Silent Jet fighters

[rov1200]

How do modern jet fighters flying at low level towards you conceal the noise emitted until they are directly overhead? Once passed the noise can be heard for ages. It doesn't happen with airliners.

[Vagrant]

This only happens when they are supersonic - they are flying ahead of the noise they make.
At subsonic speeds, you hear them coming.

[rov1200]

I didn't hear a bang though? Breaking the sound barrier, etc.

[BertiWooster]

Speaking of aircraft noise -you normally hear the sound of a aicraft becoming increasingly louder as it approaches overhead .

However there is a passenger aircraft which flies over where we are from time to time , where there is no gradually increasing of the sound as it approaches - with this aircraft you only hear the sound suddenly , when it's almost directly overhead .

i would love to know who operates this aircraft and the make .

[Vagrant]

As far as I know, the bang is a one-off event, as the sound barrier is broken. Once it's broken, the aircraft flies faster than the sound it produces, and thus approaches in silence. I'm no expert on aviation, but I live on the East coast, and see fighter jets several times a day - the faster they fly, the less you hear them coming.
I believe they normally only break the sound barrier over the North Sea - Occasionally, I presume in emergency, they break the sound barrier over land - the noise is truly terrifying, it makes the ground shake, and prompts many calls to emergency services!

[modeller]

I thought it was the compression of the air that that created the bang and that when Concorde flew over the Atlantic there were a whole sucession of bangs. ?

[modeller]

What happens with rockets. During WWII the V2s were supersonic .The sound was heard after they had passed but no reports of supersonic bangs .

[ChuckFickens]

Vagrant is correct with his first answer, if the plane is subsonic you do hear it coming, though it will be a lot quieter coming towards you due to the simple fact the noisy parts of a plane (the engines) point backwards.

Beti.... Passenger Aircraft have very large diameter engines that shift a lot of gas at a fairly slow rate, whereas military aircraft have small diameter engines that shift a small amount of gas at very high rate, meaning that all passenger aircrafts engines are a lot quieter than military aircraft, some modern passenger planes actually have the sound of the engines drowned out by the noise made by air turbulence over the wings (the 777 for example) so it's this sort of plane you are probably hearing.

As for sonic booms, they are continuous and the shockwave travels along the ground at the same speed the aircraft is flying at, so every point the plane flies over at supersonic speed (and several miles either side as it spreads out) hears the boom some time after the plane has passed (thought the effect at ground level decreases as the planes altitude increases.

For the V2 rockets, two factors meant you didn't hear a sonic boom from them, firstly rockets are pretty close to being the perfect aerodynamic shape with no wings as such, meaning the produce virtually no shockwaves behind them at low altitudes and also as they flew at about 80-100miles up where there is effectively no atmosphere anyhow so there is nothing to produce a shockwave. (and by the time they got into low atmosphere they were pointing straight down pretty much so any little shockwaves would be travelling parallel to the earth rather than coming down to meet it)

[AlBags]

To put the above right ...
Most passenger turbojets have wide-chord and very long blades on the first compressor stages. The blade tip speed is a lot slower so there is a lot less noise.
Coupled with the above, approximately 80% of the air taken in at the intake is not used AT ALL for combustion .. only 20% is. It is used for cooling and noise abatement reasons. It is also tapped off to provide cabin conditioning.
These are a couple of design features that make pass. aircraft very quiet.
Military jets do not need the area drag from large engines, so generally are noisier. Although they use bypass engines, everything is optimised for drag, weight and performance.

[rov1200]

thanks!!!