Altitude Effects On TAS & LSS

[2XL]

What are the effects on TAS and local speed of sound with an increase in altitude ?

Do you have a simple table that summarises these effects ?

[Manflex55]

The term "local speed of sound" is used because the speed of sound's propagation through the air is a function of the velocity of the air molecules themselves. Since the Temp of the air reflects this average molecular velocity, the LSS is a function of air Temp (not altitude). The formula to find the LSS (in kt) is 39xsqrt(T), where T is the Temp expressed in Kelvin (Celsius + 273). Hence @ the standard SL Temp of 15°C the LSS is approx 662kt.

MF

[2XL]

So, in an ISA enviroment, where temp decreases 2deg/1000ft up until the tropopause we could expect the LSS to reduce as altitiude increases then ?

Does anyone have a simple table to summarise the expected changes ?

[OneDotLow]

You are correct in saying that LSS decreases as you climb. That is why at sea level, when (using the above figures) you travel at 650KIAS (@ sea level = 650 TAS), you are just below the speed of sound, where as when higher up, you might have a TAS of 500 and a M0.92 (92% of the LSS). Obviously multiplying 500 by 100/92 equals approx 540 TAS.

With regards to the effects of TAS...
True Air Speed is exactly that. Your airspeed through the air, so it does not vary with altitude or temperature.

[olivasnooze]

As already pointed out, LSS is temperature related only.
Used to see this practicaly demonstrated on the B767. Cruising at a high CI and encountering a steep temp gradient it was easy to exceed MMO. ie temp reduces 20deg almost instantly, LSS of sound reduces, next thing the Mno is through the roof.

[john_tullamarine]

or more usefully ..

A/A(SL) = sqrt(T/T(SL))

temps in degree absolute.

[SOHCAHTOA]

Not sure what kind of table your interested in but it would be fairly straight forward to set up a spreadsheet to display the numbers for each temperature. The only formula you need is :

38.94 X sqrt of temp (Absolute)

If its a case of what is the relationship between speed/altitude

Climb at a constant RAS and TAS/Mach increase

Climb at a constant TAS and RAS decreases and Mach increases

Climb at a constant Mach and RAS/TAS decrease


Been ages since I read about that stuff and now I've got a headache

[ 23 August 2001: Message edited by: SOHCAHTOA ]

[dolly737]

@2XL:

Try: http://www.grc.nasa.gov/WWW/K-12/airplane/atmosi.html

[2XL]

To all who have responded, I am impressed and appreciate your posts.

Any more insights most welcome.

[Rote 8]

I am familiar with the equation:

LSS = 38.94 x sq rt(temp K)

The implication is that at absolute zero the speed of sound is also zero, is this the case?

[Manflex55]

Yes it is. To make it simple : when a substance is cooled to 0 Kelvin (absolute zero), its molecules stop moving completely. With no molecule movement, the speed of sound obviously drops to zero. For example, if we assume that the temperature in space is 0 Kelvin (it is actually slightly higher), U could "stand" right in front of someone (provided U both could breathe !!) & talk to him (even shout)... yet he wouldn't B able to hear U !

MF

[Checkboard]

That is for an ideal gas - one without any attractive forces between the molecules. In pratice the gas suddenly becomes a liquid, then a solid well above zero K!