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rnewman(at)tcwtech.com
Guest

Posted: Sat Nov 10, 2012 5:33 pm Post subject: Problem with Fuel flow transducer in the tunnel

Fellow builders, 

  

 Some of you may be aware that Iâ€™ve seen some deviations in fuel pressure  and flow in cruise flight in my RV-10:   Iâ€™ve got to root cause.

  

 After the past 10 months of investigation and discussions and general head  scratching I finally have come to a conclusion.   

  

 Executive summary:  Donâ€™t mount the fuel flow sender in the tunnel per  the plans!  It has too much pressure drop and gets too warm in this  location, this can lead to vapor lock.    The onset seen as  deviations in fuel pressure and fuel flow.

  

  

 The full report can be downloaded at:

  

 http://tcwtech.com/Fuel%20flow%20write-up.pdf

  

  

 Bob Newman

 N541RV

  

   [quote][b]

ddddsp

Joined: 22 Sep 2012
Posts: 3

Posted: Sat Nov 10, 2012 8:52 pm Post subject: Problem with Fuel flow transducer in the tunnel

Bob, 

Are you running AVGAS or MOGAS? 

Dean  



____________________________________________________________

Woman is 53 But Looks 25

Mom reveals 1 simple wrinkle trick that has angered doctors...

ConsumerLifestyleMag.com [quote][b]

charliewaffles

Joined: 15 Mar 2011
Posts: 18

Posted: Sat Nov 10, 2012 9:01 pm Post subject: Re: Problem with Fuel flow transducer in the tunnel

Thanks for sharing your experience and the data. The charts and actual data are great to see. I don't doubt typos have fixed your problem, but I wonder if this is coincidental to the tunnel issue. Perhaps you can help me understand the following questions?



1) if moving the red cube out of the tunnel (amoung other things) resulted in no more problems, is your hyphothesis that the cube is heating up and causing vapor lock?



2) all of the other people that have posted about red cube issues in the tunnel have said the reporting of fuel consumption is high when the pump is on, but no one else has indicate the say pressure drop and flow increase relating to a cube in the tunnel. 



3) is it possible insulating the lines and even the cube in the tunnel would do just as good of a job? Is any where mounted to the engine or the firewall hotsidemreally going to be cooler than the tunnel?



4) if the red cube heating in the tunnel caused the vapor lock, wouldnt that still occur when it was on the engine and exposed to heat there?



5) it sounds like several things wee changed at once and undoubtedly fixed your problem.  How sure of you it was the red cube and not say a bend in a line, the routing or one of the other variables that were changed.



I've gone back and forth on moving the red cube during my build. Most of the data and issues so far with the tunnel have been reporting issues, not performance related. If the cube IS causing vapor lock (or almost) and LOP is affected, then that is a great reason to move it. I just wonder if we have enough info yet to say that. 



Just my initial thoughts and perhaps you are onto something here.

rleffler

Joined: 05 Nov 2006
Posts: 680

Posted: Sun Nov 11, 2012 12:09 am Post subject: Problem with Fuel flow transducer in the tunnel

Bob,

Thanks for documenting your findings!   Thankfully, I had already placed my red cube fwf.   

I'm disappointed in Van's traditional "not invented here" response.   Your empirical data, along with the many RV-10s with the cubes fwf, you would think Van's would listen and consider that there may be another way of doing things that may be better.

Bob



Sent from my iPad



On Nov 10, 2012, at 8:32 PM, "bob-tcw" <rnewman(at)tcwtech.com (rnewman(at)tcwtech.com)> wrote:

[quote]    Fellow builders, 

  

 Some of you may be aware that Iâ€™ve seen some deviations in fuel pressure  and flow in cruise flight in my RV-10:   Iâ€™ve got to root cause.

  

 After the past 10 months of investigation and discussions and general head  scratching I finally have come to a conclusion.   

  

 Executive summary:  Donâ€™t mount the fuel flow sender in the tunnel per  the plans!  It has too much pressure drop and gets too warm in this  location, this can lead to vapor lock.    The onset seen as  deviations in fuel pressure and fuel flow.

  

  

 The full report can be downloaded at:

  

 http://tcwtech.com/Fuel%20flow%20write-up.pdf

  

  

 Bob Newman

 N541RV

  

    	  Quote: 	
		  



===================================

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uilthelp.com
	


matronics.com/contribution

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[b]

_________________
Bob Leffler
N410BL - Phase I
http://mykitlog.com/rleffler

Tim Olson

Joined: 25 Jan 2007
Posts: 2872

Posted: Sun Nov 11, 2012 4:46 am Post subject: Problem with Fuel flow transducer in the tunnel

It may be that the red cube sensor is more picky than others. I've never had a complaint about it, but mine is a flo scan.  It seems the complaints all seem to come from those with the cube. So may e there is a difference.  I *think* the floor bracket holes were a perfect match for the flo scan, so that may be what they used and designed around. 

Tim



On Nov 11, 2012, at 2:08 AM, Bob Leffler <rv(at)thelefflers.com (rv(at)thelefflers.com)> wrote:



[quote]Bob,

Thanks for documenting your findings!   Thankfully, I had already placed my red cube fwf.   

I'm disappointed in Van's traditional "not invented here" response.   Your empirical data, along with the many RV-10s with the cubes fwf, you would think Van's would listen and consider that there may be another way of doing things that may be better.

Bob



Sent from my iPad



On Nov 10, 2012, at 8:32 PM, "bob-tcw" <rnewman(at)tcwtech.com (rnewman(at)tcwtech.com)> wrote:

 	  Quote: 	
		      Fellow builders, 

  

 Some of you may be aware that Iâ€™ve seen some deviations in fuel pressure  and flow in cruise flight in my RV-10:   Iâ€™ve got to root cause.

  

 After the past 10 months of investigation and discussions and general head  scratching I finally have come to a conclusion.   

  

 Executive summary:  Donâ€™t mount the fuel flow sender in the tunnel per  the plans!  It has too much pressure drop and gets too warm in this  location, this can lead to vapor lock.    The onset seen as  deviations in fuel pressure and fuel flow.

  

  

 The full report can be downloaded at:

  

 http://tcwtech.com/Fuel%20flow%20write-up.pdf

  

  

 Bob Newman

 N541RV

  

    	  Quote: 	
		  



===================================

ctric.com

 	  Quote: 	
		  www.buildersbooks.com

uilthelp.com
	


matronics.com/contribution

===================================

://www.matronics.com/Navigator?RV10-List

===================================

cs.com

===================================



	






===================================

ctric.com

 	  Quote: 	
		  www.buildersbooks.com

uilthelp.com
	


matronics.com/contribution

===================================

://www.matronics.com/Navigator?RV10-List

===================================

cs.com

===================================



	


[b]

jkreidler

Joined: 13 Feb 2008
Posts: 151
Location: Sheboygan Falls WI

Posted: Sun Nov 11, 2012 5:52 am Post subject: Re: Problem with Fuel flow transducer in the tunnel

Great post Bob - thanks for sharing.  One of the important factors to note about vapor lock is that the boiling point of fuel is considerably higher with higher pressure.  So the fuel before the mechanical pump is at a much lower pressure and therefore has a much lower boiling point.



This brings back memories of problems we used to have before moving our Flow Scan transducer to the location between the servo and spider as you did.  We also saw our fuel pressure drop on long high climbs, we added a cooling shroud to the mechanical fuel pump.  At some point we also moved the sensor, we no longer see fuel pressure drops.  I think it is fair to say that we may have experienced the same issue as you had.



We did not see this change 'fix' our LOP stumbles, but since we were not looking for this to effect our LOP issues I can not tell you if it made an improvement.  I still wonder if the fuel in the lines between the spider and the injectors are vaporizing at the very low flows and low pressures.



Bottom line, your data backs up your hypothesis, thanks for sharing.  There are a lot of aircraft out there with the transducer running between the servo and the spider.  The readings are more stable, and you have offered yet one more data point in support of moving the transducer out of the tunnel.  Vans seems to reason that they do not like the sensor up front since it is in a hotter, higher vibration area.  I wonder if either of these are actually fact, the tunnel is quite hot. When you directly attach a piece of aluminum to the hot bottom surface of the aircraft and then bolt the sensor tightly to that nice heat sink subjected to all of the high frequency vibrations of the airframe, I really don't know if it is any better off.  The sensor between the servo and spider is suspended in air mounted between two hoses which I think would do a good job of dampening vibrations.  I have NO data to say which location is higher in vibration or temperature.  I can only say that both locations are used today, and I do not see reports of these sensors failing in either location.



If you are still building you really need to ask yourself what the BENEFIT to mounting the sensor in the tunnel is.  I struggle to see any downside to mounting the sensor between the servo and spider, especially after seeing Bob's well done data driven report.  The issues are real, the solution is known, just my two cents worth - Jason

_________________
Jason Kreidler
4 Partner Build - Sheboygan Falls, WI
Tony Kolar, Kyle Hokel, Wayne Elser, Jason Kreidler
N44YH - Flying - #40617

bill.peyton

Joined: 19 Sep 2010
Posts: 198
Location: St. Louis, MO

Posted: Sun Nov 11, 2012 6:06 am Post subject: Re: Problem with Fuel flow transducer in the tunnel

Bob,

Thanks for the great post.  I had the similar issues with another (certified) aircraft when the flow transducer (floscan) was installed between the electric and mechanical fuel pump.  It is my belief that the transducers don't like to be on the suction side of the mechanical diaphragm pumps due to the pulsing of the pump.  

When we looked at the RV-10 plans it was clear to us that putting it on the output side of the mechanical pump was the correct way to go.  We also have it mounted between the servo and the spider and it is working just fine.

_________________
Bill
WA0SYV
Aviation Partners, LLC

rv10flyer(at)live.com
Guest

Posted: Sun Nov 11, 2012 6:36 am Post subject: Problem with Fuel flow transducer in the tunnel

I had the exact same thought, Bob is a smart guy so I take his hard work serious and have thought this through.

I have had a psi issue maybe 4 times no real consistency on when however in 110 hours of flying.



Here is my thought on moving

1 Dynon tells installers NOT to install cube FWF for the EMS install

2 Vans has theirs in the tunnel and told

me to not put the cube there

3 it isn't broke



The general and Tim both mentioned

It only is an issue with the fuel pump on and even then not Big issue



3.2 hour flight yesterday, added 48.2 gallons when I returned because I was empty my computer said I would need

48.6, it was dark and I don't think I filled it to the top. Regardless close enough.



I have no issues, not even with hot starts

If it isn't an issue why all the work to move it?



Good information however. Thanks Bob!



On Nov 10, 2012, at 9:01 PM, "charliewaffles" <mcooper(at)live.com> wrote:



 	  Quote: 	
		   

 

 Thanks for sharing your experience and the data. The charts and actual data are great to see. I don't doubt typos have fixed your problem, but I wonder if this is coincidental to the tunnel issue. Perhaps you can help me understand the following questions?

 

 1) if moving the red cube out of the tunnel (amoung other things) resulted in no more problems, is your hyphothesis that the cube is heating up and causing vapor lock?

 

 2) all of the other people that have posted about red cube issues in the tunnel have said the reporting of fuel consumption is high when the pump is on, but no one else has indicate the say pressure drop and flow increase relating to a cube in the tunnel. 

 

 3) is it possible insulating the lines and even the cube in the tunnel would do just as good of a job? Is any where mounted to the engine or the firewall hotsidemreally going to be cooler than the tunnel?

 

 4) if the red cube heating in the tunnel caused the vapor lock, wouldnt that still occur when it was on the engine and exposed to heat there?

 

 5) it sounds like several things wee changed at once and undoubtedly fixed your problem.  How sure of you it was the red cube and not say a bend in a line, the routing or one of the other variables that were changed.

 

 I've gone back and forth on moving the red cube during my build. Most of the data and issues so far with the tunnel have been reporting issues, not performance related. If the cube IS causing vapor lock (or almost) and LOP is affected, then that is a great reason to move it. I just wonder if we have enough info yet to say that. 

 

 Just my initial thoughts and perhaps you are onto something here.

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=387531#387531

rnewman(at)tcwtech.com
Guest

Posted: Sun Nov 11, 2012 6:38 am Post subject: Problem with Fuel flow transducer in the tunnel

I received a bunch of comments off-list as well as on-list.   I'll try to 

summarize here for everyone.



1) My tunnel has always been insulated (on the bottom) since day 1 and 

comparing to what others reported early on about hot tunnels I always 

thought mine was relatively cool since in any operating condition I could 

easily and comfortably keep my hand on it.  Also, my firewall is insulated 

on the engine side with koolmat silicone membrane.



2) I did change two things at once, I realize its not the absolutely 

scientific way to do an experiment, but given the work to get in and out of 

my tunnel, I had to do every thing I could in one shot.

    I moved the fuel cube and the I insulated the replacement line I 

installed from the electric fuel pump to the firewall bulkhead fitting.



3)  Yes I used to see elevated fuel flow readings with the electric fuel 

pump on just like everyone else with the fuel cube in the tunnel.  However, 

these high readings are totally an artifact of turbulence in the line caused 

by the fuel pump.  Switching the fuel pump on and off had no noticeable 

effect on egt, so the fuel flow was definitely NOT actually changing.



4)   The problem with the fuel cube is that of all the fittings in the 

system it has the most significant pressure drop for a given flow rate.    I 

did dig into this extensively,  the electric fuel pump in the OFF state and 

fuel cube each have a similar pressure drop, followed by the pressure drop 

associated with each elbow and each length of tubing.     Each of these 

pressures drops moves the fuel closer to its vapor pressure level.



Here's the engineering details:  Avgas has a maximum reid vapor pressure of 

49kPa (at) 38 C  or about 7.1 psi absolute pressure,   a standard day at sea 

level has an absolute pressure of 14.7 psi,   as we know, as we go up in 

altitude and/or up in temp the absolute pressure falls.   When the absolute 

pressure meets the vapor pressure, the fuel begins to vaporize and bad 

things happen to fuel flow.    So know if we go up to 11,000 ft and we raise 

the temp of the tunnel to 40 C which is nice comfortable temperature as 

detected by putting one's hand on the tunnel,   we would have effectively a 

density altitude of 16,050 ft inside the tunnel as far as the fuel would 

know.   So now converting the 16,050 ft to absolute pressure we get 7.9 psi. 

Therefore, to avoid vapor lock issues the total pressure drop at our flow 

rate of about 13 gph better be less than 0.8 psi.      The fuel cube and the 

pump each are in the 0.15 to 0.25 psi range depending on flow rates and how 

you look at the charts I could dig up.  So subtracting those two pressure 

drops from the absolute pressure we are down to pressure margin of maybe 0.3 

or 0.4 psi for all other fittings, tubing and the selector valve.



So clearly the best two things to do are to insulate everything possible 

from additional heating and remove any pressure drops you can from the 

suction side of the fuel system.



The benefit to moving the fuel  cube to the pressure side of the fuel system 

is now the  0.25 psi drop is subtracted off of the gauge and absolute 

pressure,  so at 26 psi gauge for the same conditions above we would be at 

33.9 psi absolute minus the 0.25 psi drop,  which is completely 

inconsequential.



If I missed any of the comments, please tee them up again.



Bob Newman

N541RV

bill.peyton

Joined: 19 Sep 2010
Posts: 198
Location: St. Louis, MO

Posted: Sun Nov 11, 2012 6:57 am Post subject: Re: Problem with Fuel flow transducer in the tunnel

Bob,

In your calculation wouldn't you have to include the flow rate of the fuel and the OAT and the heat absorption of the fuel.  Obviously the heat absorption would be different at various flow rates and would be at it maximum amount at 0 GPH.  This would be more indicative of the fuel temp. in the lines during flight.  While the tunnel may be at 40C the fuel will be at a much lower temp while it is flowing and should provide a more significant margin, assuming it is not 40C outside and the fuel has hot soaked to 40C!

I too insulated my firewall and the floor of the tunnel with fibrefrax and fireproof black 1/2 foam on top of it.  My tunnel is cool to the touch during flight with the heat off.  I have not used the heat yet and will see how it behaves with the heat valve open.  I also firesleeved the steel fuel line out of the pump

_________________
Bill
WA0SYV
Aviation Partners, LLC

Kelly McMullen

Joined: 16 Apr 2008
Posts: 1188
Location: Sun Lakes AZ

Posted: Sun Nov 11, 2012 7:17 am Post subject: Problem with Fuel flow transducer in the tunnel

I agree that the sending unit NEEDS to be downstream of both fuel pumps.

I think it is instructive to look at EI's install instructions for their fuel flow instrument, because they are the supplier of the red cube, that is STC approved on most engines. Either EI or Flowscan can be the manufacturer, but the install instructions are the same.

 Note that for ALL engines with fuel injection that do NOT have a return line to the fuel tank(that would not include purge lines), they specify locating the sending unit between the mechanical fuel pump and the fuel servo. 

 The only time they specify between the fuel servo and the spider is for injected engines with a fuel return line. I don't believe anyone is using such a system on the IO-540. Van's location is directly contrary to EI's STC approved instructions. Such a deviation would not be approved on a certified aircraft.

 http://www.buy-ei.com/Information/II%20-%20FP-5.pdf



On Sun, Nov 11, 2012 at 7:06 AM, bill.peyton <peyton.b(at)sbcglobal.net (peyton.b(at)sbcglobal.net)> wrote:

 [quote]--> RV10-List message posted by: "bill.peyton" <peyton.b(at)sbcglobal.net (peyton.b(at)sbcglobal.net)>

  

 Bob,

  

 When we looked at the RV-10 plans it was clear to us that putting it on the output side of the mechanical pump was the correct way to go.  We also have it mounted between the servo and the spider and it is working just fine.

  

 --------

 Bill

 WA0SYV

 Aviation Partners, LLC

 

 

 

 

  

  

  

 [b]

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor
KCHD

Kelly McMullen

Joined: 16 Apr 2008
Posts: 1188
Location: Sun Lakes AZ

Posted: Sun Nov 11, 2012 7:27 am Post subject: Problem with Fuel flow transducer in the tunnel

I just check my current Dynon Skyview install manual. It actually says location of the fuel sending unit, which is the EI FT-60 (they no longer supply the floscan.) They say the location is up to the builder, and that it is common for it to be between the electric and mechanical pumps. Their only recommendation with regard to the firewall is for the EMS unit, not the fuel flow sending unit. I agree with those that have concern as to the response of the sending unit to suction of the mechanical pump.

 Given that EI is the manufacturer, I would say best performance will be obtained following their guidance to install between mechanical pump and fuel servo.



On Sun, Nov 11, 2012 at 7:35 AM, Pascal <rv10flyer(at)live.com (rv10flyer(at)live.com)> wrote:

 [quote]--> RV10-List message posted by: Pascal <rv10flyer(at)live.com (rv10flyer(at)live.com)>

 

  

 

 Here is my thought on moving

 1 Dynon tells installers NOT to install cube FWF for the EMS install

 2 Vans has theirs in the tunnel and told

 me to not put the cube there

 3 it isn't broke

 

 The general and Tim both mentioned

 It only is an issue with the fuel pump on and even then not Big issue

 

 3.2 hour flight yesterday, added 48.2 gallons when I returned because I was empty my computer said I would need

 48.6, it was dark and I don't think I filled it to the top. Regardless close enough.

 

 I have no issues, not even with hot starts

 If it isn't an issue why all the work to move it? 

[b]

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor
KCHD

drfred1960(at)gmail.com
Guest

Posted: Sun Nov 11, 2012 11:37 am Post subject: Problem with Fuel flow transducer in the tunnel

Bob,



Thanks for your excellent write up.  I have a very similar set up and have occasionally noted the fluctuations in fuel flow just as you have described.  I also, have not been able to run LOP secondary to the engine running too rough.  I don't feel that I have a "hot tunnel" either.  Does anybody know if the flow is an optical pick up or is there a rotor sensing the fuel flow?



I just started my condition inspection and am seriously thinking about moving the transducer.  Might as well do it before I have a real problem in flight or on take off.  



Did you use "hard lines" or did you go with the flex lines and fire sleeve them in the tunnel?

Thanks



Fred Williams 

500 hrs and 4 years.



Sent from my iPad

jesse(at)saintaviation.co
Guest

Posted: Sun Nov 11, 2012 1:36 pm Post subject: Problem with Fuel flow transducer in the tunnel

It has a rotor. 



Jesse Saint

I-TEC, Inc.

jesse(at)itecusa.org

www.itecusa.org

www.mavericklsa.com

C: 352-427-0285

O: 352-465-4545

F: 815-377-3694



Sent from my iPhone



On Nov 11, 2012, at 2:36 PM, Fred Williams <drfred1960(at)gmail.com> wrote:



 	  Quote: 	
		   

 

 Bob,

 

 Thanks for your excellent write up.  I have a very similar set up and have occasionally noted the fluctuations in fuel flow just as you have described.  I also, have not been able to run LOP secondary to the engine running too rough.  I don't feel that I have a "hot tunnel" either.  Does anybody know if the flow is an optical pick up or is there a rotor sensing the fuel flow?

 

 I just started my condition inspection and am seriously thinking about moving the transducer.  Might as well do it before I have a real problem in flight or on take off.  

 

 Did you use "hard lines" or did you go with the flex lines and fire sleeve them in the tunnel?

 

 

 Thanks

 

 Fred Williams 

 500 hrs and 4 years.

 

 Sent from my iPad

rnewman(at)tcwtech.com
Guest

Posted: Sun Nov 11, 2012 1:52 pm Post subject: Problem with Fuel flow transducer in the tunnel

Bill,  You are correct about the flow rate and OAT temp,  however,  there's 

a  lot of aluminum tubing, fittings, the pump and fuel cube all collecting 

the heat of the tunnel and delivering it to the fuel.  Without doing 

elaborate measurements, the answer lies somewhere between the OAT temp and 

the tunnel temp.   The issue I saw was definitely correlated with reduced 

fuel flow rates,  as soon as I increased the flow rate from 12 to about 13 

gph the issue went away.



Bob



--

rnewman(at)tcwtech.com
Guest

Posted: Sun Nov 11, 2012 1:55 pm Post subject: Problem with Fuel flow transducer in the tunnel

Fred,   The red fuel cube has a little turbine wheel that spins on some type 

of jeweled bearing,   I suspect that the turbine then acts on a hall effect 

sensor or an optical sensor.     The new fuel line I installed from the 

electric pump to the bulkhead fitting was a fire sleeved flex line.



Bob



--

billz

Joined: 08 Jul 2009
Posts: 26
Location: Clinton, New York

Posted: Sun Nov 11, 2012 3:34 pm Post subject: Re: Problem with Fuel flow transducer in the tunnel

Bob,

Thank you for the excellent analysis.



1. Bob, What did you use for insulation on the fuel lines in the tunnel??



2. Has anyone used insulation on the heat runs in the tunnel?  I'm thinking this adds a significant amount of heat (with uninsulated SCAT) when the heat is turned on.  If so, what type of insulation did you use??



FWIW, Every fuel flow sender installation I've seen in certified aircraft has had the sensor mounted ahead of the firewall and usually between the fuel servo and flow divider.



Great discussion.

rv10flyer

Joined: 25 Aug 2009
Posts: 364

Posted: Sun Nov 11, 2012 8:30 pm Post subject: Re: Problem with Fuel flow transducer in the tunnel

My red cube is in the tunnel and has worked great for 95 hours from -10 to 105F OAT's. I am within .5 gallons at every fuel up. I run boost pump until 1000' agl and have taken off with pump off flowing 25.5 gph. I occasionally see fuel pressure drop about 3-5 psi during a 115-120 kt climb to 10500-13500'. The engine did not show any signs of roughness. If I am running cabin heat my fuel temp is much lower and still provides plenty of margin. If I was building again I would intall it fwf, but will not change it as long as it is working properly.

_________________
Wayne G.

rnewman(at)tcwtech.com
Guest

Posted: Mon Nov 12, 2012 9:47 am Post subject: Problem with Fuel flow transducer in the tunnel

Bill,  I insulated the new line between the electric pump and firewall with 

standard fire sleeve.   The rest of the original lines remain un-insulated.



Bob



--

Bob Turner

Joined: 03 Jan 2009
Posts: 885
Location: Castro Valley, CA

Posted: Thu Nov 15, 2012 5:12 pm Post subject: Re: Problem with Fuel flow transducer in the tunnel

Bob,



I haven't thought this thru entirely, but I notice you have one other thing which is non-stock: filters at the wing root. Is it possible they are introducing an extra pressure drop (more than the standard filter in the tunnel) and this makes the problem at the cube in the tunnel worse?

_________________
Bob Turner
RV-10 QB

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