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A Question About Early Jet Engines  
User currently offlineJetBlue320 From United States of America, joined Feb 2005, 0 posts, RR: 0
Posted (12 years 11 months 1 week 4 days 22 hours ago) and read 1471 times:

Why are earlier High-Bypass Jet Engines so loud? The JT9D is so loud, but the BR715 is so quiet. What makes a Jet Engine so quiet/loud. Look at the JT8D-9, and the JT8D-209, The -209 is much quieter than the basic JT8D... Why?

6 replies: All unread, jump to last
 
User currently offlinePositive rate From Australia, joined Sep 2001, 2143 posts, RR: 1
Reply 1, posted (12 years 11 months 1 week 4 days 21 hours ago) and read 1440 times:

It depends on how much of the air is bypassed. On the newer generation of engines-RB211/CFM56 etc the majority of air that bypasses the core of the engine whereas with the older JT8D most of the air went through the engine core and so created more noise. The 737-200 compared to the 737-300 engines produce very little bypass.

User currently offlineJetBlue320 From United States of America, joined Feb 2005, 0 posts, RR: 0
Reply 2, posted (12 years 11 months 1 week 4 days 21 hours ago) and read 1429 times:

But why is it the JT9D is louder than the BR715. The JT9D is LARGER, and therefore, More Air should bypass than in the BR715

User currently offlineDavid B. From United States of America, joined Jun 2001, 3148 posts, RR: 5
Reply 3, posted (12 years 11 months 1 week 4 days 21 hours ago) and read 1421 times:

Not really. Size doesn't matter. It depends on the bypass ratio.A large engine can have a small bypass ratio while a small one can have a high bypass ratio.
 Nuts



Teenage-know-it-alls should be shot on sight
User currently offlineJetBlue320 From United States of America, joined Feb 2005, 0 posts, RR: 0
Reply 4, posted (12 years 11 months 1 week 4 days 21 hours ago) and read 1418 times:

Oh, Ok... Now I am understanding it a little more

User currently offlineTimz From United States of America, joined Sep 1999, 6896 posts, RR: 7
Reply 5, posted (12 years 11 months 1 week 2 days 21 hours ago) and read 1368 times:

Did you mean JT9D? It ought to be louder than the BR715, with more than 3x the thrust.

JT8D-9 bypass ratio is about 1; newer high-bypass engines have a ratio of 5 and up. That's the main difference.


User currently offlineVC-10 From United Kingdom, joined Oct 1999, 3704 posts, RR: 34
Reply 6, posted (12 years 11 months 1 week 2 days 12 hours ago) and read 1338 times:
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On the early jets most of the noise was generated by the high temp & shearing action on the surrounding air as the gas stream came out of the jet pipe. The thrust was achieved by accelerating a small mass of air to high velocity.

Todays high by-pass engines move a large mass of air at low temp & velocity, hence low noise.


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