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facet pump vapor lock follow up question

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airvair601(at)yahoo.com
Guest

Posted: Wed Apr 08, 2009 4:21 pm Post subject: facet pump vapor lock follow up question

Thanks to all who responded to my original post.  But let me be more specific in my 

concerns and situation.  First we have a corvair and an MA3 carb from a Cessna 150. 

Second our concern is vapor lock in the fuel system from the tank to the carb.



For instance, in an UN coordinated turn - like in an emergency situation when you are 

NOT coordinated in a properly banked turn, the pump will be asked to draw fuel up-hill 

from the lower wing tank;  such as in a barrel roll like maneuver or unusual attitude

 situation.   Pumps at the centerline of the aircraft must draw fuel against the vertical 

distance to the tank, AND against he centrifugal force pushing the fuel toward the 

wingtip. 



In an emergency, who knows what  attitude the aircraft will be  in.  My question is   . . .  Is 

there a danger in trying to suck fuel all the way to the firewall, through the selector valve, through the  gascolator, and through a (potentially) dead fuel pump to get to a working 

pump located 12" away from the carb ??? Chris Heinz, has the pumps in the wings - he's

 the aircraft  designer.  However, the design also calls for a rotex engine, which I believe 

has a  mechanical pump at the engine to back up the single electrical pump in each wing.



From a simple physics standpoint, it seems like you should push the fuel.  However, in 

this situation sucking fuel will allow fuel pump redundancy with the same number of

 pumps, if the pumps suck rather than push fuel.  We also have the option of using 

electrical pumps in the wings to bring pressurized fuel through the cabin, to a valve,

 through the valve to the gascolator directly to the carb, or to  the  mechanical corvair 

fuel  pump in  parallel and then to the carb.  Obviously, the mechanical pump adds weight forward and has some fuel weeping issues.



Any thoughts on this?  Am I making a mountain out of a mole hill? 

 It is easier to address this now in the garage than later at the airport, or at altitude.

Thanks again.  Phill Hartig

         [quote][b]

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notsew_evets(at)frontiern
Guest

Posted: Wed Apr 08, 2009 5:49 pm Post subject: facet pump vapor lock follow up question

Never saw a Chris design showing the pump in the  wing..

 Maybe I missed it somewhere.

 The low wing Pipers that I ve owned never had a  pump in the wings either.

 Hummmmm.

 S.

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ashleyw(at)gvtc.com
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Posted: Wed Apr 08, 2009 5:59 pm Post subject: facet pump vapor lock follow up question

Phil,

  

 I believe you are making a mountain out of a mole  hill.

  

 Per the specs, they are self priming to 18 inches  so in theory they could be located above the fuel.

 I know on my first power up with no fuel in the  lines, they produced pressure in approx 2 seconds.

  

 If you are so un coordinated as you suggest, the  fuel could slosh to the outside of the tank and unport the  inlet. 

 In this case, no matter where the pump is, it will  not work.

  

 In the Piper Cherokee line, the pumps (electric and  engine driven), are forward of the fire wall and mounted higher  then 

 the electric ones on the 601.  I am quite  certain that if this was a problem, the FAA would not have certified  it.

  

 I know the pumps on the firewall have been working  just fine in my 601 but as you are the builder and would be happier

 with pressurized fuel in the cockpit, build it that  way.

  

 Floyd Wilkes

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bryanmmartin

Joined: 10 Jan 2006
Posts: 1018

Posted: Wed Apr 08, 2009 6:59 pm Post subject: facet pump vapor lock follow up question

 	  Quote: 	
		   For instance, in an UN coordinated turn - like in an emergency  

 situation when you are

 NOT coordinated in a properly banked turn, the pump will be asked to  

 draw fuel up-hill

 from the lower wing tank;  such as in a barrel roll like maneuver or  

 unusual attitude

  situation.   Pumps at the centerline of the aircraft must draw fuel  

 against the vertical

 distance to the tank, AND against he centrifugal force pushing the  

 fuel toward the

 wingtip.


	


In this case, the fuel will be pulled away from the tank outlet port  

and the pumps will be sucking air anyway.



Personally, I would put the pumps as low in the system as possible and  

avoid any significant restrictions in the lines on the suction side of  

the pumps. Any restriction to flow will cause a pressure drop in a  

flowing fluid and the lower the pressure, the lower the vaporization  

temperature of the fuel. I would put the pumps in parallel so you will  

never be sucking through a dead pump. Most Facet pumps allow so little  

back flow that separate check valves are not necessary for two pumps  

in parallel. The gascolator is not a significant restriction to flow  

so I wouldn't worry too much about putting pumps downstream of that. A  

filter, if used, should be placed downstream of the pumps because a  

partially clogged filter can significantly restrict fuel flow. Keep  

the fuel on the suction side of the pumps as cool as possible. Put  

firesleeve insulation on the fuel lines in the engine compartment to  

keep them cool. Another point to keep in mind: avgas is much less  

prone to vapor lock than auto gas. If you only plan on using avgas,  

you don't have to worry as much about vapor lock. Also, auto gas  

varies in composition from winter to summer. Auto gas sold in the  

winter is more easily vaporized than summer auto gas.

-- 

Bryan Martin

N61BM, CH 601 XL,

RAM Subaru, Stratus redrive.

do not archive.

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_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive.

paulrod36(at)msn.com
Guest

Posted: Thu Apr 09, 2009 12:06 pm Post subject: facet pump vapor lock follow up question

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  Sorry guys, I just HAVE to jump in on this endless thread. Can we have a  show of hands of those who have ever had vapor lock? And can we have a show of  hands of those who have ever had vapor lock in the air? Vapor lock requires the  following: Temperature at or near the boiling point of the liquid; Atmospheric  pressure a good bit lower than 29.92; And one other factor, which escapes me at  the moment. (my oldtimer's disease is kicking in.) In 51 years, I have heard of  it maybe 5 times, and every one of those was on the ground, when the engine,  already hot, sat and cooked on the ramp for a while. And, come to think of it,  they were fuel-injected engines, where the fuel about to go into the cylinders  was cooking on top of the engine.  Personally, I'd rather debate whether  the addition of external wire bracing,  or AN-15 bolts would keep the wings  on in a tornado, or a 10G pullout.    Sorry for the rant. We  return you to your program already in progress.

  

 Paul Rodriguez

 [quote]   ---

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bryanmmartin

Joined: 10 Jan 2006
Posts: 1018

Posted: Thu Apr 09, 2009 12:50 pm Post subject: facet pump vapor lock follow up question

Actually, all that is required for vapor lock to occur is for the  

temperature-pressure combination of the fuel near the intake of the  

pump to fall within the vapor region. It usually requires a high  

ambient temperature, but gasoline will vaporize at a much lower  

temperature than its usual boiling point if the pressure is low  

enough. A low pressure can occur when there is a significant  

restriction to the flow of fuel and the rate of flow is high. The  

restriction will cause a pressure drop in the flow downstream of that  

point.



I have had vapor lock occur in an old carbureted car on a hot day  

while climbing a steep hill. It is very rare in aircraft in flight  

because certificated aircraft fuel systems have been designed to  

prevent it and avgas has a much higher boiling point than auto gas.  

About the only time vapor lock is at all likely in an aircraft in  

flight is during a steep climb at full throttle on a hot day (i.e.  

right after takeoff). It usually takes a very bad fuel system design  

to have much risk of vapor lock.



On Apr 9, 2009, at 4:05 PM, <paulrod36(at)msn.com> <paulrod36(at)msn.com>  

wrote:



 	  Quote: 	
		   Sorry guys, I just HAVE to jump in on this endless thread. Can we  

 have a show of hands of those who have ever had vapor lock? And can  

 we have a show of hands of those who have ever had vapor lock in the  

 air? Vapor lock requires the following: Temperature at or near the  

 boiling point of the liquid; Atmospheric pressure a good bit lower  

 than 29.92; And one other factor, which escapes me at the moment.


	




-- 

Bryan Martin

N61BM, CH 601 XL,

RAM Subaru, Stratus redrive.

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_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive.

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