big snip

Bill, 2 tanks are in each wing. Each wing is to be connected to the
Andair fuel selector with return. The left wing tanks will NOT be 'T'd
to the right wing tanks.

Attached is a sketch.

Rich

Now I understand. TWO tanks in each wing. This is dangerous Rich.
If one tank in one wing has its line uncovered you risk not
feeding from that wing at all.

The outer tank should feed to the inner tank and then only
feed from the inner tank to the selector valve. Usually this is done
with a large diameter pipe between the two and a one way flapper valve that
prevents fuel from flowing from the inner tank back out to the outer tank.
Also the outer tank should be vented. Return fuel can go to both
OUTER tanks in each wing.

When you are low on fuel don't slip the airplane as you might
slip with the selected tanks down. I usually keep my fuel selector
on both and if I have any fuel in the upper wing the engine keeps
running. Naturally! If you have to think about which way you are slipping
and which wing is selected there is of course  more chance for error.

High wing Cessna's have far fewer tank selection based accidents
than a low wing system like you describe. Probably because
of the both position on the selector valve.

What would be even better is two small tanks in both inner wing
tanks with one way valves so if you slip you will have enough fuel
to keep the engine running for a few minutes.

Paul Lamar

Rich,

I have to agree with Paul.  Your system will almost certainly
result in fuel interruption.  Your two wing tanks T'd together
will act exactly like a left and right tank T'd together, i.e. it
will empty one and leave the other with fuel in it.

One possibility is to drain the outboard tank into the inboard,
or have a four position selector and run off of each in turn.

Simplest is to have all four tanks drain into a VENTED sump
and pump from the sump.  Each tank should have a separate
vent, not co-located (in case of FOD, it can't destroy all
four vents).  This is simple and reliable.  Run your return
to the sump, but watch for sump fuel overheating with
exceptionally long ground ops.  Please be careful.

My C182 does some very weird stuff, like empty the left
tank to about 1/8 while the right has 3/4+ in it (according to
the fuel guages).  At 1/8th, the left stops getting lower until
the right goes down to about 1/4, where they sort of get
back in synch.  This is from a VERY simple gravity system.
I believe the primary problem is the single vent used on
old C182s.  Late models have dual vents, I assume they
do better, but don't know.

Low wing systems (pumped) and esp many-tank low wing
systems are even more treacherous.  I repeat - fuel systems
do *not* behave like you expect them to.

Bill
Long EZ builder & pilot
mechanical engineer

 
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