Re: Fuel Vent Line

chrishazlitt <chrishazlitt@...>

I have been cut and paste'ing the posts in that thread myself,
thanks for sharing on this subject, I had no idea there was that
much to know.


--- In Q-LIST@..., Mike Perry <dmperry1012@...> wrote:

First, thanks to everyone who keeps writing on this issue. Fuel
is one of the classic killers in early flights of experimental
aircraft and
well worth a lot of attention.

Second, Dave Gall, you seem to be advocating a large diameter down
narrowing to a small diameter tube as it exits the airframe. I
concern we could still end up with a fluid column trapped above
the narrow
vent tube. Perhaps some experiments are in order.

(Lets see, mock up of fuel tank, 2 or 3 vent tube designs, mount
on top of
the pickup truck, drive down the freeway at 100 mph, see if ram
pressure will clear the vent tube . . . "Gee officer, I wasn't
speeding . .
. this is a scientific experiment . . . yes, I'm working on an
design . . . No, I'm not crazy. Well I don't think I am . . .
sure, I'll
talk to the nice psych tech . . . " NOT GONNA HAPPEN!)

Also, Dave you wrote: "or else have a shunt to capture the
fuel . . . in a
secondary tank . . . That is a bad idea for lots of
reasons . . ." Here is
an idea that might work:

Run a fuel vent line from the header to the fuel fill tube, then a
vent line from the fuel fill to below the canard. In essence, I
am using
the main fuel tank as the "secondary tank" in your earlier
suggestion. Drawback: Complexity, weight. Advantages: short
vertical in
each of two separate tubes, unlikely to develop a significant
siphon effect
in either. Should develop positive pressure in main and header
tank in all

In other words, why not replace a long vent line that can generate
significant negative pressure thru siphon effects with two short
vent lines
much less vulnerable to siphon effect negative pressure?

I will be off line for 36-48 hours (work) but look forward to any
when I get home.

Mike Perry
(18 months away from needing this question answered but still very
interested in a safe fuel system when I get there)

At 12:03 PM 8/11/2006 -0800, Dave Gall wrote:


The design parameter for the fuel tank vent line is that it
either be
impervious to fuel entering the line (ever - not realistic) or
else be
of performing its vent function when fuel gets into it. To do
that requires
that it either be self-purging when it gets fuel in it, or that
it be able to
flow air (in sufficient volume) even when there's fuel in it. A
short vent
line extending up from the top of the tank or filler cap has the
capability with the added advantage that gravity works with ram
pressure to
purge the line in normal flight.

A long vent line extending down through the bottom of the
fuselage has the
distinct disadvantage of having gravity working against ram
pressure as
previously discussed. The pressure differential needed to purge
the line by
forcing fuel up into the tank is apparently not available, so the
line must
either be self-draining against ram pressure or else have a shunt
to capture
the fuel (up to some assumed reasonable amount) in a secondary
tank of some
kind for later return to the main tank. That is a bad idea for
lots of
reasons, so we're left with designing a self-draining vent line.

As already noted, the diameter of a tube has a big effect on its
ability to
pass air and fuel in opposite directions simultaneously. The
length of the
tube and the relative heights of its ends have an effect on its
ability to
trap and retain a slug of fuel (siphon effect). A short length of
diameter tubing at the airframe exit is tolerable because ram
pressure could
force a small slug of fuel up against the action of gravity;
likewise, a
length of small diameter tubing is tolerable at the top of the
tank to "turn
the corner" and start the vent line in a downward direction.

Its the long vertical part of the small-diameter vent line that
is the
in the QAC design. This section of the vent line can hold a slug
of fuel just
like when you cap off the top of your soda straw and pull a slug
of your
favorite beverage up out of the cup. Try the same stunt with a
piece of 5/8"
tubing and you'll find all the soda (fuel) draining back into the
cup as you
withdraw the tubing.

So, replace the long vertical part of the fuel tank vent line
with a larger
diameter line and you should have a self-draining line that is
capable of
passing air even when there's a bit of fuel in it.

That's my opinion.

David J. Gall

----- Original Message -----
From: <>bfarnam@...
To: <>Q-LIST@...
Subject: Re: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)

I wonder if the pressure differential on the vent tube with a
fuel plug
allow bubbles to get through into the tank. Mark Summers and I
that last evening. What do you think?
Bob F.

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