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 Need Help Converting Tsfc At Cruise To Sea Level
 Fishfqtv From United States of America, joined Dec 2008, 2 posts, RR: 0Posted Mon Dec 29 2008 22:21:02 UTC (4 years 5 months 3 weeks 1 day 18 hours ago) and read 3981 times:

 I'm tryng really to learn about TSFC calculations but this is far from simple, at least for a beginner. Can some one help me on this one and explain the steps in layman terms so I can do this again for more engines? I have a few that I want to figure, I'm hoping some of you here can get me on the way. I have the TSFC for an engine at cruise and need to get the TSFC for it at take off, sea level. I don't need this to be 100% exact because as I understand it air temp will affect this as well, just need to get in the right ballpark. Here is the data I have, is all in metric now. I don't know how much of this is needed, hopefuly this is enough to make the calculation. Assume max weight and fuel at takeoff. Thrust, kg take-off: TH< +30°C , PH > 730 mm Hg, H=0 16000 cruise: Н=11 km, М=0.8 3500 Specific fuel consumption - kg/kgf hour 0.595 Airflow kg/s - 470 BPR - 4.6 Engine Weight, dry, kg - 2,950 Aircraft operating weight empty,kg - 58,800 Max Fuel weight, kg - 32,800 Max payload, kg - 21,000 Max T-O weight, kg - 103,000 Thanks
 Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80Reply 1, posted Tue Dec 30 2008 10:58:35 UTC (4 years 5 months 3 weeks 1 day 6 hours ago) and read 3955 times:

 Quoting Fishfqtv (Thread starter):Can some one help me on this one and explain the steps in layman terms so I can do this again for more engines?

Basically, you need two numbers...fuel flow and thrust, at the same conditions. TSFC is then just the fuel flow divided by the thrust (units of mass / time / force)

 Quoting Fishfqtv (Thread starter): Specific fuel consumption - kg/kgf hour 0.595

I feel like I must be missing something...this number looks like the TSFC, in which case you don't need to calculate anything.

There's no real way to convert TSFC at one operating condition to another that I'm aware of...every engine is going to have a unique TSFC curve that shows the TSFC at different operating points.

Tom.

 Fishfqtv From United States of America, joined Dec 2008, 2 posts, RR: 0Reply 2, posted Tue Dec 30 2008 12:39:53 UTC (4 years 5 months 3 weeks 1 day 4 hours ago) and read 3944 times:

 Quoting Tdscanuck (Reply 1):I feel like I must be missing something...this number looks like the TSFC, in which case you don't need to calculate anything.

Yes, it was the TSFC but at cruise (I'm assuming... that's too high to be TSFC at takeoff on a modern jet engine) I'm needing to find the TSFC at takeoff for this engine...

Ok, if there is no way to convert then how does one find out the fuel flow of a given engine at various thrust levels? It seems that the engine manufacturers tend to only give it at one level that I can find and its either at Takeoff or Cruise but rarely can I find both. I can't seem to find any more comprehensive info other than just what the manufacturers have on their websites.

Just for example how does one do an apples to apples comparison of a RR engine (who publishes at cruise) and a P&W engine or someone else who publishes at T-O?

 Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80Reply 3, posted Tue Dec 30 2008 15:21:02 UTC (4 years 5 months 3 weeks 1 day 1 hour ago) and read 3926 times:

 Quoting Fishfqtv (Reply 2):Ok, if there is no way to convert then how does one find out the fuel flow of a given engine at various thrust levels? It seems that the engine manufacturers tend to only give it at one level that I can find and its either at Takeoff or Cruise but rarely can I find both. I can't seem to find any more comprehensive info other than just what the manufacturers have on their websites.

This type of data is unlikely to be easily available in a public format, but it's essential for flight planning. If you can get ahold of the FCOM for an airplane with the engine you're interested in, the Performance Inflight section will, I think, have the data you need. Although it will probably involve extracting values from multiple tables.

tom.

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