"Thomas J. Paladino Jr." wrote in message
...
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?

I just read Peter Garrison (of Flying Magazine) claiming that, no they don't
scale down well. On the other hand, we just saw in this newsgroup links to
the Cri Cri twin turbojet airplane, so obviously it can be done.

Some issues however... Turbines operate much more efficiently at altitude,
and planes with less than 300 hp (especially those with significantly less
than 300 hp) just aren't flown that high normally. Also, while they are
more reliable, they are also more complex in certain ways (in spite of the
fundamental concept being simpler), and they are more finicky about proper
operation.

With respect to your understanding of turbine engines:

* "Much more reliable" -- probably true, but turbine engines in service
are almost exclusively operated under a different maintenance standard than
most piston engines. It's hard to do an apples to apples comparison.

* "Fuel efficient" -- not down low where most light airplanes are
flying. If the engine isn't significantly compressing the intake air, the
turbine is doing a lot of unnecessary work, wasting fuel in the process.

* "Smoother running" -- without a doubt. But this is probably lowest on
the engine priority list, and piston engines can be made that run pretty
smoothly as well.

* "Easier to maintain" -- for whom? My mechanic might have a
theoretical understanding of turbine engines, with some small amount of
practical experience (for all I know), but I am sure that he doesn't work on
them on a regular basis. I don't even know where I'd go to get a turbine
engine worked on, but I'll bet wherever it is, it costs a LOT more than my
mechanic charges.

Of course, the biggest reason is probably simply the one related to
certifying small turbines for light plane use. The turbines that *have*
made it to small aircraft are ones that are already certified for other
installations, and are higher power than what you're talking about. As far
as I know, no one's certified a 100-300hp turbine engine for any airplane,
so the first one is going to be really expensive, and will require a lot of
sales just to break even.

As for the theoretical advantages you mention, I'm not convinced those would
be as significant as you're implying, nor that they would offset the added
expense of going with a turbine. As far as I am aware, engine vibration has
a negligible effect on airframe health, and on avionics lifetime. Heat due
to poor ventilation kills avionics much more than engine vibration does, and
most modern avionics are pretty hard to kill in the first place. Airframes
break after they are overstressed, or they corrode, or they are flown in
heavy turbulence for tens of thousands of hours. I've never heard anyone
suggest that engine vibration breaks airframes.

I would guess that weight would be the biggest real advantage for using a
turbine, but that may be offset by having to carry more fuel (it certainly
is in the existing single-engine turbine variants), and certainly would be
offset by the added complication of changes to the aircraft design to
accomodate the change in weight distribution and other things required to
work with a turbine engine.

Bottom line: to reiterate what I wrote above, I suspect the single biggest
reason turbines aren't used is expense. For a Normal certificate airplane,
the certification process for the first small turbine would cost a fortune.
For experimentals, it sounds like (from one of George's earlier posts) that
people ARE looking to incorporate small turbines into light airplanes, but I
doubt it's cost effective. As near as I can tell, for a given horsepower,
turbines are simply more expensive and for sure it's harder to find someone
qualified to work on them.

Maybe one day they'll be ubiquitous in a wide variety of applications, and
they'll start showing up in light airplanes too. But it seems to me that
until there's a huge market for certificated low-horsepower turbine engines,
no one's going to bother working on them.

Pete