My aeroplane has developed an embolism

[Old Koreelah]

Despite my best efforts to design my fuel system so that it could not trap air bubbles, there just isn't the space to set up a constantly-rising fuel line like I had hoped. Yesterday I found the filter half full of air and it can't get out because the line travels downhill from there to the Mizer sensor.

 

Needless to say I removed it, but it can happen any time a tank runs out of fuel. Despite the air bubble, fuel seems to happily travel thru the system, but perhaps there will come a situation, such as during severe turbulence where the bubble starves the engine of fuel. Has anyone else had experience of this?

 

 

[fly_tornado]

how does the bubble starve the engine of fuel?

 

 

[Guest Maj Millard]

Fly tornado, They don't seem to like running on air for too long !

 

Old Koreelah, Don't know if your situation warrant changes or not. Generally operation of a boost pump should purge the system of any air pockets. And, assuming you are running a standard bowl type carb it should also continue to supply fuel, even if air is pushed into the bowl momentarily. If it's continous air it's going to eventially stop your engine. I would suggest a redesign or realignment of your fuel lines to eliminate the accumulation of air, and additionally you may want to carry out a fuel-flow check by disconnecting your fuel-supply line and checking that you have a good strong continous flow of fuel, with or without the boost pump on...................................Maj...

 

 

[Old Koreelah]

Thanks for the reply, Maj. I've done lots of flow tests and all seems reliable, but the possibility of an air "embolism" blocking flow in some situation is worthy of thought.

 

Because the mizer turbine shaft needs to be mounted vertically means there are more bends in fuel lines than I'd like.

 

Ideally, the boost pump should be upstream of everything, but I don't want it in series with the engine pump.

 

First I have the filter, then the miser turbine, then the fuel branches into parallel lines- one straight to the engine pump, the other to the boost pump. This means the boost pump is downstream of the bubble in the filter and can't force the air through.

 

Can you see the dilemma? Any suggestions?

 

 

[Guest Maj Millard]

Ok, did I read somewhere that the fuel-mizer turbine wheel can have a tendency to airate the fuel in some situations ? I had one fitted in the lightwing when I bought it and I removed it. Looking through the turbine just did not install confidence in me as far as reliable fuel flow goes. I also seem to recall reading the mizer instructions saying to not put the turbine wheel after the filter, but before it ??? (it's been a while!) If your boost pump is in parallel, do you have a check/one way valve backstream of it /...so that it can't force pressure backwards into the fuel lines. A recent post discussed all this in detail. Keeps things simple, there is no need to reinvent the wheel with fuel systems, they need to be super- reliable in all sceneros.....................hope this helps ........................................Maj...

 

 

[Old Koreelah]

"Keeps things simple, there is no need to reinvent the wheel with fuel systems, they need to be super- reliable in all sceneros.....................hope this helps "

 

Maj keeping things simple is what I have aimed to do.

 

I did try non-return valves, to stop fuel being forced back thru one pump by the other. To open the NR valve required too much pressure for my liking, so I ditched the idea and went for "simple"- each pump seems to have inbuilt NR valves anyway.

 

The Mizer instructions call for a filter upstream to ensure no grit can foul the turbine.

 

It seems the only way to totally prevent the build up of an air bubble is to have the pump upstream of every thing- even the filter- that means the two pumps are in series. How do other people set up theirs?

 

 

[fly_tornado]

My Tornadoes have an electric fuel pump low in the airframe near the bottom of the fuel tank and a vacuum fuel pump set @ carb level. I can see that air in the fuel lines.

 

 

[facthunter]

If you can arrange a small bleed from the high point in the fuel run and vent it back to the fuel tank, you will get rid of the air. Nev

 

 

[Guest rocketdriver]

Hi OK

 

I am, perforce, completely redesigning my fuel sytem with parallel electric pumps. I am struggling but will succeed in putting the gascolator at the lowest point and generally arrange for rising or level lines from there, with no "hill crests"anywhere (thats what traps the air). Then I will be putting a fuel return bleed (which came with the new pumps) back to the tank.

 

BTW, I've also modifyied / arranged the water circuit for the radiator in the same way ....

 

One other "solution" is to put a tee in the line at the high point leading up to a small "airpot" reservoir which should have a drain/bleeder on it to occasionally get rid of accumulated embolisms ....

 

But, Like the MAJ says, keep it simple is best in my view and a passive system that works because it is designed and installed right beats anything that requires periodic fiddling ..... IMHO

 

Cheers and good luck!

 

RD

 

 

[Old Koreelah]

Thanks, people for the replies.

 

I particulaly liked planedriver's One other "solution" is to put a tee in the line at the high point leading up to a small "airpot" reservoir which should have a drain/bleeder on it to occasionally get rid of accumulated embolisms ....

 

I plan to regularly inspect for air bubbles, and avoid running dry. To keep it simple I may install a simple bleed point at the top of the offending loop.

 

 

[Guest rocketdriver]

I plan to regularly inspect for air bubbles, and avoid running dry. To keep it simple I may install a simple bleed point at the top of the offending loop.

Running it back into the tank with a restrictor to minimise liquid flow whilst allowing vapour through?

 

 

[Lloyd Kay]

Despite my best efforts to design my fuel system so that it could not trap air bubbles, there just isn't the space to set up a constantly-rising fuel line like I had hoped. Yesterday I found the filter half full of air and it can't get out because the line travels downhill from there to the Mizer sensor.Needless to say I removed it, but it can happen any time a tank runs out of fuel. Despite the air bubble, fuel seems to happily travel thru the system, but perhaps there will come a situation, such as during severe turbulence where the bubble starves the engine of fuel. Has anyone else had experience of this?

Simply, put your boost pump near the outlet from the fuel tank. If too many air bubbles get into any high spot in the fuel line, they will simply be pushed through into the carby float chamber and be vented overboard through the vent line to rejoin all their happy little air molecule cousins floatin freely, and will leave you to enjoy your flying. Filter can be mounted anywhere, any air bubbles: read above again. Cheers.

 

 

[robinsm]

If the airbubbles are only small, they will settle out in the bowl of the carby, no problems. ISharp bends in the lines, restrictyions etc will generate small bubbles but these aren;t a problem (at least I have found this). The large bubble (like an inch or so of line, would concern me greatly, but small bubbles, if not caused by holes or leaks in the fuel system are fairly normal for pumped fuel.

 

Just my 0.2c worth (do not take this as gospel, check before actioning.)

 

Cheers

 

Maynard

 

 

[Old Koreelah]

What happens if you accidentally allow a tank to run dry? To find out, a few months ago I deliberately did this within gliding distance of home base.

 

I waited for the coughing to become pronounced before switching over to the other wing tank. The engine died and I was surprised how long it took for fuel to reach the carby to allow a restart. A useful lesson.

 

That's how the "bubble" got into my fuel system. It was enormous and occupied the top half of the fuel filter (which is mounted horizontally). It caused no problems for weeks before I noticed and bled it out, but had I been performing wild manoeuvres it may have travelled further along the system.

 

Presumably air can only enter from an empty tank, so avoiding that will be my first defence.

 

Mounting the boost pump upstream of all other components would push air bubbles through, but I can't see how I can then have a parallel system (in case the engine pump fails or develops a leak).

 

I'd like to keep my system simple, but will look at some other installations at Temora.

 

 

[facthunter]

Air can enter from any small hole (loose clamp, crack, perforation) in the fuel system on the "suck" side of the pump. Nev

 

 

[Guest Maj Millard]

Bit off subject, but fuel related. One of my previous work buddies in the States had worked on the Saturn 5 rockets when they were built in Houston. This was the biggest rocket made in the US, and the one used to take the Apollo astronauts to the moon.

 

He told me the 1st stage fuel lines were big enough for a small man to crawl through !!..................................

 

He regreted that he never got to see one launch, as they were too busy building them for the moon shots, and couldn't spare the time to go to Cape Canaveral (as it was then called)...................................................................Maj...

 

 

[Guest rocketdriver]

Bit off subject, but fuel related. One of my previous work buddies in the States had worked on the Saturn 5 rockets when they were built in Houston. This was the biggest rocket made in the US, and the one used to take the Apollo astronauts to the moon.He told me the 1st stage fuel lines were big enough for a small man to crawl through !! .

now. lets see, 1 small man, 700mm dia?

 

my fuel lines, 5mm internal dia approx .......

 

So, To get to green cheese land, I need (700/5 squared) = 19,600 suby motors in parallel ...

 

Thats 19,600 fuel puimps at $65 ea and a lot of radiators for the waste heat! !!!!

 

No wonder they used rockets!!!

 

Actually its an awesome thought isn't it ..... 700 mm or thereabouts just to pump the fuel ....

 

 

[Guest Maj Millard]

Don't know how much fuel the 1st stage burned at lift off, but it had to be a lot !. Those saturn 5s always just seemed to sit on the pad shooting flame, and would then just slowly lift off and accelerate.....................................Maj...