Why are there no small turboprops?

On Mon, 24 May 2004 00:28:44 -0500, Greg Copeland
wrote:

On Mon, 24 May 2004 05:16:31 +0000, Shiver Me Timbers wrote:

Thomas J. Paladino Jr. wrote:

I have always wondered why there are no small GA turboprops.

GOOGLE is your friend.

http://www.mauleairinc.com/Our_Planes/Maule_M-7-420AC/

I've always wondered the same. To take his question and run with it, why
are small turbo props not the defacto engine used throughout small GA
planes?

Seems to me that a variety of small jets and turbo props could be made,
which are just as safe and have slightly better performance envelopes than
currently exist while having less failures and vibration to boot.

Is simple economics the answer? $30k piston versus something like $80k
turbine, or something like that?

Turbines drink fuel like crazy at low altitude. They are more
reliable, run smooth, and have more reserve power than piston
engines, but what a thirst.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

On Thu, 27 May 2004 at 02:26:52 in message
, Roger Halstead
wrote:
Turbines drink fuel like crazy at low altitude. They are more
reliable, run smooth, and have more reserve power than piston
engines, but what a thirst.

Can you direct us to any quantitative information on this please?
--
David CL Francis

"Shiver Me Timbers" wrote in message
...
Thomas J. Paladino Jr. wrote:

I have always wondered why there are no small GA turboprops.

GOOGLE is your friend.

http://www.mauleairinc.com/Our_Planes/Maule_M-7-420AC/

Yeah, but that Maule is a pretty big and specialized aircraft, and the
turbine is 420hp; I specified from 100-300hp. Enough to power the average
Cessnas and Pipers, or any other average GA plane. The Maule is almost more
on par with a Cessna Caravan, not a 182 or 172.

Either way, I'll rephrase the question a bit; why are there not more small
turboprops available for GA, and why are they not standard accross more
models?

Thomas J. Paladino Jr. wrote:

why are there not more small turboprops available for GA,
and why are they not standard accross more models?

Well as the newsgroup knows.... I'm just a little armchair lurker,
but if I had to stick my tongue out and make a guess, I would
say that right across the board from purchase, operational costs
which includes that dirty word fuel, to that pesky but necessary item
called insurance that the biggest reason that you can't find those
turbine jobbies is because of that dreaded word.... money.

"Thomas J. Paladino Jr." wrote in message The Maule is almost more
on par with a Cessna Caravan, not a 182 or 172.

The Maule M-7 series is about the size of a C-172. It is a 4 seater. The
Caravan can have up to 14 + 2 seats.

Either way, I'll rephrase the question a bit; why are there not more small
turboprops available for GA, and why are they not standard accross more
models?

In general, turboprop engines don't match fuel consumption with piston
engines until over the 300 horsepower range. The metals required for a
durable, efficient turbine engine are expensive. If a manufacturer produced
a turbine such as you describe, airframe manufacturers would be clamoring
over each other to install them on their airframes. You don't see it
happening because such an engine doesn't exist (yet).

D.

"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

On Sun, 23 May 2004 at 23:35:35 in message
, Peter Duniho
wrote:
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.

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

"David CL Francis" wrote in message
...
[...]
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

On Mon, 24 May 2004 at 17:31:12 in message
, Peter Duniho
wrote:
"David CL Francis" wrote in message
...
[...]
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).

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.

--
David CL Francis

"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