D180 Fuel Flow, Rotax 912, and Red Cube sensor?

[charlievictor102]

I’m using a D180 with a Rotax 912 and a Red Cube flow sensor. I have the fuel return line installed from the fuel rail back to the tank. With the return line blocked off I get a pretty reasonable fuel flow and consumption with a K-Value of 65000. With the return line restored, I get a K-Value of 108750. So what do I enter in the Fuel Flow setup screen for K-VALUE and RETRN?

 

[dynonsupport]

Curt:

All of Van's recommended settings for the FlightDEK-D180 in an RV-12 are detailed in a document called 12-D180-PRE-SETS.pdf available from the Van's website - http://vansaircraft.com/public/download/RV-12/Dynon/12-D180-PRE-SETS.pdf.

However, without ALSO measuring the fuel being returned from the engine to the tank (with a second fuel flow sensor installed), the FlightDEK-D180 cannot display an truly accurate fuel flow.

Unfortunately, the FlightDEK-D180 has a single fuel flow sensor input. We explain this in the FlightDEK-D180 Installation Guide, Page 3-10 - 912(ULS) or 914 Installation.

In the FlightDEK-D180, with engine type set to ROTAX, the RETURN setting is a static "offset" applied to the pulses being received from the fuel flow sensor. By experimentation with the K-factor (increase K-factor to lower the fuel flow, decrease K-factor to increase the fuel flow), or the RETURN value, you can get a fuel flow indication that "more closely approximates" your overall fuel usage, but it won't be (can't really be) accurate without measuring the ACTUAL fuel that is being returned to the tank (which varies depending on the conditions - idle vs takeoff vs cruise, etc.).

I’m using a D180 with a Rotax 912 and a Red Cube flow sensor.  I have the fuel return line installed from the fuel rail back to the tank.  With the return line blocked off I get a pretty reasonable fuel flow and consumption with a K-Value of 65000.  With the return line restored, I get a K-Value of 108750.  So what do I enter in the Fuel Flow setup screen for K-VALUE and RETRN?

 

[charlievictor102]

My plane is not an RV-12 but a Zenith CH701. I’ve seen that PRE-SETS document and found it extremely useful for D-180 setup. I actually have a Red-Cube sensor in the return fuel line but gave up trying to use it. I spent months of experimenting trying to get it to work correctly. I have a circuit that subtracts the pulses of the return fuel path from the forward path. I moved the sensor around before and after mechanical fuel pump. I added a pulsation damper. Change the orientation of the sensors. Used exactly the same fuel line geometer for the forward and return fuel sensors. Static, on the ground, electric fuel pump on; it made sense. With the Rotax 912 running no such luck. I just could not get it to give meaningful results. Probably the relatively small flow in the return path and the engine vibration. As I said, with the return line plugged, the forward flow reading alone does really good.

So, since the fuel pressure is pretty constant for idle and cruise, I figured that the return flow would be pretty constant too. What I was hoping to find is that the RETRN parameter in the setup menu was in units of a certain amount of sensor pulses per unit of time. For example, SENSOR PULSES PER HOUR. Then that number of pulses per hour would be subtracted from the pulses coming from the forward sensor by the D180.

So this is what I measured for one calibration interval:
Fuel Filled (actual) = 10.4 gal
Fuel Used (computed) = 17.4 gal
Hobbs Duration = 5.8 hours.

So the resulting number of pulses during this interval would be:
10.4 X 65000 = 676000 pulses (actual)
17.4 X 65000 = 1131000 pulse (computed)
With a difference of 455000.

So it seems reasonable that the RETRN setup parameter would be:
455000 pulse / 5.8 hours = 78448 pulses/hour

So does the K-FACTOR need to stay at 65000 or should it go to 108750 (too large to enter in setup).

It would be really nice to know exactly what the D180 is doing with the fuel flow calculation. It seems that this is something that should be knowable.

 

[Dynon]

The return parameter is a constant rate (pulses per hour) that is subtracted from the fuel flow Return_flow_number / K-factor = GPH

For example: If K-factor is 68000 (which means there are 68000 pulses per gallon), then a return flow rate of 6800 pulses would represent 1 gal/hr of return fuel flow.

So with a red cube, where 1 gallon is 68000 give or take, you only have about 1.5 gph of return flow that you can accommodate. If you use a Flo-Scan, which has a k-factor of about 30,000, you can accommodate about 3gph in return. (This feature was actually designed when we were using/selling Flo-Scan sensors)

 

[charlievictor102]

Excellent info! Since it looks like I’m returning about 1.2 gph I think I can make this work.