View Full Version : Can an airliner break sound barrier with an extreme tailwind?
MaverickPrime
9th Aug 2017, 16:12
Ok, before I start, I know I'm missing something/ a lot!
I'm thinking about airliners flying eastward across the atlantic during the winter usually have a high groundspeed due to the strong jet streams.
Say the OAT is -60 and using the equation below...
Local Speed of Sound = 38.94 x square root of temperature converted to Kelvin
... the LSS would be 568kts TAS.
Why would actual the TAS of the a/c not be close to that speed?
Genghis the Engineer
9th Aug 2017, 16:20
Because of the tailwind. The speed of the aeroplane that matters in terms of Mach number is the speed through the parcel of air within which it is flying that that time, not the groundspeed.
And LSS is the usual acronym for Longitudinal Static Stability. The normal abbreviation for the local speed of sound is "c".
Airliners actually at those sort of altitudes are typically flying by reference to the speed of sound. Say a Mach number of 0.8 or so - which would be TAS = 454kts for your example. Groundspeed will be TAS +/- wind, but that won't change the Mach number.
MaverickPrime
9th Aug 2017, 16:37
:ok: I see!
So, with a strong tailwind, what is stopping the a/c achieving a higher Mach Number? Is it due to compressibility or density of the parcel of air?
andytug
9th Aug 2017, 16:50
No, just the speed of the local air relative to the aircraft, which is slower than normal by however much the tailwind is.
Genghis the Engineer
9th Aug 2017, 17:04
Speed of sound is relative to the motion of the medium through which the sound is passing, not like speed of light which is an absolute. This is because c is based upon transmission of energy between adjacent molecules, not a transmission through space of energy.
When you start to worry about the speed of light, in any context than GPS or DME accuracy, let me know ;)
keith williams
9th Aug 2017, 20:19
If you are currently studying for the EASA ATPL exams, then your use of the term LSS to represent the Local Speed of Sound is quite understandable, because this term is commonly used in this course. Using any other symbol which is technically correct for any higher level of study will not be helpful.
I'm thinking about airliners flying eastward across the Atlantic during the winter usually have a high groundspeed due to the strong jet streams.
CAS, TAS and Mach Number are all measured relative to the air through which the aircraft is flying. It is only our ground speed which is affected by any headwind or tailwind.
Let’s suppose that we have an aircraft flying at 500 kts TAS in a 100 kt tailwind (due to a jetsream). Our TAS is 500 kts (measured relative to the air around us) and our mach number is 500 kts / LSS. Our ground speed will be 500 kts TAS + 100 kts Tailwind = 600 kts.
Whether or not we are able to increase speed to exceed the LSS will depend upon the design of the aircraft and it’s engines. But it will not be affected by any headwind or tailwind.
eckhard
10th Aug 2017, 01:10
I have flown a B747 in these two conditions:
A/ Mach 0.85 TAS 500kts Groundspeed 720kts
B/ Mach 0.85 TAS 500kts Groundspeed 360kts
I don't think I caused a sonic boom in A/
Several large transport category aircraft have exceeded speed of sound usually in a screaming emergency. No pun intended.
Some as part of their certification process.
http://www.pprune.org/spectators-balcony-spotters-corner/207766-citation-x-supersonic.html
https://en.m.wikipedia.org/wiki/TWA_Flight_841_(1979)
http://www.airspacemag.com/history-of-flight/i-was-there-when-the-dc-8-went-supersonic-27846699/
sudden twang
10th Aug 2017, 16:12
Or to put it another way, a balloon in a jet stream could be travelling at 150kts with a TAS of zero and you wouldn't feel a breath of wind.
Best GS in an airliner 733kts
I'm sure it'll be beaten by someone on here 787/VC10 perhaps.
Officer Kite
10th Aug 2017, 18:26
And LSS is the usual acronym for Longitudinal Static Stability. The normal abbreviation for the local speed of sound is "c".
Whilst not undermining your experience, I must say that everywhere I have come across the acronym 'LSS' it has been in reference to the local speed of sound. It has never been used in reference to static stability on the longitudinal axis according to the materials and exam questions I have come across.
Genghis the Engineer
11th Aug 2017, 10:03
As I did my BEng, PhD and ETPS before my CPL, I may have just mentally mapped this bit of terminology to the technically correct "c" used everywhere else to my CPL groundschool notes without noticing I suppose. I'll take a glance at my old CATS groundschool notes at some point just out of curiosity.
Speed of sound is relative to the motion of the medium through which the sound is passing, not like speed of light which is an absolute. This is because c is based upon transmission of energy between adjacent molecules, not a transmission through space of energy.
When you start to worry about the speed of light, in any context than GPS or DME accuracy, let me know ;)
Well every aeroplane on the planet is doing over 68,000mph if you choose the appropriate frame of reference - as Professor Eric Idle put it:
Just remember that you're standing on a planet that's evolving
And revolving at 900 miles an hour.
It's orbiting at 19 miles a second, so it's reckoned,
The sun that is the source of all our power.
Now the sun, and you and me, and all the stars that we can see,
Are moving at a million miles a day,
In the outer spiral arm, at 40,000 miles an hour,
Of a galaxy we call the Milky Way.
Mind you, he then went on to say:
Our universe itself keeps on expanding and expanding,
In all of the directions it can whiz;
As fast as it can go, at the speed of light, you know,
Twelve million miles a minute and that's the fastest speed there is.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
So you might think carefully before taking him too seriously...
:}
Officer Kite
11th Aug 2017, 15:12
[I]Just remember that you're standing on a planet that's evolving
And revolving at 900 miles an hour.
It's orbiting at 19 miles a second, so it's reckoned,
PDR
That is certainly an interesting point to ponder upon
As I did my BEng, PhD and ETPS before my CPL, I may have just mentally mapped this bit of terminology to the technically correct "c" used everywhere else to my CPL groundschool notes without noticing I suppose. I'll take a glance at my old CATS groundschool notes at some point just out of curiosity.
c = speed of light, not sound.
E=mc^2, remember? :8
They might not cover that at PhD level - it's saved for the advanced courses...
:rolleyes:
+TSRA
11th Aug 2017, 17:38
I have a couple hours between flights, am quite bored sitting in the terminal here, and I am in that sort of mind to go down this path, so this is more for my own amusement:
c = speed of light, not sound.
E=mc^2, remember?
Well, that's kind of putting the cart before the horse. "c" only denotes speed of light in that equation as that is how it has come to be defined over time:
...where c denotes the velocity of light.
That is taken from the English translation of Einsteins "Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?"
However, if we look at the original 1905 paper:
wobei V die Lichtgeschwindigkeit bedeutet.
it is clear that Einstein stated that "V," and not "c," would denote the speed of light.
The equation was rewritten in 1946 to the current form and has since been backdated in many English translations to fit the narrative.
Therefore, "c," in of itself, does not distinguish the speed of light for all equations in perpetuity, even though that is how it has come to be taken. I can write any equation where I include "c" as a variable, as long as I define it as such. Algebra has no specific set variables.
The same can be said of "F" for force or "v" for velocity. I can use either in any other equation for any other term, as long as I state what each will mean. Indeed, any algebraic equation always starts with something along the lines of "Let "x" equal..." for this very reason.
Alright, I have that out of my system. Have a great day all!
Genghis the Engineer
11th Aug 2017, 17:39
c = speed of light, not sound.
E=mc^2, remember? :8
Both in fact, they are sufficiently different in value that using the same letter doesn't seem to cause too many problems.
Genghis the Engineer
11th Aug 2017, 17:42
Oh +TS let us not start on the use of V without a qualifier!
+TSRA
11th Aug 2017, 19:11
Oh heck no Genghis, I wouldn't think of it!
keith williams
11th Aug 2017, 19:42
One of the curious facts regarding the speed of light is that virtually everyone knows that it is constant. This is curious, in that the speed of light is not constant, but it depends upon the material through which the light is passing.
The speed of light in a vacuum is constant, but when light passes through any transparent material it's speed is slower.
Also while I am being pedantic.
It has never been used in reference to static stability on the longitudinal axis
Longitudinal stability acts around the lateral axis.
Lateral stability acts around the longitudinal axis.
Pedantic mode OFF!
...what if light is on a threadmill that matches it's speed...
Ok, I'll get mi coat :)
+TSRA
11th Aug 2017, 21:05
Well, the special theory of relativity postulates that the speed of light is the same for all observers. So, it doesn't matter if a particle of light is on a treadmill, it will still see other light particles passing at the local speed of light.
But good for light trying to get back in shape. It really has let itself go for far too long.
andytug
11th Aug 2017, 22:55
...what if light is on a threadmill that matches it's speed...
Ok, I'll get mi coat :)
Not possible unless the treadmill has zero mass or infinite energy.
Officer Kite
12th Aug 2017, 08:10
Also while I am being pedantic.
Longitudinal stability acts around the lateral axis.
Lateral stability acts around the longitudinal axis.
Pedantic mode OFF!
nothing pedantic about that at all, in actual fact thank you for picking up on it and correcting! ... exams still ahead of me so need all the learning i can get.
Not possible unless the treadmill has zero mass or infinite energy.
...or like most things athletic these days had been indulging in a comprehensive drug/doping regime (allegedly).
:E