Why are multiple engines different?

Mxsmanic wrote:
Mark writes:

On a light twin, that second engine will have just enough power
to get you to the scene of the accident.

I keep reading that, but I wonder to what extent it's actually true.
Apparently some twins are much more handicapped by a lost engine than
others. It seems to me that if a twin is seriously crippled by the
loss of an engine, it may be better to just go with a single, since
the statistical probability of an engine failure is higher for a twin.
On the other hand, if the twin can fly in a useful way for a time even
after losing an engine, it would give you an extra margin of safety
over a single.

And there you have the crux of the arguments for and against twin
engine piston aircraft. In general, the fatality rate for twins is
higher than that of singles, until you include turboprops. In piston
aircraft, the basic function of a second engine is to give you somewhat
better performance at an enormous cost in fuel and safety. A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.

cjcampbell wrote:


And there you have the crux of the arguments for and against twin
engine piston aircraft. In general, the fatality rate for twins is
higher than that of singles, until you include turboprops. In piston
aircraft, the basic function of a second engine is to give you somewhat
better performance at an enormous cost in fuel and safety. A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.


It also gives you some redundancy in systems: generators, vacuum, etc...

cjcampbell writes:

A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.

Why are turboprops so much more expensive? I thought gas turbines
were supposed to be simpler and more efficient.

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Mxsmanic wrote in
:

Why are turboprops so much more expensive? I thought gas turbines
were supposed to be simpler and more efficient.

Look it up on Google and you will find your answer. Don't waste our time
on answers you can look up yourself. Isn't that what you told me to do????

Allen

Mxsmanic wrote:
cjcampbell writes:

A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.

Why are turboprops so much more expensive? I thought gas turbines
were supposed to be simpler and more efficient.

They are simple, but much less efficient than piston engines. Every
teaspoon of fuel has a fixed number of calories. Efficiency is measured
by what percentage of these calories is translated to thrust. The
reason turbines generate so much power despite their inefficiency is
that they can burn a lot more fuel even though they waste much of the
energy in the fuel. The inefficiency translates into incompletely
burned fuel, waste heat, exhaust, and pollution. Basically, this means
that you have to burn more fuel to generate 100hp in a turbine engine
than you do in a piston engine. A jet engine loses even more efficiency
in the translation of hp to thrust. A turboprop is more efficient than
a pure jet because of its propeller, but it still is not as efficient
as a piston engine. Turbines will probably never be as efficient as
piston engines. This is why gas turbine automobiles have never become
popular. People don't want a car that gets less than 10mpg unless it is
a Rolls Royce. Plus, acceleration is terrible. Chrysler built a batch
of gas turbine concept cars back in the early '60s and lent them to
ordinary consumers as a test. People hated them, not least because of
the annoying, high-pitched whine. I remember seeing them at car shows
back then. But, hey: it would burn anything -- gas, diesel, jet fuel,
vegetable oil, even perfume (and how long will it be before the price
of gas approaches that of perfume, either as fuel or otherwise -- and
what is it with cars and perfume, anyway?).

The reason we use jet engines is that they are inherently more powerful
and they can operate at high altitudes where the efficiency penalty
compared to piston engines is less. At high speeds, drag is a more
important factor in fuel economy than engine efficiency, so jet
airliners get their best fuel economy at high altitude. But for short
hauls where it would just be a waste of fuel to climb to high altitude
and descend again, a turboprop will deliver more power than a piston
engine with greater fuel economy than a jet.

cjcampbell wrote:
Mxsmanic wrote:
cjcampbell writes:

A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.
Why are turboprops so much more expensive? I thought gas turbines
were supposed to be simpler and more efficient.

They are simple, but much less efficient than piston engines.

Plus, parts are a lot more expensive and when things go very bad, the
maintenance costs are a lot more than a piston. That alone scares a lot
of operators off.

Ok, actually, I don't know much about turboprops, but that's the case
for turbofans.

Emily wrote:
cjcampbell wrote:
Mxsmanic wrote:
cjcampbell writes:

A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.
Why are turboprops so much more expensive? I thought gas turbines
were supposed to be simpler and more efficient.

They are simple, but much less efficient than piston engines.

Plus, parts are a lot more expensive and when things go very bad, the
maintenance costs are a lot more than a piston. That alone scares a lot
of operators off.

Ok, actually, I don't know much about turboprops, but that's the case
for turbofans.

From a maintenance standpoint, just think of a turboprop as being a
turbofan with a lot less blades.

cjcampbell wrote:
Emily wrote:
cjcampbell wrote:
Mxsmanic wrote:
cjcampbell writes:

A turborprop
increases safety, but now you are talking real money, both in
acquisition cost and in fuel and maintenance.
Why are turboprops so much more expensive? I thought gas turbines
were supposed to be simpler and more efficient.
They are simple, but much less efficient than piston engines.
Plus, parts are a lot more expensive and when things go very bad, the
maintenance costs are a lot more than a piston. That alone scares a lot
of operators off.

Ok, actually, I don't know much about turboprops, but that's the case
for turbofans.

From a maintenance standpoint, just think of a turboprop as being a
turbofan with a lot less blades.

LOL...yeah, the whole gas generator and power tubine thing seems a
little to complex. I'm sure at some point I understand it, but you
forget what you don't use.

All internal combustion engines work the same. A turbine
just does it as a series of continuous events in different
sections of the engine and a piston engine does one at a
time so power is produced only 1/4 of the time in a 4 cycle
and 1/2 the time in a two cycle.
I'm going to print some T-shirts...

"SUCK
SQUEEZE
BANK and
BLOW

Get your mind out of the gutter, it is an engine"


The P&W PT6 is perhaps the most popular turboprop. It uses
air coupling between the power and reduction gear section.
Makes it better in many ways, but there is a loss of
efficiency.


"Emily" wrote in message
...
| cjcampbell wrote:
| Emily wrote:
| cjcampbell wrote:
| Mxsmanic wrote:
| cjcampbell writes:
|
| A turborprop
| increases safety, but now you are talking real
money, both in
| acquisition cost and in fuel and maintenance.
| Why are turboprops so much more expensive? I thought
gas turbines
| were supposed to be simpler and more efficient.
| They are simple, but much less efficient than piston
engines.
| Plus, parts are a lot more expensive and when things go
very bad, the
| maintenance costs are a lot more than a piston. That
alone scares a lot
| of operators off.
|
| Ok, actually, I don't know much about turboprops, but
that's the case
| for turbofans.
|
| From a maintenance standpoint, just think of a turboprop
as being a
| turbofan with a lot less blades.
|
| LOL...yeah, the whole gas generator and power tubine thing
seems a
| little to complex. I'm sure at some point I understand
it, but you
| forget what you don't use.

cjcampbell writes:

The reason we use jet engines is that they are inherently more powerful
and they can operate at high altitudes where the efficiency penalty
compared to piston engines is less. At high speeds, drag is a more
important factor in fuel economy than engine efficiency, so jet
airliners get their best fuel economy at high altitude. But for short
hauls where it would just be a waste of fuel to climb to high altitude
and descend again, a turboprop will deliver more power than a piston
engine with greater fuel economy than a jet.

There are still the questions of simplicity and reliability, which I
thought were both higher for gas turbines. They are certainly more
reliable; and I should think they'd be simpler, too.

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