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Old 07-13-2018, 06:33 AM
LBunch LBunch is offline
Join Date: May 2018
Location: Venice FL
Posts: 3
Default RV-12 needs two Fuel Flow Transducers?

Since the fuel flow transducer is in the supply line from the tank, but there is a return line direct from the engine pump to the tank, it would seem the fuel flow indicated to the EFIS will always be higher than actual fuel consumed by the engine if there is ANY fuel returned to the tank. My plane demonstrates this, since the totalizer quantity remaining is always low relative to the two float gages.

I did see a schematic for a two-transducer system somewhere. Is this a VAN's design? If so, where can I find it?

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Old 07-13-2018, 09:26 AM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 8,147

In the case of an orifice, like we have in the fuel return system of the RV-12, if the fuel pressure stays constant, the actual flow through the orifice will be relatively constant.

Because of this, the K factor can be adjusted to calibrate the system and artificially compensate for the return flow. Do this by utilizing the standard calibration procedure in your EFIS documentation. This procedure is always required whether there is a return flow or not because simply using the K factor of the sensor does not account of the many other installation variables that exist.

When properly calibrated, the fuel computer in the RV-12 is quite accurate... in cruise flight (the only time it is really all that important..... for use on longer cross country flights). It does have some error when doing a lot of pattern work because idle and lower power settings do result in a different fuel pressure, but even then it is not off by very much, and will always have error in the conservative direction (will say you have less fuel remaining than you really do).
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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Old 07-15-2018, 06:30 PM
Azjulian Azjulian is offline
Join Date: Dec 2015
Location: Scottsdale, Arizona
Posts: 150

Works exactly as Scott posted. I calibrated it based on a cross country flight and get very accurate results.
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Old 07-17-2018, 10:30 AM
HansLab HansLab is offline
Join Date: Jan 2015
Location: Netherlands
Posts: 67
Default Helpful

This is very helpful! So - what IS your current K-factor (asks the builder, not yet into flying his awful machine)? I could start with that figure, for sure - better then the 18000 advised.
Hans Labruyere
build & sold project Kitfox IV
Bought project & building RV12 #120742
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Old 07-17-2018, 03:05 PM
DHeal DHeal is offline
Join Date: Sep 2006
Location: Windsor, California
Posts: 797

I have been using 107,510 as my K-factor for the past 300+ hours. This value tracks well with my actual fuel use. My flights typically average about one hour in length -- if you take many multi-hour flights your K-factor might be different. I find quite a bit of variance in the indicated fuel flow (gph) value in flight but at the end of the flight the overall fuel consumption numbers are very close (e.g., within 0.1 - 0.2 gallons per 4.5 gallons used).
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 (circa 1965) - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B 2020) - N124DH flying since March 2014 - 760+ hours (as of May 2019)!
VAF donation through June 2019.

Last edited by DHeal : 07-17-2018 at 03:15 PM.
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