Jim's problem description is the perfect example of the problems of fuel system design - the flow just doesn't necessarily perform as we would expect. Bill Freeman (a ME who works for Martin Marietta and no longer contributes to ACRE - another Lamar victim) expounded on this problem extensively with the high wing Cessnas and LongEZ's while noting that even some of them still have venting and flow problems. If you can't EXACTLY copy an existing system then you have to really test for uneven fuel flow and vapor lock. I feel so little confidence in deviating from a known design that I'm going to copy Tracy's system EXACTLY for my RV-4. Even here you have to be careful because if the system isn't EXACTLY the same you are again testing an unknown system.

Ken Powell
Bryant, Arkansas
501-847-4721

    -------------- Original message --------------
    Paul,
    Paul wrote:

    Hi, Jim....is it possible there is a restriction in one of the
    fuel lines? Are there any bends in the aluminum fuel line that
    might have kinked during the bending process?

    This problem drove me NUTS for two years. I tried everything. For
    example, I checked the vents as best I could, leveled the airplane
    (checked the ball to ensure it was centered - it was),
    disconnected the fuel line from the engine side of the firewall
    and let it drain into a bucket. I turned off one strake at a time
    (with manual maintenance valve) and timed how long it took for the
    other strake to gravity drain ten gal of fuel into the sump, out
    of the sump through the boost pump, through the filter and then
    through the firewall into the bucket(s). Then I turned it off and
    did the same thing with the left tank. They both drained 10 gal
    each within 5% of the same time. Then I reconnected all the
    plumbing, taped over the fuel caps so there's no possible way they
    could leak and went flying. Right tank drained 20 gal while the
    left tank maybe 2 or 3. Level balanced flight on a cross country.
    All fuel lines, vent lines, etc. are as identical as I am able to
    make them. To this day, I've not met *anybody* who can explain
    what happened. I tried *everything*. Back and forth on the
    Velocity list much longer than you've been inquiring after your
    vapor lock. NOTHING. I figured out a workaround - I installed
    electric shutoff valves in place of the manual maintenance valves
    between the strake and the sump. Now, when one tank doesn't
    transfer, I turn off the tank that does and let the "reluctant"
    tank catch up. The pump sucks it through the system just fine (the
    head pressure of 6" of fuel is about 0.16 psi - a problem that
    would inhibit that flow you can't even MEASURE. Assymmetric
    transfer is common as dirt in the Velocity community (but not
    nearly as persistent as mine). It was also a problem a long time
    with the Vari-EZ.

    Lastly, have you tried blowing backwards into the two fuel lines
    to see if either had more restriction than the other? Just
    looking for possible reasons for your uneven fuel flow. Even my
    Cessna 172 and 150 had fairly even fuel consumption from the two
    gravity fed tanks.

    I understand that Cessnas had a lot of trouble around this too.

    Just searching for answers. Paul Conner

    ----- Original Message ----- From: "Jim Sower"
    <canarder@frontiernet.net>
    To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
    Sent: Thursday, February 24, 2005 5:57 PM
    Subject: [FlyRotary] Re: Vapor Lock

    In my Velocity the strakes gravity feed to the sump. About a
    foot of 3/8" Al / rubber fuel line on each side, downhill all
    the way. I can run one tank plumb dry and have only a gallon or
    two gone out of the other. Like when there's a slight leak in a
    fuel cap or something.
    If they're significantly uneven, you WILL get air if you select
    "Both" ... Jim S.

    Bulent Aliev wrote:

    If you're almost home and have "Both" selected, and one
    tank runs dry, what do you end up with in the fuel line? I
    believe mostly air. Late in the trip (traffic pattern?)
    where trouble shooting time is at a premium. Use L or R.
    Not B.

    I don’t think so: connect two tanks (one full with water and
    one empty) to gravity feed Into Y connection. Than suck on the
    end and try to breathe. Let me know how it works :)
    Buly

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