Thanks, I'm starting to feel that there's no single "right answer" to my
original question about what components the thrust acts through to
finally push on the airframe.

Probably if there were one, then given your position you would know it
for sure and wouldn't have qualified your statement about where the
pressure is concentrated. (The compressor disks.)

Thanks again, I've learned quite a bit more from your's an other's posts
on this subject.

Jeff

--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"My luck is so bad that if I bought a cemetery, people would stop dying."


Dan Thomas wrote:

Lots of misconceptions about turbine engines. I teach a College-level
course on Aircraft systems, and the turbine is one of the subjects.

The axial-type compressor is a series of fan disks, with stator
(stationary blades) disks between each rotating disk to redirect the
air thrust back by each stage. In moving air back it is accelerated,
and the stators, besides removing the rotating action of the air and
directing it at a given angle into the next stage, slows the air and
therefore increases its pressure. After enough stages, perhaps 8 to
13, the air has reached a pressure of 350 psi and is directed into a
diffuser, which is a divergent duct that slows the air and thereby
increases its pressure further. The maximum pressure in the engine is
at this point, believe it or not. Airflow speed is in the neighborhood
of 30 feet per second.
This air enters the combustor can (or cans) through various
holes, and fuel is sprayed by injectors into the airflow and ignited.
Once lit, it stays lit, and only about 25% of the oxygen is consumed.
The rest of the air is directed over the combustor can surfaces to
keep flame off them, or they'd burn out quickly.
Combustion increases volume which is converted into velocity, NOT
pressure. If the pressure was to rise at this point, the air would
blow back out the compressor and stall it. Pressure drops a bit as the
air moves through the combustors. The hot, high-speed gases are run
through the turbine stages, which are more rotating blade disks with
stators in front of and between them to direct flow. Various air
channels are built into the engine and through shafts and blades to
keep them relatively cool, or the hot gases would destroy them. Some
use tiny air holes that squirt cooler air over each blade surface to
keep the combustion gases away from the metal.
The turbine section drives the compressor, and extracts about 75%
of the energy from the gas flow in doing it. The remaining velocity
and pressure is what drives the engine forward. If I was to say where
the pressure is concentrated, I'd have to say it's against the
compressor disks.
Turboprop, turbofan and turboshaft engines have more turbine
stages to remove almost all the remaining energy and use it to drive a
fan or prop or helicopter transmission. In a high-bypass turbofan as
used on newer airliners, the fan produces most of the thrust. Four or
more times as much air goes around the engine as goes through it.
Some smaller engines use centrifugal compressors, one or two
stages, and many use a hybrid compressor setup that has three or four
axial compressor stages and a centrifugal compressor. Some engines are
"free turbines," in which there are two separate compressors and two
turbine sections, with coaxial shafts so that the second turbine stage
drives the first compressor stage. Easier to start. Many turboprop
engines are free turbines, with one or two stages of turbine driving
the compressor, and a second set of turbines, not connected in any
mechanical way to the first, that drive the prop through a gearbox.
Again, easier to start. A example is the Pratt and Whitney Canada PT-6
series of engines used in airplanes like the Beech King Air,
deHavilland Twin Otter, Cessna Caravan, Piper Cheyenne, and many
others.
The beauty of the turbine engine is its reliability. Unlike the
piston engine, there are no reciprocating parts, and the pressures in
the engine are relatively constant so that the fatigue that piston
engine suffer isn't there. A typical piston aircraft engine has a
useful life of between 1500 and 2400 hours, sometimes more, but the
turbine is good for at least 3500 and some have run 10,000.
The ugliness of the turbine is its terrific cost. Because of the
high rotational speeds (66,000 RPM or more in small engines and 10,000
in the biggest) everything has to be finely balanced and very strong.
Metals are rather exotic, to take the heat and forces, and machining
is very expensive. The bigger they are, the more efficient they get,
so we don't see turbine-powered small airplanes or cars. Yet.

Hope this helps.

Dan