Re: Engine noise question

Date:         01 Dec 96 04:08:51 
From:         Reid Fairburn <cr_king@cr_king.seanet.com>
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At 06:18 AM 11/25/96, you wrote:
>I was walking my dog in the park this afternoon in Toronto and
>listening to the sound of passenger jets going over - a common
>experience as I am about 20 miles from Pearson airport.
>
>Some of the planes had a high-pitched whine and some a dull roar.
>All were above the cloud cover.

========Right off hand, I would say that this is all good news!

>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?

========The older turbojets make the most noise because the jet gases flow
out of the pipe at a very high speed and cause a lot of stress and pressure
pulses as they mix with the slower external flow of air around the engine.
The newer fan engines kind of hide the fast gas intermixing by placing a
slower moving mass of air around the core flow...this makes the noise less
of a roar.  However, the big fan blades cause the whine that you
hear...hopefully you don't hear metal on metal or I would run for cover.
The whine is just an aerodynamic pulse frequency of the rotating blades
mixing it up with the air.

>These may be absurdly inaccurate guesses, but I've often wondered
>and just found this newsgroup where I might find answers.

====Hopefully, some of our real experts will give some better technical
answers.  By the way, you are not alone in your quest for this info,
engineers have been looking for the mysterious answers for years...somewhat
successfully I think as of today.

Reid Fairburn
Creative Kingdom, Inc.
cr_king@cr_king.seanet.com
206-946-4815