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my classmates and I are trying to find a mathematical model for a low bypass turbofan engine, we ended up finding a model given by Snorri on "General Aviation Aircraft Design", but the behaviour of the graph is totally different to the ones given by engine manufacturers. What we want to achieve is the behaviour of a low bypass turbofan engine from sea level at Mach 0 to an altitude of 15000 meters and Mach 2.1. The first graph is taken from Snorri, while the second graph is from other textbook. I would like to know why they have such a different behaviour, thanks!

This is the graph shown by "Snorri", in which we see that when increasing the mach number we actually increase thrust. Does anyone knows why this happens?This other graph has a completley different behaviour to the one suggested by the textbook.*

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    $\begingroup$ Did you notice the first graph is not thrust, but thrust ratio? Is it intentional? $\endgroup$
    – mins
    Dec 8, 2021 at 22:54
  • $\begingroup$ Yes, I did notice. What I was wondering is that it gets values bigger than 1, so that means that the thrust is bigger at certain mach numbers when keeping the same altitude. While the second graph never increases the thrust more than the sea level at Mach 0. $\endgroup$ Dec 9, 2021 at 0:36
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    $\begingroup$ When the plane picks up speed there is more airflow through the engine, function on inlet shape etc. Hence my question about a reference of the setup of the 2nd graph. $\endgroup$
    – Koyovis
    Dec 9, 2021 at 9:15

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The graphs have different vertical axes and units. The first one plots Thrust@Altitude/Thrust@SeaLevel on the vertical scale, which is dimensionless. The different lines depict this ratio at different altitudes.

The 2nd graph plots thrust in pounds. Period. (And gives a value for Specific Fuel Consumption). Different graphs for different altitudes. In order to convert into a graph like the first one, all values at altitude need to be divided by the corresponding value at Sea Level.

What we want to achieve is the behaviour of a low bypass turbofan engine from sea level at Mach 0 to an altitude of 15000 meters and Mach 2.1

Easiest done with the 2nd graph type, since you need the following steps:

  • Determine required thrust @ TakeOff, climb, cruise
  • Make a time scale of Mach, altitude, thrust. So at each time interval, determine these 3 values.
  • Look up the maximum available thrust in the graph - throttle setting is a variable as well.

There is no further information on what the circumstances are of the measurements in the 2nd graph. Is it engine only in a ground based test stand, or aeroplane mounted data in flight? A reference would be useful.

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