Assessment of Numerical Models for Thrust and Specific
Fuel Consumption for Turbofan Engines
Author: Oliver Schulz
A study was undertaken to investigate the variation of thrust and specific fuel consumption due
to different parameter changes. The main goal was to find empirical models describing the relationships
in the public domain and to evaluate them by comparison with actual engine data.
31 models were found, 14 describing the variation of thrust and 17 describing the variation of
specific fuel consumption (SFC). By using Excel spread sheets the equations where calculated for their
different parameters and compared with actual engine data. Generally all models where found to be accurate
but in this process a polynomial model over the Mach number was found to be most accurate for
the description of the take-off thrust. An equation given by Denis Howe (in Aircraft Conceptual Design Synthesis, 2000)
was found to be most accurate for the evaluation of the climb thrust.
Additionally equations describing the variation of thrust with bleed air extraction,
temperatures other than the international standard atmosphere (ISA) and for the evaluation
of cruise thrust with take-off thrust for preliminary design were found and partly evaluated.
Due to the lack of reference date the models describing the variation of specific fuel consumption
could not be evaluated but their general trend was found to be correct. An equation also found in
the book of Howe 2000 was found to be especially useful for the change over height and speed.
The usually unknown starting point of all equations describing the specific fuel consumption
was approximately given for his equation. There were also models found describing the variation
of specific fuel consumption with reduced power, power off-take, bleed air extraction and temperature other than ISA.