Re: Do airliner engens run at approximately constant speed, varying only torque?

From:         Steve Lacker <>
Organization: applied research laboratories
Date:         10 Jul 95 16:36:53 
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  or MIME structure wrote:
>When one of two engines quits and the pilot decides to ask the other engine
>for 95% power instead of something in the 60% range, is it the case that
>the RPMs don't increase much, in fact they might decrease when the plane
>inevitably slows down, but that fuel flow of course increases?

This is pretty much the case for *turboprop* engines, where the propellers have
variable pitch blades (in fact, piston engine aircraft with variable props run
pretty much at constant rpm). When more power is needed, the pitch is changed
to take a bigger "bite" of air, the throttle is opened more (an anachronism for
a turbine, I know ) and the engine produces more torque keeping the revs
constant. As far as I know, turbojets and turbofans don't have the luxury of
variable pitch. I defer to those who know more about turbofan design, because
there may be another way to modify thrust without changing turbine speed too
much, or else (more likely?) modify the combustion dynamics to allow good
efficiency across a wider band of actual turbine shaft speeds. (bleed doors,
etc.???) I'd like to know the answer to that one myself.

Steve Lacker	/	Applied Research Laboratories, The University of Texas
512-835-3286	/	PO Box 8029, Austin TX 78713-8029