Thanks Ed, #1
hope not, but pump 1 has aprox. 20 hrs., pump 2 maybe 10 min. max., #3 dna, will
prob. try #4 first easyest to do + high qual. mechanical pressure
tester, David
From: "Ed Anderson" <
eanderson@carolina.rr.com>
To:
"Rotary motors in aircraft" <
flyrotary@lancaironline.net>
Sent:
Wednesday, June 13, 2012 7:17:33 PM
Subject: [FlyRotary] Two
possible causes was : [FlyRotary] Re: Fuel presure question.
David, don't know whether you have considered this - as
you are aware the two pumps are able to work in parallel because the pumps have
check valves. So if one pump is off and the other on, the on pump does not
pump fuel backwards through the off pump. Since each pump produces the 42
psi pressure by itself, one would assume the pumping elements in both pumps are
working properly. Also assuming the pressure change is NOT due to a
malfunctioning pressure sensor.
Here are some things that you might want to
consider:
1. That somehow when both pumps are on - one of the
pumps check valves is not closing properly. Not a pump expert so not
certain this could even happen - but the thought occurred to me.
2. The effect of Increased fuel flow rate due to
both pumps may be causing a pressure drop through an upstream orifice
(like perhaps your "T").
3. Turning on both pumps (assuming on the same
circuit) might cause sufficient voltage drop in the line sufficient to decrease
the pumping performance of both pumps - pump flow rates are sensitive to voltage
variations.
4. The pressure return regulator might be
malfunctioning and opening due to the higher flow rate - easy to check, just
temporarily block off the return. If the symptom disappears then it’s the
regulator.
Theoretically, if the fuel flow rate through the T's and
pumps does not change with both pumps on, then the pressure should remain
the same - which is what you are expecting, but not seeing.
So since the pressure does change (drops) - what could
cause this (assuming the pressure drop is real and not the result some strange
malfunction of your gauge)?
One thing that could cause it, is if the fuel flow
rate increases through an upstream orifice in your fuel line (say at the
"T" which feeds your two pumps) then the pressure on the other
(downstream) side will drop. Turn one of the pumps off, the fuel flow rate
drops, therefore there is less pressure drop across this orifice and your
fuel pressure on the downstream side of the orifice will again go
up.
Its similar to a resistor in a circuit. The resistor
value will remain static - the same, but if you pull more current thought it,
the voltage drop across it will increase resulting in less voltage on the
downstream side. Reduce the current flow back to previous rate and
the voltage drop across the resistor decreases and therefore voltage on the
downstream side increase.
Another possible cause might be if both of your pumps are
fed off the same electrical line and there is sufficient resistance that when
both pumps are on the voltage drop causes less performance by both
pumps.
Just some WAGs that you might consider, if the
suggestions make by others do not result in identifying the cause.
Good Luck
Ed
Sent: Wednesday, June 13, 2012 6:43 PM
Subject: [FlyRotary] Re: Fuel presure question.
Fuel is gravity
routed from tanks to ganged fuel selector out to T-fitting with each pump on the
sides of the T, to another T, into main line out to eng.,
pressure sender is mounted directly after the T where the two pumps
come together. All this is in front of the wing spar near the floor
in an RV6A. This would make the pumps in parallel & the pressure is the same
from either pump, but both together pressure drops.
David
From: "Mark Steitle"
msteitle@gmail.comTo: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Sent: Wednesday, June 13, 2012
8:36:19 AM
Subject: [FlyRotary] Re: Fuel presure
question.
David,
How are your pumps plumbed... in parallel or
series? What kind of
pressure do you get when running the backup pump
only?
Mark
On 6/13/12, Tracy <rwstracy@gmail.com>
wrote:
> Hmmmm... Mine goes up about a pound when I switch on the
second pump. What
> happens when you turn both off?
>
>
Tracy
>
> Sent from my iPad
>
> On Jun 12, 2012, at 7:11
PM, hoursaway1@comcast.net wrote:
>
>> Question about fuel
pressure indications, at fast idle 2200 RPM, fuel
>> pressure is 42 PSI
on EM2, if I switch on the back-up pump with the
>> primary pump the
pressure drops to 35 PSI, can here both pumps running, if
>> I switch
off either pump pressure goes back to 42 PSI. My system is
13B,
>> stock red injectors all four, pressure regulator is stock Mazda
on the end
>> of fuel rail with vacume connection from dynamic chamber.
Thanks, David
>> R. Cook RV6A
Rotary.
>
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