At another thread / forum, in the mid of a discussion regarding a completely distinct subject ( or maybe not .. ) there was this post of mine where I refer to a video about the "internal mechanics" of a PT-6 turbine engine:

 

"

It includes a very good explanation of why turbines ( in this case a free-running PT-6 ) cancel torque so effectively:

https://youtu.be/hATrgAsgevY?t=139

The full video here:

https://www.youtube.com/watch?v=hATrgAsgevY"

I did send Austin Meyer the link to that video, and he promptly replied in the same base as @Murmur did in that thread:

which is totally correct ( Murmur knows what he's talking about 🙂 )

But this whole subject brought a question I have posed to many airline pilots, seeking for their insight into the physics of a jet / turbofan engine.

@Janov, in that same thread ( he's an airline pilot ) also raised it down the thread:

 

and, I thought it might be interesting to get more opinions from the community 🙂  So here's a dedicated thread ....

Interesting topic. I think Murmur has provided the explanation in his previous post.

In simple terms, airflow from a propellor is uncontained. The air flows backwards,  undergoing rotation, as well as flowing outwards which increases the torque effect. It's like increasing the length of a lever. It's worth noting that the torque effect from the propellor itself, which is a fairly light chunk of material being rotated, is minimal. The effect comes from the way it moves the air.

The older axial flow jets are using high velocity airflow through a relatively small diameter container, so the lack of torque is not surprising. Even the largest engines of that type have the same characteristics of no noticeable torque effect..

The more interesting case is the high bypass turbofan, because they are moving huge volumes of air at relatively low speed, quite seperate from the core of the engine and those fans can be of a huge diameter.  But unlike a propellor the airflow is still contained and then carefully directed by the stators, as Murmur pointed out, to minimize rotation of the flow. Further stabilisation is achieved by  venturi effect and guide vanes. End result, no torque effect.

 

The word "torque" is used (or misused) to describe a number of effects related to rotary motion, however if I understand the video correctly the guy's referring to the gyroscopic effect of the rotating parts of the engine, rather than any airflow or aerodynamic effects.  In most cases - with some notable exceptions such as the rotary engines in WWI fighters such as the Sopwith Camel - these effects are small compared to the mass (or  I should say moment of inertia) of the aircraft.  On the other hand the PT-6 gets fitted to some small airplanes, too.

Many many years ago I visited the Rolls Royce plant at Filton where they built the Pegasus engines for the Harrier.  The engineer giving us the tour went into some detail into the efforts made to cancel out the angular momentum of the two shafts, as the gyroscopic effects can apparently make manoeuvring in the hover quite interesting.  He gave me the impression that  it wasn't something that jet engine makers usually worried about.