I am writing a model for LM Prepar3D to simulate the flight model of a commercial airliner.

The specification I am following requires some parameters from the engine (CFM56).

PRF(j)           Fan compression ratio of engine j (PRF = Ptf/Pt∞)
Ptf        Pa    Fan exit stagnation pressure
Pt∞        Pa    Infinite upstream stagnation pressure

j is the engine number (left or right)

I have googled these parameters, but I haven't found any way to calculate these. Do they depend on the throttle setting? Or are they fixed on a per engine model basis?

EDIT: These data are needed to model the aerodynamic coefficients (coeff lift, coeff drag and moments) and not the engine itself. My problem is that I need to understand how to calculate PRF(j) but I don't know whether it's easier to find PRF(j) or to obtain it from the ratio between Ptf and Pt∞.


closed as unclear what you're asking by FreeMan, Ralph J, Gerry, Sean, xxavier Nov 6 '18 at 6:44

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.


Your question doesn’t make complete sense, because to model the performance of an engine, you need a lot more parameters than the 3 listed. In addition, the first one can be calculated from the second two, so, aside from missing many parameters from the list, you also have unnecessary duplication.

Let’s start at the end:

  • Infinite upstream stagnation pressure: This is the ambient total pressure. It’s a function of ambient static pressure (which is a function of altitude) and Mach number of the aircraft.

  • Fan exit stagnation pressure. This is the total pressure at the exit of the fan. It’s a function of the fan inlet total pressure (which is equal to the item above, except for small losses that occur in the intake duct), and rpm of the fan (and hence throttle angle).

  • Fan pressure ratio. As shown in you question, this is given by the division of the exit pressure by the first term. Typically, for a single stage civil fan, this is about 1.8.

  • $\begingroup$ What I am modelling are the aerodynamic coefficients (coeff lift, coeff drag and moments) and not the engine itself. PRF(j) where j is the engine number (left or right) is the number that I need in the formula but I don't know whether it's easier to find PRF(j) or to obtain it from the ratio between Ptf and Pt∞. Your answer is very helpful! So when it comes to calculate the fan exit stagnation pressure: the rpm of the fan is a direct function of N1, is it right? $\endgroup$ – Afe Nov 5 '18 at 11:36
  • $\begingroup$ @Afe then you should state so in your original question. Please edit your OP to include all the detail you can about what you're after, otherwise people will be just guessing as Penguin did. $\endgroup$ – FreeMan Nov 5 '18 at 13:54
  • $\begingroup$ @Afe. OK, but I don’t see how the engine performance has any connection whatsoever with aerodynamic aspects of the wing or aircraft (coefficient of lift, drag and moments). So, still confusing. Nevertheless, I would suggest if the desire is for PRF(j), the most logical way to obtain that is from Pt infinity and Ptf. The rpm of the fan and N1 are exactly the same thing, just different names. The 1.8 figure I quoted I should have indicated is a typical fan pressure ratio of a single stage fan at max rpm. $\endgroup$ – Penguin Nov 6 '18 at 11:30
  • $\begingroup$ I haven't personally written the equations of the flight model (217 pages to be accurate), but I am writing the code to test them in Prepar3D (as I have written in the introduction): this is why I am asking for clarifications. The PRF is required multiple times i.e. to calculate Cl due to engines jet effect on the HTP or the incremental drag due to thrust reversers. Pt∞ is a given data from the simulator, but now what is the relation between rpm of the fan and Ptf? $\endgroup$ – Afe Nov 7 '18 at 12:01
  • 1
    $\begingroup$ @Afe. The relationship between rpm of the fan and Ptf is given by the compressor map of the fan. See: en.m.wikipedia.org/wiki/Compressor_map $\endgroup$ – Penguin Nov 15 '18 at 11:18

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