Re: Thrust in idle engines

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Organization: Netcom
Date:         03 Nov 95 04:23:14 
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Jean-Francois Mezei <MEZEI_JF@Eisner.DECUS.Org> wrote:

>When an engine is "idle" (eg: when aircraft awaiting takeoff, when aircraft
>finally parks at the gate etc), how much thrust is being generated ?

>If brakes are not applied, will the aircraft start moving right away with
>engines "idling" ? When idling, do the compressor blades spin slowly enough to
>be seen ?

>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 ?

>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).

Mr. Mezei,

     Engine  thrust is a funtion of %rpm, ambient temp, and relative
humidity (air density).  Most jet engines idle at around 60%-70% of
max rpm.  Which, if no brakes are applied, is quite sufficient to move
the aircraft.  It must be noted that, even at idle rpm, the engine is
still rotating at several thousand rpm,  which is too fast to see
individual blades.
Jet engines may be started using several methods.  One of which is air
pressure.   Compressed air is forced through a small turbine which is
splined  via a gearbox to the engine.  When the small turbine rotates
fast enough and brings the engine to 18-28%rpm, you "light off" the
engine.  This is usually done by advancing the throttle lever to the
idle position., and if all goes well, the engine will start, and the
starter turbine will disengage at about 50% of engine rpm.   This air
pressure may be aquired from an APU,  bleed air from another engine,
or from some types of ground power units.
DC-9's and 727's ( and many other airliners ) have thrust reversers.
These, in the case of the -9 and 727, are a pair of doors on the
engine nacell which open out and rearward to form a "v" inthe exhaust
path, directing the airflow forward. Each airline and airport have
regulations dictating when, where, and under what conditions 'reverse
thrust' may be used to facillitate backing away from the terminal.
I hope that this has helped you, and if you have any further questions
or need further clarification about my reply, please feel free to
e-mail me or post to the news group.