Re: Engine noise question

Date:         05 Dec 96 02:27:20 
From:         b17864@vaxc.phx1.aro.allied.com
Organization: AlliedSignal Engines, Phoenix, Az
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In article <airliners.1996.2590@ohare.Chicago.COM>, kls@ohare.Chicago.COM (Karl Swartz) writes:
> In article <airliners.1996.2589@ohare.Chicago.COM>, faurecm@halcyon.com (C. Marin Faure) wrote:
>>The "snarl" or "buzz" you hear from a large plane like a 747, 777,
>>MD-11, etc. as they take off is really the fan noise, not the jet
>>core noise, which is the roar part.
>
> With regard to the DC-10, at least, I've heard that the distinctive
> buzz is due to miniture sonic booms the tips of the fan blades go
> supersonic.
>
> When I flew on a United A320, I noticed this same buzz, louder than
> inside a DC-10 and quite annoying.  Since United's A320s have IAE
> V2500 engines, and CFM56s on both 737s and DC-8s don't exhibit this
> same buzz, I'm assuming this is a function of the V2500's design.

There are a number of theories around regarding fan buzz (that buzz saw
wail you hear on some engines). The one that I tend to believe is that it
is due to the vortex from the blade tip or the mid span damper (more
likely) hitting the downstream vanes.  Why some engines do it and some
don't is not an easy answer.  About 10-12 years ago we were investigating
why some of our TFE731 engines would buzz and some would not.  We
identified that the buzzing engines all had fan blades made by one
particular vendor and the non-buzzing engines were made by another.  One
vendor was producing blades to the nominal blueprint dimensions and the
other was making them to the maximum material condition (i.e.; a minimum
of material removal).  Tightening up the tolerances solved the problem,
but to my recollection, no one ever gave a satisfactory explaination of
why this difference was significant.

Mark Johnston
Sr. Development Specialist
Military Engine Enterprise
AlliedSignal Engines
Phx, AZ