I fail to understand (one of my inherent characteristics, some might say!)
how one flow transducer in the supply line to a Rotax 912-type engine can
measure the engine's consumption, PROVIDED that the return line is OPEN.
As has been covered on this List previously, the return flow through the
factory-supplied restrictor is substantial - as it should be, to serve the
intended purpose.
Adjusting the K-factor to compensate for the return flow will give correct
net flow (= consumption) at only ONE specific flow rate: The flow rate at
which the K-factor has been calibrated.
If the instrument also has a zero offset adjustment (which the E.I.
instrument does not have), one flow transducer in the supply line would
work, PROVIDED that the return rate is constant, regardless of the fuel
pressure and flow rate past the return line off-take - which it is not.
The K-factor is only a multiplier, telling the electronic logic how much
fuel is going through with each full rotation of the flow transducer's
paddle wheel. It cannot cause any flow subtraction to compensate for the
returned fuel (even if the return flow rate were constant). If plotted in a
graph, the flow rate vs. transducer output is always a straight line going
through zero, where the K-factor sets the line's inclination.
For my 912 ULS, I have GRT's digital engine monitoring system (great!),
which has one fuel flow input channel. I could therefore read the total
flow going to the engine. No way of telling how much went to the carbs and
how much returned to the tank, however. Only then did I realize how much
fueI is actually returned. I fitted a shut-off valve in the return line
(NOT recommended - may be bad if you forget to re-open!!) to determine the
consumption of the engine at various power settings during the test flying
period, for fuel range calculations. The valve was subsequently removed,
and I fitted the E.I. instrument with two flow sensors and the associated
differential module (which subtracts electronically the return flow count
---From the supply count).
In my opinion, one single transducer will serve only one useful purpose:
With the engine stopped and the electric feed pump running, one can check
that the return line is open ............
The E.I. with two transducers and diff. module shows zero flow (only net
flow is displayed) when priming the engine with the electric pump before
start. Total flow is returned.
Regards,
Svein
LN-SKJ
|