Re: Fuel Vent Line


David J. Gall
 

Mike,

I'm not advocating this installation at all: I'm trying to get people to see
the difficulties of it and to opt for the simpler vent going out the top of
the fuselage. We're all so used to seeing a fuel tank vent under the wing of
the Cessna we rent that we may not be putting much thought into the "simple"
act of "copying" that vent. Indeed, that vent is very different from the QAC
vent. And you're right, the transition from large diameter to small in my
suggestion does pose a potential problem.

Let me jump to the conclusion regarding your suggestion to use the filler
pipe as a catch tank. In essence, the Q2/200 fuel tank(s) are actually (in a
fluids sense) just one tank. The header tank is connected to the main tank
via the 5/8" standpipe down the middle, through which the "two" tanks
communicate both fuel and air.

The advent of the header tank is that it provides a reliable head pressure
to feed the carb. The header tank is kept full by the fuel transfer pump.
You can think of this transfer pump as a part of a "live" tank as opposed to
a passive tank. In a passive tank like on a Cessna, when the tank is nearly
empty the head pressure is still good because the bottom of the tank is
above the carb. In the Q2/200, the "bottom" of the tank is artificially
moved up to the level of the bottom of the header tank, because as the main
tank empties we transfer the fuel to the header. This "live" transfer makes
the tank seem to the carb (in a fluids sense) to be mounted higher in the
airframe.

Consider the act of filling the tank: If you fill the main tank until the
filler neck is full, doesn't the fuel also rise within the 5/8" standpipe to
the same level as in the filler neck? Now, mind you, I haven't got
operational experience with a Q2, but it seems to me that you either have to
run the electric fuel pump during refueling in order to fill the header
tank, or else the header tank fills when the fuel level reaches the top of
the 5/8" standpipe. Either way, when the header tank is full so is the 5/8
stand pipe and so is the filler pipe. Therefore, ANY vent connected to this
tank system anywhere had better be above the fuel level or it will have fuel
in it. Ergo, your proposed vent from the filler pipe would need to be just
as long (in the vertical dimension) as the existing vent from the header
tank. Moreover, the filler pipe vent would be just as susceptible to fuel
getting into it as the existing vent line is.

Keep up the good work,


David J. Gall

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of Mike Perry
Sent: Saturday, August 12, 2006 9:20 PM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Re: Fuel Vent Line

First, thanks to everyone who keeps writing on this issue.
Fuel starvation 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 pipe narrowing to a small diameter tube as it exits the
airframe. I have 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 air pressure will clear the vent tube . .
. "Gee officer, I wasn't speeding . .
. this is a scientific experiment . . . yes, I'm working on
an airplane 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 second 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 situations.

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 comments 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)

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