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MSFS N1 and N2 do not change with Altitude

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spain
So I keep testing and working on the A320 Neo engines.cfg.

I was able to get the right Engine parameters at Altitude 0ft, ISA conditions and Mach 0, for Idle and Maximum Take-off (all static, with parking brakes on).

When I move to a 5000 ft airport, I expected N1 to go higher (and thus N2) to maintain idle and maximum TO thrust. But what I got was the same N1, N2, FF, etc. as I did get at 0ft but with lower thrusts. Temperature was increased to mantain a constant 15C.

So it seems we cannot use the tables for Modern model, and also we cannot correlate CN1 to Altitude.

Any advice?
 

Roy Holmes

Resource contributor
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I have not looked into the new engine tables in detail yet, but I have noticed that the equivalent to 1503 and 1504 ,
mach_0_corrected_commanded_ne_table and mach_hi_corrected_commanded_ne_table only have two altitude lines for 0 and for 22.57 inverted pressure ratio which relates to sea level and 70,000 ft. For intermediate altitudes the program carries out a straight line interpolation whereas pressure ratio is anything but linear with altitude. What I recommend is to have intermediate results at say 10,000 ft intervals and for something like a Neo you do not have to go higher than about 40,000 which has an IPR of around 5.38. Using the two values that are in all the standard MS jets you cannot adjust anything at 5000 ft because you have no data point there only a very inaccurate interpolated value.
In fact the N1 value should stay close to its sea level value throughout the altitude band, but only if you have accurate IPR inputs which does not happen with only two IPR extreme values. FF in the sim should track with thrust and the ThrustSpecificFuelConsumption.
In the real world if you want to maintain a thrust level at higher elevation that is achieved by setting higher EPR on take-off.
Roy
 

Roy Holmes

Resource contributor
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mach_0_corrected_commanded_ne_table is the same as Record 1503 Turbine LoMach CN2 vs Throttle and Pressure ratio
Did you compare N1 vs CN1 There is no equivalent table and you would have to call up the values individually
Since the engine structure is I believe the same as in FSX/P3D just called up using different terms (to confuse the enemy) I would think a piece of read-out software would give the same sort of answers.
Roy
 
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spain
Thank you both for your comments.

Just a little feedback from my analysis in the past month:

- I did match ALL 150x tables from legacy FSX/ P3D .air files to the new engines.cfg (among with most variables from every .cfg file)

- As you probably know, you can choose between MODERN and LEGACY flight models within the Simulator

- It seems (since SDK and documentation is still poor), with MODERN flight model only the table relating CN1 and Mach to Thrust is being used by such flight model. Other tables, relating CN1 and CN2, Mach and Altitude to CN1, etc. are not being used at all (tested by deleting them, putting all 0s, etc.).

- That means these curves, or other similar characteristics, have to be hardcoded. The only parameters that seem to be used to these hardcoded characteristics are low-high values for N1, N2, FF, etc.

@WarpD, I did compare N1 & N2 to corrected counterparts. They are exactly the same in static (parking brakes on) tests @ Idle and TOGA thrust. I tested at 0 ft and 5000 ft airports, at the same Temperature (15 C).

@roy, it would be great to be able to use those tables so that I could work them out with your paper at hand. But it seems not possible if we are to use Modern flight model.

Its not just something that I tweaked and broke. I have tested three different "vanilla" Airliners: A320Neo, B747 and the Dreamliner. All three behave EXACTLY the same.

So maybe we just need to wait until we get all the information from Asobo/ MSFS.

Thanks again!
 
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