norm2 wrote:
<<For instance, suppose your pump puts out 30 GPH and the engine uses 15 GPH
at cruise power.>>
The drawing for the Bosch pump that I found shows that it pumps 48 gallons
per hour. Are there injection pressure pumps available that have a lower
flow rate? The other thing to watch is the electrical power consumed by
having multiple pumps running at the same time.
Start with full 30 gals left and 30 gals right, and all returned fuel
goes
to the
left tank, you would start, taxi, TO and climb on the left tank. This
simplified example ignores the extra fuel burn in climb.
LEFT RIGHT
START 30 30
1 hour on L -15 0
@ end of 1 ho 15 30
1/2 hr on Rt +7.5 -15
@ end of 1.5 hr 22.5 15
1 hr on Left -15 0
@ end of 2.5 hr 7.5 15
1/2 hr on Rt +7.5 -15
@ end of 3 hr 15 0
time to look for field, with all remaining
fuel in the left tank, no more fuel mgmt req'd
except landing before you run out. :-)
If the pump is putting out 48 gallons per hour, you have a net transfer of
33 gallons per hour. Your schedule then looks like 1 hour left, 15 minutes
right, 1 hour left, 15 minutes right, ... The large flow rate makes the
timing more critical, or you are going to pump fuel overboard. Another
solution is to vent the left tank to the right tank, and the right tank to
atmosphere. That way if you do overfill the left tank, it overflows to the
right side.
This seems reasonable, but I really like the redundancy of two
independent vents. Cessnas with interconnected vents are notorious ( fly
them, too and can attest to it) for NOT draining anything like equally. It
is not unusual in a C182 to run on "both" and have one tank at 1/4 while
the other is a 7/8, due to the common venting that Cessna used for a long
time. They eventually went to individual vents for each tank.
I agree that the more the fuel flow exceeds the engine requirements, the more
difficult this becomes, since you can run into relatively huge imbalances. It
seems a bit rediculous to have your fuel pump put out 3-4 times the fuel
required, but perhaps that is what is necessary to cool the fuel system in
the car under worst conditions. I wonder if those pumps can be run at
reduced voltage to reduce the flow rate yet retain the output pressure?
Many aircraft fuel injection systems have two speed boost pumps, with high
being only used for TO and climb, and normal used for everything else.
A voltage regulation system to lower the voltage to 8 volts for cruise and
use 12 volts for TO & climb might work, depending on the motor
characteristics.
The last fuel tank system I designed had dual tanks (300 gallon total) with
a 4 inch diameter crossover hose so that we could fill at 50 gallons per
minute. These systems are setup to draw from one tank, return to the
opposite side (for heat dissipation), crossover hose between tanks to
equalize fuel level, crossover vent lines at top to equalize air pressure,
and single atmosheric vent (with rollover check valve for upset protection).
If you can buy into a single sump system, a few crossover lines takes care
of the return fuel issues quite nicely.
My god, what kind of aircraft are we talking about?? This sounds like an
8 seat twin or something like that. A FOUR INCH fuel crossover !!! Wow.
Perhaps I missed something, but most of us are thinking more in terms of
40 -80 total gallons and 3/8" or at most 1/2" lines. It is very easy to get
20GPH
thru a 3/8" (AN-6) line.
To further this discussion we really need to get some good numbers on flow
rates of the available pumps. A pump that flows about 5 gallons per hour
over what the engine consumes at max power would make the timing much
easier.
I agree wholeheartedly. The cars can put up with these huge overflows due
to a single tank.
Bill Freeman
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