Hi all,
I'm currently drafting some cruise performance charts to be inserted into the Flight Manual of a new airplane model powered by the Rotax 912ULS coupled with a GT Propeller GT-2/173/VRR/FW101-SRTC two bladed, fixed pitch propeller.

I've derived the power required for level flight, propeller efficiency and power available using the 'Bootstrap Approach' technique from T.Lowry article. Now I'm trying to figure out what fuel flow should I use for the 912ULS so to be able to produce the TAS vs fuel flow cruise charts.

From the Rotax Operator manual paragraph 1.10 I get the following figures:

Specific consumption at max continuous performance = 285g/kWh

Fuel flow at take-off performance = 27 lt/h
Fuel flow at max continuous performance = 25 lt/h
Fuel flow at 75% continuous performance = 18.5 lt/h

In the performance data section of the manual I get:

Max continuous power = 5500RPM and 69kW

If I use the specific fuel consumption quoted for the max continuous performance I get

69kW x 285 g/kWh = 19665g/h

considering a fuel density of 720g/lt I get:

Fuel Flow at Max continuous performance = 19665 g/h / 720 g/lt = 27.3 lt/h

This number does not match with the fuel flow indicated in section 1.10 that, for the max continuous power is 25 lt/h so, apparently, it seems the Operator Manual is somewhat inconsistent.

What am I doing wrong?

Additionally, if the specific fuel consumption of 285g/kWh is only valid for MCP rating, what number should I use for all the other ratings?

Thanks for your help.
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