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
Most all the comments here are valid but I must caution against trying to
"fight the system" here. Auto EFI pumps generally put out a minimum of 30
GPH. This is what we have to work with. This is partly to insure that the
regulated pressure stays very stable. If you reduce the flow, regulation is
not as good. I would not gamble my life trying to run it at reduced voltage
when the higher flow rate is not needed. (This in itself would be a
complication)
Another point is that fuel flow varies a lot. I burn as much as 17+ GPH
(top speed runs) to as little as 3 GPH when sightseeing at max efficiency
cruise. It would be a BIG pain to remember to adjust my tank switching (in
the previously suggested scheme) to suite this large a range.
Tracy Crook
rws@altavista.net
www.rotaryaviation.com
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