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  #1  
Old 26th October 2016, 08:56 PM
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Default Sound Barrier Question



I recently came across some pictures of aircraft breaking the sound barrier.
What puzzles me is why is there a visual component, what are we seeing?
Excuse my ignorance, I'm not very clever with this stuff.
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Old 26th October 2016, 09:05 PM
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Default Re: Sound Barrier Question

I'm glad you asked this question, see here
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Old 26th October 2016, 09:10 PM
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Default Re: Sound Barrier Question

A nice clear description:

https://www.scientificamerican.com/a...-when-an-airc/

Quote:
Because aircraft wings generate both low-pressure regions (because of lift) and amplified low-pressure disturbances, large low-pressure regions exist near the aircraft, especially under sonic flight conditions. The lowered pressure condenses the water in the air, creating a vapor cloud. As the jet produces these pressure waves and propagates ahead of them, the regions of lower pressure are usually strongest behind the nose of the jet, on the wings and body. As the aircraft continues to speed up, the vapor cloud will appear farther toward the rear of the aircraft. Then, just as the aircraft bursts through the sound barrier, the air is locally disturbed by the resulting shock wave and the condensation/vapor cloud disappears.
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Old 26th October 2016, 09:12 PM
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Default Re: Sound Barrier Question

Wow, thanks, sadly most of the links on that page seem to be dead.
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Old 26th October 2016, 09:22 PM
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Default Re: Sound Barrier Question

Thank you, this is very interesting to me.
So, if I understand correctly, the sound barrier is literally a barrier, a thing that must be overcome? It is not just a number that happens to be greater than the speed of sound?
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Old 26th October 2016, 09:32 PM
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Default Re: Sound Barrier Question

Not a photo of a plane breaking the sound barrier.
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Old 26th October 2016, 09:33 PM
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Default Re: Sound Barrier Question

It's not really a thing (as it depends on temperature, air pressure, humidity) but rather it's mostly to do with the compression of air, and various aerodynamic factors like friction generating heat thereby lowering the efficiency of the jet engines. The reason it was called a barrier is because people in the early 20th century found they couldn't get faster than the 700 odd mph, and seriously considered whether it was the maximum possible speed (ignoring, of course, that you can find such speeds in nature, and in other man-made objects).

Really, it just came down to engineering and understanding aerodynamics - interestingly, planes are a lot more stable and efficient beyond supersonic speeds than subsonic!
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Old 26th October 2016, 09:35 PM
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Default Re: Sound Barrier Question

Well the speed of sound varies slightly due to pressure/temperature and off course altitude (pressure) it's not a barrier in the sense of the speed of light.

The barrier thingy was more to do with aircraft design and thrust generated we have had aircraft that fly at multiples of the speed of sound.

What it does mean though is that after the speed of sound has been exceeded you don't get the hear the aircraft approach, well not until well after the aircraft has past. A long time ago ain another universe I was in the army and on some of the field training we were sound bombed by F1-11's.

2 take home points.
You wouldn't know what the fuck hit you if they were really bombing your position cause the bombs would go off before you even heard the aircraft.

By time you heard the aircraft it was visually long gone, by several seconds.

Well fuck it 3 things It would have been an awesome experience piloting those F1-11's supersonic at a couple of hundred feet.
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Old 26th October 2016, 09:37 PM
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Default Re: Sound Barrier Question

Quote:
Goldenmane said View Post
Not a photo of a plane breaking the sound barrier.

Can you explain further? I was under the impression that, under certain conditions like high humidity, the lowered air pressure created by the wings condenses water creating a vapour cloud which literally moves down the length of the plane, then disperses immediately after break into supersonic speed.
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Old 26th October 2016, 09:54 PM
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Default Re: Sound Barrier Question

Sort of yes (this post has crossed with others).

The term "sound barrier" comes from concerns that it might be impossible safely to fly an aircraft faster than the speed of sound combined with knowledge that aerodynamics change at the speed of sound, as ST has noted. The keyword is "safely"; the 'sound barrier' was well known not to be impassable in principle: the X-1 fuselage design was based around the shape of a 0.50" bullet which was known to go supersonic relatively easily. Anti-tank ammunition would achieve multiples of the speed of sound.

During WWII a number of aircraft were lost, killing pilots, due to controls becoming unusable in dives as local airflows around control surfaces prevented their use. Propellor aircraft of course don't get anywhere near the speed of sound but advanced piston-propellor aircraft could get fast enough that some local airflow around some parts of the aircraft would come close to the speed of sound. In short, steep dive, controls become unusable, fly into the ground.

Another factor was that in the absence of computer simulations or even supersonic wind-tunnels, there was a large amount of trial and error. Some of this was fatal. Some of it was strangely ad-hoc. See this picture of the F-102. Note the odd bulges beside the jet exhaust. These were tacked on as aerodynamic devices as the design without them couldn't break Mach 1, no matter how pointy or powerful.

Generally the 1940s and 50s were a period when aeronautical design pushed at and past the limits of knowledge, with information bought dearly: for example inertia coupling.
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Last edited by wearestardust; 26th October 2016 at 09:55 PM.
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