What the Sonic Boom actually is

[QFCPT747]

Hey guys,
I was wondering what the sonic boom actually is and why it occures. What is the Cone shaped cloud exactly? is it a build up of condensation?, but what has that got to do with speed?

Cheers

[Will M]

It is caused by an object moving faster than sound, about 750 miles per hour at sea level. An aircraft traveling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship's bow. When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves which travel forward from the generation or "release" point.

As an aircraft flies at supersonic speeds it is continually generating shock waves, dropping sonic boom along its flight path, similar to someone dropping objects from a moving vehicle. From the perspective of the aircraft, the boom appears to be swept backwards as it travels away from the aircraft. If the plane makes a sharp turn or pulls up, the boom will hit the ground in front of the aircraft.

The sound heard on the ground as a "sonic boom" is the sudden onset and release of pressure after the buildup by the shock wave or "peak overpressure." The change in pressure caused by sonic boom is only a few pounds per square foot, about the same pressure change we experience on an elevator as it descends two or three floors, in a much shorter time period. It is the magnitude of this peak overpressure that describes a sonic boom.

[QFCPT747]

Wow, thanks for that, any idea to what the actual cone shaped cloud is consists of?
Thanks

[Dmmoore]

The cloud is water vapor that condenses due the the pressure drop behind the shock wave.

You will see the same cloud on top of a wing during takeoff on a rainy day.

[JordanD]

I'm assuming by cone shaped cloud you're talking about is this: [photoid=5612885]
That's called a Prandtl-Glauert condensation cloud (or something). The plane doesn't have to be going supersonic for that to happen. They're usually going just under supersonic, since they can't go supersonic at an airshow.
Here's a good article about it:http://en.wikipedia.org/wiki/Prandtl...rt_singularity
It seems like there's still some debate about the causes etc.

[Jerrycobra Boeing]

and just to add, crossing the barrier is barely or non existent felt in the plane, and it cannot be heard in the cabin ether

[bob12312357]

Damn,and I thought all along it was the sound of the planes sheer sexiness parting the air b4 it just by sheer will. lol

[bob12312357]

The speed of sound goes changes at altiude. So at sea level it may be 760,but at 45,000 its closer to 660. This posses a problem in higher altitude sub-sonic planes and was a real big problem with the U2. When a plane designed for sub-sonic flight approaches 2 close to the sound barrier bad buffeting can set in. Now just cause the plane is traveling at say 500+mph ground speed,that doesn't mean that its safe to exceed more then a couple of degrees of bank at 80,000 feet as your Indicated Air Speed doesn't care if your at 8,000 feet or 80,000' if your stall speed is XXX on the indicator that is the speed on the indicator you will stall or there abouts. On the U2 there were a number of incidents where one wing would be suffering from stall buffet, while the other wing would be suffering from the buffet of approaching the speed of sound. Quite an intresting phenomenon.

[QFCPT747]

wow, thats very interesting actually. I never though about the wings travelling at different speeds. I know its an open minded concept, but if the plane travels at one, isn't it all travelling at the same speed?

And wouldnt the wing closer to the ground travel faster seeing as it is lower?

thanks for your input.

[FireLight]

From Wikipedia on the U-2:

High-aspect-ratio wings give the U-2 some glider-like characteristics, with a lift-to-drag ratio estimated in the high 20s. To maintain their operational ceiling of 70,000 feet (21,336 m), the U-2A and U-2C models (no longer in service) must fly very near their maximum speed. (U-2A maximum speed around 528 mph) However, the aircraft's stall speed at that altitude is only ten knots (18 km/h) less than its maximum speed. This narrow window was referred to by the pilots as the "coffin corner". For 90% of the time on a typical mission the U-2 was flying within only five knots above stall, which might cause a decrease in altitude likely to lead to detection, and additionally might overstress the lightly built airframe.[2]

QFCPT747: I don't know if you've ever given your kid sister a "helicopter ride", or of course even if you have a kid sister, but it is a good way to consider the different speeds of the wings. If you hold her hands as you spin her around, they are travelling relatively slowly relative to her feet. It is a similar relationship between the inner and outer wings relative to the turn.

The plane could also be considered to be travelling at one speed if you look at it from the point of view of centre of mass if you choose.

A little OT relative to sonic booms - maybe we should talk about the SR-71 instead.

[QFCPT747]

Haha, great anology. I do understand the conept of the wings travelling at different speeds, because one is travelling more distance in the same amount of time, i just find it rediculously cool and love to question it.