Why are there no small turboprops?

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

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 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

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

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