Mach-Number to Airspeed Conversion Above 65,000 Feet

[DERG]

hahaha

you win some..sometimes

[Ex Cargo Clown]

I'm not a huge fan of thermodynamics, I hate physical chemistry.

you could quite easily use pV=nRT, the more accurate equation though would be the Van der Waal equation

(p+(n^2a/v^2)) * (v-nb)=nRT

you can easily find the a and b constants for air as you know the mole fractions, I'll help you out here, as I have the text in front of me with a nice table. a=1.3725 b=0.0372 and you can class air as having a molecular weight of 28.85 at those figures.

You can now plug your figures into the equation.

[Jane-DoH]

Ex-Cargo Clown

That formula provides the gamma figure?

[Ex Cargo Clown]

You can get the gamma value from the ratios, but you are still making assumptions. I don't know how many accurate temperature and pressure readings there are at 70,000ft.

I'd suggest you are looking at a gamma value of between 1.3-1.4, as soon as you get that high you can use pV=nRT a little better as the air is so "thin" there are less molecular collisions and so these are negated, but are still present.

You might want to email NASA, I'm sure they have some data on this. You can get up to 80,000 ft on here, temp there is 199K U.S Standard Atmosphere

Don't forget when you do your calculations to use only SI units, ie Pa, K etc

[Jane-DoH]

DERG

your my kinda women Jay....always asking questions...most don't ask..you single?

First of all, before we go any further my name isn't Jane or Jay -- it's Robyn. I did intend to register under that name but when I did, the e-mail message to activate the account never came. When I attempted to create another account, it wouldn't let me claiming that I already had an account (one that I couldn't activate). So, I then created an account called Jane-DoH, which was effectively a parody of Jane Doe pronounced as if Homer Simpson was saying it.

Secondly, yes I am single.


Ex Cargo Clown

You can get the gamma value from the ratios, but you are still making assumptions. I don't know how many accurate temperature and pressure readings there are at 70,000ft.

But that ratio covers specific heat at constant pressure vs specific heat at constant volume right?

Don't forget when you do your calculations to use only SI units, ie Pa, K etc

Thank you.

[DERG]

Robyn

Thanks for your reply. May I ask if you are a formal student of aviation?

[Upper Air]

Why not. . . ? LSS then comp?

Why not just calc Machno to get your TAS, then convert your TAS to EAS using your whizzwheel computor. . ?

The formula for Machnumber, as you know is (grab a calculator - its sssooooo easy)

the sqrt of 273 (+/- T) x 38.94

So stab in 273 add or subtract you outside temperatur (+/-T)
sq rt the result

then times it by 38.94 (you van prob do it by calculas too . . . but I really don`t wanna know, ok?)

then - you will have a number whic is you True Airspeed, ok?

Now, take this True Airspeed and using your metal or plastic flight computor you can calculate the EAS from the Tas.

Happy?

[Jane-DoH]

DERG

Thanks for your reply. May I ask if you are a formal student of aviation?

No, I'm just an aviation enthusiast. Still I've always been fascinated by how things work.


Upper Air

The formula for Machnumber, as you know is (grab a calculator - its sssooooo easy)

the sqrt of 273 (+/- T) x 38.94

So stab in 273 add or subtract you outside temperatur (+/-T)
sq rt the result

then times it by 38.94

So at 273 K, you get 643.3944 kts, correct?

[DERG]

I am a big fan of the USN. Do you like military aviation?

This link is a good read:
http://compass.seacadets.org/pdf/nrtc/fn/14104_ch6.pdf

Regards

[Upper Air]

Machno / EAS

So at 273 K, you get 643.3944 kts, correct?

Yes and that is TAS, just one more step and slide for EAS then.

But, after about 36,000 feet its -56.6*C

Concord flew backwards and forwards regularly at 70,000ft

[mm43]

Upper Air;

This may be getting pedantic, but when you start quoting the Speed of Sound as 643.3944 kts at standard sea level, something didn't look right.

Standard Sea Level conditions according to the ISA are:-

101.325kPa and 15°C / 288.15°K, and the accepted speed of sound is 661.4788kts / 340.2941m/sec. Just refer to the ISA Chart in Post #5.

It would appear that a digit or two got transposed in your "magic" multiplier and the standard temperature of 15°C got left behind in the calc.

√ 288.15 * 20.0468023 = 340.2941m/sec
or
√ 288.15 * 38.9678655 = 661.4788kts

So 38.97 seems closer for a memory number.

[Jane-DoH]

mm43

So 38.96 seems closer for a memory number.

No wonder my result didn't seem right. Off memory, Mach 1 ~ 660 kts.

[DERG]

Question

Robyn...

Do you know how many decimal places the crew use when they enter the "numbers" into the flight computer?

Well I always round mine to 2 decimal places for general use, but I could imagine the third decimal place might make a difference on long flights.
In civil engineering we work to plus or minus 5mm over a 6000m distance, well we did when I was trained, but these days they user lasers and GPS stuff..so maybe even "tighter".

So the question is... how many decimal places are used when a crew inputs the data for mapping the flight?

Regards

p.s. the laser spelling came out like that when I entered the text..I mean LASER. done it again..why does it do that?

[HazelNuts39]

Confusion?

Originally Posted by Jane-DoH

So at 273 K, you get 643.3944 kts, correct?

Correct, but ... the SLStd temperature is 15 °C or 288.15 K

regards,
HN39

[john_tullamarine]

the l@ser spelling came out like that when I entered the text..I mean l@ser. done it again..why does it do that?

The site has some background text checks for various reasons .. l-a-ser becoming l@ser is one of them. Hopefully no-one has a need to know as I would then have to dig deep to find out the specific reason.

[mm43]

Laser pinpoint accuracy in cutting steel is ...

Laser pinpoint accuracy when cutting steel .... , aah, looks like more to do with keyboard idiosyncrasies than anything else. The top line was pasted in from a text editor, and the next line was typed into the text-box. In fact the "L" was entered as "upper-case", but was also changed - to "lower-case".

Further investigation seems to confirm that laser is a reserved word in the program - as JT has already indicated.

Solution: Use a text editor.

[chris weston]

Sorry all, coming to this late.

pV=nRT is a longish wave length Clupea Harengus

Watch V - its m3

Trust me - I'm a chemist

Units of pressure?

Try N/m2 or Pa or kPa or mmHg or inches / feet of water or Bar or mBar or Atmospheres or psi etc ad nauseum

We use kPa mostly where 1 Bar/Atmosphere = 101.3 kPa

Mods will decide if this adds anything!


CW

[Upper Air]

MACHNO and Hazelnuts39 + mm43

Jane-Doh was confirming Mach 1 at 273K - nobody was talking about ISA or AMSL! If you took the time to look at the previous text you would have seen this and Jane, yes you are right, that is the Local Speed of Sound (LSS) at 273K - and mm3 - it is 38.94 because the Captain says so.

Insulting words removed by UA.


bits deleted - we really don't need to be gratuitously insulting ? JT

[HazelNuts39]

Originally Posted by Upper Air

Obviously, Hazelnut 39 if you apply the ISA temp of 15*C then K will be plus 15 higher - duh.

Sorry, you missed the point of my post, which was twofold: First to point out that Jane's sonic speed was for 273 K, not 288.15 K (hence the title: Confusion?), and second that the standard sealevel temperature is 15 °C per definition, not 15.2. Thought that would be obvious.

regards,
HN39

[Jane-DoH]

DERG

Do you know how many decimal places the crew use when they enter the "numbers" into the flight computer?

Well I always round mine to 2 decimal places for general use, but I could imagine the third decimal place might make a difference on long flights.
In civil engineering we work to plus or minus 5mm over a 6000m distance, well we did when I was trained, but these days they user l@sers and GPS stuff..so maybe even "tighter".

So the question is... how many decimal places are used when a crew inputs the data for mapping the flight?

Two to three decimal places when mapping the flight depending on the length of the flight.

p.s. the l@ser spelling came out like that when I entered the text..I mean l@ser. done it again..why does it do that?

Well, I think you just think I'm a chatterbot so you are entering odd characters and text as well as switching the topic midway into civil engineering, using ambiguous measurements (m can be in meters or miles though in this case it's obviously meters), then switching back to the original topic in some bizarre attempt to "screw me up".

I'm not a robot. Sure, I can be odd, and be socially inept, kind of quirky sometimes, and take things a bit too literally but those things are all the product of having a pervasive developmental disorder such as asperger syndrome, high functioning autism or PDD NOS (which means you have a pervasive developmental disorder which does not neatly fit into any of the following). I've been diagnosed, at different times, over the past 14 years with one of the three (PDD NOS was the most common diagnosis that came up).


HazelNuts39

Correct, but ... the SLStd temperature is 15 °C or 288.15 K

So at 273 K, Mach 1 = 643.3944 kts, and at 288.15 K, is 661.4788 kts.


john tullamarine

The site has some background text checks for various reasons .. l-a-ser becoming l@ser is one of them. Hopefully no-one has a need to know as I would then have to dig deep to find out the specific reason.

That's a really weird quirk. I honestly just thought DERG thought I was a chatterbot and did that as some kind of "test". Still, I'm going to leave what I wrote to him anyway.


chris weston

We use kPa mostly where 1 Bar/Atmosphere = 101.3 kPa

Even though 1 atm and 1 bar are supposed to be 1 atmosphere, why does a bar translate out to 100 kPa, and an atm translate out to 101.3 kPa? (I checked the defintion of 1 bar to make sure it was the same as 1 atm)

BTW: As you have "second law" written as a title under your name, I assume you mean entropy. Here's something that's rather fascinating. Being that the total amount of energy in a closed system is zero, and gravity being negative potential energy, this actually means that given enough time every unit of positive energy which includes energy and matter, and every unit of negative energy will actually neutralize out to nothing (if our universe is a closed system, which I wouldn't be surprised if it is).