[aphextron]

>A modern jet engine is just a shrouded propeller that happens to be powered by a jet turbine.

No, it's not. A propellor creates thrust by slicing the air and creating lift just like a wing, but in a forward direction. A jet engine's fan blades create no thrust in themselves. They provide the compression needed for combustion to occur within the engine, and thrust is created by the expelling of hot exhaust gasses.

[agurk]

That is a description of a turbojet - the earliest type of aero jet engines[0]. GP is referring to modern turbofans which are defined by having a bybass - i.e. air that is compressed and then expelled rearwards without passing through the main turbine body[1].

This is done to trade the high speed of the jet exhaust for a larger amount of air moving more slowly. Making the speed of the jet plume closer to the true airspeed of the aircraft makes it more efficient.

[0] https://en.wikipedia.org/wiki/Turbojet

[1] https://en.wikipedia.org/wiki/Turbofan

[agurk]

And to be more detailed, in any jet turbine, the compressor blades will create power as the energy spent compressing the air will be recovered when it's exhausted out of the tailpipe. So it's not quite correct to say they will produce no thrust.

Looking at design of turbines, a lot of air is needed above the amount for combustion to help keep the post combustion chamber turbine from melting (or just weakening to the point of failure). Some of this will be the unburnt (nitrogen, etc) components of the ingested air but a lot of design goes into moving air around components to move heat away from them.

The less air needs to be compressed for the engine then the more efficiently it will run. This is often achieved by designing one that can run hotter. This is also one of the other ideas behind having the bypass on a turbofan, reducing the amount of air that is compressed.

[darksaints]

No, the exhaust is not the primary thrust generator, unless you're talking about military jets. The fans themselves are creating the majority of the propulsion. Modern high bypass designs have bypass ratios of ~10:1, meaning for every 1 lb thrust generated from the exhaust, 10 lbs comes from the fan.

https://en.wikipedia.org/wiki/Bypass_ratio

[catalogia]

Note that modern military jets have low bypass ratios, but not zero. They use low-bypass turbofan engines, not turbojets. Even cruise missiles use turbofans rather than turbojets. I think the only place you might see a turbojet flying today is in RC scale aircraft.

[Tuna-Fish]

Most of (>80%) the thrust of modern turbofan engines is directly produced by the fan, which mostly just pushed air past the engine, not into it.

[throwawaysea]

The bypass part of a turbofan does generate thrust.