Re: aircraft noise

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works
Date:         04 Feb 93 02:36:35 PST
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>why is it that small-ish 727s and 737s seem to make so much noise on
>take off and even fly-by, compared to the apparently MUCH quieter
>747s? This has been my distinct impression on numerous occasions. Is
>is just more efficiently designed engines, or better noise insulation,
>or what ?

By 737 you probably mean the 737-100/-200 variants, as the newer
-300/-400/-500 versions are much quieter.

Anyway, the 727s and older 737s use older engine designs which are
inherently noisier than the newer designs used on the 747 and newer
aircraft.  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.

In a turbojet, all of the air coming into the engine passes through
the combustion chamber.  In a turbofan, or "fan jet" as Madison Ave.
called them when they first appeared, some of the air goes through the
low-pressure compressor but then bypasses the remainder of the engine.
In effect, a turbofan is like a turboprop, except the propellor (the
LP compressor) has a duct around it, has many more blades, and turns
at engine speed rather than being geared down.  It's also fixed pitch.

The early turbofans had a fairly low bypass ratio, less than one if I
recall correctly -- around 20% or about .2 : 1 sounds right.

In contrast, the JT9D of the origianl 747 is more like 6:1 or 7:1, and
new engines for the A330 and 777 will exceed 10:1.

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.

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