Starship Asterisk*

by **JohnD** » Wed Sep 12, 2007 12:02 pm

Good point, CC!
It made me go and look for the actual pressures involved, and they are well set out here: http://www.af.mil/factsheets/factsheet.asp?id=184 The highest ever recorded is said to be 0.8psi, 114lbs/ft^2, 100 feet from the aircraft. Biphasic, of course, so high and low.
Doesn't seem anything like the change you might see in your belljar, but probably much greater, less than a wingspan away.

I'm not too keen on the analogy therein, "dropping sonic boom along its flight path, similar to someone dropping objects from a moving vehicle."

This research led me to another site, which I don't think has figured on this thread before: http://www.flatrock.org.nz/topics/odds_ ... hrough.htm

It shows one of the best known of this pics, but also one of two shock waves along the fuselage of a 'plane, showing how there can be two booms (Concorde used to have a double boom, probably because it was much longer than most other supersonic aircraft) and one of the shuttle with associated shock wave.

John

by **craterchains** » Wed Sep 12, 2007 10:17 am

I guess most people haven't had the opportunity to play with a clear pressure vessel and watch what happens to different gas mixtures when pressure is applied at various rates of speed and released at various
rates of speed.

Air, as an example, with 60 - 70 % humidity develops water droplets when pressure is quickly released. Thus you have the "why" we see these streamers and bubble clouds around high speed objects.

by **Andy Wade** » Wed Sep 12, 2007 7:37 am

JohnD wrote:And I've seen the vortices that used to be visible above the rear wing of an F1 (or NASCAR?) car, before new regs reduced the downforce. Clearly those cars DON'T "break the sound barrier"! But the air in that vortex may move a lot faster than the car's linear velocity, and even approach sonic?
John

I remember our school physics teacher demonstrating how he could 'break the sound barrier' by cracking a whip and said how the end of the whip actually travelled fast enough to break the speed of sound resulting in the 'crack' sound which was the same as a 'sonic boom'. (yes, I went to a particularly tough school

)
No-one dared asked him where he got the whip from...

by **Case** » Wed Sep 12, 2007 4:52 am

Pop culture and science are often worlds apart, but...

Most likely CG, the new trailer for Iron Man has a cone shaped cloud (at 00:02:23, near the end), just like the APOD, when flying high speed (supersonic?).

by **craterchains** » Wed Sep 05, 2007 3:11 pm

There is another thread all ready started on this topic.
Posters should check, and or do a search for a thread before starting a new one.

http://asterisk.apod.com/vie ... 6fdf1c5947

by **bystander** » Wed Sep 05, 2007 1:55 pm

SmartAZ wrote:BTW, I'm told that saucer shaped clouds sometimes form over mountains by a somewhat similar methods. The cloud looks like it's just sitting there but it actually forms in fast moving air.

I think what you are refering to are lenticular clouds http://antwrp.gsfc.nasa.gov/apod/ap030430.html. Although they are formed in areas of high turbulence, I don't think they have anything to do with the sound barrier.

lucysommer wrote:...is there anyway I can download to my desktop?

Left click the picture in the apod, this will load just the picture into a new window. Right click and save picture as...

by **SmartAZ** » Tue Sep 04, 2007 8:33 am

BMAONE23 wrote:Another interesting question along that same venue

Does the aircraft engine noise get quieter for the pilot given that he is moving faster than the sound waves that would normally reach him in the cockpit?

Well, I'm just guessing here. No, the engine sound travels through the metal of the aircraft, which is not moving relative to the engine.

BTW, I'm told that saucer shaped clouds sometimes form over mountains by a somewhat similar methods. The cloud looks like it's just sitting there but it actually forms in fast moving air.

by **NoelC** » Tue Sep 04, 2007 1:52 am

These might interest you...

http://youtube.com/watch?v=-d9A2oq1N38

http://youtube.com/watch?v=s24F5Qmehj8
http://youtube.com/watch?v=_0hlJm6EbSw
http://youtube.com/watch?v=OSrsqH7x8Q8
http://youtube.com/watch?v=QjpikTn2qAk

-Noel

by **BMAONE23** » Mon Sep 03, 2007 11:31 pm

Another interesting question along that same venue

Does the aircraft engine noise get quieter for the pilot given that he is moving faster than the sound waves that would normally reach him in the cockpit?

by **Andy Wade** » Mon Sep 03, 2007 11:21 pm

lucysommer wrote:Andy and William. Thank you both for your answers and not making me feel foolish for the asking. I'm sure if I'd given it any thought at all, I might have figured it out for myself, but it is always nice to be validated.

Glad to be able to help.
That's exactly why this forum is here. And welcome to you, by the way.

You can ask all sorts of 'foolish' questions and there's usually a real clever dicky hanging around who knows the answer
These answers are often subject to discussion of course, but reading the discussions gives you loads of related information (and weblinks) that help to further your knowledge about the fascinating subjects on here.

I used to have an inferiority complex until it was pointed out to me that I was just inferior.
Now I'm happy because I'm no longer worried about the complex I used to have.

by **lucysommer** » Mon Sep 03, 2007 9:12 pm

Andy and William. Thank you both for your answers and not making me feel foolish for the asking. I'm sure if I'd given it any thought at all, I might have figured it out for myself, but it is always nice to be validated.

by **Andy Wade** » Mon Sep 03, 2007 5:36 pm

lucysommer wrote: Hello. I saw the picture of Aug. 19, 2007. When the pilot breaks the sound barrier, does he hear it like we on the ground do? Does the pilot hear anything? Also, is there anyway I can download to my desktop?

Thank you.

No.
The point is that he's travelling faster than the sonic boom his aircraft makes, so the sound of it cannot catch him up.

by **William Roeder** » Mon Sep 03, 2007 5:25 pm

The shock wave move away from the aircraft.
When the Concord went supersonic, the only thing to tell the passengers was the digital meter on the bulkhead.

by **lucysommer** » Mon Sep 03, 2007 4:11 pm

Hello. I saw the picture of Aug. 19, 2007. When the pilot breaks the sound barrier, does he hear it like we on the ground do? Does the pilot hear anything? Also, is there anyway I can download to my desktop?

Thank you.

by **JohnD** » Wed Aug 22, 2007 7:29 am

Wow, markk, what an excellent video (they are usually so crude), that shows both phenomena = vapour cone and water surface disturbance. I think we are arguing at cross purposes, for which I will take the blame.

Of course a boom is heard after the plane passes; however fast the aircraft, the sound still travels at the same speed (more or less). The further you are from the plane, the longer the delay.
And 'breaking the sound barrrier' is just semantics. In the transition zone, the shock wave forms a cone with an apex on the length of the aircraft. As you (I think) say, it is the presence of the shock wave on control surfaces that makes it a 'barrier'. As the aircraft accelerates, the origin of the cone moves further back along its length. The way this has been expressed above implies that the cone forms as the aircraft pops some membrane in the sky.

Maybe even craterchains and I can agree on this! The cloud, ie the shock cone, does indeed move to the rear of the aircraft, but 'slips behind' implies that it is left floating in the blue, like wisps of tissue paper from some clown's hoop, broken by the aircraft. Passes close over water show an invisible cone dragged along and disturbing the surface.

And I've seen the vortices that used to be visible above the rear wing of an F1 (or NASCAR?) car, before new regs reduced the downforce. Clearly those cars DON'T "break the sound barrier"! But the air in that vortex may move a lot faster than the car's linear velocity, and even approach sonic?

Argument aside, this is a dramatic and even beautiful sight!

John

by **markk** » Wed Aug 22, 2007 2:31 am

ColCampbell50, the story you read could not possibly be true. An observer only hears a sonic boom from an airplane after it passes (a supersonic plane is by definition, "moving faster than its sound"). Keeping this in mind, and factoring in even amazingly fast (dare I say, "supersonic"

)reactions on the photographer's part, a photo taken in the situation you read about would result in an image of the rear of the airplane.

JohnD, look at the cloud in the POD. Notice that it is occurring midway along the fuselage of the aircraft? Now watch the video I linked. The clouds are indeed traveling with the planes but they are positioned behind them.

Bottom line is that these clouds can occur without breaking the sound barrier, and breaking the sound barrier can happen without creating one of these clouds. So, logically, how can it be stated that these clouds are sonic booms?

Cheers!

edit: Here's a video of a Blue Angel high-, but subsonic, speed pass during an airshow where you can see the vapor cone forming as the jet passes.
http://my.break.com/media/view.aspx?ContentID=351527

by **craterchains** » Wed Aug 22, 2007 1:49 am

JohnD wrote:craterchains says that the cloud, "forms as they are about to break the sound barrier, and slips behind as they actually go through the sound barrier."
markk agrees and posts a link to a compilation that shows exactly the opposite. A cone of condensation that travels with the aircraft.
John

Your eyesight is as bad as usual John D, watch the movies more closely, there are more than just the one available to see. Or, you could study the effects a bit more and become aware of other interesting aspects of breaking the sound barrier in close proximity to something else. Care to step closer?

by **JohnD** » Tue Aug 21, 2007 11:36 pm

Strange how people cannot believe what they see.

craterchains says that the cloud, "forms as they are about to break the sound barrier, and slips behind as they actually go through the sound barrier."
markk agrees and posts a link to a compilation that shows exactly the opposite. A cone of condensation that travels with the aircraft.

John

by **ColCampbell50** » Mon Aug 20, 2007 7:47 pm

This type of phenomenon can also be encountered as moisture laden air is condensed coming over the rear wing of an open-wheel racing car, such as an Indy-type car.

I saw this picture a number of years ago. The explanation that accompanied it at that time inferred that the photographer heard the sonic boom and then turned and took the photograph showing the condensed cloud as depicted here.

I am no sort of scientist, just reporting what I read about the circumsatnces of the photo.

by **markk** » Sun Aug 19, 2007 9:45 pm

That's what I thought. So today's picture is not a sonic boom, but instead a shock wave occurring in the transonic range. A true picture of a sonic boom should be seen with the vapor cone occurring behind the aircraft, as in this video compilation of supersonic passes: http://youtube.com/watch?v=GEftDfjzNRo

This actually explains the situation in pre-supersonic days, where aircraft would experience buffeting and vibration as they approached the speed of sound. The vibration would begin as the nose initially "punched" through the shock wave and would continue as the wave passed over the length of the aircraft. Once an aircraft broke through the sound "barrier", and the shock wave moved behind it, the flight would become smooth again.

by **craterchains** » Sun Aug 19, 2007 9:07 pm

It forms as they are about to break the sound barrier, and slips behind as they actually go through the sound barrier.

by **markk** » Sun Aug 19, 2007 8:36 pm

Curious - if this cloud is, in-fact, the visual expression of a sonic-boom, then how is it that these clouds have been photographed at airshows all around the US, situations where breaking the sound barrier is expressly forbidden by FAA and military regulations?

a couple of examples:
http://www.tedandrami.com/bbs/view.php?id=news&no=314
http://www.airliners.net/open.file/1084861/M/

Cheers!

by **sharon** » Sun Aug 19, 2007 7:01 pm

ooh! Put that in its side and voila! "disk shaped atmospherical phenomena"
becomes "flying saucer?" Oh, you naughty boys at Groom Lake!
It IS an awesome sight to behold! America Rules!

by **FieryIce** » Sun Aug 19, 2007 10:44 am

I don't know if the word pressure is very accurate. I understand it as more of a threshold, approaching the threshold, then saturation and release.
I think the same would apply to the speed of light, when the threshold of saturation is reached the release would be a flash of light in comparison to the threshold of sound is a release of sound, the boom.

by **Roquentin** » Sun Aug 19, 2007 9:43 am

The author implies that as the pressure drops, the temperature is lowered to such a level that water forms droplets.
But is it not equally probable that as the pressure increases in another part of the wave, it is this increase that makes the water condense?

Starship Asterisk*

A Sonic Boom (APOD 19 August 2007)

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