How exactly does a gas turbine engine produce thrust?

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  • #1
ank_gl
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Hi
I have been trying to figure out the force experienced by individual components of a gas turbine engine.

Generally, the thrust of the engine is defined / calculated as the change in the momentum of the fluid. Usual formula is mass flow X (C_jet - C_intake) + the pressure difference in case the nozzle is under/over expanded.

What I am interested to know is, how is this thrust transferred to the engine mounts? The only way the fluid can interact with the hardware is through pressure (& shear, but let’s forget it here). I reckon if pressure is integrated all over the surface of the engine, we should get the thrust value.

Now let’s go through various parts of the engine(let’s assume a single spool turbojet):

1. Intake
More or less, the intake is cylindrical, so the pressure integral will not have any large component in the axial direction. If compressor is supported on 2 bearings, front bearing housing (covered by nose cone) will be pushed backwards, ie it is a dragging component.

2. Compressor
Compressor rotor blades can be thought of as being airfoils. Lift & drag produced by the airfoil can be thought as being equivalent to forward force & torque (strictly force about the shaft) requirement respectively. This forward force is transferred to the shaft, to the bearings, to the casing & to the engine mounts.

Similarly, compressor stator blades due to their orientation will also experience a forward force & counter torque.

3. Combustion Chamber
Due to the geometry of the combustion chamber, it also is a dragging component.

4. Turbine
Similar to the compressor rotor & stator blades, turbine blades will experience backward force; turbine is also a dragging component.
Exhaust cone though experiences force in forward direction.

5. Jet pipe & nozzle
Owing to their shape & continuously decreasing pressure, jet pipe does not contribute to any force and pressure distribution on the nozzle pushes it backwards.

From these considerations, only major component which pushes the engine forward is the compressor. Rest all are dragging components. Turns out that gas ‘turbine’ engine is more of a misnomer, compression propulsion system would be a better choice it seems. Turbine is merely driving the compressor. So the question is, what exactly is the nozzle doing?

From the control volume point of view, formula relating change in momentum to the net force makes perfect sense, but how is that force transferred to the engine mounts?

Regards,
Ankit
 

Answers and Replies

  • #2
russ_watters
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The nozzle increases the pressure.
 
  • #3
Travis_King
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1. If the compressor powers the combustion, and the turbine powers the compressor...then isn't the name ok?

2. The components being propelled are connected to the fuselage, that's how the thrust is transmitted.
 
  • #4
AlephZero
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The only way the fluid can interact with the hardware is through pressure (& shear, but let’s forget it here). I reckon if pressure is integrated all over the surface of the engine, we should get the thrust value.

Correct, and jet engine designers do the sort of modelling that you did to find the stress levels in the engine casings, as part of the design process.

but how is that force transferred to the engine mounts?

You forgot to take account of the forces on the rotor and stator blades in the compressor and turbine separately, but that doesn't affect the general principle.

I think you also forgot that the pressure acts normal to the surface of the casings, so it produces a resultant force on the conical compressor and turbine casings independent of the aerodynamic forces on the blades.

The resultant force on the rotor gets into the engine outer casing through the thrust bearing and the structure that supports it. Mechanically, you can think of the stator vanes as cantilever beams fixed to the outer casing, so the force on the vanes is transmitted into the casing through the reaction force where the stators are fixed.

FWIW getting all the forces to add up the "correct" thrust for a real jet engine is hard to do. The total axial force on the compressors and the turbines is much bigger than the net thrust, which is the difference between those two big numbers.
 
  • #5
ank_gl
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The nozzle increases the pressure.

Hi russ,

You mean to say that nozzle increases the back pressure. Right?

If yes, I get that part, that is to match the turbine & compressor characteristics.

1. If the compressor powers the combustion, and the turbine powers the compressor...then isn't the name ok?

2. The components being propelled are connected to the fuselage, that's how the thrust is transmitted.

Hi Travis,

name is just fine. It was secondary.

I am not talking about the transmission of force to the aircraft structure. I am concerned about the engine component loading.

You forgot to take account of the forces on the rotor and stator blades in the compressor and turbine separately, but that doesn't affect the general principle.
I think you also forgot that the pressure acts normal to the surface of the casings, so it produces a resultant force on the conical compressor and turbine casings independent of the aerodynamic forces on the blades.
The resultant force on the rotor gets into the engine outer casing through the thrust bearing and the structure that supports it. Mechanically, you can think of the stator vanes as cantilever beams fixed to the outer casing, so the force on the vanes is transmitted into the casing through the reaction force where the stators are fixed.

Hi AlephZero,

Yes I did take all that into account. I mentioned both in OP. That's how I am analyzing the force.

FWIW getting all the forces to add up the "correct" thrust for a real jet engine is hard to do. The total axial force on the compressors and the turbines is much bigger than the net thrust, which is the difference between those two big numbers.
Correct.

My question was about the purpose of nozzle. The standard 'change in momentum' formula uses the jet velocity. Hence it is assumed that faster the jet leaves, more the thrust, hence the impression that nozzle is used to increase the velocity of the jet.

However, the sole purpose of nozzle is to match the characteristics of turbine & compressor, as russ pointed out.

My next question was to be(had i received replies for OP), how does the afterburner increase the thrust?

As per the Thermodynamics, afterburner affects nothing upstream. Let's us take a simple model with flame holder, jet pipe & nozzle. Heat addition does not affect the pressure at the flame holder plane(pressure loss is negligible), the expansion is still between the same pressures. However the temperature at the flame holder plane increases, simple equations would show that nozzle outlet temperature is also increased, which implies that the jet velocity would be higher with the afterburner. Using the 'change in momentum' formula, we see an increase in the thrust. But how is this increased thrust 'felt'?

As I understand it, the static pressure distribution change due to the change in velocity along the axis. This is the only possible way thrust can increase, but this has to be proved quantitatively.

I hope we can build up a discussion & gain a better understanding.

Regards
Ankit
 
  • #6
navier1120
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The nozzle increases the pressure.

no no no no no, the nozzle increases velocity and reduces pressure!
 
  • #7
navier1120
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...So the question is, what exactly is the nozzle doing?

the nozzle increases velocity which increases thrust. thrust is defined as mdot x V so increasing v increases the thrust.
 
  • #8
Travis_King
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ank, thrust can be boiled down to the difference in the exhaust vs. intake velocity. Faster exhaust jet results in more thrust. The nozzle makes more exhaust velocity, thus creating more thrust. Same with the afterburner.

I mean all of these components do other things to ensure proper functioning and efficiency of the engine, but basically the more you speed up the exhaust, the more thrust you get.
 
  • #9
ank_gl
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@navier1120, nozzle provides higher back pressure. Let's take an example, consider a pipe with P1 at one end. If pipe exhausts to the atmosphere, static pressure drops linearly from P1 all the way to the Pa. If a nozzle is provided at the outlet, P2 at pipe outlet(& nozzle inlet) is increased. This is equivalent to say that nozzle has increased the back pressure.

In the nozzle, yes the bla bla happens. But that is not relevant to my question.

@Travis, I get the momentum equation & I understand it perfectly. What I am trying to figure out is the force distribution (or contribution) of separate components.

The way I understand it is that forward thrust is "felt" by the compressor. Everything else is dragging along, but everything else is needed for the compressor to work. But what happens when afterburner is switched on? I reckon nothing changes upstream, only the exit plane jet velocity increases. But how is this "extra" thrust transferred to the engine mounts?
 
  • #10
Travis_King
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In basically the same way the air is transferred when the afterburner is not on.
 
  • #11
ank_gl
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Hi Travis,

what changes when afterburner is switched on?
 
  • #12
Travis_King
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The afterburner adds fuel to the exhaust which ignites due to the exhaust temperatures. This increase in the exhaust jet velocity provides more thrust.

Afterburners
 
  • #13
Phrak
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no no no no no, the nozzle increases velocity and reduces pressure!

Which one; the inlet or exhaust? There are two, ya know.
 
  • #14
ank_gl
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The afterburner adds fuel to the exhaust which ignites due to the exhaust temperatures. This increase in the exhaust jet velocity provides more thrust.

Afterburners
Hi Travis,

The momentum equation is one way of calculating thrust at the surface of the control volume. It replaces the messy fluid-solid interaction with a simple formula. I am not contending the validity of this method. I am trying study the events inside the blackbox.

To put it into perspective, would you say that power of an IC engine increases because you pressed the accelerator?
 
  • #15
Travis_King
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Yes. The power output of an engine increases and decreases. They have curves (hence power bands).
 
  • #16
ank_gl
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umm.. what?? :(
 
  • #18
ank_gl
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Travis, you can stop now
 
  • #19
Pkruse
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An engine may have one, two, or three shafts to which the blades are attached.

Each shaft has one thrust bearing located near the front end.

The thrust loads are transmitted from the shaft to the case structure through the thrust bearings.

Vanes are stationary, and those loads are transmitted directly to the case.

The case is bolted to the air frame through the engine mounts.

Think of the augmentor as a separate rocket engine bolted to the back of the gas turbine. Those thrust loads apply to the back of the low speed shaft, and from there to the thrust bearing at the front of the same shaft.

The big push in design for the last couple of decades is to minimize exhaust exit speed. More efficient that way, with much less noise. Most of the design of the nozzle is for effecient mixing with ambient air with minimum noise. Such engines depend on a higher mass flow rate rather than velocity for the thrust. That is the purpose of that really big fan in the front, and the incredibly high bypass ratios seen in the newer designs. That fan is responsible for most of the thrust in civilian engines. Military engines typically have lower bypass ratios, but the principle is the same.
 
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  • #20
kkaliber44
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First of all it is so great to stumble upon this forum. Especially that I am looking for the answer to the same question, which ank_gl is trying to find.

Second of all I am faced with the same amount of misunderstanding that ank_gl is finding within this thread. Google and people I ask keep talking about the "momentum change" and the Mdot*deltaV being the thrust.

The equation above is a simple way to calculate net thrust.
It is not, however, what produces the thrust, what pushes on the surface of the metal to create forward force.

If you think of a released balloon, you can calculate its thrust measuring the jet velocity and mass flow, but the actual force propelling the balloon forward is the pressure imbalance created by the fact that on the rear face of the balloon you have one less spot (the opening), which has a pressure drop on it (pressure difference between the high pressure inside the balloon to the lower atmospheric pressure). In this case you can easily point to/calculate the surface, for which delP*A ~ thrust.

In a propeller engine, there is a pressure difference developed accross the propeller blades, which is transferred to the shaft, to the bearing, to engine frame, to the thrust link, and then to the wing.

In a turbojet or turbofan engine, an easy argument to make would be that the compressor and/or the fan act the same way as the propeller.
I was, however, told that the shafts in a jet engine are "balanced" i.e. the shaft is pulled with about equal forces in the forward and back directions, and it is often pulled in the backward direction more than fwd - it is creating drag for the engine.

What remains to push on the engine structure in the fwd direction would be the stators/cases. In this case, which stators and how much are contributing to the thrust in a jet engine? ...This may require an intimate knowledge of a given engine design, down to pressure balance cavities, etc. and may be also specific to a given engine model.

I hope somebody out there could shine more light on this.
 
  • #21
ank_gl
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First of all it is so great to stumble upon this forum. Especially that I am looking for the answer to the same question, which ank_gl is trying to find.

Second of all I am faced with the same amount of misunderstanding that ank_gl is finding within this thread. Google and people I ask keep talking about the "momentum change" and the Mdot*deltaV being the thrust.

The equation above is a simple way to calculate net thrust.
It is not, however, what produces the thrust, what pushes on the surface of the metal to create forward force.

Its good to know that at least somebody wants to know what is happening.

If you think of a released balloon, you can calculate its thrust measuring the jet velocity and mass flow, but the actual force propelling the balloon forward is the pressure imbalance created by the fact that on the rear face of the balloon you have one less spot (the opening), which has a pressure drop on it (pressure difference between the high pressure inside the balloon to the lower atmospheric pressure). In this case you can easily point to/calculate the surface, for which delP*A ~ thrust.
Exactly. Just a minor addition though. Thrust is the net pressure integral over the surface of the balloon.

In a propeller engine, there is a pressure difference developed accross the propeller blades, which is transferred to the shaft, to the bearing, to engine frame, to the thrust link, and then to the wing.
Exactly.

In a turbojet or turbofan engine, an easy argument to make would be that the compressor and/or the fan act the same way as the propeller.
Yup. Exactly.

I was, however, told that the shafts in a jet engine are "balanced" i.e. the shaft is pulled with about equal forces in the forward and back directions, and it is often pulled in the backward direction more than fwd - it is creating drag for the engine.
Umm. No. The shaft is not "balanced" in the sense that you used. The compressor blades transfer the axially forward pointing force on to the shaft, thus producing thrust, whereas the turbine blades transfer the force axially backwards. One can observe that usually the compressor stages are far more than turbine stages, something like 6 stage compressor driven by a single stage turbine, intuition says that force on compressor blades should be far more than the turbine blades. This is the net axial force, which is transferred to the engine casing via the thrust bearing, typically a ball bearing, which in turn transfers it to the engine mounts & ultimately to the vehicle.
Another simple argument to prove this point would be that pressure ratios across the compressor & the turbine on "same" shaft are never same. A typical value would be about 4-7 for compressor & about 2-2.5 for turbine for an aero gas turbine engine.

What remains to push on the engine structure in the fwd direction would be the stators/cases. In this case, which stators and how much are contributing to the thrust in a jet engine? ...This may require an intimate knowledge of a given engine design, down to pressure balance cavities, etc. and may be also specific to a given engine model.
Again, the preceding argument can be applied to stator.

I hope somebody out there could shine more light on this
I hope too. Would love to discuss more of this awesome stuff.
 
  • #22
kkaliber44
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About the shaft not being balanced - do you know this for a fact? If you only look at the pressure balance of the airfoils, I agree, the shaft would experience a net forward axial force; There are, however, pressure balance seals/cavities/pistons inside the jet engine's guts (closer to the engine's centerline than the airfoils). I was told that overall the shaft is either balanced, or often is actually being pushed aft throughout the engine's operating range. I think that the engine may be designed this way to control (keep fairly constant?) and/or minimize the amount of axial force on the thrust bearing.
Maybe this would make the system lighter? More durable? ...?

I don't know too much about what I said above :) I am rather trying to provoke a discussion that maybe will get me closer to the answer.
 
  • #23
ank_gl
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About the shaft not being balanced - do you know this for a fact?
Yup. Just for reassurance, I checked up with the guys at work, and it is so. There might be a moment while revving up the engine during starting when the rotor experiences no axial force, but it is never the design intent to keep it so during the operation. If it were to be "balanced" (axially), there would be no need for thrust bearings.

By any chance, are you confusing the balanced state of rotor in terms of torque? Torque is the tangential force, & it is equal for both ends of the rotor.

If you only look at the pressure balance of the airfoils, I agree, the shaft would experience a net forward axial force; There are, however, pressure balance seals/cavities/pistons inside the jet engine's guts (closer to the engine's centerline than the airfoils).
Pressure balancing pistons are used to reduce the net loading on the component, it has no effect on the overall thrust.

Although I suspect we might have a different opinion of what we consider as balancing pistons. I have not heard about it anywhere else except at my work.

I was told that overall the shaft is either balanced, or often is actually being pushed aft throughout the engine's operating range. I think that the engine may be designed this way to control (keep fairly constant?) and/or minimize the amount of axial force on the thrust bearing.
Maybe this would make the system lighter? More durable? ...?
Well that doesn't make sense to me. Can you elaborate a bit more? I suggest you discuss this with your source.

I don't know too much about what I said above :) I am rather trying to provoke a discussion that maybe will get me closer to the answer.
To be honest, what is the question?:uhh:
 
  • #24
kkaliber44
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...Last things first - my question still relates to making sure which surfaces in a jet engine really contribute to producing thrust.

As for "thrust bearings" - sure they are there to take out the axial force on the shafts.
Can you, however, confirm that the thrust bearings see an axial force in the FWD direction of the engine, and if so that they contribute to a "significant" (e.g. >50%) of the engine's thrust? ...It is said that the fan produces "more than 80% of a jet engine's thrust (wouldn't the ~80%) be transferred through the LP thrust bearing?

...I am not considering the torque on the shaft for now, only looking at net axial forces.
 
  • #25
ank_gl
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There are two ways to look at it. First is the general momentum change equation which gives you the net force applied by the fluid on the control volume. The other is the pressure integral all over the wetted surface (gas flow path) inside the control volume. The key is to understand that both are same. Since it is difficult to find out the pressure on the entire wetted surface, it is difficult to measure the net force (thrust) this way. I feel that this difficulty is the main reason why there is almost no mention of this concept anywhere. Whereas it is relatively easy to find out the variables required in the momentum equation, hence it is generally assumed that momentum change causes the thrust. However, it is the pressure distribution all over the wetted surface which causes a net force. Momentum change just happens owing to the laws of physics.
Consider an analogy. In potential flow, lift over an object can be calculated using circulation, & indeed it is done that way. But, here also, it is the pressure distribution which causes lift, not the circulation.

A word of caution, thrust produced by a jet engine is practically not calculated this way. Infact, it is not at all calculated. It is simply put on load cells & thrust is measured. In an aircraft, there is no indication for thrust, only spool speeds are indicated.

...Last things first - my question still relates to making sure which surfaces in a jet engine really contribute to producing thrust.
The compressor rotor & stators contribute towards forward force, whereas turbine & nozzle contribute towards rearward force. The difference of the these two is the thrust produced. Note that I am have not considered the combustion chamber, what do you think about its contribution?

As for "thrust bearings" - sure they are there to take out the axial force on the shafts.
Can you, however, confirm that the thrust bearings see an axial force in the FWD direction of the engine, and if so that they contribute to a "significant" (e.g. >50%) of the engine's thrust?
Yes. The load transfer takes place through the thrust bearings.

...It is said that the fan produces "more than 80% of a jet engine's thrust (wouldn't the ~80%) be transferred through the LP thrust bearing?
I think you are talking about a large bypass turbofan. If so, LP thrust bearing would indeed be heavily loaded.
 
  • #26
Pkruse
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I need to correct or clarify my earlier post.

Most the thrust of most engines come from the fan.

But some of it does come from the exhaust, especially in the case of a turbojet that has no fan.

I will design a nozzle, but another group will determine what the shape of the nozzle should be on order to meet the intended performance spec of the engine. So I went to present this question to them. Thrust produced depends upon the shape of the nozzle, which is proprietary so I'll quit talking about that. But the thrust loads are transmitted through the nozzle structure.

The compressor disks pull forward on the shaft and the turbine disks pull aft. These are more or less balanced in most cases. But the lowest speed shaft is also driving the fan, which transmits large thrust loads to the thrust bearing.
 
  • #27
ank_gl
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Most the thrust of most engines come from the fan.
For a large bypass commercial turbofan, yes. For a general gas turbine engine, no.

But some of it does come from the exhaust, especially in the case of a turbojet that has no fan.
Fan is just a term. Anything that imparts kinetic energy to a fluid stream is a compressor. However, there are many people who would contend this definition. For those who do, its just a definition, nothing more, a mere convention.


I will design a nozzle, but another group will determine what the shape of the nozzle should be on order to meet the intended performance spec of the engine. So I went to present this question to them. Thrust produced depends upon the shape of the nozzle, which is proprietary so I'll quit talking about that. But the thrust loads are transmitted through the nozzle structure.
Nozzle's shape is hardly a practical design criteria. I think I have stated this fact in this particular thread many times, nozzle is a device to match the characteristics of the turbine & compressor. Think of it as a pressure regulator, it regulates the work produced by the turbine.
Commercial engines do not even need a variable nozzle, as there operating conditions are constant for majority of the engine life.
Even for military class engines, nozzle's geometry is more or less fixed for a range of rotor speeds. Its main purpose is to open up when afterburner is switched on.

And for a fact, nozzle is a dragging component.
 
  • #28
MrIfixplanes
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I was also trying to find the answer to the question posted by the OP. It led me to dig my training notes out and revisit some calculations. Anyway, not a diffinitive answer by any means but maybe a small insight may be found in the diagram below.

http://sphotos-h.ak.fbcdn.net/hphotos-ak-frc1/903359_10151583082451823_1971400927_o.jpg [Broken]
 
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  • #29
Groobler
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How depressing (but not surprising) that after all these posts only one person managed to understand the question, let alone answer it! Perhaps the jet engine is an engineering enigma? This illustrates what is fundamentally wrong with education. The only thing a modern engineer can do is to throw theoretical mathematics at a problem; he, or she, can no longer visualize.
I'm afraid I too cannot answer the question; the above diagram (an Avon from RR's book) shows that the combustion chambers (though crude and draggy) act like a rocket and the cone like an aerospike. The rotor and stator blades make a net loss (the axial force on the turbine blades being more than a match for their fewer stages), and the 'propelling nozzle' is a complete misnomer (more a 'drag-limitation device'). So we are left with a combustion rocket complicated by the need to use low pressure atmospheric oxygen.
I was actually looking for an explanation of afterburning (i.e., like Ankit, what it reacts against) but it looks as if no-one on the internet (or any published work) will be able to explain it.
 
  • #30
SteamKing
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I was actually looking for an explanation of afterburning (i.e., like Ankit, what it reacts against) but it looks as if no-one on the internet (or any published work) will be able to explain it.
What exactly is not clear about thrust augmentation in GT engines? You burn more fuel in the tailpipe which creates additional mass flow coming out the engine and increases the velocity of the exhaust relative to the engine. The difference in momentum of air coming into the engine and the momentum of the gases leaving the engine produces a net thrust, which drives the aircraft forward.

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

https://www.grc.nasa.gov/www/k-12/airplane/turbth.html

A jet engine creates thrust by heating and accelerating the air which it ingests thru the compressor section. The afterburner adds additional mass to the exhaust by burning raw fuel in the tail pipe after the turbine section.
 
  • #31
Groobler
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Thanks SteamKing but again you (and your links) have missed the point. I fear I will suffer the same frustrations as the chap who started this post.

Newton's 3rd law is simple to understand when applied to a released party balloon, or stepping off a skateboard, but where does the thrust of an afterburner act? If acting against the Turbine Exit Pressure (and its upstream surfaces) the turbine would slow and the compressor would surge. I'm sure the flame-holder can't be resisting 10,000 lb or so - in fact, by decelerating the air, I suspect its drag rises in full reheat! I used to think the solution lay entirely in the con-di nozzle of modern turbojets, until I found that older designs weren't equipped with this luxury.
 
  • #32
SteamKing
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Well
Thanks SteamKing but again you (and your links) have missed the point. I fear I will suffer the same frustrations as the chap who started this post.

Newton's 3rd law is simple to understand when applied to a released party balloon, or stepping off a skateboard, but where does the thrust of an afterburner act? If acting against the Turbine Exit Pressure (and its upstream surfaces) the turbine would slow and the compressor would surge. I'm sure the flame-holder can't be resisting 10,000 lb or so - in fact, by decelerating the air, I suspect its drag rises in full reheat! I used to think the solution lay entirely in the con-di nozzle of modern turbojets, until I found that older designs weren't equipped with this luxury.
I don't know what you mean by decelerating the air.

The whole point of the GT is to accelerate the flow through it, which is one reason it burns fuel. The gases created by the burning fuel are going to want to exit the engine that much faster, producing additional momentum and consequent thrust. The afterburner just makes additional gas come out the back end of the engine and more thrust.

You're going to have the same problem trying to figure out what a rocket pushes on when it's in outer space. Change in momentum producing thrust is an established concept. You just have to persuade yourself to accept it. But if you take the attitude that no one has explained or can explain something, that's a mighty large obstacle to understanding something yourself.
 
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  • #33
RonL
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For anyone that might want to spend some study time and get some answers, this might be of use.
A 292 page pdf, but it has everything. (the picture above reminded me of the book it is in)

http://airspot.ru/book/file/485/166837_EB161_rolls_royce_the_jet_engine_fifth_edition_gazoturbinnyy_dviga.pdf
 
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  • #34
Groobler
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RonL - thanks, I have this book but have not found the answer in it. Perhaps I need to study it again, although I'm fairly pessimistic.

SteamKing - I'm sorry for being frustrated, but if you see how unsuccessful the chap who started this post was. Many years ago I did an HND and then a degree in Aeronautical Engineering and came to the conclusion that, on many fundamental principles, very few people (often not even the lecturers) have a clue of how things actually work.
Firstly, on decelerating the air, if you read carefully I was simply referring to the flame holders.
Secondly, you have just repeated the basic concept of which we are all well aware. To make my question more obvious (and repeat my predecessor), what physical parts are taking the push (in my case for the afterburner, in his for the dry turbojet (answered partly by the RR Avon diagram))?
 
  • #35
Nidum
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Most engines with reheat have variable area nozzles . Flow area is increased when reheat is in use . Increased flow area means that area of reaction surface for the exhaust gasses is greater .
 

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