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Hello Terrence,
I agree with you that the fuel system should
require minimum management. The header tank type system is an area where just a
little low tech, light, cheap and dependable equipment will do the trick
for you. I do, however, see some possible problems with your proposed
system.
Running the output from the Facet pumps to a tee
will require check valves. Without them, a failure of one pump would allow the
working pump to back flow through the
failed pump to the other wing. Rather than adding equipment (check valves) that
itself could fail, my suggestion is to run a separate line from each Facet pump
to the header.
To really work well, you'll need not one but two
float switches (or optical sensors) in your header. One should be positioned to
close at the point where you'd like to have the pumps turn on. In ours, that's
at the 8 gallon level. The other should be placed high and positioned to close
when the tank is nearly full. In ours, it's at the 10 gallon level. We also have
a float switch in each wing tank that closes when the tank is nearly
empty.
To integrate the information from all these float
switches into a fuel management system, we built a simple locking circuit that
turns on the transfer pumps when the 8 gallon switch is closed and keeps the
pump(s) running until the 10 gallon switch closes. We have 2 dpdt switches, one
to control each Facet pump. They are labeled "auto" "off" "manual". In the auto
position, which is where we operate almost 100% of the time, the fuel system
runs itself. The header tank stays between 8 and 10 gallons and if we run a wing
tank dry we get a "wing tank low" light. The "wing low" light only
functions if the respective transfer pump is on. In "auto" the system
requires very little attention. With two separate switches, we can leave one in
auto and turn the other off in order to balance the wings, if necessary
(rarely).
We also have a "crossfeed" valve which I highly
recommend as a safety item. Here's how it works. The lines from the wing tanks
each flow to an "on-off" valve in the center. We built a long shaft that
controls the valve from a knob on the console. On either side of the valve is a
tee that goes to the respective Facet transfer pump. Normally the valve is
closed and the left Facet pumps fuel from the left tank, the right pumps from
the right wing. In the event of a transfer pump failure, the valve is opened
which allows either Facet to pump from BOTH wings at once. This happens by
gravity because of the dihedral of the wings (all of this plumbing is at floor
level under the seats). On several occasions, for testing purposes, I have
made long flights using only one pump and it works perfectly.
Without a crossfeed valve, the loss of one transfer
pump would not only make the fuel in one wing unusable, it would turn it into
ballast that may make it difficult to use all the fuel on the "good" side due to
aileron authority.
The system has self test circuits built in so
that if the 8 gallon header tank switch closes and power is not being supplied
to at least one transfer pump, you'll get an "auto fail" light. If the 10 gallon
switch is closed and power IS being supplied to either transfer pump,
you'll get the same "auto fail" light. We also put in a momentary
contact "auto fill" switch which shorts the 8 gallon float
switch. This tells the system to start pumping until full. We'd use
this while taxiing to get gas. By the time we shutdown, the header is
full and we just have to fuel the wings. In almost 1200 hrs, I've only had
the fuel cap off the header tank a handful of times.
Contact me, if you'd like, for further details.
Here's the circuit and a photo of the console,
let me know if you want more info.
Bill Harrelson
(540) 372-8738
N5ZQ 320 1,175 hrs
N6ZQ IV being built with a header tank
and essentially the same system
To those who've been there-done
that on early Lancairs -- request your comments. I want a minimum-management
on-off fuel supply.
For my L235 w/ IO320, I'm now
running the fuel line from each wing tank through a filter on
each Facet pump, then from each pump -- both into an open Tee, up to the
header tank --
Control of the Facet wingtank
pumps' is on-off with an optical level-sensor in the header,
to break or complete the
plus-circuit 12VDC to each/both tanks' power-on/off
switches.
Left and right tank Facet
pumps individual switches are normally left on, and powered when a/c master is
on. A red 'pumping' light is in the wing tank pumps' power
circuit, cycles on and off as sensor demands fuel for the header.
When light stays on, attention is called to the
How can power to the
L and R wing tank Facet pumps automatically switch off when the
wing tanks are dry?
From header, fuel
flows through manual on-off valve, through filter to boost pump
(recirculate pressure circuit) , to engine driven pump, through fine filter to
fuel servo, to spider and injectors (airflow performance.)
Would it hurt the Facets to
run if one runs dry before the other? (No check valves; Flow is one-way
through the pumps).
Terrence
L235/320
N211AL
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