Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnews!military
From: gwh%typhoon.Berkeley.EDU@ucbvax.Berkeley.EDU (George William Herbert)
Newsgroups: sci.military
Subject: Re: Turbo{jet,fan}s
Summary: Get it right :-) [COrrection to turbojet defn. and more...]
Message-ID: <14307@cbnews.ATT.COM>
Date: 25 Feb 90 06:12:24 GMT
References: <24932@ut-emx.UUCP>
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From: gwh%typhoon.Berkeley.EDU@ucbvax.Berkeley.EDU (George William Herbert)
In article <24932@ut-emx.UUCP> rdd@walt.cc.utexas.edu (Robert Dorsett) writes:
>
>> What are the important differences between turbojet and turbofan
>> airplane engines?
>>
>>My limited understanding is that a turbofan is a turbojet with a
>>bypass added. So some of the air goes around the ignition area and
>>then rejoins the exhaust flow. Is there any other basic difference?
>
>All jet engines in service work more or less as follows: incoming air is
>compressed via a set of "low-pressure" compressor blades.  Airflow leaving 
>the low-pressure compressor ultimately constitutes the effective thrust of
>the engine.  The air leaving the low-pressure compressor then either goes
>straight into the high-pressure compressor (after which it is used to 
>maintain the turbine) or "bypassed" around the high-pressure compressor
>and turbine.  Turbine exhaust does *not* comprise the thrust of the engine; 
>the engine thrust comes from the "bypassed" air (after-burning is a special 
>case).  


In a turbojet, this is not the case at all.  In most turbofans, yes most
of the thrust is from the low pressure compressor, but not all by
a longshot.  You need to do some more reading on jet engines...


>Early engines ("turbojets") had relative low bypass ratios (1:1, 2:1).
>These would also generally recombine exhaust and bypass air into a common 
>exhaust port.  "Turbofans" have much higher bypass ratios (generally 5:1 
>through 10:1).  In these engines, a far higher proportion of air from the 
>low-pressure compressor is bypassed.  


Not quite right:
A turbojet puts all of the air into the combustion chamber at one point or
another.  Turbofans duct some of this around and [barring an afterburner] 
never combust it.  A Turbojet has a bypass ratio of zero.

Turbofans can come with a bypass ratio of whatever they want.  The 
10 or 15:1 is for commercail engines...most military ones are much lower.


>Fans provide much better fuel economy than turbojets, and are much quieter;
>hence, this is where nearly all civil R&D is headed.  The current "hot"
>research topics are development of the unducted propfan, and development
>of "small fans" (such as the International Aero Engines V.2500, which is being
>sold as an option on the Airbus A320).  Fans, however, tend to have poor
>performance at very high speeds.  Military aircraft tend to use turbojets.


Fans have low performance at supersonic speeds relative to turbojets.
Most military high-performance planes use low bypass turbofans, like
the TF30 or F100 or F404.  Pure turbojets are too fuel-guzzling, and
the only plane [modern] that i know of using one was the MiG-25, which
was designed for such high speeds [mach 3+] that it NEEDED a turbojet,
because the turbofan effeciency drop really hits after mach two.


*******************************************************************************
George William Herbert     JOAT For Hire: Anything, Anywhere: My Price
UCB Naval Architecture undergrad: Engineering with a Bouyant Attitude :-)
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