N231YE From , joined Dec 1969, posts, RR: Posted (5 years 6 months 2 weeks 6 days 21 hours ago) and read 2717 times:
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)?
Skookum From Canada, joined Jul 2006, 115 posts, RR: 0 Reply 1, posted (5 years 6 months 2 weeks 6 days 19 hours ago) and read 2699 times:
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.
N8076U From United States of America, joined Jun 2006, 425 posts, RR: 12 Reply 2, posted (5 years 6 months 2 weeks 6 days 18 hours ago) and read 2689 times:
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.
ElpinDAB From United States of America, joined Apr 2005, 414 posts, RR: 6 Reply 3, posted (5 years 6 months 2 weeks 6 days 3 hours ago) and read 2618 times:
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.
Avioniker From United States of America, joined Dec 2001, 1106 posts, RR: 13 Reply 5, posted (5 years 6 months 2 weeks 5 days 10 hours ago) and read 2544 times:
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
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
Corey07850 From United States of America, joined Feb 2004, 2495 posts, RR: 6 Reply 7, posted (5 years 6 months 2 weeks 3 days 22 hours ago) and read 2452 times:
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 From United States of America, joined Dec 2004, 333 posts, RR: 0 Reply 8, posted (5 years 6 months 2 weeks 3 days 19 hours ago) and read 2439 times:
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 From United States of America, joined Aug 2003, 3025 posts, RR: 3 Reply 9, posted (5 years 6 months 2 weeks 7 hours ago) and read 2344 times:
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.
HAWK21M From India, joined Jan 2001, 29818 posts, RR: 61 Reply 10, posted (5 years 6 months 1 week 6 days 15 hours ago) and read 2301 times:
Out here The Few B732s that operate sure wake up the Airport when they Get Airborne
But I think another 2-3 yrs for Freighters & another year for Pax B732s in the country.
regds
MEL
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From , joined Dec 1969, posts, 
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- I thought my mom's Hoover sounds like a PT6A turboprop more than a JT8D. Anyways, nice one
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