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Fuel pump tests

 
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Gilles.Thesee(at)ac-greno
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PostPosted: Wed Mar 14, 2007 11:10 am    Post subject: Fuel pump tests Reply with quote

Hi all,

Just conducted some test on our Rotax 914 fuel system, just to learn more.
The idea was running a brand new electrical pump with clear plastic
tubing from different points of the circuit, and observing what was
going on.
We noted that slugs of bubbles appear on the suction side of the pump
when it is connected downstream of the fuel valve (large pressure drop).
On the pressure side of the pump, very few bubbles appear.

My interpretation is that when the pump draws fuel from a restricted
portion of the circuit such as the fuel valve and all the fittings and
elbows, the most volatile fraction of the fuel tends to vaporise, and
then condenses when under pressure downstream of the pump.
I believe this is a normal phenomenon, but it was the first time I
observed it through clear hoses.
Has anyone already seen such a phenomenon ? Any comments ?

Some info on our engine fuel system here :
http://contrails.free.fr/engine_regul_en.php
Thanks,
Regards,
Gilles Thesee
http://contrails.free.fr


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frank.hinde(at)hp.com
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PostPosted: Wed Mar 14, 2007 1:23 pm    Post subject: Fuel pump tests Reply with quote

Your right that is exactly what is happening. It also gets worse as the
temperature of the fuel increases, namely thru a hot engine driven
pump...I gets worse further with altitude and the harder you suck on the
fuel. When it gets really bad its called vapour lock and your engine
quits. It will also happen more with autofuel than with 100LL due to the
higher vapour pressure of the mogas. Low wing airplanes (with wing tanks
and no header tank are much more susceptable because the engine driven
pump is sucking uphill.

This coindiently may happen at the worse possible time i.e a hot Summer
day at a high altitude airport where your fuel has been heat
soaked....You run full power, the engine gets real hot and Here come the
trees!

So, what to do?....Mount a low pressure electric pump as close to the
outlet of tha tank as possible with no pressure drop on the inlet side
of the pump.

My Zenair Zodiac (and my current Fuel injected RV 7) did not have an
engine driven pump, just a Facet fuel pump in each wing root. Thus the
pump always pushed on the fuel, never sucked. The pumps were (are) wired
to independent circuits such that if a pump failed the other pump will
take over.

Lycomings typically have a low level booster pump to augment the engine
driven pump and that's with 100LL.

Frank
RV7a no engine driven pump, mainly cus one day I want to use Mogas in
it....but only below about 15,000feet.

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Gilles.Thesee(at)ac-greno
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PostPosted: Thu Mar 15, 2007 12:22 pm    Post subject: Fuel pump tests Reply with quote

Hinde, Frank George (Corvallis) a écrit :
Quote:


Your right that is exactly what is happening. It also gets worse as the
temperature of the fuel increases, namely thru a hot engine driven
pump...I gets worse further with altitude and the harder you suck on the
fuel. When it gets really bad its called vapour lock and your engine
quits. It will also happen more with autofuel than with 100LL due to the
higher vapour pressure of the mogas. Low wing airplanes (with wing tanks
and no header tank are much more susceptable because the engine driven
pump is sucking uphill.

This coindiently may happen at the worse possible time i.e a hot Summer
day at a high altitude airport where your fuel has been heat
soaked....You run full power, the engine gets real hot and Here come the
trees!

So, what to do?....Mount a low pressure electric pump as close to the
outlet of tha tank as possible with no pressure drop on the inlet side
of the pump.


Frank,

Thank you for responding.
Actually we have wing tanks and our pumps are on the firewall, some
inches higher than wing root level. To date we've reached FL 145 in
winter on Avgas, and recently flew at FL 135 on Mogas without any problems.
I also flew at 37°C OAT (only up to 6000 ft).
But you're right on the pumps in the wing roots in a perfect circuit.

Best regards,
Gilles
http://contrails.free.fr


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Float Flyr



Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

PostPosted: Thu Mar 15, 2007 1:57 pm    Post subject: Fuel pump tests Reply with quote

I use a siphon to defuel my plane after every flight. I made the siphon
specifically out of clear fuel line and a squeeze bulb the squeeze bulb is
considerably below the highest point on the siphon but it always seems to
find a bubble or two while the fuel is flowing. If I stop the fuel flow by
plugging the out flow the bubbles completely disappear to re appear again as
soon as the fuel starts to flow again. Go figure.

Noel

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frank.hinde(at)hp.com
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PostPosted: Fri Mar 16, 2007 6:41 am    Post subject: Fuel pump tests Reply with quote

But didn't you say you saw bubbles with the clear hoses? This implies the onset of fuel boiling on the suction side of the pump. I guess my concern is if you saw bubbles on the ground then these bubbles are likely to be larger at the conditions I described. In other words my concern is you might be getting a little close to a true vapour lock situation.

Cheers

Frank

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Jess
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PostPosted: Fri Mar 16, 2007 6:51 am    Post subject: Fuel pump tests Reply with quote

Dear List:
I have just gotten in on the tail end of this problem, but we have been working with people to come to a solution. We found the fuel selector valve on low wing planes to be leaking air around the shaft but not leaking fuel. IF it were a high wing with a lot of head pressure fuel would seep out. But on low wing planes fuel does not seep out but air in. We built a manometer to prove the theory. We changed the .06 cent "O"ring around the shaft then put a drop of engine oil to make sure it did not pull in. We experienced engine roughness at high power settings returning to normal at lower power settings. If the aux fuel pump were turned on, it made the condition worse. In early 2006 we went back 5 years in the NTSB files and found 99 instances where the engine quit then would run when it was removed from the wreckage. Fuel injected engines are worse due to the high suction. It equates to putting a hole in a straw and trying to drink. This was found on the "Imperial" type valve. This valve was and is used in most GA and experimetal craft.
Jess

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frank.hinde(at)hp.com
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PostPosted: Fri Mar 16, 2007 7:16 am    Post subject: Fuel pump tests Reply with quote

Excuse my sceptisism but it seems you are now relying on a 25cent O ring... and a single point of failure to boot.

Personally i would rather redesign the system to be "hydraulically correct" i.e no sucking anywhere in the system as a fuel leak is a minor problem that has good warning qualities (stink). And air leak has almost no warning qualities until your engine stumbles or quits.

Of course if you have big lines and very low pressure drop through the valve on the inlet side to the pump the problem is much reduced on a suck type system, but if you have electric pumps anyway then its worth putting them in the correct place if space permits.

As you say, this has resulted in wreckage!

Frank

From: owner-engines-list-server(at)matronics.com [mailto:owner-engines-list-server(at)matronics.com] On Behalf Of BELTEDAIR(at)aol.com
Sent: Friday, March 16, 2007 7:51 AM
To: engines-list(at)matronics.com
Subject: Re: Engines-List: Fuel pump tests

Dear List:
I have just gotten in on the tail end of this problem, but we have been working with people to come to a solution. We found the fuel selector valve on low wing planes to be leaking air around the shaft but not leaking fuel. IF it were a high wing with a lot of head pressure fuel would seep out. But on low wing planes fuel does not seep out but air in. We built a manometer to prove the theory. We changed the .06 cent "O"ring around the shaft then put a drop of engine oil to make sure it did not pull in. We experienced engine roughness at high power settings returning to normal at lower power settings. If the aux fuel pump were turned on, it made the condition worse. In early 2006 we went back 5 years in the NTSB files and found 99 instances where the engine quit then would run when it was removed from the wreckage. Fuel injected engines are worse due to the high suction. It equates to putting a hole in a straw and trying to drink. This was found on the "Imperial" type valve. This valve was and is used in most GA and experimetal craft.
Jess

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href="http://www.matronics.com/Navigator?Engines-List">http://www.matronics.com/Navigator?Engines-List
href="http://forums.matronics.com">http://forums.matronics.com

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n801bh(at)netzero.com
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PostPosted: Fri Mar 16, 2007 1:06 pm    Post subject: Fuel pump tests Reply with quote

Jess makes a good redrive unit and lives where is is" hot" in the summer, in fact it is in the 90's down there now. YUCK. Jess,I am assuming you are running mogas in your V-6 Chevy.If so have you ever had a vapor lock condition in the extreme heat of Las Vegas? I made a test of mogas on my beast last year and it hiccuped at 13,000 and 45f degrees. That got my attention but I safely made it back on the ground.I am not sure the fuel was the problem though because I made some other changes at the same time trying to get the oil cooler to shed more heat and probably elevated the under cowl temp too much which heated up the fuel lines going to the carb. Plane goes up to 17,999 msl perfectly on 100LL. I would like to hear stories from people on past experiences of runnig mogasand vapor lock issues.
thanks in advance.
do not archive
Ben Haas
N801BH
www.haaspowerair.com

-- BELTEDAIR(at)aol.com wrote:
Dear List:
I have just gotten in on the tail end of this problem, but we have been working with people to come to a solution. We found the fuel selector valve on low wing planes to be leaking air around the shaft but not leaking fuel. IF it were a high wing with a lot of head pressure fuel would seep out. But on low wing planes fuel does not seep out but air in. We built a manometer to prove the theory. We changed the .06 cent "O"ring around the shaft then put a drop of engine oil to make sure it did not pull in. We experienced engine roughness at high power settings returning to normal at lower power settings. If the aux fuel pump were turned on, it made the condition worse. In early 2006 we went back 5 years in the NTSB files and found 99 instances where the engine quit then would run when it was removed from the wreckage. Fuel injected engines are worse due to the high suction. It equates to putting a hole in a straw and trying to drink. This was found on the "Imperial" type valve. This valve was and is used in most GA and experimetal craft.
Jess

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esmith6(at)satx.rr.com
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PostPosted: Fri Mar 16, 2007 4:33 pm    Post subject: Fuel pump tests Reply with quote

OK, I hate to mention this because I haven't tried it myself...I used to use mogas in my RV-4(Lyc. 0320/160) and lots of folks at OSH do the same, now I only use 100LL in the summer, S. Tx is just too hot, plus the additives in mogas scare me, and as you know some companies are using 10% ethanol, and don't count on the pump to be marked as such.

In a discussion about vapor-lock and etc., this one builder showed me what he did to prevent V/L...He sprayed the fuel pump(mechanical), fuel lines, and gascolator with a thick coating of SPRAY INSULATION FOAM all the way to the fire-wall...He did a neat job of applying the goop...I assume he used some form of shielding while spraying because there was no overspray anywhere else...I don't know if he sprayed the boost pump or not, he didn't mention it or show it to me...I asked about working on the units, and he said no problem, you use a knife and cut the foam away so you can get a wrench on the nuts and bolts or a screwdriver on the screws...When you get through, you simply squirt it some more with foam.

Some of you experts out there must have an opinion on this or maybe some sort of experience in this area.

I don't advocate this for anyone out there, and I repeat, I have never used this foam or a insulated wrap........Good Luck........................CHEERS!!!!................................Gene Smith, 1994 RV-4.








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