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Inside the Red Cube

DanH

Legacy Member
Mentor
A while back my buddy Tom asked if I'd like a red cube fuel flow transducer for a bit of "forensic examination", as he had recently replaced one which wasn't working right.

Of course I said "Oh yeahhhh"

The manufacturer has designed the device so it cannot be dismantled for overhaul, at least not without a lot of special effort. First, the allen screws on the cover plates are installed with a thread locker, and being long, small diameter screws, most just twist off before the locker will let go. Second, the electronics are potted. Third, one wire is routed and potted in such a way that removing a cover plate breaks the wire, and it can't be repaired. In this case I didn't care about repair, so brute force methods were employed. Here we go:

There are cover plates on both side of the cube. One side has two stacked plates, and the other has a single plate. The single plate side just covers an potted electronics bay. I did not investigate the electronics.



On the other side, removing the topmost plate exposes the outer face of this machined assembly. The potted groove contain a wire:



The inside face contains a jewel bearing in the center, and an LED (or a receiver, don't know which, see below). You can also see the broken stub of the previously mentioned wire. Removing this assembly from the cube body breaks the wire, which was potted in both the cube and the plate after they were assembled. Clever.



Inside the main body we find another bearing, and a matching LED (or receiver). You can see the (broken) potted wire at about 5 o'clock:



There are two ports for fuel flow, arranged tangent to the circle:



Suspended in the circular cavity between the two bearings is a plastic rotor with 20 teeth and four impellers:



The "teeth" break the light beam between the LED and receiver, 68,000 times per gallon, as the rotor spins in the flow:



When the cube is installed between a boost pump and the engine driven pump, most builders report an excessive fuel flow indication with the boost pump running. I don't think any of us fully understand the flow mechanics with precision, but looking at the rotor, it's not hard to see how a little bit of pulsing in the flow might result in very small reversals of the wheel rotation, which would cause extra counts.

That's all folks.
 
Interesting to see inside one of these. Thanks Dan.

For SDS EFI users, we offer a $100 option to output a similar FF signal without any Red Cubes needed. I know several people have asked whether they need these our not with our system.
 
When the cube is installed between a boost pump and the engine driven pump, most builders report an excessive fuel flow indication with the boost pump running. I don't think any of us fully understand the flow mechanics with precision, but looking at the rotor, it's not hard to see how a little bit of pulsing in the flow might result in very small reversals of the wheel rotation, which would cause extra counts.

That's all folks.

Which makes sense. Without taking a stab at the math, guessing that the (rotational) inertia of the wheel is substantially less than the (linear) inertia of the fuel passing through the device. Small changes in the latter would affect the former.
 
Thanks for showing the magic inside the Red Cube. My cube is between my Airflow Performance fuel pump and the mechanical engine driven pump. Your illustration/tutorial helps me to better understand why I get a higher fuel flow indication with the electric pump on. I had no idea the Red Cube was a electro/mechanical device.

Thanks Dan.
 
Red Cube Math

Dan, you are amazing!

I offer a question and then a comment:

1) Where did you get the factor of 68,000 counts per gallon for the unit? I assume it's from the manufacturer but where?

2) In the interest of accuracy, and I know you like to be accurate, I offer the comment that the term "impeller" is usually used to describe a part that transfers energy TO the fluid. A better term for the part that receives energy FROM the fluid might be "vane" or "blade." I know it's a nit. :eek:

Thanks for doing the forensics. I think you should offer your services to the NTSB as a field resource for aviation accident investigations.

P.S. My ancient educational methods yield a rotational speed of the little rotor of around 57 RPM for a steady state flow rate of 10 GPH through the unit. That's based on your calibration factor of 68,000 counts per gallon.
 
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Dan,

Two questions related to flow characteristics:

- What's the approximate size of those internal flow passages?

- In your estimation, how large of a particle do you think it would take to restrict fuel flow through the unit?

Thanks for posting this!
David
 
1) Where did you get the factor of 68,000 counts per gallon for the unit? I assume it's from the manufacturer but where?

Sorry Don, missed your post. The 68,000 per gallon is from the EI datasheet for the cube:

http://buy-ei.com/wp-content/uploads/2013/06/Info-FT160.pdf

... the term "impeller" is usually used to describe a part that transfers energy TO the fluid. A better term for the part that receives energy FROM the fluid might be "vane" or "blade."

Thanks! Yes, I like accuracy, in particular with technical terms.

Two questions related to flow characteristics:

- What's the approximate size of those internal flow passages?

The inlet is 0.141" diameter, give or take a fuzz.

In your estimation, how large of a particle do you think it would take to restrict fuel flow through the unit?

0.142" ;)

Seriously, any shape that could possibly span the 9/64" entrance hole would result in some fuel flow reduction. A rod shaped chunk might not block much, while a sphere might result in total blockage. Interestingly, the exit hole is slightly larger, and the manufacturer says a locked rotor will not slow the flow. I believe it.

Jon; sorry don't know anything about a FloScan. Anybody have a bad one to spare?
 
I removed my red cube after seeing the size of the lines through the red cube and worrying about them getting plugged. I had it mounted after the boost pump inside the RV-4. I either need to put a filter before it or mount it after the gascolator. But when looking everything is so tight I inside the cowling, I just don't see room to mount it there.
 
Are you cousins with "Macgyver" :D

While Dan does indeed possess "McGyver" skills, tearing things apart was not McGyver's thing. MAKING a red cube from a rock and some duct tape, now THAT would be some mad McGyver skills, and I think well within Dan's capacity! :D
 
Someone needs to fire up Solidworks and spit out a modified design and 3D print an impeller so that it will read correct with a slightly pulsed flow.
 
I removed my red cube after seeing the size of the lines through the red cube and worrying about them getting plugged. I had it mounted after the boost pump inside the RV-4. I either need to put a filter before it or mount it after the gascolator. But when looking everything is so tight I inside the cowling, I just don't see room to mount it there.

Can it be mounted between the engine fuel pump and the carb?
 
red cube

I mounted it on the firewall between engine pump and carb w/ 90 degree fittings on both sides, I was told it won't flow right with the elbows. anyone have the answer ? Thanks Dan for the surgery
 
An old post of consolidated info from the horse's mouth:

As far as I know Dynon distributes both the FloScan 201B as well as EI's FT-60.

The FloScan unit is much more sensitive to the angles/fittings entering and exiting the unit. EI's unit does not care. You can run 90 degree fittings in and out of ours without problems.

JPI does not sell EI's FT-60:cool:

Maybe you can get Dynon to trade you for a FT-60?

Good luck!

Matt


You do NOT want to rigid mount the transducer to the motor (any part) using just a fitting. It is a huge safety issue. The fitting could theoretically fatigue and break. You absolutely need to have flexible line on both sides of the Floscan 201B or the EI FT-60 (that Dynon now utilizes).

After manufacturing/supporting flow instruments for a good number of years we have seen that flow transducers accuracy is typically better when mounted after both fuel pumps. They simply seem to prefer to have fuel pushed through them, not pulled through. The truth of it though is that many installation drawings still read as though we were still using the older Floscan 201 transducers. I have seen installs that mount the transducer between the pumps with claimed success. The Floscan units were much more sensitive in regards to mounting location, angles of fittings in and out, and attitude. Our new design will tolerate a lot more. Frankly we don't care if the thing flows straight up, or if you put 90-degree fittings in and out (FT-60 only) of the transducer. Just don't mount it upside down or flowing sharply downhill.



Hope this helps,

Matt Sharp


Hello All!

Your friendly neighborhood EI Tech support rep here!

There are really two issues in regards to EI?s specifications on flow transducer placement.

1. Accuracy of the output of the flow transducer.
2. Safety.

Issue one is pretty basic. After manufacturing flow instruments for a good number of years we have seen that flow transducers accuracy is typically better when mounted after the mechanical fuel pumps. They simply seem to prefer to have fuel pushed through them, not pulled through. The truth of it though is that many installation drawings still read as though we were still using the older Floscan 201 and 231 transducers. I have seen installs that mount the transducer between the pumps with claimed success. The Floscan units were much more sensitive in regards to mounting location, angles of fittings in and out, and attitude. Our new design will tolerate a lot more.

The other more serious issue is in regards to safety. We want to make absolutely sure that the fittings on the transducers never are subjected to conflicting vibration planes. The engine will flex in the motor mounts creating conflicting vibration planes between the engine and the airframe. This is why the ?flexible line in? and ?flexible line out? is called out so often. This is also why we don?t want the transducer rigid mounted, via a fitting, to a carburetor or fuel pump. A contradicting vibration plane will focus the energy directly to the fitting. It is our sincere concern that with the two conflicting vibration planes here could cause the fitting to fatigue and crack. This would be bad?and we have seen it happen?.

On the bright side, we don?t care if the thing flows straight up, or if you put 90-degree fittings in and out of the transducer. Just don?t mount it upside down, flowing sharply downhill, or before the pumps. :eek:

Please let me know if you have any questions or if we may be of any assistance.

Matt Sharp
 
Sorry Don, missed your post. The 68,000 per gallon is from the EI datasheet for the cube:

Jon; sorry don't know anything about a FloScan. Anybody have a bad one to spare?

I might actually have a FloScan that we can dig into... I'll see if I still have it.
 
A little insight

I have posted here as it relates to the "red cube". A recent post by Lan Vinh Do (http://www.vansairforce.com/community/showthread.php?t=149243) showed he relocated his flow transducer to FWF downstream of the mechanical pump. This (along with other changes) stabilized his fuel pressures at altitude.

We have long talked about the fuel flow difference and the tiny pressure drop. While the pressure drop from end to end is relatively low, Dan has shown the flow area of the interior. A 3/8" fuel supply tube (.297" ID) at 12 GPH enters at ~11 in/sec, but accelerates to 49 in/sec in that small diameter. The Bernoulli calculator shows that this is a 0.08 psi change in local pressure. Maybe enough to cause bubbles if it is near the vapor pressure due to temps already.

I had hoped this calculation would give some insight as to why it is likely to result in fuel pressure issues at altitude (boost pump off) when located prior to the mechanical pump, but with the corrected values, it is only a small benefit. At least, now (with bubbleheads assistance) we have quantified this parameter.

Edit: The values above have been corrected - I missed a critical division by 12in/ft in the velocity and input the wrong values. Thanks to bubblehead for checking and sending me a nice email.
fXPlwQWZcbeu3s604kKx92crHT91k5h_0tx2XtZ2WxIDTjl4IIQxmXNsALigw4hF36bNdHF7_Hi2b5DKB6I-F2shC8PvNnt8SRaEAswKkVFWCcb1YbZJU_LvxqRastj02gRg6miUiET_RSNkCjCrOgbmmqW_Hbkh_tgILAZeYqdz-OP4mYZyWbASihdxgKoMawVUYp2BaHy0VQhr-y50rR8hdBxRwitqrCJJn_2jh2RHtN1nfXDTEoedF8kCBnQBE_xZMSHkOcvQsWfXPhBsnLuA8SzsSwkEI0o0a_dXj7LSRu9arQXsxpBlgQfCkhi5Q_bngVuOWGMTetWwTtSj3tfivQlBfO0N0_6en2rClEESc_AaBwfGDbEjQj9O9VqUX1b5A9jp9LtuiUcnX-NrrnExotOp-YUOY_7QDzfFEfrNaVwC3XPuJ7P_WTMl74I30Ao2iAnV7_tINysPVm6BNALjdNk3DPWNdMy7nLsqtuiv3evo08PAsG6WyS2iq0uT_L73e9j6vwB2-9QlzC1A5w6jr6tIVtSN7-HXCLuPYZIAGVBl624jFyyWD1BxOVdcCGVlbRRsjgiL6StFr0bCv58p0rs7HX0GvQ1TdzDHmBAz02E81gXZ2Z-hCNXAr7h1UufI4SQBpznQrg0lZrzbruI4LDuayiPyhQiNXgEciw=w700-h575-no
 
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Since the pressure is sensitive to flow rate, it also may indicate why the RV10 is more susceptible to this effect than other RV's

I suspect the big factor with most RV-10's is a hot tunnel containing several feet of fuel line, i.e. they are delivering warmed fuel to the transducer, greatly increasing vapor pressure.

Put another way, a stainless belly panel over 1/8" of fiberfrax would probably cure most of this stuff. Instead we see builders insulating the inside of the tunnel....
 
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When the cube is installed between a boost pump and the engine driven pump, most builders report an excessive fuel flow indication with the boost pump running.
That's exactly the case with my JPI EDM-700 fuel transducer which works on the same principle. Turning on the boost pump makes the fuel flow read 20% high. Matronics offers a Pulsation Damper which is supposed to fix that problem.
 
I suspect the big factor with most RV-10's is a hot tunnel containing several feet of fuel line, i.e. they are delivering warmed fuel to the transducer, greatly increasing vapor pressure.

Put another way, a stainless belly panel over 1/8" of fiberfrax would probably cure most of this stuff. Instead we see builders insulating the inside of the tunnel....

After getting a math lesson from Bubblehead (THANKS), you could be right, with the low velocity the fuel has some time to collect heat there, and a good bit of area. From one graph .1 psi is equal to 1 deg F in terms of vapor pressure, so with a few 3-4 seconds in the tunnel, it could be more than that.
 
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Flo-Scan Pulsation Damper

Matronics offers a Pulsation Damper which is supposed to fix that problem.
Two weeks after I ordered it, the unit arrived via UPS today but I'm not sure I'm going to install it. I was expecting it would have a bladder inside to dampen the pulses, but it's just a hollow SS sphere. The unit is supposed to be mounted with the sphere facing up so that the vapor in the sphere acts as the cushion.

After some consideration, I'm really not sure I want to add this device to my fuel system. It would have to be insulated so as to not warm the fuel. The Tee fitting that came with it would add two more places to leak. It would add more weight cantilevered off the 1/8" NPT pipe fitting on my Facet boost pump.

The problem with my fuel flow reading high with the boost pump on hasn't really caused any problems so I may just live with it. My EDM-700 fuel totalizer was only off by 0.1 gallon after the last 30 gallons I burned.
 
Pulsation Damper didn't help

I went ahead and installed the Flo-Scan pulsation damper anyway, just to see what it would do. I tee'd it into the outlet of the Facet boost pump, and wrapped it with firesleeve for thermal insulation. I went flying with it today and it made absolutely no difference in the original "fuel flow reading high when the boost pump was on" problem. On climbout the fuel flow went from 10.1 to 11.5 with the boost pump on. At cruise it want from 6.5 to 7.1 GPH. I'll be removing the unit next time the cowling is off.
 
Dan,

I have a floscan that is of no use and has been replaced with a red cube. I'll get it to ya when we meet up.
 
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