening of 2 or 3 twists

i also have stuck my tanks and all into my swimming pool to check .....using a bycycle pump ( 1 or 2 strokes only....no electric pumps ) where as i use low impact tape aronud my tank....1 or 2 pushes and the tank swells and splits the tape and thus its presurized ( most of my tanks are 100 oz ) ...then i dunk it...no bubbles....no problems


use some sense here and dont blow them up....but i have found a weak tank once that was nib and flawed at the neck...just waitng to cause a problem.....it just happen to be in a series system and would of blown upon fueling from tank to tank

now before anyone goes off on me about presurizing my tanks....think about it....in a series system the fuel has to go from tank to tank and the vent tube inside the tanks determines the flow from tank to tank...thus tanks do get pressurized to some degree.....thus i dont use a 12 volt fuel can filling system as it delivers too much fuel too fast and can and will split tanks that are series linked if the j tubes causesd too much restriction

hornetdriver

nony, mark, all - thanks guys! I'm not a jet guy yet, but watching (and learning) with LOTS of interest... maybe one of these days. [8D]

I had one that had a small pin hole in the UAT up at the top, put it in some water and shoot some air in it and see if you have a air leak.



Thanks
Mike Grant

Dave
How is it mounted vertical 90 degrees /45?
Semper fi
Joe

I saw Dave posted in the Reaction 54 thread that he found the problem.

http://www.rcuniverse.com/forum/fb.asp?m=3047859

Yup...problem solved completely. Rear facing clunks are the only way to go. At least this is my experience. "Your mileage may vary"

Dave Rigotti