Rule of thumb equation or graph for sfc for turbofans
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Rule of thumb equation or graph for sfc for turbofans
Hi folks,
Hard to believe, I know but our company yanked all our fuel consumption tables out of all of our manuals, so on a long flight all we have is our flt plan listing a number of different altitudes. Does any one have any rule of thumb formulas for fuel / sfc consumption on turbojet aircraft working down from your optimum to say 10,000'. I scanned my Aerodynamics for Naval Aviators and Handeling the Big Jets 2nd edition to no avail. Yes I know jets love high rpm and high altitude but I was looking for a formula or two that could be worked from a baseline known quantity.
The reason for the question is: when all goes to sh**t it really can bite you.
Thanx,
Phil
Hard to believe, I know but our company yanked all our fuel consumption tables out of all of our manuals, so on a long flight all we have is our flt plan listing a number of different altitudes. Does any one have any rule of thumb formulas for fuel / sfc consumption on turbojet aircraft working down from your optimum to say 10,000'. I scanned my Aerodynamics for Naval Aviators and Handeling the Big Jets 2nd edition to no avail. Yes I know jets love high rpm and high altitude but I was looking for a formula or two that could be worked from a baseline known quantity.
The reason for the question is: when all goes to sh**t it really can bite you.
Thanx,
Phil
Join Date: Feb 2005
Location: flyover country USA
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Fuel flow baseline
One simple check is to compare eng #1 vs #2 (vs #3 vs #4) at matched EPR (or N1 as the case may be). Keep track of the FF difference from flight to flight - this will require setting up an informal system for sharing data between crews.
Note that the thirstier engine never will decrease FF - if the delta value decreases it will ALWAYS be the "good" engine getting thirstier, "catching up" to its mate.
Also - many years ago the engine makers used to produce slide rule (maybe circular slide rule) computers to represent a nominal engine on wing. I developed one that fit in the F/E's shirt pocket.
They worked pretty well as a crude trend monitoring tool.
But personally, I'd insist that the dunce who pulled the tables ride along on every flight so he understood what he hath wrought.
Note that the thirstier engine never will decrease FF - if the delta value decreases it will ALWAYS be the "good" engine getting thirstier, "catching up" to its mate.
Also - many years ago the engine makers used to produce slide rule (maybe circular slide rule) computers to represent a nominal engine on wing. I developed one that fit in the F/E's shirt pocket.
They worked pretty well as a crude trend monitoring tool.
But personally, I'd insist that the dunce who pulled the tables ride along on every flight so he understood what he hath wrought.
Join Date: Feb 2005
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There is a great site from NASA that has a free program that calculates SFC etc etc for many different engines for you and if you carry a laptop on board with you it might help.
Its a good program anyway!
http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
Its a good program anyway!
http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
Join Date: Jul 2004
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well i'm darned!
phil
tell me which a/c and i might just have done what you are doing. by the way i have issues with both the nasa prog and jack mattingly's that haven't been fully addressed so take care as always.
tell me which a/c and i might just have done what you are doing. by the way i have issues with both the nasa prog and jack mattingly's that haven't been fully addressed so take care as always.
