Re: Fuel Vent Line

David J. Gall

Ya know, Bob, I thought about that too. Here's the way I see it: the lowest
pressure the carb can pull on the fuel line will be at high rpm with the
throttle closed. The highest ram pressure will be at max airspeed. Dive with
the throttle closed to purge the vent line. Not a good plan.

So there you are on climbout and the engine sags due to fuel starvation. The
rpm drops off, you push the throttle open, and the airspeed decays. What
good will it do to know that you would be able to clear the vent line at Vne
in a power-off dive if the powerlines ahead and every nerve fiber in your
body are screaming that you'd better add power and get the nose up?

I already know that the correct response to a power loss in climb is to push
the nose down to retain flying speed and that is already contrary to my
instincts. Please don't make me learn to hope for power restoration by
diving at the ground and chopping the throttle, too! I don't care if the
theory says there "ought to be" some combination of airspeed and throttle
setting (closed!!?) and rpm that will clear that slug of splashed fuel from
the vent line, I won't have time to diagnose that THAT particular problem
is, indeed, THE problem.

I would much prefer a fool-proof system that works in an intuitive way that
I can count on time and again to be reliable, predictable, and repeatable.
("Gee, would zero-gee help the ram pressure to purge the vent line," Robin
queried Batman as the Bat-plane plummeted perilously earthward.) If my fuel
tank vent line does NOT require me to perform acrobatics in order to restore
proper engine operation then I'm more likely to have time to apply carb heat
and squeeze the manual fuel pump bulb -- oh, yeah, we took that out of the
system last month, didn't we.... :)

Not to make too much light of the situation, your Pacer wing tank collapse
was caused by gravity feed PLUS pressure differential, working together, and
there's a good bit of head pressure from a Pacer's high wings to the
low-mounted carb. And there are other factors to consider. Even if the fuel
flow from that tank to the engine were shut off, simple contraction of the
fuel with decreasing temperature could have collapsed that tank. Top off an
already almost full tank in the morning, leave it setting in the sun all
day, then the mud dauber builds his nest in the fuel vent in the
afternoon... And you may never even have needed to start the engine!

As opposed to your Pacer's gravity feed PLUS pressure differential, purging
a slug of fuel from the QAC fuel tank vent requires determining the
difference between the action of gravity and the pressure differential,
working against each other, in other words, gravity MINUS pressure

In fact, the "gravity" portion of the equation might just be zero, or even a
little bit negative, if the slug of fuel in the vent line is taller than the
height of the fuel level in the header tank is above the carb inlet. So we'd
be left with just the pressure differential to move fuel through several
feet of small diameter (0.22") fuel line at approximately six gallons per
hour, or 0.85 ft/sec. Given that ram pressure at 60 mph is 1.5 inches of
water, and assuming that the carb is in the high pressure ram recovery
region of the cowl, the pressure difference available to drive the fuel flow
in climb might be quite small indeed. Certainly not equivalent to the head
pressure of approximately 14 inches of fuel (10 inches of water) that the
header tank normally gives....

I think you get my point by now. Thanks for helping me to have to think
through this problem. Now I know that there's another thing I want to do
differently on my airplane than what the plans call for.... :)

David J. Gall

-----Original Message-----
From: []
On Behalf Of Bob Farnam
Sent: Friday, August 11, 2006 3:01 PM
Subject: RE: [Q-LIST] Re: Fuel Vent Line

No, not teasing! I agree as far as it goes. But there is
another factor to consider. As an unvented tank supplies fuel
through gravity flow, it will begin to pull a negative
pressure on the tank system. I know about this because a
Pacer of which I was part owner partially collapsed a wing
tank due to negative pressure because of a blocked (top)
vent. Now the question
is: Does the stagnation pressure due to ram air plus the
suction generated in the tank constitute enough pressure
differential to purge the fuel plug from the vent line? I may
see if I remember enough manometry to put some numbers to the problem.
-----Original Message-----
[]On Behalf Of David Gall
Sent: Friday, August 11, 2006 1:04 PM
Subject: Re: [Q-LIST] Re: Fuel Vent Line


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:
> To:
> Subject: Re: [Q-LIST] Re: Fuel Vent Line
> Date: Wed, 9 Aug 2006 23:39:44 -0700 (PDT)
> Dave,
> I wonder if the pressure differential on the vent tube
with a fuel plug
> would
> allow bubbles to get through into the tank. Mark Summers
and I were
> discussing
> that last evening. What do you think?
> Bob F.

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