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Thread: Fuel draining at uneven rates

  1. #21
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    Default Re: Fuel draining at uneven rates

    Like John, I appreciate this discussion so thanks to you all for your contributions. If I understand it correctly, there would appear to be no downside to making the feed hoses from each tank the same - even if the difference in lengths is a minimal contributor to uneven flow. The other challenge appears to be getting the cap vent angles correct on both sides so they are receiving even pressure from their angle of meeting the airflow. I imagine that the offsets would be minimal so will experiment a little with that.

    Again thanks guys.

    ross
    Ross
    Mt Beauty, Vic
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    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
    Landcruiser and Cub off road camper (doesn't get any kudos on this forum!)

  2. #22
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    Default Re: Fuel draining at uneven rates

    All things being equal, atmospheric pressure and gravity work on this simple fuel system. Just fly straight and level. Fuel seeking it's own level is going to have more effect than ram air in those small vent tubes in my humble and non scientific opinion. The sky is falling all over Washington state today so I'm doing this instead of flying.

  3. #23
    Senior Member jiott's Avatar
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    Default Re: Fuel draining at uneven rates

    It will be interesting if Ross experiments with vent tube angles. I have never heard of anyone doing this before. One idea I had was to put a short length of some soft bendable tubing like copper over the end of the cap vent steel tubes (tight fit with good seal) to be able to try many different angles without work hardening the steel tubes. I might try this myself one of these days, just to try to verify the cause, not because of any worries.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #24
    Senior Member jiott's Avatar
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    Default Re: Fuel draining at uneven rates

    Yes John, I tend to agree that the header tank vent tube has nothing to do with this issue. It is simply a path for any air bubbles trapped in the header tank to work their way up out of the system without having to try to go upwards against the flow of fuel in the main hoses. This vent tube is nearly always unported unless you have the tanks topped off full. The tube remains full to the level of fuel in the right tank, but there is zero flow rate in it when it is unported, thus allowing air bubbles to rise.

    This fuel system is actually quite sophisticated IMHO, it has been developed and tweaked over the years to what it is today as I understand it. At times I wish for dual shutoff valves for maintenance purposes only; like recently when doing my annual I decided to replace a piece of vent tubing near the header tank. I had to go thru the process of completely draining both wing tanks before I could open up a fitting at the header tank-how nice it would be to only have to shut of a couple of valves. I know I could have clamped off the two hoses, but I have a personal aversion to tightly clamping these critical hoses-just me. But this kind of maintenance rarely occurs and I rather keep the system simple-just me.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  5. #25
    Senior Member PapuaPilot's Avatar
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    Default Re: Fuel draining at uneven rates

    Uneven fuel burn is a very common problem on planes that can burn off 2 tanks. Once you figure out which tank is going down quicker you can fly with the ball slightly out to that side. Often 1/4 ball is enough to even the burn or even transfer fuel to the other side. It is much easier to do on planes that have rudder trim so you don't have to hold the rudder.

    This worked with the Caravans and Kodiaks that I flew.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  6. #26
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    Default Re: Fuel draining at uneven rates

    Argh, it's not complicated.

    It's hydraulics, and the vent has much to do with it, and so does the
    different length tubing to the CP side offset header.

    The biggest mistake most people are making, is thinking of this system as
    a liquid level system which it is not (in flight). It's an alive hydraulic system
    with fluid flow.

    The only way to balance it is - One install vent to both tanks, AND/OR
    increase the size of the fuel feed tubing on the longer length feed line
    This typically the pilot, side from tank to header. OR lengthen the CP
    side feed to match the flow rate of the pilot side.

    Super simple explanation - Fuel will take the path of least resistance -
    The shorter feed from the CP side, with the double lines (feed and vent).
    this will cause the CP side tank to drain faster. At some point the head
    pressure (the weight of the fuel in the fuller tank) will balance against
    this. Which is why when you approach zero, both tanks empty at the same time.

    However when you sit on the ground, there is no fuel flow, so what
    happens? The fuel cross feeds, and levels itself.

    Jeff

  7. #27
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    Default Re: Fuel draining at uneven rates

    Thanks for all the posts guys - I have learned a lot - yes, really Jeff!

    cheers

    r
    Ross
    Mt Beauty, Vic
    OZ
    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
    Landcruiser and Cub off road camper (doesn't get any kudos on this forum!)

  8. #28
    Senior Member jrevens's Avatar
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    Default Re: Fuel draining at uneven rates

    Quote Originally Posted by n85ae View Post
    Argh, it's not complicated.

    It's hydraulics, and the vent has much to do with it, and so does the
    different length tubing to the CP side offset header...

    ...Super simple explanation - Fuel will take the path of least resistance -
    The shorter feed from the CP side, with the double lines (feed and vent).
    this will cause the CP side tank to drain faster...
    .
    Jeff
    Jeff,

    Not to beat a dead horse, but with all due respect you are not correct. I'm know that Jim (jiott) can defend himself, but I would just like to repeat a couple of points...

    First of all, Jim (jiott) explained that the different lengths of the feed lines on the left & right tanks and the ID size and flow rates that we are talking about, are a non-issue for all practical purposes - no measurable frictional difference. I believe that Jim is a retired engineer who dealt with hydraulic design for a living.

    Secondly, Jim & I both explained that the vent line does not and can not feed any fuel to the header tank when the level in the wing tank is below the level where the vent line connects to the wing tank (which is up at the very top of the tank of course). The feed and vent lines are not a set of "double lines" that are both feeding the header tank.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  9. #29
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    Default Re: Fuel draining at uneven rates

    Wow! This has turned into quite an discussion.
    Put me down as being in the "fly straight and level" camp. This seems to pretty much solve the problem (if there ever was one) in my Vixen.
    Cheers Don

  10. #30
    Senior Member PapuaPilot's Avatar
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    Default Re: Fuel draining at uneven rates

    It really is that simple. Fly straight and level, if that doesn't give an even burn then kick the ball out just a little to one side to compensate.

    Also, make sure your ball is perfectly centered when the plane is on a level surface.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

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