Sort of. The efficiency of a turbine engine is related to EGT (actually ITT
but we measure EGT and then compute ITT). The efficiency peaks at peak
temperature. In practice you are right because you generally can't reach
peak EGT at low altitudes since most turbine engines are flat rated. Also
because of the relationship between EGT and efficiency, turbines are really
inefficient at low power settings (with corrasponding low EGTs). As an
example, TPE331-10 engines (1020hp flat rated to 776eshp) use about
220lbs/hr at sea level just to keep the engine running (0 effective hp),
240lb/hr to produce 10% power and they use about 475lb/hr to produce 100%
power, so it takes half the fuel to produce 10% of the power. This makes
sense when you think about it. All the things that consume power
(compressor, gearbox, accesories) are consuming just as much power at 10% as
at 100% so the all the additional fuel is going into power production.

The airplane efficiency is related to altitude. It takes a lot less thrust
to move an airplane at any given TAS at 30K' where the are is only 30% as
dense than at sea level.

Mike
MU-2

"Peter Duniho" wrote in message
...
"David CL Francis" wrote in message
...
[...]
But is that efficiency? I would have thought that efficiency was
measurement by a parameter like pounds of fuel used per effective shaft
horsepower per hour. That certainly changes with altitude but not so
much.

Sorry, I didn't realize this was a scientific forum, where there's only
one
definition of "efficiency".

Are you trying to say that turbine engines are just as efficient to use at
the lower altitudes as they are at higher altitudes? I would disagree
with
that. If you're not saying that, I'm at a loss as to what your point is.

Even if you want to measure efficiency only by something like specific
fuel
consumption, small turbines still don't win out, regardless of altitude.
They are inherently inefficient, due to reasons already mentioned in this
thread.

Or looked at another way, a low horsepower engine intended for use only at
lower altitudes is too small to be efficient, while one intended for use
at
higher altitudes will be severely derated when operated at low altitudes
if
the engine is to provide sufficient power at the higher altitudes, which
is
again, a waste (and waste implies low efficiency).

In aviation (or any other application, for that matter), you cannot look
simply at one single aspect of efficiency. For an engine to be viable, it
needs to provide an overall efficiency greater than competing engines.
Low
horsepower turbines simply don't meet that requirement, and for an
installation intended to be flown at higher altitudes, the overall
efficiency suffers at lower altitudes.

We are talking about the real world here, not a laboratory.

Pete