my235
Topic Author
Posts: 94
Joined: Mon Mar 23, 2009 10:21 pm

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
Posts: 39
Joined: Wed Mar 07, 2012 10:56 pm

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
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

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
Posts: 12085
Joined: Thu Jul 31, 2003 1:58 pm

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!
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
David L
Posts: 8551
Joined: Tue May 18, 1999 2:26 am

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
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

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
Posts: 12085
Joined: Thu Jul 31, 2003 1:58 pm

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.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

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
Posts: 35
Joined: Fri Feb 11, 2011 8:02 am

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?  
 
User avatar
HAWK21M
Posts: 30007
Joined: Fri Jan 05, 2001 10:05 pm

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.
I may not win often, but I damn well never lose!!! ;)
 
User avatar
jumbojim747
Posts: 2426
Joined: Sat Oct 16, 2004 8:05 pm

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
On a wing and a prayer
 
LH707330
Posts: 1742
Joined: Fri Jun 15, 2012 11:27 pm

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....
 
User avatar
tb727
Posts: 1893
Joined: Thu Jun 30, 2005 1:40 pm

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   
Too lazy to work, too scared to steal!
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

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.
 
User avatar
HAWK21M
Posts: 30007
Joined: Fri Jan 05, 2001 10:05 pm

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.....
I may not win often, but I damn well never lose!!! ;)
 
MQTmxguy
Posts: 129
Joined: Sun Nov 11, 2007 3:58 pm

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
Well at least we can all take comfort in the fact that NW will never retire their DC-9s
 
okees
Posts: 358
Joined: Tue Sep 20, 2005 2:41 pm

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
 
User avatar
jumbojim747
Posts: 2426
Joined: Sat Oct 16, 2004 8:05 pm

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
On a wing and a prayer

Who is online

Users browsing this forum: SheikhDjibouti and 1 guest

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos