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3 Answers

TAS – CAS Corrected for Nonstandard Temp?

Asked by: 6659 views
Aerodynamics

I understand how altitude and density affects TAS. I do not understand why CAS needs to be corrected for nonstandard temperature.

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3 Answers



  1. John D Collins on Jul 21, 2014

    CAS is calibrated airspeed. IOW, it provides the corrections for installation errors of the pitot static system as installed on a particular aircraft type to adjust the indicated airspeed to obtain the effective airspeed. As an example, a C172P shows the following corrections from flaps up indicated airspeed to calibrated airspeed at 110 IAS the CAS is 107. If you were to indicate 110, you would correct 107 for the temperature effect and not the 110 value

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  2. Drew on Jul 23, 2014

    Up at altitude, the air is less dense, so the aircraft must have a lower CAS for a given TAS. So, if CAS is corrected for density, I get that it would yield TAS. I dont understand why non-standard temperatures are taken into account when calculating TAS. Is TAS calculated based on the standard temperature lapse rate? Why does temperature even matter?

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  3. Best Answer


    John D Collins on Jul 24, 2014

    For low speeds and altitudes that most light piston aircraft fly at, the calculation to determine the true airspeed can be simplified to be equal to the CAS times the square root of the ratio of the standard air density at sea level (p0) divided by the air density at the altitude (p), TAS = CAS SQRT (p0/p).

    You can derive the density of dry air from the ideal gas law PV=NRT where P is the pressure, V is the volume, N is the number of molecules, R is the gas constant for dry air, and T is the absolute temperature. The density is the number of molecules (N) per unit volume (V). Restating the ideal gas equation, N/V (density) = P/RT = p. As you can see, the air density is an inverse function of temperature, the higher the temperature, the lower the density.

    This means that the higher the temperature at altitude, the higher the TAS since p is a smaller value and appears in the divisor of the TAS calculation. Dividing by a smaller value yields a higher result.

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