FS2004 Airfile table 1502

[vololiberista]

I'm having a little problem with my n1 curve in that at high revs it seems to overspeed .
For example at 96%n2 it runs at 99%n1 which is too low as it should be 102%n1
But at max emergency n2 105% n1 is 115% whereas it should be 105%
I have extrapolated the numbers from 1502 into a spreadsheet graph which suggests I should add another column 100 with a figure of 124. Would this not make n1 overspeed even more?
These are the actual number in the table.
btw my aired shows rows and columns in a different manner to that I have seen elsewhere. The columns are rows the rows are columns i.e 3 rows 10 columns

0 0 0.9
10 1.1 1
20 2.7 2.4
30 5.7 4.7
40 10.1 8.4
50 17.5 12.8
60 28.9 21.2
70 43 37.7
80 63 60.4
90 90.6 85.9

vololiberista

 

[jx_]

The first issue I see is your CN1 above 90%CN2 is not mapped, so the sim is extrapolating the CN1 curve. You would need to add a few more rows of CN2. CN2 rows should extend to or beyond your highest CN2 value in table 1505 which should also be at or above your highest reachable CN2 speed. I would suggest adding 3 rows, 96, 100, and 105 CN2. This may also be a small contribution to your throttle issue.

If you're still having trouble aligning things, map CN2 in 1503 & 1504 against N2 first.

Here's how:

1503 --- Static (Mach 0): CN2 = N2
--- @ high IAP: CN2 = N2 / sqrt(Temperature Ratio)


1504 --- High Mach: CN2 = N2 / sqrt(Total Temperature Ratio at 1504 mach)
--- @ high IAP: CN2 = N2 / sqrt(Total Temperature ratio at 1504 mach at IAP altitude)

For Mach 1.0 this would be 1.0958 at sea level and 0.95029 above 36089 feet.


Once this is done, CN1 = CN2 + (N1 - N2)

So using your numbers for example, CN2 + (102n1 - 96n2) = CN1 or CN2 + (105n1 - 105n2) = CN1


This yields accurate fan speeds at all altitudes and conditions. If your thrust is set right in 1506, you're good to go.

 

[vololiberista]

Well, I'll have a stab at doing the maths!!!! But there are some anomalies which don't look right. Firstly in table 1503
these are the numbers:-
0........55
0.42...78.8
0.5.....82.5
0.61...87.4
0.7.....92
0.79...96.2
0.93...103.1
1........107



55 and 107 are wrong as they need to be 56 and 105. But you will also see that the numbers produce a strange looking graph. I would assume that the curve needs to be smoother and perhaps the first entry higher.
vololiberista

 

[jx_]

What you are looking at is CN2 versus throttle input for that IAP/Mach line.

It appears okay. Throttle in jets are very low resolution on the low end, then gain higher resolution as it goes up. From 10% to 20% throttle gives 55-80% CN2, while 62-82% throttle covers 97-107% CN2. Correct it if needed but the curve can be weird. It should in fact be convex curving upward in the lower RPM and reducing across the higher.


I'll simplify the math for you:

1503-
Mach 0.0 at IAP 1.000 = 1.000000 CN2 ratio
Mach 0.0 at IAP 6.242 = 1.153241 CN2 ratio

1504-
Mach 1.0 at IAP 1.000 = 0.912871 CN2 ratio
Mach 1.0 at IAP 6.242 = 1.052760 CN2 ratio


N2 readout desired * CN2 ratio = CN2 entry in table.

So your 100 percent N2 would be 100, 115.3, 91.2, and 105.2 CN2 for those four .


You really don't need anything other than static sea level in table 1503, but MSFS provides the other options for customization, and if they are present, they must be set correctly.

 

[vololiberista]

What you are looking at is CN2 versus throttle input for that IAP/Mach line.

It appears okay. Throttle in jets are very low resolution on the low end, then gain higher resolution as it goes up. From 10% to 20% throttle gives 55-80% CN2, while 62-82% throttle covers 97-107% CN2. Correct it if needed but the curve can be weird. It should in fact be convex curving upward in the lower RPM and reducing across the higher.


I'll simplify the math for you:

1503-
Mach 0.0 at IAP 1.000 = 1.000000 CN2 ratio
Mach 0.0 at IAP 6.242 = 0.867121 CN2 ratio

1504-
Mach 1.0 at IAP 1.000 = 0.528281 CN2 ratio
Mach 1.0 at IAP 6.242 = 0.950209 CN2 ratio


N1 readout desired * CN2 ratio = CN2 entry in table.


You really don't need anything other than static sea level in table 1503, but MSFS provides the other options for customization, and if they are present, they must be set correctly.

Thanks for that. I'll work on it over the weekend. I have in fact made the curve convex. I lifted some of the lower values and smoothed the transitions higher up.
So far it's looking like this:-
0.00......56.00
0.42......85.70
0.51......95.20
0.61......98.80
0.70....100.70
0.79....102.30
0.93....103.70
1.00....105.00

Displaying the numbers graphically in excel really helps one see what's going on.
vololiberista

 

[Roy Holmes]

Throttle in jets are very low resolution on the low end, then gain higher resolution as it goes up

I believe this is true for modern engines with electronic control. It would be an attempt to match thrust change to throttle change.

However older engines like the Conway had a linear relationship between throttle position and RPM. If the idle was 50% then half throttle would give 75%.

This meant that a certain amount of throttle movement gave a small thrust change at lower RPM and a higher thrust change at higher RPM. Electronic controls could fix this to make thrust changes better track the amount of throttle movement, which greatly simplifies accurate setting of the engines.

But I do not think the old VC10 had this
Roy

 

[vololiberista]

I believe this is true for modern engines with electronic control. It would be an attempt to match thrust change to throttle change.

However older engines like the Conway had a linear relationship between throttle position and RPM. If the idle was 50% then half throttle would give 75%.

This meant that a certain amount of throttle movement gave a small thrust change at lower RPM and a higher thrust change at higher RPM. Electronic controls could fix this to make thrust changes better track the amount of throttle movement, which greatly simplifies accurate setting of the engines.

But I do not think the old VC10 had this

Roy

Has Roy Has!!!

On the original sim model the throttle was at 82.5% at 51% throttle. This meant that to go directly to 96% took about 10 days whereas directly to firewall 105% 6.5 seconds.
In fact the old 1053 had a slight concave curve. I've improved it a little still getting 6.5 secs to max emergency power but 9.5 seconds to 96%. I'm currently investigating with VC10 FE's what the throttle response actually is. They have their own throttles and in fact do most of the throttle control on landing take-off etc. Leaving the pilots free to admire the view. (How advanced is that!!)
vololiberista

 

[vololiberista]

Well jx, confirmation my brain is made of porridge. Because the numbers I have in the tables don't seem to match your maths. If they do I can't see it.
My original table 1502
0........0........0.9
0........0.0......0.0
10......1.1......1
20......2.7......2.4
30......5.7......4.7
40......10.1....8.4
50......17.5....12.8
60......28.9....21.2
70......46......46
80......63......60.4
90......90.6....85.9
100....105.5..118
110....102.4..114.0

My original table 1503
0.00......1.00......22.57
0.00......55.00......82.50
0.42......78.80......92.40
0.51......82.50......98.50
0.61......87.40......101.10
0.70......92.00......103.10
0.79......96.20......105.70
0.93......103.00....108.06
1.00......107.00....108.06

My original table 1504
0.00......1.00......22.57
0.00......82.........94
0.42......82.........94
0.51......83.4......98.5
0.61......84.8......101.1
0.70......89.1......103.1
0.79......92.2......105.7
0.93......96.0......108.06
1.00......98.2......108.06

I would be grateful if you would walk me through the maths as to how the third column of numbers are arrived at. I'm sort of halfway there in understanding. but what doesn't make sense is that the maths are dividing yet the results are greater!!
I'm obviously missing something obvious!
vololiberista

 

[Roy Holmes]

You could look at the explanation in
http://www.fsdeveloper.com/forum/showthread.php?t=327725

Roy

 

[vololiberista]

I'll do that Roy, thanks.
I never thought that my original intention just to correct the engine surge on start-up would open up such a can of worms! It seems that any kind of messing with 1503 mucks up the reverse. I'm getting something like 103%n2 even with the airfile set to "-0". The only way to solve that seems to reduce all the values back to where they were :-(
So the lack of throttle response has returned. :-((
It certainly seems to be a game of swings and roundabouts!
vololiberista

 

[jx_]

Correction:

1503-
Mach 0.0 at IAP 1.000 = 1.000000 CN2 ratio
Mach 0.0 at IAP 6.242 = 1.153241 CN2 ratio

1504-
Mach 1.0 at IAP 1.000 = 0.912871 CN2 ratio
Mach 1.0 at IAP 6.242 = 1.052760 CN2 ratio


N2 readout desired * CN2 ratio = CN2 entry in table.

So your 100 percent N2 would be 100, 115.3, 91.2, and 105.2 CN2 for those four

1. Open a spread sheet.
2. Enter the throttle percents across the columns from 0 through 100.
3. Start a row for static N2.
4. In the table fields, enter the n2 you would like to see for a given throttle setting.
5. Be sure your N2 entries reach 105% where appropriate

Once this spread sheet is complete, take the numbers and apply the simple equation:

N1 readout desired * CN2 ratio = CN2 entry in table 1503 or 1504.


Copy the N2 spreadsheet, and do the process again for N1, except this time, find the difference between N1 and N2. Using our sea level (IAP 1.00) CN2 data for the appropriate mach:

CN2 + ( N1 - N2 ) = CN1 entry in table 1502 for given mach.



When you are done, you will find CN2 at Mach 1/sea level for 100%N2 is 52% while at high altitude it will be as much as 115.3 at static or about 108 in flight near stall speed.

While this may look odd, it is correct. This is how a corrected thrust look up works.

 

[jx_]

I'll do that Roy, thanks.
I never thought that my original intention just to correct the engine surge on start-up would open up such a can of worms! It seems that any kind of messing with 1503 mucks up the reverse. I'm getting something like 103%n2 even with the airfile set to "-0". The only way to solve that seems to reduce all the values back to where they were :-(
So the lack of throttle response has returned. :-((
It certainly seems to be a game of swings and roundabouts!
vololiberista

NEVER go back to the wrong thing or you'll never fix the real problem.

What do you mean by 103% while set at -0??? If you're talking about reverse thrust, that is not controlled in any of these tables. Forget about reverse for now, and set the tables up correctly and I'll show you how to fix that later.

 

[jx_]

I would be grateful if you would walk me through the maths as to how the third column of numbers are arrived at. I'm sort of halfway there in understanding. but what doesn't make sense is that the maths are dividing yet the results are greater!!
I'm obviously missing something obvious!
vololiberista

I believe that was because of my mistake in typing it. I corrected the error above.


Also, those tables all appear to be static.

 

[Roy Holmes]

Correction:

1503-
Mach 0.0 at IAP 1.000 = 1.000000 CN2 ratio
Mach 0.0 at IAP 6.242 = 1.153241 CN2 ratio

1504-
Mach 1.0 at IAP 1.000 = 0.528281 CN2 ratio
Mach 1.0 at IAP 6.242 = 1.052400 CN2 ratio


N1 readout desired * CN2 ratio = CN2 entry in table.

So your 100 percent N1 would be 100, 115.3, 52.8, and 105.2 for those four

I think 52.8 should be 91.29

Roy

 

[jx_]

yes Roy is correct... I screwed up that original post from beginning to end!

Correction to the correction of the correction!!!

1503-
Mach 0.0 at IAP 1.000 = 1.000000 CN2 ratio
Mach 0.0 at IAP 6.242 = 1.153241 CN2 ratio

1504-
Mach 1.0 at IAP 1.000 = 0.912871 CN2 ratio
Mach 1.0 at IAP 6.242 = 1.052760 CN2 ratio


N2 readout desired * CN2 ratio = CN2 entry in table.

So your 100 percent N2 would be 100, 115.3, 91.2, and 105.2 CN2 for those four

My fault. I don't know why I typed those numbers... Carelessness.

 

[jx_]

It should also be noted that tables 1503 & 1504 are for setting throttle relationships, not N2 by altitude. All altitude work is done automatically by the correction process.

 

[Roy Holmes]

It should also be noted that tables 1503 & 1504 are for setting throttle relationships, not N2 by altitude.

Tables 1503 and 1504 set CN2 according to throttle, IAP and Mach. What does your statement mean in that context?
Roy

 

[jx_]

It means do not mistake those tables for anything more than fan speeds commanded by throttle at a given mach by altitude.

Those tables are not for the purpose of aligning fan speeds to altitude or speed, but for aligning fan speeds to throttle, or better yet, IAP and mach inputs are there to allow you to change the throttle alignment to RPM based on either or both altitude and mach. Tables 1503, 1504, and 1505 are for adjusting commanded RPM.

If a designer's goal is to achieve 100% throttle = 100% N1 at all altitudes, then all entries should match SSL n1 / sqrt total temp ratio.

But in many aircraft, the throttle should be increased or reduced to maintain a constant n1. For these types, the mach & IAP lines would differ from SSL n1 / sqrt temp ratio, telling MSFS to realign the throttle map as altitude or speed changes.

In FADEC equipped aircraft, there may be multiple scenarios that need to be programmed.


In practice, you need to know the target n1 for a given throttle scenario first, correct that target to sqrt total temp ratio, and input that corrected number (which is usually different from SSL n1 / sqrt temp ratio) into the table.

So it is important to understand the goal of these three tables is to tell MSFS how you want your throttle relationship to vary with speed and altitude (and what fan speeds you are hoping to see at a given throttle setting).

If you look at it as adjusting CN2 against speed or altitude you will confuse yourself. Those are constant and cannot be changed.

 

[Roy Holmes]

If a designer's goal is to achieve 100% throttle = 100% N1 at all altitudes, then all entries should match SSL n1 / sqrt total temp ratio.

This goal would be achieved predominately in 1502, not in 1503/1504 because they do not manipulate N1 or CN1

Because 1503 and 1504 are linearly interpolated by Mach against a non-linear function, if the designer wants to have 100% throttle =100% N1 he will need to have multiple Mach columns in 1502 to correct for the linear interpolation errors in 1503/1504. This in addition to having multiple IAP columns in 1503 and 1504 for similar reasons.

Roy

 

[jx_]

This goal would be achieved predominately in 1502,

Exactly how does one set throttle commanded position in 1502??? You can't.

Set 1502 to the difference you want between your fans using CN2 + (N1 - N2) = CN1. In your case with a single spool, that equation yields CN2 + 0 = CN1.

table 1502 = 1503~1504 output + difference of (desired N1 - desired N2)

Because 1503 and 1504 are linearly interpolated by Mach against a non-linear function, if the designer wants to have 100% throttle =100% N1 he will need to have multiple Mach columns in 1502 to correct for the linear interpolation errors in 1503/1504. This in addition to having multiple IAP columns in 1503 and 1504 for similar reasons.

The issue here is throttle position should be aligned to Total T ratio at IAP = X where IAP follows a different curve. If throttle alignment is the goal, the solution is to draw a Total T ratio table against IAP. Mach entries in 1502 aren't needed for this. The effect of velocity on Total T Ratio is linear so the low 1503 and high 1504 table are sufficient.

Temperature lapse rate is linear up to 36,089 feet and this is what controls your fan speeds directly. This cannot (nor should it) be changed by the airfile. Corrected N is uncorrected by FS internally as it should be. 1503 & 1504 have no affect on the relationship between corrected and uncorrected fan speeds, altitude effects included.

Focus on setting static fan speeds for each throttle setting, then applying any changes to those throttle settings based on speed or altitude. It is best to visualize tables 1503 & 1504 as changing the throttle scale against a constant CN2 scale instead of the other way around.