Re: Engine noise question

Date:         05 Dec 96 02:27:20 
From:         Steve Lacker <slacker@arlut.utexas.edu>
Organization: applied research laboratories
References:   1 2 3
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Karl Swartz wrote:
>
> 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, 7
> >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.

I can't *deny* that, but it sounds fishy for several reasons:
- The tip speed is easily calculated, and an engine designer should easily
be able to ensure that it never goes supersonic under expected operating
conditions.
- Supersonic tips on conventional propellors will drastically reduce
efficiency, and so I assume supersonic tips on a fan would do the same
- I've heard the "buzz" on many high-bypass engines, not just on DC-10s

One thing I've noticed is that the buzz seems to depend a lot on air
conditions.  I first noticed that sound while working in Ft. Lauderdale,
directly under the east departure path for Fort Lauderdale/Hollywood
international. *Every* high-bypass engined plane, from CFM-powered 737s
to 767's to L-1011s to 747s made the sound. I've stood in similar proximity
to departing planes at other airports, and *not* heard the buzz as plainly
except on hot humid days (its ALWAYS hot and humid at FLL!). All that
said, I don't know the exact mechanism by which that sound is generated-
it may be doppler-modulation (due to rotation) of the sound from each
individual airfoil in the fan, but thats just a guess. If it is doppler
induced, then it should NOT be audible if you are looking right down the
throat of the engine (the blades would not be advancing and receding from
the observer as they are if the observer is off-axis) but then, thats a
tough position for an observer to be in for a climbing airliner!

-
Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029
512-835-3286	slacker@arlut.utexas.edu