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  #1  
Old 12-13-2021, 05:10 PM
avrojockey's Avatar
avrojockey avrojockey is offline
 
Join Date: Aug 2019
Location: Appleton, WI
Posts: 482
Default Red Cube Install Recommendation

So...I've read nearly all FT-60 "red cube" threads. I want to get some opinions on my proposed install. I'm going to knock this out Wednesday as it suppose to hit 60 in WI...in December!

The pictures represent what I'm thinking, based on the following requirements:
  1. Cube installed between mech fuel pump and carb
  2. Cube not affixed to different rotational plane, if it's secured, then it will be affixed to the engine only (no the motor mount, firewall, etc)
  3. Cube installed after fuel filter/screen.
  4. EI says inline/floating installation is ok
  5. EI says 45/90 adapters are ok but prefer straight or 45 at input to ensure accuracy
  6. No downslope from Cube outlet
  7. Cube wires not pointed down
  8. EI wants a flexible in and out of cube

I've picked this spot because it meets the install criteria, but also because there's no exhaust pipe down low on that side.

I would prefer to mount it to engine to give it more security, but there's nowhere logical to mount it without rerouting a bunch of fuel line. I did think about securing it to the FAB with some reinforcement, but that would leave a pretty short fuel line form Cube to carb.

I still have concerns about the inline/free floating installation. Even with the limited mass of the cube, the movement on the lines concerns me.

Priorities:
#1 Safety: I don't want fuel lines breaking or coming loose. I will run MOGAS from one tank and need to mitigate vapor lock (though post mech fuel pump install should take car of that)
#2 Accuracy. I want to minimize potential for error in flow/totalizer computations
#3 Reliability
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Appleton, WI
RV-9A N904DC
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  #2  
Old 12-13-2021, 05:38 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
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A small thing, but I'd put reliability ahead of accuracy.

I'm approaching the Red Cube installation for my O-320 and am interested in this thread.

Dave
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  #3  
Old 12-13-2021, 06:09 PM
BoydBirchler BoydBirchler is offline
 
Join Date: Jul 2020
Location: Indianapolis
Posts: 163
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Originally my RV7 was carbureted. The fuel flow red cube had an immediate 90 3 inches after the output (mounted on cabin side of firewall). This caused turbulence and gave erroneous readings at take off power, but was reasonably accurate during cruise.

Flash forward I changed the engine to Bendix fuel injection. This required a different red cube, a higher pressure boost pump and a few changes in plumbing behind the firewall.

I understood it was best to place the red cube in line with a section of 6" to 8" of straight run before and after the cube. I was able to accommodate this "best" placement configuration. Accuracy is perfect; within 2 or 3 tenths on a fill up after a 3 hour flight.

If memory serves me, the directions said if you had to have a 45 or a 90 adjacent to the cube the long sweeping turn of a "goose neck" is superior to the immediate tight turn with a 45 or 90 AN fitting; thought being the immediate turn caused more turbulence and that turbulence affects accuracy.
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  #4  
Old 12-13-2021, 06:37 PM
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avrojockey avrojockey is offline
 
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Location: Appleton, WI
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Quote:
Originally Posted by BoydBirchler View Post
Originally my RV7 was carbureted. The fuel flow red cube had an immediate 90 3 inches after the output (mounted on cabin side of firewall). This caused turbulence and gave erroneous readings at take off power, but was reasonably accurate during cruise.

Flash forward I changed the engine to Bendix fuel injection. This required a different red cube, a higher pressure boost pump and a few changes in plumbing behind the firewall.

I understood it was best to place the red cube in line with a section of 6" to 8" of straight run before and after the cube. I was able to accommodate this "best" placement configuration. Accuracy is perfect; within 2 or 3 tenths on a fill up after a 3 hour flight.

If memory serves me, the directions said if you had to have a 45 or a 90 adjacent to the cube the long sweeping turn of a "goose neck" is superior to the immediate tight turn with a 45 or 90 AN fitting; thought being the immediate turn caused more turbulence and that turbulence affects accuracy.
After reading all the threads I believe the fitting restrictions are for floscan units. Current guidance from EI has no restriction on fittings for the cube...they have tested the the current versions with 90 degree fittings in an out. The instructions make no mention of fitting or line restrictions

The explicit guidance says: after the mechanical fuel pump (for vapor lock), no drop after outlet (prevents bubbles from collecting in cube and causing error)
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Last edited by avrojockey : 12-13-2021 at 06:41 PM.
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  #5  
Old 12-13-2021, 06:42 PM
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Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Recommend not mounting the cube on the engine. The per plans engine mount location for an RV-14 I worked on proved to be a bad choice as fuel flow measurement was way off - and changed with conditions.

From the install instructions: “Due to vibration issues, never connect the fuel flow transducer directly to the engine”.

We moved it off the mount and the problem went away.

This is what I did on the RV-8 to meet all the installation requirements. It works very well there.
Carl
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  #6  
Old 12-13-2021, 06:47 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 272
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My red cube is installed similar to how you propose, except I have the hose from the fuel pump dropping down to a 90-degree fitting into the cube, then a 90-degree fitting out of the cube to go forward and slightly up to the carb. The cube itself is just hanging from the hoses and is not attached to any structure. So far (~60 hours) no issues. The reading can be a little jumpy, but I attribute that to the carb - after I got the k-factor dialed in my fill ups are consistently within one tenth of a gallon from what my fuel computer says.
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  #7  
Old 12-13-2021, 10:22 PM
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avrojockey avrojockey is offline
 
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Location: Appleton, WI
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Quote:
Originally Posted by Carl Froehlich View Post
Recommend not mounting the cube on the engine. The per plans engine mount location for an RV-14 I worked on proved to be a bad choice as fuel flow measurement was way off - and changed with conditions.

From the install instructions: “Due to vibration issues, never connect the fuel flow transducer directly to the engine”.

We moved it off the mount and the problem went away.

This is what I did on the RV-8 to meet all the installation requirements. It works very well there.
Carl
Thanks Carl...this is what is confusing...my instructions don't say this, and several threads seem to indicate that they fixed the vibration reliability issue.

Is yours free-floating now, in approximately the same location as I'm proposing?

My main concern is that I'm overlooking a safety issue with this arrangement.
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  #8  
Old 12-14-2021, 05:39 AM
TS Flightlines TS Flightlines is offline
 
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Location: Ridgeland, SC
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Tim---there are literally 100's of ways to do this and everyone is right, and a few wrong. EI, nor anyone else will give you and absolute fool proof location. We've done ALOT of plumbing for these, and even with that there are some variances. WE just use the best information we have, based on literally 100's on plumbing installs ans came up with several locations. NOT ALL WORK FOR EVERYONE---hense all the posts about it.

Tom
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  #9  
Old 12-14-2021, 06:25 AM
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DanH DanH is online now
 
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Location: 08A
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Carl, admittedly one fella's opinion...but that is not a great example.

Black oxide steel fittings are fine for oil, but water in fuel can make them shed rust. I've seen it.

Here steel is pointless anyway, as it is coupled to an aluminum tube end. The installation links a moving object (the engine) to a fixed object (the cube) with a material (aluminum) subject to high cycle fatigue. A steel tube end has no such fatigue issue if stress is kept below the knee in the S-N curve.

Both issues will take time to develop. That's the ugly part.
.
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  #10  
Old 12-14-2021, 06:27 AM
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Auburntsts Auburntsts is offline
 
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I opted for the "floating" install between the mech fuel pump and the throttle body. I covered the whole thing in a firesleeve that I bought from Aircraft Spruce that they sell just for this purpose.
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