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Thrust Reverse Noise.  
User currently onlinemy235 From United States of America, joined Mar 2009, 92 posts, RR: 0
Posted (2 years 2 weeks 1 day 17 hours ago) and read 4998 times:

Thrust reverse noise from modern high bypass turbofans is as loud as it is because the bypass air is no longer surrounding and muffling the hot center jet exhaust. Am I correct?

17 replies: All unread, jump to last
 
User currently offlineak907 From United States of America, joined Mar 2012, 42 posts, RR: 0
Reply 1, posted (2 years 2 weeks 1 day 16 hours ago) and read 4981 times:

My guess would be because instead of going straight out the back with little resistance, the air is now being rerouted vanes and going through vents, some of it eventually hitting the ground. All of this will cause turbulence.

User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 2, posted (2 years 2 weeks 1 day 7 hours ago) and read 4839 times:

Quoting my235 (Thread starter):
Thrust reverse noise from modern high bypass turbofans is as loud as it is because the bypass air is no longer surrounding and muffling the hot center jet exhaust. Am I correct?

Not really. The major contributor is the terribly turbulent flow coming through the cascade vanes...the air is being wrenched around fairly violently. You're also bypassing all the acoustic treatments in the aft nacelle that help shut up the fan flow in normal operation.

There is some increased noise from the core flow but reverse thrust isn't as high a core flow as full forward thrust and, since noise scales roughly by RPM to the 8th power, small drops in core flow cause big drops in noise.

Tom.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 10036 posts, RR: 26
Reply 3, posted (2 years 2 weeks 1 day 4 hours ago) and read 4764 times:
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Quoting tdscanuck (Reply 2):
noise scales roughly by RPM to the 8th power

Wow, really? Very interesting - I learned something today!



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 41
Reply 4, posted (2 years 2 weeks 1 day 3 hours ago) and read 4721 times:

Quoting vikkyvik (Reply 3):
I learned something today!

Me too. I think it's the first time I've heard of the 8th power of something doing anything!


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 5, posted (2 years 2 weeks 1 day 2 hours ago) and read 4705 times:

Quoting vikkyvik (Reply 3):
Quoting tdscanuck (Reply 2):
noise scales roughly by RPM to the 8th power

Wow, really? Very interesting - I learned something today!

It's a bit complicated by the fact that decibels are a log scale...technically, it's the noise power that scales with the 8th power of speed. Perceived noise is a whole different animal.

Quoting David L (Reply 4):
I think it's the first time I've heard of the 8th power of something doing anything!

Me too. I first learned that back in undergrad when we were trying to design a quiet leaf blower.

Tom.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 10036 posts, RR: 26
Reply 6, posted (2 years 2 weeks 18 hours ago) and read 4583 times:
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Quoting David L (Reply 4):
I think it's the first time I've heard of the 8th power of something doing anything!

Seriously! This sounds like perfect trivia material.

Quoting tdscanuck (Reply 5):
It's a bit complicated by the fact that decibels are a log scale...technically, it's the noise power that scales with the 8th power of speed. Perceived noise is a whole different animal.

Gotcha. Still very interesting. Thanks Tom.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 7, posted (2 years 2 weeks 17 hours ago) and read 4563 times:

Found a reference! Turns out it's called Lighthill's Eights Power Law:
http://en.wikipedia.org/wiki/James_Lighthill

The noise power from the jet jet scales with the 8th power of the jet speed. At the time I was working with it we were playing with a centrifugal fan, for which jet speed and RPM were basically linear so it also scaled with the 8th power of RPM. I'm not sure that relationship holds true for axial fans.

Tom.


User currently offlineC46 From United States of America, joined Feb 2011, 43 posts, RR: 0
Reply 8, posted (2 years 2 weeks 13 hours ago) and read 4529 times:

Quoting tdscanuck (Reply 5):
Me too. I first learned that back in undergrad when we were trying to design a quiet leaf blower.


You mean a rake?  


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 9, posted (2 years 2 weeks 11 hours ago) and read 4505 times:

The blocking of Airflow & then deflecting it thru cascade vanes forward contribute to the noise.


Think of the brighter side!
User currently offlineJumboJim747 From Australia, joined Oct 2004, 2464 posts, RR: 44
Reply 10, posted (2 years 3 days 2 hours ago) and read 3882 times:

Does over use of the reverser's make for higher maintance.
I'm asking this as some companies tell pilots if they can do without useing then do so.
The Qantas 747 that overshot the runway never used reverse thrust



On a wing and a prayer
User currently offlineLH707330 From United States of America, joined Jun 2012, 784 posts, RR: 0
Reply 11, posted (2 years 3 days 1 hour ago) and read 3859 times:

Quoting JumboJim747 (Reply 10):
Does over use of the reverser's make for higher maintance.

I think it's got more to do with brake cooling and wear than anything else, but I always find it annoying when they use reverse just because of the noise....


User currently offlinetb727 From United States of America, joined Jun 2005, 1598 posts, RR: 9
Reply 12, posted (2 years 2 days 20 hours ago) and read 3791 times:

Quoting JumboJim747 (Reply 10):

Does over use of the reverser's make for higher maintance.
I'm asking this as some companies tell pilots if they can do without useing then do so.
The Qantas 747 that overshot the runway never used reverse thrust

Reverse thrust is more effective at higher airspeed than brakes and vice versa as you get slower, at least on what I fly. I will very rarely touch the brakes above 70 knots if I have a long enough runway. I have heard the 727's brakes aren't the easiest to change so I try my best and not abuse them. Plus I like the noise   



Too lazy to work, too scared to steal!
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 13, posted (2 years 2 days 19 hours ago) and read 3752 times:

Quoting JumboJim747 (Reply 10):

Does over use of the reverser's make for higher maintance.

Most modern thrust reversers are on-condition maintenance and have no set maintenance tasks tied to thrust reverser cycles (they've got tasks tied to aircraft cycles but that happens whether or not you use the reverser).

That said, the moving parts in the reverser wear, primarily, with cycles (mostly the actuators, actuator attach fittings, and hinges). So more reverser cycles will tend to drive you to on-condition replacement of the moving parts more often.

Whether that costs more or less than the increased brake wear depends a lot on the aircraft and the operations; with carbon brakes becoming more common the economic case for the reversers goes down because the carbon brakes wear by cycle, not by amount of energy absorbed (steel brakes are the opposite).

If you have a structure problem on the cascades (notorious on a few designs) that will also go with cycles but that's really a design problem.

Tom.


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 14, posted (2 years 2 days 15 hours ago) and read 3709 times:

The low bypass Turbofans like the JT8Ds used to reverse the entire airflow and were noisier.....


Think of the brighter side!
User currently offlineMQTmxguy From United States of America, joined Nov 2007, 197 posts, RR: 0
Reply 15, posted (2 years 2 days 12 hours ago) and read 3679 times:

Quoting JumboJim747 (Reply 10):
Does over use of the reverser's make for higher maintance

My company has on occasion pressured our flight crews to keep T/R usage to a minimum on the ERJ fleet in the name of maintenance costs and reliablity so now...

...you guessed it, brakes are being changed at a record pace.

I hate bean counters



Well at least we can all take comfort in the fact that NW will never retire their DC-9s
User currently offlineokees From Canada, joined Sep 2005, 424 posts, RR: 5
Reply 16, posted (2 years 2 days 7 hours ago) and read 3638 times:

Quoting LH707330 (Reply 11):
I always find it annoying when they use reverse just because of the noise....

I find it annoying when they do NOT use the reverse thrust.. because of the noise! I love it, it's the second best part about landing (first would be finally reaching your destination).



mobs jakis
User currently offlineJumboJim747 From Australia, joined Oct 2004, 2464 posts, RR: 44
Reply 17, posted (2 years 2 days 7 hours ago) and read 3632 times:

Quoting HAWK21M (Reply 14):

That was how they could reverse the aircraft from the gate although dangerous



On a wing and a prayer
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