Re: Are "jets" really jets?

From:         barr@netcom.com (Keith Barr)
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
Date:         07 Sep 95 02:49:27 
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In article <airliners.1995.1383@ohare.chicago.com>,
Daniel P. B. Smith <dpbsmith@world.std.com> wrote:
>Is a modern-day "jet" really a jet at all, or is it just a kind of turboprop
>where the propellor has lots of little blades and is concealed within
>a cowling?

I guess you could think of modern engines as cowled turbopros, but the
cowling serves a number of important purposes, not the least of which are
flow straightening, and flow slowing.

Modern day engines are actually turbofan engines, and have a large amount
of air that bypasses the core of the engine.  Turbojet engines, those where
all the air passes through the turbine, have not been used widely for a
number of years.

A turbofan has a large fan disk on the front of the engine which compresses
the air.  This compressed air is then separated--some just flows out the
back of the engine, and some passes through a number of other compressors,
the combustion chamber, and then through the turbine section that provides
the power to turn all of the compressors.  The core exhaust is then blown out
the back to provide some thrust, but most of the thrust is provided by the
bypass air.

Turbofans are more efficient because they need less fuel, since there is
less air being mixed into the combustion section.  They are also much
quieter.  Thrust can be created by moving a small mass of air through a
huge acceleration (turbojet), or by moving a large mass of air through a
small acceleration (turbofan).  Noise is roughly related to the seventh
power of the difference in flow speed between the thurst stream and the
local air velocity, so even small decreases in thrust stream velocity can
result in large changes in noise level.
--
Keith Barr                                                    barr@netcom.com
Aerospace Engineer                ftp://ftp.netcom.com/pub/ba/barr/index.html
COMM AS&MEL/IA/A&IGI                                    Westminster, Colorado
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