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

From:         king@reasoning.com
Date:         07 Jul 95 14:26:24 
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Help me out a bit with a problem i'm having understanding aircraft
turbines.

It is my understanding that the rotation speed of a turbine can't vary
much if you care about fuel efficiency lest the compression ratio vary as
well, either overheating the engine [when the compression ratio is "too
high"] or making the combustion happen into too cold an airstream letting
the Carnot efficiency go into the dumps at least for the first bolus of
fuel that burns in each packet of air [when the compression ratio is "too
low"].

Is this correct, that the turbine speed is always just a wee bit more than
that speed that would be implied by the fan's geometry and the plane's
air speed [to add momentum to the air stream] and that that's of course
relatively constant?

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?

At the low speeds near takeoff i assume that engine efficiency indeed
suffers and everyone accepts this?  Or do airliner bypass fans have
variable pitch?

What measurement is presented to the pilot as an indication of the power
he has asked for and what he has received?  Turbine inlet temp, perhaps?

-dk