Re: Thrust in idle engines

From:         ehahn@bass.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         03 Nov 95 04:23:13 
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>In article <airliners.1995.1626@ohare.Chicago.COM> Jean-Francois Mezei <MEZEI_JF@Eisner.DECUS.Org> writes:

<don't know answers about "idle thrust" characteristics>

>   Also, in a previous posting, someone said that without an APU, an aircraft
>   would require power and "air" from the ground to start its engine. Are we
>   talking about compressed air ? What for ?

Yes, compressed air would be required in most cases.  Because a
turbine engine needs quite a few RPMs before the thrust from
combustion can allow the compressor to enable continuous operation,
an external supply of energy must be available to spin the high
pressure spool.  While one could probably design an electric starter
motor to perform this task, it would probably drain the batteries to
an unsatisfactory level, not to mention probably being pretty heavy.
Therefore, one uses compressed air to get the spool "windmilling" to a
high enough rotational velocity to allow a start.

In flight, if the engine were to flame out, the windmilling rotation
induced by the freestream air allows the spools to remain spun up, so
this isn't a safety problem.  Thus, assuming everything is working
properly in the engine and fuel systems, the only time one couldn't
start the engine would be when the aircraft is standing still (which
should be acceptably safe).  :-)

>   Also, why are DC9s and 727s capable of backing up from the gate without being
>   pushed ? Is it a technical or legal consideration ? (eg: engines being far
>   enough away from terminal for noise control etc).

This is called a "powerback", and only American Airlines and TWA (that
I'm aware of) in the US perform it.  The reason that these aircraft
are allowed to do so is:  1) relatively low weight, and 2) rear engines,
mounted higher off the ground compared with wing-mounted engines.
This topic has been discussed in depth in the past, so see the
archives.

ed

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.