Mach conversion

[Captain Cessna]

Can anyone tell me how to convert Kts or Km/h into Mach, Assuming ISA? It would really help out with my coursework :)

Thanks

CC

[Genghis the Engineer]

Divide the local True Air Speed by the local speed of sound to get Mach number.

To work out the local speed of sound you can (a) look it up, or (b), in metres per second it is the square root of [1.4 x 287 x (OAT in °C + 273.15)]

To work out TAS, divide CAS (which should be about IAS) by the square root of the relative density.

At ISA sea level conditions, relative density=1, and the local speed of sound is 340 m/s, which is 662 knots.

Alternatively, you can do it all on a flight computer.

G

[Captain Cessna]

Thanks Genghis,

I'll try that and see if i can get a reasonable answer.

(left my flight computer at home - doh!)

CC

[m&v]

quick rule of thumb=multlply mach by 6.1,take away ram rise: m.80x6.1=488-28=466
m77x6.1=469-26=444
Check it out on the Wheel

[Check-in]

Or get really rough and ready and go Mach No x 6 plus half the temp rise, which will nearly always come out at plus 4, so Mach 0.60 = 60 x 6 plus 4 = 364KTAS. Working back the other way subtract 4 knots from the TAS then divide by 6, though why anyone would ever need to work it backwards is beyond me.

[The Nr Fairy]

By a straightforward calculation,

LSS = 38.94 x sqr( temp )

WHERE :

LSS = local speed of sound ( knots )
temp = Ambient temperature in degrees Kelvin
sqr = square root function

OR

put the ambient temperature on the inner bit of your whizz wheel against the mach index mark, then read off LSS ( in knots ) as required. LSS is related ONLY to ambient temperature.

Conversions between knots and kmh are also a piece of piss on the whizz wheel.

[DrSyn]

CC, if you are looking for simple keystrokes using every-day units of aviation, the following will give an accurate answer:

TAS=(643 + (1.25 x DegC)) x Mn
Mn =TAS / (643 + (1.25 x DegC))

Where: TAS=Knots, Mn=Mach No, DegC=Degrees C.

You can run that on a pocket calculator, or feed it into a simple computer program on your PC or Organiser.

If you really want Kms (Russia perhaps?),

Km = Nm x 1.8518518 (Nm = Nautical miles)

Our milkman is called Kelvin. He says it's perishing cold whenever the OAT drops below 6 Deg C.

Hope this helps.