my235
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Thrust Reverse Noise.

Fri Sep 07, 2012 4:43 am

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?
 
ak907
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RE: Thrust Reverse Noise.

Fri Sep 07, 2012 5:16 am

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.
 
tdscanuck
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RE: Thrust Reverse Noise.

Fri Sep 07, 2012 2:27 pm

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.
 
vikkyvik
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RE: Thrust Reverse Noise.

Fri Sep 07, 2012 5:13 pm

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

Wow, really? Very interesting - I learned something today!
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David L
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RE: Thrust Reverse Noise.

Fri Sep 07, 2012 6:34 pm

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!
 
tdscanuck
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RE: Thrust Reverse Noise.

Fri Sep 07, 2012 7:21 pm

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.
 
vikkyvik
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RE: Thrust Reverse Noise.

Sat Sep 08, 2012 3:29 am

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.
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tdscanuck
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RE: Thrust Reverse Noise.

Sat Sep 08, 2012 4:37 am

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.
 
C46
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RE: Thrust Reverse Noise.

Sat Sep 08, 2012 8:50 am

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?  
 
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HAWK21M
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RE: Thrust Reverse Noise.

Sat Sep 08, 2012 10:28 am

The blocking of Airflow & then deflecting it thru cascade vanes forward contribute to the noise.
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jumbojim747
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RE: Thrust Reverse Noise.

Wed Sep 19, 2012 7:14 pm

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
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LH707330
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RE: Thrust Reverse Noise.

Wed Sep 19, 2012 8:28 pm

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....
 
tb727
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RE: Thrust Reverse Noise.

Thu Sep 20, 2012 1:25 am

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   
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tdscanuck
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RE: Thrust Reverse Noise.

Thu Sep 20, 2012 2:48 am

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.
 
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HAWK21M
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RE: Thrust Reverse Noise.

Thu Sep 20, 2012 6:25 am

The low bypass Turbofans like the JT8Ds used to reverse the entire airflow and were noisier.....
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MQTmxguy
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RE: Thrust Reverse Noise.

Thu Sep 20, 2012 9:51 am

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
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okees
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RE: Thrust Reverse Noise.

Thu Sep 20, 2012 1:54 pm

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
 
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jumbojim747
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RE: Thrust Reverse Noise.

Thu Sep 20, 2012 2:17 pm

Quoting HAWK21M (Reply 14):

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