A few years ago at Arlington I saw a turboprop made from an APU. 150
hp, $20,000 or so, 18 GPH. Would have gone nicely on my Jodel, but
with a 15 gallon tank I sure wouldn't have gone far. Not too many were
sold, I think, but others were converted for small homebuilt
helicopters, where the power-to weight ratio was more welcome.
A small propeller has poor efficiency, especially in takeoff and
climb, so serious propeller-driven aircraft use large, slow-turning
props to get the most out of the available horses. It's more efficient
to accelerate a large volume of air to a low speed than a small volume
to a high speed, since prop drag increases with the square of the
increase in speed.
A small turbine has the same drawbacks. That small diameter has a
tiny area, so the gases must be accelerated to a really high speed to
get any useable thrust. That same small diameter also applies more
drag to the flow, the same way a small pipe impedes flow more than a
large one for a given rate of flow. The power turbine that converts
exhaust gas flow to shaft torque is similarly handicapped, so
efficiencies fall off dramatically as diameter goes down. The most
efficient turbines are the really big ones that are driving large,
slow-turning props or large fans (which are often also geared).
So for the money a piston engine is still a better bet, and
probably will be until some totally different principle is invented. I
wish we weren't still burning stuff (1600's steam engine technology)
to get motion, whether turbines or pistons or rockets, but I don't
suppose anyone will have a workable nuclear fusion engine, built by
Lycoming, in my lifetime.
It would probably still have magnetos.

Dan

Jeff wrote in message ...
there is a company making small turbo props, I cant remember the name of them,
but they have a 200 HP one

"Thomas J. Paladino Jr." wrote:

I have always wondered why there are no small GA turboprops. It seems like
most of the major problems & maintenance issues associated with GA aircraft
are related to the piston motor, and as far as I can tell, turboprops are
much more reliable, fuel efficient, smoother running and easier to maintain.

So it begs the question, why are there no small turboprops in the 100-300hp
range for use on GA aircraft? I would think that turbine engines of this
size would be relatively easy to produce, and would be ideal for GA
applications. The smoother operation and lower vibration levels would also
ease wear and tear on the entire airframe and avionics components. So what's
the deal? Does turbine technology not translate downwards very well? Would
it be cost prohibitive? Am I entirely missing something?