when mach becomes a varible in flight verses just plain airspeed?

Answer 1

Airspeed and mach are just two different measuring sticks for the same thing. Sort of like knots versus miles per hour or klicks per hour. Since the advent of high speed jets, mach has become the favorite measuring stick because things get a little different in the trans-sonic range and beyond.

Late note for MiG 31: The man did not stipulate a constant airspeed and I stand by my answer. Airspeed also changes as a function of density altitude. If you can prove me wrong, please do so. I'm always willing to learn.

Answer 2

Basically indicated airspeed is limited by dynamic pressure and heat, so if the speed of sound was 100,000 mph you'd still be limited by friction heating and dynamic loads on the airframe. The maximum indicated air speed is called Vne (velocity never exceed).

The mach number becomes an issue when you approach M1 and start to see standing wave effects. These cause undesireable control and drag effects which limit the maximum speed of subsonic jets. Aircraft have a Mmo (Mach max operating) that they should not exceed.

Because the speed of sound decreases with altitude there is an altitude at which you switch over from being limited by dynamic pressure to being limited by mach number, this varies with the maximum Mach number and the maximum IAS.

Above the changeover point you fly by mach number and slowly reduce indicated airspeed. For a constant true air speed the indicated air speed decreases more slowly than the mach number with increasing altitude, so as you continue to climb at constant mach number your indicated and true air speed decrease.

Where is the changeover? 25k-30k feet depending on current conditons.

Answer 3

I would have to mildly disagree with the anwer above. Mach number is a ratio, which means it is a dimensionless number and comes to importance in aerodynamics as the fluid behaves in a similar way at the same Mach number. Airspeed (or just speed), has a dimension and cannot be considered similar to Mach Number. However, since the Mach number is a ratio of two speeds, the Mach number can be expressed as airspeed using proper conversion constants.

Mach number is the ratio of the speed of the aircraft with respect to the speed of the sound within the same reference volume. Now speed of sound in air is a function of temperature and this varies with change in altitude (The colder it is, the slower sound travels). So for a constant speed (say 330 meters/second, measured with respect to ground) the Mach number varies across altitudes. As tempereature decreases with altitude, the Mach number for the same speed increases as you gain altitude.
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After reading the reply below, BitsBurgerPilsman, you got airspeed confused with groundspeed. Airspeed (True Airspeed) is used for aerodynamics computations and Groundspeed is used for navigation. Groundspeed is never expressed in terms of Mach numbers (... er, the Thrust SSC is an exception!)
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Colonel Sir, I disagreed (mildly too!) only on the direct interchangeability between a scalar quantity (speed) with a ratio (Mach number). And as the question had its ambiguities, I assumed a constant speed just for the sake of explanation. The statement is modified for more clarity now.

Answer 4

I don't know if this is what you are looking for, but...

If you are travelling at 600 knots and grab a 200 knot tail wind, your true airspeed would be 800 knots. You would not be at mach though, because the medium you are in is moving with you.

Answer 5

I agree the two are not interchangeable. I believe flying by Mach, which includes density, involves airframe limitations.

At altitude, throttles are set by engine pressure ratio, EPR and would also be affected.