Mach Number vs Speed

How come I can get the Mach number in the HUD showing around 1.9 but the speed on the left of the hud is showing 600 (ish)

 

the 2 seem to be disconnected

 

assume I'm being stupid in some way :)

It depends on your altitude and temperature. And check your TAS please.

Edited October 12, 2019 by billeinstein

Mach depends on altitude (and temperature).

 

M1.0 at sea is like 661 knots.

 

M1.0 at 40K feet is like 573 knots.

 

https://en.wikipedia.org/wiki/Mach_number

 

Then, also take a look at this: https://en.wikipedia.org/wiki/Indicated_airspeed

How come I can get the Mach number in the HUD showing around 1.9 but the speed on the left of the hud is showing 600 (ish)

 

the 2 seem to be disconnected

 

assume I'm being stupid in some way :)

 

The speed on the left is IAS (Indicated Air Speed) not True Air Speed.

 

In your example the forces on the aircraft are equivalent to 600 knots at SL, even though the aircraft is likely at higher altitude and moving much faster though thin air.

 

This chart shows ~600 CAS = Mach 1.9 @ 42,000 ft = ~1095 KTAS

 

Mach depends on altitude (and temperature).

 

M1.0 at sea is like 661 knots.

 

M1.0 at 40K feet is like 573 knots.

 

https://en.wikipedia.org/wiki/Mach_number

 

Then, also take a look at this: https://en.wikipedia.org/wiki/Indicated_airspeed

 

You are talking TAS not CAS as indicated on the airspeed dial and normally the HUD (indicated with a C next to the speed tape). In calibrated airspeed M1.0 at 40000ft is around 300kts.

thanks all

 

the chart in post 4 is very helpful to bring all the comments together

 

never knew there was such a difference between Mach and CAS

Mach depends on altitude (and temperature).

According to theory, in fact Mach number only depends on temperature. Obviously altitude changes means temperature changes, but altitude per se does nothing.

 

 

S!

How come I can get the Mach number in the HUD showing around 1.9 but the speed on the left of the hud is showing 600 (ish)

 

the 2 seem to be disconnected

 

assume I'm being stupid in some way :)

What aircraft and conditions? Sometimes probes get stuck and readings are false. Turn on pitot heat, for instance.

 

 

 

S!

the higher up you go the more sparse the air molecules are the faster you have to go (true airspeed) to collect the same number of air molecules per second (indicated airspeed) as down lower

 

now even though you're catching fewer air molecules, because your airplane is impacting them so fast the air molecule still can only get out of your way so quickly (mach) so 200kt up high has your aircraft behaving differently than 200kt down low

 

the interaction between ias and mach meets at the phenomenon known as the coffin corner where the quantity of air molecules are so few that your wings can't keep you afloat, yet the few that you encounter are impacting too fast for your aircraft to deal with, too. this is a major concern for "slow flying" sailplane-like designs like the u-2 near the top of their service ceiling

 

so for matters of divining your aircraft's aerodynamic capabilities at a given altitude, ias and mach are both relevant, they just describe two different factors that your airfoil cares about -- the quantity of air, and the quality of the air.

Edited October 15, 2019 by probad