Fuel Flow Jabiru Engine
Posted: Sat Nov 28, 2020 3:53 pm
Hi All,
I thought I would post this here as reference for anyone that is interested.
I have been working with MGL kit since about 2013 and have used and installed a fair few systems for myself, friends in our group and some other people that asked for help. The people I deal with mostly have Jabirus or Jabiru engine aircrafts. For the most part the installs work great and there have been very few problems. Most were people that didn't really know how to use the units.
One of the things I always struggled with on a Jabiru engine (and there were a fair few posts from other people on the old forum) was getting fuel flow to read anywhere near accurate on a Jabiru. I am sure some people got a good, reliable figure but I have never managed it. Until now!
I have lost count of how many fuel flow senders I have installed in a jabiru aircraft from the MGL one, FloScan and RedCude. In virtually every instance I have never been able to get an accurate reading. I always thought it was the fuel system or types of fuel pipe used. I tried everything from dampers to re-positioning the sender in various places. Different pull-up resistors etc. In virtually all instances, the only way I could get a reading that was anything like it should be was to change the k-factor to 2 or even 3 times what it should be and even then it was only accurate at certain power settings. It got to the point where I gave up using the senders.
Then a while ago, someone gave me a FlightData Systems FC-10 as part of a clear out. I didn't bother with it for a while and then I decided to try it in my aircraft expecting much of the same. To my surprise, when I installed it, I got fairly accurate readings straight out of the box. This was using the same wiring and the sensor in the same position as it was before and no physical changes to the fuel system.
This got me wondering, what was so different about how the FC-10 did things and how the MGL did things. After all, fuel flow is only a matter of counting pulses. I didn't do anything else for a while and then curiosity got the better of me.
So I dug out my oscilloscope and wired it in to my aircraft so that I could watch the pulses while I was flying. First I did it on the MGL for a while and then I did it using the FC-10. To my surprise, there was a vast difference in the pulses being received. The MGL was very jagged with spikes and troughs whereas the FC-10 was a nice, consistent square wave. I just couldn't understand how this could be the case as both units just purely receive a pulse from the sensor. From this, I could see why I had to play with the k-factor on the MGL and it was so variable. The only time it was reasonable smooth was when the engine was off with the electric pump on or sometimes I got a reasonable figure with the engine idling.
After further testing, I discovered the difference. On a standard installation using MGL, the most likely installation was to power the sender from the aircraft bus and then put a 5K6 resistor on the RDAC to pull the sensor up to 5v for the pulse wire. The FC-10 does this differently. The FC-10 powers the sender directly. All 3 wires from the sender (PWR, GND and Sensor Wire) go directly to the FC-10 d-sub connector. You do not power the sender from the aircraft bus. What I discovered was that the FC-10 provides a constant, regulated 10v supply to the sender with the signal line pulled up to 10v via a 10k resistor.
So I decided to this a try on the MGL. I made a little 10v regulated power supply using standard electronic components. So the regulator was powered from the aircraft bus but provided a constant 10v supply to the sender and a 10v, 10k pulled up voltage on the signal wire. The signal wire went directly in to the FuelFlow input on the RDAC with no resistors on the RDAC.
I wasn't really expecting much but to my surprise, the MGL then started to display very accurate FuelFlow figures with the k-factor as it should be.
The Jabiru electrical system is not great. The charging runs from a stator that is all over the place and very noisy. The voltage varies between battery power and about 13.5v and it very erratic and noisy. This variable and noisy power I can only assume affects the sender causing the pulses to be erratic as well.
Just to prove this was not a fluke, I made the power supply for a couple of other Jabs at the airfield that also had inconsistent fuel flow readings. Once installed, both started showing very accurate figures.
Anyway, I thought I would share this as it has been quite a victory for me.
I thought I would post this here as reference for anyone that is interested.
I have been working with MGL kit since about 2013 and have used and installed a fair few systems for myself, friends in our group and some other people that asked for help. The people I deal with mostly have Jabirus or Jabiru engine aircrafts. For the most part the installs work great and there have been very few problems. Most were people that didn't really know how to use the units.
One of the things I always struggled with on a Jabiru engine (and there were a fair few posts from other people on the old forum) was getting fuel flow to read anywhere near accurate on a Jabiru. I am sure some people got a good, reliable figure but I have never managed it. Until now!
I have lost count of how many fuel flow senders I have installed in a jabiru aircraft from the MGL one, FloScan and RedCude. In virtually every instance I have never been able to get an accurate reading. I always thought it was the fuel system or types of fuel pipe used. I tried everything from dampers to re-positioning the sender in various places. Different pull-up resistors etc. In virtually all instances, the only way I could get a reading that was anything like it should be was to change the k-factor to 2 or even 3 times what it should be and even then it was only accurate at certain power settings. It got to the point where I gave up using the senders.
Then a while ago, someone gave me a FlightData Systems FC-10 as part of a clear out. I didn't bother with it for a while and then I decided to try it in my aircraft expecting much of the same. To my surprise, when I installed it, I got fairly accurate readings straight out of the box. This was using the same wiring and the sensor in the same position as it was before and no physical changes to the fuel system.
This got me wondering, what was so different about how the FC-10 did things and how the MGL did things. After all, fuel flow is only a matter of counting pulses. I didn't do anything else for a while and then curiosity got the better of me.
So I dug out my oscilloscope and wired it in to my aircraft so that I could watch the pulses while I was flying. First I did it on the MGL for a while and then I did it using the FC-10. To my surprise, there was a vast difference in the pulses being received. The MGL was very jagged with spikes and troughs whereas the FC-10 was a nice, consistent square wave. I just couldn't understand how this could be the case as both units just purely receive a pulse from the sensor. From this, I could see why I had to play with the k-factor on the MGL and it was so variable. The only time it was reasonable smooth was when the engine was off with the electric pump on or sometimes I got a reasonable figure with the engine idling.
After further testing, I discovered the difference. On a standard installation using MGL, the most likely installation was to power the sender from the aircraft bus and then put a 5K6 resistor on the RDAC to pull the sensor up to 5v for the pulse wire. The FC-10 does this differently. The FC-10 powers the sender directly. All 3 wires from the sender (PWR, GND and Sensor Wire) go directly to the FC-10 d-sub connector. You do not power the sender from the aircraft bus. What I discovered was that the FC-10 provides a constant, regulated 10v supply to the sender with the signal line pulled up to 10v via a 10k resistor.
So I decided to this a try on the MGL. I made a little 10v regulated power supply using standard electronic components. So the regulator was powered from the aircraft bus but provided a constant 10v supply to the sender and a 10v, 10k pulled up voltage on the signal wire. The signal wire went directly in to the FuelFlow input on the RDAC with no resistors on the RDAC.
I wasn't really expecting much but to my surprise, the MGL then started to display very accurate FuelFlow figures with the k-factor as it should be.
The Jabiru electrical system is not great. The charging runs from a stator that is all over the place and very noisy. The voltage varies between battery power and about 13.5v and it very erratic and noisy. This variable and noisy power I can only assume affects the sender causing the pulses to be erratic as well.
Just to prove this was not a fluke, I made the power supply for a couple of other Jabs at the airfield that also had inconsistent fuel flow readings. Once installed, both started showing very accurate figures.
Anyway, I thought I would share this as it has been quite a victory for me.