Your projected maximum fuel burn rate for the 912 is probably correct and is
consistent with a specific fuel consumption of 0.5 lbs/hp/hour.
A subsequent article in the Europa Flyer highlighted the hazards of using flow
gauges based on the RS Components type of flow sensor. These are:
1. The jet in the sensor is 1.8 mm in diameter. The problem here is not
so much one of blockage (because the fuel should have been through a filter by
then) but........
2. The pressure drop across the sensor at a flow rate of 25 litres/hour
is about 0.3 bar (see data published by RS for this device).
0.3 bar just happens to be the fuel pressure that Rotax require at the
carburrettor! So if your pump is producing that pressure then the flow sensor is
removing it again (the sensor must of course be positioned downstream of the
pump)
I accept that you have had good operational experience of the device on your
Shadow, but did you know what your fuel pressure was at the carbs. and did you
know what margin of safety you had left?
My point is not that the device should not be used. Rather, the system is
unlikely to tolerate any weakness (eg gradual aging) in the effectiveness of the
fuel pump.
The fuel flow check demanded by the PFA upon completion of a project should
also include measurement of fuel pressure ( all in a climb-out attitude ) so
that a system containing the fuel flow sensor could be checked prior to first
flight. A fuel pressure gauge could then be used to monitor what happens
thereafter.
It might also be possible to bore-out the jet in the flow sensor and recalibrate
via the electronics thereby removing some of the above hazard.
On the subject of flow sensors which somebody else raised, I have been
researching alternatives for some time and have come up with nothing other than
the very expensive US products (Flowscan etc.). Smiths used to do one in the UK
but don`t anymore. Bosch may do one but so far haven`t owned up to it! Most
other units are "industrial" and intended for other duties (large and heavy).
Rgds. Duncan McFadyean
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