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RV-12 needs two Fuel Flow Transducers?

LBunch

I'm New Here
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?

Thanks,
Les.
 
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).
 
Works exactly as Scott posted. I calibrated it based on a cross country flight and get very accurate results.
 
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.
 
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).
 
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