Diesel aircraft engines and are the light jets pushing out the twins?

wrote in message
...
And it looks like the small jets are pushing the turbo props and
the twin piston engines. Is it a matter of time before it will
be cheaper to just buy a small jet?

I'm not holding my breath on that one.

Me either. But if you believe the manufacturer's claims, it is just a
matter of time. Several of the "mini jet" designs under development are
cheaper than the existing turboprop models, single or twin, and cost about
the same as new piston twins.

Of course, you can't actually buy any of them right now, and it remains to
be seen what they will actually cost if and when they make it to market.

What puzzles me is why there doesn't appear to be anyone working on
turbines in the range of 160 to 250 HP for aircraft.

GA Flyer just included an "engines in development" article as part of their
Oshkosh coverage, and had a picture of exactly that, as well as a mention in
the article of the company producing the small turbines (I think they said
all for turboprop installations).

They are out there...you just need to look. Don't get distracted by the
lack of certificated engines, or lack of interest in certification. Not all
of the engine research and development going on is aimed at the certificated
market.

Pete

"Peter Duniho" wrote in message
...
wrote in message
...
And it looks like the small jets are pushing the turbo props and
the twin piston engines. Is it a matter of time before it will
be cheaper to just buy a small jet?

I'm not holding my breath on that one.

Me either. But if you believe the manufacturer's claims, it is just a
matter of time. Several of the "mini jet" designs under development are
cheaper than the existing turboprop models, single or twin, and cost about
the same as new piston twins.

Of course, you can't actually buy any of them right now, and it remains to
be seen what they will actually cost if and when they make it to market.

What puzzles me is why there doesn't appear to be anyone working on
turbines in the range of 160 to 250 HP for aircraft.

Small turbines are inherently inefficient so you are unlikely to see them in
this power range. The fuel consumption might be double that of a diesel.

Mike
MU-2



GA Flyer just included an "engines in development" article as part of
their
Oshkosh coverage, and had a picture of exactly that, as well as a mention
in
the article of the company producing the small turbines (I think they said
all for turboprop installations).

They are out there...you just need to look. Don't get distracted by the
lack of certificated engines, or lack of interest in certification. Not
all
of the engine research and development going on is aimed at the
certificated
market.

Pete

"Mike Rapoport" wrote in message
news
Small turbines are inherently inefficient so you are unlikely to see them
in this power range.

You're not listening. I already HAVE seen them in that power range. The
likelihood of having done so is irrelevant, since it's already happened.

The fuel consumption might be double that of a diesel.

It might be be, I don't know. Nevertheless, they do exist...

Pete

I meant that you are unlikely to see them on production aircraft. Sorry I
wasn't clearer.

Mike
MU-2


"Peter Duniho" wrote in message
...
"Mike Rapoport" wrote in message
news
Small turbines are inherently inefficient so you are unlikely to see them
in this power range.

You're not listening. I already HAVE seen them in that power range. The
likelihood of having done so is irrelevant, since it's already happened.

The fuel consumption might be double that of a diesel.

It might be be, I don't know. Nevertheless, they do exist...

Pete

"Mike Rapoport" wrote in message
nk.net...
I meant that you are unlikely to see them on production aircraft.

I'll buy that.

Though, I wouldn't go so far as to say it could never happen. Who knows?
Maybe there's an application where reduced weight or increased reliability
is more important, or perhaps the "fundamental" inefficiencies of small
turbines will turn out to not be so fundamental after all.

But you are right, for now the existing low-power turbines show no sign of
being targeted for certified, production aircraft.

Pete

"Peter Duniho" wrote in message
...
"Mike Rapoport" wrote in message
nk.net...
I meant that you are unlikely to see them on production aircraft.

I'll buy that.

Though, I wouldn't go so far as to say it could never happen. Who knows?
Maybe there's an application where reduced weight or increased reliability
is more important, or perhaps the "fundamental" inefficiencies of small
turbines will turn out to not be so fundamental after all.

But you are right, for now the existing low-power turbines show no sign of
being targeted for certified, production aircraft.

Pete


My understanding is that the reason that small turbines are less efficient
than large ones is because of efficiency losses at the tips of both the
compressor and turbine and the internal drag of the engine surfaces. Both
of these issues get more pronounced as things get smaller. Similiar to a
pipe with a cross sectional area of 1"sq flowing less than half as much
fluid as one with a cross section of 2" sq.

That is not to say that the engines will not get more efficient, it just
means that small turbines will be less efficient than large ones.

Mike
MU-2

Small turbines are inherently inefficient so you are unlikely to see them in
this power range. The fuel consumption might be double that of a diesel.

It's not true, first off. Although bigger engines have advantages of
Reynolds numbers and such, small and large are relative terms. The
relationship of BSFC of heavy diesels and industrial gas turbines in
steady state peak operation is pretty constant across engines from the
size of an 855 cid Cummins to the really big guys with four foot
bores. The turbocharged diesels are somewhat more efficient but
nowhere near 2:1.

The "secret" of linearizing gas turbine performance across a wide
range of output power is thermal feedback, or regeneration. Look
carefully at the real progenitor of the Cruise Missile turbojet...

On 29 Sep 2004 12:35:58 -0700, (Ted Azito)
wrote:

Small turbines are inherently inefficient so you are unlikely to see them in
this power range. The fuel consumption might be double that of a diesel.

It's not true, first off. Although bigger engines have advantages of
Reynolds numbers and such, small and large are relative terms. The
relationship of BSFC of heavy diesels and industrial gas turbines in
steady state peak operation is pretty constant across engines from the
size of an 855 cid Cummins to the really big guys with four foot
bores. The turbocharged diesels are somewhat more efficient but
nowhere near 2:1.

The "secret" of linearizing gas turbine performance across a wide
range of output power is thermal feedback, or regeneration. Look
carefully at the real progenitor of the Cruise Missile turbojet...

Spent my life around turbine aircraft, so I don't know a thing about
large piston engines. I don't understand what you mean by "Reynolds
numbers and such" I thought that Reynolds numbers are used in airfoil
calculations, but I may be wrong.

From my experience, a turbine is most efficient when operated near its
max temperature. That's why we cruise them at over 95% RPM. The other
way we can operate them efficiently is to go high - drag goes way down
and the thrust required goes down with it. One of the major
improvements to efficiency has been because of the metallurgy and
running them at a higher temperature. Years ago we used 150 degree
thermostats in our cars. They're probably at least 200 degrees today.
The only reason - better thermal efficiency (gas mileage).

A turbine engine doesn't have any touching parts in the working
sections. What that means is there are huge air gaps between blades,
rotors etc. In other words, no piston rings. Static, you can blow
right through them. The tips of the rotating parts don't touch
either, so there are gaps. I'm no engineer, but I would think that
as a turbine gets smaller the ratio of air leak to "working stuff"
would be greater and there would reach a point that fuel specifics
wouldn't be in your favor. That's probably why most of the small
engines down to micro-turbines use centrifugal compresses instead of
axial flow. In other words, the centrifugal, by design, leaks less.

There is a reason that airliners are almost all two-engine. A large
engine of 100,000 lbs thrust is much more efficient than two 50,000 lb
engines. That's why they are parking 747's and buying 777's. It's
not the cost of the engines. Over their service life, the engine
costs are nothing compared to the fuel they burn.

The increased fuel efficiency of the 777 engines is not strictly due
to their size. They are a newer generation design with very high
bypass and advanced FADEC controllers. The 747 engines are an older
design. Also, the 777 is a more aerodynamically efficient airplane
than the 747.

The other big advantage of two engines vs. 4 is cost of ownership in
terms of maintenance and spares. Its less expensive to maintain two
engines per plane than 4. Also, statistically speaking, the
probability of an engine failure per flight hour is lower for the 777
than it is for the 747 since it has fewer engines to fail. Believe it
or not... this was demonstrated to me when I worked at Boeing on the
777 development.

Dean

Don Hammer wrote in message . ..
On 29 Sep 2004 12:35:58 -0700, (Ted Azito)
wrote:

Small turbines are inherently inefficient so you are unlikely to see them in
this power range. The fuel consumption might be double that of a diesel.

It's not true, first off. Although bigger engines have advantages of
Reynolds numbers and such, small and large are relative terms. The
relationship of BSFC of heavy diesels and industrial gas turbines in
steady state peak operation is pretty constant across engines from the
size of an 855 cid Cummins to the really big guys with four foot
bores. The turbocharged diesels are somewhat more efficient but
nowhere near 2:1.

The "secret" of linearizing gas turbine performance across a wide
range of output power is thermal feedback, or regeneration. Look
carefully at the real progenitor of the Cruise Missile turbojet...

Spent my life around turbine aircraft, so I don't know a thing about
large piston engines. I don't understand what you mean by "Reynolds
numbers and such" I thought that Reynolds numbers are used in airfoil
calculations, but I may be wrong.

From my experience, a turbine is most efficient when operated near its
max temperature. That's why we cruise them at over 95% RPM. The other
way we can operate them efficiently is to go high - drag goes way down
and the thrust required goes down with it. One of the major
improvements to efficiency has been because of the metallurgy and
running them at a higher temperature. Years ago we used 150 degree
thermostats in our cars. They're probably at least 200 degrees today.
The only reason - better thermal efficiency (gas mileage).

A turbine engine doesn't have any touching parts in the working
sections. What that means is there are huge air gaps between blades,
rotors etc. In other words, no piston rings. Static, you can blow
right through them. The tips of the rotating parts don't touch
either, so there are gaps. I'm no engineer, but I would think that
as a turbine gets smaller the ratio of air leak to "working stuff"
would be greater and there would reach a point that fuel specifics
wouldn't be in your favor. That's probably why most of the small
engines down to micro-turbines use centrifugal compresses instead of
axial flow. In other words, the centrifugal, by design, leaks less.

There is a reason that airliners are almost all two-engine. A large
engine of 100,000 lbs thrust is much more efficient than two 50,000 lb
engines. That's why they are parking 747's and buying 777's. It's
not the cost of the engines. Over their service life, the engine
costs are nothing compared to the fuel they burn.

I don't disagree with you Dean on the spares issue etc., but as I see
it is you add up the cost of the engine and spares and then the fuel
used over 40K hours or so and see where the real $ are.

I also don't disagree about two different vintage engines; newer are
more efficient, but look at the SFC on a CF6 of about 40K thrust and a
CF34 of 9K thrust (same vintage) and I bet the larger engine has the
upper hand; maybe not.

Good discussion subject and it would be interesting to hear from
others who know what they are talking about. Obviously anything I say
comes from being around these things, not because I profess to be any
kind of authority. Any aircraft turbine engineers out there? Neat
thing about aviation is there is always something to be learned.