Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnews!military
From: henry@zoo.toronto.edu (Henry Spencer)
Newsgroups: sci.military
Subject: Re: Turbo{jet,fan}s
Message-ID: <14306@cbnews.ATT.COM>
Date: 25 Feb 90 06:12:22 GMT
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From: henry@zoo.toronto.edu (Henry Spencer)
>Here is a question for you aero experts:

(This probably would have been better in sci.aeronautics, actually.)

>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... Is there any other basic difference? 

No.  The sole difference, in principle, is that the turbofan adds one
or more "fan" stages ahead of the compressor, with some of the air
passing through those stages and then going straight out the back,
not passing through the rest of the engine.

>Why is bypassing a "good thing"? What does it accomplish?
>How does changing the "bypass ratio" affect the engine performance?

Basically, if you look at the equations for the physics of propulsion,
you get more push per penny :-) at low speed if you push a lot of air
out at low velocity, rather than a little at high velocity.  The fan
pushes more air through.  On the big turbofans used on subsonic transports,
the fan is large, most of the air does not go through the engine core (i.e.
the bypass ratio is large), and the result is a relatively massive exhaust
at fairly low velocity.  The engine core is mostly there to spin the fan,
and makes only a small thrust contribution itself.  In the low-bypass
turbofans used in a lot of fighters, only a little bit of air bypasses
the core, but it's still enough to help fuel economy (and hence range
and endurance) at subsonic speed.  Which is better depends on requirements,
notably speed.  At very low speeds you're better off enlarging the fan
until it becomes, surprise surprise, a propellor, and you end up with a
turboprop.  At very high speeds you forget the fan altogether.

>Does turbofan performance tend to be limited by different factors than
>turbojet performance? 

It depends on what you mean by "performance".  Different customers care
about different things.  The underlying physics say that the most basic
performance parameter is how hot the combustion gas is when it hits the
turbine, and that applies to both forms.  (The limit is set by turbine
materials, mostly.)

                                    Henry Spencer at U of Toronto Zoology
                                uunet!attcan!utzoo!henry henry@zoo.toronto.edu