Mxsmanic wrote:
cjcampbell writes:

A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.

Why are turboprops so much more expensive? I thought gas turbines
were supposed to be simpler and more efficient.

They are simple, but much less efficient than piston engines. Every
teaspoon of fuel has a fixed number of calories. Efficiency is measured
by what percentage of these calories is translated to thrust. The
reason turbines generate so much power despite their inefficiency is
that they can burn a lot more fuel even though they waste much of the
energy in the fuel. The inefficiency translates into incompletely
burned fuel, waste heat, exhaust, and pollution. Basically, this means
that you have to burn more fuel to generate 100hp in a turbine engine
than you do in a piston engine. A jet engine loses even more efficiency
in the translation of hp to thrust. A turboprop is more efficient than
a pure jet because of its propeller, but it still is not as efficient
as a piston engine. Turbines will probably never be as efficient as
piston engines. This is why gas turbine automobiles have never become
popular. People don't want a car that gets less than 10mpg unless it is
a Rolls Royce. Plus, acceleration is terrible. Chrysler built a batch
of gas turbine concept cars back in the early '60s and lent them to
ordinary consumers as a test. People hated them, not least because of
the annoying, high-pitched whine. I remember seeing them at car shows
back then. But, hey: it would burn anything -- gas, diesel, jet fuel,
vegetable oil, even perfume (and how long will it be before the price
of gas approaches that of perfume, either as fuel or otherwise -- and
what is it with cars and perfume, anyway?).

The reason we use jet engines is that they are inherently more powerful
and they can operate at high altitudes where the efficiency penalty
compared to piston engines is less. At high speeds, drag is a more
important factor in fuel economy than engine efficiency, so jet
airliners get their best fuel economy at high altitude. But for short
hauls where it would just be a waste of fuel to climb to high altitude
and descend again, a turboprop will deliver more power than a piston
engine with greater fuel economy than a jet.