Mach #

[Lake77]

Can anybody tell me why the Mach number listed sometimes is higher than the maximum Mach number for the particular aircraft. Thank You very much. PS Flightaware Rocks!!!

[mduell]

It’s calculated using the groundspeed (instead of the airspeed) and using the standard atmosphere model for temperature (instead of actual temp aloft).

[CessnaCitationX]

@mduell

It’s calculated using the groundspeed (instead of the airspeed) and using the standard atmosphere model for temperature (instead of actual temp aloft).

What’s the equation?

[mduell]

The speed of sound is sqrt(gammaRtemperature), where gamma is the ratio of specific heats, R is the gas constant, and temperature is in absolute units. I calculated it every 100 feet in the 1976 US standard atmosphere and came up with a nice quadratic curve fit (R^2 > 0.9995) up to FL360 (above that it’s a constant).

[CAFlier]

@CessnaCitationX

What’s the equation?

Not a simple answer: mathpages.com/home/kmath282/kmath282.htm

[G4Driver]

@mduell

The speed of sound is sqrt(gammaRtemperature), where gamma is the ratio of specific heats, R is the gas constant, and temperature is in absolute units. I calculated it every 100 feet in the 1976 US standard atmosphere and came up with a nice quadratic curve fit (R^2 > 0.9995) up to FL360 (above that it’s a constant).

I think my head just exploded.

[cfijames]

@G4Driver

@mduell

The speed of sound is sqrt(gammaRtemperature), where gamma is the ratio of specific heats, R is the gas constant, and temperature is in absolute units. I calculated it every 100 feet in the 1976 US standard atmosphere and came up with a nice quadratic curve fit (R^2 > 0.9995) up to FL360 (above that it’s a constant).

I think my head just exploded.

Mine too, my equation is to go to OTHR page 9, get the true airspeed from the air data computer, then go to CALC page 3 and put it in there to compute the mach number. Much easier.

[CessnaCitationX]

@mduell

The speed of sound is sqrt(gammaRtemperature), where gamma is the ratio of specific heats, R is the gas constant, and temperature is in absolute units. I calculated it every 100 feet in the 1976 US standard atmosphere and came up with a nice quadratic curve fit (R^2 > 0.9995) up to FL360 (above that it’s a constant).

How would I correct for nonstandard temperature? Just find the Mach # for the temperature at the current altitude?

@cfijames

@G4Driver

@mduell

The speed of sound is sqrt(gammaRtemperature), where gamma is the ratio of specific heats, R is the gas constant, and temperature is in absolute units. I calculated it every 100 feet in the 1976 US standard atmosphere and came up with a nice quadratic curve fit (R^2 > 0.9995) up to FL360 (above that it’s a constant).

I think my head just exploded.

Mine too…

There are some embarrassed History or English majors, who… [screams] [running away]

Sorry, I couldn’t resist.

[mduell]

@CessnaCitationX

@mduell

The speed of sound is sqrt(gammaRtemperature), where gamma is the ratio of specific heats, R is the gas constant, and temperature is in absolute units. I calculated it every 100 feet in the 1976 US standard atmosphere and came up with a nice quadratic curve fit (R^2 > 0.9995) up to FL360 (above that it’s a constant).

How would I correct for nonstandard temperature? Just find the Mach # for the temperature at the current altitude?

Two ways come to mind: Either calculate the speed of sound using the previously mentioned formula and divide the airspeed by that figure, or multiply the Mach number at standard day conditions by sqrt(actual_temp/standard_day_temp).