FS2004 Airfile table 1502

[Roy Holmes]

For simplicity make full throttle give 100% CN2 in 1503

Take the case where 1504 is for Mach 1.0.

The SL Mach 1.0 value for CN2 would be 91.2871% of a static 100%.

At Mach =0.5 CN2 should be 97.5900%. The relationship

SSL n1 / sqrt total temp ratio

is non-linear. (M)^2 is non-linear

FS takes for 0.5M a value midway between 91.2871% and 100%.
This would be 95.6435% and it is 2% low.

If CN2 and CN1 were the same in 1502, N1 would be 98% at 0.5M.
To compensate for that the designer needs a 0.5M column in 1502 where the values are 2% higher approximately to get back to 100% N1

In practice a Mach column every 0.25M will still yield N1 values that are not 100% when between the column values, but nothing like as obvious as having only one Mach column.

It is the old problem of FS doing a linear interpolation of a non-linear function. The same reason why you need multiple IAP columns in 1503 and 1504.

Roy

 

[vololiberista]

Well, Guys !!
My curves look a bit more decent now despite still not fully understanding the maths or politics!!!!
Engine run-up tests at sea level give me all the correct published n2/n1 figures. Idle / max take-off / overspeed / max continuous.
Reverse thrust is still a bit out. I think I'll have to increase the negative value even further in the airfile.
In flight n1 is temperature limited above 25,000ft. The figures come in at just under or on the nail using the climb n2 of 93%.
Slamming the throttle on the ground gives just under 7 seconds to max n2/n1.
The bottom end of the throttle is still too slow in my opinion (though it should be slowish) I'm probably about 5 secs off there. However, the a/c in spite of what the manuel says is usually not taxied at idle but a bit above at around 61%n2 (it being not empty!). This allows good positive braking without having to continuously adjust the throttle. Such that one can put the throttle to 96% without having to wait until the end of the year for acceleration. (2013 that is!)
vololiberista

 

[vololiberista]

Forget about reverse for now, and set the tables up correctly and I'll show you how to fix that later.

Ok Now's the time Whatever value I put into table 313*Reverse Thrust throttle Percent* n2 spools up to 96% ( I am holding the button down) I don't want it to go beyond 93%. I've gone from -32 up to -58 without any noticeable change. What does the scalar "actually" do? Is there any way I can limit the n2 value?
vololiberista

 

[jx_]

If throttle alignment is the goal,

I am discussing how to align CN2 to throttle, not how to make it "appear" to be aligned by tweaking CN1. Any changes made to CN1 have no affect on commanded CN2. I may have accidentally said CN1 somewhere when I meant CN2, I tend to do that....

You should only modify 1502 Mach values to account for changes in the difference of n1 vs n2 due to ram airflow, which can be significant.

FS takes for 0.5M a value midway between 91.2871% and 100%.
This would be 95.6435% and it is 2% low.

Not really. It only does that if you "draw" a straight line. It is true tables 1503-1504 should be using either 1/Delta2 or 1/Theta. This does complicate the process, but it's not as linear as you think.

the solution is to draw a Total T ratio table against IAP

Try this Roy,

Were going to do this at 25,000. In practice, each altitude (or altitude range) would need to be tweaked separately.

Make an IAP row at 25,000 feet (2.694691).
Set your low mach value to 0.33 and high mach to 0.87
Set your >99% Throttle columns to:

108.7126 %CN2 @ IAP 2.694691 @ Mach 0.33
102.4116 %CN2 @ IAP 2.694691 @ Mach 0.87

note: It is preferable during this kind of testing to buffer the altitude and RPM by simply adding another IAP row and making both rows +/- 500 feet, while setting the next lower RPM range to the same values. This will ensure no outside interpolation.

Now, I eyeballed these numbers and threw all this together in a few minutes without taking the normal time to tweak and test, and I get +/- 0.5 n2 from Mach 0.2 up. Static sea level is +2.5 %n2. There are probably much better numbers to be used if someone took the time to plot them.


These numbers should give you:

102.5 %n2 @ static...

100.5 %n2 @ Mach 0.1,
100.3 %n2 @ Mach 0.2,
100.2 %n2 @ Mach 0.3,
100.0 %n2 @ Mach 0.4,
99.80 %n2 @ Mach 0.5,
99.7x %n2 @ Mach 0.6,
99.9x %n2 @ Mach 0.7,
100.0 %n2 @ Mach 0.8,
100.2 %n2 @ Mach 0.9,
100.5 %n2 @ Mach 1.0,


note#2: For me personally, I would optimize the flight envelope mach range (Flaps Vmin at lightest weight to Vmo/Mmo) of each IAP line, but I understand some aircraft, especially military jets, have a broad envelope. I have not tried the supersonic envelope, nor am I experienced with supersonic engine performance. Also, my experience is with engines where fan speeds don't stay constant against throttle command-vs-mach, or FADEC equipped aircraft, so I have never cared about precisely matching n2 to a given throttle setting. But I know it can be achieved from the CN2 tables alone to within some small margin of error. I haven't taken the time yet to investigate what FS is doing to the data curves mathematically, but I do know it is not strictly linear. The typical aircraft I would work on has an envelope from mach 0.25 through mach 0.85, and can maintain +/- 0.5 n2 throughout, using this method.

 

[jx_]

Ok Now's the time Whatever value I put into table 313*Reverse Thrust throttle Percent* n2 spools up to 96% ( I am holding the button down) I don't want it to go beyond 93%.

Open the aircraft config...

Under [GeneralEngineData], find (or add) min_throttle_limit=-0.25

This limits the reverse throttle range to -25%. I don't remember the exact numbers right now but as a rule of thumb, -0.25 means the reversers will be full open at 25% throttle and the engine will be commanded to +5 %n2 from that throttle position (50%)

I've gone from -32 up to -58 without any noticeable change. What does the scalar "actually" do? Is there any way I can limit the n2 value?

0.32 or 0.58! Percentage of throttle. Set 313 to match aircraft.cfg entry. it is not used unless the .cfg file is missing the info.


Limit the n2 value by reducing the min_throttle_limit=-0.25


When this is set you will notice reverse thrust force is minimal until the reversers are fully deployed. This is a stupid MSFS bug.

If you need to scale the reverser force:

under [TurbineEngineData] find (or add) reverser_available=3.00

3.00 = 3 * normal reverse force
1.00 = unity
0.5 = half

Reverse thrust is still a bit out. I think I'll have to increase the negative value even further in the airfile.

which value? 313? be sure to not add negative throttle to 1503-1504. I tried for years to get that to work and it never did.

 

[vololiberista]

No, I've only ever played with table 313 in the airfile or numbers in the cfg. I've activated a button on my control column to select reverse. As you say that produces a feeble response. Pressing and holding the button sends the n2 all the way to 96%. I'm trying to find a way that limits that to 93%. The actual value I put into table 313 and/or cfg doesn't seem to make any difference. I'll have a look at your other solution being:-
3.00 = 3 * normal reverse force
1.00 = unity
0.5 = half
and see what that does.
vololiberista

 

[jx_]

min_throttle_limit= doesn't work?

 

[vololiberista]

Reverse does work. I just don't seem to be able to find the point at which n2 won't exceed 93% when full reverse is commanded.
vololiberista

 

[jx_]

I meant to say

Adjusting min_throttle_limit= doesn't change n2???


Try min_throttle_limit= -0.XX where XX= throttle percent position in table 1503 of 88% CN2. That should produce 93%CN2.

 

[vololiberista]

That does look like the "obvious" solution. I'll try that thanks.

The only thing is it would be -101.6 !!!!!!
vololiberista

 

[jx_]

Just to be sure,

The min throttle limit is a negative percent of throttle in decimal form. -101.6 is not possible. If it were it would be -1.016. -1.0 is the highest usable value. (I think...)

If 88%CN2 is aligned to 65% throttle in table 1503, then you should set min_throttle_limit to -0.65

Hope that helps.

 

[WarpD]

Just to try to make this really clear (I hope).. the value represents a percentage of the physical throttle range itself from 0 to 100% in decimal form... thus it would be from 0 to 1.0.

That's what jx is trying to describe to ya.

 

[vololiberista]

These are my base values for table 1503
0.00 1.00
0.00 56.00
0.42 78.80
0.51 85.50
0.61 90.60
0.70 95.50
0.79 99.70
0.93 103.80
1.00 107.00

As you can see 88% throttle is over 100%n2
I'm going to have a little play with some other numbers later for the reverse value.
vololiberista

 

[WarpD]

Your reverser setting should be around 0.6 based on your 1503 data.

 

[jx_]

Your reverser setting should be around 0.6 based on your 1503 data.

Yes WarpD is correct.

Try min_throttle_limit= -0.XX where XX= throttle percent position in table 1503 of 88% CN2. That should produce 93%CN2.

Our goal is 93%CN2 in reverse.

93%CN2 - 5% N = 88%CN2

Look at 1503. We are looking for the position where we find 88%CN2.


At 51% throttle percentage I see 85.50%CN2

At 61% throttle percentage I see 90.60%CN2


So our min_throttle_limit will be between -0.51 and -0.61.

88%CN2 is interpolated linearly between 85.5 and 90.6, so a throttle percentage of 55.90 will yield 88%CN2


Set min_throttle_limit= -0.559


This should be real close to 93%CN2.


Tweak min_throttle_limit to adjust CN2, then, if needed, adjust reverser_available= scalar to adjust forward thrust force output.

 

[vololiberista]

The cfg. overrides the airfile??

Currently I have reverser_available= 1
I always thought that was 1 or 0
vololiberista

 

[vololiberista]

Sorry to say Guys that the numbers had no effect. I went speeding off the end of the rwy. With reverse selected the n2 went down instead of up!!!
It went down to idle peaked briefly at about 80+% then ran down to 71% then slowly increased to over 113% by which time I was well off the rwy
It all used to be ok before I changed everything.
vololiberista

 

[jx_]

You have something set wrong somewhere.

Post your tables 1502, 1503, 1504, 1505, 1506, and [GeneralEngineData] & [TurbineEngineData] sections.

FS9 or FSX?

keyboard throttle, joystick throttle, or throttle quadrant?



You can also send me the airfile and config file via PM if you like.

 

[vololiberista]

Thanks for your offer jx,
I'll have another play with the numbers and if unsuccessful I'll pm the files to you.

I use a CH yoke with a single throttle lever controlling four engines. And a button to select reverse. I've tried programming one of the other levers through FSUIPC but never had great success.

The basic throttle n2 relationship seems to be about right though after take-off it's slow to get to 180kts in the climb so the power's not quite there yet.


vololiberista

 

[vololiberista]

The setting that gives me 93%n2 reverse is -41 still with the same tables.
The only thing is that it's quite slow to get there. I suppose because it starts from 56% idle and thus follows the 1505 curve back up to 93%.
vololiberista