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
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.
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.
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
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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.
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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
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
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
velocities.
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
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:
"John Ings" wrote
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.
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.
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
When the air makes a right angle turn towards the combustor ;)
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.)
Probably something like putting your thumb over the end of the garden hose.
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.
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...)
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:
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.
snipped-for-privacy@ev1.net
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)
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
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