Wschertz@aol.com wrote:
In a message dated 4/22/0 7:15:35, you wrote:
I woke up with this idea that since this system is totally dependent
on a working electrical system no selector valve is necessary for
the main feed lines or the return lines. Tank selection is
done entirely with the pump switches. This again further simplifies
the system. It also eliminates the possibility of a high pressure leak
in the selector valve.
----
Paul, et.al
I disagree that you can have a system with no valve without getting into
trouble on a low-wing plane. I will use an electrical analogy to the fluid
flow situation to make my point. It is easier than doing the fluid flow
calculations, but I believe it will show why I am concerned.
My analogy is a back-voltage of 4 volts for the left tank, and 8 volts for
the right tank, simulating 4" and 8" respectively of fuel head. I assume
equal resistance to flow to each tank (perfectly centered T).
See attached diagram, I think you will end up pumping fuel overboard with out
a valve and pilot intervention.
Bill Schertz
It is hard for me to make that leap to the electrical analogy Bill.
I like to think in terms of pressure.
A: Sucking fuel out of left tank. Right tank full.
Pressure on right tank 6" of fuel. Pressure on left
tank 5" of fuel. Return goes to left tank.
OK I think I see your point. And that is; not all return fuel will
go to left tank. Sounds possible. Perhaps a one way valve in the vent
lines can prevent pumping fuel overboard. A drawback to this is
the heat expansion of the fuel on a hot day could over pressure the tanks.
Perhaps the one way valves have an over pressure pop off. I recall
seeing something like that in the ACS catalog called a "free flow
one-way check valve". Not exactly sure how it works but it sounds like
it would do the job. The Andair check valve also sounds like it will do
a similar job. These are on page 153 of the 2000 catalog.
Also on my Cessna the common vent line is facing forward so it is pressurized
by dynamic pressure. That amounts to about .188 psi at 100 MPH
which is of the same order of magnitude of about 5 inches of water
and maybe 6 inches of fuel. If the two vent lines are teed together
and one check valve is used with dynamic pressure fed into the one
vent line I think it highly unlikely that fuel could be pumped
overboard but I could be wrong. As Bill Freeman says it never does what
you think it is going to do :-)
In any event a selector valve in the return fuel line sure would not hurt
as long as you did not forget to change it when you selected the wing
high pressure pump. Mike Will's idea of using an electrical solenoid to do
the return line switching along with the pump selection would be fine
if it did not fail. In any event the expensive duplex valve would still
not be needed.
Paul Lamar
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