Re: Are "jets" really jets?

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         08 Sep 95 02:35:35 
References:   1
Next article
View raw article
  or MIME structure

In article <airliners.1995.1383@ohare.Chicago.COM> (Daniel P. B. Smith) writes:
>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?

If you go to a library and search for "jet," you will likely come up with
only a few pubs.  If you search for "gas turbine engine," you will come
up with an extensive education in thermodynamics, including many, many
applications to airplane propulsion.

"Jet" is not a well-defined term; it never has been.

In propulsion, there are two issues: how to generate thrust, and how to
keep the engine running.

Reciprocal engines are big, unreliable, and unsuitable for providing
significant power.  Therefore, gas turbine engines are used for modern
designs.  These include a series of compressors, a combustion chamber to
help keep the thing running, and an exhaust.

If you hang a gas turbine engine on an airplane (just a series of
compressors), you have a turbojet.  This is not an efficient design.

Modern designs are turbofans.  They add one or more fan blades at the front.
Depending on the bypass ratio of a turbofan engine, the thrust produced by
the fan can range as much as 30 to 75 per cent.  The first turbofan designs
did not have much of a bypass ratio, and were similar to the turbojets which
preceded them; the latest designs have huge bypass ratios.

"Jet" engines are significantly different from turboprops:

1.  There may be more than one fan blade in front.  The JT8D series, for
example, has two.

2.  The ducting and stator vane effect helps direct airflow and control

3.  The fan blades are not controllable as prop blades are.  They are fixed
in their relative locations.   There are also many more blade elements,
compared to props: the ducted and fixed design allows them to be used at
higher rotational velocities than props, which permit them to be used at
much higher speeds.

4.  The fan blades (which are part of the low-pressure compressor) help
provide controlled airflow for the turbine section.

5.  They are EXTREMELY reliable.    The gas turbine accounts for much of
this, and would be a characteristic shared with a turboprop, but the engine
itself can withstand significant damage.

6.  They are MUCH quieter.

Bottom line: Turbofans might be easy to dismiss as a ducted prop, but in
reality, they're much more.

>Daniel P. B. Smith

Robert Dorsett                         Moderator, sci.aeronautics.simulation