Which speed should be used in calculating the mach number?

[Natstrackalpha]

Mach Number / LSS

Eagleflier

hin027, Hi,

As Eagleflier says Mach Number is the product of LSS
LSS is the Local Speed of Sound, and, the LSS is based on Temperature (your Local, that is Outside Air Temperature (OAT))

Its a really easy calculation to do - just do it loads of times over and over again for different temperatures.

Base all your calcs on the magic number Kelvin which at 0 degrees C is 273 Degrees Kelvin.

So, if LSS is based on temperature then Mach Number is based on temperature - remember this and you will never go far wrong.

So say the OAT is -20

the calculation is the Sq Rt of 273+/-T x 38.94

So, using your calculator (just for now) enter 273-20 and hit the Sq Rt button times that by 38.94 = a TAS of 619 knots (kts) therefore the Speed of Sound in your Locality is 619kts TAS, (or LSS), and this is Mach1.

if you multiply this 619 by .78 then you get a TAS of 483 kts.

So, at your OAT Mach .78 is a TAS of 483 kts (obviously this is important for flying jets because with this information added to the windspeed you get a ground speed giving you time and hey, you can now work out how much fuel you need).

The calculation is based on ambient air temperature or OAT so the aispeed is True Airspeed TAS

Now to mess it up. Calibrated airspeed and Equivelant Airspeed have nothing to do with True Airspeed and therefore have nothing to do with mach number, UNLESS you are having to calc through CAS to get TAS,
in which case, go back to your books, find the definition of CAS and learn about CAS only. Then do the same for EAS only.

Then when they try to faze you by asking stupid questions, you will be able to calculate it all back to TAS, ok? But don`t go directly from CAS or EAS to Machno or you will do yourself a head injury.

When you have done a few LSS Machno calculations for different air temperatures - then just look at the little book that came with your whizz wheel or computor and learn all about how to determine Mach number by putting in the temp, looking at where the little 10 number is (Mach1) which should be bang on your TAS.

A word on CAS and EAS one of these is due to compressibility error, i.e., the error in airspeed cause by the air ramming into the pitot tube and getting squashed in the process - the difference in CAS to TAS is not that much, and you can use your whizz wheel to do this = just as you can for EAS.
The other one is due to Temperature Rise, i.e., the rise in temperature of the air as it is going so fast into the pitot tube. What you must do is go to your little book that came with the computor and find out which is which.

Aeronautical engineers (and pilots) came up with a brilliant idea and invented a nice pitot tube/temp sensor that could do all these calculations for you this was called a Rosemont Probe - unfortunately, because it was logical and worked very well, they have stopped using them.

Machno TAS Geddit?!

Now, you should really know the difference between IAS and TAS - because if you do not, then you should not be at a pilot interview or even out on the street!
Also - you should know very well how to get your TAS from your IAS, if not and due to the possibility you are using American terminology instead of Brit terminology then you need some help further on this. For example the Yanks (said with much love and affection for the Yanks) use Course, whereas the Brits don`t (don`t worry about Europe, they just follow us Brits) the Yanks also use CAS moreover than TAS - which is not a bad thing but is a different way of hitting the problem - in short, if your schooling and studying is done Stateside - then I cannot help you, but an American pilot can and will, watch this space.