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Red Cube failure mode

Paul 5r4

Well Known Member
I've been having some weird indications regarding ROP/PK/LOP lately. Flying along and at full rich it will sometimes float between the three. I have the Dynon D100/D120. I've checked everything I can. Facet Fuel pump seems to be working good... about 2 inches of fuel in a 5 gal bucket after only 20-30 seconds. I've verified there are no obstructions/kinks in fuel lines all the way to the tanks. Tank vents are good. Gascolator opened up and checked... nothing as well in the final screen going into the carb. Also verified the mixture, (and throttle), cables are working properly. Rechecked the carb mixture set point and I'm getting 50 rpm rise at shutdown. No induction leak. No changes in CHT/EGT's. Only things In the past during takeoff I remember seeing 12.5-13+ or so GPH fuel flow.
Since I've started having this issue I'm now seeing fuel flows of 11-12.

I was suspecting the fuel pump but have since changed my mind. I'm now kind of suspecting the red cube. (I took it out and made sure it was functioning properly and seems to be).

Here's my question... If the red cube fails, what are the failure modes??? Quit all together or start sending bad numbers. Any thoughts are appreciated.

Edited: I did talk with the Dynon tech guys and they said the numbers for ROP/PK/LOP come from EGT/CHT/FF. His final thought was that if I'm seeing those numbers on the dynon then it's what is actually happening. In other words they don't lean toward anything with the instrument itself.
 
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I've been having some weird indications regarding ROP/PK/LOP lately. Flying along and at full rich it will sometimes float between the three. I have the Dynon D100/D120. I've checked everything I can. Facet Fuel pump seems to be working good... about 2 inches of fuel in a 5 gal bucket after only 20-30 seconds. I've verified there are no obstructions/kinks in fuel lines all the way to the tanks. Tank vents are good. Gascolator opened up and checked... nothing as well in the final screen going into the carb. Also verified the mixture, (and throttle), cables are working properly. Rechecked the carb mixture set point and I'm getting 50 rpm rise at shutdown. No induction leak. No changes in CHT/EGT's. Only things In the past during takeoff I remember seeing 12.5-13+ or so GPH fuel flow.
Since I've started having this issue I'm now seeing fuel flows of 11-12.

I was suspecting the fuel pump but have since changed my mind. I'm now kind of suspecting the red cube. (I took it out and made sure it was functioning properly and seems to be).

Here's my question... If the red cube fails, what are the failure modes??? Quit all together or start sending bad numbers. Any thoughts are appreciated.

Edited: I did talk with the Dynon tech guys and they said the numbers for ROP/PK/LOP come from EGT/CHT/FF. His final thought was that if I'm seeing those numbers on the dynon then it's what is actually happening. In other words they don't lean toward anything with the instrument itself.


How many hours are on the Red? Some had the problems around 500 hours. I wonder if yours fall within the statistical failure rate of the Redcube.
 
Red cube shouldn't have anything to do with the Dynon deciding ROP/LOP. That I believe is based on EGTs not fuel flow.

Ed Holyoke
 
Red Cube

I've been having some weird indications regarding ROP/PK/LOP lately. Flying along and at full rich it will sometimes float between the three. I have the Dynon D100/D120. I've checked everything I can. Facet Fuel pump seems to be working good... about 2 inches of fuel in a 5 gal bucket after only 20-30 seconds. I've verified there are no obstructions/kinks in fuel lines all the way to the tanks. Tank vents are good. Gascolator opened up and checked... nothing as well in the final screen going into the carb. Also verified the mixture, (and throttle), cables are working properly. Rechecked the carb mixture set point and I'm getting 50 rpm rise at shutdown. No induction leak. No changes in CHT/EGT's. Only things In the past during takeoff I remember seeing 12.5-13+ or so GPH fuel flow.
Since I've started having this issue I'm now seeing fuel flows of 11-12.

I was suspecting the fuel pump but have since changed my mind. I'm now kind of suspecting the red cube. (I took it out and made sure it was functioning properly and seems to be).

Here's my question... If the red cube fails, what are the failure modes??? Quit all together or start sending bad numbers. Any thoughts are appreciated.

Edited: I did talk with the Dynon tech guys and they said the numbers for ROP/PK/LOP come from EGT/CHT/FF. His final thought was that if I'm seeing those numbers on the dynon then it's what is actually happening. In other words they don't lean toward anything with the instrument itself.

Hi Paul,

Is your Red Cube mounted with at least a 3 degrees up angle at the fuel OUT fitting? If not you are likely getting air in the flow through the transducer and can and has created fuel flows all over the scale.
Good luck.
 
Fuel Pump

Frankly, sounds similar to the issues I was having that turned out to be mogas vapor pressure. The pump pulls a partial vacuum in the fuel lines and actually "boils" vapor out of the gas at fairly moderate temperatures. This causes bubbles that mess up fuel flow readings as well as fuel pressure. Put avgas in one tank and your mix in the other and go fly. Be aware it can take up to two or three minutes after you switch tanks for the all the new tank selection takes effect.
Ed
 
To answer a couple of questions. Total time since new nine years ago is about 550 hours. I’ve never burned anything except avgas. I would say yes the red cube is mounted correctly. It’s been mounted The same for nine years. Until this problem occurred a week ago I’ve never had any weird readings.
 
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Paul, I have had a FF wander on my 7 in cruise at 8-10k . It was only when leaned nearly to its limit. The FF was wandering and confirmed by airspeed and EGT, and fuel pressure (IO360). This was after cruising for about 2.5 hrs. You might plot your data under these wandering conditions to see how it all tracks. The fuel temp rise was measured from fuse bulkhead at wing to fuel pump inlet. It was over 40F at a higher fuel flow rate during testing. I insulated above my exhaust pipe which is very close to the fuse skin in the 3" aft of the firewall and right under the fuel tube in my (7) tunnel.

I believe that careful analysis of your data can help confirm if your cube is part of the problem.

Also, a plugging filter could lower fuel rate too, right?
 
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