What's the thrust path in a jet engine?

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:

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.

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.

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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!

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

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:

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

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

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

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?

1. 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

< 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.

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