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Red Cube Install Recommendation

avrojockey

Well Known Member
Patron
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! :D

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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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
 
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.
 
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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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
10067771-BAE6-44-A5-A96-D-A300169-F43-CE.jpg
 
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.
 
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
10067771-BAE6-44-A5-A96-D-A300169-F43-CE.jpg

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.
 
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
 
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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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.
 
How I did it

This is how I did it.
Note the bracket from the Red Cube mount to the inlet fuel line. This is because the fuel pump moves and it will apply torque to the pipe fitting in the Red Cube housing due to the 90 degree fitting. When using 90 degree fittings, line movement needs to be taken into account.
 

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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.
.
Dan,

Good point. I’ll swap out with aluminum fittings.
Carl
 
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.
.

I understand the issue with aluminum hose end, but isn't it recommend to have steel fittings FWF? I have stainless hose from Tom and Steve.
 
I’m going to throw this out there mostly for comment and my own continuing education. I think the poster was saying he perceived two problems with Carla installation. First was the black steel fittings and corrosion. But the second is that one end of the fuel line is mounted to the engine, and the other to the engine mount. I believe it is important in this case that the line be secured to each part by some means (clamp) other than an aluminum fitting.

If Carl replaced both the fittings and the hose ends with steel/stainless steel, that would be better but the threads in the fuel cube are still aluminum, so shouldn’t there just be clamps near each end of this hose?

Gordon
 
Update:

I have temporarily installed. I used some 3/8 auto hose laying around for mock-up…old hose I had was too hard to assemble fittings. Another hose order to Tom/Steve when I figure the length.

I attempted to use the original hose from pump to carb, it’s only a little over a year old, but to keep a 4” radius I had to make a long sweep outboard. It kind follows the path of the engine ground. Since that ground moves with the engine tied the two together to support the hose a bit. Not sure if there’s any threat to this, such as a lightening struck to prop traveling through ground and jumping to fuel line and exploding :)

Anyway…what say yea?
 

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I suggest builders not to use 90 degree fittings on the cube (per the instructions). This fitting provides for a 90 degree bend, but not abruptly like with the standard 90 degree fitting.
https://www.aircraftspruce.com/catalog/appages/phnx-comp-swvl90.php?clickkey=3007980

Carl

Carl...is there any other reason not to use the 90?

The latest version of the instructions (Revision F) do not state any fitting restrictions other than a max toque of 12 ft-lb, and several posts here say current guidance from EI for the update version of the cube is no fitting restrictions and engine mounting is allowed.
 
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Not familiar with such changes. I can report my install has been dead on accurate, and as a general rule I avoid 90 degree fittings in fuel lines if I can.

Carl
 
Tim

Using a 90 degree fitting onto something that’s unsupported with a hose that’s unsupported is just asking for a loose fitting and leak. Using a tie rap onto the ground wire is not ideal either. The tie rap can cut into the cable and fuel line.
Is it not possible to use straight fittings? And have the hose supported each side for the cube? It might mean getting a couple of new hoses.
The risks of fuel leaks and engine fires are real, please be careful.

Regards

Peter
 
Tim

Using a 90 degree fitting onto something that’s unsupported with a hose that’s unsupported is just asking for a loose fitting and leak. Using a tie rap onto the ground wire is not ideal either. The tie rap can cut into the cable and fuel line.
Is it not possible to use straight fittings? And have the hose supported each side for the cube? It might mean getting a couple of new hoses.
The risks of fuel leaks and engine fires are real, please be careful.

Regards

Peter

Thanks Peter...that's what I'm most worried about.

Adel clamps to replace the cable ties after routing is finalized.

It is possible to use straight fittings but I was attempting to reuse one of my new-ish hoses and have minimum 4" bend radius. Your right...couple new hoses may be in order.
 
Just as a point of reference to all----Our angled hose ends from -3 through -8 are 304 stainless and mandrel bent. The stem ID AFTER crimping is a larger ID that the average of the 3 TOP Aerospace brands for teflon hose, on purpose. The ID at the center of the bend is also larger than the average of the top 3.

MilSpec says what the FLARE HEAD ID should be, but not the body or the stem, because those are proprietary dimensions of the individual manufacturer. Some use a larger OD body to help compensate for the ID reduction while bending.
(if someone wants that data, I have it).

Tom
 

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Just as a point of reference to all----Our angled hose ends from -3 through -8 are 304 stainless and mandrel bent. The stem ID AFTER crimping is a larger ID that the average of the 3 TOP Aerospace brands for teflon hose, on purpose. The ID at the center of the bend is also larger than the average of the top 3.

MilSpec says what the FLARE HEAD ID should be, but not the body or the stem, because those are proprietary dimensions of the individual manufacturer. Some use a larger OD body to help compensate for the ID reduction while bending.
(if someone wants that data, I have it).

Tom

Thanks for all the info lads.

Tom...once I figure this out I'll be placing another order through Steve (he's a little closer so I can pick up).

Quick question for you though...

You list minimum hose radii for -6 as 4", assuming the hose has similar characteristics (Milspec) to aeroquip 666 or even 303, can the minimum radius be reduced for operating pressure per AC43.13? This line is on O320 so the most pressure it's seeing is 6 psi. If I can use 3" radius on the hose from engine pump to cube I can go back to straight fittings
 
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The bend radius is based on the hose at the prescribed working pressure, with is 3000 psi in this case. Since you are using a carbed install, at 5+- psi, yes you can bend it tighter. But remember, with firesleeve, you'll probably be back near the 4 inch radius.
Tom
 
Here's my semi-final install for your critique.

Couple things about this install...
  1. I'm trying to use my original pump to carb hose
  2. Orientation and position is optimized to account for engine rotation, e.g. B-nuts not is same rotational plane, and hoses can't act as levers on fittings
  3. The pump to red cube FT-60 line and fitting are temporary until I finalize location. This will be replaced with a hose with a 90 on one end attached to straight fitting into cube.
  4. Steel AN fittings replaced with stainless.
  5. Hose clamped to engine ground cable not to support but to maintain distance with engine mount. Both move with engine but obviously the engine ground is fixed to firewall on one end. Not sure if this is a big deal
  6. Pump to cube hose will be about 1" shorter to give me 2" clearance from bottom cowling...I didn't want to shorten this hose since it's being replaced
  7. Wire is not as tight as it looks slack for movement a little higher up
 

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Tim----I dont know what clearance you have between the cable and the hose-----which is a 303 rubber lined hose---but I would not clamp the 2 together even with adels. NOT SAYING that you will get some cross current between the 2, but its generally not a good idea. YES the rubber will insulate it some, but I'd personally still separate them.

Transducer that low may pick up some extra exhaust heat--and at 5 psi may create some vapor.

Tom
 
Tim----I dont know what clearance you have between the cable and the hose-----which is a 303 rubber lined hose---but I would not clamp the 2 together even with adels. NOT SAYING that you will get some cross current between the 2, but its generally not a good idea. YES the rubber will insulate it some, but I'd personally still separate them.

Transducer that low may pick up some extra exhaust heat--and at 5 psi may create some vapor.

Tom

Thanks Tom...I'll find another way to keep the hose from laying on engine mount. AC43.13 mentions not securing fuel lines to wires, but it reads like as to support the hose or wire. It also mentions if fuel lines are routed with electrical then do so underneath. (ref. 8-31b Routing)

There's no exhaust on that side other than coming from #4 and going forward to a crossover. Aircraft has a unique exhaust that's made for Glasair folks and all exhaust leads to and goes out the right/passenger side.
 
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So on further review...according to AC43.13 you can secure fuel lines to electrical...I just need to make sure there's 1/2" spacing in my arrangement

11-126. FLAMMABLE FLUIDS AND GASES. An arcing fault between an electrical wire and a metallic flammable fluid line may puncture the line and result in a fire. Every effort must be made to avoid this hazard by physical separation of the wire from lines and equipment containing oxygen, oil, fuel, hydraulic fluid, or alcohol. Wiring must be routed above these lines and equipment with a minimum separation of 6 inches or more whenever possible. When such an arrange- ment is not practicable, wiring must be routed so that it does not run parallel to the fluid lines. A minimum of 2 inches must be maintained between wiring and such lines and equipment, except when the wiring is positively clamped to maintain at least 1/2-inch separation, or when it must be connected directly to the fluid-carrying equipment. Install clamps as shown in figure 11-10. These clamps should not be used as a means of supporting the wire bundle. Additional clamps should be installed to support the wire bundle and the clamps fastened to the same structure used to support the fluid line(s) to prevent relative motion.

A couple other things working in my favor...the main concern in 11-126 is arcing between electrical and metal line. I have fire sleeved hose, and this is a ground wire so there's no arcing/fire potential.
 

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...and this is a ground wire so there's no arcing/fire potential.

Give it a flotation test. Replace it with a braided ground strap on the other side of the engine, attached to the Lycoming ground lug. Stein will make one any length you wish. The braided strap never fails, unlike the hard wire, which tends to break the crimped-on eyelets.

https://www.steinair.com/product/ground-braid-cable/

The ground lug point is just above the lower right motor mount.
.
 

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Give it a flotation test. Replace it with a braided ground strap on the other side of the engine, attached to the Lycoming ground lug. Stein will make one any length you wish. The braided strap never fails, unlike the hard wire, which tends to break the crimped-on eyelets.

https://www.steinair.com/product/ground-braid-cable/

The ground lug point is just above the lower right motor mount.
.

Dan...what's a floatation test? I'm pretty electrically ignorant. Is this to test the floating ground, like a voltage drop across somewhere testing the engine ground?

Edit: If I failed to see the sarcasm and you mean chucking it in the lake to see if it "floats" then I'll need to wait until spring. ;)

This is sorta off-topic but I was thinking about installing a second ground from the starter as a backup and provide better cranking (not that it's an issue).
 
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This setup complies with all the Cube recommendations, easy to access and works great. Uses bolt that holds airbox to fuel servo.
 

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If I failed to see the sarcasm and you mean chucking it in the lake to see if it "floats" then I'll need to wait until spring. ;)

Approved Flotation Test Facility below. All the best ones are in the temperate states.

Fun aside, seriously consider replacing the large gauge hard wire with a braided ground strap. Yes, the wire is working on lots of airframes, but they do break eyelets, and the eyelets themselves are often poorly crimped to the wire. I've found both during inspections.

If you do stay with the wire as shown, be sure to fixate it with an adel (at minimum) at the firewall end. Right now, you've clamped the hose and wire together, significantly increasing the shaking mass. The fixed end of the wire will catch a lot of cyclical stress, which you don't want applied to the crimped-on terminal end.
 

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This setup complies with all the Cube recommendations, easy to access and works great. Uses bolt that holds airbox to fuel servo.

Thanks Joe…I liked your install but I’m carbed so I would need some longer hose runs. I was considering going straight into carb with a bracket to support the red cube but instructions poo poo that idea. Although I see several installations directly to the spider in FI systems
 
Approved Flotation Test Facility below. All the best ones are in the temperate states.

Fun aside, seriously consider replacing the large gauge hard wire with a braided ground strap. Yes, the wire is working on lots of airframes, but they do break eyelets, and the eyelets themselves are often poorly crimped to the wire. I've found both during inspections.

If you do stay with the wire as shown, be sure to fixate it with an adel (at minimum) at the firewall end. Right now, you've clamped the hose and wire together, significantly increasing the shaking mass. The fixed end of the wire will catch a lot of cyclical stress, which you don't want applied to the crimped-on terminal end.

Ha! Thanks Dan!

While doing all this I was thinking about adding a ground from starter direct to battery with #2 welding cable…I would assume this would provide some redundancy with better cranking. Regardless I’m going to remove clamps between hose and ground
 
Ha! Thanks Dan!

While doing all this I was thinking about adding a ground from starter direct to battery with #2 welding cable…I would assume this would provide some redundancy with better cranking. Regardless I’m going to remove clamps between hose and ground
Most of the experts like Bob Nuckolls recommend two ground wires to the engine, so this would be a good idea, based on what I've read. I have two braided ground wires to my common ground point.

http://www.rv8.ch/ground-point-on-firewall/

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