I've just moved from fs9 to fsx-se.

I have two add-on planes, cs L1011 and cs 707.

I've noticed that the IAS is not in sync with mach number eg. at FL 370 at .842 the IAS should be 301 but the L1011 is showing 308 - 7 knots out.

In fs9 this could be corrected in the air file and in the aircraft .cfg by adding a section on speed indicator.

I've tried this in fsx-se but no change is seen.

Is the speed hard-coded in the panel gauges or can this discrepancy be corrected in fsx-se.

 

Thanks in advance

Trevor

From my memory IAS isn't to be trusted above FL290 - Mach is more accurate.

Maybe I did not make myself clear.

 

In a RW L1011 manual, it states that at FL330 at Mach .842 the IAS is 301 and the EPR is whatever at a given weight.

The CS L1011 is showing 308 KIAS for M.842

In fs9 one could correct this to get the speed indicator to read 301.

How do you do this in FSX-SE?

Let's try this the reverse way......In FS9 what did you change in the aircraft.cfg to correct the speed indicator?  Maybe

 

FSX has the same thing in a different location.

In a RW L1011 manual, it states that at FL330 at Mach .842 the IAS is 301 and the EPR is whatever at a given weight. The CS L1011 is showing 308 KIAS for M.842

 

Who says that 308 KIAS is wrong? The relationship between KIAS and Mach number is dependent on air pressure. FL330 is not the same air pressure all the time (that's why we use local QNH at low altitude). I would presume the L1011 figures are based on ISA, if you are using real world weather in the sim you're not set to ISA.

Who says that 308 KIAS is wrong? The relationship between KIAS and Mach number is dependent on air pressure. FL330 is not the same air pressure all the time (that's why we use local QNH at low altitude). I would presume the L1011 figures are based on ISA, if you are using real world weather in the sim you're not set to ISA.

No you're wrong! It doesn't depend of air pressure in this way(QNH). Wathever the plane, the relation between IAS and Mach vs altitude is always the same. TrevorS is right, at FL 330 for a Mach of .84 the IAS is 301kt.

Forgive me for playing devil's advocate but Mach 1 is the speed of sound, which is dependent on air density. Temperature is by far the biggest factor, although air pressure and humidity have a small but probably unnoticeable impact impact too.

 

At FL300 ISA temperature is -44.4°c ((30 x -1.98) + 15). But sea level temperature isn't always at +15°c ISA, so that's where the temperature variation creeps in (and thus the Mach number will change even though altitude and IAS are the same).

 

See http://www.sengpielaudio.com/calculator-airpressure.htm

 

This also explains why people have issues with Mach when using FS INN weather because it sets sea level temperature at all altitudes.

The relationship between IAS and Mach number at a given pressure altitude (referenced to 29.92) is fixed, and does not depend on air temperature. At 33,000 feet, Mach 0.842 equates to an IAS of 301 knots.

 

What DOES vary with temperature is TRUE airspeed.

 

At FL330, 301 knots IAS, Mach 0.842 - true airspeed will be 506 knots with an OAT of -35C.

 

At an OAT of -54C, TAS will be 486 knots.

The relationship between IAS and Mach number at a given pressure altitude (referenced to 29.92) is fixed, and does not depend on air temperature. At 33,000 feet, Mach 0.842 equates to an IAS of 301 knots.

 

What DOES vary with temperature is TRUE airspeed.

 

At FL330, 301 knots IAS, Mach 0.842 - true airspeed will be 506 knots with an OAT of -35C.

 

At an OAT of -54C, TAS will be 486 knots.

Exactly!

 

 

At FL300 ISA temperature is -44.4°c ((30 x -1.98) + 15). But sea level temperature isn't always at +15°c ISA, so that's where the temperature variation creeps in (and thus the Mach number will change even though altitude and IAS are the same).

 

See http://www.sengpielaudio.com/calculator-airpressure.htm

 

This also explains why people have issues with Mach when using FS INN weather because it sets sea level temperature at all altitudes.

When you cruise at M.84 at FL 330(for example) Mach number and IAS never change(till you move the throttles or change the aircraft attitude :wink:  ). What is changing is your TAS as explained by JRBarrett. The hotter the air the faster you are(in TAS).

I did not use any weather - everything standard.

 

In fs9 one could add an entry to the aircraft.cfg which would correct this discrepancy.

 

[airspeed_indicators]

airspeed_indicator.0=0.982, 0.0  //  calibrates IAS to CAS

 

In the .air file, one made changes in the primary aerodynamics section:

 

speed weight factor = +32000 with a base=0

http://www.aero.sors.fr/

 

Cheers

Finally, problem solved. Have everything in sync.

In teresting thread. Just made the same observation yesterday. The IAS values given in a 727 cruise chart don't match up with the one indicated on the ASI.

 

Since I have access to the instrument source files, I'm can at least tackle the problem at the root though.

I hope that Roy's equation posted in the FSDev thread on IAS/CAS* works.

 

 

(A:Ambient pressure,millibar) 1013.25 / sqrt (A:AIRSPEED mach,machs) * 661.48 * (>L:AIRSPEED_EQUIVALENT,number)

 

 

P_amb / 1013.25 * sqrt(M) * 661.48 = V_EAS

 

* http://www.fsdeveloper.com/forum/threads/is-a-airspeed-mach-mach-correct.20276/page-2#post-161674

 

 

The relationship between KIAS and Mach number is dependent on air pressure.

 

Absolutely.

 

THIS might be of some help.

 

 

 

Best wishes!