Re: Fuel Vent Line

David J. Gall


I think you and Mark are teasing me! :)

Well, OK, I'll bite.

If the vent tube were big enough to be able to pass bubbles through a slug of
trapped fuel, I don't think you'd be able to get enough bubbles per hour to
replace the five to six gallons per hour of fuel volume your engine wants to
consume during climb.

You can't get bubbles through a soda straw, but you can through a garden hose.
The diameter matters a lot. The tube from the main tank to the header tank is
designed to do essentially what you suggest, but sized appropriately to allow
fuel and air to flow simultaneously in opposite directions. Dumb design, but
QAC apparently made it work.

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 capable
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 self-purging
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 small
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 short
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 problem
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 were
that last evening. What do you think?
Bob F.

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