Gold Cube FT90 issue

[Nev]

I've got a Gold Cube fuel flow transducer installed. Generally it's run well so far, but has required recalibrating at about the 50 hour mark after it started under-reading. Obviously under-reading is very undesirable because the fuel remaining in tanks (Totaliser) is lower than Calculated. I put this down to the transducer "running in", and recalibrated the K factor.

More recently it's been over-reading (safe but undesirable). But when I switch the electric fuel pump ON, after a brief initial increase it then settles to a lower and more accurate fuel flow.

In other words, if I ran with the electric fuel pump ON continuously, it would appear to be reading very accurately. Once I switch the pump off, it gradually returns to over-reading the fuel flow, and displaying a higher fuel fuel. So my thoughts are that this isn't related to the K factor.

Any ideas ?

[Mark Dickens]

Nev, if I recall correctly, your gold cube is located between the electric fuel pump and the engine driven fuel pump, right? That's a pretty turbulent area in regards to the fuel flow. When you have the electric pump on, it pressurizes the line between it and the engine driven fuel pump resulting in not only higher readings at first, but a smoother flow because it is constantly pressurized. If the electric pump is off, the engine driven pump sucks the fuel through the electric pump bypass, which creates drag in the flow, and if the engine driven pump isn't perfectly sealed against back feeding fuel up stream during its pressure stroke, you could possibly see some very small backward movement of fuel.

I have the red cube on my RV-8 and it's mounted directly before the fuel distribution divider and downstream from the injector itself. The injector meters the fuel and smooths the flow at the same time so the red cube is getting a smooth flow and reading. I have the calibration down to where fuel remaining as calculated by the red cube is within a tenth or two of a gallon at fillup. It took some time to nail the k factor but once nailed, it's stayed that way for years so far.

Bottom line is take a look at the location of your gold cube and see if you think the above might be an issue. Alternatively, consider just running the electric pump continually as these pumps are rated for continuous operation.

[Nev]

Thanks very much Mark, I suspect that you're thoughts are probably exactly what is happening. I might look into relocating the gold cube to the correct location.

[Mark Dickens]

This general subject has been beat to death on the Vans Air Force site and there are agreements and disagreements of course: https://vansairforce.net/community/s...d.php?t=163771

My installation is almost identical to this:

IMG_2165.jpg

Since this installation, Tom Swearingen at Aircraft Specialty has developed a kit of brackets facilitate the installation. I think he has one for the RV-10 that might work for you

[robcaldwell]

My Red Cube is similarly mounted on top of the engine just before the spider. Don Rivera (Airflow Performance) recommended the Red Cube over the Gold Cube for my system.

I did not know Tom Swearingen now has a mounting bracket for this location. I will get one of those for sure as created my own and I am sure Tom's is better!

[Battson]

They literally say "Important! Follow these instructions!"

https://grtavionics.com/media/fuel-flow-EI-FT-60.pdf

Practically all aspects of this installation contradict a manufacturer's installations instructions, from orientation, fittings, attaching directly to the engine... (although I note several aircraft manufactures have been successfully ignoring these requirements for decades now...)

[Nev]

That's very helpful - I was trying to picture how you did this. Mark, is your red cube as shown above attached to anything other than the plumbing connections?

[Mark Dickens]

The only difference between that photo and my installation is that I used all steel fittings.. The installation shown has an aluminum nipple, which I do not recommend. The elbow shown and that I used is all stainless steel (see https://www.titanfittings.com/product-p/ss-6501.htm)

My installation is probably about 5 years old now, and I know Tom Swearingen has come up with some other designs and I'd contact him. Here's another good discussion on this: https://vansairforce.net/community/s...d.php?t=190539. The main thing in my opinion is to get the fuel flow sensor downstream from the injector to smooth the flow, and there are probably better ways to do this now than what I've done

[TimTall]

I can't remember the exact number, but the instructions for the Red Cube recommend a certain minimum length of straight hose before and after the sensor. They specifically recommend against having a angle fitting directly on the sensor. The flow needs time to smooth out after a bend. Not sure if the Gold Cube is the same, but every flow meter I've worked with in water distribution wanted a straight run to smooth the flow. Something like 8x the radius of the ID of the pipe rings a bell.

[Battson]

But when I switch the electric fuel pump ON, after a brief initial increase it then settles to a lower and more accurate fuel flow.

In other words, if I ran with the electric fuel pump ON continuously, it would appear to be reading very accurately. Once I switch the pump off, it gradually returns to over-reading the fuel flow, and displaying a higher fuel fuel. So my thoughts are that this isn't related to the K factor.

Any ideas ?

I have seen this a lot. Everything you describe above. The first sentence is just par for the course, that'll be what it'll be (as far as I can imagine).

The second paragraph, well, that's just a behaviour which resolves itself in my aircraft. I think this is caused by air bubbles collecting in the fuel transducer, and then working their way out. It normally takes a quarter hour or so, to fully solve itself - but it has hung around for an hour on occasion. It only happens occasionally for me, not sure exactly how it gets started.

This from the installation instructions:

Pulsation Dampener
A pulsation dampener may be required if the fuel flow readings are erratic, or if they read higher when an electric fuel pump is turned on. This is mostly likely when the fuel system is composed of mostly metal fuel lines, and there is no trapped air in the system. A pulsation dampener can be fashioned by installing a tee in the fuel line between the electric fuel pump, and the flow sensor, and connecting a 1 or 2 foot piece of tubing to this tee, oriented vertically up. Cap the end of this tubing to trap air in this line. Most often no provision is required for a pulsation dampener, as the fuel line leading to the fuel pressure sensor traps air and performs this function.

https://grtavionics.com/media/fuel-flow-EI-FT-60.pdf

[Battson]

Alternatively, consider just running the electric pump continually as these pumps are rated for continuous operation.

- Which pump are we talking about, they aren't all rated at 100% duty.
- Great way to wear out your pump more quickly, nothing lasts forever.
- Does your pump have a re-circulation bypass, consider fuel heating.

[svyolo]

I contacted EFII and Airflow a while back, and both claimed their boost pumps are "continuous duty" rated, but neither are "certified". The pumps for both are off the shelf EFI parts that run continuous almost forever in other applications, but not with the re-circ function that both EFII and Airflow use. So they almost certainly run at least a little warmer than full flow returning to the main fuel tanks. I don't know how much warmer.

I can't confirm how much either company tested pump life with even slightly elevated pump temps.

I know a lot of the certified boost pumps are continuous duty rated, as some applications require the pumps run that way.

[Battson]

I like the people at EFII and their products, same for Airflow Performance.
Unless they'll give you a guarantee, then it is your risk - whatever they say. Being rated for continuous duty is not the same as running almost forever, it just means the pump isn't going to destroy itself if it runs non-stop. It will still wear out faster.
My pump is EFII, its showing signs of wear already, it only runs maybe 5 minutes every hour - worst case. So perhaps 85 hours total run time on the pump so far.

[Bcone1381]

Nev, I think you have mechanical Fuel Injection. You have a Gold Cube. Have you considered a Red Cube?

Electronics International (I think they are the manufacturer) says the Gold Cube is suitable for gravity fed engines or engine between 350hp - 550 hp. The Red Cube is for fuel injected engines up to 350 hp. I wonder if that might be part of the problem. I don't know. I don't have any experience. But I got a red cube for my IO-360 from EI when I got my CGR-30P engine monitor.

https://www.iflyei.com/?s=fuel+flow

[rodsmith]

I believe the only difference is the gold cube is less restrictive to flow. That is why I am using one with mechanical fuel injection.

[robcaldwell]

That's very helpful - I was trying to picture how you did this. Mark, is your red cube as shown above attached to anything other than the plumbing connections?

Here is a picture from TSI FlightLines. It is uncommon for a IO-540 to have the boss under the fuel divider in order for this bracket to be mounted. Tom says almost all IO-360's have the boss. My IO-540 DOES have the boss.

EDIT: I should add that Tom Swearengen (TS FlightLines) collaborated with Don Rivera (Airflow Performance) on this setup.

AFP 5.12.2020 2.jpg

AFP 5.12.2020 3.jpg

Screen Shot 2022-06-20 at 8.39.20 AM.png

[Mark Dickens]

Next time I'm under the cowling, I'm going to look for that boss and if I can, I'll probably rework my installation

[Nev]

I wrote to EI to see if they had encountered this issue before and received the following reply:

Even though the system has worked well for the first 50 hours, our installation instructions prohibit our flow transducer from being installed between two fuel pumps. In doing so, the flow transducer's impeller can vibrate back and forth rapidly, due to the differential in the pump's cycling rates. This basically causes a "hammer effect" sometimes heard in residential plumbing. As such, I recommend installing the flow transducer after all fuel pumps...

I did realize at the time that I was deviating from the instructions. The problem is, I couldn't find any way to complete the installation in accordance with all the instructions, it's not actually possible. The later GRT instructions are somewhat more flexible, but only a little. I think that what Tom from TSFlightline has done is to properly test the unit in a number of different installations. He has now completed over 100 installations similar to Marks one.

I'm going to sit on my hands for a while (that's a figure of speech ) but will look into the idea of moving the transducer forward of the fuel servo, either above the cylinders as per Marks installation, or below the cylinders just above the fuel servo.

[Battson]

Well - to be clear, their original 2003 instructions - which still appear to be published, say you must install downstream of all pumps. Or were there further instructions?

[Nev]

They do. Mine is installed between the pumps.

[Mark Dickens]

I was not totally in love with my installation. Tom and I discussed it at long length in re to vibration and possible metal fatigue. I would much rather had installed it under the cylinders after the servo between two pieces of flexible fuel line but unfortunately, there wasn't nearly enough room for that. Many people have found that finding a location that meets all of the EI instructions AND puts the sensor in the right place downstream of the servo to be impossible as did I. It's a case of finding the best of a set of suboptimal choices.

[rv8bldr]

This is where I installed mine. The fuel line on the right will connect onto the FT-60 once I put the fitting on. Then, it will go up to the fuel spider from the other side.

FT-60 1.png

I made a bracket out of 4130 (.063 I think....) that attaches to the two front fuel servo bolts (one nut is missing in this pic). The moment arm of this bracket is small enough that I shouldn't have any bending or cracking issues. However, I am already thinking about a doubler ...

FT-60 2.png

Note that I am a LONG way from flying :-)

[Battson]

That looks like a smart decision. Maybe a little hotter down there, and the usual vibration from the engine being attached to the engine itself, being the only downsides. As everyone finds, complying with all the advice from the manufacturers is hard for the FT-60. Mine is upstream from both pumps, and that has proven rather reliable apart from the odd surge (short term issue only).