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Jet Engine Performance

M

mgh

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unitedkingdom
I've reposted this from a post in the Simconnect forum because I hope it wll get more coverage here.


There are tables in the .air file and values in the .cfg relating to control of engines. My incomplete understanding of the realtionship between the tables based on the descriptions given in the ESP/P3D file samples is shown in the figure - note the big question mark! In particular what is NFFC and where does it come from?

The .cfg file contains static_thrust (used to calculate Gross Thrust), inlet_area (used to calculate Ram Drag), ThrustSpecificFuelconsumption (used to calculate fuel flow from Gross Thrust), rated_N2_rpm (to convert) from % to actual), thrust_scalar (presumably a "fiddle factor), and fuel_flow_gain (??????)

The figure implies that Corrected N2 varies directly with throttle lever position. That's obviously not the case because it lags. I assume that it's the commanded Corrected N2 and that lag is modelled separately The only an engine's rpm can be increased is by increaing its fuel flow. I assume that fuel_flow_gain is effectively a time constant to model this lag.

I understand that in the real world, Corrected N2 is non-dimensionalised and is the actual N2 divided by the square root of ambient temperature ratio. Is this the case in FSX?

Can anyone expand or clarify on these points?
 

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TBL 1505 controls the turbine spool-up rate. I believe it's X value is actually CN2 and the Y value is the percent of fuel flow.

As for your other question mark location... that's the CN2 you obtained from right above it.
 
The ESP/P3D jet sample .asm file contains a table which I have assumed is Table 1505 beacase I can relate all the others to the .air file. The heading for this table is:

Code: 
   ;N2 (Corrected) vs. FuelFlow constant 
    ;  where the constant is a normalized ratio of actual fuelflow parameter to static thrust 
    ;IN:  Normalized Fuelflow constant 
    ;OUT: N2 (corrected) 
        TOKEN_BEGIN     AIR_70_CORRECTED_N2_FROM_FF

That's unambiguous that the output is N2 (corrected) as shown in my diagram.

I'm aware of the mudpond chart but that doesn't include Table 1505.

I am still not clear how FSX uses Table 1505, even assuming it's used to introduce a lag into N2, and hence into N1, Thrust and Ram Drag.
 
If you look at the typical values inside 1505... most of them have an X value of 0.54 with a corresponding Y value of 100. In what way would 0.54 of fuel flow equate to 100% CN2? Also at 108% CN2... it's only 0.68 fuel flow?? That would mean you could never, ever obtain 100% fuel flow... which seems a bit odd, don't you think?

The only area of engine performance I've seen any significant difference with regards to this table involves engine start times. I really think this table is inverted and thus the Y value is the index and the X value is the result.

I know what the .asm file states... but I will simply say this: Not even Microsoft has any documentation regarding the contents of the .air file. I know this because I discussed obtaining better documentation for the .air files with Lockheed-Martin and they informed me that there was no documentation for the .air file contents available. I would take the .asm sample comments with a grain of salt.
 
I agree, the chart does appear inverted. AAM calls it CN2 vs Turbine Corrected Fuel Flow and its plot looks like turbine thrust vs engine rpm or fuel flow vs engine rpm, since thrust and fuel flow are proportional to each other.

The chart starts out shallow and climbs steeply at high rpm. The fuel flow constants are normalized ratios of actual fuel flow to static thrust. Therefore you can change their values according to the engine fuel flow thrust efficiencies. This should let you tune FS fuel flow to match the airplane Manual figures. An old inefficient jet would have higher values than a modern efficient turbo fan. It is a bit like TSFC.

Roy
 
The values for Table 1505 are not surprisingly the same in both my copies of ESP and P3D SDKs. Both are identical to the FSX default B747 .air file.

The table from the SDKs is:

Code: 
;N2 (Corrected) vs. FuelFlow constant 
    ;  where the constant is a normalized ratio of actual fuelflow parameter to static thrust 
    ;IN:  Normalized Fuelflow constant 
    ;OUT: N2 (corrected) 
        TOKEN_BEGIN     AIR_70_CORRECTED_N2_FROM_FF 

        UINT32 10 

        REAL8   0.00000,       0.0   
        REAL8   0.00294,      10.0 
        REAL8   0.01180,      30.0 
        REAL8   0.02066,      40.0 
        REAL8   0.03397,      50.0 
        REAL8   0.05317,      60.0 
        REAL8   0.13391,      80.0 
        REAL8   0.25771,      90.0 
        REAL8   0.54000,     100.0 
        REAL8   0.68686,     105.0 

        TOKEN_END

This is plotted in the attached figure.

I still don't know its purpose - does anyone?
 

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Mgh
See my answer above. The chart as plotted does not help understand it, but if you transpose the axes, it becomes clearer. Essentially, it is the shape of thrust vs CN1, therefore fuel consumption vs CN1. However it relates to the static thrust so it gives you something like Thrust Specific Fuel Consumption

By adjusting the values of fuel consumption upwards you can simulate a less efficient engine
Roy
 
If you look at the values in the first column they start at zero and only go up to 0.68... whereas the values in the right column go in a steady climb from zero to 105.

If you treat the lefthand column as CN2... 0.68 is a typical CN2 idle value. Note the jump to 80 at 0.13 which is above the CN2 value that the FS starter will rotate the engine to without fuel.

I suspect that this table actually controls the fuel-air ratio in the jet engine, especially during start.
 
I dont think Table 1505 determines overall fuel flow.

For the default FSX B747 the The fuel flow at N2 = 81% and 106% is given by multiplying the actual thrust by the SFC in the .cfg file (0.4000). At N2 = 55% the SFC has risen to 0.5679 - all at M = 0 at sea level.

I haven't managed to relate the variation in Table 1505 to any variation of thrust, fuel flow, SFC etc.
 
Last edited:
Just a thought.
The FA-18 has a display of N2, thrust and fuel flow so you might try it rather than the B747. both have the same 1505 numbers.
I know folks who have used 1505 to get fuel flow to match the Manual figures, but they could be wrong.
I would be very interested in your findings
Roy
 
*tap* *tap* This thing on?? :confused:

Manipulate the values and check engine start. It affects engine start.
 
Please give some data on what you found. The only odd thing I have noticed on jet start is that "mixture" is off until the moment of combustion start. Maybe it affects that timing.

Really interested in what you find

Roy
 
For one of my simulated turbojet models, combustion starts at 19.5% N2 and it idles at 29.5% N2. CN2 values are about 1% higher at a 750 AMSL field.

Roy
 
I used SFSD to obtain my engine data direct from FSX. I've always found it reliable, I understand it's basically a front end for FSUIPC.

I'm setting up a spreadsheet with the various tables from the .air file and entries from the .cfg file to try to gain further understanding.
 
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