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220876 - How is Mach determed without temperature?

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  • 220876 - How is Mach determed without temperature?

    Question 220876 in Instruments:

    The input(s) of a VMO/MMO warning system is (are):
    Static pressure only
    Static pressure and aircraft configuration
    Static pressure and total pressure.
    Static pressure and SAT

    The correct answer is C
    The M
    Last edited by Flippert; 06-02-2019, 15:05.

  • #2
    Mach number is defined as the ratio (TAS / LSS). However thanks to the magic of isentropic flow theory mach number can also be expressed as a function of of the ratio (total pressure / static pressure).

    For subsonic flight: M=sqrt(5((Pt/Ps)^(2/7)-1))

    CAS is defined as a function of the difference (total pressure - static pressure)

    Therefore both CAS and Mach are functions of total and static pressures. This also explains why the relation between CAS and Mach does not depend on temperature.
    Last edited by Ortac; 18-03-2019, 20:06.