P3D v4 FS Thrust vs Altitude calculations Version 2 doubt + table 1524

[Roy Holmes]

Sergio,
The Draken engine is a RR Avon derivative of the 200 series Avon in the Hunter. The Hunter was the airplane I used in the paper. I flew it for 9 years and had over 1800 hours on it.
Since N1 is tied to N2 the table structure I used in my paper 100% applies. Intake size which controls thrust would need some recalc but otherwise the only things that would vary from the paper could be the shape of the thrust versus RPM curve plus you should use 105.5 as a multiplier in the tables
Roy

 

[Draken]

Sergio,
The Draken engine is a RR Avon derivative of the 200 series Avon in the Hunter. The Hunter was the airplane I used in the paper. I flew it for 9 years and had over 1800 hours on it.
Since N1 is tied to N2 the table structure I used in my paper 100% applies. Intake size which controls thrust would need some recalc but otherwise the only things that would vary from the paper could be the shape of the thrust versus RPM curve plus you should use 105.5 as a multiplier in the tables
Roy

Roy,
pardon me: why are you talking about "The Draken engine is a RR Avon derivative of the 200 series Avon in the Hunter"?
I PM you the motor I'm talking about.

Sergio

 

[Roy Holmes]

Since you gave me no clue about which aircraft this mysterious engine was used in and a guy called Sergio used Draken as his website name, I put two and two together and as usual, got 5. "The SAAB Draken engine is a RR Avon derivative of the 200 series Avon in the Hunter" is a true statement. If you want to know about the engine model you PM'd me, talk to Mario Motta. He spent a lot of time just in front of it.
Roy

 

[Draken]

Roy,
I quit.

For what is worth (probably nothing!)....as feedback:

your paper is pretty straightforward, that means one can understand the overall process.

Table 1502 and 1506 should be revised in their description because it is not clear the way the data are built and stored.
A great issue for me is to understand table syntax.
I put down some notes.

Tables 1503 and 1504 are well explained.

Please just tell me where I can find official literature about the jet engine tables.
I saw the SDK link you gave me, but it's hard to believe that the authors explained JET AIRFILE just through https://www.prepar3d.com/SDKv4/sdk/simulation_objects/AIR_samples/jet_sample.html


Thanks!

Sergio

 

[Roy Holmes]

Any comprehensive text book on aerodynamics will have a section about Corrected properties. The technique is widely used in civil aviation and to my knowledge has been used in Flight Simulator for a very long time. The corrected properties are:
corrected rpm, corrected gross thrust, corrected TSFC and corrected airflow. I could write down the equations but they are as given in my paper.

The use of corrected properties in the original Microsoft Flight Simulator is covered in this paper

Aircraft Simulation Techniques Used in Low-Cost, Commercial Software​

Michael Zyskowski
Session: MST-20: Simulation History and Case Studies
Published Online:25 Jun 2012 https://doi.org/10.2514/6.2003-5818

This covers how the tables are different for the different types of engines but gives very little data.

The structure of an airfile for FSX/P3D is given in the P3D SDK as JetSample.asm. You can open this with Visual Studio.

However, nothing in the SDK details how to derive the engine table values and as I mentioned earlier none of the stock engine tables are accurate. That is why I researched the subject and produced the paper.

Incidentally, It was only just before P3D was launched that anything official was published about FS airfiles. Unofficially there were decoders etc but their accuracy was never confirmed. However, when the official guide was published, as I recall, there was only one table that had an error in the assumptions of how it operated. Everything else had been correctly described.

Roy

 

[Draken]

Any comprehensive text book on aerodynamics will have a section about Corrected properties. The technique is widely used in civil aviation and to my knowledge has been used in Flight Simulator for a very long time. The corrected properties are:
corrected rpm, corrected gross thrust, corrected TSFC and corrected airflow. I could write down the equations but they are as given in my paper.

The use of corrected properties in the original Microsoft Flight Simulator is covered in this paper

Aircraft Simulation Techniques Used in Low-Cost, Commercial Software​

Michael Zyskowski
Session: MST-20: Simulation History and Case Studies
Published Online:25 Jun 2012 https://doi.org/10.2514/6.2003-5818

This covers how the tables are different for the different types of engines but gives very little data.

The structure of an airfile for FSX/P3D is given in the P3D SDK as JetSample.asm. You can open this with Visual Studio.

However, nothing in the SDK details how to derive the engine table values and as I mentioned earlier none of the stock engine tables are accurate. That is why I researched the subject and produced the paper.

Incidentally, It was only just before P3D was launched that anything official was published about FS airfiles. Unofficially there were decoders etc but their accuracy was never confirmed. However, when the official guide was published, as I recall, there was only one table that had an error in the assumptions of how it operated. Everything else had been correctly described.

Roy

Roy,
my roadblock is now much more "basic".
I'm mainly struggling with the tables syntax and conventions.
I understood what you wrote about the formulas and concepts that are at the base of the reasoning.
I got lost when I read your tables (apart 1503-4 that are clear).
If you have time, taking 1502 as example, please describe me the table syntax (+ what is first row + why second row is filled by 0 + why upto120?).
That's it.

Are you available for a WhatsApp call?

Sergio

 

[Roy Holmes]

I used identifiers like 1502 in the paper because they were/are better known than the official identifiers. Here is the cross reference.
1502 Turbine CN1 versus CN2 and Mach number. = Air_70_N2_to_N1 Table.
1503 Turbine LoMach CN2 vs Throttle and Pressure ratio = Air_70_Mach_0_Corrected_Commanded_NE
1504 Turbine HiMach CN2 vs Throttle and Pressure ratio = Air_70_Mach_HI_Corrected_Commanded_NE
1505 is not concerned with thrust vs altitude so I did not address it
1506 Turbine Thrust Scalars Vs CN1 and Mach No.= Air_70_N1_and_Mach_on_Thrust
1507 Ram Drag Factor vs CN1 and Mach No. = Air_70_Corrected_Airflow

Because I chose to have a single stage turbine, CN1 the output from 1502 is always the same as CN2, the input. When using a two or more stage turbine that is not the case, usually at lower RPM, N1 is lower than N2. Thrust comes from the N1 value so having N1 low with the throttle closed makes stopping on the runway and taxying a lot easier.

1502 for single stage engines just makes N1 equal to the N2 input.
The table top row is a zero space holder, a zero followed by a 1 for Mach number. As in all tables involving Mach the actual value is interpolated linearly in the calculation between the tho column values. In my case Mach has no effect, CN1 is the same as CN2.
The left column is the CN2 input and not throttle position so it can go to 120 if that is what CN2 is.
The second column is the CN1 output at M=0. The third column is the CN1 output at M=1, in this case.
The second row of zeros is because 0 in gives 0 out.

The JetSample.asm has a bit more detail on the purpose and syntax of the tables compared to the old "1502" layout. However, if you edit the text airfile you will be using a compiler/decompiler which is set up using a fixed convention so you have to follow the old layout

Hope this helps. The main advantage of following the procedure given in my paper is that your max speeds and performance will change with altitude as it does in real life. Just about all of the stock airplanes have performance that drops off with altitude quicker than it should. That is not the case if your tables are constructed as described in the paper. The reason is that the sim was designed to operate that way.
Roy

 

[Draken]

Roy,
your reply really helped to understand tables layout.

A) Just a last clarification on 1502:
(The left column is the CN2 input and ......) so it can go to 120 if that is what CN2 is
what do you mean?
I'm not able to replicate the maths: is it related to the 8000 data of your engine?
Is 120 calculated?
If so, how?
(idle value 31.25=2500:8000×100
120=9600:8000×100 but 9600 where does come from?
The other values in between 31.25 and 120 are just freely interpolated?


B) Now, in case of a jet engine (N1=N2) with afterburner, is it correct to replicate your method in a military conditions (that means Mach <= 1) and, once tuned, get the final adjustments to achieve supersonics performances by means of 1524?

3) Finally, I plan to use AirUpdate to edit tables.

Thanks!

Sergio

 

[Roy Holmes]

!502 for a single stage engine just outputs CN1 values the same as the CN2 inputs. If it is a two stage engine then the CN1 outputs are usually not the same as the CN2 inputs. The values are just off a straight line and I included values up to 120 because that should be high enough to cover all CN2 values. I could have just used 3 values zero, idle and max. If you want to see other values, here is the 1502 table from the P3D version of Dino's F-35
Record: 1502 Turbine CN1 vs CN2 and Mach No.
columns: 3 rows: 13
0.000000 0.000000 1.000000
0.000000 0.000000 0.000000
10.000000 10.000000 10.000000
40.000000 15.000000 15.000000
60.000000 40.000000 40.000000
70.000000 60.000000 60.000000
80.000000 80.000000 80.000000
85.000000 85.000000 85.000000
87.000000 87.000000 87.000000
90.000000 90.000000 90.000000
92.000000 92.000000 92.000000
95.000000 95.000000 95.000000
115.00000 115.00000 115.000000
Left column is CN2 inputs, middle one is CN1 output at zero Mach, right is CN1 output at zero Mach 1.
For example the Mach 0 idle value of CN2 happens to be 60%, from 1503, 1502 outputs a CN1 of 40%.

Supersonic thrust is a bit of a fudge. Yes you use 1524, but the problem is that the engine 1502, 1504 and 1506, air file table Mach value is limited to 1.0 max. This means the dry thrust control values above Mach 1.0 stays the same at higher Mach. This does not apply to afterburner and 1524. Afterburner is controlled separately from the core engine controls.

Transonic and supersonic performance is dictated by 1524 and 154a Mach Drag. 154a is AIR_10XPACK_CD0_MACH in the SDK. Fine tuning is required to get the right transonic drag rise and keep the thrust above drag until max Mach is reached. You need to work on both drag and thrust at the same time and make lots of notes.

If you can get a program like Air Wrench which will give you instant, accurate results you will speed up your fine tuning compared to flying the sim. You can just set a Mach value and altitude and get a far more accurate reading than flight testing. You do need to test, but preferably in a confirmation sample mode after you have mapped out the theoretical performance envelope.
Roy

 

[Draken]

I have AirWrench 111 but, since I'm currently travelling, I have no clue if this version can do what you're suggesting and what are the differences with the latest version.
I was told it is a great application and I'll try to get as much as I'll be able from it.
Unfortunately, AirWrench has an embedded instructions manual very very lean .... (too much lean).

It's time for me to make some homeworks 🥵

You'll be safe until mid September
once I'll be back home and ready to go.
THANKS !!!!!!

Sergio

 

[Draken]

Roy,
before leaving you in peace.....

You wrote:
1502 for single stage engines just makes N1 equal to the N2 input.

Do you mean stage=shaft ?

In a turbojet, single shaft (= multiple compressor stages run the same multiple turbine stages RPM, no matter their number) N1 should be equal to N2 even it is not a-single-stage scenario.

Am I wrong?

Still struggling to learn....

Thanks!

Sergio

 

[Roy Holmes]

Shaft is a better description. However, you can say that a turbine has 3 stages, compressor, ignition and exhaust. You may find different names for the same thing if the engines were made in different countries. 1502 is the one table that tells you if N1 and N2 are the same. If it is a two-shaft turbine and you do not have accurate data, you have to make things up a bit. Generally, N1 lags behind N2 at lower RPM and catches up as power comes in.
1503 and 1504 obey set math equations, 1505 is a swag. 1506 is where you fine tune the thrust versus drag to match performance tables. 1507 is a bit of a joke, I have seen huge page long 1507 tables that I suppose give you accuracy when you do not need it. Getting the speeds right is the name of the game and having a program that can give instant answers is a boon. I think AirWrench may be hard to find and it had a registry code so for example if you get a new computer and copy over the program it probably needs a new key. Not sure how that works, but, in any case it will not work in MSFS which is where my development efforts are involved these days. My fault, I volunteered.
Roy

 

[Draken]

Man,
to me you are a generous genius!
My comment was not at all a criticism, but a double checking for me to understand better.

I'll stay on P3Dv4 for a long time.
MSFS has a long way to go for a debugging completion and I don't perceive any value adding compared with the old-fashion FS.

I'll do my homeworks and I'll submit to you, hoping to get valuable feedbacks.

I renew the invitation for a WhatsApp call.
It would be my pleasure "meeting" you somehow.

Sergio

 

[Draken]

Roy,
I did some homeworks and the aircraft performance is now coherent with the real one at high altitudes and Mach from 0.9 to max.
Now, I would like to correct take-off performance which is a bit over performing WHILE is a bit under performing at SL at his max compatible speed.
Obviously both status are under full AB.
Drag appears to be already well tuned.
How would you act to achieve?
Which tables?
Thanks.

Sergio

 

[Roy Holmes]

Sergio,
Take-off. Over performing take-off performance could have a few causes. You need to always be as specific as possible. What are you measuring? Take-off ground run distance? Lift off speed? Do you have figures for different weights and configurations? Is your documentation minimum performance or expected performance.? Is it with or without flap?
Put another way could it be from too much thrust and/or too little drag or could a short run be caused by too much lift? Was there any wind?

Max speed. Do you mean Mach Number or indicated air speed. Mach is much more accurate usually. There are compressibility errors with "airspeed" readings and I'm assuming you are in the Mach 2 region. If so you are probably exceeding Max IAS.

If you give me some more data I can be specific on what to look for and correct.
Roy

 

[Draken]

Take-off. Over performing take-off performance could have a few causes. You need to always be as specific as possible. What are you measuring? Take-off ground run distance? Lift off speed? Do you have figures for different weights and configurations? Is your documentation minimum performance or expected performance.? Is it with or without flap?
Put another way could it be from too much thrust and/or too little drag or could a short run be caused by too much lift? Was there any wind?

Roy,
take-off ground distance @ correct lift off speed is less than manual data (S.L., no wind, clean configuration, take-off flaps).
Thrust appears correct.
Can drag be corrected in the speed range 0 to 190 knots?
If so, how?

Sergio

 

[Draken]

Max speed. Do you mean Mach Number or indicated air speed. Mach is much more accurate usually. There are compressibility errors with "airspeed" readings and I'm assuming you are in the Mach 2 region. If so you are probably exceeding Max IAS.

Roy,
I use Mach.
@ S.L. (clean, no wind, max AB thrust) manual reports Mach 1.15.
Model reaches 0.9.
I'm thinking to add new data rows to 1524 (within its 10 rows limit) to increase the resolution in the range up to Mach 1.15

Sergio

 

[Roy Holmes]

A lot of flight manuals are pessimistic because they can be used by airplanes with engines due for overhaul tomorrow. That means they tend to quote longer take-off runs than airplanes usually get.
However flight thrust = static thrust minus intake drag, so getting it right can be difficult.
Max speed with AB of 0.9 is pretty awful and almost certainly due to 154a being wrong. This is a drag curve with variable Mach intervals that was introduced with FSXA.
Values like these work fairly well
Record: 154a Mach Drag
Points: 16
0.0000 0.0000
0.8400 0.0003
0.9100 0.0015
0.9700 0.0195
1.0000 0.022
1.1000 0,022
1.2000 0.020
Followed by another 9 lines as you wish.

Roy

 

[Draken]

Roy,
about table 1524.

1) It is Mach vs Scalar on thrust for max AB, where thrust is the value in engine section of aircraft.cfg. Is it correct?

2) as example, first 3 rows 0.0, 1.58 0.2, 3.54 0.8, 7.89 does it mean that in the interval 0<Mach<0.2 the scalar is 1.58 and in the interval 0.2<Mach<0.8 the scalar is 3.54?

Thanks,

Sergio

 

[Roy Holmes]

For 1524, if the engine static thrust at sea level and zero Mach was 10,000 lbs and in AB it was 15,000, the scalar would be 1.5. That means the AB added another 5000 lbs to the dry value
The scalar is the factor by which the dry thrust is increased in AB. Your figure of 7.89 at Mach 0.8 means that AB is increasing the thrust by 7.89 times. This value is probably 4 times too high.
If the factor was 1.5 at zero Mach and 1.7 at Mach 1 it would be 1.6 at Mach 0.5. The intervening values between lines are linear interpolations.
The thrust value in the aircraft.cfg is static max at standard sea level conditions.
The assumption is that the total thrust with AB varies in the identical way to thrust without AB over the speed/altitude envelope. In other words, you work out the dry thrust for a speed/altitude situation and then multiply it by the 1524 factor for the ambient Mach number.
Roy