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Some Thing About Numbers  
User currently offlineLastordu From United States of America, joined Aug 2005, 367 posts, RR: 0
Posted (10 years 8 months 3 weeks 2 days 20 hours ago) and read 2265 times:

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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4 replies: All unread, jump to last
User currently offlineCURLYHEADBOY From Italy, joined Feb 2005, 962 posts, RR: 2
Reply 1, posted (10 years 8 months 3 weeks 2 days 20 hours ago) and read 2259 times:

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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User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2620 posts, RR: 25
Reply 2, posted (10 years 8 months 3 weeks 2 days 20 hours ago) and read 2253 times:

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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User currently offlineDl757md From United States of America, joined May 2004, 1564 posts, RR: 15
Reply 3, posted (10 years 8 months 3 weeks 2 days 20 hours ago) and read 2240 times:

Try this.


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User currently offlinePihero From France, joined Jan 2005, 5174 posts, RR: 78
Reply 4, posted (10 years 8 months 3 weeks 2 days 19 hours ago) and read 2226 times:
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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.

Contrail designer
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