Re: aircraft noise

From:         weiss@curtiss.SEAS.UCLA.EDU (Michael Weiss)
Organization: SEASnet, University of California, Los Angeles
Date:         12 Feb 93 11:12:06 PST
References:   1 2
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In article <airliners.1993.137@ohare.Chicago.COM> kls@ohare.Chicago.COM (Karl Swartz) writes:
>The most significant difference is probably that the new
>designs are "high bypass ratio" turbofans, while the 727/737 use a
>lower bypass ratio turbofan and the early 707 and DC-8 used an even
>noisier pure turbojet.
[Some stuff deleted]
>This has a dramatic impact on noise because the majority of the noise
>comes from the hot exhaust gasses of the combustion process.  With
>more of the thrust coming from the cooler bypass airflow the engine
>is quieter.  In addition, the cool bypass air surrounds the hotter
>combustion gasses and tends to contain the noise.  Further improvement
>have come as the manufacturers have learned how to better control this
>phenomenon.

I'd like to elaborate on this a bit.  In order to understand the noise, you
need to understand a bit about how an aircraft engine produces thrust.  In
greatly simplified terms, thrust is produced by pushing air toward the rear of
the aircraft.  Thrust is equal to the momentum of the moved air (every action
has an equal and opposite reaction), but is, of course, in the opposite
direction.

Now, momentum is equal to the mass of the air multiplied by the velocity of
the air.  Therefore, you can do two things to increase thrust.  Either move
the air faster, or move more air at the same speed.  Turbojets are required to
move air at supersonic speeds to produce thrust adequate for takeoff.  As
anyone who has heard a sonic boom can attest, shock waves from supersonic
airflow is NOISY!  Therefore, to reduce noise, you can run the exhaust at
lower speeds, and put a big fan on the engine to move a greater volume of air
at the lower speed, keeping the added momentum the same.  The thrust is the
same, and the noise is lowered.

As a side benefit, running the engines at a lower speed reduces heat and fuel
consumption.  Sounds great all around, doesn't it?
-- 
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-  Michael  weiss@seas.ucla.edu   |  School of Engineering & Applied Science  -
-   Weiss   izzydp5@oac.ucla.edu  |   University of California, Los Angeles   -
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