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Sadly, my airplane is set up exactly to Velocity
plans. I modified it from there to make it work. Assymmetric flow is
a very common problem with Velocitys. But that's not the purpose of my
post. I was trying to point out that if Paul selects BOTH on his
system and one tank runs dry, he will be sucking air into his
plumbing. If he's venting his sump into one tank, that tank will
likely be slightly more than the other and could transfer significantly
faster, emptying significantly sooner, recirculating air/vapor into his
main fuel line. I was merely trying to encourage him to monitor his
fuel and select one tank or the other tank when either of them get low.
We've already discussed this too much - it's all been said more than
once ... Jim S.
kenpowell@comcast.net wrote:
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
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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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Archive: http://lancaironline.net/lists/flyrotary/List.html
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