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IAS vs TAS

JB3DG

JB3DG

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Can anyone here tell me how to calculate IAS from TAS? I need to use IAS to simulate the windmilling RPM of my J47 simulation however I cannot use the IAS variable as it becomes zero if the pitot ices up. Anyone got some curves of how FSX does it? I have heard some reports that it is not accurate.

P.S. Knowing how CAS can be calculated from TAS would be handy as well.
 
If your pitot ices up you will get no airspeed or Mach readings that are accurate.

In FS IAS at highish speed and low altitude can exceed TAS, which means they must have some compressibility factor built into the calculation. IAS is suspect IMHO.

I prefer to use EAS which you can get ftom Mach with this code
Code: 
<Value>(A:Ambient pressure,millibar) 1013.25 / sqrt (A:AIRSPEED mach,machs) * 661.48 * (>L:AIRSPEED_EQUIVALENT,number)</Value>

Roy
 
In FS IAS at highish speed and low altitude can exceed TAS, which means they must have some compressibility factor built into the calculation. IAS is suspect IMHO.
Click to expand...

Roy, I think you're right that they've included compressibility. When I was performance testing the Buccaneer I noticed the difference between the IAS I expected for the Mach number and what I was actually getting. Being a rotary type it took me a while to twig what was going on, but after a few hours with the high speed Dalton*, I think the figures make sense if you take compressibility into account. I.e. they matched the values I came up with.

*Why the Navy issues them is beyond me as I can't imagine the Sea Harrier pilots ever touched them either...
 
No it doesn't include compressibility, it's just uncalibrated. Once calibrated it does what it should. how table 1101 is set affects the uncalibrated readout but I have yet to determine what it is doing exactly.



Naruto,

Try True Airspeed * sqrt Density Ratio = IAS


TAS to CAS is a pain, but here you go:

Vc = 1479.1 * sqrt { ( Pressure ratio * [( 1 + 1/Temperature ratio * (TAS/1479.1)^2 )^3.5 - 1] + 1 )^1/3.5 - 1 }


Mach to CAS is similar:

1479.1 * sqrt { [ Pressure ratio * {[(Mach^2 * 0.2) + 1]^3.5 - 1} + 1 ] ^1/3.5 - 1 }
 
Thanks a bunch guys. I plan to use the velocity variables that can be controlled via simconnect and which are unaffected by pitot icing. Just needed the calcs....

@jx I imagine the density ratio calculation would be ambient density / standard sea level density....any chance you can give me the value of the latter?
 
There is a nice free calculator called ADACALC that has conversions and atmospheric data.
A:SIGMA SQRT,number is sqrt density ratio

Roy
 
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Naruto-kun said:
@jx I imagine the density ratio calculation would be ambient density / standard sea level density....any chance you can give me the value of the latter?
Click to expand...



0.002377 slugs per cubic foot at ISA/MSL



Roy, isn't there an A:DENSITY RATIO,number or something like that in FSX???
 
jx_ said:
0.002377 slugs per cubic foot at ISA/MSL

Roy, isn't there an A:DENSITY RATIO,number or something like that in FSX???
Click to expand...
No, but there is (A:AMBIENT DENSITY,Slug/ft3) which should return the appropriate density for the given altitude.
 
Bill is correct, but there is as I said three posts up the sqaure root of ambient density ratio (A:SIGMA SQRT,number) and you can also get the ratio by dividing (A:AMBIENT DENSITY,Slug/ft3) by 0.002377

Roy
 
For information, FS makes some wrong calculations here. All details are available in this detailed and well documented thread:
http://www.flightsim.com/vbfs/showthread.php?224097-Is-IAS-indicated-wrongly-by-FS

Following is the relationship between TAS and IAS in FS for subsonic flights (IAS from TAS), that is wrong
mach = TAS/a [a=speed of sound at observed static temperature)
QcP = (1+0.2*mach*mach)^3.5 - 1
CAS= Sqr(Qcp*spr/0.7)*asL ; [spr=Pa/Po (standard pressure ratio) - asL=661.48 kts)
IAS=(CAS+Airspeed_Offset)*Airspeed_Scalar (offset ans scalar from aircraft.cfg)

Exact calculation should be
mach as before
Qcp as before
CAS= Sqr (5*((Qcp*spr+1)^(1/3.5)-1))*aSL
IAS as before
 
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Hey,

I have found that the error is not with the formula for CAS but that it is in fact not scaled. I believe the scaling will depend on the aircraft's indications.

By setting the scalars correctly in the cfg, I have been able to match a real world calibrated airspeed indicator by +- 0.003 Mach --- which puts me around +- 0.2 KTAS.

I don't have that cfg in front of me but it was along the lines of: * 0.93 + 14
 
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