I’ll take your total failure to address any of the points I have posted on this subject as concession that you were misinformed, due to a misunderstanding arising from your use of layman’s terms to argue in a technical discussion. Better men than I have made worse mistakes than that.
Just shredding this post on principle - the guy is such a tool he won’t understand it anyway, but I’ll try to stick to words of no more than one syllable to give him a chance.
As a rule, to get decent compression ratios and flow rates centrifugal compressors will be larger in diameter than axial compressors. This is the main reason axial compressors were pushed so hard immediately postwar - they weren’t more efficient than centrifugal compressors (at the time - they are now) but did give much smaller frontal area, critical for the early jets to reach high speeds.
Eightfold increase in fuel economy for an eightfold increase in pressure ratio? Almost certainly not true unless the initial pressure ratio was very low indeed - the efficiency graph is actually an exponential type curve, with an asymptote at about 45%. Pressure ratio in any case is determined by turbine metallurgy, not compressor design.
Quite right it’s absurd - nobody with the first clue about what the contents of a jet engine are would make such a statement, and it’s proof that you haven’t even read the sources you’re quoting at us.
Both axial and centrifugal compressors will typically use some form of stator.
In a centrifugal compressor, this takes the form of a diffuser - although in this case the stators are often omitted to form a vaneless diffuser. This is the source for my comment about doubling the size, or a bit less - a vaneless diffuser has to be somewhat bigger than a vaned diffuser for similar efficiencies.
An axial compressor will typically (read universally for efficiency and size reasons) use stator blades to deal with the swirl velocity between stages, and depending on the degree of reaction for pressure reasons as well. The comment I made was that it would probably be possible to remove these stators and still have a working engine, but that the resulting thing would be huge due to the requirement for the casing and flow viscosity to do the job of the stators in damping out swirl. This is where the at least 10 times the size comes in, but due to the weight of the stators the saving from removing them will be to a first approximation much the same as the extra weight requirements in casing and shaft.
And you say diameter is not proportional to engine weight? Dude, I just showed you real examples (not fantasy examples) that show that in the real world (not the imaginary one), it is indeed directly related.[/quote]
You gave some examples. They don’t actually show anything unless you provide a much larger sample and do the appropriate statistical analysis on it. To quote the old example, “9 out of 10 cats preferred Whiskas” actually means nothing statistically speaking.
I have not deleted the post at all (yet more demonstrable lies - you really are slipping here). The post in question is on page 4 of this very thread, time stamped Thu May 12, 2005 8:24 am. Now, you expect us to believe anything you say when you are lying on such childish matters as this, and can be caught out in seconds?
…is also absurd, because an induction fan in a turbofan engine requires highly aerodynamic fan blades which are intricately formed. A “household” type of fan would not have it’s blades conforming to the shape of the fan housing. Clearly, a 10% decrease in efficiency is so far from reality as to be absolutely ridiculous as well.[/quote]
I’m quite well aware of that (indeed I’m also aware of exactly why they are that shape and what the restrictions on pressure flow across them are). I’m also well aware of exactly why the fact that jet engine operation has nothing to do with fans has nothing to do with blade shape - were it to operate on the same principle as a fan, the blades would actually be of a very similar shape (identical?) to some designs of jet engine (0% or 100% reaction, I can never remember which way around it goes).
- I am not “your boy”
- Please translate “desing” into the English language
Except I’m not wrong - this site has multiple engineers in it, including one who services jet engines for a living, and none of them have picked up a single thing I have said as wrong. You have posted a bunch of irrelevant websites coupled with a chronically bad understanding of what they mean (you can’t even tell the difference between a stator and a centrifugal impeller FFS!) and flamed both me and all the other engineers on this site on that basis. We’ve posted technical reasons, data and supporting arguaments, along with sources that you could go and check up on our data from. You’ve posted inane drivel.
I understand that you are squirming now because it’s all in the light. It will pass. Be patient.
I understand that you are squirming now because it’s all in the light. It will pass. Be patient
I have addressed your points in a comprehensive and detailed manner. I have referenced my arguments to sources which are beyond question. I have also explained how the misunderstandings have arisen, which (in my first post on this forum!) gave you room to back down with your honour intact.
If you will not do me the courtesy of replying to my arguments rather than just posting nonsense, then please do the rest of this forum a favour and do not post at all.
You can’t change the facts from the experts on the sites I have provided. I know you want desperately to try to turn it into a whole new debate by trying to change the substance of it. But you cannot. That is folly. The claims were incorrect, I have proven them to be false. Live and learn from it.
You can’t change the facts from the experts on the sites I have provided. I know you want desperately to try to turn it into a whole new debate by trying to change the substance of it. But you cannot. That is folly. The claims were incorrect, I have proven them to be false. Live and learn from it.[/quote]
Your claims: taken from websites that explain the jet engine to the layman, in layman’s terms.
My claims: supported by auhtoritative textbooks that explain the technical detail of the jet engine for technical users, written by tehnical / academic authorities.
My sources trump yours. You can flog the dead horse all you like, but all the sources you posted actually support my point of view unless you do very selective and out of context posting. But no one could be so low as to do that.
So you are claiming that “jet engines have no components that could be in any way described as a fan” again? Good Lord boy. That has been done a long time ago. It was proven to be a false statement using quotes from jet engine manufacturers and NASA.
:lol: :lol: :lol:
Start taking gensing or something.
You can’t change the facts from the experts on the sites I have provided. I know you want desperately to try to turn it into a whole new debate by trying to change the substance of it. But you cannot. That is folly. The claims were incorrect, I have proven them to be false. Live and learn from it.[/quote]
Your claims: taken from websites that explain the jet engine to the layman, in layman’s terms.
My claims: supported by auhtoritative textbooks that explain the technical detail of the jet engine for technical users, written by tehnical / academic authorities.
My sources trump yours. You can flog the dead horse all you like, but all the sources you posted actually support my point of view unless you do very selective and out of context posting. But no one could be so low as to do that.[/quote]
So you are claiming that “jet engines have no components that could be in any way described as a fan” again? Good Lord boy. That has been done a long time ago. It was proven to be a flase statement. Start taking gensing or something.[/quote]
That is a straight out lie, that is below even a fat war-dodging social reject like you. I am enormously offended that you would dare misrepresent me in such a blatant and obvious manner. You have, again, proven yourself to be an absolutely shameless liar.
I have never, ever said that jet engines have no components that could in any way be described as a fan. I refer you to my first ever post on this site, which you have already written off as blathering. I reproduce it below, unedited, in its entirety for your reference.
I’ve been lurking here for some time now, and I’ve been moved to contribute because of constant, seemingly wilful misunderstandings on the subject of jet engines. For all the good it will do me, I’m throwing my weight in with many other members of this forum, including two with Master’s Degrees in Engineering from Oxbridge. To this I add my own experience, which is currently three weeks short of an MEng of my own, in Aeronautical Engineering, with a dissertation on Gas Turbine performance.
To avoid misunderstandings:
By ‘Jet Engine’ I refer to the incarnation of the Open Cycle Gas Turbine in which thrust is generated by the expulsion of gas from the engine, due to momentum and pressure considerations.I risk repeating what has been said before, but I want to be sure that there is no room at all for misunderstanding (you see the theme here).
There are two distinct forms that the open cycle aero gas turbine takes - the turbofan and the turbojet. The turbojet consists of an intake, a compressor (possibly multi-stage), a combustor, a turbine (again, possibly multistage) and an exhaust nozzle. The turbofan has all these stages, except that there is a fan between the intake and the first compressor stage. In the context of the gas turbine, this is what a ‘fan’ is. Thus, a turbofan has a fan, whilst a turbojet does not.The nature of the fan is also different to that of a compressor. A compressor compresses air which will pass through the combustor, raising its temperature and pressure (at the risk of adding confusion, up to 25% by mass of this air will be drawn from the high pressure combustor to be used for cooling of the HP turbine stage). A fan will compress all the air that enters the engine, not all of which will progress to the compressor / combustor / turbine, but will instead bypass them, providing thrust only by virtue of the work done on it by the fan. This is why turbofans are also called ‘bypass engines’. On a large Civil engine, up to about 90% by mass of the flow will bypass the core.
It is true to say that a part of the compressor can also be called a ‘compressor fan’, but this would never be referred to as a ‘fan’ without the qualifying ‘compressor’ term, and is unusual terminology when referring to overall cycle aspects, as the discussions on this forum are. I feel I can say ‘unusual’, as I have heard it used no more than once in a reputable academic or industrial context, despite having read in excess of 200 journal articles and textbooks on the Gas Turbine in the past nine months. Thus, it is perverse to use ‘fan’ to mean ‘compressor fan’ in the context of the previous discussions in this forum, as it is not a fan as is understood in this context, any more than ‘gas’ in the American automotive sense can be thought of as referring to butane.
In summary:
No fan: turbojet.
Fan: turbofan.Please can we have no more of this silliness so we can spend more time arguing about why playing Call of Duty gives more insight into modern warfare than actually fighting in real wars.
I have proven that you have lied, outright and shamelessly. I look forwards to you replacing your previous post with an apology for spreading such malicious lies. I also expect a PM, apologising for this despicable behaviour. I suggest the mods also take a close interest in this.
:roll: Uh huh. Sure.
But “Noddy’s Layman’s Guide to Jet Engines” trumps any authoritative text, and layman’s speak is superior to technical vocabulary, of course!
Actually, this has inspired me.
Enid Blyton, c. 1955ish
Noddy and Big Ears went to the airport. A nice man there showed them the aeroplanes. “I understand how a propellor pulls a plane along, but these planes have no propellors. They’ve just got those tubes under the wings. How do they fly?” asked Noddy, inquisitively.
“Well, you see Noddy”, said the nice man, “these planes don’t need propellors - they’ve got turbojet engines instead!”
“What’s a turbojet engine?” asked Big Ears, “I’ve never heard of one of those before”
The nice man replied, “Well, Big Ears, a jet engine is like a very clever tube that squirts hot air out the back to push the plane along”
Big Ears was satisfied with this description, but Noddy wanted to know more.
“How does it squirt the hot air out the back, Mr. nice man?”
“Well, noddy - look in the front of the engine - you see all those blades, that’s called a compressor. It sucks in air and squeezes it. This makes it warm.” said the nice man.
“It looks like a big, strange fan” exclaimed Noddy.
“Ho ho ho!” chuckled the nice man, “Yes, it does rather, doesn’t it? If it helps you to understand how it works, you can think of it that way. Just remember that engineers won’t know what you’re talking about if you call it that!”
Noddy, Big Ears and the nice man walked to half-way along the engine.
“In here, we burn fuel” explained the nice man, “This makes the squeezed air very very hot”
“Ooooh,” said Noddy, “does it get really really hot?”
“Yes, it does. In fact, it can get hotter than normal metals can stand, so engineers have made some very very clever metals to withstand it” said the nice man.
The three then walked around the back of the engine.
“Ooh!”, exclaimed Noddy, “that looks just like the thing on the front of the engine, but smaller!”
“That’s right”, said the nice man, “The hot, squeezed air from the middle of the engine goes through those blades very very fast, where it squirts out the back of the engine to push the plane along. Those blades are connected to the blades on the front of the engine too, so the hot air coming out the back makes the compressor work too”.
“Wow”, said Noddy, “It’s amazing what science can do!”
And they all went to the airport terminal to have an over-priced cup of tea.
All entirely technically accurate!
If only 3rd year Propulsion was that user friendly. 
Very good,
but no toast? 
Was thinking just tea and medals. However, given the time medals take to arrive nowadays I think we can let you have some toast in the meantime to go with your tea…
I think it’s very interesting that General Electric, maker of jet engines, says that today’s commercial jets are about 30% more efficient than those of the previous era. On their site they have a multimedia presentation that tells of how the shape of the fans in a turbofan engine are formed with precision, and that the next generation of engines will likely use materials that are even more high tech, thus lightening the engine further. Ofcourse, that kind of efficiency is acheived in part by having the compressor clades in close tollerance with and shaped to take advantage of their close tolerance with the fan housing. Can you imagine what pathetic efficiency you’d get using a compressor fan that is shaped like a household fan blade. It would be hard to get it to run at all I guess, since it’s ability to efficiently induce aur into the engine would be downright pathetic in comparison!
Of particular interest was the comparison chart of their engines which shows that engines that are twice the size of other engines are as much as 3.8+ times as heavy. It’s pretty amazing that one of their engines which is 57 inches in diameter weighs almost 9,000 lbs less than one that is under twice it’s size at 105 inches diameter. I guess you’d have to remove some critial components in the engine to make the bigger one as light as the smaller one. But if you did that, you’d have a jet engine that would be non-functional. Obviously, you can’t strip almost 9,000 lbs of components from an engine and expect it to function.
What was really astounding, however, is that when looking at their turboshaft engines, a change of only 1 inch in diameter can relate to a difference of more than 80 lbs in weight! WOW! Imagine if a jet engine were twice the size as one of those! How much more would it weigh? 4 times as much? 5 times? 6 times as much? My guess is at least 4 but possibly over 5 times as much, judging by the differences seen with the turbofan engines. Even though those engines use axial axial compressors, which makes them lighter than engines which have centrifugal compressors, the dramatic increase in weight with small changes in the diameter of the engine are tremendous. I’s pretty amazing how the weight (mass) increases with small increases in physical size.
Now, I guess you could build an engine with an axial compressor, make it long and thin (sorta counterproductive) and end up with an engine that weighed closer to 1/3 as much as one with twice the diameter. But that would be offset somewhat by the longer engine the casing and at least the shaft and compressor being longer, even with fewer blades. However, if it had no stators, you would be starting with an engine that is very, very inefficient because you have elonbgated it and that would make compression more difficult, if you spanned out the compressor blades. Good information about compressors can be found at NASA btw.
I have quoted you in full merely to stop you from editing after the fact.
Seriously, how many threads do you have to post the same claptrap in before you realise that you are wrong?
What you are doing is really demeaning to yourself. Get a life, get a girlfriend (or boyfriend if that is what floats your boat) and get laid! You are a sad strange little man with unhealthy obsessions.
Hey, I didn’t make this stuff up you know! Complain to GE and NASA!
Yet another utter misunderstanding. The statement wasn’t that an engine using blades shaped like a household fan would be less efficient, rather that an engine operating on the same principle as a household fan would be less efficient and require a major redesign. The GE engine you are describing (and ALL the compressor links you’ve posted to date) do not describe equipment operating on the same principle as a household fan. And blade shape has nothing to do with it.
Yet another utter misunderstanding. The statement wasn’t that an engine using blades shaped like a household fan would be less efficient, rather that an engine operating on the same principle as a household fan would be less efficient and require a major redesign. The GE engine you are describing (and ALL the compressor links you’ve posted to date) do not describe equipment operating on the same principle as a household fan. And blade shape has nothing to do with it.[/quote]
When you get a clue as to what is going on, fill us in.
I now concede that I am wrong. I thought you were either stupid or a pathological liar. It seems you are both stupid and a liar.
The liar part is undisputed - I still want the retraction of your ridiculous statement (above on this post) that I have shown to be false, and maliciously so. A PM’d apology for this has not yet been sent. It’s a shame you are unable to admit when you are wrong, even when the proof is there in black and light brown.
And now for (more) on the stupid bit:
We are discussing turbofans and turbojets. You’ve added turboshafts, which are not actually relevant to the subject under discussion. But anyway, here we go.
Turboshaft Engines
CT7-2A
Diameter: 27 in.
Weight: 429 lbs.CT7-8
Diameter: 26
Weight: 537 lbs.That’s a difference of 1 inch and 108 lbs! Imagine if one were twice the diameter of the other! Whew!
Well observed that the diameter and weights are different. What you have missed is that the heavier engine is in fact the one with the lower diameter. But, even better than that, the heavier engine is more efficient. The specific fuel consumptions are as follows:
CT7-2: 0.481 (units not quoted by GE, but assumed to be consistent)
CT7-8: 0.450
Below, my commments in bold:
Turbofan Engines
CF34-3A
Diameter: 49 inches
Weight: 1,625 lbs.CF34-8C1
Diameter: 52 inches
Weight: 2,350 lbs.That is an increase of only 3 inches diameter, and an increase in weight of 2,185 lbs, which is more than the smaller jet engine weighs! 2185 eh? Looks more like an increase of 725 to me, but lets not let matters like being wrong by a factor of about 3 bother us too much. And now, your second contention - that the increase in weight is more than the weight of the smaller engine is now also wrong. Why bother posting comments that are so obviously wrong? Is this an indication of the rigorous standard of work you usually post?
Let’s look at a few more.
CF34-10D
Diameter: 57 inches
Weight: 3,800 lbs.CF6-50C1/C2
Diameter: 105 inches
Weight: 8,966 lbs.!!!CF6-50E2
Diameter: 105 inches
Weight: 9,047 lbs.!!! Why do engines need exclamation marks? Is there something surprising about them? You’ve shown two engines of the same size weighing different amounts. Whoop-de-do, Basil.
While I was at that link (GE Engines), I also noticed that the Fan and Compressor stages were listed separately. Will you take links provided by yourself as evidence that the fan and the compressor are two different stages in the engine?
In any case, what you have posted is not relevant to the discussion at hand*, which refers to a modification to a reference axial flow turbojet which will result in a large increase in length with negligible increase in weight due to the elimination of the stators. The efficiency would also be reduced. At the time, it was stated that this was not a sensible thing to do. You won’t find an engine designed like this, because it presents no advantages over the conventional stator/rotor layouts. Nobody, except IRONMAN in his ‘build a straw man’ mode, has ever said that changing the size of an engine will never affect the weight. It so happens in te specific example given, that the weight increase due to extending the engine casing is likely to be similar to the weight saved by removing the stators.
- I’ll take the 'compressor fan argument as finished, because I have demonstrated ad nauseum that although there are situations where you might describe a component in a stage as a fan, it is highly unusual. The exception to this would be in a very basic explanation of the concepts involved, aimed at the lay person. It goes without saying that such an explanation has no technical merit. I won’t repeat the argument, as it is now posted in two separate threads, where both times IRONMAN has point blank refused to read it, describing a polite, technically accurate and otherwise well-recieved post as ‘blathering’.
Without wanting to be drawn in to a flamewar:
For the record, comparing engine mass and max. diameter of a turbofan is somewhat wide of the mark. Only one stage - the fan, will be the maximum diameter in a turbofan engine. Not only that, but being the cold end of the engine, you can more reasonably use lighter weight materials - the choice of materials on the fan being driven by the forces generated along the blades, due to rotating at such high speed - without the constraint of high temperature performance.
In the turbine however - which is of much lower diameter than the fan, granted - you will need to be using nickel. This is a material ~1000kg/m^3 denser than steel, twice as dense as titanium, three times as dense as aluminium and almost SIX times as dense as CFRP, the likes of which is now being used for the monstrous fans on GE90’s and the ilk.
So taking the diameter of the least dense part of the engine against which to measure mass is a bit back to front. Engines are optimised heavily for the job they are asked to do - e.g. primarily the thrust required. The thrust required is what really drives the weight of an engine. It will define the by pass ratio (and hence fan size - which is why higher thrust turbofans will appear to be increasing in weight as the fan size goes up - but that’s not the full cause of weight increase), loading on components number of stages, the pressure ratios and temperatures, and therefore materials. The turbofans posted should actually have their mass compared against thrust. I’ve taken the liberty of doing this for the CF6, CF34 and GE90 families of engines.
Comparing mass to thrust produces a pretty-damn-linear relationship (R-squared correlation for an Excel trendline is 0.9879 for a linear trend, 1 being perfect for those sad enough to care 
) despite the fact that there are three distinct families.
Plotting mass against diameter shows a different story. Yes mass increases for those larger engines, but the relationship isn’t nearly so linear (R-squared correlation 0.913… close, but no cigar) between the three families as it is for mass-v-thrust.
So what does this tell us? Basically - the required size of output - e.g. thrust - is what drives the mass of a turbofan engine of a given technology level, as well as the fan size (to a lesser extent). No surprises really but important to point out when the max diameter of one stage (out of around 20), is being thrown around as though it is responsible for the entirety of huge fluctuations in mass for an engine family.
As for the turboshafts - note the 1 inch change in diameter and large change in mass… Then note the large change in power output and the large change in mass. What do you think is really driving the mass of the engine? I also wonder if those mass figures include the gearbox to run the prop/rotor but as yet haven’t been able to find out.