speedracer1407
Posts: 330
Joined: Fri Dec 24, 2004 4:19 pm

Mach 1, Temperature, And Pressure

Mon Nov 13, 2006 4:54 pm

After browsing these forums for a few years, I've learned that the speed of sound is entirely dependent on temperature, not pressure. If this is wrong, please correct me. However, I do recall numerous posts from reputable members who harp on this fact.

I fail to understand this fact in the context of what I presuppose about the TAS necessary for aircraft to reach Mach 1 at sealevel versus high altitude.

If a,.....oh I donno...let's say F15 were to fly at Mach 1 over the north pole at sealevel in calm -40F weather conditions, would his TAS be the same as if he were flying at 40k feet since the temperature is somewhat similar? Is the primary difference between these two flight regimes the power requirements for reaching such speeds? Is this where pressure (and thus air density/drag/friction) comes into play?
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
chksix
Posts: 336
Joined: Mon Sep 26, 2005 6:16 pm

RE: Mach 1, Temperature, And Pressure

Mon Nov 13, 2006 4:56 pm

You can use this calculator to simulate different situations:
http://www.aerospaceweb.org/design/scripts/atmosphere/
The conveyor belt plane will fly
 
Pihero
Posts: 4206
Joined: Mon Jan 31, 2005 5:11 am

RE: Mach 1, Temperature, And Pressure

Tue Nov 14, 2006 3:30 am

Hello again, Speedracer.

Quoting Speedracer1407 (Thread starter):
After browsing these forums for a few years, I've learned that the speed of sound is entirely dependent on temperature, not pressure.

That is correct.
With a static temperature of -40°c, Mach 1.0 equates to a true airspeed - TAS - of 595 kt.
The indicated speeds will be vastly different, though : 315 kt for the high altitude plane at 40,000 ft and some 665 kt for the ground hugging one. The latter value will vary with the aerodynamic sensors'installation.
Contrail designer

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