Fuel flow fluctuations- normal or not

kaweeka · #1 02-05-2018, 07:59 AM

I have an IO 320 B1A in my -9a with dual Lightspeed ignition and Airflow Performance injection with the purge valve installed. Since upgrading to the G3x, I have wanted (and finally got around to doing it) to download the engine data and see what it looks like. I am NOT an engineer! The plane regularly flies great. Cross country flights usually around 8500-10500 ft., 60-65% power, 6.7gph at 147kts TAS. That seems to be pretty reliable. She flies and feels smooth but something I noticed is this constant fluctuation in fuel flow between 6.2-7.1 gph. When I downloaded the data to SavvyAnalysis, This fluctuation matches exactly a small fluctuation in %power, so it is not just the Floscan transducer being erratic. Again, I cannot sense any changes. I posted in another thread but I think it got lost so I'm throwing it out for anyone to comment. The injectors, as far as I know, were not GAMI but everything seems pretty tight. I would love anyone with more insight to take a look and share any thoughts.
Thanks,
David

https://savvyanalysis.com/my-flights...b-2bffab95582d

vic syracuse · #2 02-05-2018, 08:58 AM

Depending upon where you have the flow sensor it is possible to see the fluctuations you mention. As for the % power matching it, that is a red herring in this case, as the % power calculations also use fuel flow as a determining factor, so it will fluctuate due to the fuel flow fluctuations.

Vic

airguy · #3 02-05-2018, 09:01 AM

I had the same thing happening to me in late 2016 - I searched and eliminated several possible causes, and was left with a Bendix servo that was in need of overhaul. I sent the servo and flow divider in during my annual January 2017 and that cured the problem.

Just like you I was able to correlate my fuel flow with an actual power change, both as computed and as a TAS indication during stable cruise flight.

kaweeka · #4 02-05-2018, 09:09 AM

The Floscan is after the electric AND engine driven pumps but before the flow divider/purge valve combination. It is rigidly mounted to the firewall with level plumbing before and after, and no bends close to the unit.

JohnInReno · #5 02-05-2018, 09:12 AM

My plane is similar to yours with completely different vendors. I have an RV9A with a Superior IO-320, dual Pmags, and AFS panel.

I also see a fluctuation in Fuel Flow even though my "Red Cube" is mounted in the tunnel between the fuel electric fuel pump and the mechanical pump on the other side of the firewall. The cube location is a compromise. It should be a long straight run close to the engine.

On the percent of power issue, it is not measured, it is calculated using a table that includes Manifold Pressure and RPM. It should correlate with FF.

I see vic beat me to it +1

kaweeka · #6 02-05-2018, 01:59 PM

I just had a very nice discussion with Kyle at Airflow Performance. He looked at the data and noted the small fuel flow pulses are likely a result of the pulses developed by the diaphragm of the engine driven pump. Since my transducer is BETWEEN the electric and engine driven pumps on the firewall, he suggested moving and re-arranging the system to go from the electric pump to the engine driven pump directly and to locate the fuel flow transducer in line between the throttle body and flow divider. He also noted a rigid mounted fuel transducer will fail more quickly due to the vibration. Thanks for all the responses. I'll proceed and see how things shake out.