I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

I've been wondering about this ever since Machine Design's editor Ron
Kohl wrote in a recent column that he wasn't certain about it either.

Thanks guys,

Jeff


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

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

Jeff Wisnia wrote:
I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

I've been wondering about this ever since Machine Design's editor Ron
Kohl wrote in a recent column that he wasn't certain about it either.

It seems to me that the thrust couple from
the burner cans to the pylon. As far as
thrust goes, the burner cans are where it's
at. Everything else is just an auxilary.

"Jeff Wisnia" wrote in message
...
I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

I've been wondering about this ever since Machine Design's editor Ron
Kohl wrote in a recent column that he wasn't certain about it either.

Thanks guys,

Jeff


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

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


Wow. Good question. I don't know but I'm not afraid to put my foot in my
mouth.

Turbojets are all about having a low pressure at the front and high pressure
at the rear. Since the only things that have surfaces normal to the flow of
air are the turbines it pretty much has to be them. The rear turbines would
actually thrust backward -- the job of the rear turbines is to extract
energy from the airflow to turn the front turbines, which actually do the
compressing.

In article , Jeff Wisnia says...

I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Some of the thrust comes from the exiting gas stream, but
in modern high-bypass engines, there's a large fan that
is blowing air around the outside of the engine. I think
a large amount of thrust will be taken by the shaft bearings
for that bypass fan.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

Well we were taught in the USAF that over 45% of the thrust produced
by the GE 110 engines Turbo Fan (F-16 and F-15 acft) and the previous
P & W eninges as well as the 100 engines was produced by the fan that
actually blows air over the engine for cooling properties etc. The
balance came from the exhaust gasses out the rear, and what air was
not used directly to provide air for the combustion was bypassed from
these fans.


On Tue, 02 Mar 2004 14:30:55 -0800, Jim Stewart
wrote:

===Jeff Wisnia wrote:
=== I think I understand how a (non-turbofan) gas turbine jet engine works
=== and that the engine's thrust comes from an "equal and opposite" reaction
=== to lots of air molecules being flung out the rear at very high velocities.
===
=== What I'm not sure of is the specific path through which that thrust is
=== "collected" and makes its way to the engine pylon and thence to the
=== aircraft itself.
===
=== Is it mostly through the rear turbine rotor blades and their bearings,
=== and maybe the front compressor blades too?
===
=== I've been wondering about this ever since Machine Design's editor Ron
=== Kohl wrote in a recent column that he wasn't certain about it either.
===
===It seems to me that the thrust couple from
===the burner cans to the pylon. As far as
===thrust goes, the burner cans are where it's
===at. Everything else is just an auxilary.
===

Visit my website: http://www.frugalmachinist.com
Opinions expressed are those of my wifes,
I had no input whatsoever.
Remove "nospam" from email addy.

jim rozen wrote:

In article , Jeff Wisnia says...

I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.


Some of the thrust comes from the exiting gas stream, but
in modern high-bypass engines, there's a large fan that
is blowing air around the outside of the engine. I think
a large amount of thrust will be taken by the shaft bearings
for that bypass fan.

You might have missed my parenthetical "non-turbofan". It's pretty
obvious the thrust from those big fans has to go through the bearings,
same as it does on a turboprop engine. It's the plain turbojet which I'm
curious about.

I have the same kind of "whazzat" thoughts about ramjet engines, it's
hard for me to see what the thrust pushes against when both ends of the
engine are open.

Twas easier to understand things when I looked at the rusty remains of a
German V1 "Buzz Bomb" pulse jet engine circa 1961. It was lying on the
beach at Eglin AFB near where we were launching some scientific sounding
rockets. That crazy "Flying Stovepipe" had several louvre like shutters
inside the squared off front end. They flapped closed when the fuel went
off and sealed off the front.

So, for each pulse of flaming fuel, the thing acted in an easy to
understand way. When the flame went out, the shutters got pushed open by
the incoming air (maybe there were some springs on them too?) and the
cycle repeated.

Jeff

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

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

Jeff Wisnia wrote:

I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

I've been wondering about this ever since Machine Design's editor Ron
Kohl wrote in a recent column that he wasn't certain about it either.

Thanks guys,

Jeff

Basically the reactive force pushes the turbine spool forward, the
thrust acts on the bearings to transfer this force to the engine case,
where the mounts are. The mounts bolt to mounts on the pylon.

That reasonably clear? The engine pushes the gas backward, the gas
pushes the engine forward.

Cheers
Trevor Jones

Jim sez: "I think a large amount of thrust will be taken by the shaft
bearings
for that bypass fan."

A reasonable presumption wouldn't you say? And, said thrust registers on
the pylon, thence on the wing, etc. See Roy's answer, above, and remember
Roy was one of "them". All this impinges on the efficiency of liquid rocket
engines compared to solid fuel rocket engines - which have none of those
pesky blades in the ass end.

Bob Swinney





"jim rozen" wrote in message
...
In article , Jeff Wisnia says...

I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high
velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Some of the thrust comes from the exiting gas stream, but
in modern high-bypass engines, there's a large fan that
is blowing air around the outside of the engine.
Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

"Jeff Wisnia" wrote in message
...
jim rozen wrote:

In article , Jeff Wisnia says...

I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high
velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.


Some of the thrust comes from the exiting gas stream, but
in modern high-bypass engines, there's a large fan that
is blowing air around the outside of the engine. I think
a large amount of thrust will be taken by the shaft bearings
for that bypass fan.

You might have missed my parenthetical "non-turbofan". It's pretty
obvious the thrust from those big fans has to go through the bearings,
same as it does on a turboprop engine. It's the plain turbojet which I'm
curious about.

I have the same kind of "whazzat" thoughts about ramjet engines, it's
hard for me to see what the thrust pushes against when both ends of the
engine are open.

Twas easier to understand things when I looked at the rusty remains of a
German V1 "Buzz Bomb" pulse jet engine circa 1961. It was lying on the
beach at Eglin AFB near where we were launching some scientific sounding
rockets. That crazy "Flying Stovepipe" had several louvre like shutters
inside the squared off front end. They flapped closed when the fuel went
off and sealed off the front.

So, for each pulse of flaming fuel, the thing acted in an easy to
understand way. When the flame went out, the shutters got pushed open by
the incoming air (maybe there were some springs on them too?) and the
cycle repeated.

Jeff

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

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

Actually there is an alternating high and low pressure in the combustion
chamber of a pulse jet engine the frequency of which is mostly controlled by
the length of the tail pipe, think organ pipe & resonate frequency.

The low pressure event in the combustion chamber caused by the wave front of
the last combustion explosion moving down the tail pipe causes the valves to
open and to help suck in air and fuel for the next event.

Interesting that the V-1 engine produces enough thrust to carry a 1000 pound
warhead for a distance of several hundred miles but that the airframe would
suffer structural failure in about 10 hours of powered flight due to the
vibration of the engine. One or more were converted to carry a pilot for
testing flights.

Always thought an expermintal using a V-1 sized pulse jet would "get some
attention" at any airport.

Dr. Lipish, the designer of the Komet rocket powered airplane of WW-II lived
and worked in NE Iowa in the 1960's and was experminting with pulse jet
powered boats there. Fast, hot and loud! Some of his engines appeared to be
roughly the size of the V-1 engine.

Hugh
Old Dyna Jet speed flyer.

No one has mentioned the concepts of static and dynamic pressure exerted by
the gas. The static pressure changes as the gases move out from the
combustion chamber.
My guess is there has to be some sort of force component on walls and
chambers at right angles to the flow of gases. Since you don't have any
vertical brick walls so to speak the thrust forces would be distributed on
the walls of the passages as the speed and pressures vary. A ram jet would
have to have some sort of reactive force on the walls of the passages.
Just my 2 cents,
Randy

"Jeff Wisnia" wrote in message
...
I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

I've been wondering about this ever since Machine Design's editor Ron
Kohl wrote in a recent column that he wasn't certain about it either.

Thanks guys,

Jeff


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

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

On Tue, 02 Mar 2004 17:20:45 -0500, Jeff Wisnia
wrote:

I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

All the answers I've seen so far seem to be forgetting a few things.

1. If it's thrust against the front compressor blades, how did engines
with centrifugal compressors ever get an aeroplane off the ground?

2. What about afterburners? All they do is dump lots of fuel into the
hot gasses just behind the turbine. How does this increase thrust?

3. Many military engines have variable orifices at the very end of the
tailpipe to adjust thrust.

4. In plain rocket engines, like those on the shuttle, there are no
fan blades at all, but lotsa thrust.

So for engines that don't use bypass fans:

"Gas turbine engines for aircraft have an exhaust system which passes
the turbine discharge gases to atmosphere at a velocity in the
required direction, to provide the necessary thrust. The design of the
exhaust system, therefore, exerts a considerable influence on the
performance of the engine. The cross sectional areas of the jet pipe
and propelling or outlet nozzle affect turbine entry temperature, the
mass flow rate, and the velocity and pressure of the exhaust jet."

So I say the thrust is against that whole tailpipe assembly, including
the cone just behind the turbine. Probably the combustion chamber
takes some too.

See: http://www.geocities.com/nedu537/turbine/

"John Ings" wrote

1. If it's thrust against the front compressor blades, how did engines
with centrifugal compressors ever get an aeroplane off the ground?


Focus on the combustion chamber, and what goes on in it. There's high pressure
in the combustion chamber. There's a hole in the front for air to come in, and
a hole in the back for air to go out. The pressure of the air into, within, and
exiting the chamber is constant. What changes is its _temperature_, and
therefore its volume.

So you take a compressor of any kind -- even a piston compressor. You pump air
up to combustion chamber pressure with it. (If you don't pump it up to at least
that pressure, it won't go into the chamber.) The air you're pumping is cold,
so its volume is small, per unit mass. So you can shove it into the chamber
through a fairly small opening, at some velocity. In the chamber, you heat it
like hell, but at constant pressure. (It heats up at constant pressure because
you're allowing it to expand in volume.) The hot, expanded air leaves the back
end of the chamber.

We assume, for simplicity, that the air's velocity is the same going out the
chamber as it was coming in. Now, the _mass_ flow of air through the chamber is
constant. If you have a larger _volume_ flow out the back, because the air is
hotter, then you need a bigger hole in the back than you had in the front. So
look what you have: a pressurized chamber with a small hole in front, and a
large hole in back. Which way will it want to move?

Bottom line: the thrust comes from pressurized air pushing on the bigger area
at the front of the chamber. The front area is bigger because the front hole is
smaller. You get to pull off this neat trick because you burn lots of fuel to
_heat_ the air.

In a more realistic (but still cartoonish) jet engine, where the compressor in
front and the turbine in back are on the same spindle, you'll note that the
compressor pulls forward on the spindle, but the turbine pulls backward. The
_net_ force on the spindle bearings is the difference between them, and I bet
it's small, in practical engines.

-- Tony P.

You might have missed my parenthetical "non-turbofan". It's pretty
obvious the thrust from those big fans has to go through the bearings,
same as it does on a turboprop engine. It's the plain turbojet which I'm
curious about.
In a pure ram jet engine, the engine (and plane) have to get up to some
speed before the jet will work.
One way of doing this is to rocket launch the plane - like (I think) the
Regulus winged missile.
Lets guess the speed is 200 miles per hour.
The air coming in the front is compressed because the opening is cone
shaped with the smaller end at the burner site.
When the fuel is ignited, the pressure is suddenly increased inside the
chamber. The forces set up are forward - where the gases meet the
compressed wall of incoming air and the forward walls of the chamber,
around, where they meet the strong structure of the chamber and aft, where
they meet no resistance at all. This is unbalanced forces and the engine is
pushed forward, leaving the gases behind. The force is applied to the walls
of the burner chamber which is attached to the structure of the engine,
which is attached to the pylon, which is attached to the ankle bone, which
is attached to the shin bone ............. whoops
Now getting a passenger jet up to 200 mph without the engines running is
a bit tricky. So ....
Lets put a turbine in the path of those exhaust gases, which are just
blasting out the back anyway and connect it with a shaft to a compressor
wheel in front. Lets put a little starter motor to spin the thing up
perhaps. Now at 0 miles per hour plane speed, when we ignite the fuel,
pressure builds up in the chamber and because it is more open and there is
some pressure from the compressor, most of the gas goes out the back,
spinning the turbine, which increases the compression, which makes the
imbalance greater until there is enough force to move the plane.
As for increasing the compressor even more and bypassing air flow, see
other replies.


--
Mike Firth
Hot Glass Bits Furnace Working Website
http://users.ticnet.com/mikefirth/hotbit46.htm Latest notes


You might have missed my parenthetical "non-turbofan". It's pretty
obvious the thrust from those big fans has to go through the bearings,
same as it does on a turboprop engine. It's the plain turbojet which I'm
curious about.

I have the same kind of "whazzat" thoughts about ramjet engines, it's
hard for me to see what the thrust pushes against when both ends of the
engine are open.

Twas easier to understand things when I looked at the rusty remains of a
German V1 "Buzz Bomb" pulse jet engine circa 1961. It was lying on the
beach at Eglin AFB near where we were launching some scientific sounding
rockets. That crazy "Flying Stovepipe" had several louvre like shutters
inside the squared off front end. They flapped closed when the fuel went
off and sealed off the front.

So, for each pulse of flaming fuel, the thing acted in an easy to
understand way. When the flame went out, the shutters got pushed open by
the incoming air (maybe there were some springs on them too?) and the
cycle repeated.

Jeff

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

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

"Mike Firth" wrote in message
...

In a pure ram jet engine, the engine (and plane) have to get up to some
speed before the jet will work.

That's not true. If you start them by compressed air, or other means, they
will keep running even when sitting stationary, although they run much
better with speed. The low pressure in the combustion chamber after
ignition causes the valve to open and draw in air and fuel, which is then
ignited and the cycle repeats. Apparently many things influence their
operating frequency, but in small models its hundreds of cycles per second,
at least from what I've read. I gather that's the source of the "buzz"
sound.

Harold

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

No. It's been awhile but IIRC, the compressor stages compress the air
coming in the inlet. The compressed air is fed into the combustion chamber
where fuel is added and ignited. The rear turbine blades steal a bit of
this to drive the compressor stages and the rest exits as exhaust producing
thrust. I've never really thought about it before but I assume thrust is
acting on the combustion chamber itself much like a rocket engine.

Here's a good place to start digging if you REALLY want to understand what
goes on. :-)

http://www.grc.nasa.gov/WWW/K-12/air...btyp/ettp.html

Or you can back up to here for even more info:

http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html

Best Regards,
Keith Marshall


"I'm not grown up enough to be so old!"


"Jeff Wisnia" wrote in message
...
I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

I've been wondering about this ever since Machine Design's editor Ron
Kohl wrote in a recent column that he wasn't certain about it either.

Thanks guys,

Jeff


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

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

"John Ings" wrote in message
...
All the answers I've seen so far seem to be forgetting a few things.

1. If it's thrust against the front compressor blades, how did engines
with centrifugal compressors ever get an aeroplane off the ground?

When the air makes a right angle turn towards the combustor

2. What about afterburners? All they do is dump lots of fuel into the
hot gasses just behind the turbine. How does this increase thrust?

There's still pressure after the turbine so might as well add some air (um,
I've never heard of an engine being ran extra lean when afterburner is
added, but it would have to be, no?) and fuel, plus some extra exhaust
nozzle to make use of the burning, expanding gas and, um there you have it.
(I'm too tired to correct that paragraph gramatically.)

3. Many military engines have variable orifices at the very end of the
tailpipe to adjust thrust.

Probably something like putting your thumb over the end of the garden hose.

4. In plain rocket engines, like those on the shuttle, there are no
fan blades at all, but lotsa thrust.

But you *are* moving thousands of pounds of fuel from zero (relative the
engine) to several mach, aft-ward. That makes for a nice reaction force.
Same goes for *any* other jet engine, or fluid mover (propeller in air or
water) for that matter: the net effect is the fluid medium being thrown
backwards with respect to what's throwing it. Note that drag (parasite
drag, induced drag, drag of a turbine to spin the compressor, etc.)
displaces this air foreward, or at least less aftward than the thrust. So
thrust has to be that much more to counter it.

So for engines that don't use bypass fans:

"Gas turbine engines for aircraft have an exhaust system which passes
the turbine discharge gases to atmosphere at a velocity in the
required direction, to provide the necessary thrust. The design of the
exhaust system, therefore, exerts a considerable influence on the
performance of the engine. The cross sectional areas of the jet pipe
and propelling or outlet nozzle affect turbine entry temperature, the
mass flow rate, and the velocity and pressure of the exhaust jet."

Makes sense because for a given source of limited pressure and flow rate,
there is an ideal nozzle dimension which produces a maximum velocity output
(without compromising flow rate by restricting, nor pressure by being too
open). If you had some detailed spec's on the engine's output behavior, I
bet it'd be pretty easy to find with some calculus. (What can I say, I'm in
a calc. class, everything's starting to look like a derivative, erm, slope
now...)

So I say the thrust is against that whole tailpipe assembly, including
the cone just behind the turbine. Probably the combustion chamber
takes some too.

I would bet that the places of main thrust production are those most highly
pressurized: the compressor, because it's producing the pressure in the
first place; the combustor, because pressure again increases here; the
exhaust nozzle because the air is able to do more physical work before
exiting the engine.
In each place there are surfaces whose normals are pointed in the general
direction of thrust, although some mildly. Obviously these won't contribute
much thrust, instead having to simply retain their internal pressure. (Take
a piece of pipe for instance: blow air through it -- its walls are parallel
to the flow direction so despite the pressure inside it, what net force, if
any, is acting on the pipe? However, if you curved the pipe around 180° so
it points back at the source, the back side will be contributing a net
outward force. You might also be able to argue that the inside of the bend
is contributing "negative" force (um... double negative kind of "negative")
if the conditions are able to reduce its pressure below outside pressure.)

That was too long. I'm going to sleep. lol

Tim

--
"I have misplaced my pants." - Homer Simpson | Electronics,
- - - - - - - - - - - - - - - - - - - - - - --+ Metalcasting
and Games: http://webpages.charter.net/dawill/tmoranwms

Jeff Wisnia wrote in message ...
I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

In a quick nutshell, the thrust is applied to the mounts on the engine
casings through numerous paths including the burner cans, all the
rotating componets in the hot gas section and their associated
bearings, the stators between individual stages of the engine (ramjet,
scramjet and rocket engines excepted), case walls, between stage webs
and inlet structure as well as afterburner flame holders and any flow
straightening devices or nozzle aperature systems.

Afterburners generate additional thrust by burning raw fuel injected
in the exhaust stream to add temperature and flow mass to the engine
nozzle.

BTW..there's nothing like sitting on top of 30K+ pounds of thrust in
a 30K pound aircraft and being paid to run fuel through it...

Craig C.

Harold & Susan Vordos wrote:

"Mike Firth" wrote in message
...

In a pure ram jet engine, the engine (and plane) have to get up to some
speed before the jet will work.

That's not true. If you start them by compressed air, or other means, they
will keep running even when sitting stationary, although they run much
better with speed. The low pressure in the combustion chamber after
ignition causes the valve to open and draw in air and fuel, which is then
ignited and the cycle repeats. Apparently many things influence their
operating frequency, but in small models its hundreds of cycles per second,
at least from what I've read. I gather that's the source of the "buzz"
sound.

Harold

Harold, you're thinking of a pulse jet there, not a ram jet. A ram jet
uses the shock wave fron the inlet air as a barrier to the flame front
moving too far forward. No forward speed = no run at all. Pulse jets
have valves. Ram jets are pretty much open from one end to the other.

Cheers
Trevor Jones

Blow up a party balloon and let it go. What happens? The air rushing out
pushes the balloon along, but how does it do this?
The pressure of the air inside is distributed equally over the entire
internal surface, and across the hole where it comes out. The only way the
pressure across the orifice can be maintained is by accelerating the air,
thereby converting the potential energy stored in the air and the rubber
into kinetic energy in the moving air. The action at the orifice of
accelerating the air causes a reaction on the balloon that manifests itself
as thrust. And the thrust must be transferred to the balloon by the
distribution of the pressure to the internal surface. Inside the balloon
the air is stationary (relative to the balloon), but outside it is moving
rather quickly. Somewhere in between it is just on the point of moving, and
at that point the pressure is just on the point of dropping. That is the
point where the thrust is transferred. As soon as the air starts to move,
the pressure starts to drop.
In the jet engine, the thrust is transferred to the engine in the same way.
--
Regards, Gary Wooding

(Change feet to foot to reply)
"Jeff Wisnia" wrote in message
...
I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

Is it mostly through the rear turbine rotor blades and their bearings,
and maybe the front compressor blades too?

I've been wondering about this ever since Machine Design's editor Ron
Kohl wrote in a recent column that he wasn't certain about it either.

Thanks guys,

Jeff


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

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

"Craig" wrote in message
om...
Jeff Wisnia wrote in message
...
I think I understand how a (non-turbofan) gas turbine jet engine works
and that the engine's thrust comes from an "equal and opposite" reaction
to lots of air molecules being flung out the rear at very high
velocities.

What I'm not sure of is the specific path through which that thrust is
"collected" and makes its way to the engine pylon and thence to the
aircraft itself.

In a quick nutshell, the thrust is applied to the mounts on the engine
casings through numerous paths including the burner cans, all the
rotating componets in the hot gas section and their associated
bearings, the stators between individual stages of the engine (ramjet,
scramjet and rocket engines excepted), case walls, between stage webs
and inlet structure as well as afterburner flame holders and any flow
straightening devices or nozzle aperature systems.

Afterburners generate additional thrust by burning raw fuel injected
in the exhaust stream to add temperature and flow mass to the engine
nozzle.

BTW..there's nothing like sitting on top of 30K+ pounds of thrust in
a 30K pound aircraft and being paid to run fuel through it...

Craig C.


There's nothing_really_like having all four P&W R2800's at the firewall on a
full power run, rocking in the chocks, and then hitting the water/meth
injection switches on a cold day. Alll the BMEP indicators peg out and
there's not another sound like it. Long live round engines.

Garrett Fulton







-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
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I've never really thought about it before but I assume thrust [ in a turbojet
engine ] is acting on the combustion chamber itself much like a rocket engine.


The first is "internal reaction", the second is "external reaction".

"Trevor Jones" wrote in message
...
Harold & Susan Vordos wrote:

"Mike Firth" wrote in message
...

In a pure ram jet engine, the engine (and plane) have to get up to
some
speed before the jet will work.

That's not true. If you start them by compressed air, or other means,
they
will keep running even when sitting stationary, although they run much
better with speed. The low pressure in the combustion chamber after
ignition causes the valve to open and draw in air and fuel, which is
then
ignited and the cycle repeats. Apparently many things influence their
operating frequency, but in small models its hundreds of cycles per
second,
at least from what I've read. I gather that's the source of the "buzz"
sound.

Harold

Harold, you're thinking of a pulse jet there, not a ram jet. A ram jet
uses the shock wave fron the inlet air as a barrier to the flame front
moving too far forward. No forward speed = no run at all. Pulse jets
have valves. Ram jets are pretty much open from one end to the other.

Cheers
Trevor Jones

Yep, right you are! I stand corrected, and thanks for bringing it to my
attention. Must learn to read more carefully! :-)

Harold

But that's a pulse-jet engine (like the V-1), NOT a ram-jet engine,
which has NO 'valves' ... it's just an open pipe (of particular shape,
with a varying cross section as you traverse the tube).

Dan Mitchell
==========

Harold & Susan Vordos wrote:

"Mike Firth" wrote in message
...

In a pure ram jet engine, the engine (and plane) have to get up to some
speed before the jet will work.

That's not true. If you start them by compressed air, or other means, they
will keep running even when sitting stationary, although they run much
better with speed. The low pressure in the combustion chamber after
ignition causes the valve to open and draw in air and fuel, which is then
ignited and the cycle repeats. Apparently many things influence their
operating frequency, but in small models its hundreds of cycles per second,
at least from what I've read. I gather that's the source of the "buzz"
sound.

Harold

"Trevor Jones" wrote in message
...
Harold & Susan Vordos wrote:

A ram jet uses the shock wave fron the inlet air as a barrier to the
flame front
moving too far forward.

Not correct. Ram jets have flame holders (as do turbojet afterburners) to
keep the flame from moving forward -- and typically, the flame problem is
that it tends to move backwards -- it's called "blowing out". That's not
at all the way a ramjet works. A ramjet has a compressor -- it is called
the inlet. It is a converging-diverging nozzle. In supersonic airflow, the
mach number and air velocity DECREASES when going through a converging
nozzle (conversely, its speed increases through a diverging nozzle -- just
the opposite of subsonic flow. As the air velocity decreases, the air
pressure necessarily increases -- hence the compression. In an ideal
inlet, the mach number at the inlet throat is 1.00 so that the transition
from supersonic to subsonic flow is very gentle and the compression is the
most efficient it can possibly be. Generally, it is nigh impossible to get
the shock wave to stay at that ideal point and it is allowed to occur at
some higher mach number, such as 1.4 and forward of the throat -- Ramjets,
despite their superficial simplicity are very ticklish and tender beasts.
There's there the problem of getting it up to a speed where the compression
is adequate to run the engine. Then there's the problem that at subsonic
speeds, the inlet works the wrong way for supersonic speeds -- so geometry
and things must be adjusted as the jet speeds up. Practically speaking,
there aren't any realistic subsonic ramjets -- and no supersonic ramjet has
made it into serial production either. Nasty little beasties. You'd never
think an empty pipe (except for the flame holders, throat geometry, exhaust
nozzle geometry, etc.) would be so complicated.


No forward speed = no run at all.

That's correct. And hardly that below mach 1.

Boris

Who in his mispent youth got driven into computers trying to make sense out
of ramjet engine controls and who left the aircraft industry because the
task was nigh impossible.

-------------------------------------
Boris Beizer Ph.D. Seminars and Consulting
1232 Glenbrook Road on Software Testing and
Huntingdon Valley, PA 19006 Quality Assurance

TEL: 215-572-5580
FAX: 215-886-0144
Email bsquare "at" sprintmail.com

------------------------------------------

Years ago I saw a brochure from Rolls Royce as to how their engines worked. An
engineer who designed afterburners for them showed it to me.

They said that the way a turbine engine works is in four stages: suck, squeeze,
bang, blow.

I told him that a good date has the very same stages!

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

"Dan Thomas" wrote in message
m...
The ugliness of the turbine is its terrific cost.

And inefficiency, as I recall. No problem of course, when you're burning
cheap kerosene. (Or is JP-whatever jacked up in refinement and/or price?)

Great post, thanks.

Tim

--
"I have misplaced my pants." - Homer Simpson | Electronics,
- - - - - - - - - - - - - - - - - - - - - - --+ Metalcasting
and Games: http://webpages.charter.net/dawill/tmoranwms

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

Hey
Pure ram jet, not pulse jet, pure ram jet.

--
Mike Firth
Hot Glass Bits Furnace Working Website
http://users.ticnet.com/mikefirth/hotbit46.htm Latest notes

"Harold & Susan Vordos" wrote in message
...

"Mike Firth" wrote in message
...

In a pure ram jet engine, the engine (and plane) have to get up to
some
speed before the jet will work.

That's not true. If you start them by compressed air, or other means,
they
will keep running even when sitting stationary, although they run much
better with speed. The low pressure in the combustion chamber after
ignition causes the valve to open and draw in air and fuel, which is then
ignited and the cycle repeats. Apparently many things influence their
operating frequency, but in small models its hundreds of cycles per
second,
at least from what I've read. I gather that's the source of the "buzz"
sound.

Harold

"Garrett Fulton" wrote in message ..


There's nothing_really_like having all four P&W R2800's at the firewall on a
full power run, rocking in the chocks, and then hitting the water/meth
injection switches on a cold day. Alll the BMEP indicators peg out and
there's not another sound like it. Long live round engines.


Haven't had the thrill of 4 R2800's under my control, just 2 in the
A-26 that I used to crew....It's fun playing with the jets, doing
burner runs in our run stations, but you are right , radial motors and
big V's are a blast. I've got two aircraft projects with flat sixes,
one project with V-12's and some radial projects in works.... Now to
get the shop building up and moved into so I can get them all
airworthy........sigh......

Craig C.

"Dan Thomas" wrote

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.


Thanks for a truly informative post, Dan. A couple of questions:

1. Do you have a rough idea of the ratio of _axial_force_ on the compressor
disks vs. the turbine disks? Is the (backward) force on the turbine about 75%
of the (forward) force on the compressor, for instance?


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.


2. So, does this mean that in a turbofan engine the thrust really is indeed
mostly transmitted through the 10K+RPM spindle bearings? And are those
rolling-element bearings, or do they use some clever fluid-dynamical bearings in
modern engines?

-- Tony Prentakis

"tonyp" wrote in message ...
"Dan Thomas" wrote

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.


Thanks for a truly informative post, Dan. A couple of questions:

1. Do you have a rough idea of the ratio of _axial_force_ on the compressor
disks vs. the turbine disks? Is the (backward) force on the turbine about 75%
of the (forward) force on the compressor, for instance?

Coudn't answer that with any accuracy, but I would assume
something along those lines.


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.


2. So, does this mean that in a turbofan engine the thrust really is indeed
mostly transmitted through the 10K+RPM spindle bearings? And are those
rolling-element bearings, or do they use some clever fluid-dynamical bearings in
modern engines?

They are all rolling bearings, operating in sythetic oils
squirted in at low pressure.
The fan doesn't turn at 10,000 RPM. More like 3,000, depending on
model and size of engine. There's a gearset to reduce the turbine
speed and increase torque for the fan. The fan has similar limitations
to the airplane propeller, where the blade tip speeds have to be kept
under the speed of sound. The engine air inlet is designed as a
divergent duct, like the diffuser, to take the high-speed ram air (in
cruise) and slow it down and increase its pressure even before it hits
the fan (which is also the first compressor stage). Supersonic
airplanes use special intake designs that create a shock-wave aross
the inlet to slow the air to subsonic speed before compresor entry.
Don't tell modern passengers this, but that fan is a throwback to
propeller technology. Contained within a duct, it becomes much more
efficient, though, and many more blades can be added than would work
on an open propeller, and higher cruise speeds are possible. Straight
turbojet engines are really inefficient and noisy, while large
turbofans have efficiencies as good or better than piston engines. The
secret is diameter: bigger is better.

Check out
http://www.mtu.de/Projects/wi/cda/Mt...0-1--0,00.html

The fan gearset isn't visible but would be inside the fan hub. Many
are driven by an independent set of turbine disks via a coaxial shaft
withing the main spool shaft.

A much better explanation of pressures and combustion processes is
found he

http://www.chevron.com/prodserv/fuel...t_engines.shtm


Dan

Dan Thomas wrote:


snip of very informative post

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.

I would guess the turbine wheel.

You didn't mention "balance air".
The air bled from the compressor, & fed to the front of the turbine
wheel, to balance the loads.

--
Gary A. Gorgen | "From ideas to PRODUCTS"
| Tunxis Design Inc.
| Cupertino, Ca. 95014

Boris Beizer wrote:

"Trevor Jones" wrote in message
...
Harold & Susan Vordos wrote:

A ram jet uses the shock wave fron the inlet air as a barrier to the
flame front
moving too far forward.

Not correct. Ram jets have flame holders (as do turbojet afterburners) to
keep the flame from moving forward -- and typically, the flame problem is
that it tends to move backwards -- it's called "blowing out". That's not
at all the way a ramjet works. A ramjet has a compressor -- it is called
the inlet. It is a converging-diverging nozzle. In supersonic airflow, the
mach number and air velocity DECREASES when going through a converging
nozzle (conversely, its speed increases through a diverging nozzle -- just
the opposite of subsonic flow. As the air velocity decreases, the air
pressure necessarily increases -- hence the compression. In an ideal
inlet, the mach number at the inlet throat is 1.00 so that the transition
from supersonic to subsonic flow is very gentle and the compression is the
most efficient it can possibly be. Generally, it is nigh impossible to get
the shock wave to stay at that ideal point and it is allowed to occur at
some higher mach number, such as 1.4 and forward of the throat -- Ramjets,
despite their superficial simplicity are very ticklish and tender beasts.
There's there the problem of getting it up to a speed where the compression
is adequate to run the engine. Then there's the problem that at subsonic
speeds, the inlet works the wrong way for supersonic speeds -- so geometry
and things must be adjusted as the jet speeds up. Practically speaking,
there aren't any realistic subsonic ramjets -- and no supersonic ramjet has
made it into serial production either. Nasty little beasties. You'd never
think an empty pipe (except for the flame holders, throat geometry, exhaust
nozzle geometry, etc.) would be so complicated.

No forward speed = no run at all.

That's correct. And hardly that below mach 1.

Boris

Who in his mispent youth got driven into computers trying to make sense out
of ramjet engine controls and who left the aircraft industry because the
task was nigh impossible.

-------------------------------------
Boris Beizer Ph.D. Seminars and Consulting
1232 Glenbrook Road on Software Testing and
Huntingdon Valley, PA 19006 Quality Assurance

TEL: 215-572-5580
FAX: 215-886-0144
Email bsquare "at" sprintmail.com

------------------------------------------

Well, I guess I got taught the dumbed down, minimum physics version.
:-) Never dealt with the stuff at a practical level, but I know there
are no valves in one.

Cheers
Trevor Jones

have too,IIRC some guy makes and sells bikes/trucks that run on fixed up
helicopter ones, gulp gas but apparently the acceleration is smooth and
nice, as is the heat off the backside(prevents tailgaters)
so we don't see turbine-powered small airplanes or cars. Yet.

Hope this helps.

Dan