A jet engine's shaft is neutral in the direction of thrust?

  • #1
I think i read somewhere that jet engine shaft is neutral,because
forces at turbine and compressor
cancle out,so that mean axial berings dont have force on it in direction of thrust!?
(this cant be true,becsue in that case thrust will be zero...)

But if this is true ,against what pressure produce thrust??
 

Answers and Replies

  • #3
he said turbine has more force backwards than compressor has forward and so thrust at thrust bearing is backwards!!!??
this cant be true..

(i think i must put this topic at classic physics,here is not people to talk about,moderator can you do a switch?)

 
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  • #4
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I think i read somewhere that jet engine shaft is neutral,because
forces at turbine and compressor
cancle out...
The expansion of the combustion gasses react against the blades of the turbine, pushing it upstream (towards the compressor).
The shaft(s) transfers torque to the compressor(s), which does work over incoming air, pushing it downstream (towards the turbine).
That air reacts pushing the compressor upstream (same direction the turbine was pushed).

For both, turbine and compressor, there is more pressure downstream than upstream; hence, both are impulsed upstream.
You need thrust bearings that transfer that forward mechanical impulse from the shaft(s) to the rest of the airplane.

Please, read:
https://en.wikipedia.org/wiki/Components_of_jet_engines

:cool:
 
  • #5
The expansion of the combustion gasses react against the blades of the turbine, pushing it upstream (towards the compressor).
i think you are wrong, blades from turbine has force pointing backwards,so forces at blades of turbine and compressor act in opposite direction,just force on compressor is greater .
thrust=force at compressor - force at turibne

see in animation lift is pointg backawards on turbine blades (2:22sec)
 
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  • #7
689
404
i think you are wrong, blades from turbine has force pointing backwards,so forces at blades of turbine and compressor act in opposite direction,just force on compressor is greater .
thrust=force at compressor - force at turibne
I may be wrong, as many times before. :smile:
It seems to me that the orientation of the blades of that compressor in the animation has been incorrectly represented.

Nevertheless, consider these factors conditioning the shape of those blades:

1) The turbine is extracting all useful work it can from the stream of combustion gases: those blades are being driven.

2) The compressor is giving all work it can to the stream of incoming air: those blades are driving a compressible fluid.

Please, see these three links:

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

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

https://ocw.mit.edu/ans7870/16/16.unified/propulsionS04/UnifiedPropulsion9/UnifiedPropulsion9.htm

:cool:
 
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  • #8
berkeman
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(i think i must put this topic at classic physics,here is not people to talk about,moderator can you do a switch?)
No, this is the best forum for this question. I see that you have now cross-posted the question in the other forum. I will merge that in here now. Please do not ever cross-post or multiple post the same question.
 
  • #9
BvU
Science Advisor
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how diffuser and combustion chamber can produce forward thrust
pressure decreases as gas speed increases (Bernoulli) so the force against the front is higher than against the rear
 
  • #10
I may be wrong, as many times before. :smile:
It seems to me that the orientation of the blades of that compressor in the animation has been incorrectly represented.

Nevertheless, consider these factors conditioning the shape of those blades:

1) The turbine is extracting all useful work it can from the stream of combustion gases: those blades are being driven.

2) The compressor is giving all work it can to the stream of incoming air: those blades are driving a compressible fluid.

Please, see these three links:

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

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

https://ocw.mit.edu/ans7870/16/16.unified/propulsionS04/UnifiedPropulsion9/UnifiedPropulsion9.htm

:cool:
if we look at my picture,turbine backward thrust is greter than compressor forward thrust,that mean shaft push in thrust bearing in backward direction!!!
 
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  • #11
689
404
if we look at my picture,turbine backward thrust is greter than compressor forward thrust,that mean shaft push in thrust bearing in backward direction!!!
Then the airplane moves backwards?
 
  • #12
Then the airplane moves backwards?
NO :)
it seems that difusser and chamber walls produce lots of thrust forward and nozzle backward so net force is forward..

but difusser walls is facing bakcwards so it is unclaer to me how can produce thrust forward,because pressure act only prepedicular to wall surface..
so all this is very strange..
 
  • #13
689
404
NO :)
it seems that difusser and chamber walls produce lots of thrust forward and nozzle backward so net force is forward..

but difusser walls is facing bakcwards so it is unclaer to me how can produce thrust forward,because pressure act only prepedicular to wall surface..
so all this is very strange..
Again, the function of the turbine wheel and blades is to steal energy from the combustion gases that are rapidly expanding and trying to produce the reactive forward thrust (same reactive thrust that you will get in a ram jet).

Because the interaction with those turbine blades, those gases lose some energy in form of pressure; hence, they produce less forward thrust than the amount they could without that thief turbine.

That turbine is a necessary evil.
Without it, you could not increase pressure and temperature (via compressor) enough to induce ignition and combustion of the gases in an engine that is not moving forward (zero velocity incoming air).
 
  • #14
Again, the function of the turbine wheel and blades is to steal energy from the combustion gases that are rapidly expanding and trying to produce the reactive forward thrust (same reactive thrust that you will get in a ram jet).

Because the interaction with those turbine blades, those gases lose some energy in form of pressure; hence, they produce less forward thrust than the amount they could without that thief turbine.

That turbine is a necessary evil.
Without it, you could not increase pressure and temperature (via compressor) enough to induce ignition and combustion of the gases in an engine that is not moving forward (zero velocity incoming air).
i find here answer:

https://books.google.hr/books?id=bgN2cAxF9OAC&pg=PA27&lpg=PA27&dq=turbo+fan+there+is+no+rotor+force+in+axial+direction&source=bl&ots=_cgQvBtD7F&sig=ACfU3U0nHFwTClXYVQZM_SIOTmRKenoy2g&hl=hr&sa=X&ved=2ahUKEwjsm-e_78XqAhVIw4sKHabmD2wQ6AEwEnoECAkQAQ#v=onepage&q=turbo fan there is no rotor force in axial direction&f=true

so rotor (turbine-shaft compressor) is almost neutral(zero net force),a little thrust is just need for thrust bearing working properly..
so thrust comes from all walls which are faceing forward (even a litte bit) because pressure act only prepedicular to the surface so this is only why how air can exert force/thrust in forward direction...
 
  • #15
689
404
i find here answer:
...
Yes, that book proves my post #4 to be incorrect, as you have previously stated.
My apologies for my incorrect interpretation of the direction of the forces in the turbine section.

Note that the book refers to a CFM turbo-fan engine, which has the added forward axial force of the fan.
 
  • #16
anorlunda
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Axial_compressor.gif


Most illustrations of axial turbines show only the rotor blades. Actually, between each row of rotor blades is a set of non-rotating stator blades. They don't show them in most illustrations because having both makes both the rotor and stator blades impossible to see. Look carefully at that animated gif. You see alternating rotating and non-rotating blades.

You don't want the full thrust on the rotor, because it would need a big thrust bearing to handle it. So low, or neutral thrust on the rotor is desirable. However, all those stator blades are fixed to the airframe so they can carry thrust.

I'm not saying that the stator carries all the thrust, your illustration with the ramjet clearly shows that you can make thrust with no turbine at all.
 
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  • #17
View attachment 266237

Most illustrations of axial turbines show only the rotor blades. Actually, between each row of rotor blades is a set of non-rotating stator blades. They don't show them in most illustrations because having both makes both the rotor and stator blades impossible to see. Look carefully at that animated gif. You see alternating rotating and non-rotating blades.

You don't want the full thrust on the rotor, because it would need a big thrust bearing to handle it. So low, or neutral thrust on the rotor is desirable. However, all those stator blades are fixed to the airframe so they can carry thrust.

I'm not saying that the stator carries all the thrust, your illustration with the ramjet clearly shows that you can make thrust with no turbine at all.
yes stator "wings" lift vector is pointing slighty forward so they also add some thrust

rotor-stator-blades.jpg
 
  • #18
256bits
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i understand compressor has foraward thrust and turbine and nozzel has reward thrust.
but how diffuser and combustion chamber can produce forward thrust??
The diffuser slows the velocity of the air due to the greater volume, so basically it is the change in momentum that produces the force. But,
Since the pressure goes from lower to a higher value I think, your guy with the picture is incorrect in direction.

The combustion chamber increase the velocity of the gas from the entrance to the exit of the chamber, without a change in pressure ( ideally ). Again a momentum change, so a thrust. This is a kinetic energy input into the gas.

These trusts are acting on the air parcels immediately ahead and behind, and not on the physical parts labelled diffuser or combustion chamber. These thrusts are reflected towards the compressor and to the rear towards the nozzle ( the turbine extracts a percentage of the energy within the flow to run the compressor ).

The whole system acts as a a whole unit , and can be designed to operate under different criteria, with different pressures and gas velocities. Changing one unit will change how the other units operate.
ie -as a stationary gas engine where all engine power is utilized for say running a generator.
- a turbo prop, where no engine thrust is developed, and all engine power is utilized to turn the propeller which gives the thrust force - there should still be an axial load on the shaft with this engine, as well as all others.
- as a turbo fan, (consider this as a turbo prop with a much reduced flow bypass ), where some energy is extracted by the turbine to run the compressor - featuring high and low compressor sections.
- as a turbojet where only enough power is extracted to run the compressor, and the rest of the flow exits through the nozzle.

You have to study nozzles for the how the change in momentum from a high speed gas velocity can give a thrust.

It pretty much all starts with the compressor.
Take for instance the balloon.
it has a space of high pressure gas and a nozzle.
The nozzle serves two purpose
1. It is a restriction so that all the pressurized air does not exit all at once.
2. It increases the velocity of the air exiting - a large change in momentum

Then,
You could for instance attach a compressor driven by an electrical motor to keep a charge of the high pressure gas within a balloon and call it "an aircraft engine" - no need for a turbine in this case to run the compressor.
Where is the thrust produced by the nozzle from in the scenario?

What about adding fuel to the nozzle to heat up the gas
That should add some more kinetic energy to the gas, and increase thrust. One might consider a re-design of the nozzle to reflect this change in flow condition to get the same thrust as above.
( And possibly a re-design of the compressor as well if we want to really get the most out of the added fuel )
Of course, the batteries will still discharge.

Well what if somewhere near the nozzle we add a turbine to run a generator to charge the batteries
This will extract some energy from the gas flow leaving less for the nozzle.
You would have to change your compressor design to increase the pressure within so as to end up with the same exit thrust. Consider the turbine as a flow restriction in the simpler sense.

Why don't we just get rid of the generator and electrical motor.
Well, the solution for the compact unit is the Brayton thermodynamic cycle.
And we have the aircraft engine.
Where do you think the change in momentum of the gas through the nozzle acts to give the turbo jet engine its thrust?
If we take the insides of the engine as a black box of which we know nothing about ....
 
  • #19
Tom.G
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jet engine shaft is neutral,because forces at turbine and compressor cancle out,so that mean axial berings dont have force on it in direction of thrust!?
Seems to be a straight-forward conservation of energy.

Conceptually you have two fans on the same shaft, but they are oriented in opposite directions.

Try this Gedanken (thought experiment).
  • Consider that the two water wheels are oriented facing opposite directions
  • The second water wheel is 'turned around', facing the other way, in a different body of water (still water.)
  • This second water wheel is used to move the water in an otherwise still canal.
  • Obviously (ignoring friction) the energy output matches the energy input. (Though in this instance there will be some side-thrust because of the tangential, versus axial, flow.)
  • Q.E.D.

Cheers,
Tom
 
  • #20
cjl
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I don't think that's necessarily equivalent though. In this case, there's energy addition happening between the compressor and turbine, and as a result, the pressure rise through the compressor is actually higher than the pressure drop through the turbine, and based on this, I'd actually expect a net forward thrust on the spool overall. I suppose you could probably move some of this force around based on clever shaping of rotor vs stator stages (effectively, transferring thrust to the stators rather than the rotors), but I'd definitely expect a larger net forward force on the compressor than the net backwards force on the turbine.

Then again, the diagram above in post 6 seems to indicate that it's actually the other way around, and I guess I could envision that too, since the flow area through the turbine will be larger than the flow area through the compressor at equivalent pressure levels. This larger flow area means larger turbine blade area than compressor blade area (again at equivalent pressure areas), which would tend to cause a larger backwards load on the turbine than forwards load on the compressor. For a turbofan though, the fan load would be pulling the other way, and might be enough to bias the overall load forwards again.

Basically, there's no reason the force has to be balanced due to the energy addition, but I could see it ending up approximately balanced depending on how you set up the internals. I haven't looked at jets in detail since my propulsion class in grad school back in 2011 though, so I'm definitely a bit rusty.
 
  • #21
256bits
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Then again, the diagram above in post 6 seems to indicate th
The diagram also indicates that the combustion chamber has a "thrust" on it far exceeding the thrust on the compressor or turbine, at first impressions for anyone looking at it, but that is completely incorrect. The chamber does not support a mechanical load., and there is no static pressure rise through the chamber, but a velocity increase.
The labeling in that picture is deceiving.
 
  • #22
cjl
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The chamber could have a thrust - if there's an area increase from front to back (which is very likely, given the expansion as heat is added), you'll have a force imbalance even though the pressure is constant throughout (actually a slight pressure loss, but this is often neglected in simple analysis).
 

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