What's the thrust path in a jet engine?

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?

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

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

1. 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

and maybe the front compressor blades too?

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,

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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

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