Local speed sound
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Local speed sound
Hi guys,
Ii know it can seem a strange question but...... for instance if:
my LLS (local speed sound) at 33000 feet is 480 knots and temperature is ISA+10..... whats the formula to calculate the LLS at 28000 feet?
Thanks I really appreciate your help
Ii know it can seem a strange question but...... for instance if:
my LLS (local speed sound) at 33000 feet is 480 knots and temperature is ISA+10..... whats the formula to calculate the LLS at 28000 feet?
Thanks I really appreciate your help
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Hi,
I guess google doesn't work out there - but this is from their first hit. If you can't do the maths - the table will give you a look up result for temperature.
wikipedia.org/wiki/Speed_of_sound
I guess google doesn't work out there - but this is from their first hit. If you can't do the maths - the table will give you a look up result for temperature.
wikipedia.org/wiki/Speed_of_sound
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lets see...
at 28000 ft, the temperature at ISA+10 will be -31 degree celcius.
from the calculator that i found here :
Speed of sound in air and the temperature calculator air pressure - table density of air calculation acoustic impedance air density sea level velocity ideal gas 20 degrees or 21 degrees Celsius C - sengpielaudio Sengpiel Berlin
the speed of sound will be = 605.945 knots....
at 28000 ft, the temperature at ISA+10 will be -31 degree celcius.
from the calculator that i found here :
Speed of sound in air and the temperature calculator air pressure - table density of air calculation acoustic impedance air density sea level velocity ideal gas 20 degrees or 21 degrees Celsius C - sengpielaudio Sengpiel Berlin
the speed of sound will be = 605.945 knots....
In metres per second, speed of sound is the square root of 401.8 x OAT in Kelvin.
For Kelvin, add 273.17 to °C
For knots, divide metres per second by 0.514.
And remember that your result is in TAS, so you need to make any normal adjustments to CAS.
So, at 28,000ft, ISA +10
OAT~-30.4°C = 242.7K
c = SQRT (401.8 x 242.7) = 312.3 m/s = 607.6kn.TAS
You can use a whizz wheel, etc, but I'll just use a data table:
607.6kn TAS = 607.6 x SQRT (sigma for FL280 sHp) = 607.6 x 0.63534 = 386kn EAS ~~ 386kn IAS
G
For Kelvin, add 273.17 to °C
For knots, divide metres per second by 0.514.
And remember that your result is in TAS, so you need to make any normal adjustments to CAS.
So, at 28,000ft, ISA +10
OAT~-30.4°C = 242.7K
c = SQRT (401.8 x 242.7) = 312.3 m/s = 607.6kn.TAS
You can use a whizz wheel, etc, but I'll just use a data table:
607.6kn TAS = 607.6 x SQRT (sigma for FL280 sHp) = 607.6 x 0.63534 = 386kn EAS ~~ 386kn IAS
G
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.......or if you don't have a planet brain like the previous posters or you are in the air, here are some ballpark figures (assuming ISA)
SL 660kts (+15C)
FL250 600kts (-35C)
FL360 570kts (-56C)
Add 1kt/degree above ISA: subtract 1kt,degree blw ISA (appx)
M.78 TAS in ISA (-56C) 447kts
0.01M = 6kts TAS (appx)
From these, you can work out/verify most things (approximately) if you need to.
With apologies to the clever!!
mcdhu
SL 660kts (+15C)
FL250 600kts (-35C)
FL360 570kts (-56C)
Add 1kt/degree above ISA: subtract 1kt,degree blw ISA (appx)
M.78 TAS in ISA (-56C) 447kts
0.01M = 6kts TAS (appx)
From these, you can work out/verify most things (approximately) if you need to.
With apologies to the clever!!
mcdhu
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Local Speed of Sound
LSS = 38.94 * (T + A)^0.5
(Above should read the square root of T + A, but there is no way of putting in the root symbol!
Putting (T + A) to the power of '0.5' has the same effect.)
The temperature at 33000' should be -51 degrees. But since it is ISA +10, it is actually -41 degrees. Thus, assuming a lapse rate of 2 degrees per thousand feet, at 28000' the temperature should now be -31 degrees. This is now your value for 'A'.
T= 273 K and A= The actual OAT at your current FL
LSS = 38.94*(273 - 31)^ 0.5
LSS = 606 kts @ 28000'
By my reckoning... Do check, though!
(Above should read the square root of T + A, but there is no way of putting in the root symbol!
Putting (T + A) to the power of '0.5' has the same effect.)
The temperature at 33000' should be -51 degrees. But since it is ISA +10, it is actually -41 degrees. Thus, assuming a lapse rate of 2 degrees per thousand feet, at 28000' the temperature should now be -31 degrees. This is now your value for 'A'.
T= 273 K and A= The actual OAT at your current FL
LSS = 38.94*(273 - 31)^ 0.5
LSS = 606 kts @ 28000'
By my reckoning... Do check, though!
Last edited by Poose; 19th May 2010 at 16:01.
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but there is no way of putting in the root symbol!
you mean like this √ symbol ?
Mac probably has something similar - for PC use C:/windows/system32/charmap.exe which will give you copy and paste access to the full character sets, not just the keyboard simple stuff.
Depending on your windows version it could be somewhere else in the system files area.
you mean like this √ symbol ?
Mac probably has something similar - for PC use C:/windows/system32/charmap.exe which will give you copy and paste access to the full character sets, not just the keyboard simple stuff.
Depending on your windows version it could be somewhere else in the system files area.
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Ha, ha, ha!
Square root symbol! How'd you do that?!
I shall be chasing you with pitchfork and lantern for bringing out such devilish tricks as mathematical symbols!
Poose-the-Luddite
Square root symbol! How'd you do that?!
I shall be chasing you with pitchfork and lantern for bringing out such devilish tricks as mathematical symbols!
Poose-the-Luddite
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Square root symbol! How'd you do that?!
Back in the early days of ASCII only, the typical character set was 2^8 = 256 characters, of which only a subset was available on the keyboard. Hence the system set up the ability to put the other characters in by odd means and devious.
These days, the easiest way (in Windows) is to run the charmap program - see the previous post for the location. This little utility allows you to copy and paste any character into your application, including a bunch of math symbols. I don't know what, if any, is the limit these days but the sets have heaps and heaps of characters.
Back in the early days of ASCII only, the typical character set was 2^8 = 256 characters, of which only a subset was available on the keyboard. Hence the system set up the ability to put the other characters in by odd means and devious.
These days, the easiest way (in Windows) is to run the charmap program - see the previous post for the location. This little utility allows you to copy and paste any character into your application, including a bunch of math symbols. I don't know what, if any, is the limit these days but the sets have heaps and heaps of characters.
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More than you need to know about characters
These days, the easiest way (in Windows) is to run the charmap program - see the previous post for the location. This little utility allows you to copy and paste any character into your application, including a bunch of math symbols. I don't know what, if any, is the limit these days but the sets have heaps and heaps of characters.
By the way, character sets for which codes have recently been defined include Game Tiles, Egyptian Heiroglyphics, Phoenician and Phags-pa. (I don't know what that last one is either.)
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Poose et al.,
Once you know a character code (from looking it up once in a table), there is another simple way of entering it:
* Hold down the [Alt] key and type in the code (4 figures, so add a zero if your table has 3 figures) on the numeric keypad.
Simple and useful if you use only a few codes regularly; I use it for Ç (capital ç), Alt 0199 and ß (German sz, or beta), Alt 0223.
The √ code unfortunately is font/keyboard specific, so can't help you there....
CJ
Once you know a character code (from looking it up once in a table), there is another simple way of entering it:
* Hold down the [Alt] key and type in the code (4 figures, so add a zero if your table has 3 figures) on the numeric keypad.
Simple and useful if you use only a few codes regularly; I use it for Ç (capital ç), Alt 0199 and ß (German sz, or beta), Alt 0223.
The √ code unfortunately is font/keyboard specific, so can't help you there....
CJ
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When I studied for my commercial license 30 years ago, the speed of sound, expressed in meters per second, was 20 times the sqare root of the absolute temperature in Kelvin..