Re: 747 with engines on the wings?

Date:         19 Feb 98 01:34:00 
From:         Steve Lacker <no@spam.thanks>
Organization: Texas Networking, Inc.
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Lion's Pers Agentschap wrote:
>  What is the reason that airliners have engines under the wing instead
> of on it?
>  Possibly it will take a little bit more aluminium to construct
> airliners this way, but with overwing engines the wing can deflect a
> big portion of the noise for people living around airports. (This can
> be reached even without improving the engines themselves!)
>  The 777 is rather silent, but could be even better with engines on
> the wing.
>  Why are Boeing and Airbus Industrie developing the wrong airplanes
> now: 737-600/700/800/900, 757-300, 767-400, 777-300, A340-500/600 and
> A3XX?

Why do you think such an arrangement would be quiter? I believe you are
being fooled by your intuition here: intuition says that the engines
make noise, so if you put a big chunk of metal (the wing) between the
engines and the ground, and airplane will seem quieter to people on the

But intuition is often wrong and its 90% wrong in this case.

Most of the very loudest noise associated with jet engines does NOT come
from the engine nacelle itself, but rather from the long (many meters in
length) region behind the engine where high-velocity exhaust gasses are
mixing with lower-velocity air and creating tremendous turbulence. Since
this region extends well behind the wing, mounting the engine above the
wing wouldn't block downward propagation of this sound at all.

Furthermore, the loudest components of jet noise are low frequency,
which means long-wavelength. This means that the loudest sounds, even if
generated above the wing, would be able to diffract around the wing
quite easily.

Some slight shielding of the higher-freqency and highly localized noise
created by the engine *inlet* could indeed be shielded by placing the
engine above the wing, but this is a minor component for modern engines.
(Older engines, like early JT8D's had much more inlet noise, but hushkit
makers have addressed this with inlet guide vane respacing kits.)

Therefore, there is minimal to no acoustic advantage of placing the
engines above the wings. Furthermore, there is an increased cost of
maintaining engines that are difficult to access on the ground.

Short answer: Boeing and Airbus are NOT developing the "wrong" aircraft-
at least not for the reason stated.

Steve Lacker
sglacker at texas dot net

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