In my bachelor in Italy we defined a jet engine bolted down as producing zero "useful" power (or a very expensive air drier). Because we needed this "useful power" to calculate efficiency and understand why a jet during takeoff has much less efficiency than one at cruise.
@@DerKrawallkeks how many meters an aircraft can fly with a specified fuel load (in level flight at constant speed), I'm referring to the Breguet equation.
Put a windmill behind it with an alternator on it, -there, myth dispelled. There is a shit-ton of work being put into the pounds and pounds of air being processed, -even forgetting about static thrust measurements that aren't "tied to time".... Very excellent explanation by the way! Love your videos!
One of those times the brilliant guy making the video reveals the standard jet engine textbook author stating air has no mass. Excellent video as always!
This is why i like the expression. "Useful work" "A jet engine on a test stand is doing no useful work." It can make all the power it wants, unless it's connected to what it's intended to be connected to, it's doing nothing useful. Could you do something useful with that power? Sure, but you aren't, because it's on a test stand.
Thanks for bringing that up JayZ ! When I was taught that "Stationary Jet = No Power" at school, I thought I was turning insane. You just gave my sanity back!
There is a large difference between "work" and its time weighted derivative "power". The details are covered in so many physics and engineering books. what is missing in the original logic is acceleration. in the test cell the acceleration of the engine is zero which allows us to make errors in our model, we looked at the wrong end. work over time is power, great job
The only difference between work and power is just what you said -- power is the time derivative of work, and work is the time integral of power. In either case you must be very careful to define *exactly* what you mean by "work" and "power".
I have actually taken & passed 2 Physics courses from Purdue University, so that makes me a semi-educated IDIOT on this subject! Here is my best Graham (grahamj9101) answer (let's hope he replies & straightens all of us out!): Let's separate "making power" from "work done" in the sense that unless something is moving, no work is being done. It is nearly impossible to 'make power' without doing work! Actually, as long as you are burning fuel, you are "making power", just all in heat/thermal energy. Work is done when some of that thermal energy is converted to kinetic energy via a piston/crankshaft or a turbine. An internal combustion engine has to do work just to run; about how much, (two-thirds?) of the fuel burned is consumed just to run the compressor section? Since the gas turbine engine has to move & accelerate air to run, it by definition has to do work (it takes power to do work) to operate. Conclusion: To use Jay's airliner 'power-brake' at take-off example, If an F-4 Phantom is sitting on an aircraft carrier, awaiting launch, its J-79s screaming at full power, lots of fuel is being burned, air being heated, accelerated & moved! Yet the plane is going nowhere because of restraints, even though there IS thrust being produced (gases accelerated/thrust produced). This force (thrust) is being applied to a 'structure system' consisting of the aircraft & the carrier to which it is connected. These forces twist/stretch/compress/pull/spring-load EVERYTHING between the engine case & the ship itself! Once the restraints are removed, the thrust is acting on the aircraft structure alone but the engine was making the SAME POWER the whole time, it is just that the power made was moved from/applied towards a different system! A previous comment mentioned a leaf blower as an example; if you are holding it as it runs, is it not doing any work? Do you have to mount it on a skate board & use the discharge air to 'thrust' the board down the street to actually do any work ? It is accelerating air as it is burning fuel (making power/doing work). What you do with/where you direct that power is up to you. Fuel is being burned all the same. In Jay's airliner power-braking before take-off example, the 'power made' is being transferred to the airliner's air frame, tires & the runway (& the planet!), as a system. Once the brakes release, we removed the wheels, runway & planet from the system! However you try to visualize (visualise?) this, power is being made from moment of start-up, as indicated by the continually decrease amount of fuel on board, until the engines are shut-down. This is a fuzzy explanation, for sure. My apologies. Yes Jay, your are dangerously close to engineering theory & philosophy, where 'relative to what?' must always be stated! We all know air has mass & it takes energy to move one molecule, therefore, the "If a tree falls in a forest/woods & no one is there to hear it, does it make a sound?" conundrum does not apply here! Actually, unless the tree falls in a complete vacuum, it certainly makes a sound; just ask the insects or forest creatures! What say you, Graham? I say Cheers! DD
I agree, context is the key here. There is all kind of work done from this engine, but what you want to measure is _useful_ work. If you want a power turbine, you can measure the mechanical work done on moving the air, so you can easily express it in watts or hp. If you want a jet engine, you measure the mechanical work done on moving the engine. A stationary engine makes no power as an engine. It does produce constant thrust though, so if its powering a jet at MACH 2 its useful power will be twice that of flying the same thing at MACH 1. This is why its not useful to measure power of a jet engine.
That’s an old engineer “joke” that no one in the trade should be confused but somehow when it made it out to public it’s completely taken out of context. Another favorite joke of mine is supersonic inlet generates thrust, maybe you or Graham can try to explain that, too.
I see some comments using "usefulness" as a criteria, and some claiming that everything the engine does is losses because it doesn't move relative to you, i.e. the "frame of reference" argument. The latter would mean that if you fly alongside another aircraft - i.e. it doesn't move relative to you - that aircraft produces no power. That would clearly be wrong. The question of generating power does not depend on whether you consider it useful. It takes a lot of power to move a large mass of gas at a high velocity in a medium of (also) gas, which provides resistance that must be overcome, and that's what happens when the engine is bolted down (as well as when it's moving); there's a lot of mechanical power in that. Work is being done over time. The losses, i.e. the energy that isn't converted to mechanical power is in the heat, and that is also the case if the engine moves. In other words the engine doesn't care if it moves relative to you or not - it's moving a mass of gas in any case, and if it happens to be attached to an aircraft that moves, it's moving both air and metal. For those who want it as a criteria, there's even shaft horsepower being generated, which is used to drive the compressor which in turns moves the air - and around we go. It matters not a jot if you _consider_ it to be useful or not (the stream of moving gas might be "useful" to some - AgentJayZ uses it to clear gravel from behind the test stand and move a portion of the local air closer to the next county). The "usefulness" and "frame of reference" (restricted to the metal parts and not to the gas, for some reason) becomes arguments of _semantics_ , while what's the real argument here AFAIC is that of _physics_ .
I would file this simply as a misunderstanding: The book author (more or less obviously) talks about "net power", i.e. power that actually serves the device's ultimate purpose. Since a jet engine's ultimate purpose is not to move air, but rather to move stuff _through_ air, its _net_ power on a test stand _is_ zero. But measuring the net power of an aircraft engine would be rather useless anyway, as the thrust is what aircraft designers really want to know for their calculations in the first place. (Similar considerations apply to car engines, btw; horse powers are a neat marketing gimmick, but a far better measure of a car engine's ability to accelerate a vehicle is the torque.)
I remember my dad told me a story back in college where a physics professor slapped a student with both hands on both sides of his face at the same time (This was in communist China). The physics professor then stated that since his head didn't move, no work was done and therefore he was never slapped.
I love your videos, and as an aerodynamicist engineer I see sadly how much trouble that very badly written sentence in that book caused. The sentence is provocative and truly wrong. However I know exactly what he tried to write, but he wrote it very badly. First of all: AgentJayZ is completely right, gasturbines produce hundreds of megaWatts power and apart from what portion is used for driving the compressors there is still MegaWatts of power in the exhaust jet that can be further harvested. We are designing the working cycle of gasturbines (Joule or Brayton cycle) for power in Watts (or Horsepowers if you like). So we design every compressor and turbine stage for power (Watts) and so the engine produces power (Watts). As a consequence of the excess power which is leaving the gasturbine in the form of a jet a huge amount of force is also generated, which we know as thrust. Now comes into view a completely different topic which is related to airplane design and the author of the book mixed the airplane design concept with the jet engine design and here is that oversimplification what makes his sentence wrong. At airplane design process only the generated thrust force value in Newtons that would overcome the drag of the plane, is considered. Basically we no longer consider a jet engine, but we oversimplify and consider only the force vector that the engine generated. With other words we don't care at this point of design about how this force was generated, but we consider that it is simply given for our airplane. From this point on we only consider the work that that force has done for moving our airplane and so not the work and power that actually created that force. With other words: the force has been created by the gasturbine via investing an immense (hundreds of MegaWatts) power by (as you explain) accelerating a huge mass of air from zero to 500m/s velocity, but now we ask what this generated force can do for us. If this force will bring us through a distance of S, then it produced for us F*S Work, or F*Displacement_speed power. Note that this power is not the power that generated the force, but the power that the force provided to us. Imagine that the jet engine is a mother that gives birth to her son by creating an enormous power to give birth. The son is the Thrust. From now on we want the Thrust to do work for us, displace us at a given speed and we don't consider the mother anymore. If the Thrust is not moving us anywhere it will not work for us. However we will never forget that this Thrust was created by producing an incredibly immense power by its creator: the jet engine. I hope I'm understandable and it will make this topic clear.
Hello, Sir Jay-Z! Perhaps the "machines" at RUclips were upset, but I just discovered they had unsubscribed me from your channel...and your skookum logic. For those who doubt that engine is making power, just pop the engine release pins. That engine will likely fly into orbit!! Hehe. Glad to be resubbed. Bob
Hey JayZ you're right, the LM2500 PT has got 6 stages, and it is in fact based on the TF-39 LPT (C-5 Galaxy), the predecessor of all CF-6 engines which also has got 6 stages. Difference, the LM TRF is more robust because it also provides the rear GT thrust mount and it contains an additional ball bearing (No.7B) to be the PT shaft thrust bearing. In the TF-39 the LP shaft thrust bearing is the No.1 bearing at the fwd. mid shaft which an LM2500 doesn't have (but LM5000 and 6000's do). With advanced development towards the CF6-6 and -50 the LPT's became more efficient and smaller to save weight. Therefore the -6 has got 5 stages and the -50 even only 4 stages of LPT although the power output increased. Thanks for all your great videos and I am always really amazed how professional you guys handle this wide variety of different engine types in your workshop and test bed!
Although the changes are extremely small, I assure you that the engine at full power is moving the earth just a teeny bit. It's going nowhere fast but it's moving.
@@gordonrichardson2972 you obviously haven't heard of The 1st Law of Thermodynamics ;) en.m.wikipedia.org/wiki/First_law_of_thermodynamics As long as no air is leaving mother earth, we have a closed system. -> Earth's movement won't be changed by AgentJayZ 's beautiful jet engines.
The power output of a gas turbine is simply the output torque of the turbine shaft multiplied by the turbine rotational speed. HP = Torque x RPM ÷ 5252 The engine produces Power by providing a rotating shaft which can exert a given amount of Torque on a load at a given turbine RPM.
Just from the description I'd assume people are missing the obvious fact and saying that the massive volume of cold air it sucks in and the hurricane force red hot air blast it produces don't count as "power"?... Sure theres no "shaft horsepower" or anything to measure, but a force is being produced, causing the engine to push one way, and air to push the other. While the engine doesn't move, technically no "power" can be measured, just a force, however the air is moving... and so we can measure the power there, force & mass = movement = power.
Unfortunately it's a question of semantics. What is 'useful work'? The general definition is 'work (read: energy) that is not wasted'. How do we define when work/energy is not wasted? Depends on who you ask. To the aeronautical engineer, when the engine is on the test stand and stationary, all the energy is wasted because it is not going into propelling the airplane (and therefore not earning money). Ditto if you ask a mechanical engineer about a turboshaft engine hooked up to a dynamometer instead of e.g. an electrical generator. If you ask a physicist, the engine is definitely doing work moving the air and the wasted work/energy is the energy lost in heating up the engine as a whole, e.g. the energy that overcomes the friction of the turbines rotating in their bearings, or the energy that just heats up the turbine blades instead of going into accelerating the air.
Thank you for the marvelous video to enjoy on a Sunday morning. I always thought physicists should run their work through Legal and Compliance Departments like the rest of us. Rock on Jet Man.
Try walking behind the jet pipe when a J79 is running at full thrust on the test stand and if you survive, come back and tell us there is no power there...
I agree with you AgentJZ, and with Douglas Turner below--I know it sounds crazy but, I believe a screaming J79 on the stand is actually moving the Earth. The distance is immeasurably small to us, but the mass of the Earth is almost inestimably large. A very tiny distance times a huge mass could still solve to 20,000 hp. The earth is a free floating object that conceivably can be moved. A plane moved by a jet engine is merely a physics model with distance and mass within our ability to observe and measure. Bobby has an Earth whose mass is estimated at 5.972 x 10 to the 24th kg. Bobby's Earth has a radius of 3,963 miles. Bobby runs a 20,000 hp jet engine on a stationary test stand located at the radius for 10 minutes. Assume 4 meter rms accuracy of the GPS system. Calculate the distance moved at Bobby's test stand, work done, and the time a continuous run would take to verify results via GPS within a 90% confidence interval. Show all work, points off for answers with improper significant digits. ;-)
No, you’re forgetting the force applied back into the surface of the planet, from the moving stream of gasses via drag, in the opposite direction!! Unless you’re expelling material off into space or you’re accelerating the entire atmosphere, I argue that there is absolutely zero chance of affecting the rotation of the earth in any way !!
@@GTEskee - I'm listening. So, when a jet flies, does the moving stream of gasses via drag in the opposite direction, move the planet in the opposite direction? More to AgentJZ's point: what's up with a physics formula that says there's no power produced if there's no movement? I'm suggesting maybe the earth moved an infinitesimally small amount, that would qualify as power. You are saying that there would be an equivalent opposing force, acceleration, drag, to prevent movement. I would think that also must qualify as power. f=ma, but what if there's no observable a? I think we're both making AgentJZ's point.
I would say, unless the plane is accelerating, then again, there is a force applied to the stationary air (through drag on the airframe) equal and opposite to the force applied to the stationary air by decelerating the exhaust stream back to zero mph . . .
DiveTunes, And you would be wrong. As the mass goes up to "stupefyingly large", power goes down to effectively zero. Do the math! As I said else where it's like bouncing a ball off the earth. No energy is transferred, but twice the momentum. So yes, momentum is transferred to the earth by a stationary jet engine, but no energy. Exactly zero for all intents and purposes.
Thanks again Jay. Once again you demonstrate your talent for explaining the theoretical into the literal. (Does a leaf blower really do any work?) You sir are truly a man of many talents. We all benefit from your insights. You have saved us from a philosopher's conundrum. Well done.
If the J79 spits out 50 kg of air (or whatever what) at 500 m/s ... E = 1/2 m v2 to get the Kinetic energy each second P = E / t to get the power: in this case: 0.5 * 70 * 500 * 500 = 8750 000 W or 8.75 MW or 11734 hp Looks reasonable to me.
It takes power to move such large volumes of air / gas. Even just the strain exerted upon your test stand counts. If there was no power there would be no need for the test sled to be concreted into the ground.
The High Pressure Turbine of a P&W JT9D during an engine test run, at takeoff thrust, is absorbing 110,000 HP from the combustion airflow and passing it down the shaft to drive the HP compressor. It doesn't know or care whether the plane is moving or not. It certainly is doing work, ingesting and compressing air.
It is incredible how belligerence can overcome facts so clearly illustrated in practical application. Hence an aweful lot of argument out there in the name of science by people who use jargon to suit their means and sound clever- go the greens. And this is just clarifying a point between actual applied science professionals really! No power. Lol. Where is Richard Feinman when you need him!! Your explanation was awesome Jay. So clearly put a child could understand it. Yet Feinman said if I could explain QED so you could understand I wouldn’t have won the Nobel prize! 😂. He would simplify it to a point we caught a glimpse. This on the other hand seems quite fundamental. The breaks or stand and the ground are resisting a very great force and there is a lot of stuff being moved from front to back. Stand in front, you will soon be out the back in small pieces just as ignorant as when they went in. Tips for life - you can be wrong and still be clever. Clever people know this and how it is expressed. Like “I don’t know” or “ I was wrong” can be used. Cheers. Where is your NZ flag🇳🇿!!
Read the Work Energy principal on Wikipedia. If you are accelerate a mass of particles to some velocity you are doing work! “The principle of work and kinetic energy (also known as the work-energy principle) states that the work done by all forces acting on a particle (the work of the resultant force) equals the change in the kinetic energy of the particle.”
If you take the position that all of the energy leaving a heat engine can be included in its power rating, then all heat engines are 100% efficient, because when you add the energy being coupled out their shafts and the energy leaving in air velocity and temperature, they equal the heat of combustion of the fuel. So yes, we ARE talking about mechanical philosophy. In your stationary turbines, the output of the gas generator is not the output of the engine; the output of the TURBINE is. This is never 100% efficient, as there is a great deal of power that is thrown away out the exhaust of the turbine, but we do not include this when we calculate the output power of the engine, do we? I'm sure the author of the book you're pointing at is fully aware that there is power in the exhaust of an engine, and implying that he is unaware of this because the air is invisible is just insulting. There's really no reason to be a dick about this. Of course there's power being produced. It's just not being used. And YES, the WORK output of the engines on aircraft DOES go from zero to many thousands of horsepower when the pilot releases the brakes, just as the useful output of an automobile engine goes from zero to hundreds of horsepower when you engage the clutch. So please step down from that high horse.
This is right inline with my original comment. I'm glad someone else found another way to say it. Would you rate a stationary power turbine by it's generator produced power, as well as the exhaust energy flowing out? If anyone claimed such a thing on any given AgentJayZ video, they'd be ripped apart. Do we include the energy leaving with the exhaust of an automotive engine into its power output? Absolutely not. I mean, by the logic I'm seeing throughout the comments, we must include that energy as "work" because "the engine had to move the air". Well, yes, in the case of the automotive engine, the engine did move the air via the piston during the exhaust stroke, but this is *thermodynamically* not classified as work or power output *from* the engine. The piston had to do work on the exhaust as it was pushed out, but that is not the work we use to calculate thermal efficiencies or power rating, it is internal to the walls of the engine. Observing that fast moving exhaust's energy as work transfer and not kinetic energy (which is what it is) results in a thermal efficiency of 100% (as you said) which is a violation of the 2nd law of thermodynamics.
I am a mechanical engineer who works with power systems, so maybe I can help clarify what the author is saying... Theoretically speaking, he is correct in saying that no power is produced. This is because we define work transfer to be done through things like shaft work (rotating generator), boundary work (piston/cylinder), or electrical work (electric motor). It's important to use this distinction when talking about the 1st law of thermodynamics (conservation of energy) and the balance of different forms of energy. For example, if I "draw a box" around a jet engine bolted to a testbed and write out the energy balance for everything in and entering or leaving the "box" , there is technically no work (as defined previously) crossing the boundary of that box; thus, the work or power term in the equation goes to zero! This is actually a homework/test problem I have seen before! This is compared to a mainframe gas turbine for power generation for which, if I drew the same box, would indeed have a work output (or power) value equal to that of the torque*rotational speed delivered by the shaft to the generator. Now, that's not to say the bolted jet engine isn't doing anything! It is imparting a huge temperature and velocity to the exhaust gases which are leaving. These indeed find definition in "forms of energy", but they do not classify as "work" or "power" being produced. The exhaust has a large kinetic energy (velocity) value which then gets dissipated (yes, possibly through work) far downstream of the testbed and our imaginary "box".
@@tabaks I'm only defending the author insofar as he is using the definition of work or power in its true thermodynamic definition, and as far as he is talking about work being produced from the engine as a unit. It depends on where you draw the box I discussed before. If you draw it around the turbine section of the engine, there is work and power produced! There is work created from the exhaust gases which rotate the turbine to produce shaft work to drive the compressor. It depends on what you write your equations for...
@@sbbolton66 As long as no work passes through the box boundary in the forms I mentioned earlier, yes, that is correct. But that is a very extreme example. Here is something I was told in engineering school which helped me grasp the subject. The 1st law of thermodynamics is just an accounting equation. That is all; it's like totaling your debits and credits and that those take place for different types of purchases and sales. Here, the analogy is that forms of energy transfer (work, heat, kinetic, potential, etc.) are the different types of purchases or sales. So, say you are in charge of economic accounting for a large company with many departments which have their own expenses and sales. Let's also say that some departments bill other departments for labor, supplies, etc. You would not keep track of economic transactions happening between the company and outside vendors by counting up the transactions happening internally between departments.... right? Just like I would not count the work happening internal to the engine as work being done BY the engine. That's not to say internal cash flows never happen or work isn't done at all! It's just happening internal to where you drew your boundary.
If a J-79 in a test cell is not running, and you see AgentJayZ standing behind the screech pipe, nothing happens and he continues to make a video and it's a nice day outside. If the said J-79 is running full blast, you'll notice AgentJayZ does "not" stand behind it. This is because something could happen. Actually, something really bad could happen. This leads to a logical conclusion that he doesn't stand behind because there's an insane amount of power being produced even though the engine is not moving.
The thing about it is not only the movement of the air indicates that the jet is producing power but also in the movement of the internal parts of the jet. The opposite reaction to these forces are a movement to the jet as a whole. When you bolt down the jet to the sled (which in turn is anchored to the ground) and power it up the jet does move. Not only in the flexing of the sled but actual rotation to our planet and a comparative greater degree to the tectonic plate the jet is bolted too. The mass of the tectonic plate and earth is so large that this movement caused by the reactional forces on the jet causes an undetectable movement. Force / mass = acceleration. The mass of earth is so large that the force of the accelerated air produce insignificant acceleration of the bolt down jet, but there is acceleration.
If you think about how much noise a jet in afterburner makes, it would take 10,000 watts or more for a stereo system to put out that amount of noise. Just think of a concert with 10,000 watts of speakers. If you were to play that sound using a concert audio system, it wouldn't even come close to the noise of a jet engine. For instance, a 50,000 HP engine is equivalent to about 37.28 million watts. It's just incredible how much power those engines put out.
I'm surprised they let you test them fixed to the ground. Your changing the rotation of the earth.... are you making us spin faster or are you slowing us down? I hope your slowing the earth down i need all the more time to finish my chores thanks
Looking at the cracking in the sheet metal on the wall, it looks like you could use some of those industrial sound dampening blankets to hang over the walls. Might take 2 or 3 layers, spaced about 3” apart, and probably wouldn’t make engine tests sound any less deafening, but would help protect the wall.
P=Fv, it is not an argument its the definition of power in relation to force and velocity.. Its temporally not a aircraft engine its a turbo-hairdryer engine... or a hackney word game
AgentJayZ, So by that rational a Harrier, F35 or any helicopter that's hovering is making no power! Keep up the good work. I'm enjoying my edumcation in jets.
Since energy can neither be created nor destroyed, we must assume that if an engine is ingesting fuel and air, that the energy in that fuel and air mix is changing form and being released in some other form of energy. Sounds a lot like power to me.
My understanding of physics tells me that a jet engine on a stand makes thrust - place torque sensors on the stand struts you will measure force. Being generated by the engine on a wing or a stand it creates force, chemical reaction if you will. Which also alters the hydro carbon fuel. After being burned, it's composition is altered. By ratio of weight it is greatly reduced, being altered from a liquid to a gas with heavier carbon being present as a result of incomplete burning/ conversion.
I’ve carried with me vividly since jr. high science/physics the diffrence between force, work, and energy… It certainly takes effort to hold my arm out to the side holding a 20lb weight, and there’s a force on my hand and arm, there’s potential energy in the weight, but no “work” is being done… eventually the subtitles of the various models and how they interact took hold, but intuitively, that was a hard one to learn.
I am amazed as to why people get confused or become misinformed by such a simple understanding. As some have said below, if a tree falls in a forest and nobody is around to hear it, does it make a sound? Is a jet engine doing work while functioning stationary on the ground? Yes, its blasting air at incredible speed and trying to rip the bolts out of the ground (We are trying to stop the earth from turning) People just can't see it and therefore they don't believe it haha.
I think that you are stretching the point too much. What I asume Bill Gunston is trying to say is that the power generated is not useful in that particular case (a turbojet on the ground) not that no power is generated. Of course he knows that the power is used in moving a huge flow of air at high speed, but unless the objective is just that (to blow air) the power is wasted, it is not propelling any airplane and is not moving any power turbine. And of course he knows that if you install a power turbine you can get that same power to move a generator or whatever.
I'd definitely agree that neither is strictly wrong. I think if you asked the author if the book, he'd argue that a jet turbine's job is to produce propulsive power through thrust, while other gas turbines produce power by turning an output shaft.
Static engine has to make power, or else it couldn’t run. Requires power from the exhaust to compress the intake air. Yes? Thrust and power for auxiliary uses are just additional power not required to run the engine. This is why perpetual motion engines do not make power, because as soon as you apply a load to the engine, it stops.
Seems to me the non believers need to understand (if possible) the difference between thrust and Hp. Good video. Thanks. I'll revise this comment further. HP, Thrust, Newtons, Watts, Joules, Calories, BTUs, etc are man-made constructs used to understand and measure the conversion of potential energy to kenetic energy in various forms of man-made processes as well as natural processes.
In respect to earth's atmoshpere - yes, a little bit. In respect to other celecstial bodies - no. At the end even this earth-atmosphere momentum transfer thing dissipates into heat, through air-surface friction.
Those people are not considering the enormous volume of air the engine is moving. That's how people should think of it. Whether or not the engine itself is moving is irrelevant, it's how much air the engine is moving that matters.
There is some usefulness in saying that _useful_ mechanical power is force times velocity, because it allows you to quantify how thrust-efficient the engine is. The engine will always produce its commanded thrust, but some of the power used to do this will go into the exhaust (which is "wasted" because you would rather that energy go into the plane). At speeds lower than the exhaust velocity, this means that you're wasting power in order to attain the required level of thrust by accelerating the air to ridiculous speeds; that's the impetus for turbofans with their huge bypass ratios - the exhaust moves slower but there's more of it, and so you can attain the same level of thrust and hence acceleration without spending so much energy accelerating air. This makes the engine more thrust-efficient, i.e. its specific fuel consumption is lower.
Really well explained! Makes sense. As I was listening, I was like, "what's that noise?". It sounds like a squirrel .....it's the auto focus of the camera! 😂
I've never understood this fallacious thinking... just because the turbine engine isn't moving in relation to the ground, doesn't mean it isn't producing power. What it means is that the force of the mechanical devices holding it in place are greater than the force it is producing. I'm not sure why this elementary principle is so hard to understand. If you go and push on a large rock with all of your might and don't budge the rock, it doesn't mean you aren't producing any power, it simply means that the amount of power you are producing is not sufficient to overcome the forces holding the rock in place.
Not true, you for instance could be generating enormous force be it lb. (thrust) or lb.ft. ( torque) and because you have no movement in a linear or rotational fashion you are making NO power. Power requires work being actually done, static conditions means no actual work is being done. You can make all sorts of energy and do no work.
@Thomas Rowe If you wanted a giant air pump for moving the local atmosphere or track dryer, it is doing useable work and making power. Moving air for no purpose is not making useable power.
Damn this is good! I had heard this before and even had I had a person say hey I learned it in school, etc. so it has to be right (lol) no horsepower when it's not moving 🤦🏻♂️ - I had to stop discussion since it wouldn't go anywhere by saying maybe if you are looking at the reference frame of the jet or something like that it was bizarre because they did not want to grasp that work is being done on SOMETHING! This is an awesome video, it so well done and so well said as are all your videos! Hope all is well & take care! I love your line about it being invisible and being two 😂😁 -- VERY true in so many regards!
A specious argument like "If a tree falls in the forest and no one close enough to hear, is there still a noise?" Of course there is a noise. The air vibrates whether anyone is around or not.
The author left out one word. What he meant was that a propulsion engine in a vehicle that's not moving is producing no USEFUL power. The acceleration and increase in temperature of the air is not moving the vehicle, so it is doing no useful work.
@@AgentJayZ Yes, because its a ventilation fan. A jet engine is not a fan. That's the simplification you're talking about. Consider a jet engine a source of propulsion. The thrust (F) does work (W) on the engine if the engine moves (s). W = F*s. No work, no power. The kinetic energy of the thing will increase in the amount of work done. Yes, the pumping of air is also work, but is not propulsive work. That's why _as a propulsion_, a stationary engine has zero power at zero speed. And because it can maintain thrust over a high range of moving speeds, its such a great source of propulsion - its power increases with speed.
They would have no effect at all. The atmosphere is physically connected to the earth. The engine throws a lot of air out the back, so it is pushed the other way. It's on mounts to the earth so it pushes the earth. The air that was thrown out of the engine slows down through drag and friction pushing the rest of the atmosphere with it... and all that moving air eventually drags on the earth the opposite way the engine is pushing it. It's a closed system. Eventually all motion decays into heat.
... aks people why their vacuum gets louder when they block the airflow... you will be stunned what they will tell you... but non will tell you that it is because the motor turns quicker because of no-load from the air... People are shocked if you tell em Air can behave like a liquid... mannnnnn xD
I feel this is similar to the "how can you hold the engine down at full thrust?" questions with people wondering if the skid is sturdy enough. They completely ignore that the same engine is mounted to a plane with just a few (admittedly huge) bolts. That is a plane that is built as light weight as possible and not with 6" structural steel and still the engine doesn't rip out of the plane. If you then ask why the forces on the mounts should be different between a plane and a skid, the answer often is "because the plane can move, so the engine doesn't push as hard against the mount". ?!? It isn't rocket science (well, it is, though not the hard kind, in case of rocket engine tests, where you get the same argument, with rockets being even more optimized for weight than anything else). It's a little bit frustrating, because that either means that the educational system failed or people just don't really think for themselves.
Alot of concepts in physics are difficult to wrap my head around, don't quite have compressor stall yet but this one makes sense, I'm with you here. Slightly off topic question, if the engine is moving, flying, does it's speed relative to the air enhance compressor pressure? Any "ram" effect? Apologies for 2nd question, how much difference between gas generator shaft rpm and pt shaft rpm is there in a well designed system?
Yes, ram effect is always present. But depends on the inlet system for the engine to use it efficiently. One example is the Blackbird which could raise the external ambient pressure from 0.36 psi up to 14-16 psi at Mach 3.2 due to ram recovery.
I think it's helpful to take one step back and remember: Power is work per time, and work is done on some thing. It's a good idea to state what that thing is. In your test cell there is zero work being done on the stand but lots of work being done on the air. Therefore it's producing "air stream power" but not "test stand power".
@@AgentJayZ Not to be argumentative, but that's why I said _useable_ work. While the author was not very clear, that was probably the intent of his statement. The railroad would probably consider it to be useable work in the form of thrust (e.g. hot gasses accelerated out the nozzle section) and heat for removing snow and melting ice on the railroad ties, but for our purposes its doing no useable work. Its stationary. It is still doing work moving air by generating thrust, but at the end of the day its all wasted as heat since its in a test cell.
The work is plenty usable. You can use the heat to create steam or heat buildings, and you can use the moving air to drive a turbine hooked to a generator for electric power. In fact it is way easier for me to calculate this kind of power versus how fast it can make a plane go, because the would require knowing exactly how much drag the plane is creating. I can tell you the engine power is at least greater than the power used by all the electrical and hydraulic stuff inside the plane which I can easily calculate, tho. And lesser than the hypothetical power created by the rate at which it is burning fuel, which I can also easily calculate. Same for the climb rate. Gliders can fly for hours using dynamic soaring, slope soaring and thermals, even tho they have no engine, but they generate power? Silly.
@@mysock351C ... SO if you are wanting to move air, it's usable work, and if you are not wanting to move air, it's not usable work? So the physics of the device depend on the intentions of the operator? Hargle-de-hogwashery-blah.
I concur there is no (or very little) 'work' be done on the jet engine. work = force x distance. It's confusing because holding a weight still has zero work. It does take work to hold up the weight.
Unless we are ejecting mass into space, and it leaves earth orbit, then it's all a closed system. A million static jet engine tests would have zero net effect upon the rotation of the planet.
@@AgentJayZ Interesting. So I'm back to having to relocate the moon, or decrease earth's mass ? Problem seems to be photosynthesis increases our mass faster than we can burn stuff ?
The only way to slow the earth is to apply forces or add mass from outside the closed system. Meteorites do add mass to the earth, and slow down the rotation, but it's a small percentage, I tell ya.
@@AgentJayZ Not if you had enough jet engines. Probably wouldn't be enough fuel though. Now this just anecdotal but the force of one jet engine or a plane strapped down to an aircraft carrier deck can effect the speed of the ship, to the point where the Captain wants to know if you're going to be doing that sort of thing. I do wonder what would happen if you tied down all the fixed wing aircraft in the same direction and went military, augment on the ones that have it.
@@msmeyersmd8 No such material currently exists, most solid or pressed magnetic materials are very fragile and wold not survive the rotational forces when shaped as an aerodynamic blade.
How has this ever been something that needs clearing up?? The "object" that's moving doesn't have to be something the engine is bolted to, like an aircraft - by virtue of the fact that the energy isn't being expended in propelling an aircraft forward, there must be a *massive* volume of air being dragged along in the wake of the exhaust, but I suppose that's hard to observe... unless you look out the back of the test cell, which is a barren wasteland of blasted snow, gouged gravel and trees that are saying "yep, this is close enough thanks". I don't know, maybe I'm missing something, but it seems staggeringly narrow-minded to think that an engine is producing no mechanical power simply because the thing it's bolted to is stationary and can't move.
It may not be producing particularly "useful" mechanical energy, but I'm sure an engine (and the physical laws that govern how it disperses energy) don't care a whit about the purposes of its manipulators.
I am entirely with you on this, AgentJayZ - and, even though I'm a Brit, I think of you as AgentJayZee, not JayZed. Sadly, as Mr Sivhed has noted, Bill Gunston is no longer with us, so we cannot debate this with him. I might have tried to do so, had I bought my copy of the book before his passing. Whilst I recognise he might have been trying to say that a stationary jet engine on a test stand is doing no useful work, I cannot agree with the way that he actually said it, as his "zero horsepower" is essentially wrong. Part of the problem is, I believe, the term horsepower (or - typos corrected - Cheval Vapeur, or Pferdestärke) itself, which Invites the forming of a mental image of James Watt's horse hauling a weight up a mineshaft. As someone who, back in 1971, was involved in the design of the 'C' rated Industrial Olympus that you've shown us in the past, I recall the way that complete SK30 gensets were tested before being shipped offshore to production platforms. They were tested to full power with the Megawatts from the alternator, which could be measured, being dissipated between two big electrodes in a huge tank of water (the biggest electric kettle in the world?). Before that, the Olympus gas generators were tested with 'slave' jetpipes, in exactly the same way that you have tested them, and they must have produced the same power as when installed. Incidentally, and I think that I've mentioned this in the past, R-R used to quote an EGHP (equivalent gas horsepower) figure for its industrial and marine gas generators. Finally, if anyone out there wants to do the force times distance moved against the force in unit time calculation for an aircraft, try this for Concorde in supersonic cruise at 58,000 ft: 1,350 miles/hr with four Olympus 593 engines each producing 9,000 lb thrust (equals drag, of course}. Answer: 129,600 horsepower. At take-off, with a thrust of 38,050 lb from each engine, the power produced was considerably greater (obviously!): the difference between the two conditions was propulsive efficiency. PS The speed of sound in in the jetpipe of a J79 at max dry thrust? Probably somewhere over 2,000 ft/sec
Bit of a typo there, it's Cheval Vapeur and Pferdestarke. If you want to calculate a quite staggering engine output, calculate what it takes to push a nuclear submarine through the water (submerged) at 40 knots or more. By propulsive efficiency are you referring to power decrease at altitude? Jets typically become increasingly efficient with increasing altitude.
@@scheusselmensch5713 Thank you for the corrections, but my typos hardly negate the point I am trying to make - and what about the umlaut over the 'a'? Propulsive efficiency is essentially the Froude Equation, which I've quoted on this channel more than once. The greater the difference between the efflux velocity of the engine and the forward velocity of the aircraft, the lower the propulsive efficiency. 100 percent propulsive efficiency is achieved when the two velocities are equal, at which condition the thrust must be zero, as there is no momentum change.
I honestly think Gunston is saying what he described is the wrong way to calculate jet power because one would get zero power from the calculation. In fact, one would also get zero thrust since there's no acceleration of the engine, either. All the text preceding was just telling the readers to use thrust as a measure instead of horsepower for jet engine and the reason given is the sentences in debate: If one is not properly educated and applies the incorrect equations the result would be completely wrong and useless, like what he was demonstrating.
@30:10 -- Gotta love watching a guy talk about framing perceptions correctly, while wearing mirrowshades and standing in the middle of a jet engine cowling. I think you've invented turbinepunk, Alert William Gibson!
Would you say that the authors of these textbooks almost exclusively make these errors because of a lack of real world experience and a surplus of academic experience? Thanks in advance.
@@AgentJayZ I think they are just stuck thinking in the context of what they are describing. If you are just thinking about an airplane, and moving an airplane, its very useful to think of a jet engine as only producing power when it moves a plane. It perfectly describes the difference between the way a prop plane and jet plane accelerates. I dislike it when technical books and such describe something like that as literally being true because it creates confusion like that. Why not simply say "For our purposes the power generated by a stationary jet engine can be disregarded. Therefore, a jet engine effectively creates zero power until it moves a plane." Or something like that. But they don't do that.
Mr. Gunston is actually correct. The formula he presents is the standard equation for expressing thrust-horsepower, and may be found in numerous texts regarding thrust producing aircraft. J-Z is confusing the issue by conflating thrust-horsepower with power in a general sense. Thrust-horsepower is a useful measurement when discussing jet aircraft performance specifically, since fuel burn correlates with thrust, which remains approximately constant; and power varies in relation to true airspeed (for a given thrust setting). J-Z has produced an interesting video, and perhaps unintentionally illustrated why we don’t express the work performed by tug boats or water pumps in terms of thrust-horsepower. (For my source material, see chapter 2 of Aerodynamics For Naval Aviators)
That’s so simple I have never Hurd this one before! If I had I would tell them to think about it again. Air is heavy. We don’t really ever think about it that way.
Of course it's making power. I thought this discussion would be about the difference between stationary operation versus moving fast - getting the ram air from forward motion - how much difference is there?
That's one for the theoreticians. The only way to actually measure the difference would be to have specially built load sensing engine mounts in the aircraft. That may have been done before, but I have not heard of it nor read about it.
Good video. In the book the author is talking about that you can't compare lbs of thrust to horse power. How about a turboprop engine that one could measure HP off a shaft output of the engine. I'm wondering how HP at the shaft out would stack up against thrust when using a jet exhaust and turbine instead of a shaft drive off a jet engine to drive a generator to make electricity? A Shaft drive is mechanically solid connection output while jet exhaust connected to turbine powered generator is connected through air that has slippage/cavitation.
Power is a tricky concept with jet engines and rocket engines. That's why the force they produce is also often used as a metric. When the airplane is acclerating on the runway is has three times the power when going 150 than when going 50. More of less. The power of rockets can exceed the rate at which they are using the chemical energy in the fuel even. So a rocket can comsume 1 kJ of its fuel to increase its kinetic energy by 1.5 kJ which is very unintuitive but happens during every rocket flight
It makes a lot more sense if you think of it as the kinetic energy extracted from the gas jet or better yet the energy that was left over (not extracted). A whole lot more energy is extracted from a 10km/s gas jet when you are traveling at 7km/s than at 1 km/s - in the first case 3km/s is wasted and in the second 9km/s (observed from the same point of reference - energy being a square function of velocity).
In my bachelor in Italy we defined a jet engine bolted down as producing zero "useful" power (or a very expensive air drier). Because we needed this "useful power" to calculate efficiency and understand why a jet during takeoff has much less efficiency than one at cruise.
Makes sense. That's because the efficiency was defined by the mechanical work given to the aircraft only.
:)
@@DerKrawallkeks exactly. We defined useful power because we needed it to calculate range and a bolted down jet engine has a range of 0.
@@theophrastusbombastus8019 what was "range" in this case? Just curious
@@DerKrawallkeks how many meters an aircraft can fly with a specified fuel load (in level flight at constant speed), I'm referring to the Breguet equation.
@@theophrastusbombastus8019 aah, the actual range. Yes, sure, I got it.
Thx. What did you study?
Put a windmill behind it with an alternator on it, -there, myth dispelled. There is a shit-ton of work being put into the pounds and pounds of air being processed, -even forgetting about static thrust measurements that aren't "tied to time".... Very excellent explanation by the way! Love your videos!
That's actually an interesting idea, it would be fun to see how the grid would handle it :)
One of those times the brilliant guy making the video reveals the standard jet engine textbook author stating air has no mass. Excellent video as always!
I could listen to you speak tecnichal all day. I very much enjoy your accuracy of terminology. FANTASTIC!!
This is why i like the expression. "Useful work"
"A jet engine on a test stand is doing no useful work."
It can make all the power it wants, unless it's connected to what it's intended to be connected to, it's doing nothing useful. Could you do something useful with that power? Sure, but you aren't, because it's on a test stand.
... Whoooosh!... ... ...
You are doing something useful. Making sure it works before potentially delivering it to the customer!
Can't believe any thinking folk would argue with you over this. You are absolutely right.
Thanks for bringing that up JayZ ! When I was taught that "Stationary Jet = No Power" at school, I thought I was turning insane. You just gave my sanity back!
I like these “physics by J” videos. Keep it up
There is a large difference between "work" and its time weighted derivative "power". The details are covered in so many physics and engineering books. what is missing in the original logic is acceleration. in the test cell the acceleration of the engine is zero which allows us to make errors in our model, we looked at the wrong end. work over time is power, great job
The only difference between work and power is just what you said -- power is the time derivative of work, and work is the time integral of power. In either case you must be very careful to define *exactly* what you mean by "work" and "power".
I have actually taken & passed 2 Physics courses from Purdue University, so that makes me a semi-educated IDIOT on this subject!
Here is my best Graham (grahamj9101) answer (let's hope he replies & straightens all of us out!):
Let's separate "making power" from "work done" in the sense that unless something is moving, no work is being done. It is nearly impossible to 'make power' without doing work! Actually, as long as you are burning fuel, you are "making power", just all in heat/thermal energy. Work is done when some of that thermal energy is converted to kinetic energy via a piston/crankshaft or a turbine. An internal combustion engine has to do work just to run; about how much, (two-thirds?) of the fuel burned is consumed just to run the compressor section? Since the gas turbine engine has to move & accelerate air to run, it by definition has to do work (it takes power to do work) to operate. Conclusion: To use Jay's airliner 'power-brake' at take-off example, If an F-4 Phantom is sitting on an aircraft carrier, awaiting launch, its J-79s screaming at full power, lots of fuel is being burned, air being heated, accelerated & moved! Yet the plane is going nowhere because of restraints, even though there IS thrust being produced (gases accelerated/thrust produced). This force (thrust) is being applied to a 'structure system' consisting of the aircraft & the carrier to which it is connected. These forces twist/stretch/compress/pull/spring-load EVERYTHING between the engine case & the ship itself! Once the restraints are removed, the thrust is acting on the aircraft structure alone but the engine was making the SAME POWER the whole time, it is just that the power made was moved from/applied towards a different system! A previous comment mentioned a leaf blower as an example; if you are holding it as it runs, is it not doing any work? Do you have to mount it on a skate board & use the discharge air to 'thrust' the board down the street to actually do any work ? It is accelerating air as it is burning fuel (making power/doing work). What you do with/where you direct that power is up to you. Fuel is being burned all the same. In Jay's airliner power-braking before take-off example, the 'power made' is being transferred to the airliner's air frame, tires & the runway (& the planet!), as a system. Once the brakes release, we removed the wheels, runway & planet from the system! However you try to visualize (visualise?) this, power is being made from moment of start-up, as indicated by the continually decrease amount of fuel on board, until the engines are shut-down. This is a fuzzy explanation, for sure. My apologies.
Yes Jay, your are dangerously close to engineering theory & philosophy, where 'relative to what?' must always be stated! We all know air has mass & it takes energy to move one molecule, therefore, the "If a tree falls in a forest/woods & no one is there to hear it, does it make a sound?" conundrum does not apply here! Actually, unless the tree falls in a complete vacuum, it certainly makes a sound; just ask the insects or forest creatures! What say you, Graham? I say Cheers!
DD
I agree, context is the key here. There is all kind of work done from this engine, but what you want to measure is _useful_ work. If you want a power turbine, you can measure the mechanical work done on moving the air, so you can easily express it in watts or hp. If you want a jet engine, you measure the mechanical work done on moving the engine. A stationary engine makes no power as an engine. It does produce constant thrust though, so if its powering a jet at MACH 2 its useful power will be twice that of flying the same thing at MACH 1.
This is why its not useful to measure power of a jet engine.
Wow, quantum mechanics and the Heisenberg Uncertainty Principle.... didn't expect that - but certainly do like :-)
That’s an old engineer “joke” that no one in the trade should be confused but somehow when it made it out to public it’s completely taken out of context. Another favorite joke of mine is supersonic inlet generates thrust, maybe you or Graham can try to explain that, too.
What's the joke?
Applying controlled mass equation on a controlled volume problem... or are you asking about the supersonic inlet?
I see some comments using "usefulness" as a criteria, and some claiming that everything the engine does is losses because it doesn't move relative to you, i.e. the "frame of reference" argument. The latter would mean that if you fly alongside another aircraft - i.e. it doesn't move relative to you - that aircraft produces no power. That would clearly be wrong.
The question of generating power does not depend on whether you consider it useful. It takes a lot of power to move a large mass of gas at a high velocity in a medium of (also) gas, which provides resistance that must be overcome, and that's what happens when the engine is bolted down (as well as when it's moving); there's a lot of mechanical power in that. Work is being done over time. The losses, i.e. the energy that isn't converted to mechanical power is in the heat, and that is also the case if the engine moves. In other words the engine doesn't care if it moves relative to you or not - it's moving a mass of gas in any case, and if it happens to be attached to an aircraft that moves, it's moving both air and metal.
For those who want it as a criteria, there's even shaft horsepower being generated, which is used to drive the compressor which in turns moves the air - and around we go. It matters not a jot if you _consider_ it to be useful or not (the stream of moving gas might be "useful" to some - AgentJayZ uses it to clear gravel from behind the test stand and move a portion of the local air closer to the next county).
The "usefulness" and "frame of reference" (restricted to the metal parts and not to the gas, for some reason) becomes arguments of _semantics_ , while what's the real argument here AFAIC is that of _physics_ .
I would file this simply as a misunderstanding: The book author (more or less obviously) talks about "net power", i.e. power that actually serves the device's ultimate purpose. Since a jet engine's ultimate purpose is not to move air, but rather to move stuff _through_ air, its _net_ power on a test stand _is_ zero.
But measuring the net power of an aircraft engine would be rather useless anyway, as the thrust is what aircraft designers really want to know for their calculations in the first place. (Similar considerations apply to car engines, btw; horse powers are a neat marketing gimmick, but a far better measure of a car engine's ability to accelerate a vehicle is the torque.)
I remember my dad told me a story back in college where a physics professor slapped a student with both hands on both sides of his face at the same time (This was in communist China). The physics professor then stated that since his head didn't move, no work was done and therefore he was never slapped.
China is still communist.
I love your videos, and as an aerodynamicist engineer I see sadly how much trouble that very badly written sentence in that book caused. The sentence is provocative and truly wrong. However I know exactly what he tried to write, but he wrote it very badly. First of all: AgentJayZ is completely right, gasturbines produce hundreds of megaWatts power and apart from what portion is used for driving the compressors there is still MegaWatts of power in the exhaust jet that can be further harvested. We are designing the working cycle of gasturbines (Joule or Brayton cycle) for power in Watts (or Horsepowers if you like). So we design every compressor and turbine stage for power (Watts) and so the engine produces power (Watts). As a consequence of the excess power which is leaving the gasturbine in the form of a jet a huge amount of force is also generated, which we know as thrust. Now comes into view a completely different topic which is related to airplane design and the author of the book mixed the airplane design concept with the jet engine design and here is that oversimplification what makes his sentence wrong. At airplane design process only the generated thrust force value in Newtons that would overcome the drag of the plane, is considered. Basically we no longer consider a jet engine, but we oversimplify and consider only the force vector that the engine generated. With other words we don't care at this point of design about how this force was generated, but we consider that it is simply given for our airplane. From this point on we only consider the work that that force has done for moving our airplane and so not the work and power that actually created that force. With other words: the force has been created by the gasturbine via investing an immense (hundreds of MegaWatts) power by (as you explain) accelerating a huge mass of air from zero to 500m/s velocity, but now we ask what this generated force can do for us. If this force will bring us through a distance of S, then it produced for us F*S Work, or F*Displacement_speed power. Note that this power is not the power that generated the force, but the power that the force provided to us. Imagine that the jet engine is a mother that gives birth to her son by creating an enormous power to give birth. The son is the Thrust. From now on we want the Thrust to do work for us, displace us at a given speed and we don't consider the mother anymore. If the Thrust is not moving us anywhere it will not work for us. However we will never forget that this Thrust was created by producing an incredibly immense power by its creator: the jet engine. I hope I'm understandable and it will make this topic clear.
Hello, Sir Jay-Z! Perhaps the "machines" at RUclips were upset, but I just discovered they had unsubscribed me from your channel...and your skookum logic.
For those who doubt that engine is making power, just pop the engine release pins. That engine will likely fly into orbit!!
Hehe. Glad to be resubbed. Bob
Hey JayZ you're right, the LM2500 PT has got 6 stages, and it is in fact based on the TF-39 LPT (C-5 Galaxy), the predecessor of all CF-6 engines which also has got 6 stages. Difference, the LM TRF is more robust because it also provides the rear GT thrust mount and it contains an additional ball bearing (No.7B) to be the PT shaft thrust bearing. In the TF-39 the LP shaft thrust bearing is the No.1 bearing at the fwd. mid shaft which an LM2500 doesn't have (but LM5000 and 6000's do). With advanced development towards the CF6-6 and -50 the LPT's became more efficient and smaller to save weight. Therefore the -6 has got 5 stages and the -50 even only 4 stages of LPT although the power output increased.
Thanks for all your great videos and I am always really amazed how professional you guys handle this wide variety of different engine types in your workshop and test bed!
Although the changes are extremely small, I assure you that the engine at full power is moving the earth just a teeny bit. It's going nowhere fast but it's moving.
@@gordonrichardson2972 Most of the power of a CONFINED engine. At any rate, if you feel the need to repeat what I just said, you have at 'er mate.
The engine ejects a calculable mass of gas out its arse end at a calculable velocity. Newton cannot be argued with.
Nothing is moving earth. In a closed system sum of force vectors and impulses are exactly 0.
@@gordonrichardson2972 you obviously haven't heard of The 1st Law of Thermodynamics ;) en.m.wikipedia.org/wiki/First_law_of_thermodynamics
As long as no air is leaving mother earth, we have a closed system. -> Earth's movement won't be changed by AgentJayZ 's beautiful jet engines.
@@tubefish666 Read my comment very very carefully.
I can only imagine the amount of stress being applied to that engine stand to make it flex like that.
We measure the stress. No need to imagine it. For an afterburning J79, it's around 18 thousand lbs force.
Thank you, had my thinking corrected. Forgot completely about what was moving.
This man knows a lot of stuff.
By choice.
Well if it’s not making any power then why strap it down? We need this test for science!
The power output of a gas turbine is simply the output torque of the turbine shaft multiplied by the turbine rotational speed.
HP = Torque x RPM ÷ 5252
The engine produces Power by providing a rotating shaft which can exert a given amount of Torque on a load at a given turbine RPM.
Just from the description I'd assume people are missing the obvious fact and saying that the massive volume of cold air it sucks in and the hurricane force red hot air blast it produces don't count as "power"?...
Sure theres no "shaft horsepower" or anything to measure, but a force is being produced, causing the engine to push one way, and air to push the other. While the engine doesn't move, technically no "power" can be measured, just a force, however the air is moving... and so we can measure the power there, force & mass = movement = power.
Saying a engine on a stand makes 0 HP is like saying the most efficient generator gets 0 MPG
Unfortunately it's a question of semantics. What is 'useful work'? The general definition is 'work (read: energy) that is not wasted'. How do we define when work/energy is not wasted? Depends on who you ask. To the aeronautical engineer, when the engine is on the test stand and stationary, all the energy is wasted because it is not going into propelling the airplane (and therefore not earning money). Ditto if you ask a mechanical engineer about a turboshaft engine hooked up to a dynamometer instead of e.g. an electrical generator. If you ask a physicist, the engine is definitely doing work moving the air and the wasted work/energy is the energy lost in heating up the engine as a whole, e.g. the energy that overcomes the friction of the turbines rotating in their bearings, or the energy that just heats up the turbine blades instead of going into accelerating the air.
Thank you for the marvelous video to enjoy on a Sunday morning. I always thought physicists should run their work through Legal and Compliance Departments like the rest of us. Rock on Jet Man.
Try walking behind the jet pipe when a J79 is running at full thrust on the test stand and if you survive, come back and tell us there is no power there...
No debate! Fantastic clarification, thanks a lot!
I agree with you AgentJZ, and with Douglas Turner below--I know it sounds crazy but, I believe a screaming J79 on the stand is actually moving the Earth. The distance is immeasurably small to us, but the mass of the Earth is almost inestimably large. A very tiny distance times a huge mass could still solve to 20,000 hp. The earth is a free floating object that conceivably can be moved. A plane moved by a jet engine is merely a physics model with distance and mass within our ability to observe and measure. Bobby has an Earth whose mass is estimated at 5.972 x 10 to the 24th kg. Bobby's Earth has a radius of 3,963 miles. Bobby runs a 20,000 hp jet engine on a stationary test stand located at the radius for 10 minutes. Assume 4 meter rms accuracy of the GPS system. Calculate the distance moved at Bobby's test stand, work done, and the time a continuous run would take to verify results via GPS within a 90% confidence interval. Show all work, points off for answers with improper significant digits. ;-)
No, you’re forgetting the force applied back into the surface of the planet, from the moving stream of gasses via drag, in the opposite direction!! Unless you’re expelling material off into space or you’re accelerating the entire atmosphere, I argue that there is absolutely zero chance of affecting the rotation of the earth in any way !!
@@GTEskee - I'm listening. So, when a jet flies, does the moving stream of gasses via drag in the opposite direction, move the planet in the opposite direction? More to AgentJZ's point: what's up with a physics formula that says there's no power produced if there's no movement? I'm suggesting maybe the earth moved an infinitesimally small amount, that would qualify as power. You are saying that there would be an equivalent opposing force, acceleration, drag, to prevent movement. I would think that also must qualify as power. f=ma, but what if there's no observable a? I think we're both making AgentJZ's point.
I would say, unless the plane is accelerating, then again, there is a force applied to the stationary air (through drag on the airframe) equal and opposite to the force applied to the stationary air by decelerating the exhaust stream back to zero mph . . .
As f
DiveTunes, And you would be wrong. As the mass goes up to "stupefyingly large", power goes down to effectively zero. Do the math! As I said else where it's like bouncing a ball off the earth. No energy is transferred, but twice the momentum. So yes, momentum is transferred to the earth by a stationary jet engine, but no energy. Exactly zero for all intents and purposes.
Thanks again Jay. Once again you demonstrate your talent for explaining the theoretical into the literal. (Does a leaf blower really do any work?) You sir are truly a man of many talents. We all benefit from your insights. You have saved us from a philosopher's conundrum. Well done.
If the J79 spits out 50 kg of air (or whatever what) at 500 m/s ...
E = 1/2 m v2 to get the Kinetic energy each second
P = E / t to get the power:
in this case:
0.5 * 70 * 500 * 500 = 8750 000 W or 8.75 MW or 11734 hp
Looks reasonable to me.
It takes power to move such large volumes of air / gas. Even just the strain exerted upon your test stand counts. If there was no power there would be no need for the test sled to be concreted into the ground.
The High Pressure Turbine of a P&W JT9D during an engine test run, at takeoff thrust, is absorbing 110,000 HP from the combustion airflow and passing it down the shaft to drive the HP compressor. It doesn't know or care whether the plane is moving or not. It certainly is doing work, ingesting and compressing air.
Jay, I trust your explanation and I have never doubted you. Great site and for a layman I learn a lot every time I watch your videos. Great job!
It seems unimaginable that anyone could, even for a fleeting instant that an engine on a stand make no power.
Somebody said (or, possibly, didn't say), "Everything should be made as simple as possible, but no simpler."
It is incredible how belligerence can overcome facts so clearly illustrated in practical application. Hence an aweful lot of argument out there in the name of science by people who use jargon to suit their means and sound clever- go the greens. And this is just clarifying a point between actual applied science professionals really! No power. Lol. Where is Richard Feinman when you need him!! Your explanation was awesome Jay. So clearly put a child could understand it. Yet Feinman said if I could explain QED so you could understand I wouldn’t have won the Nobel prize! 😂. He would simplify it to a point we caught a glimpse. This on the other hand seems quite fundamental. The breaks or stand and the ground are resisting a very great force and there is a lot of stuff being moved from front to back. Stand in front, you will soon be out the back in small pieces just as ignorant as when they went in. Tips for life - you can be wrong and still be clever. Clever people know this and how it is expressed. Like “I don’t know” or “ I was wrong” can be used. Cheers. Where is your NZ flag🇳🇿!!
Camera focus is doing “useful work”
Read the Work Energy principal on Wikipedia. If you are accelerate a mass of particles to some velocity you are doing work!
“The principle of work and kinetic energy (also known as the work-energy principle) states that the work done by all forces acting on a particle (the work of the resultant force) equals the change in the kinetic energy of the particle.”
If you take the position that all of the energy leaving a heat engine can be included in its power rating, then all heat engines are 100% efficient, because when you add the energy being coupled out their shafts and the energy leaving in air velocity and temperature, they equal the heat of combustion of the fuel. So yes, we ARE talking about mechanical philosophy. In your stationary turbines, the output of the gas generator is not the output of the engine; the output of the TURBINE is. This is never 100% efficient, as there is a great deal of power that is thrown away out the exhaust of the turbine, but we do not include this when we calculate the output power of the engine, do we? I'm sure the author of the book you're pointing at is fully aware that there is power in the exhaust of an engine, and implying that he is unaware of this because the air is invisible is just insulting.
There's really no reason to be a dick about this. Of course there's power being produced. It's just not being used. And YES, the WORK output of the engines on aircraft DOES go from zero to many thousands of horsepower when the pilot releases the brakes, just as the useful output of an automobile engine goes from zero to hundreds of horsepower when you engage the clutch. So please step down from that high horse.
This is right inline with my original comment. I'm glad someone else found another way to say it. Would you rate a stationary power turbine by it's generator produced power, as well as the exhaust energy flowing out? If anyone claimed such a thing on any given AgentJayZ video, they'd be ripped apart. Do we include the energy leaving with the exhaust of an automotive engine into its power output? Absolutely not. I mean, by the logic I'm seeing throughout the comments, we must include that energy as "work" because "the engine had to move the air". Well, yes, in the case of the automotive engine, the engine did move the air via the piston during the exhaust stroke, but this is *thermodynamically* not classified as work or power output *from* the engine. The piston had to do work on the exhaust as it was pushed out, but that is not the work we use to calculate thermal efficiencies or power rating, it is internal to the walls of the engine. Observing that fast moving exhaust's energy as work transfer and not kinetic energy (which is what it is) results in a thermal efficiency of 100% (as you said) which is a violation of the 2nd law of thermodynamics.
I am a mechanical engineer who works with power systems, so maybe I can help clarify what the author is saying... Theoretically speaking, he is correct in saying that no power is produced. This is because we define work transfer to be done through things like shaft work (rotating generator), boundary work (piston/cylinder), or electrical work (electric motor). It's important to use this distinction when talking about the 1st law of thermodynamics (conservation of energy) and the balance of different forms of energy. For example, if I "draw a box" around a jet engine bolted to a testbed and write out the energy balance for everything in and entering or leaving the "box" , there is technically no work (as defined previously) crossing the boundary of that box; thus, the work or power term in the equation goes to zero! This is actually a homework/test problem I have seen before! This is compared to a mainframe gas turbine for power generation for which, if I drew the same box, would indeed have a work output (or power) value equal to that of the torque*rotational speed delivered by the shaft to the generator.
Now, that's not to say the bolted jet engine isn't doing anything! It is imparting a huge temperature and velocity to the exhaust gases which are leaving. These indeed find definition in "forms of energy", but they do not classify as "work" or "power" being produced. The exhaust has a large kinetic energy (velocity) value which then gets dissipated (yes, possibly through work) far downstream of the testbed and our imaginary "box".
I like how you defend ignorance. True engineering miracle. Edit: BULLSHIT!
@@tabaks I'm only defending the author insofar as he is using the definition of work or power in its true thermodynamic definition, and as far as he is talking about work being produced from the engine as a unit. It depends on where you draw the box I discussed before. If you draw it around the turbine section of the engine, there is work and power produced! There is work created from the exhaust gases which rotate the turbine to produce shaft work to drive the compressor. It depends on what you write your equations for...
Thereby, if you're a theoretical box encompasses the entire Earth, no work is ever done?
Sheldonite
Please go down fifteen or twenty comments and dress up my comment. It will improve if you work on it a bit. Thanks.
John Carroll
@@sbbolton66 As long as no work passes through the box boundary in the forms I mentioned earlier, yes, that is correct. But that is a very extreme example. Here is something I was told in engineering school which helped me grasp the subject. The 1st law of thermodynamics is just an accounting equation. That is all; it's like totaling your debits and credits and that those take place for different types of purchases and sales. Here, the analogy is that forms of energy transfer (work, heat, kinetic, potential, etc.) are the different types of purchases or sales.
So, say you are in charge of economic accounting for a large company with many departments which have their own expenses and sales. Let's also say that some departments bill other departments for labor, supplies, etc. You would not keep track of economic transactions happening between the company and outside vendors by counting up the transactions happening internally between departments.... right? Just like I would not count the work happening internal to the engine as work being done BY the engine. That's not to say internal cash flows never happen or work isn't done at all! It's just happening internal to where you drew your boundary.
It's simple, it can be a force exerted on a fixed object measuring the weight exerted
If a J-79 in a test cell is not running, and you see AgentJayZ standing behind the screech pipe, nothing happens and he continues to make a video and it's a nice day outside. If the said J-79 is running full blast, you'll notice AgentJayZ does "not" stand behind it. This is because something could happen. Actually, something really bad could happen. This leads to a logical conclusion that he doesn't stand behind because there's an insane amount of power being produced even though the engine is not moving.
The thing about it is not only the movement of the air indicates that the jet is producing power but also in the movement of the internal parts of the jet. The opposite reaction to these forces are a movement to the jet as a whole. When you bolt down the jet to the sled (which in turn is anchored to the ground) and power it up the jet does move. Not only in the flexing of the sled but actual rotation to our planet and a comparative greater degree to the tectonic plate the jet is bolted too. The mass of the tectonic plate and earth is so large that this movement caused by the reactional forces on the jet causes an undetectable movement. Force / mass = acceleration. The mass of earth is so large that the force of the accelerated air produce insignificant acceleration of the bolt down jet, but there is acceleration.
If a stationary jet engine produced no power ....... How would a stationary jet leaving the terminal ever start moving?
If you think about how much noise a jet in afterburner makes, it would take 10,000 watts or more for a stereo system to put out that amount of noise. Just think of a concert with 10,000 watts of speakers. If you were to play that sound using a concert audio system, it wouldn't even come close to the noise of a jet engine. For instance, a 50,000 HP engine is equivalent to about 37.28 million watts. It's just incredible how much power those engines put out.
I'm surprised they let you test them fixed to the ground. Your changing the rotation of the earth.... are you making us spin faster or are you slowing us down? I hope your slowing the earth down i need all the more time to finish my chores thanks
*You're
*you're
perhaps those people mean 'no useful power is generated'
Looking at the cracking in the sheet metal on the wall, it looks like you could use some of those industrial sound dampening blankets to hang over the walls. Might take 2 or 3 layers, spaced about 3” apart, and probably wouldn’t make engine tests sound any less deafening, but would help protect the wall.
Great video, btw. I’ve learned a lot from your videos.
P=Fv, it is not an argument its the definition of power in relation to force and velocity.. Its temporally not a aircraft engine its a turbo-hairdryer engine... or a hackney word game
AgentJayZ, So by that rational a Harrier, F35 or any helicopter that's hovering is making no power! Keep up the good work. I'm enjoying my edumcation in jets.
I was surprised that you know the Heisenberg uncertainty principal :D
And who is he? (www.merriam-webster.com/dictionary/principle#usage-1)
Since energy can neither be created nor destroyed, we must assume that if an engine is ingesting fuel and air, that the energy in that fuel and air mix is changing form and being released in some other form of energy. Sounds a lot like power to me.
My understanding of physics tells me that a jet engine on a stand makes thrust - place torque sensors on the stand struts you will measure force.
Being generated by the engine on a wing or a stand it creates force, chemical reaction if you will. Which also alters the hydro carbon fuel. After being burned, it's composition is altered. By ratio of weight it is greatly reduced, being altered from a liquid to a gas with heavier carbon being present as a result of incomplete burning/ conversion.
I’ve carried with me vividly since jr. high science/physics the diffrence between force, work, and energy… It certainly takes effort to hold my arm out to the side holding a 20lb weight, and there’s a force on my hand and arm, there’s potential energy in the weight, but no “work” is being done… eventually the subtitles of the various models and how they interact took hold, but intuitively, that was a hard one to learn.
I am amazed as to why people get confused or become misinformed by such a simple understanding. As some have said below, if a tree falls in a forest and nobody is around to hear it, does it make a sound? Is a jet engine doing work while functioning stationary on the ground? Yes, its blasting air at incredible speed and trying to rip the bolts out of the ground (We are trying to stop the earth from turning) People just can't see it and therefore they don't believe it haha.
I think that you are stretching the point too much. What I asume Bill Gunston is trying to say is that the power generated is not useful in that particular case (a turbojet on the ground) not that no power is generated. Of course he knows that the power is used in moving a huge flow of air at high speed, but unless the objective is just that (to blow air) the power is wasted, it is not propelling any airplane and is not moving any power turbine. And of course he knows that if you install a power turbine you can get that same power to move a generator or whatever.
I'd definitely agree that neither is strictly wrong. I think if you asked the author if the book, he'd argue that a jet turbine's job is to produce propulsive power through thrust, while other gas turbines produce power by turning an output shaft.
Static engine has to make power, or else it couldn’t run. Requires power from the exhaust to compress the intake air. Yes? Thrust and power for auxiliary uses are just additional power not required to run the engine.
This is why perpetual motion engines do not make power, because as soon as you apply a load to the engine, it stops.
an engine makes all kinds of power:acoustic power,thermal power but only useful is measured
I have the same problem at work, people are making a lot of noise while remaining stationary and the only moving parts are under their nose
You see, all they do is move air, doing no useful work at all =)
Seems to me the non believers need to understand (if possible) the difference between thrust and Hp. Good video. Thanks.
I'll revise this comment further.
HP, Thrust, Newtons, Watts, Joules, Calories, BTUs, etc are man-made constructs used to understand and measure the conversion of potential energy to kenetic energy in various forms of man-made processes as well as natural processes.
I was still sceptical (fully understanding the situation), but at 9:08 you summed it up really well, and I agree.
Does the test cell help to accelerate or de-accelerate the rotation of the earth when it is in use?
In respect to earth's atmoshpere - yes, a little bit. In respect to other celecstial bodies - no. At the end even this earth-atmosphere momentum transfer thing dissipates into heat, through air-surface friction.
Neither. Not at all. Do wind turbines slow or accelerate the rotation of the earth?
terry boyer They're in Canada, everything is already wobbly.
John Carroll Depends on where the gas turbine is mounted!
Marek Kaletka I'm sorry, are you changing the subject to global warming?
Those people are not considering the enormous volume of air the engine is moving. That's how people should think of it. Whether or not the engine itself is moving is irrelevant, it's how much air the engine is moving that matters.
There is some usefulness in saying that _useful_ mechanical power is force times velocity, because it allows you to quantify how thrust-efficient the engine is. The engine will always produce its commanded thrust, but some of the power used to do this will go into the exhaust (which is "wasted" because you would rather that energy go into the plane). At speeds lower than the exhaust velocity, this means that you're wasting power in order to attain the required level of thrust by accelerating the air to ridiculous speeds; that's the impetus for turbofans with their huge bypass ratios - the exhaust moves slower but there's more of it, and so you can attain the same level of thrust and hence acceleration without spending so much energy accelerating air. This makes the engine more thrust-efficient, i.e. its specific fuel consumption is lower.
Really well explained! Makes sense. As I was listening, I was like, "what's that noise?". It sounds like a squirrel .....it's the auto focus of the camera! 😂
I've never understood this fallacious thinking... just because the turbine engine isn't moving in relation to the ground, doesn't mean it isn't producing power. What it means is that the force of the mechanical devices holding it in place are greater than the force it is producing. I'm not sure why this elementary principle is so hard to understand.
If you go and push on a large rock with all of your might and don't budge the rock, it doesn't mean you aren't producing any power, it simply means that the amount of power you are producing is not sufficient to overcome the forces holding the rock in place.
Not true, you for instance could be generating enormous force be it lb. (thrust) or lb.ft. ( torque) and because you have no movement in a linear or rotational fashion you are making NO power. Power requires work being actually done, static conditions means no actual work is being done. You can make all sorts of energy and do no work.
@Thomas Rowe If you wanted a giant air pump for moving the local atmosphere or track dryer, it is doing useable work and making power. Moving air for no purpose is not making useable power.
Damn this is good! I had heard this before and even had I had a person say hey I learned it in school, etc. so it has to be right (lol) no horsepower when it's not moving 🤦🏻♂️ - I had to stop discussion since it wouldn't go anywhere by saying maybe if you are looking at the reference frame of the jet or something like that it was bizarre because they did not want to grasp that work is being done on SOMETHING! This is an awesome video, it so well done and so well said as are all your videos! Hope all is well & take care! I love your line about it being invisible and being two 😂😁 -- VERY true in so many regards!
A specious argument like "If a tree falls in the forest and no one close enough to hear, is there still a noise?" Of course there is a noise. The air vibrates whether anyone is around or not.
The author left out one word. What he meant was that a propulsion engine in a vehicle that's not moving is producing no USEFUL power. The acceleration and increase in temperature of the air is not moving the vehicle, so it is doing no useful work.
Does a ventilation fan, moving a bunch of air... make useful power?
@@AgentJayZ Yes, because its a ventilation fan. A jet engine is not a fan. That's the simplification you're talking about.
Consider a jet engine a source of propulsion. The thrust (F) does work (W) on the engine if the engine moves (s). W = F*s. No work, no power. The kinetic energy of the thing will increase in the amount of work done.
Yes, the pumping of air is also work, but is not propulsive work. That's why _as a propulsion_, a stationary engine has zero power at zero speed. And because it can maintain thrust over a high range of moving speeds, its such a great source of propulsion - its power increases with speed.
@@AgentJayZ Sure it does. Because that's its purpose. Really, you can't be that dense.
I’m now wondering how many jet engines bolted to the earth, facing the same way on full thrust would it take to stop the earth rotating?
They would have no effect at all. The atmosphere is physically connected to the earth. The engine throws a lot of air out the back, so it is pushed the other way. It's on mounts to the earth so it pushes the earth. The air that was thrown out of the engine slows down through drag and friction pushing the rest of the atmosphere with it... and all that moving air eventually drags on the earth the opposite way the engine is pushing it.
It's a closed system.
Eventually all motion decays into heat.
... aks people why their vacuum gets louder when they block the airflow... you will be stunned what they will tell you... but non will tell you that it is because the motor turns quicker because of no-load from the air... People are shocked if you tell em Air can behave like a liquid... mannnnnn xD
Wow.. i learned about electrons today. I always thought it was a actual ball circling the atom..
I feel this is similar to the "how can you hold the engine down at full thrust?" questions with people wondering if the skid is sturdy enough. They completely ignore that the same engine is mounted to a plane with just a few (admittedly huge) bolts. That is a plane that is built as light weight as possible and not with 6" structural steel and still the engine doesn't rip out of the plane. If you then ask why the forces on the mounts should be different between a plane and a skid, the answer often is "because the plane can move, so the engine doesn't push as hard against the mount". ?!?
It isn't rocket science (well, it is, though not the hard kind, in case of rocket engine tests, where you get the same argument, with rockets being even more optimized for weight than anything else). It's a little bit frustrating, because that either means that the educational system failed or people just don't really think for themselves.
Alot of concepts in physics are difficult to wrap my head around, don't quite have compressor stall yet but this one makes sense, I'm with you here. Slightly off topic question, if the engine is moving, flying, does it's speed relative to the air enhance compressor pressure? Any "ram" effect? Apologies for 2nd question, how much difference between gas generator shaft rpm and pt shaft rpm is there in a well designed system?
Yes, ram effect is always present. But depends on the inlet system for the engine to use it efficiently. One example is the Blackbird which could raise the external ambient pressure from 0.36 psi up to 14-16 psi at Mach 3.2 due to ram recovery.
I think it's helpful to take one step back and remember: Power is work per time, and work is done on some thing. It's a good idea to state what that thing is. In your test cell there is zero work being done on the stand but lots of work being done on the air. Therefore it's producing "air stream power" but not "test stand power".
love it jay keep up the good work
The correct term is its not doing any useable work since its stationary. Still is making internal power to compress the air, and generating thrust.
You are ignoring the air.
Oversimplification.
Error.
@@AgentJayZ Not to be argumentative, but that's why I said _useable_ work. While the author was not very clear, that was probably the intent of his statement. The railroad would probably consider it to be useable work in the form of thrust (e.g. hot gasses accelerated out the nozzle section) and heat for removing snow and melting ice on the railroad ties, but for our purposes its doing no useable work. Its stationary. It is still doing work moving air by generating thrust, but at the end of the day its all wasted as heat since its in a test cell.
The work is plenty usable. You can use the heat to create steam or heat buildings, and you can use the moving air to drive a turbine hooked to a generator for electric power.
In fact it is way easier for me to calculate this kind of power versus how fast it can make a plane go, because the would require knowing exactly how much drag the plane is creating. I can tell you the engine power is at least greater than the power used by all the electrical and hydraulic stuff inside the plane which I can easily calculate, tho. And lesser than the hypothetical power created by the rate at which it is burning fuel, which I can also easily calculate. Same for the climb rate.
Gliders can fly for hours using dynamic soaring, slope soaring and thermals, even tho they have no engine, but they generate power? Silly.
@@mysock351C ... SO if you are wanting to move air, it's usable work, and if you are not wanting to move air, it's not usable work?
So the physics of the device depend on the intentions of the operator? Hargle-de-hogwashery-blah.
I concur there is no (or very little) 'work' be done on the jet engine. work = force x distance. It's confusing because holding a weight still has zero work. It does take work to hold up the weight.
Fluid dynamics. Interesting stuff.
Great video jay you are absolutely correct.
How many mounted test stand engines would it take to increase the earth's rotational speed ? If we speed her up she aught to cool down ?
Unless we are ejecting mass into space, and it leaves earth orbit, then it's all a closed system. A million static jet engine tests would have zero net effect upon the rotation of the planet.
@@AgentJayZ Interesting. So I'm back to having to relocate the moon, or decrease earth's mass ? Problem seems to be photosynthesis increases our mass faster than we can burn stuff ?
The only way to slow the earth is to apply forces or add mass from outside the closed system. Meteorites do add mass to the earth, and slow down the rotation, but it's a small percentage, I tell ya.
@@AgentJayZ Not if you had enough jet engines. Probably wouldn't be enough fuel though. Now this just anecdotal but the force of one jet engine or a plane strapped down to an aircraft carrier deck can effect the speed of the ship, to the point where the Captain wants to know if you're going to be doing that sort of thing. I do wonder what would happen if you tied down all the fixed wing aircraft in the same direction and went military, augment on the ones that have it.
Something that does no work, but hangs on the side of a fridge 24 hours a day is called a magnet.
@@msmeyersmd8 No such material currently exists, most solid or pressed magnetic materials are very fragile and wold not survive the rotational forces when shaped as an aerodynamic blade.
A question on the boneyard, if I may: what drives the decision to keep rusty parts around? What uses do these unserviceable parts have?
I love the Heisenberg Uncertainty Principle, you should have thrown in energy state changing orbital levels.
Sound is the transmission of energy through a medium, an explosion blast wave is compressed sound waves that can kill a human if too close.
please dig out your radio mic for interior recordings. The reverb has lots of horsepower
Agentjayz what is ram pressure recovery ?
I might do a video on that.
It's in all the books I recommend in my video on books.
@@AgentJayZ sure thanks
Instructions unclear, head stuck between banister rails.
How has this ever been something that needs clearing up?? The "object" that's moving doesn't have to be something the engine is bolted to, like an aircraft - by virtue of the fact that the energy isn't being expended in propelling an aircraft forward, there must be a *massive* volume of air being dragged along in the wake of the exhaust, but I suppose that's hard to observe... unless you look out the back of the test cell, which is a barren wasteland of blasted snow, gouged gravel and trees that are saying "yep, this is close enough thanks". I don't know, maybe I'm missing something, but it seems staggeringly narrow-minded to think that an engine is producing no mechanical power simply because the thing it's bolted to is stationary and can't move.
It may not be producing particularly "useful" mechanical energy, but I'm sure an engine (and the physical laws that govern how it disperses energy) don't care a whit about the purposes of its manipulators.
Next person who says moving air is not energy gets to walk through the jet blast 🤣
I am entirely with you on this, AgentJayZ - and, even though I'm a Brit, I think of you as AgentJayZee, not JayZed.
Sadly, as Mr Sivhed has noted, Bill Gunston is no longer with us, so we cannot debate this with him. I might have tried to do so, had I bought my copy of the book before his passing. Whilst I recognise he might have been trying to say that a stationary jet engine on a test stand is doing no useful work, I cannot agree with the way that he actually said it, as his "zero horsepower" is essentially wrong. Part of the problem is, I believe, the term horsepower (or - typos corrected - Cheval Vapeur, or Pferdestärke) itself, which Invites the forming of a mental image of James Watt's horse hauling a weight up a mineshaft.
As someone who, back in 1971, was involved in the design of the 'C' rated Industrial Olympus that you've shown us in the past, I recall the way that complete SK30 gensets were tested before being shipped offshore to production platforms. They were tested to full power with the Megawatts from the alternator, which could be measured, being dissipated between two big electrodes in a huge tank of water (the biggest electric kettle in the world?). Before that, the Olympus gas generators were tested with 'slave' jetpipes, in exactly the same way that you have tested them, and they must have produced the same power as when installed. Incidentally, and I think that I've mentioned this in the past, R-R used to quote an EGHP (equivalent gas horsepower) figure for its industrial and marine gas generators.
Finally, if anyone out there wants to do the force times distance moved against the force in unit time calculation for an aircraft, try this for Concorde in supersonic cruise at 58,000 ft: 1,350 miles/hr with four Olympus 593 engines each producing 9,000 lb thrust (equals drag, of course}. Answer: 129,600 horsepower. At take-off, with a thrust of 38,050 lb from each engine, the power produced was considerably greater (obviously!): the difference between the two conditions was propulsive efficiency.
PS The speed of sound in in the jetpipe of a J79 at max dry thrust? Probably somewhere over 2,000 ft/sec
Bit of a typo there, it's Cheval Vapeur and Pferdestarke. If you want to calculate a quite staggering engine output, calculate what it takes to push a nuclear submarine through the water (submerged) at 40 knots or more. By propulsive efficiency are you referring to power decrease at altitude? Jets typically become increasingly efficient with increasing altitude.
@@scheusselmensch5713 Thank you for the corrections, but my typos hardly negate the point I am trying to make - and what about the umlaut over the 'a'?
Propulsive efficiency is essentially the Froude Equation, which I've quoted on this channel more than once. The greater the difference between the efflux velocity of the engine and the forward velocity of the aircraft, the lower the propulsive efficiency. 100 percent propulsive efficiency is achieved when the two velocities are equal, at which condition the thrust must be zero, as there is no momentum change.
I honestly think Gunston is saying what he described is the wrong way to calculate jet power because one would get zero power from the calculation. In fact, one would also get zero thrust since there's no acceleration of the engine, either. All the text preceding was just telling the readers to use thrust as a measure instead of horsepower for jet engine and the reason given is the sentences in debate: If one is not properly educated and applies the incorrect equations the result would be completely wrong and useless, like what he was demonstrating.
@30:10 -- Gotta love watching a guy talk about framing perceptions correctly, while wearing mirrowshades and standing in the middle of a jet engine cowling. I think you've invented turbinepunk, Alert William Gibson!
Would you say that the authors of these textbooks almost exclusively make these errors because of a lack of real world experience and a surplus of academic experience? Thanks in advance.
I don't think so. It's not really a text book, and this is one of the very few mistakes I've noticed.
@@AgentJayZ I think they are just stuck thinking in the context of what they are describing. If you are just thinking about an airplane, and moving an airplane, its very useful to think of a jet engine as only producing power when it moves a plane. It perfectly describes the difference between the way a prop plane and jet plane accelerates. I dislike it when technical books and such describe something like that as literally being true because it creates confusion like that. Why not simply say "For our purposes the power generated by a stationary jet engine can be disregarded. Therefore, a jet engine effectively creates zero power until it moves a plane." Or something like that. But they don't do that.
By there logic a car on a dyno wouldn't Be making any power. You did a good job explaining the obvious
*their
@nick f What are you talking about? Cars or jet engines? Either way, that's the pinnacle of gibberish.
Mr. Gunston is actually correct. The formula he presents is the standard equation for expressing thrust-horsepower, and may be found in numerous texts regarding thrust producing aircraft. J-Z is confusing the issue by conflating thrust-horsepower with power in a general sense. Thrust-horsepower is a useful measurement when discussing jet aircraft performance specifically, since fuel burn correlates with thrust, which remains approximately constant; and power varies in relation to true airspeed (for a given thrust setting).
J-Z has produced an interesting video, and perhaps unintentionally illustrated why we don’t express the work performed by tug boats or water pumps in terms of thrust-horsepower.
(For my source material, see chapter 2 of Aerodynamics For Naval Aviators)
Blah blah. Another example of oversimplifying reality and missing an important detail.
@@AgentJayZ That's as articulate, as it is insightful. Would you mind sharing your source material?
Every physics texbook I have ever looked at. To be honest that's only about ten of them.
That’s so simple I have never Hurd this one before! If I had I would tell them to think about it again. Air is heavy. We don’t really ever think about it that way.
Of course it's making power. I thought this discussion would be about the difference between stationary operation versus moving fast - getting the ram air from forward motion - how much difference is there?
That's one for the theoreticians. The only way to actually measure the difference would be to have specially built load sensing engine mounts in the aircraft. That may have been done before, but I have not heard of it nor read about it.
Good video. In the book the author is talking about that you can't compare lbs of thrust to horse power. How about a turboprop engine that one could measure HP off a shaft output of the engine. I'm wondering how HP at the shaft out would stack up against thrust when using a jet exhaust and turbine instead of a shaft drive off a jet engine to drive a generator to make electricity? A Shaft drive is mechanically solid connection output while jet exhaust connected to turbine powered generator is connected through air that has slippage/cavitation.
Power is a tricky concept with jet engines and rocket engines. That's why the force they produce is also often used as a metric. When the airplane is acclerating on the runway is has three times the power when going 150 than when going 50. More of less. The power of rockets can exceed the rate at which they are using the chemical energy in the fuel even. So a rocket can comsume 1 kJ of its fuel to increase its kinetic energy by 1.5 kJ which is very unintuitive but happens during every rocket flight
It makes a lot more sense if you think of it as the kinetic energy extracted from the gas jet or better yet the energy that was left over (not extracted). A whole lot more energy is extracted from a 10km/s gas jet when you are traveling at 7km/s than at 1 km/s - in the first case 3km/s is wasted and in the second 9km/s (observed from the same point of reference - energy being a square function of velocity).