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Red Cube shows inflated flow on high altitudes

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plippo

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Our non-RV plane (DR-107, IO-360) had a failing VM1000 (+ floscan sensor), which we suspected showing too high flow in higher altitudes (cross country flights). The VM1000 started to die on us, so we replaced it with the MGL MX1 + RDAC + red cube. The old installation had two 90 degree turns right before the floscan, so we thought that was the reason for the odd behavior. Now with new sensors and better installation between mechanical pump and servo, we see exactly the same behavior. With new EFIS we can reliably lean based on EGTs and record the data, which shows the following (altitudes are pressure altitudes):

  • At low altitudes (1000-3000 ft) the consumption is quite accurate with the factory K-factor (shows about 1.5% too much)
  • When leaning, the cylinders peak nicely within about 0.2 gal/h.
  • The consumption at 2200 ft, 2300/23, at peak EGT, shows about 36 l/h (9.5 gal/h), which is not too far from theoretical
  • When leaning at 7200 ft, the "at peak" flow showed 47 l/h
  • I suspected that there could have been bubbles and put on the boost pump for the repeat test, the flow showed 58 l/h at peak.
The engine operated normally in leaning in all cases, and EGTs showed the usual humps, so there was no extra flow into the cylinders. Also there are no visible stains from fuel leaks anywhere. Short test seems to indicate that high altitude flights, the flow is much less correct (~8% at 3500ft, vs 1.5% at 2000ft).

If the "too high" would have not shown in the old Floscan, I would have thought sensor error, but... Reading internet, the bubbles in the fuel would cause too high readings, but why would increasing the pressure make it worse?

Has anyone ever encountered similar? Any help is much appreciated!
 
Yes, the exact same problem on an RV-14A. I had a repeatable (as in every flight) erroneous red cube fuel flow in a buddy’s RV-14A. The cube worked perfectly until the aircraft climbed above 5000’ or so. At that point fuel flow when to 14+ GPH mostly independent of throttle or mixture setting. Replacing the cube had no effect, the problem remained. The cube was mounted following the FWF package instructions - in exactly the worst spot (on the engine in a high vibration and heat area). Remounting the cube following the cube instructions and the problem was fixed.

We could only guess that the original mounting location (vibration), combined with fuel line routing, lengths and such exposed the cube to an altitude dependent flow oscillation, causing cube wheel to spin faster than normal for the fuel flow conditions.

So - how is this fuel flow sender mounted?

Carl
 
Carl Froehlich said:
Yes, the exact same problem on an RV-14A. I had a repeatable (as in every flight) erroneous red cube fuel flow in a buddy’s RV-14A. The cube worked perfectly until the aircraft climbed above 5000’ or so. At that point fuel flow when to 14+ GPH mostly independent of throttle or mixture setting. Replacing the cube had no effect, the problem remained. The cube was mounted following the FWF package instructions - in exactly the worst spot (on the engine in a high vibration and heat area). Remounting the cube following the cube instructions and the problem was fixed.

We could only guess that the original mounting location (vibration), combined with fuel line routing, lengths and such exposed the cube to an altitude dependent flow oscillation, causing cube wheel to spin faster than normal for the fuel flow conditions.

So - how is this fuel flow sender mounted?

Carl
Click to expand...

Thanks Carl for the excellent info!

We've mounted the sensor to the oil sump bolts on top of the intake runners so that we get gentle curve from the fuel pump and into the servo. The previous sensor was installed in the firewall, and exhibited the same problem. Our plane is not that smooth (it could really use dynamic balancing, if we could find someone doing it), so it may be that both locations have too much vibration.

RedCube_placement.jpeg

Another thing I've been thinking is the fuel pump, if it could be getting old (shows 19.9 psi at idle, 15.x near full power), but I can't figure out a failure mode that would increase the flow readings when sensor is after the pump.
 
12vaitor said:
A few years ago, Van's published an RV-12 Service Letter on that contains a lot of information on their testing and speculation as to the cause of the higher flow rate at altitude. You can check your numbers with the red cube flow rate vs altitude graph in the SL to see if it is consistent with Van's observations.


John Salak
RV-12 N896HS
Click to expand...
Excellent information, thank you! Symptoms are identical to our issue. I found a blog post about installing the sensor between servo and spider, which should take care of it if it's in deed caused by the pump pulses:

ivankristensen.smugmug.com

Installing the "Red Cube" fuel flow transducer - Kristensen Picture Galleries

The wisdom of the RV community suggested that the best location of the Red Cube fuel flow transducer is between the Fuel Servo and the fuel flow divider. I basically copied this installation from another RV-10 builder who found it to be working well. One thing to remember is not to have any 90...
ivankristensen.smugmug.com ivankristensen.smugmug.com

Looks like there is something to do in the next annual...
 
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