Brian Trubee wrote:

I'm in the middle of designing my fuel system and wanted to see
what type of problems I might run into with the following
configuration. I've got an RV-4, plan on using two fuel pumps,
a purolator fuel filter/moisture separator, and was thinking of
using an electric 6 port switching valve. The idea was to keep
high pressure fuel lines out of the cockpit. I have since re-
thought the necessity of having the electric 6 port switching valve.
Eliminating it would eliminate one more thing that could fail along
with it's weight.

It might be useful to ask the manufaturer what is the switch's MTBF.
Perhaps it isn't anything to worry about.  As for the weight, how much does
the electric 6 port weigh versus the manual 3 port?

So here's what it would look like:
One fuel pump per tank, either in the tank or close by, each
 feeding fuel to the fuel filter. This is assuming that the fuel
pump has an integral one way valve.(the filter has two inlets)
The fuel would flow from the filter to the two fuel rails,
and the return would go to the manual three port valve mounted in
the cockpit. I'd have to select which tank to return fuel to manually.
A three position on-off-on switch would be used to select either pump,
which would in effect select tanks.

I'm somewhat leary of designing my own fuel plumbing, but I think
that this design might have merit.
Please fire away if you see  any likely problem areas.

More planes have crashed because of fuel management problems than very
nearly any other cause (VFR into IMC is the only greater cause, I believe).
I think that having two different controls (electrical switch for the fuel
source and mechanical valve for the fuel return) is setting the pilot up for
fuel management failure down the road.  If you're going to do it this way,
please build a cam onto the valve stem (of the return valve) and have the
cam turn on the pumps (via switches) so that fuel source follows fuel return
automatically.  You probably also should use a "OFF-RIGHT-BOTH-LEFT" valve
(with matching cam) so that takeoffs can be made in the "BOTH" position.
Takeoff on one pump would be a bad time to discover pump failure/screen
clogged/etc.

Russell Kent

 
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