TURBINE ENGINES

Turbine engines power both turboprops and jets, and so both these engine types are considered as essentially the same thing (the main difference is that turboprops have a propeller on the outside of the engine while jets have fan blades inside the engine housing). This is true as well for the way that Microsoft Flight Simulator simulates these engines, as the same turbine and fuel calculations are used for both, and only the way the thrust is output is different.

 

 

The Turbine Engine Simulation

The turbine simulation is more complex than that of The Piston Engine Simulation but it is in some ways much more flexible. The basic idea behind the turbine simulation is that the throttle commands a target speed, and the fuel flow controller will try to reach this target. Then - based on the engine speed - thrust is calculated. In general the simulation consists of the following calculations, in the given order:

NOTE: This is for a jet turbine engine, with the differences for a turboprop being outlined at the bottom of the section.

 

• Computing the fuel demand: Is fuel available ? How much fuel is taken from the tanks ?. This part does not really affect the engine performance, as it only computes the final fuel flow which gives the amount taken from the tanks each frame.

 

• Compute inlet flow conditions: Here the simulation essentially calculates the two "correcting values" necessary for the whole turbine model to behave as it should based on the altitude and the flight Mach number.

  NOTE: when NOT using the Supersonic Inlet (supersonic_inlet) parameter, inlet condition calculations will only consider a mach number inferior or equal to 1.0.

  • The higher the mach number is the more thrust the engine will generate
  • The higher the mach the higher the temperatures in the engine will be.
  • The higher the altitude the lower these previous two values will be, which gives a sense of equilibrium.

 

• The correcting values are used to calculate the actual N1 and N2 of the engine as well as the corrected thrust. However first the corrected Ns - as well as the corrected gross thrust - need to be calculated:
  • Calculate the effective throttle input: Rather than only using the actual throttle input the user gives, the effective throttle takes into account the effect of the autopilot and autothrottle as well. It also allows us to consider the afterburner range when applicable.
    NOTE: The effective throttle is such that the engine command will see an effective throttle input of 100% when at the afterburner (AB) threshold. AB effect will be added when going over the threshold without increasing the effective throttle over 100%.
  • The effective throttle is used for fuel flow calculation: The turbine engine command works by regulating the corrected fuel flow to control N2 which in turns drives N1 which is the value used for thrust calculation.
  Note that the corrected fuelflow is the fuel flow that will drive the corrected N2. The actual fuel flow will be "uncorrected" - as will the N2 and N1 - to give their actual values. When calculating the performance of the engine, only the corrected values are used.

 

• Calculate the N1 target: The fuel flow controller will use N1 as a reference - assuming that the Use Commanded Ne Table (use_commanded_Ne_table) parameter has not been checked  - and an N1 target will be calculated using the effective throttle input and an affine function. This target will then be compared to the reference and the fuel flow controller will try to minimize this difference by regulating the corrected fuel flow accordingly.

 

• Calculate corrected N2: This fuel flow will then give a corrected_N2 using a logarithmic curve, when not using the Use Corrected N2 From FF Table (use_corrected_N2_from_FF_table) parameter. This speed will then be used to calculate a corrected N1 from which the actual N1 can be calculated.

 

• Calculate gross thrust: The corrected N1 will determine the gross thrust which will then be corrected using the value arrived at from the inlet condition calculations.

 

• Calculate the net thrust (the actual thrust propelling the aircraft): For the net thrust, a ram drag value is needed. This is calculated using the Corrected Airflow Table (corrected_airflow_table), the actual velocity of the aircraft, and the correcting values. The ram drag is calculated and then subtracted from the gross thrust to give the Net thrust. You can find more detailed information on the ram drag calculations here: Flight Model Physics - Ram Drag.
  NOTE: If an afterburner is available, this will be used as a multiplier on the gross thrust before ram drag is subtracted to give the net thrust.

 

Differences For TurboProps

The turboprop flight model uses the same calculations as those given above for the jet engine with two main exceptions:

• N2 is now N1 (and N2 has no use)
• Shaft torque rather than thrust is calculated (as turboprops use a propeller)

 

 

Engine Tuning

Coming Soon...