Today's Message Index:
----------------------
1. 10:58 AM - CH 701/912S Fuel System Questions (Walt Cannon)
2. 11:29 AM - Final inspection scheduled for Dec 17th (Rick Pitcher)
3. 03:33 PM - Re: Final inspection scheduled for Dec 17th (Al Young)
4. 04:53 PM - Re: CH 701/912S Fuel System Questions (Thomas F Marson)
5. 06:29 PM - Re: CH 701/912S Fuel System Questions (Phil & Michele Miller)
6. 07:29 PM - Re: CH 701/912S Fuel System Questions (Thomas F Marson)
7. 08:05 PM - Re: CH 701/912S Fuel System Questions (Rico Voss)
8. 08:14 PM - Re: CH 701/912S Fuel System Questions (Thomas F Marson)
9. 09:56 PM - Re: CH 701/912S Fuel System Questions (Dave Alberti)
Message 1
| INDEX | Back to Main INDEX |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | CH 701/912S Fuel System Questions |
--> Zenith-List message posted by: "Walt Cannon" <grnlake@earthlink.net>
All,
I have recently contacted some of you directly with questions on this
topic...the responses just made me want to open the issue up to wider input.
I am nearing the completion of my 701/912S (without the fuselage tank) and
still have some degree of unease about the redundancy and failure mode of
the fuel supply system. I contacted Zenith to understand their philosophy
and basically got the following:
They do not have an auxiliary electric boost pump shown on their plans or
installed on their demonstrator airplane.
They did do a fuel flow test from wing tanks to the engine and were
convinced that the flow was enough, but apparently didn't actually verify
that the engine would run given the head pressure from the tanks. When I
raise the nose to a climb attitude, it looks like the gravity feed head is
maybe only 12 inches or so.
They didn't seem to know if the 912 carbs required a certain feed pressure
to operate.
So here are my questions for this group:
Have you run fuel flow tests from the gravity feed wing tanks to the engine
in a climb attitude and was their enough flow?
Has anybody actually tried to test the gravity flow by bypassing the engine
driven pump and going straight to the carbs?
Would the engine operate that way if the mechanical pump failed? Would it
operate in a climb attitude?
Does anyone have any information about failure modes on the mechanical
pumps? When they do fail, do they fail open so a boost pump in series could
push fuel through, or does the pump need to be in parallel?
I would be glad to hear of specific solutions that people may have come up
with. I understand from anecdotal information that I have received that the
engine driven pumps are quite reliable. Never the less, a backup system
seems to be fairly standard in Type Certified airplanes that won't run on
strictly a gravity feed system.
Thanks in advance,
Walt Cannon
Seattle, WA
Message 2
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Final inspection scheduled for Dec 17th |
--> Zenith-List message posted by: Rick Pitcher <rick.pitcher@verizon.net>
Well, I finished building the Zodiac. It's sitting out behind the barn
with the wings on and the gas tanks filled. Weighed in at 650 pounds
empty... not much more than my old Quicksilver trainer :)
The Jabiru starts so easy and runs so smooth... SWEET!!!
Tomorrow it goes out to my hangar space at WJF for reassembly and final
inspection. The EAA guys are slated for Saturday, Dec 13th, and the DAR
is gonna have a look at it too. I'll have 4 days to work back the
squawks and take care of any stray paperwork, then the DAR comes back
Wednesday look it over one last time and sign the airwothiness certificate.
It'll be cool if I can get Decemeber 17th as the date of birth on that
Centennial Dataplate
http://www.eaa.org/homebuilders/centennial.asp
Fingers Crossed,
Rick P.
Message 3
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Final inspection scheduled for Dec 17th |
--> Zenith-List message posted by: "Al Young" <armyret@one-eleven.net>
Congrats Rick- Hope you make it on the 17th, but hope also that you have a
safe first flight, no matter if it doesn't come til Feb 17th. Get ready,
get confident, and get going! Best of luck! Al Young 601XL (60% done)
Message 4
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: CH 701/912S Fuel System Questions |
--> Zenith-List message posted by: "Thomas F Marson" <tmarson@pressenter.com>
Walt,
I can see your nervous about that fuel situation. Here is what I did on a
Dakota Hawk, High wing with a Continental engine and gravity flow fuel
feeding a carb.
FUEL FLOW TESTS:
I parked the plane in an attitude of 22 degrees nose up to measure fuel
flow by gravity with only 5 gallons of fuel in the cowl tank. Cowl has
head with 5 gallons of fuel and 22degrees up (plane stalls at 16 degrees)
of only 10 inches. disconnect fuel line at carb and measured fuel flow of
more than 150 percent of full throttle demand. Actual flow was rate was 14
gallons perhour. I have all 3/8 aluminum aero fuel line and AN 6 fittings.
This passed normal requirements of 150 percent of maximum engine demand.
Next to be sure of what would happen in real world. I parked the plane in
the taildown position and had the tail tied to my tractor and put only 2
gallons of fuel in the fuel tank and had the boost pumps off. (we will talk
about boost pumps later). With the boost pumps off I ran the engine at full
throttle for a timed two minutes. It ran fine and developed full power
static. I then shut the engine off and let it heat soak for 10 minutes.
Next I restarted it and again ran it full throttle for 90 seconds, again
boost pumps off. In both cases the engine ran flawlessly. This proved it
would run with only 2 gallons of fuel in a steep climb, this is very low
head of maybe 12 inches. It was 87 octane auto fuel, and a summer day of
about 80 degrees. This told me that in the worst case of a missed approach
and a go around in a steep angle of climb with very low fuel the engine
would function normally.
Now about boost pumps: As you probably have heard the majority of forced
landings in homebuilt aircraft are from faults in the fuel delivery system.
Auto Fuel vaporizes more easily than 100LL and has a bad history of vapor
lock in aircraft applications but av gas can do it too. When it gets hot as
in the engine compartment especially with a hot engine and an engine mounted
fuel pump at low flow such as idling on a hot day it may vaporize and the
pump will not pump vapor. . One of he systems used to reduce this situation
is to plumb in a by pass system so unused fuel (that not needed by the
engine at the moment) is piped back to the fuel tank. This will insure a
continuous flow of cooler fuel in the lines and pump so in effect the hot
pump is partially cooled by a inflow cool fuel. . That is the system
recommended for RV aircraft where fuel is pumped up from the wing tanks.
(low wing plane). In the RVs there is also a 12 volt boost pump installed at
the root of the wing tank. The main source of pumping here is the engine
mounted pump ( the one subject to getting hot).
Even here with the boost pump in the wing root it is recommended to make the
bypass system discussed above.
NOW BACK TO MY DAKOTA HAWK WITH THE GRAVITY FEED SYSTEM:
For a cost of 32 dollars each I purchase 2 Facet 12 volt pumps and installed
them in parallel with each other with their output combined and then on to
the gascolater and then to the engine. These pumps each have a built in
check valve so fuel can only flow in one direction. Why two in parallel?
I tested to find that a single pump would pass enough fuel by gravity (not
operating) to run my engine at full power. Remember I have these fuel pumps
installed in a gravity flow system). I installed two so that in the rare
case where the internal check valve might stick closed and block flow the
remaining pump would have to also have the check valve fail closed at the
same time for my engine not to operate.
I also installed individual panel switches and a full pressure gage so I can
see when they are pumping.
WHY ALL THIS EXTRA STUFF IN A GRAVITY FEED SYSTEM, CUBS AND CHAMPS MOST
CESSNAS DONT HAVE THIS??
Two reasons: These planes were never designed to use auto
fuel--------- though many do use it with an STC. And the nature of auto fuel
is such that vapor lock is more likely than aviation gasoline. Secondly
they all have a greater head (distance of fuel above the carb than my
plane).
What did all this cost? Not counting the head scratching time and testing
time it added about 6 lbs and 150 dollars of cost, pumps, fuel pressure
gage and circuit breakers and just a little more fuel tubing and fittings.
I added a lot of redundancy to my planes fuel system. Was it worth it--
maybe not--------- if I never need it. But it may some day be worth my
plane or maybe my life. The only real drawback was the added weight. The
redundancy I believe is more than adequate, as neither pump needs to operate
for my engine to work perfectly and both electric pumps must have their
check valves stick and stick closed at the same time for fuel starvation to
occur. These are very common pumps (you can buy them in NAPA or other auto
parts store. ) have no history of sticking check valves. In 7 years of
operation neither of my have failed to operate and of course never stick
closed. I switch the pumps on for take off and landings.
Oh I failed to mention, during taxi testing with very low levels of fuel in
the tank (before I installed the pumps) on a moderately rough field the low
levels of fuel unported the tank occasionally and a little air would enter
the fuel line to the engine. It would quit. Then it would take 10 minutes
before I could get engine to start again. This was in warm summer and the
engine compartment was warm too. It was auto fuel. I was never completely
sure it was only unporting or if it was vapor lock. In a sense it may have
been one in the same. A bubble of air in a gravity system with very little
head above it and the engine would not start again ten minutes. This
repeated on 3 or four different days. I did note that later when I had more
fuel in the tank it never happened. I continued more taxi testing on the
same field several more days before first flight.
Bottom line of all of this is that I bought redundancy and confidence with
the dual boost pumps. When I switch them on and see the gage indicate 6 psi
I have assurance of pressure at the carb.
Tom Marson
----- Original Message -----
From: "Walt Cannon" <grnlake@earthlink.net>
Subject: Zenith-List: CH 701/912S Fuel System Questions
> --> Zenith-List message posted by: "Walt Cannon" <grnlake@earthlink.net>
>
> All,
>
> I have recently contacted some of you directly with questions on this
> topic...the responses just made me want to open the issue up to wider
input.
> I am nearing the completion of my 701/912S (without the fuselage tank) and
> still have some degree of unease about the redundancy and failure mode of
> the fuel supply system. I contacted Zenith to understand their philosophy
> and basically got the following:
>
> They do not have an auxiliary electric boost pump shown on their plans or
> installed on their demonstrator airplane.
> They did do a fuel flow test from wing tanks to the engine and were
> convinced that the flow was enough, but apparently didn't actually verify
> that the engine would run given the head pressure from the tanks. When I
> raise the nose to a climb attitude, it looks like the gravity feed head is
> maybe only 12 inches or so.
> They didn't seem to know if the 912 carbs required a certain feed pressure
> to operate.
>
> So here are my questions for this group:
>
> Have you run fuel flow tests from the gravity feed wing tanks to the
engine
> in a climb attitude and was their enough flow?
> Has anybody actually tried to test the gravity flow by bypassing the
engine
> driven pump and going straight to the carbs?
> Would the engine operate that way if the mechanical pump failed? Would it
> operate in a climb attitude?
> Does anyone have any information about failure modes on the mechanical
> pumps? When they do fail, do they fail open so a boost pump in series
could
> push fuel through, or does the pump need to be in parallel?
> I would be glad to hear of specific solutions that people may have come up
> with. I understand from anecdotal information that I have received that
the
> engine driven pumps are quite reliable. Never the less, a backup system
> seems to be fairly standard in Type Certified airplanes that won't run on
> strictly a gravity feed system.
>
> Thanks in advance,
>
> Walt Cannon
> Seattle, WA
>
>
Message 5
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | CH 701/912S Fuel System Questions |
--> Zenith-List message posted by: "Phil & Michele Miller" <millerpg@ps.gen.nz>
Walt,
This is all good advice from Tom. I have a word of caution and a couple
of comments:
Take care when selecting your boost pump. The Rotax manual recommends a
normal operating fuel pressure range of 2.2 to 4.4psi and a maximum of
5.8psi. Any more and you run the risk of flooding and engine stoppage
due to fuel pushing past the carby float valves.
Tom is right in saying that auto gas is more likely to vapour lock than
avgas. Vapour locking will also occur sooner if the pressure is lowered
as is the case when the engine pump sucks fuel up from a low wing fuel
tank. Less likely with a high wing tank due to the pressure head from
the tank. Heat protection sleeving for under-cowl fuel lines is a wise
precaution.
The bottom of the 701 wing tank is about 14 inches above the carbies in
level flight. At 40 degrees nose up (Rotax maximum recommended) the fuel
head reduces to about 10.6 inches. At 10.6 inches the unassisted
pressure at the carbies is roughly 2.7psi which is a still within the
normal operating pressure range. In level flight the unassisted fuel
pressure will only be about 3.6psi. Note that these numbers are only
approximate. You should do your own measurements and calculations.
Failure modes for the Rotax fuel pump are stated in the manual as
leaking valves or leaking diaphragm. A leaking diaphragm will cause fuel
to leak out of the fuel pump housing at the vent hole and into the
engine bay. This is not good and a boost pump will cause more fuel to be
expelled. Leaking valves will cause partial or complete loss of pump
pressure. I don't know how much pressure is required to bypass an
inactive pump (I can see another test coming up on this one).
Cheers,
Phil Miller
New Zealand
701/912S
>>I can see your nervous about that fuel situation. Here is what I did
on a Dakota Hawk, High wing with a Continental engine and gravity flow
fuel feeding a carb.
>>For a cost of 32 dollars each I purchase 2 Facet 12 volt pumps and
installed them in parallel with each other with their output combined
and then on to the gascolater and then to the engine.
>>When I switch them on and see the gage indicate 6 psi I have assurance
of pressure at the carb.
Tom Marson
----- Original Message -----
From: "Walt Cannon" <grnlake@earthlink.net>
Subject: Zenith-List: CH 701/912S Fuel System Questions
> --> Zenith-List message posted by: "Walt Cannon"
> --> <grnlake@earthlink.net>
>
> All,
>
> I have recently contacted some of you directly with questions on this
> topic...the responses just made me want to open the issue up to wider
input.
> I am nearing the completion of my 701/912S (without the fuselage tank)
> and still have some degree of unease about the redundancy and failure
> mode of the fuel supply system. I contacted Zenith to understand their
> philosophy and basically got the following:
>
> They do not have an auxiliary electric boost pump shown on their plans
> or installed on their demonstrator airplane. They did do a fuel flow
> test from wing tanks to the engine and were convinced that the flow
> was enough, but apparently didn't actually verify that the engine
> would run given the head pressure from the tanks. When I raise the
> nose to a climb attitude, it looks like the gravity feed head is maybe
> only 12 inches or so. They didn't seem to know if the 912 carbs
> required a certain feed pressure to operate.
>
> So here are my questions for this group:
>
> Have you run fuel flow tests from the gravity feed wing tanks to the
engine
> in a climb attitude and was their enough flow?
> Has anybody actually tried to test the gravity flow by bypassing the
engine
> driven pump and going straight to the carbs?
> Would the engine operate that way if the mechanical pump failed? Would
> it operate in a climb attitude? Does anyone have any information about
> failure modes on the mechanical pumps? When they do fail, do they fail
> open so a boost pump in series
could
> push fuel through, or does the pump need to be in parallel?
> I would be glad to hear of specific solutions that people may have
> come up with. I understand from anecdotal information that I have
> received that
the
> engine driven pumps are quite reliable. Never the less, a backup
> system seems to be fairly standard in Type Certified airplanes that
> won't run on strictly a gravity feed system.
>
> Thanks in advance,
>
> Walt Cannon
> Seattle, WA
>
>
=
==
direct advertising on the Matronics Forums.
==
==
==
Message 6
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: CH 701/912S Fuel System Questions |
--> Zenith-List message posted by: "Thomas F Marson" <tmarson@pressenter.com>
Hi Phil, thanks for the back up. I agree with most of your points except
fuel pressure VS/head.
I seem to remember 27 inches of water colum is 1 psi since gasoline is
lighte than water 27 inches of water would be even less than 1 psi. Where
am I wrong on this? Tom
----- Original Message -----
From: "Phil & Michele Miller" <millerpg@ps.gen.nz>
Subject: RE: Zenith-List: CH 701/912S Fuel System Questions
> --> Zenith-List message posted by: "Phil & Michele Miller"
<millerpg@ps.gen.nz>
>
> Walt,
>
> This is all good advice from Tom. I have a word of caution and a couple
> of comments:
>
> Take care when selecting your boost pump. The Rotax manual recommends a
> normal operating fuel pressure range of 2.2 to 4.4psi and a maximum of
> 5.8psi. Any more and you run the risk of flooding and engine stoppage
> due to fuel pushing past the carby float valves.
>
> Tom is right in saying that auto gas is more likely to vapour lock than
> avgas. Vapour locking will also occur sooner if the pressure is lowered
> as is the case when the engine pump sucks fuel up from a low wing fuel
> tank. Less likely with a high wing tank due to the pressure head from
> the tank. Heat protection sleeving for under-cowl fuel lines is a wise
> precaution.
>
> The bottom of the 701 wing tank is about 14 inches above the carbies in
> level flight. At 40 degrees nose up (Rotax maximum recommended) the fuel
> head reduces to about 10.6 inches. At 10.6 inches the unassisted
> pressure at the carbies is roughly 2.7psi which is a still within the
> normal operating pressure range. In level flight the unassisted fuel
> pressure will only be about 3.6psi. Note that these numbers are only
> approximate. You should do your own measurements and calculations.
>
> Failure modes for the Rotax fuel pump are stated in the manual as
> leaking valves or leaking diaphragm. A leaking diaphragm will cause fuel
> to leak out of the fuel pump housing at the vent hole and into the
> engine bay. This is not good and a boost pump will cause more fuel to be
> expelled. Leaking valves will cause partial or complete loss of pump
> pressure. I don't know how much pressure is required to bypass an
> inactive pump (I can see another test coming up on this one).
>
> Cheers,
> Phil Miller
> New Zealand
> 701/912S
>
> >>I can see your nervous about that fuel situation. Here is what I did
> on a Dakota Hawk, High wing with a Continental engine and gravity flow
> fuel feeding a carb.
>
> >>For a cost of 32 dollars each I purchase 2 Facet 12 volt pumps and
> installed them in parallel with each other with their output combined
> and then on to the gascolater and then to the engine.
>
> >>When I switch them on and see the gage indicate 6 psi I have assurance
> of pressure at the carb.
>
> Tom Marson
>
>
> ----- Original Message -----
> From: "Walt Cannon" <grnlake@earthlink.net>
> To: <zenith-list@matronics.com>
> Subject: Zenith-List: CH 701/912S Fuel System Questions
>
>
> > --> Zenith-List message posted by: "Walt Cannon"
> > --> <grnlake@earthlink.net>
> >
> > All,
> >
> > I have recently contacted some of you directly with questions on this
> > topic...the responses just made me want to open the issue up to wider
> input.
> > I am nearing the completion of my 701/912S (without the fuselage tank)
>
> > and still have some degree of unease about the redundancy and failure
> > mode of the fuel supply system. I contacted Zenith to understand their
>
> > philosophy and basically got the following:
> >
> > They do not have an auxiliary electric boost pump shown on their plans
>
> > or installed on their demonstrator airplane. They did do a fuel flow
> > test from wing tanks to the engine and were convinced that the flow
> > was enough, but apparently didn't actually verify that the engine
> > would run given the head pressure from the tanks. When I raise the
> > nose to a climb attitude, it looks like the gravity feed head is maybe
>
> > only 12 inches or so. They didn't seem to know if the 912 carbs
> > required a certain feed pressure to operate.
> >
> > So here are my questions for this group:
> >
> > Have you run fuel flow tests from the gravity feed wing tanks to the
> engine
> > in a climb attitude and was their enough flow?
> > Has anybody actually tried to test the gravity flow by bypassing the
> engine
> > driven pump and going straight to the carbs?
> > Would the engine operate that way if the mechanical pump failed? Would
>
> > it operate in a climb attitude? Does anyone have any information about
>
> > failure modes on the mechanical pumps? When they do fail, do they fail
>
> > open so a boost pump in series
> could
> > push fuel through, or does the pump need to be in parallel?
> > I would be glad to hear of specific solutions that people may have
> > come up with. I understand from anecdotal information that I have
> > received that
> the
> > engine driven pumps are quite reliable. Never the less, a backup
> > system seems to be fairly standard in Type Certified airplanes that
> > won't run on strictly a gravity feed system.
> >
> > Thanks in advance,
> >
> > Walt Cannon
> > Seattle, WA
> >
> >
>
>
> > ==
> direct advertising on the Matronics Forums.
> ==
> ==
> ==
>
>
Message 7
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: CH 701/912S Fuel System Questions |
--> Zenith-List message posted by: Rico Voss <vozzen@yahoo.com>
> I seem to remember 27 inches of water colum is 1 psi
> since gasoline is
> lighte than water 27 inches of water would be even
> less than 1 psi. Where
> am I wrong on this? Tom
Good info, Phil, but I think Tom is right. Divers use
the fact that one atmosphere (14.7 psi) occurs at
water depth of 33 feet... or roughly 1/2 psi per foot
of water.
Would be interesting to see whether the 1/2 psi of
head pressure would be enough to keep the engine
running.
Rico, 601XL, fuse, in KC
do not archive
__________________________________
http://companion.yahoo.com/
Message 8
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: CH 701/912S Fuel System Questions |
--> Zenith-List message posted by: "Thomas F Marson" <tmarson@pressenter.com>
Fuel pressure /vs/ head of gasoline. I went to Machineries Handbook,
twentieth edition. Page 2316 "one foot head of water will be 0.43 psi."
Gasoline is only 72 percent the weight of water so that would mean,
12 inches head of gasoline will develop 0.309 psi.
What I have experienced in the past is that carburetors that are stated to
need X amount of minimum pressure, will operate with much less at least for
Stromberg and Marvel Schebler. But the facts seem to be that it will take
about a little over 36 inches to develop 1 psi. Of course if you have an
airpressure tube putting blast air into your fuel tank that will raise the
pressure of what ever column you have by the amount the tank is pressurized.
You can buy these Facet pumps in several pressure ranges if you desire. I
think as low as 2 psi and as high as 8 or more psi.
----- Original Message -----
From: "Phil & Michele Miller" <millerpg@ps.gen.nz>
e care when selecting your boost pump. The Rotax manual recommends a
> normal operating fuel pressure range of 2.2 to 4.4psi and a maximum of
> 5.8psi. Any more and you run the risk of flooding and engine stoppage
> due to fuel pushing past the carby float valves.
>
>
Message 9
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | CH 701/912S Fuel System Questions |
--> Zenith-List message posted by: "Dave Alberti" <daberti@execpc.com>
A system that requires no more than 3 psi (for example) will be unable to
keep the proper fuel level in the float bowl at pressures above 3 psi. It
will over fill and essentially flood because the float system will be
overwhelmed. Although you may not notice the condition at wide open throttle
except for excessive fuel consumption. What you need is fuel flow; excess
capacity. This is governed by the smallest orifice in the lines from the
tank to the carbs. Example...3/8" lines to the fuel pump, with a 1/4" inside
diameter fitting at the fuel pump. Calculate or actually test the fuel flow
at that small fitting and make sure you get the 150% flow in gal or liters
per hour above the max consumption rate for the engine. A fuel pump will
assist this and increase the flow rate but only if gravity can deliver
enough fuel to the inlet side of the pump. What a static fuel pressure
reading will give you is an indication of the systems integrity (leaks etc.)
post fuel pump. If you have a system that is totally based on gravity feed
to the carb (no pumps at all) fuel pressure is useless, because as indicated
in earlier posts, you need multiple feet of head tube length to get a
practical measurement of pressure. A sealed fuel tank system can use
dynamic ram air pressure (like pitot) to build additional pressure in the
system, but this is airspeed and air density dependant.
And as a follow up to the electric pump as a backup to the Rotax mechanical
pump, yes you can pump through the mechanical pump. I do it on every
initial engine start to charge the system with the meager 3 psi. If the
mechanical pump dies so will your engine for sure. Don't count on gravity
pushing the fuel through the dead pump. My backup electric pump is located
below the tank level but above the gascolator on the gascolator output side.
Dave
-----Original Message-----
From: owner-zenith-list-server@matronics.com
[mailto:owner-zenith-list-server@matronics.com]On Behalf Of Thomas F
Marson
Subject: Re: Zenith-List: CH 701/912S Fuel System Questions
--> Zenith-List message posted by: "Thomas F Marson"
<tmarson@pressenter.com>
Fuel pressure /vs/ head of gasoline. I went to Machineries Handbook,
twentieth edition. Page 2316 "one foot head of water will be 0.43 psi."
Gasoline is only 72 percent the weight of water so that would mean,
12 inches head of gasoline will develop 0.309 psi.
What I have experienced in the past is that carburetors that are stated to
need X amount of minimum pressure, will operate with much less at least for
Stromberg and Marvel Schebler. But the facts seem to be that it will take
about a little over 36 inches to develop 1 psi. Of course if you have an
airpressure tube putting blast air into your fuel tank that will raise the
pressure of what ever column you have by the amount the tank is pressurized.
Other Matronics Email List Services
These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.
-- Please support this service by making your Contribution today! --
|