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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.
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
Ed
Anderson
> But why is the other pump apparently not
affected?
> If the outlet path were restricted the pressure would
increase, right?
> New hoses from common point to both pump
inlets.
> This is the path (that has been working for 100's of
hours):
> Fuselage tank -> Fuel selector valve -> Fuel flow sensor
-> fuel filter
> -> drain point -> split to two fuel
pumps.
>
> I've replaced the fuel filter in the last 3 months and
25 hours (cheap
> "G3" automotive paper filter).
>
> Any
ideas?
>
> Finn
>
>
>
> >>
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