What's the thrust path in a jet engine?



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: http://webpages.charter.net/dawill/tmoranwms
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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/airplane/Animation/turbtyp/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 snipped-for-privacy@progressivelogic.com
"I'm not grown up enough to be so old!"

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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".
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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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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
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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
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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."
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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?

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
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Dan Thomas wrote:
< snip of very informative post >

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"
snipped-for-privacy@comcast.net | Tunxis Design Inc.
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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.
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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)

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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!
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