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gardeg
11-24-2014, 05:44 AM
I recently attended an air show.
At the show a jet flew at about 40 feet above the ground very near the attendees.
As if passed it was completely silent then about a second or so later the noise of the engine followed.
My question: Is the jet flying so fast that the sound is delayed OR is the combustion occurring that far from the engine exhaust.
IMHO I do not believe it is delayed sound as the speed would have to be approaching the sound barrier. And it did not appear to be going that fast. Which of course would be very dangerous to spectators.
So is it possible the combustion is occurring outside of the primary source?

bob++
11-24-2014, 06:17 AM
40 feet? I seriously doubt that they would allow a jet to fly that low near spectators. As far as I am aware combustion happens inside the engine and I suspect that the plane may have been a lot further away than you thought. If it was faster than sound, you would have heard the boom.

engineer_comp_geek
11-24-2014, 06:53 AM
If the numbers I googled are correct (and there was no cite given where I found them) then small and slow aircraft are required to be a minimum of 500 feet from spectators, larger and faster aircraft 1000 feet, and things like jets performing acrobatics have to be 1500 feet away. The speed of sound is a bit over 1000 feet per second, so a one second or so delay puts you in roughly the right ballpark for about a 1500 foot distance between you and the jet.

Those jets are bigger than some folks think, and when you are viewing them over an open field without a lot of good reference points it is fairly difficult for your brain to accurately judge size and speed.

It was probably a bit higher off the ground than you think, too.

ETA: Combustion occurs inside the engine, with a slight exception for afterburners, which has additional combustion occur just after the turbine.

Telemark
11-24-2014, 06:58 AM
So is it possible the combustion is occurring outside of the primary source?
No, that's not possible. People are just generally pretty bad at judging distances of fast moving objects.

Machine Elf
11-24-2014, 07:20 AM
ETA: Combustion occurs inside the engine, with a slight exception for afterburners, which has additional combustion occur just after the turbine.

Actually, afterburner use can result in some (useless) combustion taking place outside the engine; this is how shock diamonds (http://en.wikipedia.org/wiki/Shock_diamonds) are formed. However, the shock diamonds rarely persist for more than one aircraft-length behind the exhaust nozzle, so it's safe to say that all of the noise is produced in the general vicinity of the aircraft's exhaust nozzle.

OP, was the jet you saw part of a Thunderbirds or Blue Angels flight team? If so, a standard component of their programs is the sneak pass (http://en.wikipedia.org/wiki/Blue_Angels#Show_routine), during which most of the team distracts spectactors with some aerobatic maneuver over the main airfield while one or two other aircraft come up behind the crowd at just a hair under Mach 1. One visible clue is that under high humidity conditions, you may see a vapor cone (http://en.wikipedia.org/wiki/Vapor_cone) form around the aircraft; localized condensation may form over the wings during subsonic high-G maneuvers, but if you see that cone forming around the entire body of the aircraft, then it's moving at something close to the speed of sound.

The altitude for a sneak pass can be less than 40 feet if they're performing a show over an unoccupied stretch of water (like this (http://www.youtube.com/watch?v=K7rAUu8djZ4)), but in that case the lateral distance to spectators is quite a bit more than 40 feet. For shows over land, I suspect (but can't confirm) that the sneak pass altitude is a good bit more than 40 feet.

GusNSpot
11-24-2014, 10:08 AM
::: snip ::::

At the show a jet flew at about 40 feet above the ground and very near the attendees.

::: snip ::::



Nitpick::::

Aircraft was about 40 above the ground.

He just said 'very near the people' which is very subjective.

Also, many times they get special waivers about altitude & distance from crowds for particular acts qualify like the Thunderbirds.

I think the FAA might consider the military routines are done by better and highly trained pilots.

Your altitude & air speed may vary. :D

LSLGuy
11-24-2014, 02:08 PM
... My question: Is the jet flying so fast that the sound is delayed OR is the combustion occurring that far from the engine exhaust. ...Neither.

The engine sound is generated almost entirely right at the point where the exhaust leaves the engine. Most of what you're hearing is that high speed gas stream interacting turbulently with the static air nearby. The passage of the airplane itself also adds a bunch of noise as it pushes air violently out of the way. It's the audible equivalent of a ship's wake.

As to why you perceive it as delayed:

All sound travels at ~1000 feet per second in our atmosphere near sea level. So any sound of any kind that is created 1000 feet from you will be heard 1 second after it is generated. Which is why you can determine how far away lightning is by counting out the seconds between flash & noise. 5 seconds ~= 1 mile.

And the same exact delay exists whether the sound source is moving or stationary. The fact the jet is going fast has NOTHING to do with the delay itself.

If someone a thousand feet from you fires a gun, you'll hear the bang 1 second after he/she pulled the trigger.

The whole difference between the jet and the gunfire is that for the gunfire, odds are you couldn't actually see the trigger-pull. So you mistakenly think the gunshot happened when you heard it. It didn't. It happened a second before you heard it.

In the case of the airshow jet, it's traveling at ball-park 600 feet per second. So during the totally ordinary 1 second delay while the sound moves to you, the jet moves forward 600 ft. So you're hearing where it was a second ago while seeing where it is now. So it looks like the sound is coming from someplace behind the jet. It isn't. It's being heard now coming from where the jet was a second ago.

Actually, you're seeing the jet where it was about a microsecond ago. Because the light traveling from the jet to your eye takes about a microsecond to cover the thousand feet. During that microsecond, the jet moves forward a small fraction of an inch. Too small for you to see.

So the true underlying reality is that from where & when you see the jet & hear the sound, the jet's actually a fraction of an inch farther ahead than what you see and the sound you're hearing now is from a second ago when the jet was 700-ish feet farther back from where you see it.

If instead of making a continuous roar, the jet emitted period puffs of smoke and explosion noises, you'd readily see the delay effect.


On a quiet day or night you can sometimes see the same effect with a cruising airliner leaving a contrail. You spot the jet in the sky & can hear the noise appearing to come from some distance behind the airplane. The travel time for the sound is ball-park 30-60 seconds to you, and meanwhile the jet has traveled 4-8 miles. So the noise appears to come from blue sky miles behind the airliner.

All of this is only obvious when you have something which makes a noise loud enough to be heard over an appreciable distance and which is moving fast enough to see the motion over the time interval the sound is enroute to you.

Machine Elf
11-24-2014, 02:20 PM
If someone a thousand feet from you fires a gun, you'll hear the bang 1 second after he/she pulled the trigger.

Unless the round is supersonic, in which case you'll be dead before you hear anything. :D

picunurse
11-25-2014, 09:31 AM
Did I miss the part where someone points out that light, the image you see of the aircraft, is faster than the speed of sound?

Amateur Barbarian
11-25-2014, 09:59 AM
No, that's not possible. People are just generally pretty bad at judging distances of fast moving objects.
See: a good majority of UFO reports that are resolved as meteorites, celestial objects, etc.

LSLGuy
11-25-2014, 11:43 AM
Did I miss the part where someone points out that light, the image you see of the aircraft, is faster than the speed of sound?My 7th paragraph makes the point, albeit indirectly. I said the sound takes ~1 second to cover the distance whereas the light takes ~1 microsecond.

Rearranging that equation states the speed of light is ~1 million times faster than the speed of sound in our typical atmosphere.

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