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914 fuel pumps

 
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VE3LVO(at)rac.ca
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PostPosted: Sun Feb 26, 2006 8:07 am    Post subject: 914 fuel pumps Reply with quote

I am intrigued by the discussion - particularly the mental picture of 2
pumps in Series or Parallel. There seems a discrepancy in some of ther
discussions in the true meanings.
If it were truly Series, what plugs the first will plug the second pump, so
I conclude (from the electric alanogy) that the present setup is
Series-parallel of a kind.
If both filters come first and are in parallel, all parts downstream should
be clean. Because they are parallel, as one begins to block, the other takes
on the whole task - so regular inspection should satisfy any precautionary
need. But my pumproom is at the rear, so I figure I need a shutoff valve
upstream of the filters so I can open them. Then, filter 1 leads to check
valve 1 and filter 2 leads to Pump 1, they combine into one line and split
again (tee) one branch to check valve 2, and the second branch to Pump 2.
They recombine into one line (which in my case leads to a lowest Gascolator
and off to the engine .
Thus (if you sketch this out corrctly), the two pumps are clearly one after
the other (series), but if any component plugs, another will feed the fuel
through an alternate line (parallel). In any case, I'm hoping the flow
pressure is not excessive, either with all bits working, or when one branch
is plugged.
I also have the excess fuel returning under the starboard sill to
the fuel filler entry on the tank. Is there any foaming involved in the
overflow fuel? If there is I'm hoping it dissipates on top of both tanks
when fuel is above the saddle, and on top of reserve when sucking from the
bottom.
I realise I'm not designing for Mars, but hope I am on track to a secure
fuel transfer plot.
Flames and flowers eagerly anticipated.
Ferg
A064


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jan.de.jong(at)xs4all.nl
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PostPosted: Mon Feb 27, 2006 7:13 am    Post subject: 914 fuel pumps Reply with quote

Hi Josok,

Quote:
Fuel pressure with both pumps running in series is double of the parallel system.
Similar to the electrical analog.

I doubt this if I may. If both pumps can be represented by a current

source and the check valves by diodes then the resulting current will be
determined by the larger of the current sources; the other is along for
the ride.

The parallel vs. series choice is probably driven by the desire for
minimal pressure and flow change when going from one to both pumps on (a
jump in reliability yes, a jump in pressure and flow no).

If the pumps are more like voltage sources then a parallel setup seems
best, if more like current sources then a series setup seems best. So it
would depend on the pressure vs. flow characteristics of the pumps.

Cheers,
Jan de Jong


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jan.de.jong(at)xs4all.nl
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PostPosted: Tue Feb 28, 2006 8:00 am    Post subject: 914 fuel pumps Reply with quote

Hi Gilles,

A very interesting website, http://contrails.free.fr/
Your radiator duct experiments are fascinating. Very nice recommended
book section too.
B.t.w. I'm missing the old tutorial bits on parallel/series voltage
regulators by you and Jrme Delamarre.

I don't quite agree with your fuel pump discussion though.

As I see it there are exactly 2 ways of connecting 2 pumps.
Parallel:
To remove a pump you replace it with an open circuit. A series check
valve for each pump makes that possible (no backflow).
Series:
To remove a pump you replace it with a short circuit. A parallel check
valve for each pump makes that possible (no blockage).
There is no 3rd way.

A pump has a flow rate versus pressure characteristic. How steep is the
curve?
At one extreme is a pump that supplies a set pressure regardless of the
flow required to achieve that.
Two identical such pumps in parallel would supply the same pressure and
flow into a certain fixed orifice as one.
Two identical such pumps in series would supply double the pressure and
flow into a certain fixed orifice as one.
At the other extreme is a pump that supplies a set flow rate regardless
of the pressure required to achieve that.
Two identical such pumps in parallel would supply double the pressure
and flow into a certain fixed orifice as one.
Two identical such pumps in series would supply the same pressure and
flow into a certain fixed orifice as one.

Where in the range between one extreme and the other the Piermont pump
falls I do not know. I suspect that Rotax found experimentally that it
is somewhat closer to being a " flow source" (the second extreme) than
to being a "pressure source" (the first extreme). Hence the series setup
recommendation. But the difference is apparently not large.

Practical questions:
Is there a likelyhood that a check valve fails stuck open? That would
weaken the case for the parallel setup.
Is there a likelyhood that a check valve fails stuck closed? That would
weaken the case for the series setup.
In both cases the pump with the failed check valve must remain on.
But maybe the backflow in the parallel case with the Piermont pump
turned-off is not too bad?
Or, maybe the blockage in the series case with the Piermont pump
turned-off is not too bad?

On your website you also mention the possibility that a failed pump may
shed debris.
That probably weakens the case for the series setup indeed.

B.t.w. The "pressure source" (first extreme case above) has the battery
as its electrical analogue: one voltage (approximately), any current
(almost). The "flow source" (second extreme case above) has no readily
available electrical analogue, although current sources are quite common
constructs in electronics. But pretending that a pump is just like a
battery is not useful in a parallel versus series discussion.

Hoping I didn't offend.

Jan de Jong
461, second wing


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Gilles.Thesee(at)ac-greno
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PostPosted: Tue Feb 28, 2006 8:44 am    Post subject: 914 fuel pumps Reply with quote

Quote:


Hi Jan,

Thank you for your message and your kind words.

Quote:
I don't quite agree with your fuel pump discussion though.

As I see it there are exactly 2 ways of connecting 2 pumps.
Parallel:
To remove a pump you replace it with an open circuit. A series check
valve for each pump makes that possible (no backflow).
Series:
To remove a pump you replace it with a short circuit. A parallel check
valve for each pump makes that possible (no blockage).
There is no 3rd way.


A true series circuits has no parallel branch. Hence the

"series/parallel" expression.
Of course the true series mounting is not desirable in an airplane.

Quote:
A pump has a flow rate versus pressure characteristic. How steep is the
curve?


You've got the flow/pressure curve on page 9 in the downloadable

Pierburg fuel systems
<http://contrails.free.fr/fichiers2/kraftstoffanlagen_en.pdf>
pdf document.

Quote:
At one extreme is a pump that supplies a set pressure regardless of the
flow required to achieve that.
Two identical such pumps in parallel would supply the same pressure and
flow into a certain fixed orifice as one.
Two identical such pumps in series would supply double the pressure and
flow into a certain fixed orifice as one.
At the other extreme is a pump that supplies a set flow rate regardless
of the pressure required to achieve that.
Two identical such pumps in parallel would supply double the pressure
and flow into a certain fixed orifice as one.
Two identical such pumps in series would supply the same pressure and
flow into a certain fixed orifice as one.


One can clearly *hear *the pumps slowing down when backpressure builds up.


Quote:
Where in the range between one extreme and the other the Piermont pump
falls I do not know. I suspect that Rotax found experimentally that it
is somewhat closer to being a " flow source" (the second extreme) than
to being a "pressure source" (the first extreme).

I have the notion the current or voltage source is too schematic when

applied to pumps. Remember this is an approximation to facilitate
reasoning on electronic device.

Quote:
Hence the series setup
recommendation. But the difference is apparently not large.


I believe Rotax took the diagram direct from the Pierburg documentation,

without questioning too much...

Quote:
Practical questions:
Is there a likelyhood that a check valve fails stuck open? That would
weaken the case for the parallel setup.


The probability is remote indeed. And consider that such a vane pump

blocks the fuel flow when stopped, apart from some leak. Sooo, if a
check valve sticks open, the parallel setup won't be too much impaired.

Quote:
Is there a likelyhood that a check valve fails stuck closed? That would
weaken the case for the series setup.
In both cases the pump with the failed check valve must remain on.
But maybe the backflow in the parallel case with the Piermont pump
turned-off is not too bad?
Or, maybe the blockage in the series case with the Piermont pump
turned-off is not too bad?


On your website you also mention the possibility that a failed pump may
shed debris.
That probably weakens the case for the series setup indeed.


That's the drawback of this setup. A stopped pump is not transparent to

fuel flow.

Quote:
B.t.w. The "pressure source" (first extreme case above) has the battery
as its electrical analogue: one voltage (approximately), any current
(almost). The "flow source" (second extreme case above) has no readily
available electrical analogue, although current sources are quite common
constructs in electronics. But pretending that a pump is just like a
battery is not useful in a parallel versus series discussion.

Hoping I didn't offend.



No problem,


Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr


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jan.de.jong(at)xs4all.nl
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PostPosted: Thu Mar 02, 2006 5:31 am    Post subject: 914 fuel pumps Reply with quote

For those not sick of the subject (as I am getting).

Series vs. parallel - a public effort at some quantification from Rotax
published data.

Page 50 of the 914 Installation Manual has a pressure-flow graph showing
a maximum flow rate of 120 l/h at zero pressure and a maximum pressure
of 1850 hPa at zero flow rate. The graph seems to be a redrawn copy of
the one in the Pierburg brochure for the E1F, pn 7.21440.78.0, with
stated rating of 95 l/h at 1 bar - thank you Gilles.
Whether the pump acts more like a "pressure source" (pressure little
dependent on flow) or "flow source" (flow little dependent on pressure)
depends on where it operates on this line. At low flow rates it acts
more like a "pressure source" at high flow rates more like a "flow source".

Estimating the maximum pressure that a pump must at least supply.

Page 9-5 of the 914 Operating Manual shows that sealevel airbox pressure
at 100% throttle is 1220 hPa, the boost above ISA pressure being 207 hPa.
Pages 10-1 and 10-2 of the 914 Operating Manual show that sealevel
manifold pressure at 100% throttle (some 40 hPa lower than the airbox
pressure) is maintained to an altitude of 4500 m where ISA pressure is
577 hPa, requiring an additional boost pressure of 477 hPa. Maximum
boost over ambient is then 207 + 477 = 684 hPa.
Page 9-5 of the 914 Operating Manual shows that sealevel airbox pressure
at 115% throttle is 1370 hPa, the boost above ISA pressure being 357 hPa.
Pages 10-1 and 10-2 of the 914 Operating Manual show that sealevel
manifold pressure at 115% throttle (some 50 hPa lower than the airbox
pressure) is maintained to an altitude of 2450 m where ISA pressure is
752 hPa, requiring an additional boost pressure of 261 hPa. Maximum
boost over ambient is 357 + 261 = 618 hPa.
It seems safe to assume a maximum boost pressure - airbox over ambient -
of 700 hPa.

Page 49 of the 914 Installation Manual specifies a maximum pressure loss
in the return line of 100 hPa.
Maybe the maximum pressure loss in the forward line (filters etc.) may
be put at 150 hPa.
Page 10-2 of the 914 Operating Manual states that fuel pressure should
be at least 150 hPa above airbox pressure (Installation Manual says
manifold pressure).
Not taken into account: hydrostatic difference and any ram tank pressure.

Pressure regulator assumptions:
It operates by adjusting its input pressure.
It adjust a restriction in the return line to set an operating point
(pressure, flow) for the pump(s).
The pressure drop from input to carburettor output lines is negligeable.

The maximum pressure a pump must be able to at least supply is then 700
+ 100 + 150 + 150 = 1100 hPa.
According to the pressure-flow graph it does this when regulated to a
flow rate of 90 l/h.
Even at the maximum fuel consumption of 36 l/h that makes for a 60%
return to the tank.
This is very close to the official Pierburg rating of 95 l/h at 1000 hPa.
The estimated pressure being the maximum required it follows that a
single pump operates to the left of the 90 l/h and 1100 hPa coordinate,
where it works much as a "flow source": flow little dependent on pressure.

It is interesting to see in the pressure-flow graph of page 50 of the
914 Installation Manual what are the effects of putting two pumps in
series and in parallel.

Series:
Say the one-pump operating point is 1100 hPa / 90 l/h.
Now double the graph in the horizontal direction - each flow rate
generating twice the pressure.
If the pressure regulator does nothing the line through the origin and
1100 hPa / 90 l/h finds a new combined operating point at about 1320 hPa
/ 109 l/h.
As the regulator regulates back to see 1100 hPa the combined operating
point then becomes about 1100 hPa / 111 l/h.
Note: the selected rightmost operating point is the worst case for the
series connection.

Parallel:
Say the one-pump operating point is 1100 hPa / 90 l/h.
Now double the graph in the vertical direction - each pressure
generating twice the flow rate.
If the pressure regulator does nothing the line through the origin and
1100 hPa / 90 l/h finds a new combined operating point at about 1350 hPa
/ 116 l/h.
As the regulator regulates back to 1100 hPa the combined operating point
then becomes about 1100 hPa / 180 l/h.
Note: the selected rightmost operating point is the best case for the
parallel connection.

Conclusion:
In the parallel setup the second pump causes a large extra flow to get
rid of. Switching it on and off causes larger pressure and flow changes
for the regulator to cope with.
In the series setup the main effect is that the second pump relieves the
first from half its load. Switching causes smaller changes.
The series setup actually saves some electric power too. Because the 2
pumps share the pressure between them, they each operate at 550 hPa in
the example (if identical - in practice one will do most of the work,
the other only a little). In the parallel case each pump supplies the
full 1100 hPa.
The graph on page 82 in the 914 Installation Manual (likely also redrawn
from the Pierburg text) suggests a (maximum) savings of about 1 A for
both pumps together.

Cheers,
Jan de Jong


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rholder(at)avnet.co.uk
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PostPosted: Thu Mar 02, 2006 7:56 am    Post subject: 914 fuel pumps Reply with quote

Jan de Jong wrote:

a whole bunch of stuff about series and parallel pumps.

But everyone is getting confused here. The parallel/series
discussion is not about flow or pressure, it is about
redundancy.

Thus the MOST important thing is that in the event of any
one problem, fuel can still flow to the engine. Both pumps
are only used together for take-off and landing, so for
most of the flight it is the single pump that does all the
work.

The problems can be a pump failure, a pump blockage, a
non-return valve blockage, a pipe blockage, a t-piece
blockage, a filter blockage (and others !). The set-up
should be such that any one "problem' as defined above can
be avoided (bypassed) by the use of the different options.

There is no guarantee that you will get to your
destination if (say) a in-line filter blocks up, but you
will have power.

Ideally any by-product of the initial problem (debris ...)
will not affect the alternative fuel supply.

Now the 914 seems to use two pumps with impellers of a
type that cannot have fuel sucked or blown through them
unless the pump is running. This is not true of the
mechanical pump or the electric pump used on a 912/S.

So (for the 914) a parallel system does not need
non-return valves except as a belt, braces and piece of
string option; as one pump running cannot recirculate
through the other.

On the other hand a series (for the pumps) must have a
pipe bypassing each pump in case it fails, and these two
pipes need a non-return valve in them.
So parallel is :

Tank--ILF--- -- pump --
selector---tpc tpc --- engine
Tank--ILF--- -- pump --
So series is :

T---ILF--- -- NRV -- -- NRV --
sel -- tpc tpc ---tpc tpc
-- eng
T---ILF--- -- pump -- -- pump -
But that diagram may not come out how I want ! In all
cases Y-pieces are better than T-pieces.

Parallel has less components, and I am a great believer in
the KISS principle. Even with the NRVs the parallel has
less components.

These components are very reliable (they are approved for
aviation) so it is really best not to go too far into the
what-if solutions as most of the compound problems will
never occur. Or might even occur because of additional
complexity.

KISS Smile

Richard ducking head beneath parapet !


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jan.de.jong(at)xs4all.nl
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PostPosted: Fri Mar 03, 2006 5:56 pm    Post subject: 914 fuel pumps Reply with quote

Hallo Richard,

Series:
Bypass valve per pump, more parts, and debris from one could get into
the other.
Total flow ranges between 90 l/h (24 gal/h) and 120 l/h (32 gal/h) - 1
to 2 pumps.

Parallel:
Return line, including regulator return line valve, must guarantee the
passage of 240 l/h (63 gal/h) without dropping more than 350 hPa (5 psi)
- otherwise carburetor overflow.
Total flow ranges between 90 l/h (24 gal/h) and 240 l/h (63 gal/h) - 1
to 2 pumps.

Your choice of chances.

Jan de Jong


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Gilles.Thesee(at)ac-greno
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PostPosted: Sat Mar 04, 2006 6:19 am    Post subject: 914 fuel pumps Reply with quote

Jan de Jong a crit :

Quote:
Parallel:
Return line, including regulator return line valve, must guarantee the
passage of 240 l/h (63 gal/h) without dropping more than 350 hPa (5 psi)
- otherwise carburetor overflow.
Total flow ranges between 90 l/h (24 gal/h) and 240 l/h (63 gal/h) - 1
to 2 pumps.


Jan and all,


I'd be interested in learning more about your numbers. They seem far
greater than ours.

In our airplane I actually flowed about 53 L/h on the ground with each
of our standard Rotax 914 pumps. I did not bother to measure with both
pumps, as the goal was to ensure we were exceeding the 125 % max fuel
consumption limit, before first flight.

No fuel problem whatsoever in one year flying.

But I'll check shortly the flow with both pumps. All I have to do is
disconnect the line at the fuel pressure regulator connection.
Another check I'll conduct is run only one of the FF pickups to compare
with physical measurements, and get a ball park idea of the return flow.
My opinion is that with two pumps flowing 53 L/h in parallel, you'll
definitely flow less than twice that value.

I'd welcom any actual measurements, and would be glad to publish them.

Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr


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jan.de.jong(at)xs4all.nl
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PostPosted: Sat Mar 04, 2006 5:04 pm    Post subject: 914 fuel pumps Reply with quote

Hello Gilles,

Very interesting.
My numbers are from the pump data published by Pierburg (on your site)
and Rotax.
The pump is supposed to deliver a flow of 120 l/h when output pressure
equals input pressure.
At what I deduced would be the maximum deliverable pressure boost (1,1
bar) it should still be 90 l/h.

Even if there where a serious restriction between your tank and your
pump (1,0 bar theoretical max. draw down) 53 l/h could not be made to
fit the published graph (the graph calls for a pressure boost of 1,5 bar
at 53 l/h).
Don't know what to think. It seems incredible that actual performance
could be less than half the published performance.

B.t.w. My 240 l/h is a little high indeed: exact total flow at 0,35 bar
is maybe 2 x 116 l/h (graph), subtract minimum flow to engine, maybe 5
l/h, result 227 l/h. Same ballpark. But only if the published data can
be believed ... And if my mental model is correct ...

I'm very interested in the results of your further experiments.

And I hope you get more flow results from people.

Regards,
Jan de Jong


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jodel(at)nildram.co.uk
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PostPosted: Sun Mar 05, 2006 7:53 am    Post subject: 914 fuel pumps Reply with quote

Further to all the discussion on parallel v series, I put fuel in my
aircraft for the first time today and found an interesting "effect" that I
had not expected.

I have a 914 plumbed the latest series way (Nov 2004 issue Cool.

With only pump 1 running the fuel flows through "filter 1" which is fairly
obviously the only way from the diagram. With pump 2 running, it pulls the
fuel through "filter 2" in preference to pulling it through filter 1 and
pump 1. However, with both pumps running all the flow is through filter
one. I had sort of expected pump 2 to still draw through filter 2 but this
is not the case. It would seem that filter 2 will only be used if pump 1
and/or filter 1 fail in some way. Has everyone else found the same or is
anyone getting a different flow pattern with the series setup?

Simon


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PostPosted: Sun Mar 05, 2006 9:48 am    Post subject: 914 fuel pumps Reply with quote

Simon Smith a crit :

Quote:

Further to all the discussion on parallel v series, I put fuel in my
aircraft for the first time today and found an interesting "effect" that I
had not expected.

I have a 914 plumbed the latest series way (Nov 2004 issue Cool.

With only pump 1 running the fuel flows through "filter 1" which is fairly
obviously the only way from the diagram. With pump 2 running, it pulls the
fuel through "filter 2" in preference to pulling it through filter 1 and
pump 1. However, with both pumps running all the flow is through filter
one. I had sort of expected pump 2 to still draw through filter 2 but this
is not the case. It would seem that filter 2 will only be used if pump 1
and/or filter 1 fail in some way. Has everyone else found the same or is
anyone getting a different flow pattern with the series setup?


Simon and all,


Not too easy to figure out your fuel system, if you're not a Europa
builder Wink
From what you're telling us, I believe you've plumbed per the Rotax
"series" diagram. What I'm not sure of is, where is your "filter 2" ?
The "series" system draws fuel from only one point, so why a "filter 2" ?

What you need is a finger mesh strainer at each tank pickup point, a
drainable gascolator upstream of the pump assembly, and a drainable sump
at the lowest point of the fuel tank.

Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr


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PostPosted: Sun Mar 05, 2006 10:43 am    Post subject: 914 fuel pumps Reply with quote

-
Quote:
Not too easy to figure out your fuel system, if you're not a Europa
builder Wink

Sorry!

See http://www.europa-aircraft.biz/pdfs/914xs.pdf

Chapter 5

Simon


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PostPosted: Sun Mar 05, 2006 10:47 am    Post subject: 914 fuel pumps Reply with quote

Hi Jan and all,

Quote:

Very interesting.
My numbers are from the pump data published by Pierburg (on your site)
and Rotax.
[...]
Don't know what to think. It seems incredible that actual performance
could be less than half the published performance.

[...]

Let's do a small "thought experiment" : if we have an adjustable valve

at the end of the fuel line, couldn't we adjust fuel flow by screwing in
or out the valve knob ?

Quote:

I'm very interested in the results of your further experiments.

And I hope you get more flow results from people.


I'm in the process of crafting a new "fuel pressure regulator" page. I'm

willing to publish results from builders experiments. Measuring flow and
pressures is easier when the airplane is not finished.

Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr


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PostPosted: Sun Mar 05, 2006 11:10 am    Post subject: 914 fuel pumps Reply with quote

Simon Smith a crit :

Quote:

See http://www.europa-aircraft.biz/pdfs/914xs.pdf



Simon,


Thank you for the link. Now I get it !

The behavior of your system is quite normal, considering the way it is
set up.

Normal operation (pump 1 is assumed to be primary pump)
Pump 1 draws fuel through filter 1, then pushes it through check valve 2.

Takeoff/landing : both pumps on
Pump 1 draws fuel through filter 1, then pump 2 adds pressure to the
already pressurize fuel downstream of pump 1.
Pressure downstream of check valve 1 is the same as downstream of pump
1, so the fuel has no reason to flow through check valve 1 and filter 2.

Emergency : pump 1 unserviceable
Pump 2 draws fuel via check valve 1 and filter 2.

This seems a bizarre setup, where filter 2 is only there for a ride,
except in the rare event of a primary pump failure. But there is nothing
wrong with it, provided you have a drainable sump in the tank, and a
drainable gascolator aupstream of the pumps.
By the way, the fuel pickup points appear to be at the front tank
bulkhead, whereas they would be much more wisely located at the REAR of
the tank, for the sake of nose-high fuel supply.

Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr


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jan.de.jong(at)xs4all.nl
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PostPosted: Mon Mar 06, 2006 5:06 pm    Post subject: 914 fuel pumps Reply with quote

Hi Gilles,

Quote:
Let's do a small "thought experiment" : if we have an adjustable valve
at the end of the fuel line, couldn't we adjust fuel flow by screwing in
or out the valve knob ?

I don't know where you want to go with this, but a restriction after the

pump would certainly decrease the generated flow.
The combination Pierburg pump plus restriction would constitute a new
pump with a graph of similar shape as the original.
A lower maximum flow at zero delivered pressure, the same maximum
pressure (1850 hPa) at zero delivered flow.

A restriction that lowers maximum flow (0 pressure) from 120 l/h to 53
l/h will not explain the huge difference between actual and published
though.
Such a restriction drops 1500 hPa at 53 l/h (operating point of the
published pump) or 28.3 hPa per l/h
At 39 l/h the published pump delivers about 1650 hPa, the restriction
then subtracts 39 * 28.3 = 1104 hPa.
Leaves you with 550 hPa at take-off fuel flow. The fuel regulator needs
up to twice that. Again - according to published figures.

Still don't know what to think.

Regards,
Jan de Jong


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