In article k.net,
Rick writes:
wrote:


Ok, that makes sense as far as it goes...I'm a little surprised
at the low speed of that turbine though but then, I'm used to
aircraft turbines that rotate much faster,

The LM2500 is an aeroderivative engine, that is it was born
to fly as the TF39 that powers the C5 transports. It was
adapted to stationary and marine use.

The power turbine sits behind the turbine that drives the
compressor. Just like the fan was driven in the aircraft
version the gases leaving the turbine drive the power
turbine at a much lower speed.

O.K. That's a Free-Turbine Turboshaft. The output shaft is driven by
its own separate turbine, and, therefore, can be sized to turn at
whaver combination of torque & RPM you want, within certain limits.
The Gas Generator (Compressor/turbinw spools, and the burners) are
then able to turn at whatever speed is best for them. I'll bet it's
quite a bit faster.
Many turboprops are set up the same way - the output shaft is run off
its own turbine, and is separate from the Gas Generator. You can see
this on some commuter airliners - they'll be sitting on the ramp,
whining away, with the props stationary.

As I remember it, Gord was an FE on airplanes with the Allison T56.
That's a bit different - there's only one shaft, and the output to the
gearbox, compressor and turbine are all rigidly connected.
In that case, the whole engine turns at some serious RPM.
(Something like 13,000). A neat deal with teh T56 is that in flight,
it's basically a constant speed engine - the propeller pitch changes
to keep the RPM constant, while the torque varies, and the fuel
control varies the temperature to produce the desired torque.

Take a google at a turbofan engine and you will see what I
mean.


Most aircraft turbines driving props
rotate at 13,000 - 14,000 RPM and that's much too fast for props
so you need reduction gearing.

You are a bit slow, the smaller engines turn around 40,000 rpm.

Rick


--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster