The published TSFC value is usually for cruise conditions, so I've made it vary with N2 to better model fuel flows at low N2, where the compressor is inefficient. I matched the results with known fuel flow values for the CFM-56, and the results are pretty good.
Do you have a better formula to vary TSFC? If so let's get it into the model.
No, I have no better formula, but I found other information:
This site says, that the TSFC is constant for a jet:
http://142.26.194.131/aerodynamics1/Performance/Page8.html
On this site I found tests results of various engines:
http://www.caa.co.uk/srg/environmental/emissions/document.asp?groupid=704
For the CFM56-5B4 I found
100 % is 117,9 kN = 26500 lbf
Power Fuel flow kg/s TSFC kg/daN *h
100 % 1.166 0,35
85 % 0.961 0,35
30 % 0.326 0,31
7 % 0.107 0.47
This seems to be constant in the range of 30 % to 100 %
At 'Klaus H�necke: Technic of modern airliners' I found at
TSFC for cruise thrust = 0.611 TSFC for static thrust = 0.35 ( with meets the ICAO values)
May be it is a good idea to meet the published TSFC values at cruise speed (N1 about 80 or 85 %), not at N1=100 % To put these conditions in a formula, I have to sleep on it one or two nights.
Kind regards
Hans-Georg
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