Re: Engine noise question

Date:         05 Dec 96 02:27:20 
From:         simonlc@ozemail.com.au (Simon Craig)
Organization: OzEmail Ltd - Australia
References:   1
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>Can anyone explain to this non-pilot what actually creates the
>sound and why the sounds are different?  I seem to recall from
>being at the airport that the high whine is more pronounced when
>close to the planes and the dull roar more pronounced from a
>distance.
>
>Is the whine a mechanical sound (turbines or something) of metal
>parts rubbing on one another?  Is the roar the actual sound of
>combustion or of air rushing over the metal surfaces of the
>engine?
>
>These may be absurdly inaccurate guesses, but I've often wondered
>and just found this newsgroup where I might find answers.

Well I can't speak for all cases, and every engine has a different
characteristic, but the "whine" is usually heard when in front of the
engine, and is the sound of the fan and compressor (ie. the bit you see at
the front).  It is often caused by the air going supersonic (or close to
it) at the tips of the fan and compressor blades.

On the same vein, the roaring noise is usually heard from the rear of the
engine, of from a/c flying overhead, and is a characteristic of the
"shearing" effect of hot, high velocity air passing cold low velocity air,
as is normally the case coming from the rear of engines when they are
powering an a/c taking off.  A "crackling" noise is usually this same hot
air going supersonic and forming some sort of shock wave that tends to
then slow it down (often called "choked nozzle thrust").

Regards,

--
Simon Craig

--- If it's not Boeing, then I'm not going!