Re: Jet Engine Rev Ups for Take Off

From:         nak@gwe486.cb.att.com ()
Organization: AT&T GBCS/Bell Labs
Date:         17 Aug 95 04:58:49 
References:   1
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In article <airliners.1995.1241@ohare.chicago.com>,
Mark Wiklund  <Mark_Wiklund@hmco.com> wrote:
>Having watched jet departures from the open air decks at
>Logan airport since a boy, and more recently having done
>much travel, I have always noticced that when a jet
>revs up for take off, it seems to go to half-power
>for a second or two before proceeding to take-off
>power - as if it was a two step process. Is this half-step
>process with all engines, or spooling up engines
>individually, or simply a misperception on my part?


Note that in a carburetted piston engine, fuel lags behind air.  You open
the throttle, letting in more air, which picks up more fuel.  In a turbine
engine, air lags behind throttle.  You add more fuel and RPM come up giving
more air.  Carb'd engines starve a bit when you punch it at low RPM.
Turbines go rich and risk dousing the flame with liquid fuel.


What you observed varies by jet engine, and I don't have all of the
specifics handy.  But in general, it's difficult to make a turbine engine
that will accept sudden throttle transitions.  In particular, they don't
like going from low to high throttle settings too fast.  Flameout is the
 big worry.

The Me262 was known for this.  Piston trained pilots wanted to slam the
throttle all the way forward when it came time for air combat, and this
would flame out an engine.

The F14 Tomcat was given new engines which were free from restrictions in
throttle movement, a welcome change.  For example, you could go directly
into afterburner without waiting for RPM to climb.

I would expect that some engine management systems might decouple the
throttle inputs from being directly tied to fuel output.  This would allow
the engine controller to add more fuel as RPM rose.  This allows the pilot
to set the throttles as a "request" and then they can concentrate on
something other than slowly advancing the throttle according to RPM.

---
Neil Kirby	DoD# 0783	nak@archie.cb.att.com
AT&T Bell Labs  Columbus OH     USA (614) 860-5304
President Internet BMW Riders
The BMW R1100RSL - Because the Britten V 1000 is not street legal.