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Sometimes we do things because we don't think about why we are doing them. Just
the other day I followed the instructions in the Lancair IV-P manual and without
thinking about it got myself into a bind. Again!
 
I am aware of the dual tank fuel selectors, and that they are used on high wing
aircraft (exclusively?). The reasons I've been given for not using such a system
on a low wing aircraft all had to do with lack of head pressure and the ability
of the fuel to drain through the valve from one tank to the other causing a
heavy wing situation during flight. These reasons however, don't hold up to
scrutiny, at least not with the information I have available to me. That's why I
started this thread.  
 
All references to fuel pump or pump in the following discussion refer to the
pump(s) that moves fuel to the engine compartment. The engine driven fuel pump
is yet another pump and is disregarded. Also, system redundancy is ignored for
simplicity until the very end.
 
The obvious: What gets pumped out one end of the fuel pump must flow in from the
other end. To some degree, the intake side of the pump vacuums fuel in to it.
Any pressure already existing at the intake side makes that job easier, but even
with 0 intake pressure, the pump will still operate. Having had fresh and salt
water tropical fish aquariums all my life, I know something about pumps and what
you can and can't do with them through experimentation. Once a pump has been
primed, it will draw a fluid at a negative pressure (below the pump intake) into
the pump chamber and eject it at the output side. The amount of negative
pressure determines when cavitation starts and pumping stops. Many pumps are
"self priming" so they can suck air through them creating a vacuum and causing
the liquid to eventually hit the intake system.
 
In a low wing aircraft, pressure at the intake port of a fuel pump is minimal at
best, at least on the ground. The fuel pump is largely getting its supply of
fuel by expelling it at the output end causing fuel to flow into the negative
area at the intake side. I believe the normal plumbing for the Lancair IV and
other aircraft puts the fuel pump below the floor boards and between the wings.
That means the pump will at some point be above the remaining fuel.
 
The typical fuel pump puts out quite a bit more fuel flow than the engine driven
fuel pump can handle so a valve is used to move the excess back into the fuel
tank. With two tanks and no common plumbing, the fuel that came from a
particular tank must be put back into that tank so as to avoid pumping fuel from
one tank into the other and causing the heavy wing situation. That complicates
all the plumbing.
 
Why not do it this way. Main tanks plumbed into a common but tiny (1 pint)
header tank located between the wings. The header tank is the source of fuel for
the fuel pump. Think of the header tank as a large diameter fuel line. Excess
fuel diverted back by the previously mentioned valve goes to the header tank not
the main tanks. This is a very simple system (KISS) and requires less plumbing
(no return lines running back to the wings) than the typical set up (less
weight). One ON/OFF valve could be placed between the header tank and the fuel
pump if desired, or each wing line could be valved. Two main tanks supply fuel
to the header so available fuel volume to the fuel pump is double what the
normal arrangement sees.
 
During normal straight and level flight, the tiny header tank would allow fuel
to flow between tanks to keep both tanks at relatively the same fuel level. The
fuel lines from the mains to the header should be large enough to allow for
this. Fuel gauges (if they are to be trusted) could monitor that status. If
there is a possibility of creating the heavy wing situation, then valves at the
main tank lines could be used to manually correct the situation. Alternately,
flow between tanks could be prevented by inserting reverse valve restrictors
into each main fuel line. If fuel levels did get out of balance, the tank with
more fuel would exert more pressure on the header tank and consequently would
add more fuel until balance is achieved.  
 
Common sense (or is it ignorance) leads me to believe that under normal
circumstances the fuel system would take care of itself reducing pilot work
load. Only under extreme conditions would fuel levels get out of balance. Out of
balance conditions are the norm in the average set up when we are asked to
supply balance control by switching tanks every so often. The system I propose
is a net work load reducer.
 
If two fuel pumps were connected to the header tank in parallel and their
outputs connected to a common line leading to two overflow valves connected in
series with both overflow lines running back to the header tank, then those two
pump could be used as a redundant system, and also as a boost pump when needed.
i.e. two fuel pumps with intake sides connected to the header tank, and output
sides Y'd to a common line large enough to take the flow. That common line feeds
the first higher pressure valve which overflows back to the header tank. The
useable output from that valve is input to the next lower pressure valve which
also overflows back to the header tank.
 
The two valves in series should be set so that the valve closest to the pumps is
set to overflow at a higher pressure than the next valve upstream. The idea is
that if both pumps are running, the first valve diverts lots of fuel back to the
header tank. The remaining fuel and pressure on the input side of the next valve
up stream is designed to be high enough to also allow it to overflow back to the
header tank so that the engine driven fuel pump always has lots of volume and
pressure.
 
The pressure that the overflow lines put into the header tank are neutralized by
the action of the negative pressure that the pumps create. I believe the header
tank will remain at a constant steady state pressure equal to whatever pressure
the wing tanks exert. The main tanks act as shock absorbers for any pressure
spikes that may get produced momentarily.
 
Why won't this work?
What have I forgotten?
Which one of my assumptions is incorrect?
 
Thank You
 
Bill Gradwohl
IV-P Builder
N858B