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Sent: Saturday, June 12, 2004 2:57 PM
Subject: [FlyRotary] Low fuel
pressure
> I put a new Facet pump in place
of the old one that had fluctuating fuel
> pressure.
> Also replaced the fuel line to the pump.
> On test flight last Sunday again had low fuel pressure, as low as
1 psi,
> so I cut the flight short and headed home.
>
> This morning I put a vent tube in the fuel cap pointing into the
wind,
> to eliminate bad vent lines as a cause.
> Pressure to outer carbs normal (about 3.3 psi), dropped a bit on
run-up.
> During take-off I noticed fuel pressure dropping more and more,
down to
> .5 then .1 psi, at which point the engine began to loose power.
> Fortunately I was ready for it and pulled back throttle to the two
outer
> carbs and was able to continue climbing and continue to a "normal"
landing.
>
> It's nice to have redundant system and be overpowered for the
airplane,
> but I really need to solve the low fuel pressure problem to my
outer
> carbs. (Center carb is shared between rotors and has it's own pump
and
> throttle).
> Pressure is OK at idle and apparently drops as fuel flow increases.
> This would indicate a restriction on the pump inlet side, right?
Finn, why
not a restriction on the outlet side of the pump- seems most likely to
me. Scenario - there is a restriction between the output of your pump
that services your two outer carbs (why don't you go EFI? never mind!)
and your outer two carbs. While there is low fuel demand by your two
outer carbs the pump is able to provide adequate fuel flow (and
pressure) through the restriction against the closed bowl plunger, but
as the demand for fuel increases, the venture of your two outer carbs
suck the fuel out of their bowls faster than your pump can pump fuel
through the restriction to the bowls and since the bowl plungers are
now full open (to allow fuel into the bowl) the pump can never build
up pressure past the restriction (as it could when the bowl plunger was
closed) to compensate. So eventually the bowls are sucked dry and
since the pump can't pump enough fuel through the restriction to meet
the demand you loose pressure and power.
Ok, I'll have to check the path between the pump and the pressure gauge.
In effect
the line area past the restriction is like pumping to open atmosphere
and becomes more like that as the fuel level in the bowl decreases
until when the bowl is nearly empty, the pump is pumping all the fuel
it can through the restriction, but the pressure shows zero (which
indicates your pressure gauge is on the carb side of the restriction -
so the restriction should be between your pump and your pressure gauge)
and while there is fuel flowing its insufficient to fill the bowl or
support the power demand.
Since the
restriction is on the pump outlet side of the pump servicing your outer
carbs, it does not affect your center pump or carb
Excellent point!