fixedwingsforever
24th Jul 2002, 07:35
When you use JSA for calculations the temperature is assumed to be 15 C with at lapse rate of 2 C pr 1000ft (as in ISA) - the difference is that there is no upper limit. So my question is - why do we use JSA and not ISA for calculations regarding the Machmeter?
Checkboard
24th Jul 2002, 08:13
What calculations are you referring to? The Machmeter doesn't use temperature to determine your mach number, as the mach number is independant of temperature (i.e. Temperature effects TAS and the local Speed of Sound in the same ratio, and as Mach Number is a ratio, temperature is cancelled out).
Have a look at these previous threads:
machmeter (http://www.pprune.org/forums/showthread.php?s=&threadid=53558&highlight=Mach+temperature)
Mach conversion (http://www.pprune.org/forums/showthread.php?s=&threadid=10282&highlight=Mach+temperature)
Mach vs. IAS (http://www.pprune.org/forums/showthread.php?s=&threadid=10128&highlight=Mach+temperature)