Re: Fuel Vent Line

Bob Farnam <bfarnam@...>

-----Original Message-----
From: []On Behalf Of
David J. Gall
Sent: Monday, August 14, 2006 1:34 AM
Subject: RE: [Q-LIST] Re: Fuel Vent Line

Ya know, Bob, I thought about that too. Here's the way I see it: the
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
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.
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
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
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
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
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
tank vent line does NOT require me to perform acrobatics in order to
proper engine operation then I'm more likely to have time to apply carb
and squeeze the manual fuel pump bulb -- oh, yeah, we took that out of the
system last month, didn't we.... :)
[Bob Farnam] Not me! Just like one good remaining engine on a twin above
its single engine ceiling, it may not be a perfect solution, but it sure
extends your glide.

Not to make too much light of the situation, your Pacer wing tank collapse
was caused by gravity feed PLUS pressure differential, working together,
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
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
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,
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
[Bob Farnam] I think you miss my point, David. Forget any possible help
from engine suction, just think about the float bowl itself as the lower
termination of one line from the fuel system. It has a certain head pressure
on it which is the height of the fuel column above it. We'll assume for the
moment that the fuel system is completely full, that is, above the stand
pipe. So the vent line has a head pressure on it which is the height of the
fuel above its lowest point. There is a net head that is the difference in
height of the float bowl and the height of the lowest point on the vent
line. The ram pressure only needs to overcome that head pressure which is
the difference in heights. This implies that the ram pressure at cruise of
around 10" water, or 13.3" gasoline would clear the vent AS LONG AS the vent
is less than 13.3" below the float bowl level. I agree that at lower
speeds, lower ram pressure reduces that margin. Maybe one fix would be to
place the ram vent no lower than the float bowl, but not necessarily in
front of the windshield. Possible a location on the side of the fuselage
would be workable.

In fact, the "gravity" portion of the equation might just be zero, or even
little bit negative, if the slug of fuel in the vent line is taller than
height of the fuel level in the header tank is above the carb inlet. So
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
in climb might be quite small indeed.

[Bob Farnam] Actually, I climb away at a speed which results in about 5"
water pressure. And my carb is in the lower pressure bottom of the cowl.
Other engines may differ. Bottom line, for me at least, is that I stick with
my existing system. My fuse side fill location ensures that I can't fill up
to the top of the overflow pipe. If I have run into the header tank during
the previous flight, I do run the transfer pump to refill it as part of the
refueling process. The design of the tank with arched top and vertical
plate baffles makes the main tank very stiff with minimal volume change when
I sit on it, so that it isn't much of a pump itself. I think it very
unlikely that turbulence could put enough fuel into the opening of a 1/4"
aluminum tube vent line to fill it, especially with ram pressure acting on
it to help keep it clear. The only way I can see to fill the vent tube is
the way that Jim Patillo described when he transfered fuel from his aux.
tank into the header without providing room in the main for the overflow. My
system has worked flawlessly for 8 years and 500 hours. Come to the
Livermore flyin this weekend and you can see (and ride in) two very
successful airplanes.

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

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