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FS2004 thrust conundrum

vololiberista

vololiberista

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italy
I have successfully constructed a polynomial curve for my P7 gauge EXCEPT
It is only correct at maximum thrust.
Currently the EPR setting in the airfile gives a range from 1 to 2.8 and it is on this i have based my calculations. The problem arises at idle thrust where the P7 pressure is always much too high.
I contructed the polynomial curve to give a correct max at all altitudes including that of SLLP but this has resulted in all the idle readings being too high.
Generally at idle the reading should be 1 inch above ambient pressure.
so for example at ISA
Shoreham by Sea ambient is 29.92 / engine running at idle sb 31
El Alto ambient is 18 / engine running at idle sb 19
In case 1 the max P7 is almost 74 at ISA
In case 2 the Max P7 is about 48 at ISA

The EPR Maxima are 2.81 in case 1 and 1.71 in case 2 (which give correct readings)

However in case 1 the EPR idle is showing 2.14 which gives a P7 of 60 at idle SB 31
and in case the EPR idle shows 1.32 which gives a P7 of about 32 at idle SB19

Any suggestions on how this could be resolved. Would increasing the EPR number in the airfile work for example?
Currently it states
*EPR Scale Factor=1
*EPR Calibrate=2.855
EPR rate of Change?=14

Only the middle one seems to have an effect.
The EPR default value is always 1

I tried changing the curve shape but is was almost vertical so something else needs to be factored in.
 
Last edited:
Volo
I think you need to get away from the stock EPR data.
I did some tests and found the following simulator results.
EPR varies with N1 according to two straight line fits, one from idle to 90% and one from 90% upwards. The max EPR static on the ground is the value of the second air file constant called MAX EPR.
In the air at constant altitude with full throttle EPR increases as a straight line according to Mach once above 0.4 Mach. At low altitude and 0.82 Mach it will be 4.0 or greater.
In a climb at constant Mach with full throttle EPR decreases according to Air Pressure Ratio.

If you change the first constant from 1 to 2 it has no effect.
If you change the second constant from 1.4 to 1.0 or 0.0, EPR is 1.0 on the ground at all throttle/ N1.
If you change the second constant to 1.6 that or slightly less becomes your max EPR on ground.

I'm not into EPR, but I think most of this is unrealistic, so better to code it separately
Roy
 
Roy Holmes said:
Volo
I think you need to get away from the stock EPR data.
I did some tests and found the following simulator results.
EPR varies with N1 according to two straight line fits, one from idle to 90% and one from 90% upwards. The max EPR static on the ground is the value of the second air file constant called MAX EPR.
In the air at constant altitude with full throttle EPR increases as a straight line according to Mach once above 0.4 Mach. At low altitude and 0.82 Mach it will be 4.0 or greater.
In a climb at constant Mach with full throttle EPR decreases according to Air Pressure Ratio.

If you change the first constant from 1 to 2 it has no effect.
If you change the second constant from 1.4 to 1.0 or 0.0, EPR is 1.0 on the ground at all throttle/ N1.
If you change the second constant to 1.6 that or slightly less becomes your max EPR on ground.

I'm not into EPR, but I think most of this is unrealistic, so better to code it separately
Roy
Click to expand...

Thanks for your input Roy,
I have been experimenting with all sorts of options and cannot get a P7 power curve to match different airport elevations, pressures and temperatures.
Only at sea level and ISA can I get a true realistic curve. I've tried reproducing that at different elevations and get too little at the bottom and too much at the top.
The only curve I can get to work in all (the majority of) situations is a top end curve on which only the maxima are plotted. Meaning the bottom P7 at idle is always much too high. I'm out of solutions at the moment!:banghead: x 10!
 
jx_ said:
use CN1 or thrust. Don't use that crappy FS EPR variable.
Click to expand...
The problem isn't really EPR as it varies with temperature and pressure. Which is what I want. It's more to do with getting the right curve to fit the right situation.
In the graph below the top black curve represents the maximum P7 (x scale) for any given max EPR (y scale). One can see straight away problems with this as it incorrectly defines the minimum. For example the model is set up to have 2.15 min EPR at sea level and ISA with a max EPR of 2.8.
So translated into real numbers I'm looking for a minimum P7 of about 30.9psi and a max of 74.25. You will see that the right hand navy blue curve fits that nicely but the top curve doesn't except at maximum. I cannot get that to repeat at other elevations. On paper it would seem that the other curves would work but in practice they are either too low or too high. and, you can see that it has become way to complex. all the other curves attempt to calculate min/max P7 from SL on the right to 14,000ft extreme left.

So currently I'm playing with two options.
1. To use only the top curve which means that maximum P7 is always correct
or
2. Do curves for every elevation and get a hit and miss result.

So taking the right hand navy blue curve or a shallower version that would represent only the minima. Is there a mathematical way of getting from that to the maximum without all the complexity of trying to define curves for every situation?

Ideas welcome.

curves.jpg
 
vololiberista said:
Is there a mathematical way of getting from that to the maximum without all the complexity of trying to define curves for every situation?
Ideas welcome.
Click to expand...

If you were using FSX, that would be quite a simple task just by coding XMLTABLES custom vars from XMLTools.

But XMLTools for FS9 does not have it available-yet.

If you are really interested, I could add the class to FS9 version and compile the new dll.

Simply let me know.

Tom
 
taguilo said:
If you were using FSX, that would be quite a simple task just by coding XMLTABLES custom vars from XMLTools.

But XMLTools for FS9 does not have it available-yet.

If you are really interested, I could add the class to FS9 version and compile the new dll.

Simply let me know.

Tom
Click to expand...

Hi Tom,
Yes, a table would probably be the best answer if you think it's possible for Fs9. Thanks.
 
Volo,

Here you have a copy of XMLTools for FS2004 (FS9). It only contains XMLVars and XMLTables classes.
Download and install it in Modules subfolder of FS2004 main folder.

To obtain the documentation, just download XMLToos 2.0 for FSX from the Resources page and read all related to XMLTables class. Also check the examples, they are valid for FS9 as well.

Tom

PS: You also need to have installed VC++ 2010 Redistributable. It can be downloaded from MS pages.
 

Attachments

taguilo said:
Volo,

Here you have a copy of XMLTools for FS2004 (FS9). It only contains XMLVars and XMLTables classes.
Download and install it in Modules subfolder of FS2004 main folder.

To obtain the documentation, just download XMLToos 2.0 for FSX from the Resources page and read all related to XMLTables class. Also check the examples, they are valid for FS9 as well.

Tom

PS: You also need to have installed VC++ 2010 Redistributable. It can be downloaded from MS pages.
Click to expand...

Many thanks Tom,
I'll start work on that shortly.
 
Volo,

You're missing an important piece. Although EPR is dependent on MACH, you are trying to use perfect numbers to simulate an imperfect engine. The real engine will have inefficiencies (very similar to the Ram Drag Curve).

So to model perfect EPR in the sim you need:

1. Sea Level Static EPR curve
2. Sea Level All MACH curve
3. Ram Pressure Error curve

Then there is no need for a table. Combine those three and convert to temperature altitude and you're good to go.
 
jx_ said:
Volo,

You're missing an important piece. Although EPR is dependent on MACH, you are trying to use perfect numbers to simulate an imperfect engine. The real engine will have inefficiencies (very similar to the Ram Drag Curve).

So to model perfect EPR in the sim you need:

1. Sea Level Static EPR curve
2. Sea Level All MACH curve
3. Ram Pressure Error curve

Then there is no need for a table. Combine those three and convert to temperature altitude and you're good to go.
Click to expand...
Thanks jx I'll see if i can put those together.
 
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