You don’t have to call yourself ‘grumpy’ when *any* human who has devoted their career and life to working on these complex machines is bombarded by misunderstandings, incorrectly formed questions on a daily basis would rightly get a little ‘gnarked’/irritated by repetitive ill-informed individuals. And just when you think you’ve dealt with the last, a flood of newcomers comes and asks all the same questions all over again. Most of us truly appreciate the effort you’ve put in *FOR FREE* over the years. For me, when you say ‘but there’ll be no Maths’, I feel a little sad cos that’s what my PhD is in. But I get that it’s a difficult subject, especially without the formal education I was so fortunate to receive. In short, we - the understandable, understanding ones - not demanding more time and more spoon fed knowledge for God damn *FREE*! We *THANK YOU*, truly we do. I was interested in jet engines, I like planes, I think they’re cool, and you’ve built a community that is somewhat mutually supportive in your spare time. K, thanks, bye!
For the people who can't follow AgentJaZ's excellent explanation... Every action has an equal and opposite reaction. Thrust equals the mass and velocity of air (& combustion byproducts) being moved. Your welcome... where's my beer! ;-)
Same kind of misunderstanding goes for the rocket engine exhaust. It is at or slightly below atmo pressure on the ground level. Exhaust expands rapidly, trading pressure for speed
I like that you advise using multiple sources. I once trained as a military aircraft engineer, but some of the things you say are becoming more understandable than what I read in books. I completely agree with you that even if you study one source perfectly, you are still don't master the topic well enough. Thank you
Love it. Explanations like this are gold. Doesn’t matter the amount of attitude or passion in which it’s delivered....as long as it’s clearly articulated. You’ve e done well. Thanks
Bernoulli's principal at its finest. Pet peeve, an analogy I suppose...the pressure washer.....lol It's the speed that the water leaves the nossel uses to creat the force for cleaning. So I deem it a velocity washer. Lol. Stay safe, merry Christmas. Good job once again.
You are right. Here's one for you: The almighty EPR that the pilots look to as the only gage they care about... it only reads anything, because the exhaust pressure probe is in the wrong place!
At 18:33, that diagram is correct. The pressure shown here is total pressure, not static pressure. Static pressure drops to close to ambient, but total pressure almost doesn't change through the nozzle. Without adding or substracting energy the only mechanism for a change in total pressure is friction. In the nozzle there is no additional energy added, and friction is very low so the total pressure coming out is almost exactly the same as coming in.
It is indeed correct, just a bit misleading. Perhaps static pressure plot would be more accurate in order to precisely display the nature of the fluid flow in the nozzle :)
13:55 The ideal case you describe is call nozzle optimum expansion Ratio. The fact that in practice it seems to overexpand a bit at ground level (at least for civil application) is because with fixe exhaust nozzle section, you can only be in this optimal situation for a particular (regime, altitude) couple (as the atmospheric static pressure keep decreasing when you get higher and higher). Civil application jet engines fixes nozzle are usually optimise for cruise regime and altitute. In a military application with variable nozzle section, you could make some control laws taking into account this phenomena in the egine calculator, but in practice I think it might be quite complicated as you must work in close loop because atmospheric static pessure conditions varies over time and location... You can also see very clearly this phenomena in rocket engines flame exhaust during launch and why the upper stages engines nozzles have much larger exhaust sections. ps : Thanks for all your excellent content, you definitly have the best jet engine youtube chanel.
One of the best explanations, thanks. I understand the math as an engineer and your ability to "bring it down" so many people can understand it is spot on.
This was a good way to spend an hour, learning something that I probably won't ever use but I want to know these things. Thank you AgentJay and I also appreciate the commenters helping to explain things. Now it's time for some microbiology, or maybe a shift cable change out on a Ford pickup.
I believe I've figured out a bit of Y-tubes formula for recommendations: "If elementary school=fail then send to AgentJayZ"! I'm suffering with you for getting all these people that refuse to learn Jay. Wish you and all tech folks a good 2021!
Jet engines and rocket engines work on the same basic principle. They both create thrust by accelerating a gas in one direction to produce a force in the opposite direction. All three of Newton's laws of motion are involved. According to the 1st law the gas has mass and therefore inertia, which will cause the mass to resist being accelerated. Therefore according to the 2nd law, F=MA, a force will be produced proportional to the volume of the mass and how much its being accelerated. Then according to the 3rd law the force will be produced in a direction that is opposite from the direction of the acceleration. As Jay said these engines produce thrust by accelerating a gas. Essentially what happens is that the mass of the gas and the mass of the engine push against each other and are accelerated in opposite directions. That's really all there is to it.
You are correct in all you wrote, but i have one question. What causes the gas to flow? As in, what causes the gas to flow in that direction? Or why the wind blows in one direction or another?
@@bogdan_n The gas, which is just air initially and then air combined with fuel residue, flows in the direction that it does due to the way the engine is designed. The compressor in the front of the engine sucks in the air and compresses it, which creates the highest pressure anywhere inside the engine at the outlet of the compressor. From there as the gas travels backward through the combustion section, then through the turbine and finally out through the nozzle. At each step the gas pressure is reduced as the gas exchanges pressure for velocity (according to the principle of the conservation of energy). So the direction of the gas flow through the engine is dictated by the pressure distribution at various points going through the engine. The compressor mechanically forces the air backwards increasing the pressure to its highest point but then from there the pressure drops step by step as the gas goes the rest of the way through the engine and out the back. Its really just basic physics. A compressed gas contains potential energy, which will be converted to kinetic energy when the pressure is released. So gas will always want to flow from high pressure to low pressure. This is what makes air move through an engine in the direction that it does and makes the wind blow in a particular direction. Its all about having a pressure imbalance and in which particular direction that imbalance exists.
@@joevignolor4u949 So, we are still talking about pressure. And even if the pressure, or better said, the pressure gradient isn't what produces thrust, it causes the gas to move. And even if the pressure in a shop compressor (120psi or about 8bar) is about 4 to 6 times greater than in the exhaust pipe of a jet engine (20-30psi or 1.38-2.07bar) the area of the hole that the air "escapes" through is about 60k times bigger in case of a jet engine. So, if we are to compare the jet engine to a compressor, let's compare what would happen to the compressor (and the surrounding objects) if it had, let's say, a 1000 cubic meter pressure bottle, and a 1m diameter hole would suddenly open in that bottle.
@@bogdan_n If you math is right, which it seems to be, and you mounted your big shop compressor and nozzle on a glider then you would have a jet powered glider. There would however be a problem. Your engine will only produce thrust until the supply tank runs out. You could of course keep the compressor plugged in but you would need a really long extension cord. Real jet engines get around this problem by burning liquid fuel (which has a much higher energy density than compressed air) and using the energy produced to drive the compressor and to heat and expand the air to create pressure, causing the hot pressurized air to be accelerated out through the nozzle.
Thanks so much for adding this added explanation. As a mechanic of course we have always had difficulty with pressure. Having it accelerate Air is a concept that it's hard to wrap your head around without it changing pressure much. I hope you had a great Christmas and have a great New Year. All the best from Surrey
For the people stuck on the nozzle pressure aspect of jet/rocket engines just remember that any exhaust that is higher pressure than atmospheric will expand radially which is wasted momentum (aka thrust). This is also why rocket engines have nozzles that are optimum for air or space and not both (except aero spike engines). Vacuum rocket nozzles have large bells to catch the expanding exhaust and keep it moving parallel to the thrust line.
Thanks Jay, I have 20,000 plus hours of turboprop time and many people have asked me about thrust and how much we get from the jet exhaust, when I answer i can see they don’t quite get it.
Strong wind can wreck houses and unroot all trees in a forrest. It doesn't take very high wind velocities until you can't stand. A jet engine is producing a hurricane. Smaller size but with higher wind velocities.
I saw a demonstration once where they parked an old school bus behind one wing of a B-747. The bus was parked parallel to the wing. They started up the two engines on that wing and when they advanced the throttles the jet blast lifted the bus up off the ground like a toy and flung it through the air. Also, when I was in the Air Force I was riding in a panel truck down the flight line. The driver didn't notice a B-52 that was running all eight of its engines and he drove too close behind it. As soon as we entered the jet blast it pushed the panel truck up onto two wheels and almost flipped it over onto its side. Luckily we came out from behind the plane just in time and ended up still sitting on all four wheels.
13:25 this is the sole reason why rifles have longer barrels to throw the bullet as long as possible. Rifles allow the pressure built by bulet firing to be converted to velocity for a longer duration
I assume that the green line on your chart is total pressure which includes dynamic pressure (~velocity) and static pressure . That's why the pressure doesn't change when static pressure is converted to velocity at the nozzle. Total pressure remains the same. Thanks for the great videos in 2020, have a great 2021.
I'm back and I'm going to take issue with the statement that the final nozzle (as I know it) "creates" the thrust. If the final nozzle were not there on the end of the jet pipe, the engine would still produce some thrust, but the pressure drop across the turbines would be way off design. The turbomachinery as an aero/thermodynamic system would, therefore, be operating way off design and the engine would be grossly inefficient. The thrust of an engine is created within the engine by the sum of all the forward pressure loads acting on the internal surfaces of the engine minus all the rearward pressure loads acting on the internal surfaces of the engine. I've stated and restated this several times on this channel. A physical force can only be produced by a pressure load acting on a given area: velocity is irrelevant. Having said that, those forces are produced by changes in pressure and velocity throughout the engine. If the pressures and areas could all be measured, calculated and integrated, then the result would be the thrust of the engine. Any 'fiddle factors', using mass flow, velocity/momentum change, intake ram drag and pressure drop across a choked nozzle would be unnecessary. OK, that's the engineer in me bursting out, like the alien from John Hurt's chest in the movie - but I digress. Of course, I recognise that Newton's laws apply: Force still equals Mass times Acceleration, the engine must accelerate the air passing through it to produce thrust, and the thrust can be calculated more conveniently using the physics. Yes, the thrust can be calculated from a summation of all the forces within the engine, but I've thrown this little spanner (OK, wrench, if you must) into the works before. What happens in my old Bristolian friend, the Pegasus engine, when a Harrier 'jump jet' is the hover, with its nozzles pointing downwards? Essentially, the thrust is produced by the pressure acting on the exit areas of the four nozzles projected onto the internal surfaces of the nozzles opposite their exits. This is effectively what propels a balloon when it is blown up and released to career around the room. It's the pressure in the balloon acting on the area of the neck projected onto the inner surface of the balloon opposite the neck. I use this to explain how my balloon-powered model cars work when I'm doing a STEM activity with young people (mustn't say 'kids'). Well, I feel better for that - so, Happy New Year to you, AgentJayZ, and to all of your subscribers.
Misunderstanding of this concept is rooted in the misunderstanding of dynamic systems vs static systems. Pressure alone as with temperature and volume are only relevant in the static realm. In dynamic systems only the time derivatives of these are relevant dP/dt, dT/dt, etc are relevant. It's a difficult jump to make without a solid base of calculus. Our human minds make an error of trying to generalize static systems to dynamic ones, when actually the general case is the dynamic case where the dt factor falls out to generalize for the static case.
@AngentJayZ, I have reached out to you a few times in the past. I work on gas turbines in the power generation industry (Siemens/Westinghouse, Mitsubishi and GE). On a normal "jet engine", I assume you probably have 2 or 3 stages for the hot gas path. In the power gen industry, we typically coat our first two stages of the power turbine with TBC along with cooling channel arrangements. Is that typical in the aviation space? Just curious how the aviation world manages the same issues we deal with in the generation business.
Hi Stephen... just looked this up so I could understand the question. "Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces operating at elevated temperatures, such as gas turbine or aero-engine parts, as a form of exhaust heat management. These 100 μm to 2 mm thick coatings of thermally insulating materials serve to insulate components from large and prolonged heat loads and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface.[1]" Wikipedia
Yes, the whole idea of TBCs was introduced in aviation engines, and also adopted by newer designs of industrial engines. What we have done is apply the process to older engines, both of industrial and aviation application. For these older engines, it can be thought of as an aftermarket upgrade.
Thank you for picking up these misconceptions. Just thinking about the velocity difference now, I get closer to understanding how reverse thrust can even work. So there is more energy in the airflow due to fuel combustion, which provides for the velocity increase. After that it's Newton's third law. Interesting fact about the nozzle section here too.
This is a very nice explanation of thrust, thanks for this. I might note that a good example of what you are saying is the difference between atmospheric and vacuum nozzles on rocket motors. Vacuum motors have an extended bell nozzle to better match the pressure of the exhaust to the very low pressure of space and thus increase their efficiency..
I have recently got into your excellent videos and are finding them very informative. Is it possible to explain the differences in thrust generation between a pure turbojet, low bypass ratio and high bypass ratio turbofans.
To my way of thinking is that if you use your thumb over the end of a water hose to make the water travel farther yes you increase the pressure behind your thumb and the water going out has it's velocity increased but the water pressure outside the hose is zero after it has exited the hose.
A thing I realized just now thanks to your excellent explanation: A turbojet and a rocket engine are basically the same: Oxidizer and fuel are fed under pressure into a combustion chamber where they are burned. The host exhaust gasses are then expelled through a nozzle that turns pressure into velocity. The lower the pressure relative to ambient the more velocity and thus efficiency you get. This is a much larger problem for rocket engines. They run from ground level atmospheric pressure up to near vacuum. It is easy to see at a rocket launch: At liftoff the exhaust leaves the rocket engine almost straight. The higher the rocket flies, the lower the air pressure and thus the exhaust plumes out bell shaped. That is the reason whey upper stage rocket engines have much larger nozzles: They expand the exhaust gas to almost vacuum and must be much larger to achieve this.
@@AgentJayZ Thanks for the correction. I now realize my error: Rocket engines use a de Laval nozzle. This accelerates the gas to super sonic speeds. Which is different from turbojets. Yet an other interesting part learned.
Yes. Jets are a reactive propulsion. You throw air molecules out the back, you will get a reaction propelling you forward. But only the molecules going straight back contribute to the reaction pushing you forward. Molecules going sideways are not useful. Those molecules moving sideways in all different directions is what makes pressure. When not all molecules go in random directions, but there is, on avarage a dominant direction of the otherwise random molecule movement - that is what gas speed is. From this you can see that, given constant energy, in order to gain speed, molecules have to be redirected from their random movement - so the amount of random movement will be less - the pressure will drop. This is why fast moving gas has low static pressure (bernoulli principle), and this is why jets exhaust is as low pressure as possible.
Interesting, but what about if there is no friction between jet air and ambient air? In that case what happened to the thrust? Is it similar to the car tyres on the ground?
There's no similarity to car tyres. Friction between ambient and exhaust gas plays no role. It's action-reaction principle. You throw something heavy in one direction, you push yourself in the other direction. And the role of that "something heavy" plays air in this case. The engine is throwing air behind it and that pushes it forward.
Well, Mr. D, You are making another of the biggest mistakes, and probably the subject of my next video: You think that the reaction force to the acceleration of the jet exhaust ( thrust ) is caused by the exhaust "pushing" against the atmosphere. This is not the case. What happens to the jet exhaust once it exits the nozzle does not matter at all. You are in good company; this is a very widespread misunderstanding.
@@kasuha I can not feel or imagine how the action-reaction principle may exist without friction! The action force act on fulcrum in order to create the reaction force, right? I believe that the fulcrum in the case of turbojet comes from the friction of high velocity exhaust gas and stagnation ambient air otherwise the thrust will be executed.
@@mrdarho4718 If you lift a heavy stone, stand on one leg, and throw the stone in front of you, you'll start falling backwards and will need to put the other leg behind you to prevent the fall. Try it. The fulcrum is you - you send the stone in one direction and at the same time you send your body in the other direction. And friction is only involved in making both you and the stone stop moving eventually. The part you can not try yourself is that it would work exactly the same in vacuum. But it would and that's why rockets can fly in space. I can understand how it may come unintuitive to you but you're wrong and if you want to do something about it, I suggest you to look up some educational videos on newton's third law.
@@mrdarho4718 This one is harder to explain. In general, this situation will never occur - jet engine works on accelerating the air. So air enters it at certain speed, and leaves it at higher speed. But that certain speed at which the air is entering the engine is the same speed as the air around the engine output nozzle. If we accelerated the engine to ultrasonic speeds and only then started it up (assuming it would start up), the air would be still escaping it at higher speed than is the speed of the surrounding air. But we may go for example with a rocket. When the rocket starts up on the ground, it's almost stationary and the exhaust gases are going out at large speeds. But high up in the atmosphere where the rocket is reaching orbital speeds (thousands meters per second) the exhaust is actually going along with the rocket - the rocket moves at 5000 m/s, the exhaust gases are escaping the nozzle at 1000 m/s, that means relative to the earth and surrounding atmosphere, the exhaust gas is moving at 4000 m/s in the same direction as the rocket. And the rocket is still accelerating, because it is accelerated by the mass of exhaust gas leaving it at 1000 m/s through the nozzle.
Jay, that was great 💖 "But eyes gots to know more" more about the flow after passing the exhaust turbine out through nozzle??? Laminar opposed to centrifugal turbulent flow? Which is prefered and why please? Is vortex generation a thing yet? Kind Regards & Keep warm
Hi, I appreciate that thrust is the velocity difference for the fluid from inlet to exhaust but what I'm always thinking about is where the force generated by thrust is transmitted through. Are the lines of force only turbine disk and blades through to the shaft thrust bearings then the housing? Or is it carried through the housing more than via the shaft/s? How much does the bell mouth contribute to the thrust? When you had that compressor section setup like a dyno those years ago for the gas generator, when it was being a load was it producing any type of thrust?
@@AgentJayZ Thanks for thank. Just had a look and discovered I was not seeing it for the first time. I know conclusively as I had previously "liked" it. Must have KRAFT though because I don't recall it. Will have to look when not tired.
1 kg of air in at ambient temperature and ambient pressure and entering at the speed of the airplane. After getting heated, the pressure goes up. Then the pressure is dropped, which means the volume goes up. So the higher volume moves at a much higher speed to have time to get out, creating a delta-v. Basically same pressure at front and back. But warmer and much speeded up when leaving the nozzle.
Well, you get 40% marks, for making the number ONE mistake of thinking that the combustion heat raises the pressure. No. Nope. Wrong. Don't give me that crap. Watch the video about the number One mistake, OK?
AJZ was kind enough to answer my question a few years ago on this issue. Max pressure is at the diffuser after the compressor, feeding air to the entrance of the combustors, and drops as the air moves to the combustor exit. It must be thus, as the air must follow the path of least resistance. Even taking into account the turbines extracting energy from the flow the pressure goes down as the air moves rearward. That is why turbojets blow out the back and not the front.
Thrust corresponds to axial velocity change to be precise. But for first assumption X*Airflow - for engines from 60s /70s/80s factor equals 50/60/70 ( without afterburner )
@AgentJayZ would it be possible to use damaged blades as channel merch? Laser etched with channel name and framed? I know that laser won't do much to blade itself but it would remove the outer gunk accumulated during its service, so close enough.
I've got a video about how to polish them to a chrome-like finish. They are nickel alloy. The idea of selling them was destroyed by douchebags many years ago. Sorry.
Your video's are great for helping to understand the details of turbine design and function. I have always been interested in jets and jet powerplants. I am gathering/guessing that if the nozzle area is too small that would increase the pressure and reduce thrust?
So the air goes in the front, the combustion process adds energy to it, and the nozzle ideally directs the excited air mass to go straight out the back and not to the sides to make forward thrust. Or am I way off?
So by Newton the engine thrust is an equal and opposite reaction to the action of the accelerating air mass. I hope that's right. I get that the engine casing connections transfer engine thrust to air air frame (or engine hold-downs). Can you please provide the definitive answer as to how, or through which engine parts, the reaction energy from the accelerating air mass transfers to the engine casing. I could try to guess but probably would be wasting your time. edit: just watched another of your videos on thrust. I see it's been hashed through already. thanks for all this knowledge.
I know you are not a design engineer of jet engines. Though your knowledge of their operation and repair makes you more knowledgeable than most jet enthusiasts. I have a question concerning the flow of air and jet exhaust through an engine. Don't you think a straight path through the engine would make more efficiency and thrust, as opposed to an engine where the air from a centrifugal compressor makes a 180º turn to enter the combustor, then another 180º turn before exiting out the rear? The engines I am talking about are the engines designed for R/C aircraft. All manufacturers of these engines follow the same design.
The PT-6 and the PW-100 series of turboprop engines have many 90 degree turns for the air that passes through them, yet they are efficient and reliable. The Garrett TFE 731 turbofan has a centrifugal compressor, and it has been a successful turbofan engine for almost 50 years. I should read your entire comment. RC engines are toys, with vastly inferior performance and longevity to real engines. The main advantage for toy engines is low cost. Centrifugal compressors, especially at the toy size, are much less expensive to make. Nobody is going to buy a $30,000 axial compressor turbojet for their RC plane when they can get a $5,000 centrifugal compressor engine. Given the choice, what would you do?
Turning the airflow actually doesn't really introduce much in the way of inefficiencies, especially because the flow velocity isn't that high in the areas where the turning is taking place. The largest reason why small engines tend to use centrifugal compressors is that axial compressors are extremely difficult to make efficient at small sizes, due to tip clearance concerns. Because of the incredible tolerance concerns, along with the fact that you need many more stages to achieve the same overall pressure ratio (axial compressors can only achieve ~1.3:1 per stage, while centrifugal can achieve over 3:1 pretty easily), an axial compressor would simply be too complex and expensive to use in model engines.
Love your videos. Just signed up on patreon. The combustion and exhaust pressure videos, as well as the video you did explaining pressure, temperature, and velocity through the engine stages have been very helpful. However, I am still not quite grasping something. I think if you could answer a question for me, it might help clear it up in my little brain. There are forces that are acting on the air molecules to accelerate it. Where, in the turbojet engine, is the equal and opposite force being applied? What part of the engine is being pushed forward, to put it in simple terms? Is it the compressor blades?
Ah, the question. THE question. There's no really complete answer, but then the answer does not matter, does it? So the best answer I have is the most convenient: all of them. If you put turbojet thrust distribution into your search bar, you will find some very informative diagrams.
@@AgentJayZ It matters, but only in the mind of a geek like me. Sometimes the hardest part of finding an answer is knowing what question to ask (e.g. knowing what terms to search for). I think turbojet thrust distribution is the question to my answer. Thank you for pointing me in the right direction. I've already found some great information.
Bernoulli's principle is critical for understanding this. I think a video just about Bernoulli's principle would help many that don't understand the function of the narrowing tail cone.
Someone started explaining this to me a long time back by pointing out that air just does not weigh much per cubic meter. 80% of it is nitrogen and 800L of nitrogen is only 0.93kg at sea level at 22C. So for say 10,000 pounds of thrust you can go two ways. Loads of air accelerated a fair bit or not so much air accelerated a huge amount. As air is in effect so light it does not take a vast forces to get each individual cubic meter moving fast. IE it has a density little more than 0.01% that of water, so for an equal velocity a cubic meter of air only needs about 0.015% the force that 1 cubic meter of water would need. You still need the mass though (to get the total thrust) so a jet has to use a lot of air. And that is why the pressure is usually far lower than people would normally guess in the exhaust (because you have a lot of air moving fast rather than a bit of air moving hyper-fast) Bernoulli effect even lowers it further. I would like to know if this is roughly correct.
Sort of. A large mass of air is passed through the engine, and so that means a huge volume of air. Hundreds of thousands of cubic feet per minute. It is compressed in order to burn the fuel efficiently, and after making the turbine turn, the leftover pressure is converted to speed, so when the air leaves the engine, it is at a pressure only a little bit higher than is was when it went in the front, but it is coming out with a lot of speed. That acceleration from low speed to high is what causes the thrust which pushes the aircraft in the opposite direction.
Hi JayZ. (I hope the coffee mug arrived ok!) I think a fundamental blockage with peoples understanding in this regard is that they don’t discriminate between static and dynamic pressure (ie are not aware of the difference). Basically, it’s the continuous versus discrete definition thing I guess ....
I don’t know if it’s right or wrong, but I think of a jet engine as a fixed pitch, multi blade in a tube. Or a propeller is almost an un-ducted two (or three or four) blade jet. Lol I can tell you from flying both that a high performance piston engine is more work than operating a jet.. the piston engine has a throttle lever or knob, propeller control, mixture, cowl flaps.. maybe carb heat or alt. air... you have to adjust climb or descend speeds for cooling issues.. with a jet, you just have one lever, and that’s it. No shock cooling..Much more simple.
A good example of this is to watch a rocket launch. All that power, but the rocket engine exhaust goes in on itself due to atmospheric pressure. As the rocket rises in the atmosphere the plume expands (and becomes less efficient) due to less pressure around it.
And that's also incorrect, a sea level optimized engine will be slightly more efficient in vacuum than at sea level since the pressure around the nozzle is 1 bar lower in a vacuum. The pressure and speed in the exaust gas stream (rear) is about the same at sea level and in a vacuum in case of a sea level engine. But the pressure around the engine (front) is not, it changes drasticly from 1 bar to 0 bar. Since pressure in front of the engine drops and pressure in the rear of the engine stays the same, you gain thrust. It does waste energy at high altitude though, that's because it could expand the exaust further than it could before as ambient pressure drops. The expansion ratio of the engine stays the same as at sea level, it just expands more after the engine. However you can run even larger expansion ratios to expand even further for higher exaust gas velocities in space to build even more efficient engines. That's why for example Merlin 1D is way smaller than Merlin 1D vacuum and the main reason for the latter beeing far more efficient despite beeing mostly the same engine up untill the nozzle extension.
@@diesistkeinname795 Sea level optimized rocket engine can't be more efficient in vacuum. The difference of 1 bar is negligible compared to the pressure in combustion chamber. Yes, the exhaust gas will go out a tiny bit faster. But then it will be expanding in all directions rather than going straight opposite to where the rocket is accelerating and that will reduce the total thrust way more than what the 1 bar pressure difference adds.
@@kasuha Nope that's wrong. Any rocket engine will produce more thrust the higher up it goes. If you take a rocket motor with a nozzle optimized for sea level and run it in space, it will have exactly the same speed coming out of it, but an extra thrust of 1 atm * nozzle exit area. The speed coming out of a nozzle is affected only by the combustion chamber's pressure and temperature, and by the expansion ratio of the nozzle. It is not affected by ambient pressure (unless there's flow separation at lower altitudes). If however you take the same exact motor, but replace the nozzle with one optimized for vacuum, you will gain a lot more thrust from that same motor. That does not change the fact that with the original nozzle it produces more thrust in space than at sea level, and that's always the case. So in summary, any rocket motor performs better at lower ambient pressures, regardless of nozzle (so becomes more efficient at altitude), but at any altitude a nozzle optimized for that altitude is superior to all other configurations (so a matched nozzle at 1 altitude is the most efficient). en.wikipedia.org/wiki/Rocket_engine#Net_thrust
good video. Thrust is the change in monument of the air flow thru. Super sonic aerodynamics is backwards because of the speed of sound. (air pressure waves)
Thanks Agent Jay Z for this amazing explanation and I have a question Depend on what we studied we know that the turbine needs the combusted mixture of pressurized air and fuel to turn and also we know that all the stages in turbojet engine (intake, compression, combustion, exhaust) takes place in the same time. My question is how do the turbine and the compressor turning in the same time before turbine have any energy to turn. I'm sorry for my English if you couldn't understand.
Nitpick: you dropped the mass flow term in some of your equations. Thrust is proportional to the amount of stuff going through the engine (mass flow) multiplied by the change in velocity (delta v), not just delta v. You're right that pressure thrust is fairly insignificant and an indication of inefficiency though.
When I said "air", of course I meant the mass flow of the air. A bit of shorthand, and it would never pass a physicists scrutiny, but we're here to get a gut feeling for things. Nits will be picked on the second floor, OK?
Yes, and in the case of the J79, one end is open, while the other is being stuffed by a 30 thousand Hp air compressor. I'm always puzzled by people who have trouble understanding which way the air is gonna go...
@@AgentJayZ hahaha! It's that y'all Canadians have an inherantly better mechanical aptitude because life up there is a closer approximation of reality, than city life here in the States. I spent a number of years in North Dakota. Good experience from the standpoint of hands-on DIY, survival of the fittest, hahaha.
@@Triple_J.1 i worked in oil and gas in the bakken shales for a number of years and I'll be perfectly happy never going back to that godforsaken place hahahaha
This video helped me understand a misconception that ive had for a long time now,thankyou. Out of interest i really want to know about the effect in supersonic flight. You spoke about it lightly in this video but i want to know what differs from super and subsonic in relation to the nozzle geometry and if there is any change in exhaust gas pressure. Sorry its not worded very well im useless when it comes to getting my thoughts into words
Jet thrust and rocket thrust are similar in this sense, the nozzle provides essentially the same purpose- trade pressure for velocity. Take "every action has an equal and opposite react", then consider the mass of the combustion products(would it be the working fluid in the case of jets?), and you figure that you must increase the velocity of your fluid to produce more work.
Talking about the engine featured in this video, do you have any film of it actually powering a plane, J ust be nice to see one being used, [ now I expect you will throw a load of links with planes flying by the engines you have built ]
Good to emphasize that thrust is proportional to delta v, and mass air flow. Not volumetric air flow. Conservation of momentum is a simple way to understand it, and that requires mass and velocity, volumetric flow is moot.
@@AgentJayZ essentially, but I heard repeated references to velocity and flow, not specifically mass flow. Not to criticize, but it's conceivable to me that the two could be mistaken if not emphasized. Thanks for the vijeo
I would guess that people get confused about the pressure because if they stood behind the nozzle it would tear them apart. It might be easier to imagine moving at the same velocity in the same direction inside the jet exhaust. In that situation the pressure certainly would not crush you. Am I right? Any additions or corrections? Good video AgentJayz.
Which jet engine in your opinion is the best over all, not the most powerfull or fuel efficient, simply which offers the best overall value for money.? I spend many hrs looking/listening to your videos, thanks for sharing your knowledge with us lesser mere mortals. Jet Engine's have come a long way since the Frank Whittle Day's . Happy new year dude
That is a very difficult question, because prices are all over the place. If you just want to have it on a stand and fire it up, then the cheapest one you can get running is best, right? You can get a full size turbojet for about 10 grand on Ebay, probably less from a scrap yard. Expect to spend that much again, and six months or so to get it running. If you want to fly a particular vintage aircraft, then the only suitable engine is the one designed for that plane, and hey, if you own the plane, your first concern is not money, it's safety. So best and value are both subjective words, and change based on what you want the engine for. Sorry, but your question was insufficiently specific.
Maybe you could show how the EPR gauge works it could help. Incoming Air at the inlet is actually negative pressure being well below the standard 29.95 inches.
Nope. In my opinion as an engine builder and tester, the EPR is meaningless, and is an imaginary variable, relied upon by folks who don't understand how jet engines work. We never use that variable, and a gage for it does not exist in all the jet powered aircraft I have experience with.
Pressure indicates resistance, so yup, bad! As I see it, it's about using "pressure" to increase velocity. Channeling the expansion into velocity through the exit.
Check yourself, please. Bernoulli is a word you need to look in to. Before further staining your rep by "correcting" the one who is trying to share knowledge.
@@AgentJayZ Oh, I wasn't trying to correct. I was just commenting to the ideas I formed. ;) Always eager to learn, have heard OF Bernoulli's Principal but now I'm going read it in full and will correct my understanding. Love the channel and info you share because I've always been fascinated with turbine engines. Thank you again!
Not to cross disciplines, but you can also visualize the optimal exhaust flow vs ambient pressure (I'm probably using the wrong terms) by checking out this article about rocket nozzles. It supports your assertion that pressure is a relatively insignificant component of thrust. Optimally, pressure at the nozzle = ambient pressure. www.quora.com/SpaceX-has-two-versions-of-the-Merlin-engine-sea-level-and-vacuum-What-are-the-big-differences-and-why
What if there was a door at the back end of the jet engine and you could shut it closed during operation. wouldn't there be an instant rise in pressure and rip the thing apart? so because there is no door, this potential pressure is let out the back end (before it can become pressure) in form of a high speed mass of air producing thrust? Or am I getting this wrong, still?
Something like that can actually happen if the afterburner nozzle gets stuck and stays closed when the afterburner lights. The jet pipe will instantly over-pressure and start to heat up and it could eventually explode. That's why they used to put an over-pressure sensor in the jet pipe that shuts the burner off if the nozzle doesn't open correctly. The F-100 originally had a poorly designed iris type afterburner nozzle that very often would get stuck closed and when it did the burner would immediately shut itself off to prevent it from exploding. This caused a lot of aborted take offs and it was an even bigger problem in flight, especially in combat. Unlike the F-100 the F-102 had a petal type afterburner nozzle that was much more reliable. Eventually when the F-102's were retired the jet pipes and burner nozzles were removed from the 102's and retrofit onto the F-100, which finally fixed the problem.
I've read accounts by pilots of the F-100 that, with it's simple two position nozzle, activating AB caused an instant increase in thrust which felt like being "kicked by a mule". I would like to see one flying in an airshow someday.
@@AgentJayZ Even though I was in aircraft maintenance while in the Air Guard I did get two back seat rides in an F-100F. What you have described is exactly what happens when the F-100 would go into afterburner. You basically feel a large thump and then the extra acceleration pushes you back in the seat. Once you are in afterburner and the speed maxes out you really don't feel much of anything until it comes out of burner. When it does the plane decelerates and you get thrown forward into the straps. Another thing that happens is that when the burner is engaged the EPR drops for a split second when the nozzle opens and then it comes back up when the burner lights. Then when you come out of burner the EPR drops for an instant and then recovers when the nozzle closes again. The pilots are trained to watch the EPR when going in and out of burner because it indicates that the nozzle is working right. The other indication is that of course the fuel flow jumps way up when the burner lights.
To my knowledge V1 is the speed at which the aircraft can not safely reject the takeoff attempt. The point of no return for the pilot, so to speak. Aircraft speed is measured by the pitot tube, which is on the airframe. I also think speed is measured by GPS, but I think the pitot-measured air speed is more relevant to takeoffs and such. As well as that, the intake cowling is part of the aircraft, not the engine, so I've never touched one or even seen one up close.... except for that one from a 767 that we made into a desk.
@@AgentJayZ Thank you for your response. My apologies for not specifying. In the video, at timestamp 4:45 you use V1 in the Force equation. Where would V1 be measured on the engine to form this equation? The reason for my question is I'm trying to get an idea of the effect the intake has on the V1 speed in the force equation and how much V1 differs from the speed of the aircraft.
@@archieobrien1 Ah, well we can see now just how much a time stamp helps clear things up. In the equation V1 is the initial speed of the air entering the engine. V2 is the speed of the gases leaving the engine. The quantity V2-V1 is the acceleration of the air. In operation, engine inlet air speed is not measured, but it is in research and development of engines. I think for The engine inlet speed will be close to aircraft speed, although air inlets to turbofans act as diffusers to reduce air speed and increase air pressure. Maybe we will be lucky enough to hear from our friendly turbine engine design engineer on this.
I would say anything big enough to move molecules of air. So that means exactly 1/32 the mass of that (in)famous dust speck upon which a certain Horton heard a certain Who...
@@stopthephilosophicalzombie9017 That's an interesting question. The answer is everything that is significantly bigger than the mean free path of air will create a shockwave if it is supersonic. There's a thing called the Knudsen number (Kn) which is the ratio of the free mean path to the geometric size of the object. The free mean path is the average distance that a gas molecule travels before it collides with another molecule. If Kn is much less than 1 (it almost always is), the flow behaves like a continuum, and in that case any impact above the speed of sound will inevitably create a thin shockwave. At high Kn, the molecules move on straight paths without colliding with each other. This is called rarified gas flow. In that case there are no shockwaves, even at supersonic speeds. At intermediate Kn, it is something in between, with very diffused, thick shockwaves. At ambient pressure the MFP is so small that really any physical object will create a shockwave. For example you can see Schlieren videos showing individual unburned gun powder grains coming out of a barrel and creating their own shockwave. Any shockwave creates a sonic "boom" that will be heard when the wave reaches the observer. The characteristic loud sound that we hear when compressed air is released through a small opening (like in canned air or machine air) is actually the result of oblique shockwaves forming at the exit (Mach diamonds).
Another interesting example of why pressure at the jet outlet is not efficient is a rocket booster. Watch the lfit off a modern two stage rocket such as SpaceX. As the first stage ascends the exhaust plume begins to expand in diameter. After seeing this reality, go to NASA or JPL and search for "rocket exhaust nozzle design" (bell outlet). You will find understandable explanations for this that are in line with AgentJayZ's explanation of jet exhaust pressure. If you don't (can't) understand............JSTFU.
Once you understand what pressure is it becomes easy. High pressure is slow where low pressure is fast. Air comes into the engine and is slowed increasing pressure. Air is heated and expands rushing out the end at a high velocity which is a low pressure.
Nozzles of first stage rocket engines are smaller compared to the nozzles of their second stage engines . For maximum efficiency , The pressure of exhaust needs to be the same as of that of the ambient atmosphere. Aerospike engines does this automatically. ( Found this info on the youtube , I couldn't remember the name of the video. It was a while back)
Yep their are some great animation visuals out their showcase the affect of nozzles shape relative to air pressure and the resulting change in efficiency since any vector not in the opposite direction of travel is wasted energy. Part of the reason aerospike engines are so nice in principle although I believe have many engineering complications making them less than ideal.
First stage, or atmosphere optimized nozzles, are small because pressure of exhaust gets so low that atmosphere pressures its way along the nozzle inner side, causing flow separation and violent turbulence leading to catastrophic engine failure with nozzle getting destroyed.
Fyi im researching how to make just a jet engine. Nothing fancy, nothing super powerful, just a jet engine... I was thinking about it as a portential solution to a problem I was grappling with. I do reserve the right to ask as many dumb questions as will get me where I need to be. Fortunately, I don't need it to last very long... so happy to use old schematics and tech and substandard materials. If I can get the general principles to work in the one unit I'll be happy...
No, that is not correct. The exact same mass of air comes out the back as goes in the front. It is of greater volume because it has been heated up, and so it must ravel faster. The thrust is due to this speed increase. Didn't we cover that?
@@AgentJayZExactly. In pure engineering /physics terms F=m*a. The jet engine accelerates the mass of air, producing force. Any pressure difference represents an internal restriction which would reduce the efficiency of the engine. Any contribution to the thrust equation would be the air expanding and accelerating after it leaves the engine.
@@AgentJayZ Not EXACTLY the same mass flow because of the fact there there is bleed air coming out of the stream and fuel injected into the stream meaning there can be times where the change in mass flow is positive or negative between inlet and exhaust. Although one dirty assumption that can be made is the fuel mass flow in is about the same as the bleed air mass flow out meaning the mass flow is about the same at inlet and exhaust.
So jet engines work like rocket engines, except the oxidizer comes from the air, gotcha. higher velocity more good, do jet engines then have problems with instability in the nozzle? If I remember correctly NASA had a bunch of problems with the F1 engine on the Saturn V, flow separating from the nozzle, causing it to oscillate and disassemble itself. I can highly recommend Scott Manley's videos on rocket engines and how they produce thrust, goes into detail as to why over expansion is inefficient and why the exhaust at the nozzle is actually below atmospheric pressure.
That was combustion instability at the fuel injector plate, not the rocket nozzle. Nothing to do with todays subject. As I mentioned in the video, it is important to have a very clear understanding of what parts you are talking about.
Yes, an imaginary, useless parameter. How useless? It is never used in engine testing. The only reason EPR gages are in the cockpit is because there is no way to measure thrust, other than fitting the main engine mounts with strain gages... which would require regular calibrations, and add expense to the aircraft.
G'day, Yay Team ! There was a very Good REASON why the Germans, who invented these devices..., called them "SQUIRT Engines..." The faster one squeezes an Orange or Lemon-Seed betwixt Finger and thumb, the further it goes during an identical Time-Interval... Because, the simple Temperature & Pressure-Difference between inside the Compressor & the Ambient Pressure of the Outside Atmosphere..., achieves no Velocity at all... But..., when the Mass (of Indrawn Atmosphere) is accelerated via the Temperature-Differential aquired during it's Combustion with Refined Transport Fuel - inside the Combustors, then the (Half) Mass(flow) times Velocity Squared going in, being significantly less than the (Half) Mass(flow) times Velocity Squared, coming out, at the Nozzle Orofice, is responsible for the (desired) "Thrust" percieved at the Engine-Mounts. Well done. You must have a lot of stupificated Viewers, if you're genuinely encountering (such) "a lot" of Commenters expressing a wrongheaded perception of what goes on. Do they also think that Rockets can't possibly function within a Vaccumn - because in a Vaccumn there'd be nothing against which, "to Push..." (!) ? Such is Life, Happy Solstice Festival... Stay safe. ;-p Ciao !
Good explanation. I think of it as the heat of combustion, and the expansion it causes, adding momentum to the air exiting the back. And for anybody not convinced that pressure at the outlet should be as close as possible to ambient pressure check out rocket man Scott Manley's video about rocket nozzle design. Unlike turbojets rocket engines need to use an expanding outlet to _lower_ the pressure of their exhaust gas to close to ambient. LINK: ruclips.net/video/l5l3CHWoHSI/видео.html And some guy on the internet showed me a link to download books, including the ones in this video, but I seem to have lost that link... not that I would ever use something like that. EDIT: found it - www.pdfdrive.com/the-jet-engine-e107007559.html
Interestingly, in both turbojets and in rockets, the purpose of the nozzle is to decrease the pressure to ambient. In a turbojet, the (static) pressure at the exit of the turbine is still above ambient, though not nearly by the same degree as in a rocket's combustion chamber, and this pressure is converted to velocity as the flow passes through the nozzle. The nozzle pressure ratio on jets is fairly small though, so you don't tend to see the dramatic nozzles like you do on rockets, and you also mostly tend to see subsonic (converging only) nozzles rather than supersonic (converging-diverging).
@@Miata822 Yes, but bernoulli isn't really applicable here. Bernoulli applies to incompressible flow, and jet exhaust is definitely fast enough that compressibility matters. Trading pressure and velocity isn't just bernoulli though - it's basic conservation of energy, so it applies even in cases where bernoulli does not.
@@clapanse Incompressible flow? Then explain venturi effect. Conservation alone would not need a nozzle. It is the _relative_ velocity of the exhaust gasses to ambient air that creates a pressure differential outside of the engine, keeping the exhaust jet coherent.
@@Miata822 the venturi effect works in both incompressible and compressible flow, and I don't know what relevance you think it has here. Also, the relative velocity of the exhaust to ambient air is what creates thrust. However, it doesn't tell you anything about pressure. The velocity of a jet of gas doesn't tell you anything about the surrounding pressure. Ideally, you exhaust the jet at whatever velocity you need such that it matches ambient pressure.
I think the penny's dropped... So basically the reason for a convergent nozzle is to produce in essence a venturi effect, accelerating the gases, AND preventing their expansion?
No. The nozzle maximally expands the volume of the gases, which makes them exit the engine as fast as possible. The word venturi is not used in this context.
@@AgentJayZ yeah but the nozzle is operating using the Venturi effect - constrict the flow, causing the velocity to increase and the pressure to drop (as close to ambient as possible).
There's no link for applying tinfoil onto sunglasses....There was a not-so-bad description of a jet engine in the 1952 movie, 'The Sound Barrier'; Sir Ralph Richardson shows you I believe, was a DH Goblin running in a test stand and throws a handkerchief into the jet blast. One of the first descriptions I ever got: "...paraffin heats the air."
![Noob To Max With DRAGON REWORK In Blox Fruits [FULL MOVIE]](http://i.ytimg.com/vi/LnBMOinoOvA/mqdefault.jpg)
You don’t have to call yourself ‘grumpy’ when *any* human who has devoted their career and life to working on these complex machines is bombarded by misunderstandings, incorrectly formed questions on a daily basis would rightly get a little ‘gnarked’/irritated by repetitive ill-informed individuals. And just when you think you’ve dealt with the last, a flood of newcomers comes and asks all the same questions all over again.
Most of us truly appreciate the effort you’ve put in *FOR FREE* over the years. For me, when you say ‘but there’ll be no Maths’, I feel a little sad cos that’s what my PhD is in. But I get that it’s a difficult subject, especially without the formal education I was so fortunate to receive.
In short, we - the understandable, understanding ones - not demanding more time and more spoon fed knowledge for God damn *FREE*! We *THANK YOU*, truly we do. I was interested in jet engines, I like planes, I think they’re cool, and you’ve built a community that is somewhat mutually supportive in your spare time.
K, thanks, bye!
Agent Grumpy Z...you know yourself well. Your a funny guy and there's nothing wrong with being passionate. Happy New Year all :-)
For the people who can't follow AgentJaZ's excellent explanation...
Every action has an equal and opposite reaction.
Thrust equals the mass and velocity of air (& combustion byproducts) being moved.
Your welcome... where's my beer! ;-)
Same kind of misunderstanding goes for the rocket engine exhaust. It is at or slightly below atmo pressure on the ground level. Exhaust expands rapidly, trading pressure for speed
I like that you advise using multiple sources. I once trained as a military aircraft engineer, but some of the things you say are becoming more understandable than what I read in books. I completely agree with you that even if you study one source perfectly, you are still don't master the topic well enough. Thank you
Love this guy !!! His speaking is super clean and knows and explain all very clear
Love it. Explanations like this are gold. Doesn’t matter the amount of attitude or passion in which it’s delivered....as long as it’s clearly articulated. You’ve e done well. Thanks
Bernoulli's principal at its finest.
Pet peeve, an analogy I suppose...the pressure washer.....lol
It's the speed that the water leaves the nossel uses to creat the force for cleaning. So I deem it a velocity washer. Lol.
Stay safe, merry Christmas.
Good job once again.
You are right.
Here's one for you:
The almighty EPR that the pilots look to as the only gage they care about... it only reads anything, because the exhaust pressure probe is in the wrong place!
At 18:33, that diagram is correct. The pressure shown here is total pressure, not static pressure. Static pressure drops to close to ambient, but total pressure almost doesn't change through the nozzle.
Without adding or substracting energy the only mechanism for a change in total pressure is friction. In the nozzle there is no additional energy added, and friction is very low so the total pressure coming out is almost exactly the same as coming in.
It is indeed correct, just a bit misleading. Perhaps static pressure plot would be more accurate in order to precisely display the nature of the fluid flow in the nozzle :)
13:55
The ideal case you describe is call nozzle optimum expansion Ratio. The fact that in practice it seems to overexpand a bit at ground level (at least for civil application) is because with fixe exhaust nozzle section, you can only be in this optimal situation for a particular (regime, altitude) couple (as the atmospheric static pressure keep decreasing when you get higher and higher). Civil application jet engines fixes nozzle are usually optimise for cruise regime and altitute. In a military application with variable nozzle section, you could make some control laws taking into account this phenomena in the egine calculator, but in practice I think it might be quite complicated as you must work in close loop because atmospheric static pessure conditions varies over time and location...
You can also see very clearly this phenomena in rocket engines flame exhaust during launch and why the upper stages engines nozzles have much larger exhaust sections.
ps : Thanks for all your excellent content, you definitly have the best jet engine youtube chanel.
One of the best explanations, thanks. I understand the math as an engineer and your ability to "bring it down" so many people can understand it is spot on.
This was a good way to spend an hour, learning something that I probably won't ever use but I want to know these things. Thank you AgentJay and I also appreciate the commenters helping to explain things. Now it's time for some microbiology, or maybe a shift cable change out on a Ford pickup.
I believe I've figured out a bit of Y-tubes formula for recommendations: "If elementary school=fail then send to AgentJayZ"!
I'm suffering with you for getting all these people that refuse to learn Jay.
Wish you and all tech folks a good 2021!
Jet engines and rocket engines work on the same basic principle. They both create thrust by accelerating a gas in one direction to produce a force in the opposite direction. All three of Newton's laws of motion are involved. According to the 1st law the gas has mass and therefore inertia, which will cause the mass to resist being accelerated. Therefore according to the 2nd law, F=MA, a force will be produced proportional to the volume of the mass and how much its being accelerated. Then according to the 3rd law the force will be produced in a direction that is opposite from the direction of the acceleration. As Jay said these engines produce thrust by accelerating a gas. Essentially what happens is that the mass of the gas and the mass of the engine push against each other and are accelerated in opposite directions. That's really all there is to it.
You are correct in all you wrote, but i have one question. What causes the gas to flow? As in, what causes the gas to flow in that direction? Or why the wind blows in one direction or another?
@@bogdan_n The gas, which is just air initially and then air combined with fuel residue, flows in the direction that it does due to the way the engine is designed. The compressor in the front of the engine sucks in the air and compresses it, which creates the highest pressure anywhere inside the engine at the outlet of the compressor. From there as the gas travels backward through the combustion section, then through the turbine and finally out through the nozzle. At each step the gas pressure is reduced as the gas exchanges pressure for velocity (according to the principle of the conservation of energy). So the direction of the gas flow through the engine is dictated by the pressure distribution at various points going through the engine. The compressor mechanically forces the air backwards increasing the pressure to its highest point but then from there the pressure drops step by step as the gas goes the rest of the way through the engine and out the back. Its really just basic physics. A compressed gas contains potential energy, which will be converted to kinetic energy when the pressure is released. So gas will always want to flow from high pressure to low pressure. This is what makes air move through an engine in the direction that it does and makes the wind blow in a particular direction. Its all about having a pressure imbalance and in which particular direction that imbalance exists.
@@joevignolor4u949 So, we are still talking about pressure. And even if the pressure, or better said, the pressure gradient isn't what produces thrust, it causes the gas to move. And even if the pressure in a shop compressor (120psi or about 8bar) is about 4 to 6 times greater than in the exhaust pipe of a jet engine (20-30psi or 1.38-2.07bar) the area of the hole that the air "escapes" through is about 60k times bigger in case of a jet engine. So, if we are to compare the jet engine to a compressor, let's compare what would happen to the compressor (and the surrounding objects) if it had, let's say, a 1000 cubic meter pressure bottle, and a 1m diameter hole would suddenly open in that bottle.
@@bogdan_n If you math is right, which it seems to be, and you mounted your big shop compressor and nozzle on a glider then you would have a jet powered glider. There would however be a problem. Your engine will only produce thrust until the supply tank runs out. You could of course keep the compressor plugged in but you would need a really long extension cord. Real jet engines get around this problem by burning liquid fuel (which has a much higher energy density than compressed air) and using the energy produced to drive the compressor and to heat and expand the air to create pressure, causing the hot pressurized air to be accelerated out through the nozzle.
This was very helpful. Thank you for your simplified explanation. I always enjoy your videos. Keep up the great work!
Thanks so much for adding this added explanation. As a mechanic of course we have always had difficulty with pressure. Having it accelerate Air is a concept that it's hard to wrap your head around without it changing pressure much. I hope you had a great Christmas and have a great New Year. All the best from Surrey
I used to live in Surrey, BC.
For the people stuck on the nozzle pressure aspect of jet/rocket engines just remember that any exhaust that is higher pressure than atmospheric will expand radially which is wasted momentum (aka thrust). This is also why rocket engines have nozzles that are optimum for air or space and not both (except aero spike engines). Vacuum rocket nozzles have large bells to catch the expanding exhaust and keep it moving parallel to the thrust line.
Thanks Jay, I have 20,000 plus hours of turboprop time and many people have asked me about thrust and how much we get from the jet exhaust, when I answer i can see they don’t quite get it.
It still amazes me how a jet engine can have so much thrust. My mind cant really grasp it to the point that I'm satisfied.
Strong wind can wreck houses and unroot all trees in a forrest. It doesn't take very high wind velocities until you can't stand.
A jet engine is producing a hurricane. Smaller size but with higher wind velocities.
I saw a demonstration once where they parked an old school bus behind one wing of a B-747. The bus was parked parallel to the wing. They started up the two engines on that wing and when they advanced the throttles the jet blast lifted the bus up off the ground like a toy and flung it through the air. Also, when I was in the Air Force I was riding in a panel truck down the flight line. The driver didn't notice a B-52 that was running all eight of its engines and he drove too close behind it. As soon as we entered the jet blast it pushed the panel truck up onto two wheels and almost flipped it over onto its side. Luckily we came out from behind the plane just in time and ended up still sitting on all four wheels.
13:25 this is the sole reason why rifles have longer barrels to throw the bullet as long as possible. Rifles allow the pressure built by bulet firing to be converted to velocity for a longer duration
I assume that the green line on your chart is total pressure which includes dynamic pressure (~velocity) and static pressure . That's why the pressure doesn't change when static pressure is converted to velocity at the nozzle. Total pressure remains the same.
Thanks for the great videos in 2020, have a great 2021.
Excellent explanation as always! Thanks a lot Jay! Love the channel been with you for years. Keep up the great work!
I'm back and I'm going to take issue with the statement that the final nozzle (as I know it) "creates" the thrust. If the final nozzle were not there on the end of the jet pipe, the engine would still produce some thrust, but the pressure drop across the turbines would be way off design. The turbomachinery as an aero/thermodynamic system would, therefore, be operating way off design and the engine would be grossly inefficient.
The thrust of an engine is created within the engine by the sum of all the forward pressure loads acting on the internal surfaces of the engine minus all the rearward pressure loads acting on the internal surfaces of the engine. I've stated and restated this several times on this channel. A physical force can only be produced by a pressure load acting on a given area: velocity is irrelevant. Having said that, those forces are produced by changes in pressure and velocity throughout the engine. If the pressures and areas could all be measured, calculated and integrated, then the result would be the thrust of the engine. Any 'fiddle factors', using mass flow, velocity/momentum change, intake ram drag and pressure drop across a choked nozzle would be unnecessary.
OK, that's the engineer in me bursting out, like the alien from John Hurt's chest in the movie - but I digress. Of course, I recognise that Newton's laws apply: Force still equals Mass times Acceleration, the engine must accelerate the air passing through it to produce thrust, and the thrust can be calculated more conveniently using the physics.
Yes, the thrust can be calculated from a summation of all the forces within the engine, but I've thrown this little spanner (OK, wrench, if you must) into the works before. What happens in my old Bristolian friend, the Pegasus engine, when a Harrier 'jump jet' is the hover, with its nozzles pointing downwards? Essentially, the thrust is produced by the pressure acting on the exit areas of the four nozzles projected onto the internal surfaces of the nozzles opposite their exits.
This is effectively what propels a balloon when it is blown up and released to career around the room. It's the pressure in the balloon acting on the area of the neck projected onto the inner surface of the balloon opposite the neck. I use this to explain how my balloon-powered model cars work when I'm doing a STEM activity with young people (mustn't say 'kids').
Well, I feel better for that - so, Happy New Year to you, AgentJayZ, and to all of your subscribers.
Misunderstanding of this concept is rooted in the misunderstanding of dynamic systems vs static systems. Pressure alone as with temperature and volume are only relevant in the static realm. In dynamic systems only the time derivatives of these are relevant dP/dt, dT/dt, etc are relevant. It's a difficult jump to make without a solid base of calculus. Our human minds make an error of trying to generalize static systems to dynamic ones, when actually the general case is the dynamic case where the dt factor falls out to generalize for the static case.
@AngentJayZ, I have reached out to you a few times in the past. I work on gas turbines in the power generation industry (Siemens/Westinghouse, Mitsubishi and GE). On a normal "jet engine", I assume you probably have 2 or 3 stages for the hot gas path. In the power gen industry, we typically coat our first two stages of the power turbine with TBC along with cooling channel arrangements. Is that typical in the aviation space? Just curious how the aviation world manages the same issues we deal with in the generation business.
Hi Stephen... just looked this up so I could understand the question.
"Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces operating at elevated temperatures, such as gas turbine or aero-engine parts, as a form of exhaust heat management. These 100 μm to 2 mm thick coatings of thermally insulating materials serve to insulate components from large and prolonged heat loads and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface.[1]" Wikipedia
Yes, the whole idea of TBCs was introduced in aviation engines, and also adopted by newer designs of industrial engines. What we have done is apply the process to older engines, both of industrial and aviation application.
For these older engines, it can be thought of as an aftermarket upgrade.
Thank you for picking up these misconceptions. Just thinking about the velocity difference now, I get closer to understanding how reverse thrust can even work. So there is more energy in the airflow due to fuel combustion, which provides for the velocity increase. After that it's Newton's third law. Interesting fact about the nozzle section here too.
King of jet engine on RUclips 🔥🔥🔥💯💯💯
This is a very nice explanation of thrust, thanks for this. I might note that a good example of what you are saying is the difference between atmospheric and vacuum nozzles on rocket motors. Vacuum motors have an extended bell nozzle to better match the pressure of the exhaust to the very low pressure of space and thus increase their efficiency..
Keeping notifications on for JayZ is like telling your math teacher yeah... I just watch YT now so bring it
I have recently got into your excellent videos and are finding them very informative. Is it possible to explain the differences in thrust generation between a pure turbojet, low bypass ratio and high bypass ratio turbofans.
It's possible. It's done very well in any of the resources I've recommended. Some of them are free.
To my way of thinking is that if you use your thumb over the end of a water hose to make the water travel farther yes you increase the pressure behind your thumb and the water going out has it's velocity increased but the water pressure outside the hose is zero after it has exited the hose.
A thing I realized just now thanks to your excellent explanation: A turbojet and a rocket engine are basically the same: Oxidizer and fuel are fed under pressure into a combustion chamber where they are burned. The host exhaust gasses are then expelled through a nozzle that turns pressure into velocity. The lower the pressure relative to ambient the more velocity and thus efficiency you get.
This is a much larger problem for rocket engines. They run from ground level atmospheric pressure up to near vacuum. It is easy to see at a rocket launch: At liftoff the exhaust leaves the rocket engine almost straight. The higher the rocket flies, the lower the air pressure and thus the exhaust plumes out bell shaped. That is the reason whey upper stage rocket engines have much larger nozzles: They expand the exhaust gas to almost vacuum and must be much larger to achieve this.
This video/discussion is not about rockets.
I speak of convergent nozzles here.
Rockets have divergent nozzles.
... some homework for you there...
@@AgentJayZ Thanks for the correction. I now realize my error: Rocket engines use a de Laval nozzle. This accelerates the gas to super sonic speeds. Which is different from turbojets. Yet an other interesting part learned.
Yes. Jets are a reactive propulsion. You throw air molecules out the back, you will get a reaction propelling you forward. But only the molecules going straight back contribute to the reaction pushing you forward. Molecules going sideways are not useful.
Those molecules moving sideways in all different directions is what makes pressure. When not all molecules go in random directions, but there is, on avarage a dominant direction of the otherwise random molecule movement - that is what gas speed is.
From this you can see that, given constant energy, in order to gain speed, molecules have to be redirected from their random movement - so the amount of random movement will be less - the pressure will drop. This is why fast moving gas has low static pressure (bernoulli principle), and this is why jets exhaust is as low pressure as possible.
Interesting, but what about if there is no friction between jet air and ambient air? In that case what happened to the thrust? Is it similar to the car tyres on the ground?
There's no similarity to car tyres. Friction between ambient and exhaust gas plays no role. It's action-reaction principle. You throw something heavy in one direction, you push yourself in the other direction. And the role of that "something heavy" plays air in this case. The engine is throwing air behind it and that pushes it forward.
Well, Mr. D, You are making another of the biggest mistakes, and probably the subject of my next video: You think that the reaction force to the acceleration of the jet exhaust ( thrust ) is caused by the exhaust "pushing" against the atmosphere.
This is not the case. What happens to the jet exhaust once it exits the nozzle does not matter at all.
You are in good company; this is a very widespread misunderstanding.
@@kasuha I can not feel or imagine how the action-reaction principle may exist without friction! The action force act on fulcrum in order to create the reaction force, right? I believe that the fulcrum in the case of turbojet comes from the friction of high velocity exhaust gas and stagnation ambient air otherwise the thrust will be executed.
@@mrdarho4718 If you lift a heavy stone, stand on one leg, and throw the stone in front of you, you'll start falling backwards and will need to put the other leg behind you to prevent the fall. Try it. The fulcrum is you - you send the stone in one direction and at the same time you send your body in the other direction. And friction is only involved in making both you and the stone stop moving eventually. The part you can not try yourself is that it would work exactly the same in vacuum. But it would and that's why rockets can fly in space. I can understand how it may come unintuitive to you but you're wrong and if you want to do something about it, I suggest you to look up some educational videos on newton's third law.
@@mrdarho4718 This one is harder to explain. In general, this situation will never occur - jet engine works on accelerating the air. So air enters it at certain speed, and leaves it at higher speed. But that certain speed at which the air is entering the engine is the same speed as the air around the engine output nozzle. If we accelerated the engine to ultrasonic speeds and only then started it up (assuming it would start up), the air would be still escaping it at higher speed than is the speed of the surrounding air. But we may go for example with a rocket. When the rocket starts up on the ground, it's almost stationary and the exhaust gases are going out at large speeds. But high up in the atmosphere where the rocket is reaching orbital speeds (thousands meters per second) the exhaust is actually going along with the rocket - the rocket moves at 5000 m/s, the exhaust gases are escaping the nozzle at 1000 m/s, that means relative to the earth and surrounding atmosphere, the exhaust gas is moving at 4000 m/s in the same direction as the rocket. And the rocket is still accelerating, because it is accelerated by the mass of exhaust gas leaving it at 1000 m/s through the nozzle.
Solid effort. Fellow engine tech to another thanks you do good work.
Jay, that was great 💖
"But eyes gots to know more"
more about the flow after passing the exhaust turbine out through nozzle???
Laminar opposed to centrifugal turbulent flow?
Which is prefered and why please?
Is vortex generation a thing yet?
Kind Regards &
Keep warm
Hi, I appreciate that thrust is the velocity difference for the fluid from inlet to exhaust but what I'm always thinking about is where the force generated by thrust is transmitted through. Are the lines of force only turbine disk and blades through to the shaft thrust bearings then the housing? Or is it carried through the housing more than via the shaft/s? How much does the bell mouth contribute to the thrust? When you had that compressor section setup like a dyno those years ago for the gas generator, when it was being a load was it producing any type of thrust?
I've got a video called Where does thrust act, where I talk about this.
@@AgentJayZ Thanks for thank. Just had a look and discovered I was not seeing it for the first time. I know conclusively as I had previously "liked" it. Must have KRAFT though because I don't recall it. Will have to look when not tired.
1 kg of air in at ambient temperature and ambient pressure and entering at the speed of the airplane.
After getting heated, the pressure goes up.
Then the pressure is dropped, which means the volume goes up.
So the higher volume moves at a much higher speed to have time to get out, creating a delta-v.
Basically same pressure at front and back. But warmer and much speeded up when leaving the nozzle.
Well, you get 40% marks, for making the number ONE mistake of thinking that the combustion heat raises the pressure.
No.
Nope.
Wrong.
Don't give me that crap. Watch the video about the number One mistake, OK?
AJZ was kind enough to answer my question a few years ago on this issue. Max pressure is at the diffuser after the compressor, feeding air to the entrance of the combustors, and drops as the air moves to the combustor exit. It must be thus, as the air must follow the path of least resistance. Even taking into account the turbines extracting energy from the flow the pressure goes down as the air moves rearward. That is why turbojets blow out the back and not the front.
Thrust corresponds to axial velocity change to be precise. But for first assumption X*Airflow - for engines from 60s /70s/80s factor equals 50/60/70 ( without afterburner )
@AgentJayZ would it be possible to use damaged blades as channel merch? Laser etched with channel name and framed? I know that laser won't do much to blade itself but it would remove the outer gunk accumulated during its service, so close enough.
I've got a video about how to polish them to a chrome-like finish. They are nickel alloy.
The idea of selling them was destroyed by douchebags many years ago. Sorry.
Your video's are great for helping to understand the details of turbine design and function. I have always been interested in jets and jet powerplants. I am gathering/guessing that if the nozzle area is too small that would increase the pressure and reduce thrust?
So the air goes in the front, the combustion process adds energy to it, and the nozzle ideally directs the excited air mass to go straight out the back and not to the sides to make forward thrust. Or am I way off?
The goal of the nozzle is to accelerate the combustion gasses as much as possible to have as much thrust as possible
So by Newton the engine thrust is an equal and opposite reaction to the action of the accelerating air mass. I hope that's right. I get that the engine casing connections transfer engine thrust to air air frame (or engine hold-downs). Can you please provide the definitive answer as to how, or through which engine parts, the reaction energy from the accelerating air mass transfers to the engine casing. I could try to guess but probably would be wasting your time. edit: just watched another of your videos on thrust. I see it's been hashed through already. thanks for all this knowledge.
That's the subject of my video called "Where Does Thrust Act?"
I know you are not a design engineer of jet engines. Though your knowledge of their operation and repair makes you more knowledgeable than most jet enthusiasts. I have a question concerning the flow of air and jet exhaust through an engine. Don't you think a straight path through the engine would make more efficiency and thrust, as opposed to an engine where the air from a centrifugal compressor makes a 180º turn to enter the combustor, then another 180º turn before exiting out the rear? The engines I am talking about are the engines designed for R/C aircraft. All manufacturers of these engines follow the same design.
The PT-6 and the PW-100 series of turboprop engines have many 90 degree turns for the air that passes through them, yet they are efficient and reliable.
The Garrett TFE 731 turbofan has a centrifugal compressor, and it has been a successful turbofan engine for almost 50 years.
I should read your entire comment. RC engines are toys, with vastly inferior performance and longevity to real engines.
The main advantage for toy engines is low cost. Centrifugal compressors, especially at the toy size, are much less expensive to make.
Nobody is going to buy a $30,000 axial compressor turbojet for their RC plane when they can get a $5,000 centrifugal compressor engine.
Given the choice, what would you do?
You are correct, centrifugal turbines become very inefficient as they scale up
Turning the airflow actually doesn't really introduce much in the way of inefficiencies, especially because the flow velocity isn't that high in the areas where the turning is taking place. The largest reason why small engines tend to use centrifugal compressors is that axial compressors are extremely difficult to make efficient at small sizes, due to tip clearance concerns. Because of the incredible tolerance concerns, along with the fact that you need many more stages to achieve the same overall pressure ratio (axial compressors can only achieve ~1.3:1 per stage, while centrifugal can achieve over 3:1 pretty easily), an axial compressor would simply be too complex and expensive to use in model engines.
Love your videos. Just signed up on patreon. The combustion and exhaust pressure videos, as well as the video you did explaining pressure, temperature, and velocity through the engine stages have been very helpful. However, I am still not quite grasping something. I think if you could answer a question for me, it might help clear it up in my little brain. There are forces that are acting on the air molecules to accelerate it. Where, in the turbojet engine, is the equal and opposite force being applied? What part of the engine is being pushed forward, to put it in simple terms? Is it the compressor blades?
Ah, the question. THE question. There's no really complete answer, but then the answer does not matter, does it?
So the best answer I have is the most convenient: all of them.
If you put turbojet thrust distribution into your search bar, you will find some very informative diagrams.
@@AgentJayZ It matters, but only in the mind of a geek like me. Sometimes the hardest part of finding an answer is knowing what question to ask (e.g. knowing what terms to search for). I think turbojet thrust distribution is the question to my answer. Thank you for pointing me in the right direction. I've already found some great information.
Bernoulli's principle is critical for understanding this. I think a video just about Bernoulli's principle would help many that don't understand the function of the narrowing tail cone.
Someone started explaining this to me a long time back by pointing out that air just does not weigh much per cubic meter. 80% of it is nitrogen and 800L of nitrogen is only 0.93kg at sea level at 22C. So for say 10,000 pounds of thrust you can go two ways. Loads of air accelerated a fair bit or not so much air accelerated a huge amount. As air is in effect so light it does not take a vast forces to get each individual cubic meter moving fast. IE it has a density little more than 0.01% that of water, so for an equal velocity a cubic meter of air only needs about 0.015% the force that 1 cubic meter of water would need. You still need the mass though (to get the total thrust) so a jet has to use a lot of air. And that is why the pressure is usually far lower than people would normally guess in the exhaust (because you have a lot of air moving fast rather than a bit of air moving hyper-fast) Bernoulli effect even lowers it further. I would like to know if this is roughly correct.
Sort of. A large mass of air is passed through the engine, and so that means a huge volume of air. Hundreds of thousands of cubic feet per minute.
It is compressed in order to burn the fuel efficiently, and after making the turbine turn, the leftover pressure is converted to speed, so when the air leaves the engine, it is at a pressure only a little bit higher than is was when it went in the front, but it is coming out with a lot of speed.
That acceleration from low speed to high is what causes the thrust which pushes the aircraft in the opposite direction.
Hi JayZ. (I hope the coffee mug arrived ok!) I think a fundamental blockage with peoples understanding in this regard is that they don’t discriminate between static and dynamic pressure (ie are not aware of the difference). Basically, it’s the continuous versus discrete definition thing I guess ....
Would an analogy be like prop wash (velocity) as opposed to prop. pressure.
Not gonna compare air and water, or props and jets.
I don’t know if it’s right or wrong, but I think of a jet engine as a fixed pitch, multi blade in a tube. Or a propeller is almost an un-ducted two (or three or four) blade jet. Lol I can tell you from flying both that a high performance piston engine is more work than operating a jet.. the piston engine has a throttle lever or knob, propeller control, mixture, cowl flaps.. maybe carb heat or alt. air... you have to adjust climb or descend speeds for cooling issues.. with a jet, you just have one lever, and that’s it. No shock cooling..Much more simple.
@@AgentJayZ I may be wrong, but the principle of thrust is the same.
I just thought it may be a good thought experiment.
A good example of this is to watch a rocket launch. All that power, but the rocket engine exhaust goes in on itself due to atmospheric pressure. As the rocket rises in the atmosphere the plume expands (and becomes less efficient) due to less pressure around it.
And that's also incorrect, a sea level optimized engine will be slightly more efficient in vacuum than at sea level since the pressure around the nozzle is 1 bar lower in a vacuum.
The pressure and speed in the exaust gas stream (rear) is about the same at sea level and in a vacuum in case of a sea level engine.
But the pressure around the engine (front) is not, it changes drasticly from 1 bar to 0 bar.
Since pressure in front of the engine drops and pressure in the rear of the engine stays the same, you gain thrust.
It does waste energy at high altitude though, that's because it could expand the exaust further than it could before as ambient pressure drops.
The expansion ratio of the engine stays the same as at sea level, it just expands more after the engine.
However you can run even larger expansion ratios to expand even further for higher exaust gas velocities in space to build even more efficient engines.
That's why for example Merlin 1D is way smaller than Merlin 1D vacuum and the main reason for the latter beeing far more efficient despite beeing mostly the same engine up untill the nozzle extension.
TL;DR
No, because it has to fight against ambient pressure at sea level and doesn't in a vacuum.
@@diesistkeinname795 Sea level optimized rocket engine can't be more efficient in vacuum. The difference of 1 bar is negligible compared to the pressure in combustion chamber. Yes, the exhaust gas will go out a tiny bit faster. But then it will be expanding in all directions rather than going straight opposite to where the rocket is accelerating and that will reduce the total thrust way more than what the 1 bar pressure difference adds.
@@kasuha Nope that's wrong. Any rocket engine will produce more thrust the higher up it goes. If you take a rocket motor with a nozzle optimized for sea level and run it in space, it will have exactly the same speed coming out of it, but an extra thrust of 1 atm * nozzle exit area.
The speed coming out of a nozzle is affected only by the combustion chamber's pressure and temperature, and by the expansion ratio of the nozzle. It is not affected by ambient pressure (unless there's flow separation at lower altitudes).
If however you take the same exact motor, but replace the nozzle with one optimized for vacuum, you will gain a lot more thrust from that same motor.
That does not change the fact that with the original nozzle it produces more thrust in space than at sea level, and that's always the case.
So in summary, any rocket motor performs better at lower ambient pressures, regardless of nozzle (so becomes more efficient at altitude), but at any altitude a nozzle optimized for that altitude is superior to all other configurations (so a matched nozzle at 1 altitude is the most efficient).
en.wikipedia.org/wiki/Rocket_engine#Net_thrust
good video. Thrust is the change in monument of the air flow thru. Super sonic aerodynamics is backwards because of the speed of sound. (air pressure waves)
Thanks Agent Jay Z for this amazing explanation and I have a question
Depend on what we studied we know that the turbine needs the combusted mixture of pressurized air and fuel to turn and also we know that all the stages in turbojet engine (intake, compression, combustion, exhaust) takes place in the same time.
My question is how do the turbine and the compressor turning in the same time before turbine have any energy to turn.
I'm sorry for my English if you couldn't understand.
You start the engine by using a starter motor.
Nitpick: you dropped the mass flow term in some of your equations. Thrust is proportional to the amount of stuff going through the engine (mass flow) multiplied by the change in velocity (delta v), not just delta v. You're right that pressure thrust is fairly insignificant and an indication of inefficiency though.
When I said "air", of course I meant the mass flow of the air. A bit of shorthand, and it would never pass a physicists scrutiny, but we're here to get a gut feeling for things.
Nits will be picked on the second floor, OK?
@@AgentJayZ Entirely fair. As an engineer, I couldn't help myself though.
My tribute to engineers in this video was real, and your viewership is appreciated.
It would be incredibly difficult to build a significant pressure inside a tube with both ends open.
Yes, and in the case of the J79, one end is open, while the other is being stuffed by a 30 thousand Hp air compressor. I'm always puzzled by people who have trouble understanding which way the air is gonna go...
@@AgentJayZ hahaha!
It's that y'all Canadians have an inherantly better mechanical aptitude because life up there is a closer approximation of reality, than city life here in the States.
I spent a number of years in North Dakota. Good experience from the standpoint of hands-on DIY, survival of the fittest, hahaha.
@@Triple_J.1 i worked in oil and gas in the bakken shales for a number of years and I'll be perfectly happy never going back to that godforsaken place hahahaha
This video helped me understand a misconception that ive had for a long time now,thankyou.
Out of interest i really want to know about the effect in supersonic flight. You spoke about it lightly in this video but i want to know what differs from super and subsonic in relation to the nozzle geometry and if there is any change in exhaust gas pressure.
Sorry its not worded very well im useless when it comes to getting my thoughts into words
All of the info sources I mention discuss supersonic exhaust gas nozzles. They are called convergent-divergent, or con-di nozzles.
Merry Xmas and Happy New Year
Jet thrust and rocket thrust are similar in this sense, the nozzle provides essentially the same purpose- trade pressure for velocity. Take "every action has an equal and opposite react", then consider the mass of the combustion products(would it be the working fluid in the case of jets?), and you figure that you must increase the velocity of your fluid to produce more work.
Talking about the engine featured in this video, do you have any film of it actually powering a plane, J ust be nice to see one being used, [ now I expect you will throw a load of links with planes flying by the engines you have built ]
Good to emphasize that thrust is proportional to delta v, and mass air flow. Not volumetric air flow. Conservation of momentum is a simple way to understand it, and that requires mass and velocity, volumetric flow is moot.
That's what I said, wasn't it?
@@AgentJayZ essentially, but I heard repeated references to velocity and flow, not specifically mass flow.
Not to criticize, but it's conceivable to me that the two could be mistaken if not emphasized.
Thanks for the vijeo
I would guess that people get confused about the pressure because if they stood behind the nozzle it would tear them apart. It might be easier to imagine moving at the same velocity in the same direction inside the jet exhaust. In that situation the pressure certainly would not crush you.
Am I right? Any additions or corrections? Good video AgentJayz.
How can you add velocity with force , two different units!!
Without a timestamp, your comment/question is meaningless.
Which jet engine in your opinion is the best over all, not the most powerfull or fuel efficient, simply which offers the best overall value for money.? I spend many hrs looking/listening to your videos, thanks for sharing your knowledge with us lesser mere mortals. Jet Engine's have come a long way since the Frank Whittle Day's . Happy new year dude
That is a very difficult question, because prices are all over the place.
If you just want to have it on a stand and fire it up, then the cheapest one you can get running is best, right?
You can get a full size turbojet for about 10 grand on Ebay, probably less from a scrap yard.
Expect to spend that much again, and six months or so to get it running.
If you want to fly a particular vintage aircraft, then the only suitable engine is the one designed for that plane, and hey, if you own the plane, your first concern is not money, it's safety.
So best and value are both subjective words, and change based on what you want the engine for.
Sorry, but your question was insufficiently specific.
Maybe you could show how the EPR gauge works it could help. Incoming Air at the inlet is actually negative pressure being well below the standard 29.95 inches.
Nope. In my opinion as an engine builder and tester, the EPR is meaningless, and is an imaginary variable, relied upon by folks who don't understand how jet engines work.
We never use that variable, and a gage for it does not exist in all the jet powered aircraft I have experience with.
Pressure indicates resistance, so yup, bad!
As I see it, it's about using "pressure" to increase velocity. Channeling the expansion into velocity through the exit.
Check yourself, please.
Bernoulli is a word you need to look in to.
Before further staining your rep by "correcting" the one who is trying to share knowledge.
@@AgentJayZ Oh, I wasn't trying to correct. I was just commenting to the ideas I formed. ;) Always eager to learn, have heard OF Bernoulli's Principal but now I'm going read it in full and will correct my understanding. Love the channel and info you share because I've always been fascinated with turbine engines. Thank you again!
Not to cross disciplines, but you can also visualize the optimal exhaust flow vs ambient pressure (I'm probably using the wrong terms) by checking out this article about rocket nozzles. It supports your assertion that pressure is a relatively insignificant component of thrust. Optimally, pressure at the nozzle = ambient pressure. www.quora.com/SpaceX-has-two-versions-of-the-Merlin-engine-sea-level-and-vacuum-What-are-the-big-differences-and-why
@ 9:54 Ah yes. The old bomb-sail. I had this idea when I was in the third grade and thought I was a genius.
Sir, I must take this opportunity to inform you... that proves you are a genius!
To see this idea taken a bit further, have a look at Project Orion, a proposed spacecraft propulsion system from the 1950s. :)
What if there was a door at the back end of the jet engine and you could shut it closed during operation. wouldn't there be an instant rise in pressure and rip the thing apart? so because there is no door, this potential pressure is let out the back end (before it can become pressure) in form of a high speed mass of air producing thrust? Or am I getting this wrong, still?
Yes.
Also, it would melt if it was made of cheese.
Something like that can actually happen if the afterburner nozzle gets stuck and stays closed when the afterburner lights. The jet pipe will instantly over-pressure and start to heat up and it could eventually explode. That's why they used to put an over-pressure sensor in the jet pipe that shuts the burner off if the nozzle doesn't open correctly. The F-100 originally had a poorly designed iris type afterburner nozzle that very often would get stuck closed and when it did the burner would immediately shut itself off to prevent it from exploding. This caused a lot of aborted take offs and it was an even bigger problem in flight, especially in combat. Unlike the F-100 the F-102 had a petal type afterburner nozzle that was much more reliable. Eventually when the F-102's were retired the jet pipes and burner nozzles were removed from the 102's and retrofit onto the F-100, which finally fixed the problem.
AgentJayZ well, I was refering to the ins made from cheese that flat out refuses to melt
I've read accounts by pilots of the F-100 that, with it's simple two position nozzle, activating AB caused an instant increase in thrust which felt like being "kicked by a mule". I would like to see one flying in an airshow someday.
@@AgentJayZ Even though I was in aircraft maintenance while in the Air Guard I did get two back seat rides in an F-100F. What you have described is exactly what happens when the F-100 would go into afterburner. You basically feel a large thump and then the extra acceleration pushes you back in the seat. Once you are in afterburner and the speed maxes out you really don't feel much of anything until it comes out of burner. When it does the plane decelerates and you get thrown forward into the straps. Another thing that happens is that when the burner is engaged the EPR drops for a split second when the nozzle opens and then it comes back up when the burner lights. Then when you come out of burner the EPR drops for an instant and then recovers when the nozzle closes again. The pilots are trained to watch the EPR when going in and out of burner because it indicates that the nozzle is working right. The other indication is that of course the fuel flow jumps way up when the burner lights.
I already have the Gas Turbine book (along with some others) do I still have to go to the corner?
Well, if you want to know how to get another, I'd suggest that may work well for you... but then, only JZ knows for sure. I'm just guessing.
Hi AgentJayZ, Where exactly is V1 measured on the intake cowling?
To my knowledge V1 is the speed at which the aircraft can not safely reject the takeoff attempt. The point of no return for the pilot, so to speak.
Aircraft speed is measured by the pitot tube, which is on the airframe. I also think speed is measured by GPS, but I think the pitot-measured air speed is more relevant to takeoffs and such.
As well as that, the intake cowling is part of the aircraft, not the engine, so I've never touched one or even seen one up close.... except for that one from a 767 that we made into a desk.
@@AgentJayZ Thank you for your response. My apologies for not specifying. In the video, at timestamp 4:45 you use V1 in the Force equation. Where would V1 be measured on the engine to form this equation?
The reason for my question is I'm trying to get an idea of the effect the intake has on the V1 speed in the force equation and how much V1 differs from the speed of the aircraft.
@@archieobrien1 Ah, well we can see now just how much a time stamp helps clear things up. In the equation V1 is the initial speed of the air entering the engine. V2 is the speed of the gases leaving the engine. The quantity V2-V1 is the acceleration of the air.
In operation, engine inlet air speed is not measured, but it is in research and development of engines.
I think for The engine inlet speed will be close to aircraft speed, although air inlets to turbofans act as diffusers to reduce air speed and increase air pressure.
Maybe we will be lucky enough to hear from our friendly turbine engine design engineer on this.
@@AgentJayZ Lesson learnt, thanks again for your response. If Mr Turbine Engine Design Engineer sees this, I'll be very interested in your input.
I'm so glad there's not a quiz at the end. Having a 15 year old Glen Fiddich, thanks to my oldest.
What is the smallest object capable of producing a sonic boom if forced to supersonic speed?
I would say anything big enough to move molecules of air. So that means exactly 1/32 the mass of that (in)famous dust speck upon which a certain Horton heard a certain Who...
Probably an atom, albeit a tiny boom.
A .17HMR bullet (4.3mm) makes a sonic boom. The tip of a bullwhip cracking is a sonic boom. Size (in this case) doesn't seem to really matter.
@@Miata822 I guess not. There apparently are supersonic airgun pellets but I don't know if they make a crack/boom noise.
@@stopthephilosophicalzombie9017 That's an interesting question. The answer is everything that is significantly bigger than the mean free path of air will create a shockwave if it is supersonic.
There's a thing called the Knudsen number (Kn) which is the ratio of the free mean path to the geometric size of the object. The free mean path is the average distance that a gas molecule travels before it collides with another molecule. If Kn is much less than 1 (it almost always is), the flow behaves like a continuum, and in that case any impact above the speed of sound will inevitably create a thin shockwave. At high Kn, the molecules move on straight paths without colliding with each other. This is called rarified gas flow. In that case there are no shockwaves, even at supersonic speeds. At intermediate Kn, it is something in between, with very diffused, thick shockwaves.
At ambient pressure the MFP is so small that really any physical object will create a shockwave. For example you can see Schlieren videos showing individual unburned gun powder grains coming out of a barrel and creating their own shockwave. Any shockwave creates a sonic "boom" that will be heard when the wave reaches the observer. The characteristic loud sound that we hear when compressed air is released through a small opening (like in canned air or machine air) is actually the result of oblique shockwaves forming at the exit (Mach diamonds).
Agent Jay Z ❤️
No math? But if I don't get at least one double integral per day my brain implodes!
This video is for people different from you. Some of the books I have recommended have many pages with nothing but equations, and lots of calculus.
Another interesting example of why pressure at the jet outlet is not efficient is a rocket booster. Watch the lfit off a modern two stage rocket such as SpaceX. As the first stage ascends the exhaust plume begins to expand in diameter. After seeing this reality, go to NASA or JPL and search for "rocket exhaust nozzle design" (bell outlet). You will find understandable explanations for this that are in line with AgentJayZ's explanation of jet exhaust pressure. If you don't (can't) understand............JSTFU.
Once you understand what pressure is it becomes easy. High pressure is slow where low pressure is fast. Air comes into the engine and is slowed increasing pressure. Air is heated and expands rushing out the end at a high velocity which is a low pressure.
22:05 savage lol.
Nozzles of first stage rocket engines are smaller compared to the nozzles of their second stage engines .
For maximum efficiency , The pressure of exhaust needs to be the same as of that of the ambient atmosphere.
Aerospike engines does this automatically.
( Found this info on the youtube , I couldn't remember the name of the video. It was a while back)
Yep their are some great animation visuals out their showcase the affect of nozzles shape relative to air pressure and the resulting change in efficiency since any vector not in the opposite direction of travel is wasted energy. Part of the reason aerospike engines are so nice in principle although I believe have many engineering complications making them less than ideal.
First stage, or atmosphere optimized nozzles, are small because pressure of exhaust gets so low that atmosphere pressures its way along the nozzle inner side, causing flow separation and violent turbulence leading to catastrophic engine failure with nozzle getting destroyed.
@@Xerkus Had forgotten about that issue, thanks.
Fyi im researching how to make just a jet engine. Nothing fancy, nothing super powerful, just a jet engine... I was thinking about it as a portential solution to a problem I was grappling with. I do reserve the right to ask as many dumb questions as will get me where I need to be. Fortunately, I don't need it to last very long... so happy to use old schematics and tech and substandard materials. If I can get the general principles to work in the one unit I'll be happy...
I have several videos about designing a jet engine by yourself.
It makes sense if you think about it, there is thrust because a greater volume of air exiting the engine than entering it
No, that is not correct. The exact same mass of air comes out the back as goes in the front. It is of greater volume because it has been heated up, and so it must ravel faster.
The thrust is due to this speed increase. Didn't we cover that?
I should have probably watched the whole video before commenting, I think I understand better now. Thank you for clarifying!
@@AgentJayZExactly. In pure engineering /physics terms F=m*a. The jet engine accelerates the mass of air, producing force. Any pressure difference represents an internal restriction which would reduce the efficiency of the engine. Any contribution to the thrust equation would be the air expanding and accelerating after it leaves the engine.
@@AgentJayZ Not EXACTLY the same mass flow because of the fact there there is bleed air coming out of the stream and fuel injected into the stream meaning there can be times where the change in mass flow is positive or negative between inlet and exhaust. Although one dirty assumption that can be made is the fuel mass flow in is about the same as the bleed air mass flow out meaning the mass flow is about the same at inlet and exhaust.
People just need to Understand the DIFFERENCE between Pressure and FLOW... THEY are NOT the same... JUST like WATER and Electricity! !
Yelling is not going to help the cause, but I appreciate your enthusiasm!
@@AgentJayZ IT'S called EMPHASIS... FYI!
It was a friendly reminder.
People mistake voltage for current and power all the time too. A gizmo generating 1000V must be mighty powerful right? Maybe. Maybe not.
U need a PHD in teaching.... this stuff is awesome
would you like me to send you some of the notes from my engineering class on the topic of nozzles?
From, or For?
Answer to both: no.
The plan for becoming an engineer:
Work hard and succeed... or try to find the easy way through, and fail.
@@AgentJayZ I have already completed the class, I am just wondering if you would want the notes for you to use in later episodes
I'm starting to think I might be a bit dyslexic. Apologies to you.
Some times you sound like the engine guy called “banks”
As Ozzy Man says: "yeah, nahh"
@@AgentJayZ maybe it was the air density
I was shocked to learn the nozzle outlet pressure on a rocket engine is lower than atmospheric pressure at sea level
Yes. The exhaust accelerating nozzle on both a rocket and a jet is there to convert pressure to velocity.
Agent Jay Zed, someday will you end your vijeo with "keep your jet running nice"
Vijeo haha nice
Is that book in my corner. :o
F=ma
So jet engines work like rocket engines, except the oxidizer comes from the air, gotcha.
higher velocity more good, do jet engines then have problems with instability in the nozzle?
If I remember correctly NASA had a bunch of problems with the F1 engine on the Saturn V, flow separating from the nozzle, causing it to oscillate and disassemble itself.
I can highly recommend Scott Manley's videos on rocket engines and how they produce thrust, goes into detail as to why over expansion is inefficient and why the exhaust at the nozzle is actually below atmospheric pressure.
That was combustion instability at the fuel injector plate, not the rocket nozzle. Nothing to do with todays subject. As I mentioned in the video, it is important to have a very clear understanding of what parts you are talking about.
My copy of "The Jet Engine", published by Rolls Royce, is my Bible, it is 41 years old!
Cool video
EPR. engine pressure ratio...
Yes, an imaginary, useless parameter.
How useless? It is never used in engine testing.
The only reason EPR gages are in the cockpit is because there is no way to measure thrust, other than fitting the main engine mounts with strain gages... which would require regular calibrations, and add expense to the aircraft.
page 53 of this show the pressure drop: www.slideshare.net/Aviationshared/jet-engines-78757226
Please activa subtítulos automatic
As Jay replied previously: They contain way to many serious errors.
Hi AgentJayZ. As it's nearly my bedtime, I'll get back to you tomorrow with my comments.
G'day,
Yay Team !
There was a very Good REASON why the Germans, who invented these devices..., called them
"SQUIRT Engines..."
The faster one squeezes an Orange or Lemon-Seed betwixt Finger and thumb, the further it goes during an identical Time-Interval...
Because, the simple Temperature & Pressure-Difference between inside the Compressor & the Ambient Pressure of the Outside Atmosphere..., achieves no Velocity at all...
But..., when the Mass (of Indrawn Atmosphere) is accelerated via the Temperature-Differential aquired during it's Combustion with Refined Transport Fuel - inside the Combustors, then the (Half) Mass(flow) times Velocity Squared going in, being significantly less than the (Half) Mass(flow) times Velocity Squared, coming out, at the Nozzle Orofice, is responsible for the (desired) "Thrust" percieved at the Engine-Mounts.
Well done.
You must have a lot of stupificated Viewers, if you're genuinely encountering (such) "a lot" of Commenters expressing a wrongheaded perception of what goes on.
Do they also think that Rockets can't possibly function within a Vaccumn - because in a Vaccumn there'd be nothing against which,
"to Push..." (!) ?
Such is Life,
Happy Solstice Festival...
Stay safe.
;-p
Ciao !
Good explanation. I think of it as the heat of combustion, and the expansion it causes, adding momentum to the air exiting the back.
And for anybody not convinced that pressure at the outlet should be as close as possible to ambient pressure check out rocket man Scott Manley's video about rocket nozzle design. Unlike turbojets rocket engines need to use an expanding outlet to _lower_ the pressure of their exhaust gas to close to ambient.
LINK: ruclips.net/video/l5l3CHWoHSI/видео.html
And some guy on the internet showed me a link to download books, including the ones in this video, but I seem to have lost that link... not that I would ever use something like that. EDIT: found it - www.pdfdrive.com/the-jet-engine-e107007559.html
Interestingly, in both turbojets and in rockets, the purpose of the nozzle is to decrease the pressure to ambient. In a turbojet, the (static) pressure at the exit of the turbine is still above ambient, though not nearly by the same degree as in a rocket's combustion chamber, and this pressure is converted to velocity as the flow passes through the nozzle. The nozzle pressure ratio on jets is fairly small though, so you don't tend to see the dramatic nozzles like you do on rockets, and you also mostly tend to see subsonic (converging only) nozzles rather than supersonic (converging-diverging).
@@clapanse As the gas is accelerated in a jet's converging nozzle the pressure has dropped when it reaches the outlet. Bernoulli.
@@Miata822 Yes, but bernoulli isn't really applicable here. Bernoulli applies to incompressible flow, and jet exhaust is definitely fast enough that compressibility matters. Trading pressure and velocity isn't just bernoulli though - it's basic conservation of energy, so it applies even in cases where bernoulli does not.
@@clapanse Incompressible flow? Then explain venturi effect.
Conservation alone would not need a nozzle. It is the _relative_ velocity of the exhaust gasses to ambient air that creates a pressure differential outside of the engine, keeping the exhaust jet coherent.
@@Miata822 the venturi effect works in both incompressible and compressible flow, and I don't know what relevance you think it has here.
Also, the relative velocity of the exhaust to ambient air is what creates thrust. However, it doesn't tell you anything about pressure. The velocity of a jet of gas doesn't tell you anything about the surrounding pressure. Ideally, you exhaust the jet at whatever velocity you need such that it matches ambient pressure.
I think the penny's dropped... So basically the reason for a convergent nozzle is to produce in essence a venturi effect, accelerating the gases, AND preventing their expansion?
No. The nozzle maximally expands the volume of the gases, which makes them exit the engine as fast as possible. The word venturi is not used in this context.
@@AgentJayZ yeah but the nozzle is operating using the Venturi effect - constrict the flow, causing the velocity to increase and the pressure to drop (as close to ambient as possible).
And making that flow as linear as possible.
I agree
There's no link for applying tinfoil onto sunglasses....There was a not-so-bad description of a jet engine in the 1952 movie, 'The Sound Barrier'; Sir Ralph Richardson shows you I believe, was a DH Goblin running in a test stand and throws a handkerchief into the jet blast. One of the first descriptions I ever got: "...paraffin heats the air."
actually, any glasses will do. The most important part is the waiting for further instructions.
You should get this, it's not rocket science!!! Oh wait, nevermind...