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Pros and Cons of independent fuel feed and vent

Old 04-11-2018, 12:49 PM
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Dieselman1220
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Default Pros and Cons of independent fuel feed and vent

Im re doing the fuel system on a jet that has two separate tanks. I previously flew it using a BVM UAT with a single Vent and the fuel
feed lines to the UAT teed into a single line. I've done it this way on several jets with both CATS and BVM UATs with no issue.
I decided to try out the Intairco air trap with dual fuel tank inputs. What would the pros and cons of running separate fuel lines into
the air trap be? Would it be better just to block of the second fuel port? Im a fan of "KISS and If it aint broke, don't fix it"
I can't really think of any problems doing it this way as long as I ensure the fuel lines and vent lines are the exact same length on both tanks. Anything else?
Old 04-11-2018, 01:29 PM
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ravill
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Sounds reasonable to me.
Old 04-11-2018, 01:36 PM
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CraigG
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I think the two input line UAT's are part of the trend, along with hi-flow tubbing/fittings (i.e. 3/16"/8mm) to minimize fuel restrictions with the larger bore turbines. If your previous teed/single line has been working, no real reason to change IMHO.
Old 04-11-2018, 02:17 PM
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being able to suck fuel through two vs one will lower pump voltages as long as your vent doesn't restrict it any. I would think running twin lines without increasing the vent side wouldn't help you any but I have never ran dual in ran dual with my turbines.
Old 04-11-2018, 02:29 PM
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Dave Wilshere
 
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Are the two tanks the same capacity?
If so, you can run the two clunk line feeds into the two bubble trap supplies, but you must T the two overflow fittings on the tanks to a single overflow outlet.

Dave
Old 04-11-2018, 02:56 PM
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Dieselman1220
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Originally Posted by Dave Wilshere View Post
Are the two tanks the same capacity?
If so, you can run the two clunk line feeds into the two bubble trap supplies, but you must T the two overflow fittings on the tanks to a single overflow outlet.

Dave
Yes Dave both tanks are the exact same capacity.
So by overflow you mean the vent right?
You are saying to T the overflow (vent) just curious what the advantage would be to this?
Old 04-11-2018, 03:06 PM
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JackD
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Originally Posted by Dieselman1220 View Post
Yes Dave both tanks are the exact same capacity.
So by overflow you mean the vent right?
You are saying to T the overflow (vent) just curious what the advantage would be to this?
The advantage of not T'ing the vents is to reduce the restriction. But it has a big disadvantage: if for any reason one of the two is overflowing on the ground, it will siphon the other tank empty without you noticing. So if you dont T them, always ensure that you defuel a little bit to avoid this from happening.
Old 04-11-2018, 05:15 PM
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gunradd
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Originally Posted by JackD View Post
The advantage of not T'ing the vents is to reduce the restriction. But it has a big disadvantage: if for any reason one of the two is overflowing on the ground, it will siphon the other tank empty without you noticing. So if you dont T them, always ensure that you defuel a little bit to avoid this from happening.

well said Jack. That exact thing happened to me before. Fueled up and was ready to fly. Someone needed help and before I made it back the sun shining on the fuel tank caused it to expand and overflow.... It drained one tank quite a bit. I did not notice and had to pull off a nice dead stick.

On any bigger turbine I always run separate vents and don't use any Ts in the entire system. If it has a saddle tanks and a single main tank I will hysol 2 vent fittings in the top and run each saddle into the main tank. Lower pump PW = happy engine.
Old 04-12-2018, 04:43 AM
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Like Keith said....independent inlets at the CAT/UAT results in a cleaner installation, less back pressure on the system. Is it necessary? No, but nice.

On the vent side, a T is perfectly fine...just use large diameter tubing. Individual vents are ok too, but if there is a minimum restriction (delta P) to either vent the tanks will fill disproportionately, which is not an issue as long as you don't mind one tank overflowing more than the other, or if you have clear visibility of the tank fuel level.

BTW, if someone develops a bypass valve you could pressure feed the fuel system via venturi effect of the vent line. This would motivate fuel flow "some" and improve system performance (pretty useful for multiple tank installation).

DavidR
Old 04-12-2018, 06:32 AM
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While we're talking about this: what do you all think about adding a (very small diameter) pipe connecting the bottom of 2 saddle tanks feeding the uat in parallel?
The little pipe would slowly balance the fuel level in the 2 tanks. Having a small diameter would ensure no single tank becomes full /overflows while the other one is empty during aerobatics.
I've had a fuel system with 3 tanks all in series with little issues, but it did cause some trim change in flight, using them in parallel would improve on that.
Old 04-12-2018, 07:54 AM
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Dieselman1220
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Thanks for the info guys.. I will run separate feed lines and T off the Vent. All my lines will be 6mm festo, not huge line I know but its only a 170 size engine.
Old 04-13-2018, 12:45 PM
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Kelly Rohrbach
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So on my PNP F-16 1/6th scale I have plumbed all four tanks for fuel. I have the front tanks feeding the aft main tanks which are on the CG. I duel vented the system with both left and right sides, T into the UAT. So far on pump, fuel in fuel out and test running the engine it seems to be drawing down almost perfect, within about 3 seconds of each. Like mentioned I made sure that EVERYTHING is the same on both sides including very accurate measurements of ALL fuel lines.
Of course I have a AMT pump, that thing could draw water from a 300ft Well.
Old 04-13-2018, 01:10 PM
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rhklenke
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Originally Posted by Kelly Rohrbach View Post
So on my PNP F-16 1/6th scale I have plumbed all four tanks for fuel. I have the front tanks feeding the aft main tanks which are on the CG. I duel vented the system with both left and right sides, T into the UAT. So far on pump, fuel in fuel out and test running the engine it seems to be drawing down almost perfect, within about 3 seconds of each. Like mentioned I made sure that EVERYTHING is the same on both sides including very accurate measurements of ALL fuel lines.
Of course I have a AMT pump, that thing could draw water from a 300ft Well.
Man, ain't that the truth! You won't squeeze it under the front engine cover anytime soon, but they sure do the job!

Bob
Old 04-13-2018, 04:56 PM
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HAHA I forgot to remove the vent off a jet one time that had a mercury on it. I did an entire flight and when I pulled the hatch off I saw the sides of my UAT we almost touch each other.
The engine never indicated any signs of a problem,.

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