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Thread: Is IAS indicated wrongly by FS?

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    Default Is IAS indicated wrongly by FS?

    Hi all,
    hope you can shed some light on an issue I've been wondering about for some time now...

    It really seems there is an error in the way FS (I have 2004) calculates IAS.

    I had thought about hijacking a recent discussion relating to a similar question, but it's probably better to open an own thread.
    http://www.flightsim.com/vbfs/showth...ent-elevations

    Mexy correctly (AFAIK) states in that discusstion that at sea level IAS and TAS should be very closely the same (and wisely indicates that the PICD speed correction terms can have a small effect).

    However, as CRJ has observed, IAS indicates much higher than TAS at sea level at high speeds in the sim.
    CRJ observes an IAS of 613 kts and a GS of 562 kts at sea level. IAS is 51 knots, or 9%, higher than TAS (=GS assuming no wind).

    Even taking into account the PICD speed correction terms there is no explanation for this large discrepancy. Position (P) and instrument (I) errors to apply to IAS are typically set to zero for most planes in FS, and at sea level density (D) and compressibility (C) correction terms to apply are zero as well.

    As far as I understand there's no physical effect which would explain why there would be this large difference between IAS and TAS at sea level.

    I'd highly appreciate if anybody could point out an explanation/physical effect that would account for this large discrepancy.

    Thanks!

    Gunter

    PS: We have discussed this in the FSDeveloper forum in the past, but without resolution.
    http://www.fsdeveloper.com/forum/showthread.php?t=20276
    I'm copying a message from there.

    Maybe somebody in the larger audience here can shed some light on this issue.

    "I did some more tests with SF8 F-86E and default B737-400 in FS2004, and Microsoft may really have got something wrong here...

    ...

    2. Bad news is: The Problem is with IAS, (the IAS speed) displayed is much too high at high IAS compared to TAS.
    Up to 200 KIAS there is no significant error.
    At higher IAS the error sharply increases and has to be considered when determining TAS from IAS indicated in the sim.

    3. Good news is: IMHO this error can be treated as an instrument error (large at high IAS).
    According to the tests the error seems directly to depend on IAS.

    When calculating TAS from IAS the ICE-T steps can be followed:

    Indicated air speed (IAS)

    + Instrument error - this accounts for the wrong indication. See attached graph from fs9 test data.
    This correction is much larger than would be expected for a normal instrument or position error, but applying this correction seems to allow to correctly correlate IAS and TAS (considering the additional corrections below).
    Below 200 KIAS this correction can be neglected.

    (Note: The difference observed by CRJ between IAS and TAS at sea level is 51 kts, exactly what would be expected considering the graph at 613 KIAS.)

    = Calibrated air speed (CAS)

    + compressibility correction. See graph at http://www.usntpsalumni.org/FTM108-appendVII.pdf
    Below 0.5 Mach this correction can be neglected.

    = Equivalent air speed (EAS)

    * Density correction. Perform with E6-B.

    = True air speed (TAS)

    Alternative methods to determine TAS from CAS:

    - Graph from F/A-18A/B/C/D FLIGHT MANUAL, SECTION XI PERFORMANCE DATA, for Low Mach and High Mach region
    Graphs attached.

    - Online Atmospheric Properties Calculator
    http://www.aerospaceweb.org/design/scripts/atmosphere/

    Comments on IAS correction:

    + Instrument error: This is where the the apparent error in the IAS displayed by FS can be accounted for.
    For the correction see the data in the attached graph, derived from flight tests in fs2004.
    The SF8 F-86E and default B737-400 have the same display error as function of IAS.
    Note that when going from IAS to CAS the correction is negative, i.e. CAS is smaller than IAS.

    It's hard to believe that MS got this wrong indeed, but I didn't find any source mentioning a correction factor that would fit the observed deviation for IAS.
    I'd highly appreciate if anybody could point out an explanation/physical effect that would account for this correction term.

    I have done tests with two planes only, but results indicate the same correction terms for both.
    I'd be interested to hear if this correction term does not work for other planes.

    Thanks,

    Gunter

    ..."
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    Last edited by teson1; 12-22-2010 at 01:02 PM.

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