Why are there no small turboprops?

"Roger Halstead" wrote in message
...
On Tue, 25 May 2004 15:27:33 +0100, "Paul Sengupta"
wrote:

"Gerald Sylvester" wrote in message
ink.net...
Did you know you can buy true turbo jets for model aircraft? They
cost
about $3000 and give about 20 lb thrust, They are around 4" in
diameter.

I think you'll find they cost a *lot* more than that.

AMT's prices (excluding tax) start from 2972.00 Euros.
http://www.amt.nl/form_pricelist.php

Or build your own... :-)
http://www.artesjet.com/components.htm

As for fuel consumption, the above site gives a thrust of 92.3N. It gives
fuel
consumption of 0.154kg/N/H. So that's 14kg per hour. About 20 litres? What
is the specific gravity of propane? One of the pages for the Cobra engine on
the
Cri-cri says 0.8kg/N/H.

Paul

Sort of. The efficiency of a turbine engine is related to EGT (actually ITT
but we measure EGT and then compute ITT). The efficiency peaks at peak
temperature. In practice you are right because you generally can't reach
peak EGT at low altitudes since most turbine engines are flat rated. Also
because of the relationship between EGT and efficiency, turbines are really
inefficient at low power settings (with corrasponding low EGTs). As an
example, TPE331-10 engines (1020hp flat rated to 776eshp) use about
220lbs/hr at sea level just to keep the engine running (0 effective hp),
240lb/hr to produce 10% power and they use about 475lb/hr to produce 100%
power, so it takes half the fuel to produce 10% of the power. This makes
sense when you think about it. All the things that consume power
(compressor, gearbox, accesories) are consuming just as much power at 10% as
at 100% so the all the additional fuel is going into power production.

The airplane efficiency is related to altitude. It takes a lot less thrust
to move an airplane at any given TAS at 30K' where the are is only 30% as
dense than at sea level.

Mike
MU-2

"Peter Duniho" wrote in message
...
"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

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

On Thu, 27 May 2004 at 18:20:14 in message
.net, Mike Rapoport
wrote:

The airplane efficiency is related to altitude. It takes a lot less thrust
to move an airplane at any given TAS at 30K' where the are is only 30% as
dense than at sea level.

Help me here. I am struggling to find out more information but I have a
bit of a problem with that statement. I probably misunderstand what you
are saying and I may have it wrong I admit. In essence it is correct but
you normally fly at much higher TAS at altitude than at sea level.

It seems to me that if you want range you fly the aircraft at the AoA
that provides the best overall lift/drag ratio.

Let us suppose that is a ratio of 10. Then at the appropriate speed for
that height the drag (and therefore the thrust requirement) will be one
tenth of the weight, since in level flight lift must equal weight.
--
David CL Francis

"David CL Francis" wrote in message
...
[...]
Let us suppose that is a ratio of 10. Then at the appropriate speed for
that height the drag (and therefore the thrust requirement) will be one
tenth of the weight, since in level flight lift must equal weight.

How is that different from what Mike said?

He basically said, keep speed constant, and required thrust is reduced.
You're saying, keep required thrust constant, and speed is higher. Those
seem to me to be two ways of saying the same thing.

Aren't they?

Pete

It's not economical to use a small turbo contrasted to the payload.
"G.R. Patterson III" wrote in message
...


Morgans wrote:

And wear out in a few hundred hours, at best, and consume vast
quantities of
fuel.

If they wear out that fast, how well does that Cri-Cri fly on one engine?
Seems to me
that would be a real problem fairly regularly.

George Patterson
None of us is as dumb as all of us.

On Fri, 28 May 2004 at 16:25:22 in message
, Peter Duniho
wrote:

He basically said, keep speed constant, and required thrust is reduced.
You're saying, keep required thrust constant, and speed is higher. Those
seem to me to be two ways of saying the same thing.

What I was trying to point out (cautiously) is that talking about huge
reductions of drag at altitude may be misleading. If you fly at best A0A
all the time then the drag is almost independent of altitude. If that is
correct then the work done per mile is also constant and range is also
almost independent of altitude. But that sounds horribly revolutionary.
:-(

Doing that you would always get there faster at altitude.
--
David CL Francis

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?

AOA and L/D curves are not based on TAS but CAS.

"David CL Francis" wrote in message
...
On Thu, 27 May 2004 at 18:20:14 in message
.net, Mike Rapoport
wrote:

The airplane efficiency is related to altitude. It takes a lot less
thrust
to move an airplane at any given TAS at 30K' where the are is only 30% as
dense than at sea level.

Help me here. I am struggling to find out more information but I have a
bit of a problem with that statement. I probably misunderstand what you
are saying and I may have it wrong I admit. In essence it is correct but
you normally fly at much higher TAS at altitude than at sea level.

It seems to me that if you want range you fly the aircraft at the AoA
that provides the best overall lift/drag ratio.

Let us suppose that is a ratio of 10. Then at the appropriate speed for
that height the drag (and therefore the thrust requirement) will be one
tenth of the weight, since in level flight lift must equal weight.
--
David CL Francis

(Rick Durden) wrote in message om...
Thomas,

There are some smaller turboprops but the economics just aren't there
to put them into mass production. The market is quite small and the
specific fuel consumption of a turbine versus a piston means that if
you put it on your Cessna 210, for example, you can't carry enough
fuel to get anywhere and still put people in the cabin. Plus, the
simple cost of the technology and the metals to handle the heat in a
turbine engine makes it almost impossible to compete with other types
of engines in that horsepower range.

Actually there is a turboprop conversion for the P210 called the Silver Eagle.
(see http://www.onaircraft.com). The stats look pretty good to me. Cruising
speed of 215 kts and a range of 1030 nm with a full fuel payload of 735 pounds,
or a range of about 780 nm with a payload of 930 pounds (if you don't use the
aux tank added in the conversion). You can even use reverse thrust to back up.
Of course it doesn't come cheap though.