Well,
After much consideration I've decided to go with my simplified fuel system
configuration on my RV.
One fuel pump per tank, with the two lines going to the two inlets on my
fuel filter/moisture
separator, and then on to "T" off to the two fuel rails. The return will
go to a "T" and then
back to the fuel return selector. I'll have a switch to turn either the
left or right fuel pump
on, and will have to select a which tank the fuel will return to. The
selector switch and valve
will be adjacent to each other and in between the fuel quantity gauges.
After looking at the
failure modes/parts counts/complexity of operation/possiblity of pumping
fuel overboard if I get
vapor lock of the brain, I've decided that this is the simplest setup with
the least number of
connections and mechanical components to fail. Worst case -if I get a fuel
pump go catty wumpuss
on me on climbout or in some critical phase of flight, I'm frightened
beyond the capacity for
rational thought, I'm in brain frozen bung puckering, eye bulging fear
bordering on hysteria, if I
switch pumps and forget to switch returns, the fuel will continue to be
supplied to the engine
and, if the returned to tank is full, I'll pump fuel overboard. Worst I
could do is pump half of
my fuel overboard. Would be noticeable a quite a while before fuel
exhaustion in the tank being
used due to the weight of the fuel on the two sides being unequal. The
cost of the wasted gas will
teach me to pay better attention in the future, if there is one for me at
that point.
Anyway, some will find this objectionable, no doubt. Am I bold or foolish?
You decide.
Brian Trubee
If one tank is empty you are down to one fuel pump. No backup.
Why don't you just get one of Bulents four way valves and use
two pumps in parallel? If one pump fails you have the other pump.
The valve is a bargain and cheap insurance. A quality piece.
It automatically sends the fuel back to the tank selected.
Put the pumps downstream from the selector valve.
The four positions are:
Both tanks off in case of a fire. Pointer facing aft
Left tank Pointer facing left tank
Right tank Pointer facing right tank
Both tanks. Pointer facing forward
Idiot proof as can be.
Mount it down low between your legs (RV4) or in the center between the
seats.
Paul Lamar
John Slade wrote:
"Worst I could do is pump half of my fuel overboard..."
I found an even worse failure mode here, Brian. If you draw from one tank
and return to the other, you're not burning much fuel, and you're vent
happens to be blocked, it's possible to collapse a fuel tank. Ask me how I
know. (: If the fuel tank happens to be a flying surface, as in a canard,
things could get difficult.
However, I don't think the loss of fuel pump redundancy when one tank is
empty is a serious concern. You arnt going to run a tank empty very often,
so you only loose redundancy only in a rare and known circumstance. I have a
system similar to yours, except that my return is switched automatically by
a solenoid powered by the pump switch. Two pumps, two filters, very short
pipes in the cabin. As you say, there's no right way to do this stuff.
Personally I prefer the parallel system with redundancy of fuel supply to
the rail in NORMAL operations i.e when there's some fuel in both tanks.
John Slade
If one vent is blocked you could collapse that fuel tank in any configuration
other than "both". Even if you return fuel to it. In many fuel systems tank vents are
connected together with only one vent to the outside. C182. In that case you could collapse
both tanks no mater how they are drawn from. In most cases I suspect the fuel
pump(s) would cavitate before the tank(s) collapsed depending on how they were made
and baffled of course. Much also depends on where you are flying. Out here in
the west the distance between airports can be further and if one pump fails
you could run out of fuel before you get to the next airport. Particularly if
you factor in weather. Not all airports are available in bad weather. The situation
is particularly critical in Western/Northern Canada, South East Alaska, Baja and
all over Australia.
You need to think of and list all possible failure modes and design a system that
has as much redundancy as possible. Study certified airplane fuel systems.
They are the way they are because of 100 years of development. Trial and
error and thousands of crashes because of fuel supply problems.
I have never heard of a fuel system with two tanks and only two pumps.
One for each tank. At the same time keep it as simple as possible.
The crash rate of low wing airplanes is higher if for no other reason
many high wing airplanes have no fuel pumps at all. Most Cessnas have
no fuel pumps. Gravity feed only. They use a fuel selector valve with left,
right, both and off. I leave mine in both 80% of the time and only go
to one or the other if they don't feed equally which happens occasionally.
I have had two in-flight fuel system failures. One in a Cessna where the carb
float sank and another in a Mooney where the pump failed. The Cessna would still
run but not well enough to preclude landing at the first airport available.
The Mooney engine stopped with a bang forcing me to find a place to land right
now. Fortunately an airport was within gliding distance. I have twenty or
thirty times the number of hours in Cessnas than I have in Mooneys.
Paul Lamar
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