Local speed sound

[Dubaiforever]

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

[rudderrudderrat]

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

[grebllaw123d]

Speed of sound

Hi,

Or try this link:

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

[syedo]

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....

[Genghis the Engineer]

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

[mcdhu]

.......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

[Poose]

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!

[john_tullamarine]

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.

[Pugilistic Animus]

JT
Finally I had the patience to bring this baby out ∫

[john_tullamarine]

oooh .. heavy stuff that be, matey.

[Poose]

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

[john_tullamarine]

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.

[Dairyground]

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.

These days, most modern computer systems use use Unicode (or ISO/IEC 10646, which is essentially the same thing) for character coding. In its fullest form, this uses 32 bits to encode characters. This could permit up to 4294967296 characters in principle, although for technical reasons not all code positions are available for distinct characters. The number of characters for which codes are defined is still growing, but few, if any charaters are yet assigned codes that require more than 16 bits. That is still 65536 possible characters. Whether your PC or MAC can display a particular code is another question, with the answer depending on the fonts available and the characters supported by any particular font.

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.)

[ChristiaanJ]

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

[Lightning Mate]

The local speed of sound is 38.94 x the square root of the temperature in degrees Kelvin/absolute.

Seemples.

[latetonite]

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..