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Parallel main fuel tanks or serial?

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Parallel main fuel tanks or serial?

Old 04-02-2024, 03:20 AM
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Default Parallel main fuel tanks or serial?

I have a jet with two 2 litre tanks. They are connected in parallel but they donīt neither fill or empty equally. The tubing is of equal length. The pick up clunks are equal and they are positioned equally on each side of the turbine. I have been over them a number of times and I canīt figure out why they donīt work in parallel
Now Iīm thinking about connecting them in series instead. Has anyone here tried somthing similar to that? What does the community think about serial fuel tanks?

Old 04-02-2024, 04:33 AM
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Have you tried running the turbine and noting if they drain equally? Pumping fuel in and out of the airplane is done at a much higher flow rate than the turbine draws. Any frictional losses thru the fuel lines is amplified with increased flow velocity. Your system may balance at the lower flow rate.
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Old 04-02-2024, 06:22 AM
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Hello Jannica.

This is a very common problem. You can totally connect them in series. The only issue with that approach is that the tank closest to the turbine (1st tank) might be under slightly more pressure than the last tank. Not a big problem if you are using quality tanks, large tubing and vent. Also make sure that each line/pickup/vent has a larger or equal diameter compared to the previous tank. Obviously this assumes the tanks are located centrally and the weight difference wont be a factor.

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Old 04-02-2024, 06:52 AM
Len Todd
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Series is O.K. But large tubing and large vent is a must. Make sure there are no other fuel system restrictions (e.g. brass tube in tank stoppers, Clunk ID, connectors, etc.)
Once you have the system installed, run the turbine as WOT and see if it still spools up to expected RPMs. If it does not spool up to WOT properly, there probably is a restriction someplace.
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Old 04-02-2024, 08:25 AM
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I have set up multiple jets of mine that run a 3 tank system, one main and 2 side/saddle tanks, F-16s, F-15s, f-18s, comp flashes, comp lightnings, I always plumb the 2 tanks in parallel. I have had the issue of uneven filling and emptying a couple times,
will share what I learned, as you stated the EVERY line from the T-fitting to the clunk needs to be exactly the same length. Also make ABSOLUTELY sure the tanks are sealed and hold pressure, I plumb them then block off the vent put a silicone hose on the clunk fitting, use my mouth to blow the tank up as much as possible (read somewhere humans can put out about 10psi with our mouth) then I submerge my tank in a sink full of water and look for any air bubbles. If all good I install in the jet.
When it comes to the vent system you will read many opinions, some say run a separate vent for each tank of the same length and size, Myself every system I have ever done with dual vents I had to be very careful as I had multiple times of fuel syphoning from the vent and could empty a tank. I do not and will not ever run separate vents again, and I have 3 different planes with a 300 engine in them, I always run a single vent, a high flow system is a must, even on my 200 size jets I run an 8mm vent, meaning the vent line from tank to the T is 8mm and exact length on both tanks to the vent, then a 8mm from the T to the vent, I also run the High flow overflow/vent system from dream works, that fitting is very large and will not restrict the venting. This is my experience and has worked every time for me over the years,,
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Old 04-02-2024, 10:28 AM
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Thank you guys for all your input. You are really very helpful. Now I have got a couple of new things to test and think about. You are great!.

Old 04-02-2024, 12:19 PM
Dave Wilshere
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Even the radius on the fuel lines between the tanks will vary the flow rate.
everything has to be equal, you should also use a twin feed bubble trap…though here is an example of how not to rig one of those!
wrong mounting angle, position and tube runs

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Old 04-02-2024, 02:54 PM
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After doing your best to make sure everything is equal they may not flow exactly the same. Shorten the tube from the tank that fills and drains the slowest in small increments, maybe a cm. at a time, until they drain about the same. You'll never get them exactly the same. If I get them within an ounce of each other that's good.
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Old 04-03-2024, 04:41 AM
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Dont waste the brain power trying to explain why it wont feed...go with series and forget about it.
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Old 04-03-2024, 06:52 AM
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Originally Posted by Vincent
Dont waste the brain power trying to explain why it wont feed...go with series and forget about it.
It’s called path of least resistance and drag.
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