Bill Freeman wrote:

As I understand the concept of a header tank, the purpose
is to permit a pumped fuel system from multiple tanks without
a selector valve.  A totally gravity feed system will feed from multiple
tanks without much problem, but a pumped system with multiple
tanks will suck from the lowest flow resistance tank, empty
it and then continue to suck air even tho a connected tank
has fuel available.  This is the standard wisdom, apparently
gained a the cost of many airframes and at least a few lives.

So, if you want to eliminate the selector valve, you add a common
header tank that the multiple tanks gravity feed to, vent it
(critical to success) and then pump from the header tank.  This
is well tested and will work.  Without the vent  you just have
a fat "T" in the multiple tanks Teed together and it will probably
suck one tank dry and the engine will quit.  Or so I have been
told by folks that seem to know more than I do, and I have been
unwilling to test this.

As to multiple pumps in the individual tanks *pushing* fuel to
a common pickup header tank, I assume that this will work fine
with any sort of header (or no header) and that you do not need,
and in fact can't have a vent, since the tank pumps would push
fuel out the vent since the header is pressurized in this case.
Personally, I would avoid having a pump in each tank from
cost and weight reasons plus the old "can't fail if not installed"
theory of reliability.

The Facet "Block" type pump is claimed to be essentially "blockage
proof", altho this may be an exaggeration.  So far, many many
thousands of hours have been flown with Long EZs with the
Facet block pump downstream of the engine driven mech pump,
and I have heard of no electric pump failures which caused a
fuel blockage.  The multiple pump systems with several check valves and
bypasses seems more likely to fail than a pair of Facet block pumps
in series with a good filter between to stop chunks from a failing
upstream pump from killing the downstream pump.

In case some of you are not familiar with this pump, it is essentially
a solenoid with a hollow piston and a valve at each end of the piston.
  A transistor switches the current on/off to pulse the solenoid against
a spring.  The oscillations pump fuel straight thru.  Cheap and
service record appears to be very good.  Doers anyone have any
contrary service experience?  The only problem I know of was a
minor materials compatibility problem many years ago when auto
fuel rated pumps were used with avgas.  Even then, apparently no
actual failures occurred, just the possibility cause them all to be
changed out in the Long EZ community.

Any contrary opinions and facts are very welcomed.  I am not an
expert in this field, but have listened to many who have claimed to
be experts and tried to learn from them.

Bill

 
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