"David CL Francis" wrote in message
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Can you explain why the efficiency of turbines is much higher at
altitude? What sort of efficiency are you talking about?

Mainly the same reason turbocharged reciprocating engines operate more
efficiently at altitude. You're carrying around a compressor that just
isn't all that useful down low. Once you get higher, where there's less
drag, you get more "bang for the buck" out of the engine. Of course, as
Mike Rapaport pointed out, there's also the issue of efficiency with respect
to the size of the engine (independent of operating altitude).

The bottom line with respect to that point is that, for the purposes a
typical 100-300hp engine would be used, reciprocating engines are more
practical.

Pete