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N231YE
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P&W JT8D Idle Sound "Vaccuum Cleaner"

Thu Jul 27, 2006 10:46 am

Just wanted to know, what causes the Pratt & Whitney JT8D to create that (in my opinion, beautiful) "vaccuum cleaner" sound when idleing. Is it possibly from the blade tips moving at a velocity higher than that of the speed of sound (like the "buzz-saw" of high bypass engines on takeoff)?

Thanks
N231YE
 
Skookum
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Thu Jul 27, 2006 12:41 pm

The tips shouldn't be going supersonic at such a low power setting. Now I'm no jet engine expert or acoustics engineer, but I think that this higher (compared to other jet engines) pitch idle sound is caused by the smaller (compared to other jet engines) diameter of the engine.

I'm sure that the others will have a much better explanation.
Good flying
 
n8076u
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Thu Jul 27, 2006 1:35 pm

Quoting Skookum (Reply 1):
The tips shouldn't be going supersonic at such a low power setting.

True.

The JT8D (and other low-bypass turbofans and turbojet engines) sucks the air in at a higher velocity than a high-bypass engine with comparable thrust output, like the CFM-56. The big fan on the high-bypass engine makes up for the lower velocity by moving a larger volume of air, and a benefit of that is less noise.

Chris
Don't blame me, I don't work here...
 
ElpinDAB
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Fri Jul 28, 2006 4:37 am

Please correct me if i'm wrong b/c i'm just taking what I learned in my turbine engines class at ERAU. I'm sure there are many (much) higher qualified people out there, but...

Spools of the compressor section are composed of rows of rotating compressor blades and stationary vanes called stators. On turbojets and early turbofans, such as the JT8D, the compressor blades spin very fast right next to the stationary vanes. This shears the air between the compressor blades and the stators, creating the 'vacuum cleaner' sound you hear when the engine is at lower power settings. This is very similar to how a siren creates noise: by a rotating a set of blades next to stationary vanes.

Newer high bypass turbofans are designed with lower noise levels as a priority, so the rotating compressor blades in the fan section of the engine are much farther away from the stators, eliminating this shearing and the the resulting sound.
 
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HAWK21M
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Fri Jul 28, 2006 6:37 pm


This sound is a very peculiar Varied noise level when the JT8D is Idling.
Low Bypass Engines sure are noisy.

The Airflow passing through the Compressor blades at that speed contributes to this Whining sound.

regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
avioniker
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Fri Jul 28, 2006 10:11 pm

Gee
I always thought Mother's Kirby sounded like a JT-3 above idle and the newer JT-8 sounds like my Hoover;
I never realized it's actually that a jet engine sounds like a vacuum cleaner
 Smile
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
N231YE
Topic Author
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Sun Jul 30, 2006 9:44 am

Thanks for the input,

Quoting Avioniker (Reply 5):
I always thought Mother's Kirby sounded like a JT-3 above idle and the newer JT-8 sounds like my Hoover;

 biggrin  - I thought my mom's Hoover sounds like a PT6A turboprop more than a JT8D. Anyways, nice one
 
corey07850
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Sun Jul 30, 2006 10:12 am

Quoting Skookum (Reply 1):
The tips shouldn't be going supersonic at such a low power setting.

True... To continue with that, the tips shouldn't be going supersonic any power setting
 
speedracer1407
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Sun Jul 30, 2006 12:30 pm

Quoting Corey07850 (Reply 7):
True... To continue with that, the tips shouldn't be going supersonic any power setting

It's my understanding, based on numerous threads about fan aerodynamics on this forum, that tip speed does indeed reach and exceed Mach 1 at high power settings.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
747400sp
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Thu Aug 03, 2006 12:35 am

Quoting ElpinDAB (Reply 3):
Newer high bypass turbofans are designed with lower noise levels



Quoting HAWK21M (Reply 4):
Low Bypass Engines sure are noisy.

At taxi a F-119 that powers F-22A are as quite as a 737 classic or NG CFM56-3 or 7. They also sound like CFM56-3 or 7. At take off, well that's a totally different story.
 
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HAWK21M
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RE: P&W JT8D Idle Sound "Vaccuum Cleaner"

Thu Aug 03, 2006 5:13 pm

Out here The Few B732s that operate sure wake up the Airport when they Get Airborne  Smile
But I think another 2-3 yrs for Freighters & another year for Pax B732s in the country.
regds
MEL
I may not win often, but I damn well never lose!!! ;)

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