Re: 747 with engines on the wings?

Date:         19 Feb 98 01:33:54 
From:         "Chris Dahler" <dahler@iglobal.net>
Organization: All USENET -- http://www.Supernews.com
References:   1
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> What is the reason that airliners have engines under the wing instead
>of on it?

The engine on the wing design has been tested on the types of aircraft you
mentioned in your post.  It is actually easier to build an airplane this way
because of the support structure of the engine.  However, one can be
absolutely sure that aircraft manufacturers would not be building a design
were it not efficient, and this is the case with over-the-wing designs: they
are aerodynamically inefficient.  The disturbance of airflow over the
portion of the wing on which the engine sits degrades performance, which all
adds up to more fuel being burned.

The noise part of your post didn't make much sense.  The majority of noise
from a jet aircraft does not originate at the engine itself, it originates
aft of the exhaust.  The roaring you hear is primarily a result of the
superhot air expanding rapidly as it leaves the engine, causing a
compression wave; this is the same principle that causes thunder following a
lightning bolt.  Other areas of noise are caused aerodynamically by the tips
of the wings and flaps and by the "buzz" heard from modern turbofan engines,
which is caused by the tips of the fan blades exceeding the speed of sound
as they spin around.  Modern turbofan engines are much quieter due to
improved core design and also due to the high bypass of relatively unheated
air around the core exhaust; this "tube" of unheated exhaust air around the
superheated exhaust gases provides a buffer to dampen the shock wave of the
exhaust, thus reducing the overall noise.  The placement of the engine on
top of the wing would not reduce noise by a noticeable amount at all.