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John Hauck
Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)
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 Posted: Sat May 13, 2006 8:08 am Post subject: Redundancy.. I am not gonna repeat myself ! but I am gonna |
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| Another reason for plumbing your redundant fuel system with two
pumps in parallel - if one pump shoots craps in a series installation,
the remaining pump has to pull fuel throught the dead pump (and check
valves). Don't be stubborn about this guys, just because you did it
wrong and haven't had a problem yet. That's probably because you
haven't had either pump fail yet. This is an issue that has been
throughly debated over the years in aviation circles. The designers
put them in parallel unless the electric pump at the fuel tank is
remotely located at a distance from the engine pump.
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| Think about it as related to a two stroke powered Kolb. You have a
pulse powered engine pump that is rated to lift fuel no more than 39
inches. You have fuel tanks that are at least 30 inches lower than
the engine pump. Fail the electric pump in a series installation, and
you are asking the engine pump already operating near the design limit
to pull fuel through the failed pump and its two internal check
valves. You are better off to forget the electric pump entirely than
to plumb them in series.
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| If this logic is lost on you, read the Rotax installation manual.
It says to plumb the pumps in parallel.
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| Dave Bigelow
| Kamuela, Hawaii
| FS2, Rotax 503 DCDI
Dave/Ralph/Gang:
I plumbed my two strokes (Ultrastar had two mikunis/FS had Facet and
mikuni) in series, as I do my 4 stroke.
Lost the "no name" electric pump on the FS, and the little Mikuni kept
on pumping.
Was my procedure to shut down the electric pump after reaching
altitude. Little Mikuni pulled fuel through the dead electric pump
for 755 hours without a burp or slurp.
Same, same the 4 stroke, pushing 2,500 hours.
It works, and it works well.
If for some reason the Facet sticks one of its two check valves in the
closed position, I will experience fuel starvation. Chances of the
engine driven pump sticking one of its two check valves is nearly
impossible on the four stroke, and impossible on the two stroke pump
(valves are fabric on the Mikuni).
Really don't see that I am doing it wrong. I am doing it my way.
There are other problems that can and have put me down, like a plugged
fuel filter. Simply sampling fuel after taking on fuel would have
prevented two forced landings on my part. Pilot responsiblity, and I
was totally wrong both occasions.
You can plumb your airplane anyway you want to. It will be fine with
me. I will not tell you, you are wrong for doing so.
Adding additional check valves increases the amount of resistance the
remaining operational fuel pump must work through should one fail.
The Rotax Installation Manual for the 912ULS calls for a return fuel
line to the fuel tank. It is not required by practical experience,
even though the book says to use one.
Fuel line leaks??? Possible if you don't check clamps and physical
condition of your fuel hoses. Clear plastic fuel lines are known not
to have much longevity and fail quite often. Urethane fuel line
usually fails on the pulse line from engine to pump. Pays to reach up
there and pull on the clear line, even when it looks good. Might be
surprised when it pulls off in your hand. Went to Gates heavy duty
neoprene fuel line many years ago. Haven't had any problems, and it
lasts for many, many hours.
Again, I am not telling anyone else to do it the way I do. Simply
sharing the way I do it, one way that works well for me.
Still trying not to complicate the system. 
john h
mkIII 2,468.1 hours
912ULS 1,122.2 hours
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_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama |
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Denny Rowe
Joined: 10 Jan 2006
Posts: 89
Location: Leechburg, PA
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| Posted: Sat May 13, 2006 9:17 am Post subject: Redundancy.. I am not gonna repeat myself ! but I am gonna |
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Low pressure check valve?
See McMasterCarr.com
I got the one I put on my Mk-3 from them and it was very reasonable.
Make sure you get one with the Viton seal .
Denny rowe
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Dave Bigelow
Joined: 29 Jan 2006
Posts: 103
Location: Kamuela, Hawaii
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| Posted: Sat May 13, 2006 8:04 pm Post subject: Re: Redundancy.. I am not gonna repeat myself ! but I am gon |
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John,
I'm not trying to tell anyone what to do - only share what I know to be an accepted aviation practice. I'm glad you had a really good Mikuni that pulled fuel succesfully through a dead electric pump for many hours. Most of the time those little pumps do great.
This list is only a very small little piece of the aviation world. There's a lot of lessons that have been learned the hard way over the years, and that's the reason for "accepted practices". The sharing of that information is one of the primary reasons this list exists. Untimately, each builder and operator can sift throught the info here and plumb his fuel system any way he wants.
From my Rotax Installation manual on page 15-1:
"If the fuel tank is considerably lower than the engine, an electric pump should be used. The pump is to be connected in parallel...."
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_________________
Dave Bigelow
Kamuela, Hawaii
FS2, HKS 700E |
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