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

I think that you can look at the market to see where the crossover
occurs.
THere are currently no production piston aircraft engines over 450hp
and
there are no aircraft turbines under 400hp.

There's lots of ground turbines under 400hp so we know there's a market
there; [...]

A ground turbine runs at almost constant speed, near its design point,
so even at small dimension can still be fuel efficient. Part load fuel
consumption of a gas turbine is a bit too high, particularly for GA
aircraft (considering their flight pprofile).

--
Fritz

"Fritz" wrote in message
...
A ground turbine runs at almost constant speed, near its design point,
so even at small dimension can still be fuel efficient. Part load fuel
consumption of a gas turbine is a bit too high, particularly for GA
aircraft (considering their flight pprofile).

Hmmm...define "fuel efficient"?

Your comment brings to mind the Toyota Prius hybrid engine. It essentially
has a "continuously variable transmission" that doesn't involve a
complicated, maintenance-hungry belt or chain and pully system.

I wonder if the answer to bringing turbine engines to small airplanes might
not be using a hybrid system. The weight of the batteries (which is
substantial) is offset by the relatively low weight of the rest of the power
train. The engine would only run during climbs, and when the batteries need
to be recharged. Biggest problem I see right off the bat is the problem of
starting and stopping the turbine frequently...my understanding is that
there are "issues" there, but I don't know what they are, or whether they
can be addressed by design.

Using such a system, a turbine could be run "at almost constant speed, near
its design point", while accomodating a variety of power settings.

All that said, someone else mentioned turbine-engined locomotives; that's a
much bigger power demand and yet somehow diesel-electric engines wound up
the standard. I suppose looking at the history of train locomotives might
offer some insight into how feasible hybrid technology might be for
airplanes. It might be that there are some unsolveable problems, or it
might be that we're at a stage in engine development now where things that
used to be problems aren't anymore.

Pete

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.

Firstly, all gas turbine rotors-compressor and turbine-are airfoils,
and that's why engine (and airframe) designs don't scale perfectly for
aerodynamic performance: air, in effect, has a finite size.

All engines are more efficient as the delta between the hot parts and
the cold parts increases, per Carnot. That said the primary reason car
engines run hotter than before is not because of direct thermal
efficiency but for emissions and also because smaller radiators can be
used.

Large turbines are actually designed to wear themselves in at times
as creepage brings the blades out and they ever so slightly lathe
themselves down, opening up the stator surface as they do. Active
clearance control using bleed air is another nifty feature of big
engines. Working against the economics of big fan engines is the fan
case being bigger than normal shipping means can handle. Also, a
serious FOD, midair, or controller failure resulting in an engine
writeoff is a much bigger capital hit:someone has to write a big
check.

The less the number of engines the greater probability that one
engine will fail. However the consequence of one engine failing
becomes directly higher. One engine out on a B-52 is a marginal
consideration: one engine out on a single turns your mission to worst
case recovery (find an airport, off airport landing, ditch, or
bailout/eject depending on the aircraft and terrain underneath.)

An exception to this rule is the light twin, most of which are really
1 1/2 engine airplanes to start with, and which tend to be flown by
hobbyists and part-timers. Statistics show that engine failures in
light twins kill more people than engine failures in singles, for a
lot of reasons. Cessna, Piper, and Beech knew this since roughly 1960
and their response for 25 years was, buy more liability insurance.
When high interest rates in 1986 made the reinsurance market for this
untenable, "they could no longer make reciprocating-engine aircraft".

ETOPS-Engines Turn Or People Swim!

Dean Wilkinson wrote:
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.

True, but the probability of losing all of the engines at the same time
is greater with only two engines as opposed to four.

Matt

Statistics show that engine failures in
light twins kill more people than engine failures in singles,

My understanding is that this statistic only applies to engine
failures that result in accidents. Left out are all the twins that had
engine failures and landed safely.

True, but the probability of losing all of the engines at the same time
is greater with only two engines as opposed to four.

Matt

Not necessarily...

There has never been a historical case of a twin engine jetliner
losing both engines at once due to unrelated failures. All twin
engine failures have been due to a common cause; fuel starvation being
the prime reason.

Here are some examples of related engine failures:

A four engine 747 had all four engines flame out at the same time when
it flew into the ash cloud of Mt. Redoubt in Alaska, and only managed
to restart three of them after losing over 10,000 feet of altitude.

A four engine Airbus A340 made a dead-stick landing at Lajes in the
Azores after running of fuel due to a combination fuel leak and fuel
system management problem.

A 767 (twin) made an emergency landing in Canada on a drag strip after
losing both engines due to a miscalculation during fueling.

The probability of an ETOPS plane losing both engines in a single
flight due to unrelated failures is extremely remote. That doesn't
mean it can never happen, but it is less likely than winning the
lottery.