Fuel flow anomaly at higher altitudes

[McSteff]

It has been bugging for me a while and I have found one old post ( http://www.vansairforce.com/communit...ad.php?t=88963
on this altitude related FF subject:
my fuel flow indication does not read accurately with increasing altitude. I am pretty happy with my k-factor of 71000 and the flow reads correctly approx. 4.5 gal at 5200RPM at lower altitudes. See graphs attached of a longer cross country flight at various altitudes:

Top chart : FF and DA
Bottom chart: FF and RPM



Measuring the fuel quantity with a dip stick shows an avg of 4.5gal burnt per hr post flight.

Appreciate your help.

[RV10inOz]

I am not sure what you mean?

AT higher altitude the fuel flow WILL be less. The carbs are pressure compensating so the flow will reduce due to lower DA.

I would also think as the float changes up and down at times the fuel flow at any given moment could be up/down due to flow surges at low demand?

[Mich48041]

Does the electric fuel pump run continuously or do you shut it off during cruise? I am thinking that maybe the fuel forms little vapor bubbles at higher altitude and that causes the flow meter to read incorrectly. But that is just a wild guess.

[McSteff]

I don't have a separate switch for the elec fuel pump, it runs continuously. As per the graph the flow rate peaks at 7500 feet but then drops slightly at 9500 feet. But still way above what I would expect at such altitude (the POH states values below 4 GALs).

Seems like not many other users have come across this phenomenon, so I'll also touch base with Dynon.

[yankee-flyer]

but at density altitudes somewhere around 6500 feet the fuel flow reading does indeed go very high. Postflight checks show the fuel burn is as usual, but the indication is very definitely wrong. Nobody ever came up with an answer, but it's probably something ion the Dynon system. A case of too much information!

Wayne 120241/143WM 311 hours

[RFSchaller]

After about six months of cruising at 5100 RPM and getting 4.5 GPH I set my K-Factor to show 4.5 GPH while on a trip. In a 3 HR flight it is now within 0.3 gallons of actual fuel used.

[McSteff]

Originally Posted by RFSchaller

After about six months of cruising at 5100 RPM and getting 4.5 GPH I set my K-Factor to show 4.5 GPH while on a trip. In a 3 HR flight it is now within 0.3 gallons of actual fuel used.

And that works for you at any altitude flown?

[Driftdown]

Originally Posted by RFSchaller

After about six months of cruising at 5100 RPM and getting 4.5 GPH I set my K-Factor to show 4.5 GPH while on a trip. In a 3 HR flight it is now within 0.3 gallons of actual fuel used.

Regarding the altitude, I'm interested in the answer to that question also.

Additionally, did you reset your k-factor while in flight, to 4.5 gph or did you figure a predetermined computation and reset it on the ground?

I still have to do it to my unit. I plan on setting it to 4.5 gph, at 5100 RPM, while in flight, at 2500 MSL (my most frequent cruise altitude in Florida). After that, I will make a couple long cross country flights and fine tune the setting. But I think that is a good place to start. Anybody concur with that?

Any ideas or advice from those who have done this before me is much appreciated.

The fuel flow computer on SkyView is a great fuel management tool to assist in determining the fuel status during cross country flying. I intend to use it to verify what I am seeing on my sight tubes.

[McSteff]

Vans provided the following response on the FF issue:

"It is a known issue - but we don't have a fix for it yet. With the G3X system in the RV-12, we do not use the Fuel FLow feature, as with the current fuel system it can't be made truly accurate under all conditions. With 20/20 hindsight, the smart thing to do would have been to leave the sensor out of the fuel system, and save customers $200.

We have conducted a number of tests with various mods, but do not yet have a solution. The tank float gauge, if correctly calibrated, will give you the most accurate level of fuel remaining."

In a follow up email, they told me that even a second transducer in the return line will not provide perfect results either due to the always on pulsating elec fuel pump.

[rv3flier]

Last week I flew from La Center, WA to McCall, ID at 9500' and observed the same fluctuation in fuel flow at 5200 rpm. The fuel flow went from 5 gph to over 7 gph and then gradually came down after about 2 hrs in flight at the same altitude and rpm to 4.8 gph. Actual fuel used during the flight start to stop was 5 gph. Gallons of fuel used indication did not match the gallons remaining indication on the D180. As previously stated, the in tank fuel gauge I installed was accurate.

It's nice to know the background of the inaccuracy and that there's no fix.