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P3D v4 Engine Variables

Jim Johnson

Jim Johnson

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43
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us-texas
I have been asking around about which Avars to use for the engine system I'm currently working on. When looking at the SDK reference on variables, I see variables for different types of engines. My question is for a single stage turbine jet engine should I use those marked with the prefix word GENERAL, ENG, or TURB ENG; or a combination (obviously its not a prop or recip) of them to simulate the engine variables?

P.S. and does it make any difference if your building aircraft for FSXA, P3D V3 and V4?
 
Typically I use the most specific variable possible, unless I find it does not provide suitable output. So for a turbine engine I use the TURB variables, unless there isn't one I need. If so, I use an ENG variable. If not there, I use a GENERAL variable.
 
And no, it makes no difference what sim your are building for.
 
Jim,
Just to put you on the right track.
For a single stage turbine engine these are the appropriate parameters.
TURB ENG N1:index. The index will be 1 and the units percent so (A:TURB ENG N1:1,percent) is what you should use.
Since it is a single stage, N2 will be the same as N1 so you do not need N2
Do not use the "CORRECTED" values. CORRECTED here means what the current real value of N1 would be if it was corrected to sea level conditions. Corrected values are used in the sim computation of thrust but only as intermediate values in the algorithm.
TURB ENG FUEL FLOW PPH:index gives you actual values of fuel flow lbs per hour. Again do not use CORRECTED values.
ENG N1 RPM:index can be used if the airplane gauges just use RPM, not percent N1.
Anything else with TURB ENG such as Jet Thrust, Afterburner etc can be used if appropriate.

Engine performance is specified in the aircraft.cfg and controlled by the air file.
[turbineenginedata] section has:
fuel_flow_gain The is related to spool up and down times/engine acceleration. Typical values would be about 0.05, larger will give shorter spool up times. Max value would be 1.0. It is also related to Record 1505 in the air file.
inlet_area This is strictly the area of the engine inlet into the combustion zone, not the whole area of the nacelle inlet area. Since you have a single stage turbine all air goes into the combustion area. It is used by the sim to calculate intake ram drag. Ram drag is subtracted from gross thrust to give net thrust which is what is effective in pushing the airplane.
rated_n2_rpm is actually the high pressure rpm but it is the same as N1 for your engine.
static_thrust manufacturers value for installed max thrust at standard sea level 15° C conditions.
afterburner_available a 1 here means it has a single stage afterburner system. Modulated AB can be simulated with a higher number. Zero means no AB
afterburner_throttle_threshold I use 90%. This gives me non AB thrust from throttle closed to 90% open and modulated AB between 91 and 100%, according to the number of stages.
thrustspecificfuelconsumption Default is 0.5 lbs per hour per 1 lb of thrust. If TSFC is not used then you can use fuel_flow_scalar in the [generalenginedata] section. A fuel_flow_scalar of 1.0 corresponds to a TSFC of 0.5. If your engine is something like a Rolls Royce Avon which has a TSFC of about .85 you can use 1.0 for the fuel flow scalar and 0.85 for the TSFC or 1.7 for the fuel flow scalar. Any fuel flow value different from 1.0 overrides the TSFC value.

I prefer to use 1.0 for all scalars in the aircraft.cfg and apply the coefficients in the air file.

The air file controls the coefficients used in the sim engine performance. They are in Records 1502, 1503, 1504, 1505, 1506 and 1507 if you use one of the air file editors or AirWrench. They are called:
AIR_70_N2_TO_N1_TABLE
AIR_70_MACH_0_CORRECTED_COMMANDED_NE
AIR_70_MACH_HI_CORRECTED_COMMANDED_NE
AIR_70_CORRECTED_N2_FROM_FF
AIR_70_N1_AND_MACH_ON_THRUST
AIR_70_CORRECTED_AIRFLOW
in the Flight Models Jet ASM section of the SDK.

At this point I'll direct you to http://www.fsdeveloper.com/forum/resources/fs-thrust-vs-altitude-calculations.43/ where the engine turbine tables are described in detail and you will find examples of 1502 through 1507 ideally suited for your single stage turbine. The only other thing I'll say at this point is that turbine tables which are not constructed per that document will be wrong, give you incorrect performance and erroneous values for N1 in flight.

Roy
 
Roy, thank you, that is some good information! I've not written any config file stuff yet, too busy trying to design and code, but at least I got the Avars right. Along with the other guys on here that have helped me get this far, I'm thankful that you all are here and willing to help a new gauge developer. Thanks again all of you.
 
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