Tech question / Airliners vs sonic boom

[lockeed]

One thing sparked a discussion this past weekend and we still can't quite come to an agreement!

Let's say: a 747 cruises at mach .85 and encouter a strong jetstream over the Atlantic bringing it's GS close to mach 1. What will happen? Yes the airspeed might still be the same, but the speed at which the aircraft is mooving relative to the ground is the one (I think) is relevent for producing a sonic boom.

Is there any specific procedures in place for such an event.

What's your thoughs?

 

[35 AoA]

One thing sparked a discussion this past weekend and we still can't quite come to an agreement!

Let's say: a 747 cruises at mach .85 and encouter a strong jetstream over the Atlantic bringing it's GS close to mach 1. What will happen? Yes the airspeed might still be the same, but the speed at which the aircraft is mooving relative to the ground is the one (I think) is relevent for producing a sonic boom.

Is there any specific procedures in place for such an event.

What's your thoughs?

Sonic booms are entirely due to the relative speed of the airplane through the air.....speed over the ground is irrelevant. Hope that helps!

 

[Ghery]

Like everything else, save take-off / landing speed, your airplane only understands air speed. Speed over the ground is great for navigation and getting somewhere (I've seen 750 mph over the ground in the back of a 777), but speed through the air is what makes a boom (or not).

 

[flyingmoose]

+1 on the above posts. GS is not relevant.

 

[Henning]

One thing sparked a discussion this past weekend and we still can't quite come to an agreement!

Let's say: a 747 cruises at mach .85 and encouter a strong jetstream over the Atlantic bringing it's GS close to mach 1. What will happen? Yes the airspeed might still be the same, but the speed at which the aircraft is mooving relative to the ground is the one (I think) is relevent for producing a sonic boom.

Is there any specific procedures in place for such an event.

What's your thoughs?

Does the ground create the sonic boom or the air? ground speed is irrelevant to Mach since it's the air being compressed.

 

[lockeed]

My point is, the speed of sound is the same, windy or not.

Imagine if there was a 200mph wind 200ft over a runway, and an airliner would make a pass at 600mph in the same direction. Do you guys thing there wouldn't be some sort of shockwave as a result of that, relative wind beeing irrelevant ?

I always though that the sonic boom was caused by the aircraft traveling faster that the sound it produce. If it's the case, due to the tailwind, the aircraft WOULD travel faster than the sound it produce therefore breaking the sound barier. (which is 340m per sec)

We need a rocket scientist here !!!!

 

[Henning]

My point is, the speed of sound is the same, windy or not.

Imagine if there was a 200mph wind 200ft over a runway, and an airliner would make a pass at 600mph in the same direction. Do you guys thing there wouldn't be some sort of shockwave as a result of that, relative wind beeing irrelevant ?

I always though that the sonic boom was caused by the aircraft traveling faster that the sound it produce. If it's the case, due to the tailwind, the aircraft WOULD travel faster than the sound it produce therefore breaking the sound barier.

We need a rocket scientist here !!!!

No, what part of the ground being completely irrelevant are you not catching on to? The speed of sound is connect to the atmosphere in this case. You will never break the speed of sound through the earth with an airplane as they don't fly through the earth. The pressure wave is purely a function of the planes speed through the air. The wind could be blowing 500 miles per hour and still ground speed would be irrelevant, the only thing that matters is the planes speed through the air. No rocket science here, this is 6 grade science.

 

[Witmo]

But what if you were on a treadmill....

 

[Jim_R]

My point is, the speed of sound is the same, windy or not.

Imagine if there was a 200mph wind 200ft over a runway, and an airliner would make a pass at 600mph in the same direction. Do you guys thing there wouldn't be some sort of shockwave as a result of that, relative wind beeing irrelevant ?

I always though that the sonic boom was caused by the aircraft traveling faster that the sound it produce. If it's the case, due to the tailwind, the aircraft WOULD travel faster than the sound it produce therefore breaking the sound barier.

We need a rocket scientist here !!!!

You've already got the right answers from others. But I've got an aerospace degree, if that makes you feel better.

Would a rocket flying at 1000 miles an hour past an asteroid in a vacuum create a sonic boom? No.

As others have already said, a sonic boom is an effect from a shockwave. Whether or not a shockwave is created has to do with how fast the plane is moving through the air, and has nothing to do with groundspeed.

Now, if there's a treadmill involved, all bets are off.

Seriously, you are forgetting what sound actually is. It's the ripple effect of air molecules hitting other air molecules. If there's a 200mph wind, then sound is moving across the ground at the speed of sound + 200mph in the direction of the wind, and at the speed of sound - 200mph in the opposite direction.

The plane would still have to fly at the speed of sound + 200mph relative to ground (if going with the wind) to create a sonic boom.

But that's just to say that it would have to be flying at the speed of sound with respect to the air around it.

 

[lockeed]

I understand the point made by the others. You guys say that a sonic boom/sound barrier is created by the speed of the aircraft going thru the air.


My question then: What creates a sonic boom exactly? Is it the air being compressed,
or the fact that the aircraft is pocking thru a virtual wall of "sound" ?


In my previous exemple, if an airliner go thru "space" at 1300mph, whether it's helped by a tailwind or not, it's still traveling at "supersonic" speeds... I guess that's what I don't understand properly...

 

[lockeed]

Jim I just saw your "edited" section. You say that the sound is affected by the wind or by the enviroment in which it evolves? Right?

 

[wanttaja]

My point is, the speed of sound is the same, windy or not.

Imagine if there was a 200mph wind 200ft over a runway, and an airliner would make a pass at 600mph in the same direction. Do you guys thing there wouldn't be some sort of shockwave as a result of that, relative wind beeing irrelevant ?

I always though that the sonic boom was caused by the aircraft traveling faster that the sound it produce. If it's the case, due to the tailwind, the aircraft WOULD travel faster than the sound it produce therefore breaking the sound barier. (which is 340m per sec)

We need a rocket scientist here !!!!

I'm a rocket engineer...is that close enough?

Ground speed...and the ground itself...is irrelevant. We have a prime, recent example of breaking the sound barrier with a near-zero ground speed: Felix Baumgartner.

Ron Wanttaja

 

[TMetzinger]

And there's no such thing as "The speed of sound".

There's:
The speed of sound through the standard atmosphere
The speed of sound through water
The speed of sound through steel
The speed of sound through aluminum
The speed of sound through concrete

And many more, and they're all different and will also vary with temperature and other changeable properties.

Think of it this way... Why did I hear the engines when flying supersonic on the Concorde? Why could I hear the captain's announcements?

 

[35 AoA]

My point is, the speed of sound is the same, windy or not.

Imagine if there was a 200mph wind 200ft over a runway, and an airliner would make a pass at 600mph in the same direction. Do you guys thing there wouldn't be some sort of shockwave as a result of that, relative wind beeing irrelevant ?


We need a rocket scientist here !!!!

Again, his speed over the ground is irrelevant. He would be going 800 mph over the ground, but still only 600 mph through the air. If you switched the wind direction, he would still only be going through the air at 600 mph, though his speed over the ground would only be 400 mph.

 

[Henning]

Jim I just saw your "edited" section. You say that the sound is affected by the wind or by the enviroment in which it evolves? Right?

The density of the environment is everything to the speed and all other qualities of sound propagated. "In space, no one can hear you scream"(that was the best movie commercial ever.) Air, steel, aluminum, copper, water, gasoline, crude oil, argon, helium.... Everything will have a different speed and sound quality, also don't forget the other end, it's not sound until you brain is finished processing these pressure waves.

The actual sound is generated as was told to me by my, the Cheif Aerodynamic Engineer and rocket scientist who was on the team that developed the nuclear powered rocket, as the pressure wave passes of the back of the aircraft and it snaps back into place.

Jack Theilander was a hell of an instructor.

 

[Skip Miller]

In my previous exemple, if an airliner go thru "space" at 1300mph, whether it's helped by a tailwind or not, it's still traveling at "supersonic" speeds... I guess that's what I don't understand properly...

All speeds must be measured relative to a reference. Two cars going down the highway in parallel, at 80 mph over the ground. Zero relative speed.

Same cars going in opposite directions. 80 miles per hour over the ground. 160 mph relative speed.

Your airliner traveling through space at 1300 mph. Relative to that rock you are standing on. Oops, the airliner's ASI reads 0. No molecules of air at all, no airspeed.

Look, there is an asteroid out there somewhere speeding along. Now put yourself on that asteroid. The earth is speeding along in the opposite direction. Is it making any sonic boom? Why?

Answer: no. Sonic booms are caused by the relative speed between the object and the air surrounding it.

-Skip

 

[Henning]

All speeds must be measured relative to a reference. Two cars going down the highway in parallel, at 80 mph over the ground. Zero relative speed.

Same cars going in opposite directions. 80 miles per hour over the ground. 160 mph relative speed.

Your airliner traveling through space at 1300 mph. Relative to that rock you are standing on. Oops, the airliner's ASI reads 0. No molecules of air at all, no airspeed.

Look, there is an asteroid out there somewhere speeding along. Now put yourself on that asteroid. The earth is speeding along in the opposite direction. Is it making any sonic boom? Why?

Answer: no. Sonic booms are caused by the relative speed between the object and the air surrounding it.

-Skip

What happens when stars collide?

 

[Skip Miller]

What happens when stars collide?

Big problem at Hollywood and Vine!

More seriously, the ejecta from the collision can be considered the equivalent of sound. But the propagation of this sound is governed by gravity, not by relative pressure as is true of sound as we mostly know it.

-Skip

 

[Henning]

Big problem at Hollywood and Vine!

-Skip

Lol, no stars left, only little boy crack hoes....

 

[poadeleted20]

I understand the point made by the others. You guys say that a sonic boom/sound barrier is created by the speed of the aircraft going thru the air.


My question then: What creates a sonic boom exactly? Is it the air being compressed,
or the fact that the aircraft is pocking thru a virtual wall of "sound" ?

Air being compressed, that is, the air plane is moving faster than the air can get out of its way, so the air gets squashed into a sort of wall. The boom is the wall sweeping over the observer. You hear nothing in the plane because the wall remains pushed in front of you.

 

[MAKG1]

My point is, the speed of sound is the same, windy or not.

Imagine if there was a 200mph wind 200ft over a runway, and an airliner would make a pass at 600mph in the same direction. Do you guys thing there wouldn't be some sort of shockwave as a result of that, relative wind beeing irrelevant ?

I always though that the sonic boom was caused by the aircraft traveling faster that the sound it produce. If it's the case, due to the tailwind, the aircraft WOULD travel faster than the sound it produce therefore breaking the sound barier. (which is 340m per sec)

We need a rocket scientist here !!!!

Will an astrophysicist suffice?

The speed of sound DOES change with relative motion. The disturbance is created at the source and moves with the medium.

It's possible to start with the assumption that transmission velocity doesn't change with motion of the medium, and figure out the consequences. The biggest one is that you can't exceed the transmission velocity without infinite energy! That's the derivation of Special Relativity -- the speed of light (in vacuum) really doesn't change with motion of the medium. But sound does, and we do have supersonic aircraft.

 

[MAKG1]

Big problem at Hollywood and Vine!

More seriously, the ejecta from the collision can be considered the equivalent of sound. But the propagation of this sound is governed by gravity, not by relative pressure as is true of sound as we mostly know it.

-Skip

Not usually.

To get a "shock wave," you need to satisfy the fluid approximation. Mainly, that the mean free paths are short compared to the viscosity. This works great with air, liquids, even Newtonian solids. It even works with (with some extra variables) with rarified plasmas.

But it doesn't work with isolated particles on ballistic trajectories.

One can make statistical models that sort of look like gas dynamics for such particles, but they have some odd properties, like a pressure tensor (it's not a number, but rather 6 of them -- a symmetric 3x3 matrix), and the equation of state is hard to pin down.

Gravitational shock waves do exist, and are observed, for instance, by the ingestion of galaxies by a cluster (usually containing some hot gas in the center), or by the passage of a globular cluster through the plane of the galaxy. In fact, where there is gas, there are often shock waves, as the relative motions are almost always hugely supersonic.

 

[Henning]

Big problem at Hollywood and Vine!

More seriously, the ejecta from the collision can be considered the equivalent of sound. But the propagation of this sound is governed by gravity, not by relative pressure as is true of sound as we mostly know it.

-Skip

but it wouldn't create a sound until the wave hit an atmosphere containing a human. interesting how it works through space though, although it's much closer to the speed of light. You can almost say the speed of sound through a vacuum approaches the speed of light.

 

[Threefingeredjack]

Big problem at Hollywood and Vine!

More seriously, the ejecta from the collision can be considered the equivalent of sound. But the propagation of this sound is governed by gravity, not by relative pressure as is true of sound as we mostly know it.

-Skip

Actually, no collision in the classical sense would occur. As they neared each other there would be an extended period of tidal stripping by the most massive body, (assuming they were not of equal mass), as the smaller one passed the Roche limit. Since stars are gas and not solid matter there would be no ejecta, just a mass transfer of gas, emitted X-rays and a neat swirly gas flow from the smaller into the more massive.

Re: the sonic boom question. Groundspeed is irrelevant. As has been noted several times above.

 

[Lance F]

All speeds must be measured relative to a reference.
-Skip

Except light.

 

[MAKG1]

Except light.

Nope. It still needs a reference. That you happen to get the same value regardless of what the reference is, is another issue.

If the medium is other than vacuum, you most definitely need a reference, and the speed is not constant. In fact, you can make "light booms" in a medium, generally called "Cerenkov radiation." You make a charged object penetrate a medium faster than the speed of light in that medium. Usually, this is done in water, though any medium transparent in the blue will do.

 

[JHW]

I'd guess everyone reading this has themseoves produced air moving at the speed of sound. Removing the valve stem core from a tire almost always produces choked flow.

 

[MAKG1]

If you've ever snapped a towel or cracked a whip, you've made a shock wave. Not terribly difficult.

 

[BillTIZ]

One thing sparked a discussion this past weekend and we still can't quite come to an agreement!

Let's say: a 747 cruises at mach .85 and encouter a strong jetstream over the Atlantic bringing it's GS close to mach 1. What will happen? Yes the airspeed might still be the same, but the speed at which the aircraft is mooving relative to the ground is the one (I think) is relevent for producing a sonic boom.

Is there any specific procedures in place for such an event.

What's your thoughs?

Mach is a value of percentage of speed relative to the sound barrier or Mach 1.0 within an air mass. Just because the airmass within which the airliner is flying may be moving over the ground at an apparent high rate of speed, perhaps greater than 0.15M. The airliner is still only doing .85M relative to the airmass, no sonic transition or "boom".

 

[Ianovic]

So the 777 at 600mph might have made a brief shock wave when it encountered abrupt wind shear to the tune of 200mph directly on the nose, depending on temp and pressure?

 

[flyingcheesehead]

And there's no such thing as "The speed of sound".

There's:
The speed of sound through the standard atmosphere
The speed of sound through water
The speed of sound through steel
The speed of sound through aluminum
The speed of sound through concrete

And many more, and they're all different and will also vary with temperature and other changeable properties.

Think of it this way... Why did I hear the engines when flying supersonic on the Concorde? Why could I hear the captain's announcements?

Why did Tim choose those last two questions when they both relate to the speed of sound through the air, after his previous examples of different materials?

 

[Clark1961]

I'd guess everyone reading this has themseoves produced air moving at the speed of sound. Removing the valve stem core from a tire almost always produces choked flow.

Perhaps more technically, critical flow. I the upstream pressure is more than twice the downstream pressure then critical flow is likely. Choked flow is simply restricted flow with no description of a particular pressure drop threshold.

 

[Henning]

So the 777 at 600mph might have made a brief shock wave when it encountered abrupt wind shear to the tune of 200mph directly on the nose, depending on temp and pressure?

Hmmm, you'd suspect there be enough inertia to make a ripple at least.

 

[MAKG1]

So the 777 at 600mph might have made a brief shock wave when it encountered abrupt wind shear to the tune of 200mph directly on the nose, depending on temp and pressure?

Yes, and it would also exceed Vne.

It's a good thing such massive abrupt windshears are unheard of outside of a tornado.

 

[Dan Thomas]

The density of the environment is everything to the speed and all other qualities of sound propagated.

The speed of sound in air is mostly dependent on temperature, not much on density.

Here's way the speed of sound changes with altitude:

And the way temperature changes with altitude:

Dan

 

[MAKG1]

The speed of sound in air is mostly dependent on temperature, not much on density.

Mostly? It is only dependent on temperature. Specifically, sqrt(gamma*R * T), where gamma=1.4, R=287 J/K kg, and T is the absolute temperature in Kelvin. The answer is in m/s; multiply by 1.94 to get it in knots.

This surprises a lot of pilots. I even get strange looks from our flight planners when I tell them our true airspeed at cruise is a function only of temperature (we fly mach numbers at cruise -- so it's just the sound speed).

Gamma itself is a (very) weak function of temperature. But still not density.

The difference between the two diagrams is that the upper one doesn't assume a standard atmosphere. The ISA is fiction. It's a widely used approximation, and not even a very good one.