Mach and Machmeter

Question 1

What is ‘Mach’?

Answer 1

Our aircraft speed (TAS) as a proportion of the local speed of sound (fluctuates with local temp).

eg: 0.82

calculated using a machmeter or an air data computer.

Question 2

How does a Machmeter work?

Answer 2

Function of Dynamic pressure divided by Static pressure.

Basically an ASI with ALT which interact using a ranging arm and a ratio arm.

We can derive: - EDIT

Question 3

The speed of sound is primarily dependant on what?

Answer 3

local air temperature

Question 4

What is the Mach equation?

Answer 4

M = TAS/A

Mach = True Air Speed / Local speed of sound

Question 5

What is the equation for the speed of sound (a) ?

Answer 5

a = sqrroot (K x R x T)

simplified to a= 39x sqrroot (273-SAT) for kts
(or 20.1)b for m/s

K = ratio fo specific heat capacities
R = gas constant
T= SAT in kelvin

Question 6

What is a cross over altitude and why do we use it?

Answer 6

The altitude at which we transition from CAS to Mach number.

as (a) decreases with alt we must switch to maintaining mach to avoid going above mach 1 using TAS.