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Thread: Kind of speed in FMB

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    Student Pilot greybeard_52's Avatar
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    Kind of speed in FMB


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    Re: Kind of speed in FMB

    Indicated Air Speed. So for same True Air Speed at higher altitudes the IAS will start to decrease, and vis-a-versa. For those who have not dealt with this before, as aircraft climb the air gets thinner and so less air molecules flowing through the pitot tube resulting in the mechanical observation that the aircraft is flying slower. It's a little more complicated than that but you get the gist. So to maintain the same true airspeed at a higher altitude will result in a slower indicated air speed. It's like 1.5% slower per 1000' (~300 metres).

    Attached is a little spreadsheet that allows one to calculate IAS and TAS at different altitudes. Just change the variables in the yellow highlighted cells. It is in metric and imperial. There is also a worksheet with some performance data for aircraft in the sim.

    Lenny

    Indicated Airspeed (IAS) to True Airspeed (TAS) Conversion.zip
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    Student Pilot greybeard_52's Avatar
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    Re: Kind of speed in FMB

    Quote Originally Posted by ATAG_Lenny View Post
    Indicated Air Speed.
    Thank you very much! I asked because often I see AI keeping a cruising speed significantly lower than that set in FMB (I check in flight during simulation on my airspeed indicator, while following my AI leader). I got the doubt that that velocity set in FMB could be TAS.

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    Re: Kind of speed in FMB

    Hello,

    Before we do any more calculations, please recall that the impact pressure is the only directly measurable quantity in a WWII single-seat fighter that relates to the aircraft's speed with respect to the air. So, with considerable accuracy to annual average conditions at sea level, nearly all WWII airspeed indicators were corrected for certain air pressure conditions in the battle area they were used.
    A very convenient way was the vertical displacement of the standard atmosphere from its standard position required to obtain the observed pressure at the height above mean sea level considered. In other words, this vertical displacement of the standard atmosphere is necessary to obtain the same weight of air above the height considered in both the actual and standard atmosphere. The approach was not considered the theoretically best method for providing an immediate display of speed based on dynamic or impact pressure measurement at that time. However, it was considered the best possible compromise solution, coming quite close to the ideal solution.

    Back to Greybeard's screenshot and the question: 'TAS or IAS?'

    Doubtless, the quantity 347 represents True Airspeed at Mean Sea Level where TAS = IAS. The following observations confirm this.

    When the mission starts, the aircraft in question will be created at 1832m above MSL with 349 km/h TAS (airspawn). 347 km/h TAS produces the same incompressible dynamic pressure q(h, TAS) at Mean Sea Level (MSL) as 349 km/h TAS at 132 meters pressure altitude above MSL. As long as the AI is the pilot in command, the aircraft accelerates to approximately 382 km/h TAS and tries to hold the speed. 347 km/h TAS produces the same incompressible dynamic pressure q(h, TAS) at MSL as 382 km/h True Airspeed at (1832+132) meters pressure altitude.
    Obviously, the airspeed indicator is corrected to standard pressure altitude plus approximately 132 meters. Where 'Standard Atmosphere' means Temperature = 288.15°K @MSL, Air Pressure = 101325 PA @MSL, Air Density = 1.225 kg/m³ @MSL.

    The screenshot shows waypoint 5/8. The AI as pilot in command accelerates to 382km/h at 1832m AMSL and will hold the speed as long as no other event forces a change.




    greybeard_52_2024-02-29.png
    Last edited by GANIX; Mar-05-2024 at 12:49.

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    Re: Kind of speed in FMB

    So does that mean it is really TAS? Everything I have read, including the manual said it was IAS.
    Lenny
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    Re: Kind of speed in FMB

    Hello Lenny,

    IAS is essential for pilots. For 'Full Mission Building' the TAS is essential to navigate AI aircraft through the 'Cartesian World of Cliffs'. So, there are a couple of good reasons why 1C:Maddox implemented this value as 'True Airspeed'. It 'really was' TAS and I have reason to believe that a couple of still hidden equations 'behind the curtain' work with it.
    If you like, I can give a more detailed answer. Please tell me what manual you refer to?

    Postscript: In the meantime, there are loads of posts dealing with airspeed, whatever it may be. It would be great if we could gather and reference them here in a subforum, like 'All About Airspeed' for example.

    Quote Originally Posted by ATAG_Lenny View Post
    So does that mean it is really TAS? Everything I have read, including the manual said it was IAS.
    Lenny
    Last edited by GANIX; Mar-06-2024 at 05:23.

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