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I think I read somewhere that the forces on a jet engine shaft is neutral because the forces at the turbine and compressor cancel out. This means that the axial bearings don't have force on it in direction of thrust!?

Is this true? In that case I think the thrust will be zero because there's noting against which the pressure can push to produce thrust.

enter image description here

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  • $\begingroup$ Not necessarily. Some jets doesn't use a straight nozzle so the thrust vector is not aligned with the shaft, with the extreme cases like VTOL airplanes (e.g. F-35) that the thrust is perpendicular to the shaft. Even when this is the case, the pressure behind the fan and compressor equalizes itself. "Cancel" here means there's no imbalance, not no force. $\endgroup$ – user3528438 Jul 10 at 20:37
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Thrust is produced in the compressor; as its blades push the air backwards to compress it, a reaction force pushes the blades forwards. These blades are fixed to the shaft. The shaft has a flange on it which pushes against a similar face on the engine mounting, in an arrangement known as a thrust bearing. This in turn transmits the thrust to the airframe.

A turbofan has extra big blades which push more air backwards, adding to the thrust.

The turbine blades draw work from the exhaust flow which slows it down, so they push against the main thrust and in a simple single-spool design can even require the thrust bearing to face backwards.

Heating the compressed air in between is what makes the thing work at all. Also, the acceleration of air out the back of the combustion chamber creates a thrust reaction against its forward wall. For a pure turbojet, and especially for a centrifugal compressor, this will be greater than the compressor thrust. It might not be as much as one might suppose because the chamber is relatively small.

For supersonic flight, the air intake acts as a shock compressor to generate most of the thrust, while the exhaust nozzle can be made to generate more by expanding high-pressure exhaust against a divergent nozzle. At high enough speeds the spinning bits just get in the way.

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  • $\begingroup$ @ guy inchbald,link: youtube.com/… he said turbine has more force backwards than compressor has forward and so thrust at thrust bearing is backwards!!!?? this cant be true.. $\endgroup$ – Aeronautic Freek Jul 11 at 13:07
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    $\begingroup$ If that was true, a bucket reverser in the tailpipe of a turbojet would have little effect wouldn't it. Or a vectored thrust nozzle. The F-35 wouldn't be able to hover, because there is all that forward thrust coming from its compressor deep in the engine so what's the point of pointing the tail pipe down? Most of the trust is happening at the exhaust nozzle, which is why you can point it in different directions and get the thrust axis you want by where you point it. Even the Harrier is the doing the same thing with its fan flow directed out nozzles. $\endgroup$ – John K Jul 11 at 13:07
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    $\begingroup$ Also if this was true, a centrifugal compressor turbo jet wouldn't make much thrust at all would it, with the spinny wheel thingy flinging air out. No "lift" there at all. $\endgroup$ – John K Jul 11 at 13:20
  • $\begingroup$ How does a ramjet work with no compressor? $\endgroup$ – Organic Marble Jul 11 at 14:30
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    $\begingroup$ I was thinking "what the heck is that word... imp-something... ah screw it, I'll go with spinny wheel" lol $\endgroup$ – John K Jul 11 at 14:42
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In a pure turbojet pretty much all the thrust is coming from the expansion of gasses from heat released by the burning of fuel in the compressed air stream. The acceleration of the gasses out the burner can and on to the tailpipe nozzle is the primary action/reaction going on that makes thrust (and mostly in the convergent nozzle of the tailpipe).

The thrust force itself is not "pushing" on anything. It's gas that is expanding and has only one way to go, so it accelerates that way and you have your Newtonian action/reaction. Like letting go of a balloon. The thrust comes from the acceleration of air, being compressed by the balloon's rubber, out the ballon's mouth after you let it go.

Thrust loads from the compressor are largely offset by drag loads on the turbine, so the spool is sort of trying to stretch itself all the time, although there may be a net forward thrust component to the extent that the compressor's thrust load exceeds the turbine's drag load, and the ball bearings will be designed to handle whatever axial load there is. I've never found any source that says that any thrust surplus produced by the compressor itself is significant in the total thrust of the engine.

Also think of a turbojet with a centrifugal compressor. No blades making forward lift, just curvy channels flinging air out. How could you get any thrust from that? The centrifugal turbojet is getting its push from the same place as an axial one; between the turbine and exhaust nozzle.

On a turbofan, a significant part of the energy in the exhaust flow is extracted by a separate turbine to make torque to drive the fan. A turbofan is more or less a free-turbine turboprop with an un-geared, fixed pitch propeller with oodles of blades. There's a bit of thrust out the tailpipe from expanding gas energy surplus to what was extracted by the fan's turbine, and perhaps a bit of surplus thrust load from the core compressor, but most is from the fan, and in that case the fan's forward bearings will be designed to handle most of the engine's thrust capability.

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  • $\begingroup$ action-reaction dont explain how fluid(air) exert force to the jet engine at all.air can exert force to the object only through pressure ,so integration of pressure at blades and walls of jet must show force,if you have two fans on same shaft ,once pull to the left one to the right with same force(shaft is neutral) now pressure integration is zero ,so thrust is zero...so if you said that shaft has almost zero load ,pressure must push aginast some walls in jet engine ,maybe at nozzle walls.. $\endgroup$ – Aeronautic Freek Jul 11 at 6:01
  • $\begingroup$ Of course compressors produce thrust. I have no books about jet engines as such, but Kermode (Mechanics of Flight) notes that the turbine produces drag but the turbine-plus-compressor is what produces thrust at zero airspeed. Think; each compressor blade works as a airfoil by "lifting" forwards to push air backwards. Think too that if that did not work, then a bypass turbofan would not work either; compressors, fans and propellors are all spinning wings doing much the same job. Also, why else would the shaft be in tension yet need a thrust bearing at the front? $\endgroup$ – Guy Inchbald Jul 11 at 9:11
  • $\begingroup$ @guy inchbald,are talking to me or to John K,because i have oppinion same like you... $\endgroup$ – Aeronautic Freek Jul 11 at 12:08
  • $\begingroup$ Of course the compressor produces a thrust load, but much of it is offset by the turbine, and whatever net thrust force in the spool remains is small in relation to the acceleration of the mass flow generated by heat expansion from fuel. In a pure turbojet most of the thrust is generated at the exhaust nozzle which converges to force the mass to accelerate further. A turbofan's, or a turboprop's, or a turboshaft's, torque is coming from all the energy still present AFTER the compressor's turbine, which is the majority of the energy produced by the engine. $\endgroup$ – John K Jul 11 at 12:51
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    $\begingroup$ So then, how do you get thrust with a centrifugal compressor? $\endgroup$ – John K Jul 11 at 15:50
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The forces almost cancel, but not quite. You might have 10,000 lbf forward and 9,500 lbf aft, for a net of 500 lbf forward. This is intentional. If the thrust on the bearings is too low, that's actually bad. It can lead to a phenomena called "skidding". Ball bearings are designed to work with some finite amount of axial load. Too much is bad, because it leads to increased stress, but too little is also bad. Engine designers can tweak the rotor thrust by adjusting various parts of the design, such as seal diameters (the part where the rotating shaft meets the stator). There will be some high pressure seals in the turbine, with maybe 750 psi on one side and 1000 psi on the other. If you increase the seal diameter a little, that adds some rotor thrust, and if you decrease the seal diameter is decreases it. The diameter will be adjusted to get the desired rotor thrust.

Edit: To clarify... I'm thinking here of the core rotor on a turbofan. I guess if you are thinking of a pure turbojet, then this might not apply

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  • $\begingroup$ if forces cancle then thrust is zero.did you see my picture with load distrubution in jet turbine? $\endgroup$ – Aeronautic Freek Jul 11 at 16:42
  • $\begingroup$ @AeronauticFreek: There is enough structure left besides the spinning blades against which the air pushes in order to create thrust. The diffusor is one such place and the nozzle another one. In supersonic engines the intake produces most of the thrust. $\endgroup$ – Peter Kämpf Jul 11 at 16:55
  • $\begingroup$ @Peter Kampf, does that mean that all walls inside jet engine which produce forward thrust,must "facing" forward even a little bit,because pressure act only prepedicular to the surface? $\endgroup$ – Aeronautic Freek Jul 11 at 17:38
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    $\begingroup$ if the forces on the rotor cancel, then the net force on rotor is zero. the net force on the entire engine is not zero. The rotor is only part of the engine. $\endgroup$ – Daniel K Jul 11 at 18:14
  • $\begingroup$ @AeronauticFreek: Yes, like a rocket chamber. $\endgroup$ – Peter Kämpf Jul 11 at 18:43

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