Ok, so now that we have got that sorted out, let’s look a bit more closely at the propelling nozzle.
Most texts tell us that the nozzle produces forward thrust by accelerating the air rearwards.
But if we examine it closely we will find that the aerodynamic force on the nozzle is actually trying to tear it off the jet pipe and push it rearwards.
Anyone who does not believe this can try the following simple experiment.
Buy an ice cream cone and throw away the ice cream (OK you can eat it if you really must)
Now bite off the pointed end to leave a convergent nozzle.
Put the narrow end between your lips and blow through it.
You will find that a slow warm airstream comes out of it. (because it is a divergent duct)
Now put the wide end between your lips and blow through it.
This time you will get a faster colder airstream. (because it is now a convergent duct)
The above results are exactly what Bernouli would lead us to expect.
Now release your grip on the cone a little bit and blow really hard until you no longer hold the nozzle.
If the nozzle is producing thrust it will force its way into your mouth.
But it will actually fly out and away from you.
This is because the aerodynamic force on the nozzle is drag acting downstream.
So if the force on the nozzle is drag, how does it increase the thrust?
This question is not unrelated to the subject of this thread.