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  #11  
Old 09-04-2019, 11:03 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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The 3 company demonstrators have flown many hundreds of hrs with the flow transducer mounted in the kit documented location.

During kit development, stick on temp sensors were mounted on multiple sides of the flow sensor. During testing the indicated temps never got close to the manufacturers specified limit for the sensor.

The fuel flow has always been steady and accurate on the company demonstrators.

There are now 108+ RV-14's and 14A's flying. As far as I am aware, the majority of them likely have the flow sensor installed in the prescribed location and have not had any problems.

There are numerous factors that could be a variable from one airplane to another, that couldn't possibly be evaluated during the test program.

Some that come to mind are -
Engine vibration - This could be a factor regardless of the sensor location, if it is mounted on the engine.
Peak EGT - If a particular engine is for some reason operated at EGT's that are higher than typical, it could expose the sensor to temps that were beyond what was expected.
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Last edited by rvbuilder2002 : 09-04-2019 at 11:08 AM.
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  #12  
Old 09-04-2019, 01:27 PM
rapid_ascent rapid_ascent is offline
 
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From the pic it appears there is some corrosion on the wiring. Could the cause of the failure not be heat related?
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  #13  
Old 09-04-2019, 01:53 PM
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hgerhardt hgerhardt is offline
 
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Quote:
Originally Posted by rapid_ascent View Post
From the pic it appears there is some corrosion on the wiring. Could the cause of the failure not be heat related?
I agree it looks like corrosion. Is the airplane parked outdoors? Could rain/dew have leaked onto the cable, and with the cable exiting the cube vertically and to the rear, could that cable have become a conduit to allow water into the cube? You can also see some black residue on the top of the cube which could have been dust on the cable which got washed down on top of the cube. Heat alone would not have caused that black residue.

And, what's with the missing/broken bolt? Did they take the cube apart to try to troubleshoot it?

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  #14  
Old 09-05-2019, 07:55 AM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
The 3 company demonstrators have flown many hundreds of hrs with the flow transducer mounted in the kit documented location. SNIP....
I hear you Scott. But as I?ve posted I have pages of data that clearly show the per plans cube mounting position did not work on this RV-14A, and I would not be surprised if others have had issues but have not shared them with the good folks at Van?s.

My point - while the per plans mounting location seems to work for most builders, why not look at the option of mounting the cube on the engine mount with adel clamps as I recommended? What is the downside?

I don?t have a photo of the final RV-14 cube mount that fixed the problem, but here is a photo of the tentative mounting spot I picked for the new RV-8 project.

Carl
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  #15  
Old 09-05-2019, 08:22 AM
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rocketbob rocketbob is offline
 
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Keep in mind these units are very precisely made and because of that there are some tolerances that have to be maintained internally, particularly the distance between the jeweled bearings and the shaft the wheel rotates upon. Radiant heat will have an effect on overall expansion which affects how tight or loose these internal components are. Some may be manufactured on the tight side and work fine when exposed to elevated temperatures, and some may not.

The best way is to mount one of these IMO is directly at the flow divider, which avoids radiant heat exposure.
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  #16  
Old 09-05-2019, 08:40 AM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by rocketbob View Post
SNIP..
The best way is to mount one of these IMO is directly at the flow divider, which avoids radiant heat exposure.
Works well but recommend taking steps to avoid creating vibration fatigue points, such as a mounting plate attached to the engine. I note that some use a 90 degree fitting on the spider and then hang the cube to that. If the connecting lower hose is restrained and this is a steel fitting, I suspect all will be well. The only remaining item is the install instructions calling for no 90 degree fittings close to the cube - but I suspect this would be a minor issue.

On my first plane (IO-360 with vertical AFP injection) I installed the fuel flow sender in the line below the baffle cylinder plate, so there was straight line between the sender and both the spider and throttle body. I covered the sender with firesleeve and used adel clamps on the line as support. This has worked perfectly for 17 years. On the new RV-8 project (horizontal injection) I found the fuel line run between the spider and the throttle body not to support this set up, thus my choice to mount the sender on the engine mount. I briefly looked at the ?90 degree fitting on the spider? option and chose not to do that.

Carl
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  #17  
Old 09-05-2019, 09:07 AM
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One measurement is worth 1000 opinions.

Put a temp sensor on the side facing the pipe, to quantify radiant heating. Strap one to the back with some heat transfer compound and see the resulting cube temperature.

Allergic to data? Rather experiment? The heat shield could be improved a lot.
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  #18  
Old 09-05-2019, 09:31 AM
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Quote:
Originally Posted by Carl Froehlich View Post
Works well but recommend taking steps to avoid creating vibration fatigue points, such as a mounting plate attached to the engine. I note that some use a 90 degree fitting on the spider and then hang the cube to that.

Carl
Always use a bracket to bolt the cube to keep stresses low on fittings.

At the flow divider the vibration forces are going to be the lowest due to its close proximity to the centerline of the crankshaft. There's a reason why the flow divider is there to begin with.
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  #19  
Old 09-10-2019, 05:30 PM
jdmrv7a jdmrv7a is offline
 
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Just for the record my Red Cube failed at 90 hours. On my 14A, it is installed at the VANs recommended location and plans. My G3X lost all fuel flow, no reading after it failed.

I installed the replacement Red Cube with these minor modification. Using the same clamps and location, I added 4 inches of fire sleeve over both the IN & Out metal fuel line fittings at the cube. I think those fittings collect a lot of heat from the exhaust pipes. I also added 10 inches of fire sleeve over the cube wire closet to the cube. I fastened the insulated wire away from the cylinder, exhaust and block. I also added another Van's heat shield on the exhaust under the Red Cube. Only the Red Cube is exposed.

The fire sleeved fuel line connectors, insulated cube wire and two exhaust shields provide far more heat protection. I have not measured the temps in that location and I currently have 55 hours on the second Red Cube. The modified insulation shielding is working well.
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  #20  
Old 09-11-2019, 05:01 AM
jliltd jliltd is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
The only remaining item is the install instructions calling for no 90 degree fittings close to the cube - but I suspect this would be a minor issue.
Carl
Carl brings up an excellent point. I have spoken with EI about this very issue. 90 degree fittings actually will work fine but will probably cause a change in K factor. The factory uses straight inlet and outlet fittings when they set the initial K factor. So the units are usually very close to good accuracy for flight right out of the box. If the builder opts for 90 degree inlet and/or outlet fittings they will almost definitely have to adjust and optimize the K factor over a number of flights. No big deal just more iterations to get the flow accurate. That is a small price to pay compared to wrangling to use perfect straight lines within the tight spaces under a cowl. I always ratio the K factor over a few flights on every installation anyway. So I use whatever style of fittings that work best for the installation. Remeber the more cross country flights used to adjust the K factor, the better the precision of the sender.

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Last edited by jliltd : 09-11-2019 at 05:04 AM.
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