Guys,
Here’s the schematic for a simple automatic
header tank system. The relay can be anything from a Radio Schlock 5 buck part
to the Perihelion Designs PowerLink (http://www.periheliondesign.com/powerlinkjr.htm).
I used the PowerLink in our IV (yes, I put a header tank in the IV) and I
have a mechanical relay in our 320. For this purpose, the magnetic float
switches from Spruce work best. The diode can be any small, light duty R.S.
diode. The diode is to prevent the pumps from
being powered through the float switches. These are light duty switches and are,
in my opinion, only suitable for small current loads such as closing the relay.
The momentary push button labeled “auto fill” is
so that you can start the fill sequence before the low float activates.
Once the low float makes contact the pump(s) will
stay on until the high float switch breaks contact. The pumps will remain off
until the low float makes contact once again. This prevents the pumps from
cycling rapidly as fuel sloshes. To further lessen any effect of fuel sloshing,
I made a little baffle box around the float switches .
I also recommend a crossfeed valve. The
lines from the wing tanks are both routed along the floor under the seats to a
ball valve mounted on the floor (low point in the fuel system). The transfer
pumps are teed off the lines on either side of the crossfeed valve. I drilled
and tapped an aluminum rod and safetied it onto the valve shaft. the rod then
runs up through the console where I turned it down to accept a knob. In the
event of a single transfer pump failure, opening this valve allows fuel to
gravity flow due to wing dihedral. All of the fuel from both wings can be pumped
by either pump.
Bill Harrelson
N5ZQ 320 2,050 hrs
N6ZQ IV under construction