jetBlue320
Posts: 319
Joined: Thu Feb 10, 2005 3:01 pm

A Question About Early Jet Engines

Tue Dec 25, 2001 8:56 am

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?
 
POSITIVE RATE
Posts: 2121
Joined: Sun Sep 16, 2001 11:31 am

RE: A Question About Early Jet Engines

Tue Dec 25, 2001 9:52 am

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.
 
jetBlue320
Posts: 319
Joined: Thu Feb 10, 2005 3:01 pm

RE: A Question About Early Jet Engines

Tue Dec 25, 2001 9:57 am

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
 
david b.
Posts: 2894
Joined: Fri Jun 15, 2001 7:18 pm

RE: A Question About Early Jet Engines

Tue Dec 25, 2001 10:01 am

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
 
jetBlue320
Posts: 319
Joined: Thu Feb 10, 2005 3:01 pm

RE: A Question About Early Jet Engines

Tue Dec 25, 2001 10:12 am

Oh, Ok... Now I am understanding it a little more
 
timz
Posts: 6100
Joined: Fri Sep 17, 1999 7:43 am

RE: A Question About Early Jet Engines

Thu Dec 27, 2001 10:04 am

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.
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: A Question About Early Jet Engines

Thu Dec 27, 2001 7:12 pm

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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