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Question Re: Engine Exhaust Noise  
User currently offlineBsergonomics From United Kingdom, joined Jan 2002, 462 posts, RR: 0
Posted (10 years 5 months 3 days 1 hour ago) and read 2635 times:

I was reading through the crew chief notes for one of the aircraft that I work on and it had the noise (dB(A)) profiles at various distances from the aircraft at various engine settings.

The curious (to me) thing was that the highest noise spike came out at about 35-40 degrees (horizontally - these are ground measurements) from the aircraft centreline. I usually do the pointy end of the aircraft (cockpits, mission data etc) so the nearest I usually get to the engines is the throttle box. In my ignorance, I would have expected the loudest noise to be directly behind the engines.

I've asked around the office, but the ones who would know have been in management for too long to remember anything useful, and the intern (who would remember) is out at the moment.

It has been suggested that the loudest noise comes from the largest area of turbulence, which would be where there is the largest speed differece (and, therefore, pressure difference) between air pockets. This would mean that the highest pressure difference would be between the stationary air at the side of the exhaust nozzles and the exhaust gas itself.

So, is the sound all generated at this point and continues outwards (I would have expected more of a hemisphere effect); or does it travel with the moving air (which is being blown backwards); or does the noise continue to be generated along the line of the dissipating exhaust flow; or have I got it badly wrong and there is something else at work here?

The aircraft is a twin-engined 'multi-role' fighter built by a European consortium. Can you guess what it is?


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5 replies: All unread, jump to last
 
User currently offlineDAirbus From United States of America, joined Nov 2003, 593 posts, RR: 2
Reply 1, posted (10 years 5 months 2 days 21 hours ago) and read 2528 times:
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I do remember when I was in college learning that the highest noise level from a jet engine being at 35-45 degrees of the exhaust. Sorry but I do not remember the exact scientific explanation for this but your explanation seems to be accurate. Maybe someone else knows.


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User currently offlineDC-10Tech From United States of America, joined Jun 2001, 298 posts, RR: 2
Reply 2, posted (10 years 5 months 2 days 15 hours ago) and read 2479 times:

You are correct. The difference in speed between the engine exhaust and the still air has a direct impact on the sound level. This is one reason that turbofan engines are generally quiter than turbo jets - their exhaust gas is traveling slower than the turbojets.

As for the angle from the tailpipes, if you look at jet blast danger areas, you'll see that the 35-45 degree angle coincides with the boundry of jet blast and still air.



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User currently offlineFly2hmo From , joined Dec 1969, posts, RR:
Reply 3, posted (10 years 4 months 2 weeks 5 days 1 hour ago) and read 2368 times:

An Eurofighter for sure  Smile/happy/getting dizzy

User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 4, posted (10 years 4 months 2 weeks 4 days 9 hours ago) and read 2323 times:

As for the angle from the tailpipes, if you look at jet blast danger areas, you'll see that the 35-45 degree angle coincides with the boundry of jet blast and still air.

Hi Dan, why does the exhaust air exit in such a cone shape rather than a straight gradually-dissipating tube of jetblast? I would have thought that it is a tubular jetblast and that the boundary is inexistent (very wide) because it would be a gradual speeding of the surrounding air, and not on a specific line. Could you please enlighten me on this? Thanks in advance

-Alfredo


User currently offlineA/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 4
Reply 5, posted (10 years 4 months 2 weeks 3 days 12 hours ago) and read 2285 times:

Went out on engine runs the other day, idle outside hangar then full power runs up the bay, ill never stop being amazed at the aweseome power and noise those RB211's produce ! wings bouncing up and down and horizontal stab shaking very violently !! yea man.  Smile/happy/getting dizzy

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