By Pass Ratios?
Thread Starter
Join Date: Nov 2000
Location: Scandiland
Posts: 480
Likes: 0
Received 0 Likes
on
0 Posts
By Pass Ratios?
Hi all!
At the moment I'm teaching powerplants to an ATP theory class. I've got a question about By Pass Ratios that I tend to have trouble explaining.
A straight jet will have a 0 BPR as no air bypasses the hot section, that one's simple.
A turbofan engine will have a bypass of anywhere around 2:1 to 5:1 as there is air bypassing the hot section, not taking part in combustion.
When it comes to a Turboprop, I allways claim that they have an even higher bypass ratio. The engine is driving heaps of air that does not take part in combustion. The students normally follow on this line, but it starts getting silly when discussing it with rotary wing pilots claiming that a turboshaft has an extremely high BPR. At this point students start asking difficult questions and I must admit that I'm not quite sure of where to draw the line. The CT7-B gas generator for example uses all air it ingests, and looking it that way, it would seem it does not have a BPR at all.
What's right and wrong in this?
/LnS
At the moment I'm teaching powerplants to an ATP theory class. I've got a question about By Pass Ratios that I tend to have trouble explaining.
A straight jet will have a 0 BPR as no air bypasses the hot section, that one's simple.
A turbofan engine will have a bypass of anywhere around 2:1 to 5:1 as there is air bypassing the hot section, not taking part in combustion.
When it comes to a Turboprop, I allways claim that they have an even higher bypass ratio. The engine is driving heaps of air that does not take part in combustion. The students normally follow on this line, but it starts getting silly when discussing it with rotary wing pilots claiming that a turboshaft has an extremely high BPR. At this point students start asking difficult questions and I must admit that I'm not quite sure of where to draw the line. The CT7-B gas generator for example uses all air it ingests, and looking it that way, it would seem it does not have a BPR at all.
What's right and wrong in this?
/LnS
Join Date: Mar 2000
Location: Arizona USA
Posts: 8,571
Likes: 0
Received 0 Likes
on
0 Posts
When it comes to a Turboprop, I allways claim that they have an even higher bypass ratio.
IF this were true, the same engine would have a different 'by-pass ratio' if it was installed on another aircraft using a slightly different propellor, one of a lesser or greater diameter, for example.
Thread Starter
Join Date: Nov 2000
Location: Scandiland
Posts: 480
Likes: 0
Received 0 Likes
on
0 Posts
Thanks 411A.
So in that case, BPR is only related to the hotstream and any coldstream airflow that is created by the first fan or compressorstage and not the coldstream airflow that the engine might produce outside the engine cowling?
In that case, how do you explain the Ultra High Bypass engine? That is, where do you draw the line between an unducted fan and a propeller?
/LnS
EDIT: I know I'm splitting hairs a bit here, but that is what my students tend to do, so I'd really like to be able to give them a specific and detailed answer.
So in that case, BPR is only related to the hotstream and any coldstream airflow that is created by the first fan or compressorstage and not the coldstream airflow that the engine might produce outside the engine cowling?
In that case, how do you explain the Ultra High Bypass engine? That is, where do you draw the line between an unducted fan and a propeller?
/LnS
EDIT: I know I'm splitting hairs a bit here, but that is what my students tend to do, so I'd really like to be able to give them a specific and detailed answer.
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,179
Likes: 0
Received 0 Likes
on
0 Posts
What you need to differentiate between, IMHO, is cases where there is a true bypass ratio, and cases where you are saying "as if it had" a bypass ratio.
You can make a assessment of the effect of bypass ratio change on the design and off-design characteristics of a turbojet/turbofan type engine - and incidentally, bypass ratios don't start at 2:1, there are cases where the bypass ration is between 1 and 2 - sometimes called, at least partly in jest, a "leaky turbojet".
You can also determine a "pseudo bypass ratio" for things like UDF and turboprops, and you'll find that the same design relationships hold true to a greater or lesser extent. So it's fair to say, when talking at a fairly high level, that a turboprop performs in some respects as if had a bypass ratio of xx.
As long as you make the distinction between a "pure" bypass ratio and a "pseudo" bypass ratio I think you should be fine.
An analogy is often a useful teaching device - just be careful that it is clear that it is an analogy, and is of restricted application.
For example, I might say that a certain size of steel beam is as strong as if it were an oak beam of a certain size, to allow a student to comprehend the relative difference in strengths. As long as its clear that's all you are doing, no-one should ask silly questions about burning steel beams, or building rafts out of them, or ...
You can make a assessment of the effect of bypass ratio change on the design and off-design characteristics of a turbojet/turbofan type engine - and incidentally, bypass ratios don't start at 2:1, there are cases where the bypass ration is between 1 and 2 - sometimes called, at least partly in jest, a "leaky turbojet".
You can also determine a "pseudo bypass ratio" for things like UDF and turboprops, and you'll find that the same design relationships hold true to a greater or lesser extent. So it's fair to say, when talking at a fairly high level, that a turboprop performs in some respects as if had a bypass ratio of xx.
As long as you make the distinction between a "pure" bypass ratio and a "pseudo" bypass ratio I think you should be fine.
An analogy is often a useful teaching device - just be careful that it is clear that it is an analogy, and is of restricted application.
For example, I might say that a certain size of steel beam is as strong as if it were an oak beam of a certain size, to allow a student to comprehend the relative difference in strengths. As long as its clear that's all you are doing, no-one should ask silly questions about burning steel beams, or building rafts out of them, or ...
Join Date: Aug 2001
Location: Dorset
Posts: 775
Likes: 0
Received 0 Likes
on
0 Posts
So in that case, BPR is only related to the hotstream and any coldstream airflow that is created by the first fan or compressor stage and not the coldstream airflow that the engine might produce outside the engine cowling?
The BPR is the ratio of the cold stream flow (which passes through the by-pass duct), to the hot stream flow (which passes through the engine core).
It is also worth noting that only a small fraction of the hot stream (core flow) is actually used in combustion, but it all passes through the turbines.
Strictly speaking the term BPR is not applicable to engine/propeller combinations, and this appears to be where you are going wrong.
But that does not mean that the matter should not be discussed in ATPL classes. It should be discussed, because it is fundamental to the subjects of stall/surge avoidance and also propulsive efficiency.
To teach the subject you should be fully familiar with how the concept of by-pass developed. It was first used to reduce the problems of compressor surge/stall, then was gradually developed to increase propulsive efficiency.
In that case, how do you explain the Ultra High Bypass engine? That is, where do you draw the line between an unducted fan and a propeller?
The engineers that designed the first ultra-low by-pass engines (they were known in some circles as "leaky trubojets" ) to reduce surge/stall couldn’t possibly have envisaged the development of high by-pass turbofans or unducted fans. So we should not be surprised if we are now coming up with problems in deciding how to classify such engines.
EDIT: I know I'm splitting hairs a bit here, but that is what my students tend to do, so I'd really like to be able to give them a specific and detailed answer.
In order to be able to teach a subject properly your level of knowledge and understanding of the subject really needs to be at least one level above that at which you are teaching. This provides a bit of a buffer to help deal with the more gifted (probing) students.
A good place to start reading would be the book “The Jet Engine” by Rolls Royce. It covers the subject at the appropriate level for ATPL studies, without burying you in too much mathematics.
Join Date: May 2002
Location: UK
Posts: 140
Likes: 0
Received 0 Likes
on
0 Posts
there are cases where the bypass ration is between 1 and 2 - sometimes called, at least partly in jest, a "leaky turbojet]
We were taught that the term by-pass referred to an engine where although the air by-passed the combustion chambers it was retained in ducting within the engine case.
Where fan /compressor air passed along the outside of the powerplant casing then this engine was called a turbo -fan
Join Date: Aug 2001
Location: Dorset
Posts: 775
Likes: 0
Received 0 Likes
on
0 Posts
Where fan /compressor air passed along the outside of the powerplant casing then this engine was called a turbo -fan
But if you look at the majority of the engines fitted to modern fighter aeroplanes, the situation is completely different.
When you open the MVP bag to take out the replacement engine, you find a large cylindrical object that comes complete with the fan, engine core, by-pass duct and reheat system. In the UK this cylindrical object is commonly called an "Engine Change Unit" (ECU). But these engines are still called turbofan engines.
If we want a description that fits all types of turbofan we need to use something like.
1. The hot air stream passes through the engine core. Some of this air is used for combustion and some is used for cooling.
2. The by-pass air stream passes around the outside of the engine core. This air by-passes most of the compressor stages and all of the turbines.
3. In some turbofans the hot and cold streams are mixed back together inside the jet pipe and in other turbofan the mixing takes place externally.
Join Date: Jan 2005
Location: France
Posts: 2,315
Likes: 0
Received 0 Likes
on
0 Posts
Nobody would seriously call the Concorde Olympus 593 a "by-pass" engine.
Yet even there some of the air from the inlet passed around the engine, not just for cooling purposes... it was injected at the back around the 'hot' exhaust, and both 'constrained' the exhaust, thereby slightly increasing thrust, and reduced noise.
CJ
Yet even there some of the air from the inlet passed around the engine, not just for cooling purposes... it was injected at the back around the 'hot' exhaust, and both 'constrained' the exhaust, thereby slightly increasing thrust, and reduced noise.
CJ
Thread Starter
Join Date: Nov 2000
Location: Scandiland
Posts: 480
Likes: 0
Received 0 Likes
on
0 Posts
Thanks all for your enlightening answers.
Indeed I'm learning as I teach, perhaps levels in this could be discussed. I'm in awe of the knowledge on this forum (some times...).
I'm getting a pretty good idea of what this is about. The question about mixing the bypassed air with the core air is brought up in the "exhaust" chapter where I teach the method of external mixing and internal mixing.
Would it be true to say that the 737 NG (with CFM engines) has external mixing? I would still claim this to be a high bypass engine, even though the outer cowling does not reach beyond the aft part of the engine core (which in turn goes one step to it being an unducted fan and then on to a propeller and so on...)
The typical Airbus with RR engines (two spool I believe?) I've understood uses internal mixing? The only difference from my point of view is that the outer cowling reaches further aft than its CFM counterpart and allows bypass air and core air to mix before exiting rearwards.
Both have fans, both have a bypass ratio, but the method of mixing differs.
/LnS
Indeed I'm learning as I teach, perhaps levels in this could be discussed. I'm in awe of the knowledge on this forum (some times...).
I'm getting a pretty good idea of what this is about. The question about mixing the bypassed air with the core air is brought up in the "exhaust" chapter where I teach the method of external mixing and internal mixing.
Would it be true to say that the 737 NG (with CFM engines) has external mixing? I would still claim this to be a high bypass engine, even though the outer cowling does not reach beyond the aft part of the engine core (which in turn goes one step to it being an unducted fan and then on to a propeller and so on...)
The typical Airbus with RR engines (two spool I believe?) I've understood uses internal mixing? The only difference from my point of view is that the outer cowling reaches further aft than its CFM counterpart and allows bypass air and core air to mix before exiting rearwards.
Both have fans, both have a bypass ratio, but the method of mixing differs.
/LnS
Join Date: Feb 2009
Location: Jungles of SW London
Age: 77
Posts: 354
Likes: 0
Received 0 Likes
on
0 Posts
Low n' slow
The typical Airbus with RR engines (two spool I believe?) I've understood uses internal mixing? The only difference from my point of view is that the outer cowling reaches further aft than its CFM counterpart and allows bypass air and core air to mix before exiting rearwards.
As for 'mixing', I always understood that the fan air - the bypass air - formed a tube of relatively slow, quite cool air that provided the vast amount of thrust typical in high bypass engines. This tube is coaxial to and surrounds the very hot, fast moving gas from the core engine, which is why the modern high bypass engine is so much quieter than its straight turbojet predecessor. I don't know, but my guess would be that in any manufacturers engine, the mixing of these two, dramatically different streams of gas happens tens, even hundreds of feet behind the aeroplane.
In the case of the turboprop engine, the useful thrust of the engine is produced by the propeller - I guess there is some pure jet thrust from the 'generator' - so, in my truly humble opinion
, the volume of air displaced by the propeller compared to the volume of gas coming out of the 'generator/core engine' must represent the bypass ratio? I'll get me coat.
As an entirely gratuitous aside, did you know that the engine Frank Whittle really wanted to build was a two spool high bypass engine? He realised that the engine he visualised would only be really powerful and efficient when it produced a very large volume of slow moving air, as opposed to the low volume of noisy, fast moving air that 'jets' do.
Of course he had champagne ideas and beer support
and had to make do with a thrice used, much battered and re-machined centrifugal compressor on a single spool. He did, however, patent water injection; prop/jets; multispool jets; (high) bypass jets and many more ideas long before they were technically possible.By the time they were possible and fashionable, the patents had all expired and, in any case, the British Government had all but given the jet engine to the Americans.
As I said, I'll get me coat.
Roger.
Thread Starter
Join Date: Nov 2000
Location: Scandiland
Posts: 480
Likes: 0
Received 0 Likes
on
0 Posts
Landroger, you haven't stepped on any toes!
It's allmost embarrasing to think that I'm supposed to teach this comparing my knowledge to what can be found on this forum...
Thanks for all replies!
/LnS
It's allmost embarrasing to think that I'm supposed to teach this comparing my knowledge to what can be found on this forum...
Thanks for all replies!
/LnS
Join Date: Aug 2010
Location: Munich, Germany
Posts: 22
Likes: 0
Received 0 Likes
on
0 Posts
What's right and wrong in this?
I could imagine, the "helicopter guys" claim a turboshaft engine in a helicopter has a higher BPR than the same engine fitted in a fixed wing aircraft and i agree in some way with them, even when most of the "bypass air" is used for lift and not for propulsion.
An example to make this more clear would be the Bell V-22, the geometric of the engine doesn't change, even if it's tilled up and the V-22 flies like a helicopter.
Of course it's very unusual to claim, turboshafts of a helicopter have a high BPR.
The amount of thrust produced by the hot-section can differ from engine to engine, on a low BPR engine it maybe more than the cold stream, on some turbo props the residual thrust amount is very low. (the RR Tyne MK22 delivers ~4200kw shaft power with a residual thrust of 5kn ).
Drawing the line between low bypass, high bypass, ultra high bypass and props isn't easy, I've some literature in which is a low bypass: ~2:1, high: 4-9:1, ultrahigh for UDF's 12-15:1 (or maybe more) and props up to 1:40 and even more.
Important for the usage, of which BPR where to install, is the operating place and for fixed wing aircrafts this means the operating speed. As you know, with a higher the bypass the engines work more efficiently but this can't be spread to all kind of A/C, the exhaust speed is also important (an A/C can't fly faster than the exhaust speed) and along with this goes the pressure ratio of the bypassed air.
So a "mega-bypass" like a helicopter would accelerate a lot of air mass with a more or less low speed, a prop accelerates less air mass but to a higher air-speed (the "thrust" or the needed shaft power might be the same) and a turbofan does even less air mass with higher air speed and with a certain fan-pressure-ratio. This pressure is converted in the nozzle to additional speed by reducing the pressure to approximately average air pressure (this ensures, that the engine operates efficiently and the exhaust gas flow doesn't spread up after leaving the engine ).
Thats also the reason why fighter jet engines with afterburner have a reduced BPR but with a higher fan pressure ratio than commercial A/C's. So turbo-props work fine up to ~M0.7, fans high BPR up to ~M0.95 (or little more) and above this speed a low BPR engine would be the choice, AFAIK the Concorde was able to fly supersonic with afterburners off, once reached supersonic speed.
rgds
Join Date: Feb 2009
Location: Jungles of SW London
Age: 77
Posts: 354
Likes: 0
Received 0 Likes
on
0 Posts
Peoplesx
Thats also the reason why fighter jet engines with afterburner have a reduced BPR but with a higher fan pressure ratio than commercial A/C's. So turbo-props work fine up to ~M0.7, fans high BPR up to ~M0.95 (or little more) and above this speed a low BPR engine would be the choice, AFAIK the Concorde was able to fly supersonic with afterburners off, once reached supersonic speed.
I'm afraid I'm a chain, hundredweight, fortnight man myself. However, having seen your location, I suspect my poor attempt at humour will have passed you by without 'troubling the scorers'. Oh dear, I've probably done it again with a cricket reference. Seriously, I'm sure we all pretty much agree with what you say, but with reference to your words that I've highlighted, may I recommend the 'Concorde thrust from intake' thread? Not only does it have a bearing on the subject of bypass and all its aspects, it is absolutely fascinating reading - if you don't mind a bit of painless brain damage.
(Well it was brain damage for me.
)Roger.
Join Date: Jan 2005
Location: France
Posts: 2,315
Likes: 0
Received 0 Likes
on
0 Posts
CJ
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
I once taught an introductory propulsion course - and I made it clear that I was comparing Propulsion systems, and not merely comparing engines.
As a result, every system from a pure jet to a helo had, by definition, a bypass ratio. And any application using variable-pitch cold stream blades indeed had a variable bypass ratio.
And the chart I used was propulsion efficiency (y-axis) vs. airspeed (x-axis). A helo could create huge thrust (ie lift) at low speed, but ran out of steam quickly as IAS increased. A pure jet was relatively inefficient until reaching high speed; and the turboprop and turbofan lay somewhere in between.
As a result, every system from a pure jet to a helo had, by definition, a bypass ratio. And any application using variable-pitch cold stream blades indeed had a variable bypass ratio.
And the chart I used was propulsion efficiency (y-axis) vs. airspeed (x-axis). A helo could create huge thrust (ie lift) at low speed, but ran out of steam quickly as IAS increased. A pure jet was relatively inefficient until reaching high speed; and the turboprop and turbofan lay somewhere in between.
