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FS2004 Complex Graph.

vololiberista

vololiberista

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Is there a simple way to model this graph? My feeling is that every line would have to be individually calculated.
P1000078x.jpg
 
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mgh said:
What is the significance of the diagonal dotted line from 68 to 73 Hg?
Click to expand...
I'm afraid I don't know that as yet. I only acquired the graph last Sunday. I have put out a query to the "eggsperts". I was told that at SL the max P7 was 74 so as you say the dotted line is a bit confusing.
 
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mgh said:
What is the significance of the diagonal dotted line from 68 to 73 Hg?
Click to expand...
It says it right on the graph...
Anti-icing on- within dotted line reduce P7 by:-
Aircraft 0.7" Hg
Engine 0.7" Hg
Both 1.4" Hg

I'm pretty certain this is because engine anti-ice on is going to impact pressure levels.
 
WarpD said:
It says it right on the graph...
Anti-icing on- within dotted line reduce P7 by:-
Aircraft 0.7" Hg
Engine 0.7" Hg
Both 1.4" Hg

I'm pretty certain this is because engine anti-ice on is going to impact pressure levels.
Click to expand...

Yes that makes sense. Any ideas regarding the original question?
 
For a standard atmosphere your P7 varies at close to atmospheric pressure ratio.
For example at sea level 15°C P7 is about 73.8, at 4000 feet it is 64.1 at 8000 ft it is 55.3
Atmospheric pressure ratio for 4000 ft is 0.864, times 73.8 gives P7=63.8, at 8000 ft use .74 * 73.8 = 54.6
Atmospheric pressure ratio (delta) is theta^5.256 where:
theta = (288.15-1.98*altitude/1000)/288.15
You could decrease the 5.256 power slightly to more closely match your chart numbers, for example if you made it 5.11 the 4000 and 8000 values are correct to the accuracy of reading the chart
I would expect that non-standard atmosphere results would be somewhat similar to model.
Roy
 
The chart is represented as distinct arrays of up to 4 sets of data where the X value is the temperature and the Y value is the P7 limit, and the arrays represent the altitude values. You would have to know how many data points were valid in each array since some of the lines have 3 distinct X/Y positions and others have 4.
 
WarpD said:
The chart is represented as distinct arrays of up to 4 sets of data where the X value is the temperature and the Y value is the P7 limit, and the arrays represent the altitude values. You would have to know how many data points were valid in each array since some of the lines have 3 distinct X/Y positions and others have 4.
Click to expand...

I was wondering if in effect I would have to maske up to 4 different calculations for each line. If I can get a sort of ball park figure that's no mopre than half a point out that would do.
 
Roy Holmes said:
For a standard atmosphere your P7 varies at close to atmospheric pressure ratio.
For example at sea level 15°C P7 is about 73.8, at 4000 feet it is 64.1 at 8000 ft it is 55.3
Atmospheric pressure ratio for 4000 ft is 0.864, times 73.8 gives P7=63.8, at 8000 ft use .74 * 73.8 = 54.6
Atmospheric pressure ratio (delta) is theta^5.256 where:
theta = (288.15-1.98*altitude/1000)/288.15
You could decrease the 5.256 power slightly to more closely match your chart numbers, for example if you made it 5.11 the 4000 and 8000 values are correct to the accuracy of reading the chart
I would expect that non-standard atmosphere results would be somewhat similar to model.
Roy
Click to expand...
Thanks Roy,
as regards the other question in the other thread http://www.fsdeveloper.com/forum/threads/it-all-works-but.434787/
what would you suggest in order for the gauge to read the correct number at full throttle?
 
You are using (A:Turb eng1 pressure ratio, ratio) and this comes from what you have in Record 1532. I have a copy of the DM VC10 air file but it has no 1532. It would be interesting to see what you have.
As you probably know, EPR is just a display value calculated after the engine data has been calculated and plays no part in the thrust equations.
The stock 1532 has:
;Engine Pressure Ratio
TOKEN_BEGIN AIR_70_EPR
REAL8 1.0 ;EPR tuning constant
REAL8 1.4 ;Max EPR
REAL8 14.0 ;EPR time constant
TOKEN_END
I have never messed with that, but I suppose the Max EPR is added to the tuning constant to give 2.4. Your 73.8/29.92 would give 2.47

The chart has pressure altitude lines which means that they are lines connecting points where the actual pressure corresponds to the given altitude pressure, even though because the temperature is non-standard the actual altitude above sea level may differ from the pressure altitude. So sea level pressure altitude may not actually be at sea level. The problem here is that the sim basically works off ISA and changing the ambient temperature, just adds or subtracts values from the standard temperature and AFAIK does not change pressure, although it does change density.

I suspect Herve may be able to throw more light on this subject
Roy
 
vololiberista said:
I was wondering if in effect I would have to maske up to 4 different calculations for each line. If I can get a sort of ball park figure that's no mopre than half a point out that would do.
Click to expand...

Assuming these values I eyeballed from the graph:
Code: 
S.L.
-40, 15, 25, 45
72.4, 73.8, 72.7, 64.0
1000
-40, 5, 25, 43
73.1, 72.7, 70.2, 61.8
We'll be using a typical slope formula of (y2-y1)/(x2-x1)*(pt-x1)+y1. Based on the data and an aircraft altitude of 350'
Code: 
ALT=0 TEMP=15
(72.7-73.8)/(25-15)*(15-0)+73.8 = 73.8

ALT=1000 TEMP=15
(70.2-72.7)/(25-5)*(15-5)+72.7 = 71.45

AIRCRAFT ALT = 350
(73.8-71.45)/(1000-0)*(350-0)+71.45 = 72.9775
 
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Hi Roy,
In my table 1532 I have
EPR Scale Factor=1
EPR Calibrate=2.855
EPR Rate of Change?=14
In a way it makes sense for (A:Turb eng1 pressure ratio, ratio) to drive the P7 gauge and for sea level it's perfect. But I need to add/subtract another factor so that at full power the correct P7 is given for a given airfield. For example Shoreham is ok but Torino at 1,000 ft asl the dial under reads. It should be around 71.5 but the current calculation gives 68.7
 
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Obviously to get the actual airfield pressure i would use QFE which I've calculated elswhere. But I would have to find a way to tell where I am.
 
For Torino, using the equations in post#7 I get 71.25, Warp D gets 71.45, both using ISA. His is closer to the chart
Roy
 
Roy Holmes said:
For Torino, using the equations in post#7 I get 71.25, Warp D gets 71.45, both using ISA. His is closer to the chart
Roy
Click to expand...

Hi Roy,
Yes I've been able to create a "table" of maxima from those equations. Thanks for that. The next step is to generate the numbers that drive the gauge.
 
= (((288.15-1.98*N118/1000)/288.15)^5.11)*73.8
What's the best way to express this in xml? N118 is thousands of feet.
 
I'm trying to get this one to work too but in the mean time may I ask you how to convert it to xml?
=(267.4242424238*D15^4-2602.7777777738*D15^3+9399.5833333178*D15^2-14868.2979797705*D15)+8711.6903030121
This represents the actual P7 gauge rather than P7 maxima. D15 represents
(A:Turb eng1 pressure ratio, ratio)
 
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