quick LSS calculations?
Thread Starter
Join Date: Sep 2006
Location: London
Age: 41
Posts: 50
Likes: 0
Received 0 Likes
on
0 Posts
quick LSS calculations?
Hi
I was asked today what a quick way of working out the local speed of sound is from 'C to knots...
643+(1.2C)
Any quicker ways in your head?
I was asked today what a quick way of working out the local speed of sound is from 'C to knots...
643+(1.2C)
Any quicker ways in your head?
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Likes: 0
Received 0 Likes
on
0 Posts
....quick way of working out the local speed of sound is from 'C to knots...
643+(1.2C)
643+(1.2C)
643 + 1.2T kts, where T is temperature measured in °C.
This is one of the best linear approximations to speed of sound in dry air. Between -40°C and 17°C it yields the speed of sound to the nearest kt. From 17°C to 37°C your formula is one kt too high, and at 40°C two kts too high. Similarly, at -50°C it is one kt too high, and at -60°C two kts too high.
I note, though, that this formula mixes up the units.
For those who wish to work it out for themselves, the exact formula is SpdSound = Sqrt(gammaDryAir x gasConstantDryAir x Temp°K), where gammaDryAir is the ratio between specific heat at constant pressure and specific heat at constant volume of dry air, about 1.4; the gasConstantDryAir is about 287 in SI units (J per kt per °K), and °K is about °C + 273. (cf. e.g., John D. Anderson Jr., Introduction to Flight, 6th edition, McGraw-Hill 2008, eqn 4.54 in Section 4.9. The value of gammaDryAir is in Section 4.6, and that of gasConstantDryAir is in Section 2.3).
What you get from this formula is speed of sound in meters per second (SI units). You can just plug the numbers in and press "Sqrt" on your calculator. If you want a series approximation, first note that the formula is approximately 20 x Sqrt(273 + x°C), or 331 x Sqrt(1 + x/273). Then you can expand this last term binomially and ignore quadratic and higher terms. Or leave the quadratic term in if you wish for better approximation at temperatures above 15°C and below -40°C. If I do this, I get SpdSnd = 331 + 0.575.x - 0.000556.x^2 + epsilon, where "epsilon" are the cubic and higher terms. The quadratic term makes a difference outside +/-20°C.
If you want to get SpdSnd in kts, you need to calculate in feet per second using gasConstantDryAir or 1716 ft-lbs per slug per °Rankine (where °Rankine is approximately (°F + 460). Then you need to convert ft/sec into kts (you won't go far wrong if you take 1 ft/sec = 0.6 kt). I am not exactly "bilingual" in SI/English units (to use Anderson's quaint way of putting it), so I prefer to calculate in SI and convert m/s to kts directly.
PBL
Last edited by PBL; 22nd Jul 2010 at 11:36.
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by fantom
That deserves a prize.
Originally Posted by Maureen Lipman
Awards are like piles. Sooner or later, every bum gets one
Originally Posted by Denti
By the way PBL, you know that Bielefeld doesn't exist, do you?
PBL
Join Date: Jun 2004
Location: Australia
Posts: 1,843
Likes: 0
Received 0 Likes
on
0 Posts
Excellent response from PBL, one that deserves a "cut and paste" to the data files.
It is a good response for those seeking the background to the derivation of the simpler, yet highly accurate formulae used daily. Gamma, for example at 1.4 does indeed vary slightly, but is used as a constant by people such as Boeing and NASA. I am a product of the Douglas (the Santa Monica Douglas people, remember them?) process, and they too used 1.4 as a constant.
What it all boils down to, if you accept constants with minor variation as a constant, is a fairly simple formula which uses Static Air Temperature (SAT) in Degrees Celsius and Mach Number (M) -
TAS = 38.975 * SQR (SAT + 273.15) *M
This can be easily accomplished with a hand-held calculator (and is also used by most FMC/FMS units). It provides for TAS by applying the Mach Number, but for LSS simply use 1 for Mach Number.
Simple, but very accurate
PBL, Denti, Bielefeld gibt es doch!
Regards,
Old Smokey
It is a good response for those seeking the background to the derivation of the simpler, yet highly accurate formulae used daily. Gamma, for example at 1.4 does indeed vary slightly, but is used as a constant by people such as Boeing and NASA. I am a product of the Douglas (the Santa Monica Douglas people, remember them?) process, and they too used 1.4 as a constant.
What it all boils down to, if you accept constants with minor variation as a constant, is a fairly simple formula which uses Static Air Temperature (SAT) in Degrees Celsius and Mach Number (M) -
TAS = 38.975 * SQR (SAT + 273.15) *M
This can be easily accomplished with a hand-held calculator (and is also used by most FMC/FMS units). It provides for TAS by applying the Mach Number, but for LSS simply use 1 for Mach Number.
Simple, but very accurate
PBL, Denti, Bielefeld gibt es doch!
Regards,
Old Smokey
Last edited by Old Smokey; 24th Jul 2010 at 04:43.
just to clarify 'c' generally is used for any gas and 'a' for air; that formula is a general formula from fluid mechanics..although totally useless for flight 
Old Smokey great to see you so active here again--- you're too damn busy
Old Smokey great to see you so active here again--- you're too damn busy
