fuel flow fluctuations

prkaye · #1 11-14-2010, 05:55 AM

Particularly at lower power settings, I am seeing significant fluctuations in my fuel flow readings, even in a steady cruise configuration. I have a carb and a red cube sensor just upstream of my carb. At around 50-55% power, I will see my fuel flow readings move up and down by up to 1GPH. At higher power settings the magnitude of these fluctuations decreases.
Other readings (MAP, RPM, engine sound) remain steady.
Does this suggest a problem, or is it typical for a carb' ed engine?

Ironflight · #2 11-14-2010, 06:22 AM

That is pretty normal for the configuration that you describe Phil - especially (and more-so) if th boost pump is on. I generally see a fluctuation about the "steady state" of maybe +/- 0.4 gph, and if I flip the boost pump on, I gain another 1.0 or so. None of it is a real (I believe) flow change, just indication. The good news is that it averages out to the actual flow - unless you leave the boost pump on all flight, in which case, you'll definitely have more fuel left onboard than your totalizer will indicate.

Paul

Bob Martin · #3 11-14-2010, 06:56 AM

Quote:

Originally Posted by Ironflight

The good news is that it averages out to the actual flow - unless you leave the boost pump on all flight, in which case, you'll definitely have more fuel left onboard than your totalizer will indicate.

Paul

Paul, Thanks for this info. I don't know why this didn't occur to me before.
I installed the red cube and set the K factor accordingly on xcountry flights.
but when flying locally the accuracy would change from .1 to 1gallon off in the safe direction. Now I understand the accuracy is changing with the amount of boost pump use. Which is more on multiple local flights. Thanks for pointing this out in a way that clicked for me!

schristo@mac.com · #4 11-14-2010, 06:59 AM

The red cube is fed with 5 volts and generates a square wave from the impeller turning with flow... the count of the breaks in the signal is very accurate but the signal is susceptible to some interference if the connections are not very solid and they are subjected to vibration, and can show up as drifting or erratic flow. Double check your connections to and from the transducer.

I chased this for a bit with my setup (fuel injected with the red cube between the fuel meter and flow divider - downstream of both fuel pumps) and only eliminated it (drifting high) after changing the connections at the red cube to inline d-sub pins with heat shrink.

I have no fluctuations with this configuration regardless of the state of the fuel pump.

Ron Lee · #5 11-14-2010, 08:06 AM

I get the same fluctuations on my carbed engine. Not sure of the flow sensor but I have a Rocky Mountain engine monitor. To get instantaneous fuel flow I have to monitor the fuel flow for a short time (30 seconds or more) and mentally average the values. Good thing is that fuel remaining is very close.

Ironflight · #6 11-14-2010, 08:48 AM

Quote:

Originally Posted by Ron Lee

I get the same fluctuations on my carbed engine. Not sure of the flow sensor but I have a Rocky Mountain engine monitor. To get instantaneous fuel flow I have to monitor the fuel flow for a short time (30 seconds or more) and mentally average the values. Good thing is that fuel remaining is very close.

Yup - I'd say it's about a 30 second period.

Ron Lee · #7 11-14-2010, 08:53 AM

I don't know what the period is. I see it go high, low, mid range. Not even sure if there is a pattern. I just have to wait for enough different fuel flow values so that my mental averaging is semi-close. It may take a minute.

I only do that if I am recording performance data since the actual value is not that important most of the time. Fuel remaining is the more important value

Ron B. · #8 11-14-2010, 09:01 AM

With the electric fuel pump on, the fuel flow indicates higher than with the pump off. Is the actual flow the same in both conditions and the reading incorrect or will fuel consumption be higher if the pump was forgotten on for the entire flight?
Ron

Finley Atherton · #9 11-14-2010, 02:01 PM

I have tried unsuccessfully for over three years to solve the big FF fluctuations I get on my VM1000C (see http://www.vansairforce.com/communit...ad.php?t=32144.) Post #4 gives a graph of the sort of fluctuations I get. I think the basic problem for me is that the VM1000C does not have any sensitivity settings to electronically damp out the fluctuations you get with a carb. Check to see if you can adjust the sensitivity on your FF gauge.
Some time ago Bob Martin installed a FC-!0 Fuel Flow gauge which has sensitivity adjustment for his carb engine and reported reasonably steady readings, so, recently I ordered the FC-10 from ACS and will be installing it as a stand alone gauge and maybe the problem will be solved?

Fin
9A 0-320 (carb)

videobobk · #10 11-14-2010, 03:35 PM

Like Paul noted, I get a higher FF reading with the aux pump on. I initially set the gauge (AFS 2500) to give me accurate average readings, but that was during the initial 40 hours. These were mostly short flight with higher boost pump usage. I had to reset once I started doing more X-C, or I would have had less fuel remaining than indicated, not a good thing. Now I am pretty much dead-on doing X-C or I have more than indicated. I don't know how to correct this, but I don't see it as a problem.

Bob Kelly