Re: Thrust in idle engines

From: (Brad Gillies)
Organization: Internex Online, Toronto, Ontario, Canada (416 363 3783)
Date:         03 Nov 95 04:23:13 
References:   1
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In article <airliners.1995.1626@ohare.Chicago.COM>,
   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 ?

Very little. Aturbine engine delivers very little thrust while sitting at
idle. They only begin to deliver significant amounts of thrust when given a
little push above around 60-65%.
>If brakes are not applied, will the aircraft start moving right away with
>engines "idling" ? When idling, do the compressor blades spin slowly enough
>be seen ?

Most large airliners will noot move when the brakes are released at idle power
as the generated thrust is not enough to overcome the inertia of the
stationary aircraft. Small aircraft like learjets will move. The large
airplanes will apply "breakaway thrust " to get the aircraft started rolling.

>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 ?
The APU supplies compressed air to start the engines. The air drives a
pneumatic starter which spins the HP compressor up to around 25% (I know
pilots usually put fuel around 15%) at this point fuel and ignition are
applied and the engine starts. The starter itself is a turbine of sorts. The
compressed air spins it at around 40,000 RPM. This rotation spins the gearbox
which spins the compressor.
Small turbines (like the apu) use electric starters due to the decreased load
on the starter.

>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).
"Powerbacks" for DC-9 and 727 aircraft are allowed because the engines are
high enough off of the ground to avoid ingesting debris from the gorund.
Also the reverser system allows for this because ALL of the exhaust flow is
diverted. On a high bypass engine only the cold stream exhaust is diverted.
This causes a problem with forward thrust and reverse thrust at the same time.
Before I get all kinds of arguments about that one, I know they work when the
airplane lands. It just causes alot more debris to fly around the ramp
besides, the ground crew would not appreciate being sucked into an engine
running at 80% in reverse.

Brad Gillies
A&P, AME (Canada), PPASEL (Canada)
Leading Edge Aviation Services