Many questions over the years... about exactly where the thrust of a jet engine is created and what part of the engine it pushes on...to propel the aircraft. Let's look into that...
So its just like a garden hose. The walls and head keep the water going out the back instead of anywhere else, and to get any real thrust out the back you narrow it down to the appropriate amount for the flow and pressure. The working bits just work to keep the hose running.
I’ve been through these arguments before, but it’s worth repeating them here, as a contribution to this discussion. It is incontrovertible that the thrust produced by an engine can only be as a result of the acceleration of the exhaust gases, initiated by the combustion process and completed in the final nozzle, courtesy of Newton’s Second Law. However, the thrust does not “act” on the nozzle. On the contrary, the engine is, in effect, trying to blow the nozzle off, as the force acting on it is rearwards. As a reaction to the rearward acceleration of the exhaust, there must be a forward force on the structure of the engine, in accordance with Newton’s Third Law. The net forward force, which is thrust, can only be as a result of a summation of the internal loads, both forwards and rearwards, caused by the effect of the various pressures throughout the engine, acting in an axial direction on the surfaces of the engine components. There is no one place where the thrust can be considered to act, least of all on the so-called thrust bearing(s). I can and have, in my career, changed the load on a thrust bearing from forwards to rearwards, by means of changes to the sealing and pressurisation arrangements on the compressor and/or turbine discs. Referring to the thrust distribution diagram of the Avon, lifted from chapter 20 of R-R’s ‘Jet Engine’ book, it is a gross simplification and, in one very particular respect, quite misleading. Firstly, the forward and rearward loads quoted for the compressor and turbine respectively are not for the rotors and the difference is not the load being applied to the thrust bearing - or as R-R Derby prefers to describe it, the location bearing. The loads are intended to represent those being applied to both the rotors and the stators of the compressor and turbine and do not necessarily account for the pressure loads on the discs. So exactly what are the loads quoted in the diagram? My best attempt at explaining what they represent is to suggest that, if you imagine the engine being instantaneously sliced up arbitrarily into the sections shown, then the loads measured on each of those sections at that instant would be as quoted. The thoroughly misleading figure is the enormous forward load quoted for the combustion section, whereas the actual loads on the combustion chambers (aka combustion liners or flame tubes - you choose your preferred term) is rearwards. Similarly, the rearward load quoted for the turbine section is equally misleading. This issue arises from the fact that the dividing plane between the combustion chamber and turbine assembly has been quite arbitrarily defined as being at the junction of the circular section of the flame tubes with their turbine entry ducts. If the dividing plane had been defined at the entry to the first stage NGVs, then both figures would have been substantially reduced, but the difference would be the same. I intend to throw a spanner in the works, as we say in Brit English, by referring to the Pegasus engine, but I’ll do that in a separate comment.
Attention to everybody who commented on this video: Unless you spent a career designing jet engines, this is your opportunity to learn something. So shut up, and read Graham's comment in its entirety. If you don't agree with what he says, then it's you against a guy who designs these engines, and a guy who builds these engines. Uh... what was your occupation again...?
We can most certainly not say that. The idea that the exhaust pushes on the atmosphere is a very common misunderstanding, so don't feel bad. But it is not correct at all. If it were true, rocket engines would not work in the vacuum of space. Actually a rocket engine works better in a vacuum.
@@HardusHavenga The "push" can only act on the structure of the engine. AgentJayZ mentioned a balloon and I refer to the way it produces thrust when I build balloon-powered cars with 8-11 year-olds. The thrust is acting on the projection of the cross-sectional area of the neck on the interior surface of the balloon opposite the neck - not on the air at the outlet of the neck.
I really have to give you credit Jay. Even if many aren't doing a great job - you're getting people to struggle a little bit with physics. That's certainly a net benefit for the world.
Hi AgentJayZ, I've watched your channel for many years and I must admit it's always been a mine of information. If not presented by you, I was just inspired to do my own research if some topic would interest me. You're making the videos better and better, great job.
Nice video. I enjoyed it, but I must correct you on a few crucial points: 1. Graph e - (local thrust) is just the total thrust generated up to a certain point. That is why the last point on the graph is marked as the net thrust. It is a bit confusing at first glance, and not of much use IMO. 2. Graph f (integrated thrust) is the arrows marked on the engine diagram designating the thrust contribution of each section. You seemed to have missed this, and IMO this is the main point of this diagram. It clearly shows that the biggest thrust contributor is the combustor, followed by the compressor. 3. At 5:01 they were not confused - the inlet is also commonly called the diffuser because that is an accurate description of its action - it diffuses the incoming airflow (slowing it down and increasing its pressure). They did not mix it with the compressor's outlet diffuser. 4. 15:00 - It is more accurate to say that the nozzle is why the thrust is generated, but not where. Look at graph f again - the nozzle is actually pulling the engine back with a negative thrust of 5587 lbf, but without the nozzle the upstream conditions at the gas generator would not be possible such that the compressor and combustor would not be able to provide the positive thrust. 5. 17:17 - that is not a mistake. You misinterpreted the graph to be static pressure, but it is total pressure. The static pressure indeed drops to about ambient in the nozzle, but total pressure remains (almost) unchanged. The same with the temperature (17:56) - this is a graph of total temperature which remains constant through the nozzle (we usually consider total figures when discussing turbines). 6. 18:17 - this increase in airspeed implies low speed (or even static) operation of the engine. At low speeds the compressor sucks in air which necessitates an increase in speed ahead of the compressor. At high speeds the ram air effect is overpowering and the reverse happens - excess air will bypass the inlet and thus the speed will actually decrease prior to entering the intake. Also, as mentioned before, the inlet is designed as a diffuser - therefore the airspeed within it decreases, so in a low speed operation you will see acceleration of the air before the intake, followed by some deceleration through it. The fact that we see speed increase to the compressor in this graph leads me to believe that this shows a static operation of the engine, equipped with a bell mouth (which is a nozzle - the opposite of the intake diffuser).
I promised to throw a spanner (wrench, if you must) in the works by referring to the Pegasus engine (so start thinking about what happens when the thrust is acting at right-angles to the axis of the engine). However, I've been busy with a little community project, but I'll discuss vectored thrust shortly.
Jet engines are difficult to understand if your mind (like mine) is fixated on pressure. Look up the Bernoulli Principal first and things will become clearer. Superb video as always!
You are awesome. There is nothing else to say. Repeat after me: Every dyne of thrust comes from a gas molecule impacting a rigid surface. But the air in the exhaust nozzle is increasing speed!!! Accelerating!!! What is pushing it? Well, the molecules being accelerated are bouncing off molecules closer to the front. The molecules towards the front are higher pressure. (Maybe higher temperature, not pressure?) You work your way forward until a molecule actually bounces off a rigid (maybe rotating) surface. THAT molecule imparts forward thrust on the engine, and ultimately through the engine mounts. The interesting thing you didn't stress in the force drawing is this: The change in force (vertical axis) in a section of the graph is seen as rising or falling. This corresponds to a rearward (rising) or forward (falling) force contribution for that section. I think this is all true. I think the gas molecule to gas molecule transfer of force means that the air in the engine better be traveling slower than the speed of sound. The force can't transfer upstream faster than the speed of sound. But what about rocket engines, with their pretty supersonic diamonds? And aren't jet engines really rockets which manufacture their own oxidizer? I think the thrust comes from the exiting gas hitting the walls of the diverging nozzle. I don't believe thinking about pressure is how engines are designed. I think engine designers think in terms of energy, entropy, static v.s. dynamic pressure. As you mention many times, designers are extremely concerned with temperature tolerance of their materials. I am not a jet or gas generator engineer. But I am a believer in atoms. Corrections gratefully anticipated. You are awesome. Dog looks like he/she is gaining weight! Lifting weights?
More speculation..... We know that the net force on the rotating elements (compressor plus turbine) is ~0. We see from the chart that the forward force increases mostly in the front half of the engine. Therefore, most forward thrust comes from ... air impacting the compressor stators. The second region which shows as a raise in the integrated force is... in the combustion area. Therefore, the next contribution comes from the front of the combustion chambers. All that gas hurling out the back has only one purpose: to increase the pressure of the gas inside the engine where it presses forward on some engine element.
I remember from "Fluid Mechanics" that you can calculate force by follow a "unit mass" through the machine. The change of momentum at each place in the stream is a net force of the system. I remember from years (& YEARS) ago by the "math" showed that in a particular situation, the net forces were the same regardless of which way the fluid flowed.
@@0MoTheG I was including All the Reactive parts of the system. Those which provide the net acceleration of mass. The confusion is in the interpretation. AgentJayZ was at pains to point out that it was not at one single point within the system that the reaction force was generated.
@@KozmykJ It is easier to answer what parts we can exclude: Turbine Any surface perpendicular to flow that is not redirecting it anything that performs no work such as the nozzle That leaves us with the compressor and not much else
Hello Agent Jay Z. Thanks for all the effort you've put in to these videos. This one in particular has been food for a lot of thought. I thought I could simplify the matter by considering Rayleigh flow in a constant area frictionless pipe. If you add heat to the pipe, the subsonic gas velocity accelerates and can reach Mach 1. This must produce thrust, but I could see no way that the thrust would be imparted to the pipe so, unfortunately, I reached a dead end. By the way, I have just visited the Bournemouth Aviation museum in the UK. I can recommend it. It's a really friendly place where you can sit in various cockpits and get to touch different engines and even an APU.
I promised to throw a spanner (wrench for my North American readers) into the works , or set a cat among the pigeons if you wish. I have been busy doing other things for the past week or two, but I will now do so. In another comment I pointed out that, in an inflated balloon that has been released, the force propelling it is the internal air pressure on the cross-sectional area of the neck projected onto the inner surface of the balloon directly opposite the neck. Now that cannot apply in a 'conventional' jet engine, as there is no single surface on which to project the cross-sectional area of the final nozzle - and yet the engine does produce thrust, just as if there is a pressure acting on a continuous surface opposite the nozzle. So what happens when a 'conventional' jet engine, which happens to be turbofan, has four vectored thrust nozzles stuck onto it and it is called a Pegasus? Nothing changes in the turbomachinery and the axial loads remain the same: OK, the engine may have to run a little hotter for a given thrust, because of the losses in the nozzles, but the turbomachinery is 'unaware' of their presence. Nothing has changed - yet everything has changed. So start by thinking about the engine without the ducting in place for the nozzles, but without the nozzles fitted. The engine will produce no forward thrust, as all the air and gas flow will be directed sideways on either side of the engine. There will be as much rearward thrust on the ducting as there is forward thrust from the turbomachinery. Now think about the the reattached nozzles pointing vertically downwards. Where is the thrust applied? It can only be on the cross sectional area of the nozzle outlets projected onto the inner surface of the nozzle elbows opposite the nozzle outlets. Just as in the case of a 'conventional' jet engine, where the force on the final nozzle is rearwards, the Pegasus is trying to blow the four nozzles sideways off the engine. An early development engine did exactly that to one of its 'cold' nozzles in flight and the aircraft went down as a result.
Jay, it makes sense in the last diagram with velocity increasing through the nacelle. The engine inlet is slightly smaller than the centerline nacelle diameter (wall thickness), so a corresponding increase in velocity is logical. Thanks for shining light into the "dark corners"! BTW: what happened to your lapel Mic? You don't have enough flags to kill the echo yet!
I HAVE FOUND THE ANSWER !!! Hello again AgentJayZ... In my initial comment (a month ago), I had unsuccessfully attempted to answer this question correctly, while simultaneously trying to add a little humor to my answer. This "humor" was very much at my own expense, but alas I am a very wealthy man in this regard and do not mind frivolously spending the currency of laughter. At any rate, I was surprised and honored you took the time to respond to my comment Sir, so let me thank you once again for your response to my comment. While it's true that I can come across a "bit silly" to most people, it cannot be said that I don't follow through on looking up or into/investigating concepts and principles that I do not understand. Without pooping the "Incorrect Answer Party" too much by writing the answer to the thrust question here, I will instead present y'all with the source material containing the answer to this question/concept. The answer can be found in the first video starting at 4:15 and continuing through 7:15 ruclips.net/video/6-GxN4t8VAE/видео.html This second video also contains a wealth of related information... ruclips.net/video/p5SCkSUijuI/видео.html
The trouble many are having comes from their understanding of thrust. In the engine the thrust is indeed forward pressure by area, but out the back end it is mass flow by speed. Even though the nozzle is responsible for increasing speed and there by thrust, the thrust does not push the engine by the nozzle.
Мои скромные познания в ТРД. Тяга в ТРД создаётся: 1) Лопатками ротора компрессора 2) Давлением на диск последней ступени компрессора 3) Давлением на стенки диффузор (он расширяется) 4) Давлением на передние стенки камер сгораний 5) Давлением на диск турбины 2 и 5 могут перераспределятся. Сопло тянет двигатель назад. Но оно повышает давление в двигателе и косвенно способствует увеличению тяги.
Love you videos. I have learned soo much about jet engines from these. I would like to suggest that the thrust is created by the difference in pressure at the last compressor stage and the tail pipe. The nozzle causes the exhaust to accelerate and thus lowers the exhaust pressure increasing the difference in the two pressures. The burning fuel just adds the energy needed to create these pressures.
I gave it a short thought at the beginning and my conclusion was that that the thrust comes from the compressor because that's the only part that actually pushes something towards the back of the engine. And your diagram essentially confirms that. The shaping of the nozzle only helps to increase the efficiency of the process, it helps the escaping gas to escape easier. Any force against walls of the engine is symmetrical in all directions and does not add to the thrust. Besides, all further parts (combustor, turbine, nozzle) are protected by injecting cooling air provided by the compressor again. The hot gases are pushing against this cooling air, not against the engine walls.
There's difference between aircraft and industrial jets: in industrial jets, the turbine extracts almost all energy from the gas - the push of the gas against compressor is almost equal to push of the gas against the turbine. The gas leaves with minimum energy needed to keep the process going, all the rest goes into turning the engine axis to power the compressor and whatever other machinery is mounted to it. In aircrafgt jets, the turbine only extracts energy needed to keep the engine going, mostly to turn the compressor. The rest is left to escape. The force pushing the compressor forwars is less than the force pushing the turbine backward. That's why there's thrust in aircraft jets.
I'll go Jet Nozzle applying force through the Thrust Bearing on to the Frame and then on to the Mounting System of the aircraft / testbed. But then what do I know?
thanks for all of your work entertaining and educating us! A fascinating area of technology. Also.. who doesn't love a faithful shop doggie?? A nice finish to a video.
my understanding: the thrust comes from the velocity of the gases exiting the nozzle. the thrust ultimately pushes against (acts on) the back of the blades of the compressor thus accelerating the aircraft in the opposite direction of the flow of the exiting gases. the velocity of the gases comes from the pressure produced by 1.the compressor plus 2.the heating of the gases (by burning fuel both in the combustion chamber and in the after-burner). firstly we produce high pressure THEN we decrease that pressure which accelerates the gases so that at the point where they exit the engine they move fast and have the same pressure as the atmospheric pressure. simplified, the engine transforms high pressure gases to high velocity gases. high pressure means the gases bounce violently around in ALL directions, high velocity means they travel fast in ONE direction. it's a 'continually rapidly deflating balloon' kind of situation.. :}
My very unprofessional opinion is the gross motive force comes from the first compressor stage, to the exit nozzle, as viewed from front to rear. Moving from rear to front, there is thrust from the nozzle to the combustion area. But a forward pulling effect created from the compressor stages trying to "fly" forward.
What I got from this, is that the thrust effect at the nozzle pushes against the column of air within the engine. That column in turn is "held in place" by the force of the compressor section, as you said, the mechanical parts which keep the air from moving forward. My take on the integrated thrust graph is the amount of thrust added throughout the engine, left to right. Like if you were to let the air out of a rubber balloon, but the air pushes against the blast of a hair dryer. As long as the hair dryer is stronger (pressure from compressor) than the balloon's power (combustion), it can only move away from the dryer, no matter how you point the balloon. Therefore, the drop of pressure from compressor to the combustion section is a necessary sign. I think there is critical limit to how narrow the nozzle can be then.
@AgentJayZ , can you explain how a theoretical turbojet (running max thrust) would behave, if you contracted the variable nozzle more and more, up to a complete shut? Will the compressor eventually stall? I guess so.
Found an explanation here: ruclips.net/video/F6J_mHUNVjs/видео.html one of your other videos. So, all right, the speed of sound is the major limit to work with. I've really come to appreciate how jet engines can push a plane supersonic now.
Before the restriction caused pressure to rise to the point where the turbine lost effectiveness, the temp would go up past the point that the metals could endure, and things would melt and fall apart. One of the primary signals used by the fuel control is Exhaust Gas Temp, so in an otherwise normally functioning turbojet, if you could somehow independently close down the nozzle ahead of its schedule ( it's physically impossible to close completely ), the fuel control would reduce fuel flow to maintain EGT, which at full power anyway is right about at the upper limit.
@@AgentJayZ Thank you! Now I can see how the engine would self-regulate in that scenario. So, the fuel flow is always controlled - I remember you pointed out how the pilot may think he controls the fuel flow directly with the levers, when in fact he only asks for the the engine to negotiate the corresponding power output.
From my understanding, there is no single point where the thrust is acting, but is the cumulative force along the center-line of the engine. Each section adds or subtracts overall thrust. A good example of this is the exhaust nozzle taper. The thrust generated at the end of the turbine section is at the highest point, but the taper helps maintain the gas velocity and extract more thrust. If you were to remove the taper, you would effectively lose overall thrust.
Imagine a liquid fuel rocket engine. It basically has a combustion chamber with three holes. Two of them is used for injecting pressurized oxygen and fuel and the third one is the exit for gases. The force is created by the gases pushing on the chamber's walls. The forces are not equal in all directions since there is a large hole on one side (no walls there). That inequality is what matters. The force can be calculated by multiplying pressure inside the chamber and cross-section of the exit. You will get the similar result by multiplying exhaust gas mass and its accelleration. Now, turbojet engine is mostly the same to me. The turbine and the compressor are just for delivering air. In large rocket engines You also have a turbo-pump to deliver fuel and oxygen into combustion chamber. In any case some energy must be used to pressurize air (oxygen) and fuel. Of course all of that is just a simplification, but it may explain something.
@@Bodi2000 OK, I've made a big simplification, maybe too big :) . I believe the rocket nozzle acts similar to converging and diverging nozzle which AgentJayZ mentioned in "Afterburner Jet Propulsion Nozzle" video two weeks ago.
In case it helps with the discussion, here's a fresh scan of the first diagram from the textbook along with related text on adjacent pages: www.dropbox.com/s/pjj7utjpl5u6rbe/jet%20engine%20thrust_scaled.pdf?dl=0 Pages 162-164 in "Anderson, J. David. Aircraft Performance and Design. McGraw-Hill. 1999." It is a reference text for upper-level courses in aerospace engineering programs.
And the relevant paragraph for those hesitant to click on a stranger's file link (apologies for any typos): "The thrust generated by the engine is due to the net resultant of the pressure and shear stress distributions acting on the exposed surface areas, internal and external, at each point at which the gas contacts any part of the device, as described in Section 3.2. Figure 3.10e illustrates how each component of the turbojet contributes to the thrust; this figure is essentially a picture of the "thrust buildup" for the engine. The internal duct of the diffuser and compressor has a component of surface area that faces in the thrust direction (toward the left in Fig. 3.10). The high pressure in the diffuser and especially in the compressor, acting on this forward-facing area, creates a large force in the thrust direction. Note in Fig. 3.10e that the accumulated thrust T grows with distance along the diffuser (1-2) and the compressor (2-3). This high pressure also acts on a component of forward-facing area in the burner, so that the accumulated value of T continues to increase with distance through the burner (3-4), as shown in Fig. 3.10e. However, in the turbine and nozzle, the net surface area has a component that faces in the rearward direction, and the pressure acting on this rearward-facing area creates a force in the negative thrust direction (to the right in Fig. 3.10). Thus, the accumulated thrust F decreases through the turbine (4-5) and nozzle, (5-6), as shown in Fig. 3.10e. However, by the time the nozzle exit is reached (point 6), the net accumulated thrust F_net is still a positive value, as shown in Fig. 3.10e. This is the net thrust produced by the engine, that is, T = F_net. A more diagrammatic illustration of the thrust distribution exerted on a turbojet is shown in Fig. 3.10f."
A hero right there, thanks. Can someone explain to me, is the net force on the shaft really towards the rear? (41000lb backwards and 19000lb forward) What is the main thrust bearing doing? Thanks guys!
Using firefox 52.6 and only getting one picture with supplied url, but cut off ?dl=0 from the link gets me a downloadable pdf with several pages. Maybe this will help others? Derk - Net force is ALL combined forward (4) and reverse (2) forces shown, you don't get to cherry pick one or two and then make up math, to declare that as a valid result. Main thrust bearing is pushing forward on engine case, but so is the diffuser, and combustion chambers also connected to engine case. Main thrust bearing would provide from 1/3 to 1/2 of total thrust would be my WA guess. They should have covered that detail a bit more since it's a fantastic real nuts and bolts question you bring up.
@@leebarnes655 thanks. Sure you can't just pick some, but in the case of my question, it's okay: the question is whether the total axial force on the shaft is forward or backwards, so I chose axial forces acting on the shaft: - compressor and turbine. And since compressor axial force is smaller, I am still wondering what I am thinking wrong:)
@@DerKrawallkeks After that clarification, I'll capitulate and join your camp. Brilliant, even the one and only JayZ suggested he wasn't sure about this issue IIRC from the video - I wasn't clear on exactly what he was speaking to either. But it does now appear to me that rearward gas load overrides forward and shaft thrust should be to the rear. I don't think you are thinking wrong at all now - it's just a very unexpected aspect to be sure. Can say the same for the thrust attributed to the combustion chambers alone - as in WoW.
Would it be possible to increase the efficiency of a turbo fan with a adjustable nozzle? It seems like it would be a good place to start for more efficiency on a modern airliner. I mean even a half a percent would be a great savings for the life of the thing.
The benefits would be small, as they would be happening at the core exhaust only, which accounts for 10 - 20% of the total thrust. It also depends on how you define efficiency. A variable exhaust nozzle would add weight, complexity, expense, maintenance requirements. So far, it has not proven to be a desirable compromise
Presumably the pressure will have to decrease as the air exits regardless of the taper, since the ambient pressure outside will be lower? I.e the taper is irrelevant?
What is missing here to make it a bit easier is the inclusion of the MASS. Accelerated and expelled (backwards) mass requires "force" and equal and opposite force pushes the engine (attached to the airplane) forward.
@5:00 you say the 4th from the top graph is integrated graph. I think that may refer to calculus integration or adding of point in the three other graphs. Correct me if I’m wrong please!
It's just Newtonian physics. Throw mass out the back and get an equal and opposite force towards the front. If the engine is stationary you can blow pinwheels with it.
You are quite right: the physics is very simple and answers the question as to how the thrust is produced. However, it does not answer the question as to where the forces are experienced. Please see my comments elsewhere.
@@numbynumb avogadro's law applies to a to a closed system. A jet engine is not a closed system. Newton's third law is why jet engines work, no matter how complicated the process.
The garden hose is an great model how it works...if you keep the end of the hose uncovered, then you can experience no significant thrust and the water just splashes out. But if you cover the end with your finger then the water is forced through a smaller area and it accelerates and you can experience a force. How strong depends on the power of the water pump.
Newton's third law. For every action there's an equal and opposite reaction. The engine accelerates a huge mass of air. The opposite reaction drives the engine forward, along with the attached airframe. Everybody's happy! The interesting stuff is how the engine accelerates that mass of air, while remaining mechanically reliable.
Seems to me that in the second diagram, for angular velocity to increase going into the compressor, you'd need a corresponding drop in pressure compared to ambient. (i.e. the green line should go down from the first to the second point on the graph, as the red line goes up)
Since this axial velocity increase in front of the compressor is caused by some intake cone, I think the pressure doesn't drop(like it normally would), since the volume is getting smaller. The intake cone is displacing air.
The "integrated thrust" diagram is telling you the total thrust contribution of each part of the engine as you move from the front (which you might say is set at "zero" on the diagram because it intersects the horizontal axis) to the back. It is only an approximation; a more precise diagram would show little "upward steps" for each stage of the compressor (including steps for each stator stage), a slight increase at each expanding section of flow through the combuster (burner), a slight decrease at points in the combustor that are either straight (due to friction) or converging (due to pressure against the gas flow necessary to compress it). There would be little "downward steps" for each stage of the turbine and each hot-section stator. The "derivative," or slope, of the integrated thrust curve would show the contribution for each stage, with positive contributions for the compressor stages, both rotating and stationary, and with negative contributions for pretty much everything else except the those places in the combustor in which the gases are expanding, and the little bits between the turbine and hot section stators where the pressure of the flowing gas has a forward (or leftward on the diagram) component as it presses against the flaring cross-section of the hot section. Almost all the forward force, or thrust, arises as a result of the compressor stages chewing their way through the air mass that flows into the engine intake. Each compressor stage acts exactly like an ordinary propeller, adding rearward momentum to the air flowing through it and by so doing experiencing a forward force on the compressor blades, just like the blades of an ordinary prop. These forces are all summed up by the compressor disks (summing the thrust contribution of each blade) and compressor shaft (summing the contribution from each disk); the shaft is strong and stiff; these summed forces are exerted against the thrust bearing, which must be present somewhere along the length of the shaft. It is the thrust bearing(s) that transmit the thrust to the engine housing, and from that to the airplane, to which the engine is obviously rigidly connected. There is a some forward (thrust) contribution from the compressor stator blades too; this force is collected by the compressor housing and adds to the total thrust force on the engine, which is also transmitted to the airplane by the engine mount. The turbine section is a "consumer" of thrust; it is therefore a "drag" on the airplane. This force, times the velocity of the gas column moving through the hot section, is what does the work that spins the turbine. Almost 100% of this work is transmitted to the compressor, at least in a turbojet engine. The hot section stator blades also "consume" thrust. In a turboprop, or if there is bypass, the compressor doesn't consume all the turbine output; some goes to the prop or first stage fan. The amount of force "consumed" by the hot section is less than the amount of force "generated" by the compressor section for two reasons: (1) the gas moving through the hot section is going a lot faster than the gas going through the compressor (you can see this in curve (d) in the diagram you showed) and (2) there is a lot more _volume_ of gas moving through the hot section than there was through the compressor, because the gas column was strongly heated by the combustors before it hit the first-stage turbine; this expanded the gas column, as you know. The difference in thrust "generated" by the compressor section and the thrust "consumed" by the hot section is what is shown in the diagram you used as _F_ net, the net thrust [curve (e)]. The product of these two increases, in speed and volume, is what allows the turbine to extract enough energy to spin the compressor _without_ _exerting_ _as_ _much_ _drag_ _on_ _the_ _gas_ _column_ _as_ _was_ _generated_ _in_ _thrust_ _by_ _the_ _compressor_ _section._ It is this difference that yields a net thrust for the engine, and allows the engine to do mechanical work on an airplane and thereby keep it moving forward. Any expanding part of the air path is a "generator" of thrust, and any converging part of the air path is a "consumer" of thrust. This is obvious if you think about it; it's why rocket engine nozzles are expanding, bell-shaped affairs. The thrust generated from a rocket engine is almost entirely exerted on the forward wall of the combustion chamber (the "power head") and to a lesser extent along the expanding wall of the nozzle. These contributions are miniscule in comparison to those of the compressor blades. I hope this helps. P.S. I know what you're thinking: "what is the effect of an afterburner, where is _it's_ thrust contribution transmitted to the airframe." Believe it or not, I've actually told you enough to figure that out by yourself, if you think about it.
@@AgentJayZ Yeah, I guess so. Oh well. BTW, I love your videos, and have watched probably every one over the last five or six years. Many things stuck in my mind about them; probably the most vivid and unforgettable was when you demonstrated the amount of fuel sprayed by one nozzle at operating pressure, I couldn't believe it. When you think of the amount of flame that would have generated if the fuel was burning, and then multiplied that by eight or nine for the combined heat output of all the combustors in an LM1500, and... wow.
Please see my reply to Mr burroaks 7, as even you appear to have been misled by the term 'thrust bearing'. The thrust bearing (R-R Derby uses the term 'location bearing') is reacting the difference between the forward load of the compressor rotor and the rearward load of the turbine rotor. In most cases that load is forwards. However, dependent on the design features of the engine and its internal air system arrangements, that load could be rearwards.
@@grahamj9101 The compressor exerts a force "forward" on the shaft, the turbine exerts a force "backward" on the same shaft. In order to provide net thrust, the force generated by the action of the compressor as it "pushes" air into the engine must be greater than the reverse force generated by the turbine as it extracts momentum from the hot gas column moving through it. This difference is transmitted to the engine by means of a thrust bearing, which is a bearing that's designed to handle large loads parallel to the shaft in addition to loads perpendicular to the shaft. If due to some transient effect, the force generated by the turbine exceeds the force generated by the compressor stages, yes, the thrust bearing will experience a backward force. In an operating aircraft engine, that condition had best not persist very long. There are engines in which the flow of gas is from the back to the front, but those (as far as I know) are all turboprop engines, an example of which is the PT6, made by Canadian Pratt & Whitney. In that engine I suppose the thrust generated by the turbomachinery would be to the rear, but would be overwhelmed by the much greater thrust produced by the prop. The PT6 is designed in this way to put the hot section at the front; this is done because the hot section needs more frequent overhauls than does the cold (compressor) section, and putting it at the front greatly simplifies the overhaul workflow, which yields a cost savings for the owner. As a matter of semantics, yes, a "thrust bearing" that's experiencing a net axial force toward the rear of the airplane might be called a "drag bearing," I suppose. But the term "thrust bearing" is applied to any bearing that's designed to safely sustain a large axial load and support that load without suffering a greatly increased rate of wear, as would an ordinary ball bearing for example. In everything I've written, both here and in my earlier comment, I've used the words "thrust" and "drag" as they are applied to the system of reference of an aircraft in flight. Thrust is to the front or fore end of the aircraft; drag is to the rear or aft end.
AgentJayZ, what is the TBO (Time between overhaul (in hrs) for an LM1500?). You have show several engines that have come in for service, that look like , the owner ran it past the check engine light, i.e they look incredibly beat up. Do you change the "plugs" (i.e high energy ignitors) when you service the turbine or do the ignitors last indefinitely because they are only used in starting)?
Ignitors are an "on condition" item for industrial use. We change them when they stop working. Some LM1500s have over a hundred thousand hours on them.
It's partly the water pressure running down the hose and partly the hose itself and partly the thumb on the end of the hose. I maybe disagree with your disagreement with the first model which showed an increase of velocity going through the compressor. The air passage tapers smaller so it should go faster due to a Venturi effect (like when air is squeezed through narrowed passage on a carburator), at least a little, shouldn't it?
The tapering down of the compressor gas path is to compensate for the fact that as air gets compressed, it takes up a smaller volume. So thew tapering is to keep the velocity the same, instead of slowing down.
I wasn’t going to say laser pointer - I thought you were tapping the diagram with a Sherlock Holmes style tobacco pipe like a 1950’s British engine designer. Right I’m heading for the jet wash.
Laser pointer! I would want one of those squirrel suits before jumping into the jet wash! Hope Graham is doing ok, haven't seen him around in a while and his channels been silent for a year.
Please see the contribution that I've just posted. I'm fine, thanks, but I've been increasingly busy with Grandpa and family duties, both here in the UK and in Singapore (and in France when they come back for their skiing). I've also been quite busy as a Bloodhound Education Ambassador, assisting a school events where the kids build and test little rocket-powered cars.
@@beachboardfan9544 Oh yes, I forgot to mention that I do some sailing too, with another trip to France planned for the last week of July. Before that, however, I'll be visiting my daughter and family in Singapore for three weeks, with another trip planned in October/November. I've also got the 54th anniversary of my 21st birthday coming up in August. I'll be celebrating it with my son (who flew an Apache for a few years) and family at an event at the UK's Army Air Corps base at Wattisham in Suffolk.
There is SO MUCH awesome in that comment its ridiculous, congrats on everything! I'll also be celebrating with you in August but it'll be my 12th, 21st birthday anniversary 😄
As far as my incomplete engineering knowledge goes, yes. The integrated force graph is just the sum of all forces before that point along the engine i believe. Each point on that graph should be the sum of all the forces acting on the engine up to that point. Inside the combustor, the air undergoes an isobaric process, this is due to the combustor being open to the compressor so the pressure must be the same (in ideal conditions) between the compressor exit and the nozzles at the end of the combustor. if it is hard to understand the accelerating air providing a force, just thing of the equation every highschool physics teacher will tell you, F = ma, accelerate a mass (in this case air) and you get a force in the opposite direction on the turbine, this is newtons 2nd and 3rd laws.
Thanks for writing this comment, I was just about to mention those points. Especially the F = m * a is important. As AgentJayZ says, the combustor really is open to the front and the back. - In the front, the compressor is keeping in the pressure with a wall of air supported by force on the compressor blades - In the open back, the pressure is kept in by the force it takes to accelerate the air (F = m * a) I learned something interesting in this video: The net force on the shaft of a turbojet engine apparently is going backwards. I thought more of it was acting on the compressor, giving the shaft a net force forward. I thought therefore the main thrust bearing takes net forward force. Can someone explain that to me please?
@@DerKrawallkeks Please see my comments else:where. In the majority of engines, the load on the thrust bearing(s) is in a forwards direction, because the net forward pressure loads on the compressor rotor are greater than the net rearward loads on the turbine rotor. However, these loads can be varied considerably by varying the arrangements of the air seals and the way they are pressurised. As I stated elsewhere, I changed the load on a thrust bearing from forwards to rearwards with the introduction of a new turbine, which had major changes to its sealing arrangements.
@@grahamj9101 thanks for that clarification, so depending on how the engine is designed, the net force of the shaft can go either way, and the thrust bearing can be designed for forward or backward facing force, depending on the engine. I was surprised in your other comment, that the force acting on the combustion chamber(s) is actually facing to the rear. Why is that? I'm imagining a combustion chamber to have a larger outlet than inlet, therefore pushing forward. Could you explain? Thanks for your great comments, it's always a great pleasure to have an expert/professional on hand
@@DerKrawallkeks Perhaps you are being mislead, as many are, by the way a piston engine works, where there is an increase in pressure as a result of the combustion process. There is no increase in pressure in a gas turbine engine operating to the Brayton cycle. There is, in fact, a pressure drop across the walls of the combustion chamber. Now I use this term, which AgentJayZ dislikes, but is the term that I was brought up with at R-R, to mean the combustion liner or flame tube or combustor, which is the term that is in common usage in N America. As there is a pressure drop across the combustor, there must be a net rearward load on it. Now what the physical loads on the complete combustion chamber section of the engine actually are will be very dependent on the design 'architecture' of the engine and the way that the entry and exit sections are defined for calculation purposes. That is where I have issues with the Avon thrust distribution diagram and the huge forward load that is quoted for the combustion section.
@@grahamj9101 Yes, thank you. The diagram might be correct, but it's not close enough defined, which parts they actually still count into each segment.
Agent JayZ: Are you more likely to experience a hung start or a hot start on a cold day at Jet City or in the middle of summer? Are turbine engines less finicky about cold starts than diesel engines?
People make this way more complicated than it actually is, the physics is pretty basic. people in the space industry know this fairly well. remove the atmosphere, remove the jet nozzle, remove all the confusing variables from the picture, in space THE ONLY WAY TO MOVE IS TO RELEASE MASS, the more energy (acceleration) you give the same amount of mass the more acceleration you get per unit of mass release. if you throw a spanner in space (insert energy into the system in form of momentum) both you and the spanner will move (exactly in opposite directions) the energy gets divided by mass to define the speed (acceleration) in with both items move away, lighter mass item will go faster and heavier item will go slower Now back to earth, the air is a fluid, it has mass, you accelerate a chunk of air from the sea of air that is around the engine and you accelerate it to one direction (by inserting energy), the inertia and conservation of momentum dictates the engine has to move the opposite direction. (replace air with water in your head and it all starts to make more intuitive sense). Think of taking a bucket of water from the front of the boat and dumping it at the back. if you transfer that mass from front to the back (relatively) faster than the water that the boat is on, you get forward motion. the water itself can be taken and dumped at exactly the same speed, no nozzles, jet pipes, none of that. just a simple conservation of momentum.
Reiterating Newton’s law wasn’t his purpose, Agent JayZ was attempting to look at the complexity of the situation and explain precisely where in the engine the thrust acts. If you’re happy with a simplified understanding this isn’t the video for you.
Can someone explain why the tapering of the nozzle matters? The way I understand it, it shouldn't, but I am obviously missing something. F=MA, right? Pressure is also related to density which is related to mass. More pressure = More Density = More mass per unit of area. So, the way I see it now, If you have a nozzle, you have higher velocity but less mass of air per unit of area. Without a nozzle, you have more mass of air per unit of area but it exits the engine slower. It seems that the two situations would equal out. What am I missing here??
Hmmm... Probably better to re-establish a basic understanding of principles than to look for specific errors. What I mean is, nothing you have said here makes any sense at all to me. Not an insult, but I think you have a couple of incorrect assumptions that are stopping you from coming to an understanding. Subsonic aerodynamics, and Bernoulli would be good search words.
While the nozzle does increase exhaust speed, the reason it has a positive impact on engine thrust comes from pressure. As it is the pressure inside the engine that actually pushes it forward. If you cut the nozzle off, engine would not be able to maintain it's internal pressure (and less pressure = less thrust). Thou it's not only nozzle that can keep pressure high in the engine, afterburner does that too, and you can see how nozzles open up to full when it's on. What you are missing is the behavior of working fluid (exhaust gases) itself. Most engines are so complex because they need to adjust for what the gases can and can't do. And while you could create two engines with same power, same mass flow, and two different size exhausts, the one with smaller exhaust would likely be more efficient. It's definitely not easy topic, so i'd recommend to see how rocket engines work first. It's much easier to find good, grounded in basic physics definitions on that.
Remember fluids: one can increase the speed by reducing the area (of course assuming the fluid is in constant flow without loss). And the equation of Bernoulli stands a balance of energy conditions, what it is needed is an increasing of speed in order to reduce pressure and cause lift in the aircraft.
The F_net is basically a summation of all the combined reaction forces in the engine along its length. Basically integrating incremental force vectors vs. distance along the centerline of the engine. Since the forces are vector quantities and thus act with a defined direction (either towards the compressor or towards the turbine/exhaust section), they will both add and subtract from the overall net force. Basically the force builds through the compressor section as each stage further compresses the air charge, builds slightly through the combustor, and then decreases as the gas expands and imparts energy to the turbine stages.
I've always figured the driving force is produced in the jet pipe , like a Rocket motor produces it's entire thrust from the cone at the very end . So , the Jet Engine is pushing from the Turbine and pipe area and all hardware it's mounted to and transfered to engine mounts .
The problem I am having is Newtons third Law... all forces between two objects exist in equal magnitude and opposite direction. So where does the rubber meet the road? You can say jet engines produce thrust through Delta V of exhaust but how does the engine move in the opposite direction if not through forward pressure on internal surfaces that are creating that rearward delta V? I would like to say any gas acceleration before the turbine is only useful because it turns the compressor through the shaft and it's the compressor blades that push the air rearward and the engine forward, but what about after burners. And what about ramjets with out compressors?
There is pressure in the jet pipe after the turbines. It is converted to velocity by the nozzle. You want to join the ages-old discussion about just where does the "push" happen on the aircraft? The answer is out there... for you to find.
Where is the thrust? It starts at the back of the compressor. It EXISTS at that location. That’s just my take on it. Just another layperson here trying to absorb. It might be easier to think of one of the space shuttle solid rocket boosters. As they burn, the combustion inside the booster moves up the inside of the booster body until it is burning damned near the top of the inside of the booster. So when all that is happening, is the thrust still only occurring at the nozzle end?
It's not a rocket engine. The Turbojet produces thrust not with pressure, but with acceleration of exhaust gases. These gases exit the rear nozzle with high speed at almost ambient pressure. Come think of it, a rocket engine converts pressure in the combustion chamber into velocity in the exhaust nozzle. Similar!
Gas moves from high to low pressure as described by Bernoulli, which is the aggregate behavior, not necessarily describing kinetic forces of gas molecules.
Bernoulli doesn't really apply anywhere for a jet engine because it assumes incompressible, irrotational, isentropic, adiabatic, no-work flow when jet flow is compressible, rotational, entropic, not exactly adiabatic, and definitely has work. You can make Bernoulli work if you make a ton of modifications, but then it's basically just a general conservation of energy equation.
The first of two examples is the Rolls Royce Avon. The air does not leave the engine at significant pressure, but at very high speed. Jet engines do not work on pressure, they work on acceleration. The full power mass airflow of an Avon is about 150 lbs per second. So, about 9,000 lbs per minute leave the compressor, at a discharge pressure of about 120 psi. Conversions between mass and volume for air can be found at different pressures. It would be inefficient to use an Avon compressor section as an air compressor to obtain 25 psi. It could be modified to do so, but we are beginning to drift away from standard practices...
@@AgentJayZ look, man, I just want to figure out how to run my nail gun at 4000 nails a minute. But back on topic, wouldn't the last compressor stage blades be providing the back pressure to force the air towards the rear? Could they be considered the part of the engine that gets pushed forward?
Sure. This is an often discussed subject, and is an irrelevant question for the people who maintain, operate, and manufacture these engines. The thrust produced by the engine is the sum of many forces acting on all of its different sections. There's a few good diagram on the distribution of forces out there. Here's my favorite: www.google.ca/url?sa=i&source=images&cd=&ved=2ahUKEwj40PvNqMTlAhWUrJ4KHeOZBr0QjRx6BAgBEAQ&url=https%3A%2F%2Faviation.stackexchange.com%2Fquestions%2F33068%2Fon-which-points-in-a-jet-engine-does-the-reaction-force-act&psig=AOvVaw1YFB0AgDG-0eQklbCU-tDn&ust=1572536366924626
I don’t know why everyone gets in such a state on this issue, it’s quite simple. An aircraft turbojet engine gets its thrust by dint of the fact that the hot stuff coming out of the back is travelling a lot faster than the cool stuff going in the front. The turbine section is there for one reason only - to supply the 30,000 or so horsepower drive for the compressor. Lets imagine a theoretical concept engine, with full engine casing and jet pipe, with only the compressor section present, driven by a 30,000 hp electric motor. Run up to speed, is there any thrust? Yes, a little because the air coming out the back is going faster than the air going in the front as the compressor has heated it, but the thrust is negligible. Now let’s add the combustion section. The electric motor is still required to drive the compressor, so it’s run up to speed and the combustors are lit. Is there any thrust? Yes, an immense amount - whatever the engine is rated at plus 30,000 hp as no turbine is present. There’s a huge amount of hot stuff coming out the jet pipe, travelling very, very fast. When the turbine section is added, that draws 30,000 hp out of the exhaust stream to drive the compressor, so the electric motor is no longer needed, bringing the exhaust stream under control. The jet pipe nozzle doesn’t generate thrust, it manipulates (multiplies) the thrust that has already been generated, narrowing to increase the speed of the exhaust stream, thus increasing thrust. But the thrust is generated by the compressor, the X vetor of the angled blades providing the forward thrust, and the Y vector providing resistance to rotation. The X vector from every compressor blade is amalgamated and becomes forward thrust, is transmitted through the thrust bearing to the engine mounts, thus the aircraft gets pulled forward. So the answer to the question “Where Does Thrust Act” is, on the X vector of every compressor blade. Industrial gas generators don’t generate thrust because their turbines are just the right size to extract the maximum amount of energy from the exhaust stream, there’s no jet pipe or nozzle to multiply the residual thrust, and they’re firmly bolted down. There, simple, can I have a job?
@Zachary Martinez you are right. The fact that you can calculate the thrust of an engine as the air mass flow times the speed difference is just an application of Newton's 3rd law that simplifies the calculation. The thrust, strictly speaking, is the integral sum of all the shear stresses and pressure forces on all the surfaces of the engine, which is of course much much more difficult to do than just the simple speed difference thingy, specially when no computers existed. Nowadays it can be done by CFD (computational fluyd dynamics). But at the end, thanks to Newton's 3rd law, they are both just the two sides of the same coin.
"thrust is generated by the compressor, the X vetor of the angled blades providing the forward thrust". I think You are ignoring the force that gas makes on the walls of combustion chamber. And what about centrifugal compressors? In this case the air travels sideways of the propulsion vector, which makes Your theory wrong. In general - the way the air is compressed has nothing to do with the thrust. In theory You could use a tank with compressed air instead of the compressor. Does that mean that the trust is generated by the air tank? No.
@@wniedzie sorry but the answer is yes, think of a balloon, the thrust is generated by the compressed air, and again you can calculate it as the integral sum of the pressure forces and shear forces on all the balloon surface, or just measure the mass flow and the speed or the air coming out.
@@laertesl4324 Baloon is not a jet engine :). A spinning compressor alone WILL give some thrust, like an ordinary fan. It WILL blow some amount of air. But it's not something You expect from a jet engine. Everytime You have a pump or compressor, the presure in the system rises only when there is a resistance, some load. Even with the biggest pump, when there's no load You have only a moving medium (gas, fluid) from one side to the other, no pressure. Pressure in a turbojet engine comes from gases expanding. The combustion gases are the load for the compressor, they do the opposite and try to escape in every directions, also through the compressor. It's the compressor's role to overcome this force and provide constantly more air. Otherwise You have a compressor stall, fire escaping in front of the engine and of course a loss of power (thrust). Try to find an old AgentJayZ movie where he shows a "dymmy" load for testing engines. It's made of a compressor with a metal plate covering the "exhaust" of the compressor. Without the cover the compressor would not have a load, it would not compress air (maybe a little).
@@laertesl4324 by that model, would the fuel combustion also be considered a thrust-modifying factor (as with the jet pipe)? Or something else? My mind wants to think about this from a momentum transfer perspective, but I'm having trouble thinking about combustion from that perspective. Where I'm at right now is: the momentum of the air doesn't appreciably change during combustion, does it? Because the mass doesn't (appreciably) change, and the increase in velocity is caused by a corresponding decrease in density, which means that your actual flux stays about constant, no?
While we're all thinking about it, where does the thrust force act upon the bell of a rocket engine? I guess fundamentally it's the same closed system with a hole out the back as in the turbojet engine or balloon.
Actually, the thrust of a rocket engine is found by integrating the pressure forces along the entire inner surface of the combustion chamber, starting at the bottom of the bell and going all the way up to and including the injector plate/manifold. When you do the math, it comes out that the force is actually mostly acting against the injector plate way up inside the engine. It's weird I know, but that's how it is.
"What part of a jet engine bears the load of the thrust?" has the same validity as the statement: "Rocket engines can never work in space because there's no air for the rocket exhaust to push against." A turbojet engine is a dynamic, reaction based system. You can take a turbojet engine at full throttle, measure all the pressures and forces inside it, then take the thrust nozzle off, turning it into a gas generator, and the pressures and forces inside it are exactly the same. The closest thing to a meaningful answer to the question is that the radial loads trying to push the thrust nozzle apart at its narrowest point is where the thrust loads are concentrated. If that doesn't make sense, then there's one key element of a jet engine you're not understanding. A jet engine is a system which *includes the air roaring through it*. With the thrust nozzle attached, the output is a narrow, high speed stream of air. Without it, the air expands in all directions as soon as it clears the end of the turbine like a constant explosion. If you can conceptually understand why this difference exists, you'll understand why the question isn't meaningful.
@@airgliderz I think what he meant was that the statement "rockets can't work in space because there's no air" is a ridiculous and laughably false statement. He was not trying to say it was true.
"Rocket engines can never work in space because there's no air for the rocket exhaust to push against." This is easy to explain, when we equalize all the pressures, only the difference between the pressure on the front wall and the back wall with the hole remains. The comparison with a gas turbine is very good, otherwise I don't know if it creates the same thrust or not, the difference to a turbojet engine is the rear nozzle, I understand, but not this sentence: radial loads trying to push the thrust nozzle apart at its narrowest point, in my opinion is distributed over all parts of the engine
Its the combined mass of air and jet fuel (typical) that is the (accelerated) thrust.. Fuel plus air > Air alone in mass (density of matter) more matter moving in a direction = more ass in the flow... More mass = more thrust... its a conservation of mass equation.. simply
This. Mass of the air times the acceleration. Acceleration can be viewed as the delta V, the change in velocity between the front and rear of the engine. Mass times acceleration equals force.
@@kevingallineauii9353 That doesn't explain where the force happens, though. You showed that there must be a thrust force on the engine because the momentum changed, but that doesn't explain where that happened.
Every step where the air experiences net acceleration? So, a little in the compressor, the combustion chamber, not the turbine, then again in the nozzle.
Yes, except the compressor accelerates and decelerates the air in steps, each step increasing the pressure. The air speed into the compressor in flight is usually higher than the exit speed.
Why roocket engines have totally different shape of exhaust nozzle than jet engines? They fundamentaly work the same - pressure of hot gasses propels attached vehicle forward. But shape is totally different?
Both afterburning jets and rockets use the same converging-diverging nozzle. Look at a cross section and you will see the same shape, but the proportions may be different. The optimal proportions are dependent on the ratio of ambient pressure to nozzle inlet pressure. Rockets have far greater combustion pressures (100 Atm is a typicall figure), and can also operate in very different ambient conditions to jets. For example with rockets engines operating in the vacuum of space the expansion ratio is much greater, so a nozzle with extreme outlet diameter is preferred. The shape is still the same con-di nozzle shape, just with different proportions. Non afterburning jets may do fine with just a diverging nozzle, so in these cases it is sometimes preferred (but not always).
Rocket exhaust speed are usually supersonic, which need a Con-div nozzle. However, the exhaust speed of a jet engine (without afterburner) are mostly subsonic. For subsonic flow we want a convergent nozzle.
I recently had a very similar discussion with a parts counter guy. I tried to simplify that a turbojet is essentially an atmospheric amplifier with an added blow dryer effect of volumetric expansion. This did not seem to compute, so I went visual and said "Imagine strapping a corset on a tornado, where the corset is containing a section of the tornado and all the boosted pressure is also expanding out the ass section with great force."
I'm fairly sure that I understand this question. I am but a lowly mechanic/problem solver/fabricator/hack and have heard the Honorable Agent JayZ mention (Ad Nauseum) that I am not to use my Piston Pea Brain when it comes to Jet Engines and I understand why and when he says this, but I am but a simple man and very likely a simpleton. If I am understanding this question properly and I believe that I am...we are looking for the final surface inside an engine that basically bears all the load of the thrust the engine produces...the last leg it stands on "if it were" AND ALSO what the thrust pushes off of, like the air? Final surface? No clue... Thats all I can come up with on that one. What is the thrust pushing off of ? For me its always been the air of the atmosphere in my mind...and if it isn't, I will probably wind up at Shady Maples weaving baskets and rocking back n forth for half a year or more...Great. For me personally, I have pondered the question of "what does thrust push off of, far more frequently than where or what is the last item in the chain that bears all the thrust load etc... I've always thought about the air as being a fluid, like water, so I quickly just assume it pushes off of this barrier. I always picture a finger pushing into a very stretchy inflated balloon...the balloon skin (atmospheric "air") this air deforms inward, giving us what the flame and violence is acting upon? OR one might try the converse and say that the atmosphere is trying to squeeze OUT this "giant finger of flame" or thrust...Like a soybean being ejected by squeezing the pod around it, launching the bean out of its slippery husk/beanpod? Does the hot gas coming from the engine create a low pressure column (i.e. the "finger") that the higher pressure atmosphere is trying to squeeze away? I think I have "Thunk myself into the weeds" guys...or was it the weed that got me to thunk? Uh Oh. Well if everyone isn't laughing too hard I'd be interested in hearing where I'm going sideways here. Christ I might as well try to come up with a theory involving DARK MATTER because how else would I explain how a rocket engine works in a vacuum or the "nothingness of space"? No atmosphere in space is supposedly the Monkey Wrench that gets thrown into the program, but MAYBE the actual wrench is Dark Matter...LOL... If space is NOT a lot of "nothingness"...if that nothingness weren't "nothing" but "something" the monkey wrench is magically removed !! I solved it ! ...LOL No? Hey man you never know, it could be Dark Matter that thrust acts upon, up there in space. 50yrs from now this comment will be studied by physicists the world over man! Thats my answer and I'm sticking to it...otherwise the "white coats" will be taking me to Shady Maples for basket weaving and honestly I have things to do, so I dont want that to occur. So, for me...its Dark Matter!
Your first answer is good: basically a combination of "everything" and "nobody really can say"... excellent. Your second answer: I stopped reading right away. Thrust is created by throwing stuff away. It is not "pushing" on anything. The acceleration of the air inside the engine pushes the engine in the opposite direction of the accelerating air. What happens to the air after it leaves the engine does not matter at all. It's easier to visualize if we think about a rocket engine, because it can run without taking in air. In the atmosphere a rocket engine throws combustion gases at high speeds to the rear (or downward if on a test stand), and the acceleration of the gases due to the burning of fuel inside the engine creates thrust in the opposite direction of the flow of those gases. The thrust is measured and the engine is given a thrust rating. In space, where there is no air or anything else surrounding the vehicle, that same rocket engine, burning the same amount of fuel and oxidizer, accelerating the same amount of gases out of the nozzle, actually creates more thrust. Since there is no air, or air pressure "in the way", the gases leave the rocket nozzle at a higher speed, and this extra acceleration creates extra thrust. The fact that the gases are being thrown into empty space proves that they are not pushing on anything. They are literally "pushing" on nothing. I hope this helps. I also have a video on this subject called Jet Engine Thrust. That one is a bit of a rant...
@@AgentJayZ I am honored you took the time to reply to my ramblings Sir ! I never expected anyone to read let alone respond but I am happy to watch the "Jet Engine Thrust" video you mention. I must admit I have thought about the questions I rambled on about more than a few times since... I uh... rambled...Its a very interesting set of variables I must admit. Keep the superb videos and content coming my friend the work you put out is valued much more than you could possibly know. I hope you at least got a chuckle from my comment. Take care!
I think the graph shown in this post is very clear when it comes to velocity, temperature and pressure: aviation.stackexchange.com/questions/33280/why-is-there-a-pressure-drop-in-the-combustion-section-of-a-jet-engine Looks to me that it is correct ("makes sense", but I'm not jet engine engineer ;) ) Don't know what the orignal source is, but it seems to be a cleaned up version of the image in this post: aviation.stackexchange.com/questions/11744/why-do-gases-in-the-combustion-chamber-only-flow-one-direction-to-the-gas-turbin They don't explain thrust though, so it's a little bit off topic.
Interestingly, this one doesn't break down the compressor blades and vanes velocities variation and only showed the turbine section row by row, just like the Avon picture AgentJayZ showed.
I've been trying to get the answer to this for years and no one can answer it. How does a jet fighter jet engine not a turbo fan deal with water? Where does the water go after it enters the intake on an F-16? Or any other jet engine. I know how it leaves a turbofan or ducted type jet engine that's quite simple I just don't know where the water goes when it enters a actual jet engine how does it stay lit?
Water becomes steam as the compressor heats up the air. This adds to mass flow, and actually increases power. Any amount of naturally occurring rainfall, even the heaviest monsoon deluge, is a puny, tiny, insignificant amount. Oh, and the engine in an F-16 is indeed a turbofan. Also in the F15, F18, F22 and F35.
@@AgentJayZ ok thanks for the information. I was under the assumption that the F-16 had an actual jet engine. Or what I consider an actual jet engine not a turbo fan or ducted fan. Is there any planes that fly using just the thrust of the engine and what engines are they? This is what I consider a jet engine. All others to me are just ducted fans with a jet engine powering it.
@@triggeredmonkey3439 by "actual jet engine", are you referring to turbojet(compressor-burner-turbine-nozzle)? If you do a google search of aircraft with turbojet engines you should find an answer to your question. Most aircraft today are turbofan due to their increased efficiency. Which to my understanding, it is easier to move more air, more mass flow versus adding lots of energy to a small amount of air(energy loss associated w/ energy conversion). So the mass flow through the fan is what creates much of the thrust on modern day aircraft.
Okay, we have developed thrust. Now how do we get that thrust into the plane? Looking at your test stand, there seems to be two large pins that are 50mm in diameter about where the thrust bearing is located. There is another attachment in the rear of the engine. That's it. All that force on three points?
As AgentJayZ has said, usually there are only two main mounting points in the turbojets that he services, which take both the thrust and most of the weight of the engine, plus a third mounting point that takes just a small proportion of the weight. In the big turbofans, all the thrust is usually taken through a single point into the engine pylon.
Hats off to AgentJayZ for this and all his other informative and interesting videos. Please refer to this image to follow my comment and quesion: ibb.co/TYVy8HQ (gas load distribution "The Jet Engine, Rolls Royce") The image is a screen clip from this link: aviation.stackexchange.com/questions/33068/on-which-points-in-a-jet-engine-does-the-reaction-force-act I am endlessly fascinated by this topic. Understanding all that is happening in a turbo jet engine is almost like taking a full physics and mechanical engineering course. Which is, of course, the source of all the contrary statements on how the engine works - I don't and it seems most people don't really understand all the physics involved. That said... My Question - AgentJayZ said at 8:35 that the majority of the thrust comes from the acceleration of gases caused by the constriction/taper of the end of the propelling nozzle (paraphrasing). Following this he said the change in gas speed from inlet to outlet is what causes most of the thrust. Many explanations I've read have talked about how the huge amount of gas roaring out the back is where the thrust happens. Or, is AgentJayZ saying the thrust (the force, or the energy, or work) is created by the change in gas velocity from inlet to outlet? The thrust (pushing/pulling) forces then act on different parts of the engine in unequal amounts as they are generated or encountered. Here's my simple take - 1) The magic of a jet engine is in the combustion of the jet fuel in a pressurized/high volume stream of air. The combustion process results in an enormous increase in: gas molecule volume (the stoichiometric ratio of jet fuel, 14.7:1), heat, and velocity but oddly enough, not pressure. 2) The hot, almost explosive expansion of gas is what provides the work, the force to spin the turbine which in turn drives the compressor which, combined, drives the entire process. 3) the thrust (the push) happens as follows as per the diagram at the top of this comment: 3a) 19,049lbs forward thrust (forward gas load) comes from the spinning compressor blades pulling massive amounts of air into the engine (much like how a propeller works pulling an airplane forward). 3b) A much smaller amount of forward thrust comes from the diffuser, 2,186 lbs. 3c) 34,182 lbs forward thrust within the combustion chamber (hot expanding gases can't go forward due to higher gas pressure from compressor stage, the only option is for the hot gas to shoot out the exit noozle(?) of the combustion chamber, where it... 3d) hits the turbine causing it to spin at a high speed. This, however, exerts a rearward thrust of 41,091 lbs (rearward gas load). 3e) Next, the hot gases are squeezed in the exhaust unit and jet pipe resulting in a forward thrust of 2,419 lbs. 3f) And finally, the gas is further compressed by the propelling nozzle resulting in a rearward thrust of 5,587 lbs. which increases the velocity and reduces the pressure to almost ambient. 3g) The net total thrust (forward) is 11,158 lbs. All of these competing forces have to be carefully balanced for the engine to run. Easy peasy :) I welcome corrections and comments.
Maybe you should make a remake of this video. - Draw a bigger cross-section of an engine showing all relevant parts but no more. - Draw the graphs to perfectly align with the relevant engine parts. Draw your own graphs to the best of your knowledge. Now the whole is a bit confusing. You use 2 engine cross-sections, different sets of graphs and divert a lot about the shortcomings of the graphs. Near the end of the video you can explain that there may be some subtle differences. - Olaf -
Maybe you can read a book or two, visit the NASA website, and then sit back and consider that what you ask is available to anyone willing to pay for it. It cost me about 20K for my one year preparatory course, a great deal of extra "unnecessary" reading, and so far 17 years of working in the field to gather the admittedly incomplete knowledge I have. If you are dissatisfied with my attempts to both share and inspire, please contact the office of bitching and whining for a full refund. Their number is 1-800-JET-FUEL
@@AgentJayZ, In fact, I am very happy with the videos you make. It wasn't meant to be complaining or just moaning in general. It is just a difficult subject. Any attempt to explain it is much appreciated. I just feel it would be clearer if you stick with one cross-section of an engine and align the graphs. That's all. Best regards. Olaf The Netherlands
@@olafv.2741 don't apologize, your initial comment wasn't rude.. it's just that some people like mr Jay decide to reply to comments when they are having a bad day.
Bernoulli is just a special form/case of the first law of thermodynamics and from 1st law you can still get to appropriate equation for turbojet. It is quite common for literature or discussion aimed toward general audiences to use the term Bernoulli as a pronoun for 1st law applying to fluid mechanical problems.
@@Skyshade yes, a special case only applicable in incompressible flows. A turbojet convergent nozzle accelerates the air to speeds quite close to Mach 1, hence compressible flow, hence Bernoulli not applicable.
@@rodgerq The usual in classical physics (we are not dealing with quantum mechanics or relativity here): Newton's laws (momentum equation), conservation of mass (continuity equation), the first and second laws of thermodynamics, the equation of state for gases. All of them applied to fluids, which is a little bit more difficult than for solids, produce some complex vector differential equations that unless you apply some simplifications (like incompressible flow, inviscid flow, isentropic flow., one dimensional flow... when possible) can only be solved by numerical methods, normally by computer. Bernoulli is just one solution of all this for a simplified case, which is very useful in some cases, but not in a turbojet nozzle. If you want to go deeper I can only recommend John Anderson's great books "Introduction to flight" and "Fundamentals of Aerodynamics". They are not cheap and they are not easy (the second requires vector calculus knowledge), but they are one of the bests books on the subject, giving not only the physics but also a very interesting historical perspective.
Since the velocity is graphed against distance, the acceleration of the gasses is the derivative of the velocity graph times the velocity. Which is useless information, because they didn't label their axes.
is it not simply related to the weight of the whatever molecules are in the exhaust and the velocity in which they are flung ?ry8? they only reaction that causes actual thrust to move an object must be a seperate mass that leaves the object ,ry8? if you stand on a skateboard and throw a heavy object you will roll the opposite direction(a very symplistic ,splanation)in relation to how hard you pitch the object and how much it weighs however if you simply fake throwing the heavy object but hang on to it you roll back and forth the object must be released to gain net thrust this is how i see it as a highschool graduate, from my head, NOT cheating and looking for written in stone answers
Yes, but that doesn't help identify where the thrust comes from. All you've done is shown that momentum increases, which necessarily implies a force was exerted on the fluid but does not mean anything about what the actual forces are.
So thrust is the result of accelerating forces out of the rear of the engine being manipulated by the shape of the rear and pushing the engine forward by pressure on the front .
the massive acceleration and release of energy out the jetpipe is imparted against the turbo-machinery of the engine onto.......The Thrust Bearing , which bears and transfers said energy produced to the engine casing to its mounts and to whatever that is attached to i dunno like a ship?? this brings up afterburner, is that release of energy, pressure or whatever directed directly on the "?structural exhaust parts?" being attached to the parts that are bolted to the mounts . I guess working backward the energy to whatever is moving is transferred thru the motor mounts so start there end go backwards??? sorry for the improper nomenclature, horrific grammar and probably countless spelling errors.
The term 'thrust bearing' is, unfortunately, very misleading in this context. R-R Derby uses the term 'location bearing'. which is more accurate, in terms of the function of the bearing. The load from the thrust/location bearing is one of several loads, both forwards and rearwards, as can be seen in the diagram that AgentJayZ has shown us ( and with which I am all too familiar), the summation of which gives the engine thrust. I've quoted this example before, but it does make the point. I was involved in the design of a certain two-spool industrial gas generator. Unfortunately, we underestimated the forward load on the IP location bearing. The solution was to introduce a thrust balance piston at the front of the IP compressor to reduce the bearing load. In the next mark of the engine, we introduced a new HP turbine, which was designed for an uprated version of the aero engine: this actually changed the load on the HP location bearing from forwards to rearwards.
So its just like a garden hose. The walls and head keep the water going out the back instead of anywhere else, and to get any real thrust out the back you narrow it down to the appropriate amount for the flow and pressure. The working bits just work to keep the hose running.
I’ve been through these arguments before, but it’s worth repeating them here, as a contribution to this discussion.
It is incontrovertible that the thrust produced by an engine can only be as a result of the acceleration of the exhaust gases, initiated by the combustion process and completed in the final nozzle, courtesy of Newton’s Second Law. However, the thrust does not “act” on the nozzle. On the contrary, the engine is, in effect, trying to blow the nozzle off, as the force acting on it is rearwards.
As a reaction to the rearward acceleration of the exhaust, there must be a forward force on the structure of the engine, in accordance with Newton’s Third Law. The net forward force, which is thrust, can only be as a result of a summation of the internal loads, both forwards and rearwards, caused by the effect of the various pressures throughout the engine, acting in an axial direction on the surfaces of the engine components.
There is no one place where the thrust can be considered to act, least of all on the so-called thrust bearing(s). I can and have, in my career, changed the load on a thrust bearing from forwards to rearwards, by means of changes to the sealing and pressurisation arrangements on the compressor and/or turbine discs.
Referring to the thrust distribution diagram of the Avon, lifted from chapter 20 of R-R’s ‘Jet Engine’ book, it is a gross simplification and, in one very particular respect, quite misleading. Firstly, the forward and rearward loads quoted for the compressor and turbine respectively are not for the rotors and the difference is not the load being applied to the thrust bearing - or as R-R Derby prefers to describe it, the location bearing. The loads are intended to represent those being applied to both the rotors and the stators of the compressor and turbine and do not necessarily account for the pressure loads on the discs. So exactly what are the loads quoted in the diagram?
My best attempt at explaining what they represent is to suggest that, if you imagine the engine being instantaneously sliced up arbitrarily into the sections shown, then the loads measured on each of those sections at that instant would be as quoted. The thoroughly misleading figure is the enormous forward load quoted for the combustion section, whereas the actual loads on the combustion chambers (aka combustion liners or flame tubes - you choose your preferred term) is rearwards. Similarly, the rearward load quoted for the turbine section is equally misleading.
This issue arises from the fact that the dividing plane between the combustion chamber and turbine assembly has been quite arbitrarily defined as being at the junction of the circular section of the flame tubes with their turbine entry ducts. If the dividing plane had been defined at the entry to the first stage NGVs, then both figures would have been substantially reduced, but the difference would be the same.
I intend to throw a spanner in the works, as we say in Brit English, by referring to the Pegasus engine, but I’ll do that in a separate comment.
Attention to everybody who commented on this video: Unless you spent a career designing jet engines, this is your opportunity to learn something.
So shut up, and read Graham's comment in its entirety.
If you don't agree with what he says, then it's you against a guy who designs these engines, and a guy who builds these engines.
Uh... what was your occupation again...?
@@AgentJayZ can we say, in laymen terms, that the air is pushing on air and that "push" is directional and causing the "thrust"
We can most certainly not say that.
The idea that the exhaust pushes on the atmosphere is a very common misunderstanding, so don't feel bad. But it is not correct at all. If it were true, rocket engines would not work in the vacuum of space.
Actually a rocket engine works better in a vacuum.
@@HardusHavenga The "push" can only act on the structure of the engine.
AgentJayZ mentioned a balloon and I refer to the way it produces thrust when I build balloon-powered cars with 8-11 year-olds. The thrust is acting on the projection of the cross-sectional area of the neck on the interior surface of the balloon opposite the neck - not on the air at the outlet of the neck.
@@AgentJayZ It's almost like someone planned it that way
I really have to give you credit Jay. Even if many aren't doing a great job - you're getting people to struggle a little bit with physics. That's certainly a net benefit for the world.
Thanks! Have look at my latest vid, and we'll see if your thoughts change...
@@AgentJayZ - I'll look - but I doubt it! Thanks for what you do. I've learned a lot - and been reminded of a lot that I haven't used since school.
Hi AgentJayZ, I've watched your channel for many years and I must admit it's always been a mine of information. If not presented by you, I was just inspired to do my own research if some topic would interest me. You're making the videos better and better, great job.
Nice video. I enjoyed it, but I must correct you on a few crucial points:
1. Graph e - (local thrust) is just the total thrust generated up to a certain point. That is why the last point on the graph is marked as the net thrust. It is a bit confusing at first glance, and not of much use IMO.
2. Graph f (integrated thrust) is the arrows marked on the engine diagram designating the thrust contribution of each section. You seemed to have missed this, and IMO this is the main point of this diagram. It clearly shows that the biggest thrust contributor is the combustor, followed by the compressor.
3. At 5:01 they were not confused - the inlet is also commonly called the diffuser because that is an accurate description of its action - it diffuses the incoming airflow (slowing it down and increasing its pressure). They did not mix it with the compressor's outlet diffuser.
4. 15:00 - It is more accurate to say that the nozzle is why the thrust is generated, but not where. Look at graph f again - the nozzle is actually pulling the engine back with a negative thrust of 5587 lbf, but without the nozzle the upstream conditions at the gas generator would not be possible such that the compressor and combustor would not be able to provide the positive thrust.
5. 17:17 - that is not a mistake. You misinterpreted the graph to be static pressure, but it is total pressure. The static pressure indeed drops to about ambient in the nozzle, but total pressure remains (almost) unchanged. The same with the temperature (17:56) - this is a graph of total temperature which remains constant through the nozzle (we usually consider total figures when discussing turbines).
6. 18:17 - this increase in airspeed implies low speed (or even static) operation of the engine. At low speeds the compressor sucks in air which necessitates an increase in speed ahead of the compressor. At high speeds the ram air effect is overpowering and the reverse happens - excess air will bypass the inlet and thus the speed will actually decrease prior to entering the intake.
Also, as mentioned before, the inlet is designed as a diffuser - therefore the airspeed within it decreases, so in a low speed operation you will see acceleration of the air before the intake, followed by some deceleration through it. The fact that we see speed increase to the compressor in this graph leads me to believe that this shows a static operation of the engine, equipped with a bell mouth (which is a nozzle - the opposite of the intake diffuser).
Great summary, specially point 4 that many people gets wrong.
This comment should be pinned on top IMHO.
@@laertesl4324 4.0 Exactly. This is the most useful contribution.
from Mike back home in UK with cat Nibbler, thanks for all your videos and education of the gas-turbine, cheers back to you
I promised to throw a spanner (wrench, if you must) in the works by referring to the Pegasus engine (so start thinking about what happens when the thrust is acting at right-angles to the axis of the engine). However, I've been busy with a little community project, but I'll discuss vectored thrust shortly.
I can say for at least forty years I have been waiting for these and this question to be answered.
Jet engines are difficult to understand if your mind (like mine) is fixated on pressure. Look up the Bernoulli Principal first and things will become clearer.
Superb video as always!
You are awesome. There is nothing else to say.
Repeat after me: Every dyne of thrust comes from a gas molecule impacting a rigid surface.
But the air in the exhaust nozzle is increasing speed!!! Accelerating!!! What is pushing it?
Well, the molecules being accelerated are bouncing off molecules closer to the front.
The molecules towards the front are higher pressure. (Maybe higher temperature, not pressure?)
You work your way forward until a molecule actually bounces off a rigid (maybe rotating) surface.
THAT molecule imparts forward thrust on the engine, and ultimately through the engine mounts.
The interesting thing you didn't stress in the force drawing is this:
The change in force (vertical axis) in a section of the graph is seen as rising or falling.
This corresponds to a rearward (rising) or forward (falling) force contribution for that section.
I think this is all true.
I think the gas molecule to gas molecule transfer of force means that the air in the engine better be traveling slower than the speed of sound.
The force can't transfer upstream faster than the speed of sound.
But what about rocket engines, with their pretty supersonic diamonds?
And aren't jet engines really rockets which manufacture their own oxidizer?
I think the thrust comes from the exiting gas hitting the walls of the diverging nozzle.
I don't believe thinking about pressure is how engines are designed.
I think engine designers think in terms of energy, entropy, static v.s. dynamic pressure.
As you mention many times, designers are extremely concerned with temperature tolerance of their materials.
I am not a jet or gas generator engineer. But I am a believer in atoms.
Corrections gratefully anticipated.
You are awesome. Dog looks like he/she is gaining weight! Lifting weights?
Oups its enthalpy, not entropy.
More speculation.....
We know that the net force on the rotating elements (compressor plus turbine) is ~0.
We see from the chart that the forward force increases mostly in the front half of the engine.
Therefore, most forward thrust comes from ... air impacting the compressor stators.
The second region which shows as a raise in the integrated force is... in the combustion area.
Therefore, the next contribution comes from the front of the combustion chambers.
All that gas hurling out the back has only one purpose: to increase the pressure of the gas inside the engine where it presses forward on some engine element.
I remember from "Fluid Mechanics" that you can calculate force by follow a "unit mass" through the machine. The change of momentum at each place in the stream is a net force of the system.
I remember from years (& YEARS) ago by the "math" showed that in a particular situation, the net forces were the same regardless of which way the fluid flowed.
Yup. Mass transfer and acceleration. Newton.
Accelerating nozzle maintains direction of flow.
@@KozmykJ You two are confusing a nozzle which is a part where no work is done with other parts such as compressor, turbine, burner.
@@0MoTheG I was including All the Reactive parts of the system. Those which provide the net acceleration of mass.
The confusion is in the interpretation.
AgentJayZ was at pains to point out that it was not at one single point within the system that the reaction force was generated.
@@KozmykJ It is easier to answer what parts we can exclude:
Turbine
Any surface perpendicular to flow that is not redirecting it
anything that performs no work such as the nozzle
That leaves us with the compressor and not much else
Hello Agent Jay Z. Thanks for all the effort you've put in to these videos.
This one in particular has been food for a lot of thought.
I thought I could simplify the matter by considering Rayleigh flow in a constant area frictionless pipe. If you add heat to the pipe, the subsonic gas velocity accelerates and can reach Mach 1. This must produce thrust, but I could see no way that the thrust would be imparted to the pipe so, unfortunately, I reached a dead end.
By the way, I have just visited the Bournemouth Aviation museum in the UK. I can recommend it. It's a really friendly place where you can sit in various cockpits and get to touch different engines and even an APU.
I promised to throw a spanner (wrench for my North American readers) into the works , or set a cat among the pigeons if you wish. I have been busy doing other things for the past week or two, but I will now do so.
In another comment I pointed out that, in an inflated balloon that has been released, the force propelling it is the internal air pressure on the cross-sectional area of the neck projected onto the inner surface of the balloon directly opposite the neck. Now that cannot apply in a 'conventional' jet engine, as there is no single surface on which to project the cross-sectional area of the final nozzle - and yet the engine does produce thrust, just as if there is a pressure acting on a continuous surface opposite the nozzle.
So what happens when a 'conventional' jet engine, which happens to be turbofan, has four vectored thrust nozzles stuck onto it and it is called a Pegasus? Nothing changes in the turbomachinery and the axial loads remain the same: OK, the engine may have to run a little hotter for a given thrust, because of the losses in the nozzles, but the turbomachinery is 'unaware' of their presence. Nothing has changed - yet everything has changed.
So start by thinking about the engine without the ducting in place for the nozzles, but without the nozzles fitted. The engine will produce no forward thrust, as all the air and gas flow will be directed sideways on either side of the engine. There will be as much rearward thrust on the ducting as there is forward thrust from the turbomachinery. Now think about the the reattached nozzles pointing vertically downwards. Where is the thrust applied? It can only be on the cross sectional area of the nozzle outlets projected onto the inner surface of the nozzle elbows opposite the nozzle outlets.
Just as in the case of a 'conventional' jet engine, where the force on the final nozzle is rearwards, the Pegasus is trying to blow the four nozzles sideways off the engine. An early development engine did exactly that to one of its 'cold' nozzles in flight and the aircraft went down as a result.
Jay, it makes sense in the last diagram with velocity increasing through the nacelle. The engine inlet is slightly smaller than the centerline nacelle diameter (wall thickness), so a corresponding increase in velocity is logical.
Thanks for shining light into the "dark corners"!
BTW: what happened to your lapel Mic? You don't have enough flags to kill the echo yet!
The nacelle has nothing to do with engine thrust.
I HAVE FOUND THE ANSWER !!! Hello again AgentJayZ... In my initial comment (a month ago), I had unsuccessfully attempted to answer this question correctly, while simultaneously trying to add a little humor to my answer. This "humor" was very much at my own expense, but alas I am a very wealthy man in this regard and do not mind frivolously spending the currency of laughter. At any rate, I was surprised and honored you took the time to respond to my comment Sir, so let me thank you once again for your response to my comment. While it's true that I can come across a "bit silly" to most people, it cannot be said that I don't follow through on looking up or into/investigating concepts and principles that I do not understand. Without pooping the "Incorrect Answer Party" too much by writing the answer to the thrust question here, I will instead present y'all with the source material containing the answer to this question/concept. The answer can be found in the first video starting at 4:15 and continuing through 7:15 ruclips.net/video/6-GxN4t8VAE/видео.html This second video also contains a wealth of related information... ruclips.net/video/p5SCkSUijuI/видео.html
Thank You! These videos were fantastic at explaining the point!
Yeah, most of this kind of charts are simplified one way or another so they are seldom 100% correct. Thanks for the good effort to infotain.
The trouble many are having comes from their understanding of thrust.
In the engine the thrust is indeed forward pressure by area, but out the back end it is mass flow by speed.
Even though the nozzle is responsible for increasing speed and there by thrust, the thrust does not push the engine by the nozzle.
Мои скромные познания в ТРД.
Тяга в ТРД создаётся:
1) Лопатками ротора компрессора
2) Давлением на диск последней ступени компрессора
3) Давлением на стенки диффузор (он расширяется)
4) Давлением на передние стенки камер сгораний
5) Давлением на диск турбины
2 и 5 могут перераспределятся.
Сопло тянет двигатель назад. Но оно повышает давление в двигателе и косвенно способствует увеличению тяги.
Love you videos. I have learned soo much about jet engines from these. I would like to suggest that the thrust is created by the difference in pressure at the last compressor stage and the tail pipe. The nozzle causes the exhaust to accelerate and thus lowers the exhaust pressure increasing the difference in the two pressures. The burning fuel just adds the energy needed to create these pressures.
That is not what causes thrust. Thanks for the kind words.
I gave it a short thought at the beginning and my conclusion was that that the thrust comes from the compressor because that's the only part that actually pushes something towards the back of the engine. And your diagram essentially confirms that. The shaping of the nozzle only helps to increase the efficiency of the process, it helps the escaping gas to escape easier.
Any force against walls of the engine is symmetrical in all directions and does not add to the thrust. Besides, all further parts (combustor, turbine, nozzle) are protected by injecting cooling air provided by the compressor again. The hot gases are pushing against this cooling air, not against the engine walls.
Except, the drag of the turbine is bigger than the push of the compressor. He said that in the video
There's difference between aircraft and industrial jets: in industrial jets, the turbine extracts almost all energy from the gas - the push of the gas against compressor is almost equal to push of the gas against the turbine. The gas leaves with minimum energy needed to keep the process going, all the rest goes into turning the engine axis to power the compressor and whatever other machinery is mounted to it.
In aircrafgt jets, the turbine only extracts energy needed to keep the engine going, mostly to turn the compressor. The rest is left to escape. The force pushing the compressor forwars is less than the force pushing the turbine backward. That's why there's thrust in aircraft jets.
我是您的崇拜者！您讲得很好！非常好！
我不太懂英文，我是用录音和翻译来听您讲解的。
我知道您不懂中文，但是我还是想说，谢谢！非常感谢！永远支持你！
Google helped me out, but I had to guess this was "traditional Chinese". Thanks!
I'll go Jet Nozzle applying force through the Thrust Bearing on to the Frame and then on to the Mounting System of the aircraft / testbed. But then what do I know?
You are yet another person who has been misled by the term 'thrust bearing'. please see my reply to Mr burroaks 7.
thanks for all of your work entertaining and educating us! A fascinating area of technology. Also.. who doesn't love a faithful shop doggie?? A nice finish to a video.
my understanding:
the thrust comes from the velocity of the gases exiting the nozzle. the thrust ultimately pushes against (acts on) the back of the blades of the compressor thus accelerating the aircraft in the opposite direction of the flow of the exiting gases.
the velocity of the gases comes from the pressure produced by 1.the compressor plus 2.the heating of the gases (by burning fuel both in the combustion chamber and in the after-burner).
firstly we produce high pressure THEN we decrease that pressure which accelerates the gases so that at the point where they exit the engine they move fast and have the same pressure as the atmospheric pressure.
simplified, the engine transforms high pressure gases to high velocity gases.
high pressure means the gases bounce violently around in ALL directions, high velocity means they travel fast in ONE direction.
it's a 'continually rapidly deflating balloon' kind of situation.. :}
My very unprofessional opinion is the gross motive force comes from the first compressor stage, to the exit nozzle, as viewed from front to rear.
Moving from rear to front, there is thrust from the nozzle to the combustion area. But a forward pulling effect created from the compressor stages trying to "fly" forward.
What I got from this, is that the thrust effect at the nozzle pushes against the column of air within the engine. That column in turn is "held in place" by the force of the compressor section, as you said, the mechanical parts which keep the air from moving forward. My take on the integrated thrust graph is the amount of thrust added throughout the engine, left to right. Like if you were to let the air out of a rubber balloon, but the air pushes against the blast of a hair dryer. As long as the hair dryer is stronger (pressure from compressor) than the balloon's power (combustion), it can only move away from the dryer, no matter how you point the balloon. Therefore, the drop of pressure from compressor to the combustion section is a necessary sign. I think there is critical limit to how narrow the nozzle can be then.
@AgentJayZ , can you explain how a theoretical turbojet (running max thrust) would behave, if you contracted the variable nozzle more and more, up to a complete shut? Will the compressor eventually stall? I guess so.
Found an explanation here: ruclips.net/video/F6J_mHUNVjs/видео.html
one of your other videos. So, all right, the speed of sound is the major limit to work with. I've really come to appreciate how jet engines can push a plane supersonic now.
Before the restriction caused pressure to rise to the point where the turbine lost effectiveness, the temp would go up past the point that the metals could endure, and things would melt and fall apart.
One of the primary signals used by the fuel control is Exhaust Gas Temp, so in an otherwise normally functioning turbojet, if you could somehow independently close down the nozzle ahead of its schedule ( it's physically impossible to close completely ), the fuel control would reduce fuel flow to maintain EGT, which at full power anyway is right about at the upper limit.
@@AgentJayZ Thank you! Now I can see how the engine would self-regulate in that scenario. So, the fuel flow is always controlled - I remember you pointed out how the pilot may think he controls the fuel flow directly with the levers, when in fact he only asks for the the engine to negotiate the corresponding power output.
HUGE thumbs up RE: Plane savers
Is the area of the inlet or outlet of the combustor larger?
From my understanding, there is no single point where the thrust is acting, but is the cumulative force along the center-line of the engine.
Each section adds or subtracts overall thrust. A good example of this is the exhaust nozzle taper. The thrust generated at the end of the turbine section is at the highest point, but the taper helps maintain the gas velocity and extract more thrust. If you were to remove the taper, you would effectively lose overall thrust.
Imagine a liquid fuel rocket engine. It basically has a combustion chamber with three holes. Two of them is used for injecting pressurized oxygen and fuel and the third one is the exit for gases. The force is created by the gases pushing on the chamber's walls. The forces are not equal in all directions since there is a large hole on one side (no walls there). That inequality is what matters. The force can be calculated by multiplying pressure inside the chamber and cross-section of the exit. You will get the similar result by multiplying exhaust gas mass and its accelleration. Now, turbojet engine is mostly the same to me. The turbine and the compressor are just for delivering air. In large rocket engines You also have a turbo-pump to deliver fuel and oxygen into combustion chamber. In any case some energy must be used to pressurize air (oxygen) and fuel. Of course all of that is just a simplification, but it may explain something.
@@Bodi2000 OK, I've made a big simplification, maybe too big :) . I believe the rocket nozzle acts similar to converging and diverging nozzle which AgentJayZ
mentioned in "Afterburner Jet Propulsion Nozzle" video two weeks ago.
In case it helps with the discussion, here's a fresh scan of the first diagram from the textbook along with related text on adjacent pages:
www.dropbox.com/s/pjj7utjpl5u6rbe/jet%20engine%20thrust_scaled.pdf?dl=0
Pages 162-164 in "Anderson, J. David. Aircraft Performance and Design. McGraw-Hill. 1999." It is a reference text for upper-level courses in aerospace engineering programs.
And the relevant paragraph for those hesitant to click on a stranger's file link (apologies for any typos):
"The thrust generated by the engine is due to the net resultant of the pressure and shear stress distributions acting on the exposed surface areas, internal and external, at each point at which the gas contacts any part of the device, as described in Section 3.2. Figure 3.10e illustrates how each component of the turbojet contributes to the thrust; this figure is essentially a picture of the "thrust buildup" for the engine. The internal duct of the diffuser and compressor has a component of surface area that faces in the thrust direction (toward the left in Fig. 3.10). The high pressure in the diffuser and especially in the compressor, acting on this forward-facing area, creates a large force in the thrust direction. Note in Fig. 3.10e that the accumulated thrust T grows with distance along the diffuser (1-2) and the compressor (2-3). This high pressure also acts on a component of forward-facing area in the burner, so that the accumulated value of T continues to increase with distance through the burner (3-4), as shown in Fig. 3.10e. However, in the turbine and nozzle, the net surface area has a component that faces in the rearward direction, and the pressure acting on this rearward-facing area creates a force in the negative thrust direction (to the right in Fig. 3.10). Thus, the accumulated thrust F decreases through the turbine (4-5) and nozzle, (5-6), as shown in Fig. 3.10e. However, by the time the nozzle exit is reached (point 6), the net accumulated thrust F_net is still a positive value, as shown in Fig. 3.10e. This is the net thrust produced by the engine, that is, T = F_net. A more diagrammatic illustration of the thrust distribution exerted on a turbojet is shown in Fig. 3.10f."
A hero right there, thanks. Can someone explain to me, is the net force on the shaft really towards the rear? (41000lb backwards and 19000lb forward)
What is the main thrust bearing doing?
Thanks guys!
Using firefox 52.6 and only getting one picture with supplied url, but cut off ?dl=0 from the link gets me a downloadable pdf with several pages. Maybe this will help others? Derk - Net force is ALL combined forward (4) and reverse (2) forces shown, you don't get to cherry pick one or two and then make up math, to declare that as a valid result. Main thrust bearing is pushing forward on engine case, but so is the diffuser, and combustion chambers also connected to engine case. Main thrust bearing would provide from 1/3 to 1/2 of total thrust would be my WA guess. They should have covered that detail a bit more since it's a fantastic real nuts and bolts question you bring up.
@@leebarnes655 thanks. Sure you can't just pick some, but in the case of my question, it's okay: the question is whether the total axial force on the shaft is forward or backwards, so I chose axial forces acting on the shaft:
- compressor and turbine.
And since compressor axial force is smaller, I am still wondering what I am thinking wrong:)
@@DerKrawallkeks After that clarification, I'll capitulate and join your camp. Brilliant, even the one and only JayZ suggested he wasn't sure about this issue IIRC from the video - I wasn't clear on exactly what he was speaking to either. But it does now appear to me that rearward gas load overrides forward and shaft thrust should be to the rear. I don't think you are thinking wrong at all now - it's just a very unexpected aspect to be sure. Can say the same for the thrust attributed to the combustion chambers alone - as in WoW.
Would it be possible to increase the efficiency of a turbo fan with a adjustable nozzle? It seems like it would be a good place to start for more efficiency on a modern airliner. I mean even a half a percent would be a great savings for the life of the thing.
The benefits would be small, as they would be happening at the core exhaust only, which accounts for 10 - 20% of the total thrust.
It also depends on how you define efficiency. A variable exhaust nozzle would add weight, complexity, expense, maintenance requirements.
So far, it has not proven to be a desirable compromise
@@AgentJayZ thanks
Presumably the pressure will have to decrease as the air exits regardless of the taper, since the ambient pressure outside will be lower? I.e the taper is irrelevant?
What is missing here to make it a bit easier is the inclusion of the MASS. Accelerated and expelled (backwards) mass requires "force" and equal and opposite force pushes the engine (attached to the airplane) forward.
Works on the engine mounts duueehh! Kidding. This Seems like the best explanation I’ve seen.
@5:00 you say the 4th from the top graph is integrated graph. I think that may refer to calculus integration or adding of point in the three other graphs. Correct me if I’m wrong please!
Please make a video about what you think the engineers from GE could do better with the j79 for example and great video!!!
It's just Newtonian physics. Throw mass out the back and get an equal and opposite force towards the front. If the engine is stationary you can blow pinwheels with it.
You are quite right: the physics is very simple and answers the question as to how the thrust is produced. However, it does not answer the question as to where the forces are experienced. Please see my comments elsewhere.
@@grahamj9101 Thank you for taking the time to share your knowledge!
@@numbynumb More molecules (more mass) = more pressure
@@numbynumb avogadro's law applies to a to a closed system. A jet engine is not a closed system. Newton's third law is why jet engines work, no matter how complicated the process.
Nope
,,, so, if we run with the garden hose facing rearward at the same velocity as the nozzle, have we reached entropy ?
The garden hose is an great model how it works...if you keep the end of the hose uncovered, then you can experience no significant thrust and the water just splashes out. But if you cover the end with your finger then the water is forced through a smaller area and it accelerates and you can experience a force. How strong depends on the power of the water pump.
Newton's third law. For every action there's an equal and opposite reaction. The engine accelerates a huge mass of air. The opposite reaction drives the engine forward, along with the attached airframe. Everybody's happy! The interesting stuff is how the engine accelerates that mass of air, while remaining mechanically reliable.
magic?
Very interesting, but unfortunately, it is difficult to understand without a translator. Thanks for the informative videos, very cool!
Seems to me that in the second diagram, for angular velocity to increase going into the compressor, you'd need a corresponding drop in pressure compared to ambient. (i.e. the green line should go down from the first to the second point on the graph, as the red line goes up)
Since this axial velocity increase in front of the compressor is caused by some intake cone, I think the pressure doesn't drop(like it normally would), since the volume is getting smaller.
The intake cone is displacing air.
@@DerKrawallkeks Ah, thanks, didn't think of the cone! 👍
The "integrated thrust" diagram is telling you the total thrust contribution of each part of the engine as you move from the front (which you might say is set at "zero" on the diagram because it intersects the horizontal axis) to the back.
It is only an approximation; a more precise diagram would show little "upward steps" for each stage of the compressor (including steps for each stator stage), a slight increase at each expanding section of flow through the combuster (burner), a slight decrease at points in the combustor that are either straight (due to friction) or converging (due to pressure against the gas flow necessary to compress it). There would be little "downward steps" for each stage of the turbine and each hot-section stator.
The "derivative," or slope, of the integrated thrust curve would show the contribution for each stage, with positive contributions for the compressor stages, both rotating and stationary, and with negative contributions for pretty much everything else except the those places in the combustor in which the gases are expanding, and the little bits between the turbine and hot section stators where the pressure of the flowing gas has a forward (or leftward on the diagram) component as it presses against the flaring cross-section of the hot section.
Almost all the forward force, or thrust, arises as a result of the compressor stages chewing their way through the air mass that flows into the engine intake. Each compressor stage acts exactly like an ordinary propeller, adding rearward momentum to the air flowing through it and by so doing experiencing a forward force on the compressor blades, just like the blades of an ordinary prop. These forces are all summed up by the compressor disks (summing the thrust contribution of each blade) and compressor shaft (summing the contribution from each disk); the shaft is strong and stiff; these summed forces are exerted against the thrust bearing, which must be present somewhere along the length of the shaft. It is the thrust bearing(s) that transmit the thrust to the engine housing, and from that to the airplane, to which the engine is obviously rigidly connected.
There is a some forward (thrust) contribution from the compressor stator blades too; this force is collected by the compressor housing and adds to the total thrust force on the engine, which is also transmitted to the airplane by the engine mount.
The turbine section is a "consumer" of thrust; it is therefore a "drag" on the airplane. This force, times the velocity of the gas column moving through the hot section, is what does the work that spins the turbine. Almost 100% of this work is transmitted to the compressor, at least in a turbojet engine. The hot section stator blades also "consume" thrust. In a turboprop, or if there is bypass, the compressor doesn't consume all the turbine output; some goes to the prop or first stage fan.
The amount of force "consumed" by the hot section is less than the amount of force "generated" by the compressor section for two reasons: (1) the gas moving through the hot section is going a lot faster than the gas going through the compressor (you can see this in curve (d) in the diagram you showed) and (2) there is a lot more _volume_ of gas moving through the hot section than there was through the compressor, because the gas column was strongly heated by the combustors before it hit the first-stage turbine; this expanded the gas column, as you know. The difference in thrust "generated" by the compressor section and the thrust "consumed" by the hot section is what is shown in the diagram you used as _F_ net, the net thrust [curve (e)].
The product of these two increases, in speed and volume, is what allows the turbine to extract enough energy to spin the compressor _without_ _exerting_ _as_ _much_ _drag_ _on_ _the_ _gas_ _column_ _as_ _was_ _generated_ _in_ _thrust_ _by_ _the_ _compressor_ _section._ It is this difference that yields a net thrust for the engine, and allows the engine to do mechanical work on an airplane and thereby keep it moving forward.
Any expanding part of the air path is a "generator" of thrust, and any converging part of the air path is a "consumer" of thrust. This is obvious if you think about it; it's why rocket engine nozzles are expanding, bell-shaped affairs. The thrust generated from a rocket engine is almost entirely exerted on the forward wall of the combustion chamber (the "power head") and to a lesser extent along the expanding wall of the nozzle. These contributions are miniscule in comparison to those of the compressor blades.
I hope this helps.
P.S. I know what you're thinking: "what is the effect of an afterburner, where is _it's_ thrust contribution transmitted to the airframe." Believe it or not, I've actually told you enough to figure that out by yourself, if you think about it.
What's that the kids are saying these days?... TL/DR?
@@AgentJayZ Yeah, I guess so. Oh well.
BTW, I love your videos, and have watched probably every one over the last five or six years. Many things stuck in my mind about them; probably the most vivid and unforgettable was when you demonstrated the amount of fuel sprayed by one nozzle at operating pressure, I couldn't believe it. When you think of the amount of flame that would have generated if the fuel was burning, and then multiplied that by eight or nine for the combined heat output of all the combustors in an LM1500, and... wow.
Thanks. I will try to read all of your big comment after work tomorrow.
Please see my reply to Mr burroaks 7, as even you appear to have been misled by the term 'thrust bearing'.
The thrust bearing (R-R Derby uses the term 'location bearing') is reacting the difference between the forward load of the compressor rotor and the rearward load of the turbine rotor. In most cases that load is forwards. However, dependent on the design features of the engine and its internal air system arrangements, that load could be rearwards.
@@grahamj9101 The compressor exerts a force "forward" on the shaft, the turbine exerts a force "backward" on the same shaft. In order to provide net thrust, the force generated by the action of the compressor as it "pushes" air into the engine must be greater than the reverse force generated by the turbine as it extracts momentum from the hot gas column moving through it.
This difference is transmitted to the engine by means of a thrust bearing, which is a bearing that's designed to handle large loads parallel to the shaft in addition to loads perpendicular to the shaft.
If due to some transient effect, the force generated by the turbine exceeds the force generated by the compressor stages, yes, the thrust bearing will experience a backward force. In an operating aircraft engine, that condition had best not persist very long.
There are engines in which the flow of gas is from the back to the front, but those (as far as I know) are all turboprop engines, an example of which is the PT6, made by Canadian Pratt & Whitney. In that engine I suppose the thrust generated by the turbomachinery would be to the rear, but would be overwhelmed by the much greater thrust produced by the prop.
The PT6 is designed in this way to put the hot section at the front; this is done because the hot section needs more frequent overhauls than does the cold (compressor) section, and putting it at the front greatly simplifies the overhaul workflow, which yields a cost savings for the owner.
As a matter of semantics, yes, a "thrust bearing" that's experiencing a net axial force toward the rear of the airplane might be called a "drag bearing," I suppose. But the term "thrust bearing" is applied to any bearing that's designed to safely sustain a large axial load and support that load without suffering a greatly increased rate of wear, as would an ordinary ball bearing for example.
In everything I've written, both here and in my earlier comment, I've used the words "thrust" and "drag" as they are applied to the system of reference of an aircraft in flight. Thrust is to the front or fore end of the aircraft; drag is to the rear or aft end.
AgentJayZ, what is the TBO (Time between overhaul (in hrs) for an LM1500?). You have show several engines that have come in for service, that look like , the owner ran it past the check engine light, i.e they look incredibly beat up. Do you change the "plugs" (i.e high energy ignitors) when you service the turbine or do the ignitors last indefinitely because they are only used in starting)?
Ignitors are an "on condition" item for industrial use. We change them when they stop working.
Some LM1500s have over a hundred thousand hours on them.
It's partly the water pressure running down the hose and partly the hose itself and partly the thumb on the end of the hose.
I maybe disagree with your disagreement with the first model which showed an increase of velocity going through the compressor. The air passage tapers smaller so it should go faster due to a Venturi effect (like when air is squeezed through narrowed passage on a carburator), at least a little, shouldn't it?
The tapering down of the compressor gas path is to compensate for the fact that as air gets compressed, it takes up a smaller volume.
So thew tapering is to keep the velocity the same, instead of slowing down.
I wasn’t going to say laser pointer - I thought you were tapping the diagram with a Sherlock Holmes style tobacco pipe like a 1950’s British engine designer. Right I’m heading for the jet wash.
Is is a wrench, not a pipe
Please put subtitles my humble request
greetings everyone. very well explained. success.
Laser pointer! I would want one of those squirrel suits before jumping into the jet wash!
Hope Graham is doing ok, haven't seen him around in a while and his channels been silent for a year.
Please see the contribution that I've just posted.
I'm fine, thanks, but I've been increasingly busy with Grandpa and family duties, both here in the UK and in Singapore (and in France when they come back for their skiing). I've also been quite busy as a Bloodhound Education Ambassador, assisting a school events where the kids build and test little rocket-powered cars.
Nice! glad to hear 👍
@@beachboardfan9544 Oh yes, I forgot to mention that I do some sailing too, with another trip to France planned for the last week of July. Before that, however, I'll be visiting my daughter and family in Singapore for three weeks, with another trip planned in October/November.
I've also got the 54th anniversary of my 21st birthday coming up in August. I'll be celebrating it with my son (who flew an Apache for a few years) and family at an event at the UK's Army Air Corps base at Wattisham in Suffolk.
There is SO MUCH awesome in that comment its ridiculous, congrats on everything! I'll also be celebrating with you in August but it'll be my 12th, 21st birthday anniversary 😄
How about critical speed on the turbo jet ?
As far as my incomplete engineering knowledge goes, yes.
The integrated force graph is just the sum of all forces before that point along the engine i believe. Each point on that graph should be the sum of all the forces acting on the engine up to that point.
Inside the combustor, the air undergoes an isobaric process, this is due to the combustor being open to the compressor so the pressure must be the same (in ideal conditions) between the compressor exit and the nozzles at the end of the combustor. if it is hard to understand the accelerating air providing a force, just thing of the equation every highschool physics teacher will tell you, F = ma, accelerate a mass (in this case air) and you get a force in the opposite direction on the turbine, this is newtons 2nd and 3rd laws.
Thanks for writing this comment, I was just about to mention those points.
Especially the F = m * a is important. As AgentJayZ says, the combustor really is open to the front and the back.
- In the front, the compressor is keeping in the pressure with a wall of air supported by force on the compressor blades
- In the open back, the pressure is kept in by the force it takes to accelerate the air (F = m * a)
I learned something interesting in this video: The net force on the shaft of a turbojet engine apparently is going backwards. I thought more of it was acting on the compressor, giving the shaft a net force forward. I thought therefore the main thrust bearing takes net forward force. Can someone explain that to me please?
@@DerKrawallkeks Please see my comments else:where. In the majority of engines, the load on the thrust bearing(s) is in a forwards direction, because the net forward pressure loads on the compressor rotor are greater than the net rearward loads on the turbine rotor. However, these loads can be varied considerably by varying the arrangements of the air seals and the way they are pressurised. As I stated elsewhere, I changed the load on a thrust bearing from forwards to rearwards with the introduction of a new turbine, which had major changes to its sealing arrangements.
@@grahamj9101 thanks for that clarification, so depending on how the engine is designed, the net force of the shaft can go either way, and the thrust bearing can be designed for forward or backward facing force, depending on the engine.
I was surprised in your other comment, that the force acting on the combustion chamber(s) is actually facing to the rear. Why is that? I'm imagining a combustion chamber to have a larger outlet than inlet, therefore pushing forward. Could you explain?
Thanks for your great comments, it's always a great pleasure to have an expert/professional on hand
@@DerKrawallkeks Perhaps you are being mislead, as many are, by the way a piston engine works, where there is an increase in pressure as a result of the combustion process. There is no increase in pressure in a gas turbine engine operating to the Brayton cycle. There is, in fact, a pressure drop across the walls of the combustion chamber. Now I use this term, which AgentJayZ dislikes, but is the term that I was brought up with at R-R, to mean the combustion liner or flame tube or combustor, which is the term that is in common usage in N America. As there is a pressure drop across the combustor, there must be a net rearward load on it. Now what the physical loads on the complete combustion chamber section of the engine actually are will be very dependent on the design 'architecture' of the engine and the way that the entry and exit sections are defined for calculation purposes. That is where I have issues with the Avon thrust distribution diagram and the huge forward load that is quoted for the combustion section.
@@grahamj9101 Yes, thank you. The diagram might be correct, but it's not close enough defined, which parts they actually still count into each segment.
Agent JayZ: Are you more likely to experience a hung start or a hot start on a cold day at Jet City or in the middle of summer? Are turbine engines less finicky about cold starts than diesel engines?
1) Same thing
2) Cold weather has no effect until about -50C on the starting of a turbine engine, so I would say yes
People make this way more complicated than it actually is, the physics is pretty basic.
people in the space industry know this fairly well. remove the atmosphere, remove the jet nozzle, remove all the confusing variables from the picture, in space THE ONLY WAY TO MOVE IS TO RELEASE MASS, the more energy (acceleration) you give the same amount of mass the more acceleration you get per unit of mass release. if you throw a spanner in space (insert energy into the system in form of momentum) both you and the spanner will move (exactly in opposite directions) the energy gets divided by mass to define the speed (acceleration) in with both items move away, lighter mass item will go faster and heavier item will go slower
Now back to earth, the air is a fluid, it has mass, you accelerate a chunk of air from the sea of air that is around the engine and you accelerate it to one direction (by inserting energy), the inertia and conservation of momentum dictates the engine has to move the opposite direction. (replace air with water in your head and it all starts to make more intuitive sense).
Think of taking a bucket of water from the front of the boat and dumping it at the back. if you transfer that mass from front to the back (relatively) faster than the water that the boat is on, you get forward motion. the water itself can be taken and dumped at exactly the same speed, no nozzles, jet pipes, none of that. just a simple conservation of momentum.
Reiterating Newton’s law wasn’t his purpose, Agent JayZ was attempting to look at the complexity of the situation and explain precisely where in the engine the thrust acts. If you’re happy with a simplified understanding this isn’t the video for you.
Can someone explain why the tapering of the nozzle matters? The way I understand it, it shouldn't, but I am obviously missing something. F=MA, right? Pressure is also related to density which is related to mass. More pressure = More Density = More mass per unit of area. So, the way I see it now, If you have a nozzle, you have higher velocity but less mass of air per unit of area. Without a nozzle, you have more mass of air per unit of area but it exits the engine slower. It seems that the two situations would equal out. What am I missing here??
Hmmm... Probably better to re-establish a basic understanding of principles than to look for specific errors.
What I mean is, nothing you have said here makes any sense at all to me.
Not an insult, but I think you have a couple of incorrect assumptions that are stopping you from coming to an understanding.
Subsonic aerodynamics, and Bernoulli would be good search words.
While the nozzle does increase exhaust speed, the reason it has a positive impact on engine thrust comes from pressure. As it is the pressure inside the engine that actually pushes it forward. If you cut the nozzle off, engine would not be able to maintain it's internal pressure (and less pressure = less thrust). Thou it's not only nozzle that can keep pressure high in the engine, afterburner does that too, and you can see how nozzles open up to full when it's on.
What you are missing is the behavior of working fluid (exhaust gases) itself. Most engines are so complex because they need to adjust for what the gases can and can't do. And while you could create two engines with same power, same mass flow, and two different size exhausts, the one with smaller exhaust would likely be more efficient.
It's definitely not easy topic, so i'd recommend to see how rocket engines work first. It's much easier to find good, grounded in basic physics definitions on that.
@@AgentJayZ Thanks for the help!
Remember fluids: one can increase the speed by reducing the area (of course assuming the fluid is in constant flow without loss). And the equation of Bernoulli stands a balance of energy conditions, what it is needed is an increasing of speed in order to reduce pressure and cause lift in the aircraft.
From my small understanding- get the exhaust as FAST as you can to increase thrust
T handle hex pointers come in a wide variety of sizes and never need batteries.
The F_net is basically a summation of all the combined reaction forces in the engine along its length. Basically integrating incremental force vectors vs. distance along the centerline of the engine. Since the forces are vector quantities and thus act with a defined direction (either towards the compressor or towards the turbine/exhaust section), they will both add and subtract from the overall net force. Basically the force builds through the compressor section as each stage further compresses the air charge, builds slightly through the combustor, and then decreases as the gas expands and imparts energy to the turbine stages.
We little people with the wrenches... yepp, that's most of us :-)
Jet City hat - Jay needs some merchandise. 113,000 subscribers - if 1% bought a hat... Jay, I'd be your US distributor/silent partner.
I've always figured the driving force is produced in the jet pipe , like a Rocket motor produces it's entire thrust from the cone at the very end . So , the Jet Engine is pushing from the Turbine and pipe area and all hardware it's mounted to and transfered to engine mounts .
Heh .. ongoing comments .. Oh yeah.. ;)
I still like your videos .. regardless of what others say. ;)
Hacky
The problem I am having is Newtons third Law... all forces between two objects exist in equal magnitude and opposite direction.
So where does the rubber meet the road? You can say jet engines produce thrust through Delta V of exhaust but how does the engine move in the opposite direction if not through forward pressure on internal surfaces that are creating that rearward delta V?
I would like to say any gas acceleration before the turbine is only useful because it turns the compressor through the shaft and it's the compressor blades that push the air rearward and the engine forward, but what about after burners. And what about ramjets with out compressors?
There is pressure in the jet pipe after the turbines. It is converted to velocity by the nozzle.
You want to join the ages-old discussion about just where does the "push" happen on the aircraft?
The answer is out there... for you to find.
Where is the thrust? It starts at the back of the compressor. It EXISTS at that location. That’s just my take on it. Just another layperson here trying to absorb. It might be easier to think of one of the space shuttle solid rocket boosters. As they burn, the combustion inside the booster moves up the inside of the booster body until it is burning damned near the top of the inside of the booster. So when all that is happening, is the thrust still only occurring at the nozzle end?
It's not a rocket engine. The Turbojet produces thrust not with pressure, but with acceleration of exhaust gases. These gases exit the rear nozzle with high speed at almost ambient pressure.
Come think of it, a rocket engine converts pressure in the combustion chamber into velocity in the exhaust nozzle.
Similar!
Gas moves from high to low pressure as described by Bernoulli, which is the aggregate behavior, not necessarily describing kinetic forces of gas molecules.
Bernoulli doesn't really apply anywhere for a jet engine because it assumes incompressible, irrotational, isentropic, adiabatic, no-work flow when jet flow is compressible, rotational, entropic, not exactly adiabatic, and definitely has work. You can make Bernoulli work if you make a ton of modifications, but then it's basically just a general conservation of energy equation.
If that jet engine was compared to an air compressor, how many cubic feet of air per minute exit the turbine at 25psi?
The first of two examples is the Rolls Royce Avon. The air does not leave the engine at significant pressure, but at very high speed. Jet engines do not work on pressure, they work on acceleration.
The full power mass airflow of an Avon is about 150 lbs per second.
So, about 9,000 lbs per minute leave the compressor, at a discharge pressure of about 120 psi.
Conversions between mass and volume for air can be found at different pressures.
It would be inefficient to use an Avon compressor section as an air compressor to obtain 25 psi. It could be modified to do so, but we are beginning to drift away from standard practices...
@@AgentJayZ look, man, I just want to figure out how to run my nail gun at 4000 nails a minute.
But back on topic, wouldn't the last compressor stage blades be providing the back pressure to force the air towards the rear? Could they be considered the part of the engine that gets pushed forward?
Sure. This is an often discussed subject, and is an irrelevant question for the people who maintain, operate, and manufacture these engines.
The thrust produced by the engine is the sum of many forces acting on all of its different sections. There's a few good diagram on the distribution of forces out there.
Here's my favorite: www.google.ca/url?sa=i&source=images&cd=&ved=2ahUKEwj40PvNqMTlAhWUrJ4KHeOZBr0QjRx6BAgBEAQ&url=https%3A%2F%2Faviation.stackexchange.com%2Fquestions%2F33068%2Fon-which-points-in-a-jet-engine-does-the-reaction-force-act&psig=AOvVaw1YFB0AgDG-0eQklbCU-tDn&ust=1572536366924626
I don’t know why everyone gets in such a state on this issue, it’s quite simple. An aircraft turbojet engine gets its thrust by dint of the fact that the hot stuff coming out of the back is travelling a lot faster than the cool stuff going in the front. The turbine section is there for one reason only - to supply the 30,000 or so horsepower drive for the compressor. Lets imagine a theoretical concept engine, with full engine casing and jet pipe, with only the compressor section present, driven by a 30,000 hp electric motor. Run up to speed, is there any thrust? Yes, a little because the air coming out the back is going faster than the air going in the front as the compressor has heated it, but the thrust is negligible. Now let’s add the combustion section. The electric motor is still required to drive the compressor, so it’s run up to speed and the combustors are lit. Is there any thrust? Yes, an immense amount - whatever the engine is rated at plus 30,000 hp as no turbine is present. There’s a huge amount of hot stuff coming out the jet pipe, travelling very, very fast. When the turbine section is added, that draws 30,000 hp out of the exhaust stream to drive the compressor, so the electric motor is no longer needed, bringing the exhaust stream under control. The jet pipe nozzle doesn’t generate thrust, it manipulates (multiplies) the thrust that has already been generated, narrowing to increase the speed of the exhaust stream, thus increasing thrust. But the thrust is generated by the compressor, the X vetor of the angled blades providing the forward thrust, and the Y vector providing resistance to rotation. The X vector from every compressor blade is amalgamated and becomes forward thrust, is transmitted through the thrust bearing to the engine mounts, thus the aircraft gets pulled forward. So the answer to the question “Where Does Thrust Act” is, on the X vector of every compressor blade.
Industrial gas generators don’t generate thrust because their turbines are just the right size to extract the maximum amount of energy from the exhaust stream, there’s no jet pipe or nozzle to multiply the residual thrust, and they’re firmly bolted down. There, simple, can I have a job?
@Zachary Martinez you are right. The fact that you can calculate the thrust of an engine as the air mass flow times the speed difference is just an application of Newton's 3rd law that simplifies the calculation. The thrust, strictly speaking, is the integral sum of all the shear stresses and pressure forces on all the surfaces of the engine, which is of course much much more difficult to do than just the simple speed difference thingy, specially when no computers existed. Nowadays it can be done by CFD (computational fluyd dynamics). But at the end, thanks to Newton's 3rd law, they are both just the two sides of the same coin.
"thrust is generated by the compressor, the X vetor of the angled blades providing the forward thrust". I think You are ignoring the force that gas makes on the walls of combustion chamber. And what about centrifugal compressors? In this case the air travels sideways of the propulsion vector, which makes Your theory wrong. In general - the way the air is compressed has nothing to do with the thrust. In theory You could use a tank with compressed air instead of the compressor. Does that mean that the trust is generated by the air tank? No.
@@wniedzie sorry but the answer is yes, think of a balloon, the thrust is generated by the compressed air, and again you can calculate it as the integral sum of the pressure forces and shear forces on all the balloon surface, or just measure the mass flow and the speed or the air coming out.
@@laertesl4324 Baloon is not a jet engine :). A spinning compressor alone WILL give some thrust, like an ordinary fan. It WILL blow some amount of air. But it's not something You expect from a jet engine. Everytime You have a pump or compressor, the presure in the system rises only when there is a resistance, some load. Even with the biggest pump, when there's no load You have only a moving medium (gas, fluid) from one side to the other, no pressure. Pressure in a turbojet engine comes from gases expanding. The combustion gases are the load for the compressor, they do the opposite and try to escape in every directions, also through the compressor. It's the compressor's role to overcome this force and provide constantly more air. Otherwise You have a compressor stall, fire escaping in front of the engine and of course a loss of power (thrust). Try to find an old AgentJayZ movie where he shows a "dymmy" load for testing engines. It's made of a compressor with a metal plate covering the "exhaust" of the compressor. Without the cover the compressor would not have a load, it would not compress air (maybe a little).
@@laertesl4324 by that model, would the fuel combustion also be considered a thrust-modifying factor (as with the jet pipe)? Or something else?
My mind wants to think about this from a momentum transfer perspective, but I'm having trouble thinking about combustion from that perspective. Where I'm at right now is: the momentum of the air doesn't appreciably change during combustion, does it? Because the mass doesn't (appreciably) change, and the increase in velocity is caused by a corresponding decrease in density, which means that your actual flux stays about constant, no?
While we're all thinking about it, where does the thrust force act upon the bell of a rocket engine? I guess fundamentally it's the same closed system with a hole out the back as in the turbojet engine or balloon.
Actually, the thrust of a rocket engine is found by integrating the pressure forces along the entire inner surface of the combustion chamber, starting at the bottom of the bell and going all the way up to and including the injector plate/manifold. When you do the math, it comes out that the force is actually mostly acting against the injector plate way up inside the engine. It's weird I know, but that's how it is.
Thank you.
"What part of a jet engine bears the load of the thrust?" has the same validity as the statement: "Rocket engines can never work in space because there's no air for the rocket exhaust to push against." A turbojet engine is a dynamic, reaction based system. You can take a turbojet engine at full throttle, measure all the pressures and forces inside it, then take the thrust nozzle off, turning it into a gas generator, and the pressures and forces inside it are exactly the same. The closest thing to a meaningful answer to the question is that the radial loads trying to push the thrust nozzle apart at its narrowest point is where the thrust loads are concentrated. If that doesn't make sense, then there's one key element of a jet engine you're not understanding. A jet engine is a system which *includes the air roaring through it*. With the thrust nozzle attached, the output is a narrow, high speed stream of air. Without it, the air expands in all directions as soon as it clears the end of the turbine like a constant explosion. If you can conceptually understand why this difference exists, you'll understand why the question isn't meaningful.
@@airgliderz I think what he meant was that the statement "rockets can't work in space because there's no air" is a ridiculous and laughably false statement. He was not trying to say it was true.
alan connelly I understood exactly what he meant
"Rocket engines can never work in space because there's no air for the rocket exhaust to push against."
This is easy to explain, when we equalize all the pressures, only the difference between the pressure on the front wall and the back wall with the hole remains.
The comparison with a gas turbine is very good, otherwise I don't know if it creates the same thrust or not, the difference to a turbojet engine is the rear nozzle, I understand, but not this sentence: radial loads trying to push the thrust nozzle apart at its narrowest point, in my opinion is distributed over all parts of the engine
Its the combined mass of air and jet fuel (typical) that is the (accelerated) thrust.. Fuel plus air > Air alone in mass (density of matter) more matter moving in a direction = more ass in the flow...
More mass = more thrust... its a conservation of mass equation.. simply
This. Mass of the air times the acceleration. Acceleration can be viewed as the delta V, the change in velocity between the front and rear of the engine. Mass times acceleration equals force.
@@kevingallineauii9353 That doesn't explain where the force happens, though. You showed that there must be a thrust force on the engine because the momentum changed, but that doesn't explain where that happened.
This is getting as confusing as Chinese algebra. But, way more interesting. Thanks for making excellent videos.
Where Does Thrust Act? Jet City.
Every step where the air experiences net acceleration? So, a little in the compressor, the combustion chamber, not the turbine, then again in the nozzle.
Yes, except the compressor accelerates and decelerates the air in steps, each step increasing the pressure. The air speed into the compressor in flight is usually higher than the exit speed.
AgentJayZ gotcha thank you!
Brilliant.
Why roocket engines have totally different shape of exhaust nozzle than jet engines? They fundamentaly work the same - pressure of hot gasses propels attached vehicle forward. But shape is totally different?
Both afterburning jets and rockets use the same converging-diverging nozzle. Look at a cross section and you will see the same shape, but the proportions may be different. The optimal proportions are dependent on the ratio of ambient pressure to nozzle inlet pressure. Rockets have far greater combustion pressures (100 Atm is a typicall figure), and can also operate in very different ambient conditions to jets. For example with rockets engines operating in the vacuum of space the expansion ratio is much greater, so a nozzle with extreme outlet diameter is preferred. The shape is still the same con-di nozzle shape, just with different proportions.
Non afterburning jets may do fine with just a diverging nozzle, so in these cases it is sometimes preferred (but not always).
@@ASJC27 that final afterbourning thougth opened mu eyes. Now I understand. So simple. And very similar. Thank you!
Rocket exhaust speed are usually supersonic, which need a Con-div nozzle. However, the exhaust speed of a jet engine (without afterburner) are mostly subsonic. For subsonic flow we want a convergent nozzle.
T handle Allen wrench is a not an OSHA approved pointing device.
I recently had a very similar discussion with a parts counter guy. I tried to simplify that a turbojet is essentially an atmospheric amplifier with an added blow dryer effect of volumetric expansion.
This did not seem to compute, so I went visual and said "Imagine strapping a corset on a tornado, where the corset is containing a section of the tornado and all the boosted pressure is also expanding out the ass section with great force."
Thumbs up for doggo!
I'm fairly sure that I understand this question. I am but a lowly mechanic/problem solver/fabricator/hack and have heard the Honorable Agent JayZ mention (Ad Nauseum) that I am not to use my Piston Pea Brain when it comes to Jet Engines and I understand why and when he says this, but I am but a simple man and very likely a simpleton. If I am understanding this question properly and I believe that I am...we are looking for the final surface inside an engine that basically bears all the load of the thrust the engine produces...the last leg it stands on "if it were" AND ALSO what the thrust pushes off of, like the air?
Final surface? No clue... Thats all I can come up with on that one. What is the thrust pushing off of ? For me its always been the air of the atmosphere in my mind...and if it isn't, I will probably wind up at Shady Maples weaving baskets and rocking back n forth for half a year or more...Great. For me personally, I have pondered the question of "what does thrust push off of, far more frequently than where or what is the last item in the chain that bears all the thrust load etc... I've always thought about the air as being a fluid, like water, so I quickly just assume it pushes off of this barrier. I always picture a finger pushing into a very stretchy inflated balloon...the balloon skin (atmospheric "air") this air deforms inward, giving us what the flame and violence is acting upon? OR one might try the converse and say that the atmosphere is trying to squeeze OUT this "giant finger of flame" or thrust...Like a soybean being ejected by squeezing the pod around it, launching the bean out of its slippery husk/beanpod? Does the hot gas coming from the engine create a low pressure column (i.e. the "finger") that the higher pressure atmosphere is trying to squeeze away? I think I have "Thunk myself into the weeds" guys...or was it the weed that got me to thunk? Uh Oh. Well if everyone isn't laughing too hard I'd be interested in hearing where I'm going sideways here. Christ I might as well try to come up with a theory involving DARK MATTER because how else would I explain how a rocket engine works in a vacuum or the "nothingness of space"? No atmosphere in space is supposedly the Monkey Wrench that gets thrown into the program, but MAYBE the actual wrench is Dark Matter...LOL... If space is NOT a lot of
"nothingness"...if that nothingness weren't "nothing" but "something" the monkey wrench is magically removed !! I solved it ! ...LOL No? Hey man you never know, it could be Dark Matter that thrust acts upon, up there in space. 50yrs from now this comment will be studied by physicists the world over man! Thats my answer and I'm sticking to it...otherwise the "white coats" will be taking me to Shady Maples for basket weaving and honestly I have things to do, so I dont want that to occur. So, for me...its Dark Matter!
Your first answer is good: basically a combination of "everything" and "nobody really can say"... excellent.
Your second answer: I stopped reading right away. Thrust is created by throwing stuff away. It is not "pushing" on anything.
The acceleration of the air inside the engine pushes the engine in the opposite direction of the accelerating air. What happens to the air after it leaves the engine does not matter at all.
It's easier to visualize if we think about a rocket engine, because it can run without taking in air.
In the atmosphere a rocket engine throws combustion gases at high speeds to the rear (or downward if on a test stand), and the acceleration of the gases due to the burning of fuel inside the engine creates thrust in the opposite direction of the flow of those gases. The thrust is measured and the engine is given a thrust rating.
In space, where there is no air or anything else surrounding the vehicle, that same rocket engine, burning the same amount of fuel and oxidizer, accelerating the same amount of gases out of the nozzle, actually creates more thrust. Since there is no air, or air pressure "in the way", the gases leave the rocket nozzle at a higher speed, and this extra acceleration creates extra thrust.
The fact that the gases are being thrown into empty space proves that they are not pushing on anything. They are literally "pushing" on nothing.
I hope this helps.
I also have a video on this subject called Jet Engine Thrust. That one is a bit of a rant...
@@AgentJayZ I am honored you took the time to reply to my ramblings Sir ! I never expected anyone to read let alone respond but I am happy to watch the "Jet Engine Thrust" video you mention. I must admit I have thought about the questions I rambled on about more than a few times since... I uh... rambled...Its a very interesting set of variables I must admit. Keep the superb videos and content coming my friend the work you put out is valued much more than you could possibly know. I hope you at least got a chuckle from my comment. Take care!
I think the graph shown in this post is very clear when it comes to velocity, temperature and pressure:
aviation.stackexchange.com/questions/33280/why-is-there-a-pressure-drop-in-the-combustion-section-of-a-jet-engine
Looks to me that it is correct ("makes sense", but I'm not jet engine engineer ;) )
Don't know what the orignal source is, but it seems to be a cleaned up version of the image in this post:
aviation.stackexchange.com/questions/11744/why-do-gases-in-the-combustion-chamber-only-flow-one-direction-to-the-gas-turbin
They don't explain thrust though, so it's a little bit off topic.
Interestingly, this one doesn't break down the compressor blades and vanes velocities variation and only showed the turbine section row by row, just like the Avon picture AgentJayZ showed.
I've been trying to get the answer to this for years and no one can answer it. How does a jet fighter jet engine not a turbo fan deal with water? Where does the water go after it enters the intake on an F-16? Or any other jet engine. I know how it leaves a turbofan or ducted type jet engine that's quite simple I just don't know where the water goes when it enters a actual jet engine how does it stay lit?
Water becomes steam as the compressor heats up the air. This adds to mass flow, and actually increases power. Any amount of naturally occurring rainfall, even the heaviest monsoon deluge, is a puny, tiny, insignificant amount.
Oh, and the engine in an F-16 is indeed a turbofan.
Also in the F15, F18, F22 and F35.
@@AgentJayZ ok thanks for the information. I was under the assumption that the F-16 had an actual jet engine. Or what I consider an actual jet engine not a turbo fan or ducted fan. Is there any planes that fly using just the thrust of the engine and what engines are they? This is what I consider a jet engine. All others to me are just ducted fans with a jet engine powering it.
@@triggeredmonkey3439 by "actual jet engine", are you referring to turbojet(compressor-burner-turbine-nozzle)? If you do a google search of aircraft with turbojet engines you should find an answer to your question. Most aircraft today are turbofan due to their increased efficiency. Which to my understanding, it is easier to move more air, more mass flow versus adding lots of energy to a small amount of air(energy loss associated w/ energy conversion). So the mass flow through the fan is what creates much of the thrust on modern day aircraft.
Okay, we have developed thrust. Now how do we get that thrust into the plane? Looking at your test stand, there seems to be two large pins that are 50mm in diameter about where the thrust bearing is located. There is another attachment in the rear of the engine. That's it. All that force on three points?
Usually only two in the aircraft.
As AgentJayZ has said, usually there are only two main mounting points in the turbojets that he services, which take both the thrust and most of the weight of the engine, plus a third mounting point that takes just a small proportion of the weight.
In the big turbofans, all the thrust is usually taken through a single point into the engine pylon.
I need that hat in my life...
Lol, I wouldn’t dare mention a laser pointer with that threat.
Hats off to AgentJayZ for this and all his other informative and interesting videos.
Please refer to this image to follow my comment and quesion: ibb.co/TYVy8HQ (gas load distribution "The Jet Engine, Rolls Royce")
The image is a screen clip from this link: aviation.stackexchange.com/questions/33068/on-which-points-in-a-jet-engine-does-the-reaction-force-act
I am endlessly fascinated by this topic. Understanding all that is happening in a turbo jet engine is almost like taking a full physics and mechanical engineering course. Which is, of course, the source of all the contrary statements on how the engine works - I don't and it seems most people don't really understand all the physics involved. That said...
My Question -
AgentJayZ said at 8:35 that the majority of the thrust comes from the acceleration of gases caused by the constriction/taper of the end of the propelling nozzle (paraphrasing). Following this he said the change in gas speed from inlet to outlet is what causes most of the thrust. Many explanations I've read have talked about how the huge amount of gas roaring out the back is where the thrust happens.
Or, is AgentJayZ saying the thrust (the force, or the energy, or work) is created by the change in gas velocity from inlet to outlet? The thrust (pushing/pulling) forces then act on different parts of the engine in unequal amounts as they are generated or encountered.
Here's my simple take -
1) The magic of a jet engine is in the combustion of the jet fuel in a pressurized/high volume stream of air. The combustion process results in an enormous increase in: gas molecule volume (the stoichiometric ratio of jet fuel, 14.7:1), heat, and velocity but oddly enough, not pressure.
2) The hot, almost explosive expansion of gas is what provides the work, the force to spin the turbine which in turn drives the compressor which, combined, drives the entire process.
3) the thrust (the push) happens as follows as per the diagram at the top of this comment:
3a) 19,049lbs forward thrust (forward gas load) comes from the spinning compressor blades pulling massive amounts of air into the engine (much like how a propeller works pulling an airplane forward).
3b) A much smaller amount of forward thrust comes from the diffuser, 2,186 lbs.
3c) 34,182 lbs forward thrust within the combustion chamber (hot expanding gases can't go forward due to higher gas pressure from compressor stage, the only option is for the hot gas to shoot out the exit noozle(?) of the combustion chamber, where it...
3d) hits the turbine causing it to spin at a high speed. This, however, exerts a rearward thrust of 41,091 lbs (rearward gas load).
3e) Next, the hot gases are squeezed in the exhaust unit and jet pipe resulting in a forward thrust of 2,419 lbs.
3f) And finally, the gas is further compressed by the propelling nozzle resulting in a rearward thrust of 5,587 lbs. which increases the velocity and reduces the pressure to almost ambient.
3g) The net total thrust (forward) is 11,158 lbs.
All of these competing forces have to be carefully balanced for the engine to run. Easy peasy :)
I welcome corrections and comments.
Maybe you should make a remake of this video.
- Draw a bigger cross-section of an engine showing all relevant parts but no more.
- Draw the graphs to perfectly align with the relevant engine parts. Draw your own graphs to the best of your knowledge.
Now the whole is a bit confusing. You use 2 engine cross-sections, different sets of graphs and divert a lot about the shortcomings of the graphs.
Near the end of the video you can explain that there may be some subtle differences.
- Olaf -
Maybe you can read a book or two, visit the NASA website, and then sit back and consider that what you ask is available to anyone willing to pay for it. It cost me about 20K for my one year preparatory course, a great deal of extra "unnecessary" reading, and so far 17 years of working in the field to gather the admittedly incomplete knowledge I have.
If you are dissatisfied with my attempts to both share and inspire, please contact the office of bitching and whining for a full refund. Their number is 1-800-JET-FUEL
@@AgentJayZ, In fact, I am very happy with the videos you make. It wasn't meant to be complaining or just moaning in general.
It is just a difficult subject. Any attempt to explain it is much appreciated.
I just feel it would be clearer if you stick with one cross-section of an engine and align the graphs.
That's all.
Best regards.
Olaf
The Netherlands
@@AgentJayZ your overreaction was so uncalled for.. what are you trying to prove?
@@olafv.2741 don't apologize, your initial comment wasn't rude.. it's just that some people like mr Jay decide to reply to comments when they are having a bad day.
9:04 Humility.
Agentjayz, be careful, Bernoulli principle can only be applied in uncompressible flow, not the case of a nozzle in a turbojet.
Bernoulli is just a special form/case of the first law of thermodynamics and from 1st law you can still get to appropriate equation for turbojet. It is quite common for literature or discussion aimed toward general audiences to use the term Bernoulli as a pronoun for 1st law applying to fluid mechanical problems.
@@Skyshade yes, a special case only applicable in incompressible flows. A turbojet convergent nozzle accelerates the air to speeds quite close to Mach 1, hence compressible flow, hence Bernoulli not applicable.
What principles do apply then?
@@rodgerq The usual in classical physics (we are not dealing with quantum mechanics or relativity here): Newton's laws (momentum equation), conservation of mass (continuity equation), the first and second laws of thermodynamics, the equation of state for gases. All of them applied to fluids, which is a little bit more difficult than for solids, produce some complex vector differential equations that unless you apply some simplifications (like incompressible flow, inviscid flow, isentropic flow., one dimensional flow... when possible) can only be solved by numerical methods, normally by computer. Bernoulli is just one solution of all this for a simplified case, which is very useful in some cases, but not in a turbojet nozzle. If you want to go deeper I can only recommend John Anderson's great books "Introduction to flight" and "Fundamentals of Aerodynamics". They are not cheap and they are not easy (the second requires vector calculus knowledge), but they are one of the bests books on the subject, giving not only the physics but also a very interesting historical perspective.
how come there are not graphs that show mass of gases, and acceleration of gases?
The mass does not change, except for the addition of fuel.
Acceleration of those gases is the slope of the velocity graph.
Since the velocity is graphed against distance, the acceleration of the gasses is the derivative of the velocity graph times the velocity. Which is useless information, because they didn't label their axes.
is it not simply related to the weight of the whatever molecules are in the exhaust and the velocity in which they are flung ?ry8?
they only reaction that causes actual thrust to move an object must be a seperate mass that leaves the object ,ry8?
if you stand on a skateboard and throw a heavy object you will roll the opposite direction(a very symplistic ,splanation)in relation to how hard you pitch the object and how much it weighs
however
if you simply fake throwing the heavy object but hang on to it you roll back and forth
the object must be released to gain net thrust
this is how i see it as a highschool graduate, from my head, NOT cheating and looking for written in stone answers
Yes, but that doesn't help identify where the thrust comes from. All you've done is shown that momentum increases, which necessarily implies a force was exerted on the fluid but does not mean anything about what the actual forces are.
Cool, but I almost can’t see the data.
air-breathing rocket engine
According to the missus, the thrust acts directly on the headboard!
Laser pointer
the credit to Bernoulli for high pressure, low velocity
Combuster? I just met her!
I guess when the gas-speed increases through the turkey-feathers, the opposing force has to act on the column of pressurized gas(???)
So thrust is the result of accelerating forces out of the rear of the engine being manipulated by the shape of the rear and pushing the engine forward by pressure on the front .
#HexPointer
I just remembered idk
the massive acceleration and release of energy out the jetpipe is imparted against the turbo-machinery of the engine onto.......The Thrust Bearing , which bears and transfers said energy produced to the engine casing to its mounts and to whatever that is attached to i dunno like a ship?? this brings up afterburner, is that release of energy, pressure or whatever directed directly on the "?structural exhaust parts?" being attached to the parts that are bolted to the mounts . I guess working backward the energy to whatever is moving is transferred thru the motor mounts so start there end go backwards??? sorry for the improper nomenclature, horrific grammar and probably countless spelling errors.
The term 'thrust bearing' is, unfortunately, very misleading in this context. R-R Derby uses the term 'location bearing'. which is more accurate, in terms of the function of the bearing. The load from the thrust/location bearing is one of several loads, both forwards and rearwards, as can be seen in the diagram that AgentJayZ has shown us ( and with which I am all too familiar), the summation of which gives the engine thrust.
I've quoted this example before, but it does make the point. I was involved in the design of a certain two-spool industrial gas generator. Unfortunately, we underestimated the forward load on the IP location bearing. The solution was to introduce a thrust balance piston at the front of the IP compressor to reduce the bearing load. In the next mark of the engine, we introduced a new HP turbine, which was designed for an uprated version of the aero engine: this actually changed the load on the HP location bearing from forwards to rearwards.