FS2004 Inverse Atmospheric Pressure

[Supercruise]

Is there a scale or calculator that states the IAP ratio at any given altitude?

 

[Flusirainer]

 

[vincentf563]

IAP is a function of the actual gauge.

This is something that is done internally in the gauge in real life. The manufacturer of the gauge compensates for differences in the gauges sensitivites over altitude. They are published in the gauges specs. In the case of altimeters this is "programmed" into the gauge if it is digital. If the gauge is mechanical the specs are there to adjust the Indicated value accordingly by altitude and differences between actual and ICAO standard air temperature/pressure. In essence this is what you are doing when you turn the knob to adjust for barometric pressure. Ie...when they say "current altimeter two-niner-niner-two" you adjust the altimeter knob to 29.92. As you increase altitude over 10,000 air traffic control reports a different altimeter setting that is lower than ground setting. In this way differences in air density are compensated so that the values reading are as close as possible to actual. As a fail safe for this in IFR your flight plan includes altitude. If you are 100 ft +/- assigned they tell you which is a redundency designed to verify that your altimeter is set correctly throghout the flight. Also mechanical gauges are tested at various altitudes to verify they work properly on the aircraft. They are placed in such a way that low pressures around the airfoil and fuselage do not interfere with the readings. They are tested to ascertain what the effects of speed have on the readings. Vacuum gauges are set to compensate for this also. All of this is in the spec sheets for the individual gauge.

 

[Roy Holmes]

FS calculates IAP as follows up to 36000 feet

theta = (288.15-1.98*altitude/1000)/288.15
delta = theta^5.256
IAP=1/delta

For example at 20,000 Ft delta, which is Atmospheric Pressure ratio is 0.4598 and IAP is 2.1750
The standard ratio for pressure at 20,000 ft is 0.4599127, so the FS calculation is not exact, but close enough for simulation purposes.

Roy

 

[WarpD]

Out of curiosity, and digging up an old discussion... how does FS calculate IAP above 36000 feet?

 

[hervesors]

I guess nobody knows
Evidently not from static temperature
But tests indicate it is in accordance with the theoretical equation for pressure decrease with geopotential altitude in the stratosphere layer (exponential decrease) at least until 60,000-70,000 ft

 

[Roy Holmes]

Out of curiosity, and digging up an old discussion... how does FS calculate IAP above 36000 feet?

Sorry I did not answer sooner.
In Appendix A to my paper I wrote:

theta is temperature ratio for a particular altitude compared to standard sea level temperature. theta = (288.15-1.98*altitude/1000)/288.15. This equation is valid for altitudes up to the tropopause 36089 ft, but FS appears to use it for all altitudes
delta is pressure ratio for a particular altitude compared to standard sea level pressure. delta = theta^5.256 and like theta is valid up to the tropopause but FS appears to use it for all altitudes.

I also mentioned in the body of the paper that the results of comparing thrust in FS, AirWrench and NASA EngineSim showed that FS and AW were similar above the tropopause but EngineSim differed. I ascribed this to EngineSim using different code. The EngineSim JAVA code is as follows. tso is ambient temperature in degrees Rankine. Pso is pressure in pounds-force per square foot

 if (alt < 36152. ) {
            ts0 = 518.6 - 3.56 * alt / 1000. ;
            ps0 = 2116. * Math.pow(ts0/518.6, 5.256) ;
         }
         if (alt >= 36152. && alt <= 82345.) {   // Stratosphere
            ts0 = 389.98 ;
            ps0 = 2116. * .2236 *
                Math.exp((36000.-alt)/(53.35*389.98)) ;
         }
         if (alt >= 82345.) {
            ts0 = 389.98 + 1.645 * (alt-82345)/1000. ;
            ps0 = 2116. *.02456 * Math.pow(ts0/389.98,-11.388) ;
         }

It can be seen that ES has three bands, up to the tropopause, from there to 82345 ft and above 82345.

This makes FS results differ from ES, but since linear interpolation is used for IAP and typically the stock FS turbofans use IAP=1 and IAP=22.57 which is about 70,000ft not much consideration was given for the need to be accurate in Records 1503 and 1504. Accuracy can, of course, be improved by using multiple IAP columns in these records.
The following table shows altitude, pressure, IAP from that pressure, theta, delta and inverse delta=IAP. It can be seen that the difference between standard IAP and that used in FS is only large above 45,000Ft where most airplanes do not dare to go.

Altitude    in. Hg    IAP      theta      delta    IAP
0            29.92    1.000    1.000     1.000     1.000
5000       24.89    1.202    0.9656    0.8321   1.2017
10000     20.58    1.454    0.9312    0.6878    1.4537
15000     16.88    1.773    0.8969    0.5645    1.7713
20000     13.75    2.176    0.8625    0.4597    2.1750
25000     11.10    2.695    0.8282    0.3713    2.6930
30000     8.880    3.369    0.7938    0.2971    3.3647
35000     7.036    4.252    0.7595    0.2355    4.2456
40000     5.541    5.400    0.7251    0.1846    5.4151
45000     4.364    6.856    0.6907    0.1430    6.9889
50000     3.436    8.708    0.6564    0.1094    9.1382
55000     3.000    9.973    0.6220    0.0824    12.1219
60000     2.700    11.08    0.5877    0.0611    16.3403

Roy

EDIT Please see my next post where I found that FS IAP was accurate.

 

[Roy Holmes]

I decided to get some more test data for this subject and went to altitudes that I had not dared to go to before.​

The results were that the simulator has accurate data for pressure at altitude and therefore correct IAP.
Since the altitude thrust results for FS indicated that it was using the below tropopause calculation method for pressures at altitudes above the tropopause I'm assuming that this only occurs for engine calculations. But I will explore that further
The first delta and IAP columns below were recorded on flight test, the second pair of columns came from AdaCalc for the indicated altitudes.
So this is a correction to my post above.
Roy

High altitude test 6/30/2015

altitude delta IAP delta IAP
40077 0.185 5.4091 0.185 5.4029
50065 0.115 8.6807 0.115 8.7140
55072 0.091 11.0433 0.090 11.0715
59816 0.072 13.9598 0.072 13.8895
65514 0.054 18.4238 0.055 18.2360
70090 0.043 23.2552 0.044 22.6752
75272 0.034 29.3477 0.035 28.9700

 

[hervesors]

Here are the results I obtained for standard pressure ratio from tests at altitudes from 40,000 to 70,000 ft (true altitudes)
FS data are those directly retrieved from AMBIENT PRESSURE variable (FSX). Theoretical data are those calculated from ISA atmosphere equations (correcting true to geopotential altitude before applying)
True altitude Delta FS Delta ISA
40,000 0.18577 0.18577
45,000 0.14616 0.14623
50,000 0.11509 0.11511
55,000 0.09061 0.09063
60,000 0.07136 0.07137
65,000 0.05616 0.05620
70,000 0.04382 0.04429

gAlt (geopotential altitude) = Alt -(Alt*Alt/(REarth+Alt))


So agree with Roy last post. I think one can consider FS uses standard atmosphere equations, at least up to around 65,000-70,000 ft (that is for tropo and stratosphere layers). Seems it could be "off" for mesosphere layer (to be rechecked) but who goes there ;-)

[Edit]: comments on troposphere, stratosphere and mesosphere data

 

[WarpD]

Ok... so I've spent a great deal of time reading, and re-reading the replies. I don't see anywhere that indicates how FS calculates the values above 36089. Did I miss something?

 

[hervesors]

Use standart atmosphere equations for troposphere above 36089 ft and below 65,800 ft. Seems FS use them (not a surprise) considering the fit is very good between calculated and FS reported values.
You'll find them in the 1976 ISA document as well as in most aerodynamical textbooks and reviews. In short

gh = ga - [(ga*ga)/(Rearth+ga)] ; gh: geopotential altitude (m), ga: geometric altitude (m) ; Rearth=6356766 m
SPR = Delta_base * Exp (-k * (gh-Layer_base))
IAP=1/SPR
with for troposphere
Delta_base=0.223361105092158
Layer_base=11000
k=(M*g0/R)/T)=0.0341631947363104 / 216.65

 

[Heretic]

A bit better to read:

IAP_tropo = [e^(0.000157689⋅(H−10998.1))] / 22.63253

H needs to be in meters.

 

[Roy Holmes]

This is the same code as in Post#7 but in Visual Basic. It uses 36152 as the tropopause rather than 36089.
As before, tso is ambient temperature in degrees Rankine. Pso is pressure in pounds-force per square foot, alt is in feet

If alt < 36152# Then
          ts0 = 537.4 - 3.56 * alt / 1000#
          ps0 = 2192# * (ts0 / 537.4) ^ 5.256
       End If
       If alt >= 36152# And alt <= 82345# Then '   Stratosphere
          ts0 = 389.98
          ps0 = 2116# * 0.2236 * Exp((36000# - alt) / (53.35 * 389.98))
       End If
       If alt >= 82345# Then
          ts0 = 389.98 + 1.645 * (alt - 82345) / 1000#
          ps0 = 2116# * 0.02456 * (ts0 / 389.98) ^ -11.388
       End If

Roy

 

[arrex2]

Dears,

Let me reopen this interesting post many years after. Did anybody ever realize that the computations of dynamic pressures, ambient pressures, IAP and whatever are completely wrong once the QNH is not set to standard ISA? And not by a tiny bit. I came to this conclusion as I am expecting that if I remain at the same pressure altitude, i should be facing the same atmosphere in terms of ambient pressure, dynamic pressure (which alters all flight dynamics), IAP (which alters the power settings...) etc...independently of the sea level pressure. Instead no, there is a huge variation. Am I doing some wrong assumptions here or the atmospheric model is completely fu**ed up? Verified on FS9, i bet the same problem applies for FSX, P3D & co....

Regards,
Antonio

 

[arrex2]

I did some more testing. It seems like that the ambient pressure is computed as
Delta_temp = theta^5.256
Ambient_press = Delta_temp*P_isa-(P_isa-P_sl)
Where P_isa is the standard ISA pressure (1013.12 hPa) and P_sl is the current sea level pressure. Only after this correction the real Delta is computed
as Delta = Ambient_press/P_isa
IAP = 1/Delta

(Note that i am not 100% sure this is the actual used formula, but works rather well (within 5%) in the many cases I tested up to now.)

The idea of correcting the ambient pressure by simply subtracting the variation from ISA at sea level is VERY wrong; does not make any sense actually. And the problem is not noticed when the pressure is set to ISA. There are absurd cases where, for very low pressures and very high altitudes, the ambient_press can become negative! In this case FS simply sets it back to 1013.12 hPa.... imagine flying at 65000 ft suddenly the ambient become again "sea level"....