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

From: (Robert Westbrook)
Organization: Louisiana Tech University
Date:         12 Jul 95 01:42:33 
References:   1 2
Followups:    1 2 3 4
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In article <airliners.1995.1026@ohare.Chicago.COM>, Steve Lacker
<> wrote:

> wrote:
> <snip>
> >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.

   As far as I know, high bypass turbofans measure their thrust output
through the N1 (rpm of the fan) gauge, as opposed to EPR (engine pressure
ratio, which compares the static pressure of the tailpipe to static
pressure of the inlet, which I suppose could be considered ambient), which
is the best measure of thrust for medium bypass engines like the JT8, or
pure turbojets. RPM will of course increase when the throttle is put to
firewall, and the fan turning faster is the primary source of thrust in
fan engines. I don't think there is any tourquemeter to be found on a
modern high bypass engine, since all (or most) of the torque goes toward
spinning that fan.
   I've never heard of variable pitch fan or rotor blades in a turbine
engine, but there are variable pitch IGV's (inlet guide vanes) and stator
vanes, but these only serve to smooth out the flow of air into and thru
the engine and to prevent compressor stalls.
   The 707-321 I used to work on did have spring-loaded blow-in doors on
the inlets, or as I've heard some people call them, "auxillary air doors."
They would really start flapping if there was a mild oscillating
compressor stall... You could see them bouncing as you heard a "whoomp
whoomp whoomp whoomp..." :-)


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