lastordu
Posts: 365
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Some Thing About Numbers

Sat Sep 10, 2005 1:57 am

I know that one knot is 1.15mph(that could be wrong). Dose anyone know how to covert mach in to knots or what ever.
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curlyheadboy
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RE: Some Thing About Numbers

Sat Sep 10, 2005 2:10 am

Quoting Lastordu (Thread starter):
Dose anyone know how to covert mach in to knots or what ever.

Mach refers to the speed of sound, the speed of sound depends on air density and so varies with altitude. I'm sure someone can put this down a lot better though  Smile
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Jetlagged
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RE: Some Thing About Numbers

Sat Sep 10, 2005 2:14 am

First you need to calculate the speed of sound, a. This is proportional to ambient temperature (OAT):

a = SQRT( OAT * 287 * 1.4) m/s (where OAT is in deg K)

Convert this speed into whatever units your true airspeed is in (mph, knots, etc.). Note this must be true airspeed, not indicated.

Mach number = TAS/a

BTW 1.15 is close enough for practical purposes to convert mph to knots.
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dl757md
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RE: Some Thing About Numbers

Sat Sep 10, 2005 2:50 am

Try this.

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Pihero
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RE: Some Thing About Numbers

Sat Sep 10, 2005 3:37 am

There is a much simpler way, valid for subsonic aircraft, that I still use to check my instruments, as it is easily done mentally.

To get a true airspeed (in knots) from a mach number, take as basis a temperature of -35 degrees celsius.At that temperature, the airspeed is exactly equal to the Mach number x 600. Then add or substract 1 kt per degree, rerspectively above or below your -35° basis.
As our mach reading are presented with three digits after the decimal, I just multiply that figure by .6.

Example :Mach .845 and OAT -45° c.
.845 x.6 = 507 kts, from which I substract 10 kts, as -45° c is ten degrees colder than my -35° c basis.

Result : TAS = 497 kts.

It's surprisingly accurate.
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