JoeCanuck
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### Big Fan, Little Fan...NEO V. RE...

I have read a number of papers and articles regarding the relative merits of fan sizes on bypass engines...(and I wish I could find them again). As I recall, the gist is that while a larger fan moving a mass of air slowly is inherently more efficient than a smaller fan moving the same mass of air more quickly but only by a relatively small amount and the effect diminishes with increased aircraft speed and altitude.

Of course, the smaller fan would produce more noise but I'm more interested in fuel efficiency.

So to the experts in here; What would the approximate fuel efficiency differences be of an 80 inch fan be over a 70 inch fan, pushing the same amount of air, (so maintaining the same BPR), at takeoff, climb and cruise? This is assuming the same size core.

What the...?

tdscanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting JoeCanuck (Thread starter):So to the experts in here; What would the approximate fuel efficiency differences be of an 80 inch fan be over a 70 inch fan, pushing the same amount of air, (so maintaining the same BPR), at takeoff, climb and cruise? This is assuming the same size core.

Insufficient data...BPR is the ratio of the bypass flow to the core flow. It depends on how big your core flow is relative to the fan.

Yes. Fan efficiency plays into the total propulsive efficiency.

Tom.

JoeCanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting tdscanuck (Reply 1):Insufficient data...BPR is the ratio of the bypass flow to the core flow. It depends on how big your core flow is relative to the fan.

Right...and theoretically, a smaller fan can achieve the same BPR as a larger fan by moving the same mass of air around the core, by having the bypass air move at a faster rate.

Smaller fan, faster bypass air....Larger fan, slower bypass air = same mass of air bypassing the core over time = same BPR...at least by the definition of BPR as the ratio of mass of air bypassing the core to the mass of air going through the core.

For example; can a leap X engine with an 70 inch fan, get close to the same fuel efficiency as the same core with an 80 inch fan? Can the smaller fan accelerate the bypass air enough to push the same mass as the larger fan to achieve the same bypass ratio?

Could this faster bypass air, (even if it can achieve the same BPR), allow similar efficiencies as the engine with the slower bypass air of the same mass?

Or, will the smaller fan always be at an efficiency handicap regardless if it manages to achieve the same BPR?
What the...?

flipdewaf
Posts: 1814
Joined: Thu Jul 20, 2006 6:28 am

### RE: Big Fan, Little Fan...NEO V. RE...

I think BPR can be a very misleading number and is better used when playing top trumps than assessing the efficiency. BPR is more of a result than a driver. (proviso here is that ther is a lot more to selecting engine than going for a big fan and it all has to integrate on the aircraft as a sytem)

I think its better to go back to basics and see why we go for big fans in the first place.

The laws of physics (on a very basic level) give thrust produced as F=Ma
or
Thrust = Mass flow rate * velocity increase through the engine (T=M'*dV)

The energy input into the air (KE) is KE = 1/2*M*dV^2

M = Mass and power is energy/time

so to get power in the air you have to replace mass (M) with mass flow rate (M')

So Power (P) = 1/2*M'*dV^2

If you put these together you can see that if you use the same tube (engine), to get twice the thrust you will require twice the change in Velocity (simples) but this will take 4 times the energy, Ruh Roh!!

Solution?

Make the pipe twice the area and keep the velocity the same, twice the trust but only twice the energy, Woop Woop!

Ok thats the begginers science lesson over

Here's how it stacks up for the different NEO/RE aircraft.

Very simply derived but provides a little insight to the differences.

NOTE: The aircraft is a system and there are other things then SFC so take with a huge pinch of salt.

Fred

keesje
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### RE: Big Fan, Little Fan...NEO V. RE...

Nearly half of the better fuel efficiency of the LEAP comes from propulsive efficiency through increased bypass ratio.

The LEAP-X will deliver 15% lower fuel burn and thus CO2 and a 50% reduction in NOx and will be up to 15dB quieter than Stage 4 while offering the same reliability and maintenance cost as the current CFM56, Brown claimed. This has been achieved by incorporating much of the GE90 and GEnx technology into the new engine, he said. The 15% lower fuel burn will come from a 45% higher propulsive efficiency through increased bypass ratio and lower weight combined with 45% higher thermal efficiency and integrated engine systems, he said.

http://atwonline.com/eco-aviation/ar...tical-success-greener-engines-0714

I guess the LeapX engines for the 737RE and A320 will have almost identical cores. Thermal efficiency will be more or less the same.

The weight reduction of few hundred pounds of a smaller fan is nice, but also neglectible on 150.000 lbs aircraft.

So if ~45% of a ~15% improvement is based on BPR that's about 7%. If one aircraft has a 35% lower BPR that makes about 2.5% structural worse sfc.

Do that over 200 a/c over 20 years with oil at \$100..
"Never mistake motion for action." Ernest Hemingway

JoeCanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

Thanks Fred...that's exactly the kind of information I am looking for.I realise at this point we can only deal with generalities but this information makes a good place to start.

If you don't mind, I'd like to probe your mind a bit further.

How might the differences in bypass air velocity, (from larger fan to smaller fan), effect sfc in different flight regimes? For example, I was under the impression that the efficiency differences in sfc were greatest at the greatest differences in intake air, (aircraft), velocities...for instance, the faster the aircraft, the smaller the efficiency difference between the smaller and larger fan.

How might the relative differences be at, for example, take off, climb and cruise? Where would the greatest advantages be and where would the greatest disadvantages be, for either fan diameter?

Or are there any differences?
What the...?

KELPkid
Posts: 5247
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### RE: Big Fan, Little Fan...NEO V. RE...

Has Boeing officially removed lengthening the 737 landing gear as an option for the RE?
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

JoeCanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

It depends on which fan size option, (out of 4 possibilities), they choose.
What the...?

flipdewaf
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### RE: Big Fan, Little Fan...NEO V. RE...

Cycling all day tomorrow and have to spend sunday doing "couple" (read girl) things so i'll see if I can bash some numbers together at some point.

Fred

JoeCanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

That would be excellent...no rush. Have a good ride...and happy antiquing...(or whatever).
What the...?

tdscanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting keesje (Reply 4):Nearly half of the better fuel efficiency of the LEAP comes from propulsive efficiency through increased bypass ratio.

That's not what the quote says. It says they have a 15% improvement in fuel burn through a 45% improvement in propulsive efficiency and a 45% improvement in thermal efficiency. It doesn't say what the split is between the two because you don't know what the original efficiencies were.

For example, if thermal efficiency was 20% before and is now ~28% but propulsive efficiency was 40% and is now ~57%, you're getting a way bigger improvement on the propulsive efficiency side.

Tom.

dynamicsguy
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting keesje (Reply 4):So if ~45% of a ~15% improvement is based on BPR that's about 7%. If one aircraft has a 35% lower BPR that makes about 2.5% structural worse sfc. Do that over 200 a/c over 20 years with oil at \$100.

You can't ignore the results of having to integrate the engine with the larger fan onto the airplane. Higher drag, higher weight, higher development costs to be recouped for greater changes to the airplane, etc. will eat into that advantage pretty quickly. Those numbers are for the engine and ignore the fact that some of the gains in the engine will be lost in the rest of the system.

astuteman
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting dynamicsguy (Reply 11):Higher drag, higher weight, higher development costs to be recouped for greater changes to the airplane, etc. will eat into that advantage pretty quickly

Nevertheless, although the fan diameter needs to be considered as part of a system in order to understand what really happens, even a cursory look at the top level tells us that fan diameters are steadily rising for a given weight and thrust, and can indicate where the "optima" on these latest generation engines seem to lie.

The CFM 56-7 on the 737 has a fan diameter of 61", on planes that have MTOW's of 77t - 85t
The CFM 56-5 on the A320 has a fan diameter of 68" on planes that have MTOW's of 75t - 93t

Pratts Geared turbofans look like this.

The 1200G on the MRJ has a fan diameter of 56" on a plane that has MTOW's of 37t -43t
The 1500G on the C-Series has a fan diameter of 74" on a plane that has MTOW's of 55t - 63t
The 1100G on the A320NEO has a fan diameter of 81" on a plane that has MTOW's of 76t - 94t

Boeing are looking at a maximum of 70" for the 737RE, (which they say will need an 8" increase in nose gear height to achieve).

It's clear there are optima, as you describe, at which point all the parts of the system work together best.

It's not clear that 70" or smaller is the optimum size using the latest technology for a plane the size and weight of a 737RE, when the C-series engine is 74". and that of the A320NEO is 81".

Unless Bombardier and Airbus just hang huge weights under the wing just for the hell of it

Rgds

Stitch
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting astuteman (Reply 12):It's not clear that 70" or smaller is the optimum size using the latest technology for a plane the size and weight of a 737RE, when the C-series engine is 74". and that of the A320NEO is 81".

Well it is an academic question since the GTF will not be available for the 737RE. Only LEAP-X.

dynamicsguy
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting astuteman (Reply 12):Nevertheless, although the fan diameter needs to be considered as part of a system

Absolutely. Just pointing out that you can't take the engine-level improvement and then expect to see that same improvement for the whole airplane. 2.5% better sfc does not equal a 2.5% improvement for the airplane as keesje seemed to be implying. That is particularly the case when adapting an existing airplane.

keesje
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### RE: Big Fan, Little Fan...NEO V. RE...

The consequences of a smaller fan for a Boeing are not welcome for some Boeing fans I guess. Better deny / question / ask proof. The reason Boeing puts bigger fans on the 747, 777, 787 and 737 is better efficiency, despite drag weight and whatever variable can be brought up to mistify a looming unwanted efficiency compromise on the 737RE's powerplants.
"Never mistake motion for action." Ernest Hemingway

tdscanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting keesje (Reply 15):The consequences of a smaller fan for a Boeing are not welcome for some Boeing fans I guess.

Generally a true statement, but incorrect statements backed by faulty engineering about the expected gains or impacts of different fan sizes are less welcome still.

I don't think anyone is denyting that, all other things being equal, more bypass is better for fuel efficiency. But all other things aren't equal.

 Quoting keesje (Reply 15):The reason Boeing puts bigger fans on the 747, 777, 787 and 737 is better efficiency, despite drag weight and whatever variable can be brought up to mistify a looming unwanted efficiency compromise on the 737RE's powerplants.

That's the reason they put more *bypass* if they can. That's not the same thing as fan diameter, and looking just at bypass or fan diameter does not tell you much about overall system efficiency, which is what customers actually care about.

I'm quite sure that Boeing would strap an A320NEO-sized fan to the 737 if they thought it made sense, but they're out to optimize the entire system. That includes everything, not just fan size.

Tom.

astuteman
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting dynamicsguy (Reply 14):Just pointing out that you can't take the engine-level improvement and then expect to see that same improvement for the whole airplane. 2.5% better sfc does not equal a 2.5% improvement for the airplane

And rightly so

 Quoting Stitch (Reply 13):Well it is an academic question since the GTF will not be available for the 737RE. Only LEAP-X.

True.

I couldn't find a fan size for the Leap-X on either the C919 or A320NEO.
But on the basis that the two GTF's on the A320NEO only weigh 0.1t more than the two Leap-X's, I'd be surprised if the fan were materially smaller...
I'd be interested to find out, to be fair

Rgds

B777LRF
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### RE: Big Fan, Little Fan...NEO V. RE...

There are indeed quite a few attempts to discount the advantages of a larger fan, primarily citing drag and weight issues. This argument is very selective indeed, and tries to make hay of the fact that a 5% increase in fan diameter does not translate directly to a 5% efficiency gain.

Now I don't have the numbers at hand, and suspect the same holds true for every other poster on this thread, but just looking at the engines offered for sale today makes it pretty damn obvious that a bigger fan = better efficiency. And it should be equally obvious that if you take an engine optimised for a large fan, and then bugger around with it to make it work with a smaller one, the result will always be a compromise, and thus yield worse performance figures.

If the 737RE doesn't get pimped to the fullest extent (raised gear front and aft, new box, revised wing) it's going to fall short of the A320neo. How far short depends on the investment Boeing and CFM are willing to make, or rather lack thereof. Cause I don't think Boeing have either the funds, owner backing or corporate inclination to do anymore than what is absolutely necessary, and that means CFM will have to spend an even bigger amount, buggering around to get a somewhat acceptable Hop-X out of a Leap-X. Or, rather, Boeing will have to pay CFM to bugger about. Which will make the RE more expensive, a fact that has been somewhat overlooked so far on this thread.
From receips and radials over straight pipes to big fans - been there, done that, got the hearing defects to prove

JoeCanuck
Topic Author
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting keesje (Reply 15):The consequences of a smaller fan for a Boeing are not welcome for some Boeing fans I guess. Better deny / question / ask proof.

Thanks for assuming to know my mind better than I. What do you get out of coming into what has been a mostly civil thread and throwing in a stink bomb? Do you feel better now?

Since when is it wrong to ask for more information? Why do you automatically assume it has to be a fanboi quest as opposed to why I asked the original question; to learn from people who actually have some technical expertise in the field and can offer information which I, and others, could find enlightening and interesting...which is the response most in this thread have given.

Of course there are differences but instead of just blindly following the "big fan is better just because", kindergarten line, I was looking for explanations from grownups...details about something which is complex, topical and interesting...at least for me, which is why I started the thread.

Why is a big fan better? Is it better in all flight regimes? In what flight regimes is it optimised? What can be done with a smaller fan to help mitigate the differences? Are there flight regimes where there are advantages for a smaller fan? What are the handicaps with matching the BPR of the larger fan by spinning the smaller fan faster, (which Airbus is doing for the 350-1000)?

These are some things I like to understand better, for no other reason that I find the topic interesting and I like to learn. Does that meet your lofty standards of thread quality control?

[Edited 2011-07-24 01:41:32]
What the...?

flipdewaf
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting tdscanuck (Reply 16):I don't think anyone is denyting that, all other things being equal, more bypass is better for fuel efficiency. But all other things aren't equal.

Not totally true,

Engine 1:
Diameter:50"
Core flow:50kg/s
Bypassflow:450kg/s
BPR:9:1

Engine 2:
Diameter:50"
Core flow:50kg/s
Bypass flow: 600kg/s
BPR:12:1

but from a propulsive efficiency stand point engine 1 will be more efficient.

BPR is an easily comparable result of trying to get the efficiency not the driver.

Fred

BrouAviation
Posts: 280
Joined: Fri Jun 05, 2009 6:31 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting JoeCanuck (Reply 19):Why is a big fan better? Is it better in all flight regimes?

AFAIK, increasing by-pass without increasing Pressure Ratio (and as such, increasing BPR) kills your take-off performance. BPR makes for efficient cruising, but what does it matter when you can't take a decent amount of payload in the first place?
Increasing pressure ratio increases your TIT (turbine inlet temperature) which means you need better, but most important more expensive turbines.
Furthermore: a larger fan is especially efficient at slow rotation speeds, while your compressors need sometimes three times as much to be at their most efficient. So when you increase your fan, it is bound to become less efficient when you want to maintain your pressure ratio with the same compressors.

That is essentially why GTF is such a good idea: Allows both your fan and compressor to operate at their most efficient speeds, making substantial increase of Pressure Ratio possible without adding more compressor discs.

If GTF really works, combined with the new CNC-technology which makes higher temperatures possible, I can see turbofans becoming 25-30% more efficient in the coming two decades, making the open rotor concept superfluous for quite some time to come I'd say.
The only thing we have to see now is if the GTF, and especially its planetary gearbox, is as reliable and easy to maintain as P&W promises us.

Never ask somebody if he's a pilot. If he is, he will let you know soon enough!

astuteman
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting tdscanuck (Reply 16):I'm quite sure that Boeing would strap an A320NEO-sized fan to the 737 if they thought it made sense

I'd be interested to know why CFM, Pratt AND Airbus would all undertake something that didn't make sense....

To me, I'd have thought that if a 66" (say) to 70" fan optimised the system on the A320NEO, then that is what the engine makers and Airbus would have done.
But they haven't. They've gone for 81"
And presumably within that done as much as they can to miminise core size, and thus the fan diameter needed for a given efficiency.

Rgds

tdscanuck
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting B777LRF (Reply 18):This argument is very selective indeed, and tries to make hay of the fact that a 5% increase in fan diameter does not translate directly to a 5% efficiency gain.

It's not so much a selective argument as not understanding where to apply efficiency numbers. A 5% increase in fan diameter doesn't even cause a 5% increase in bypass ratio. On top of that, a 5% increase in bypass ratio doesn't cause a 5% change in propulsive efficiency (and causes some negative effects on nacelle drag and engine weight). And a 5% improvement in propulsive efficiency doesn't cause a 5% change in SFC, which in turn doesn't cause a 5% change in fuel burn, which in turn doesn't cause a 5% change in CASM.

Arguments that the LEAP-X on the 737 is just as efficient an *engine* as the LEAP-X on the NEO are ignoring reality, but arguments that actually try to put numbers to the situation are frequently falling down by not correctly applying the right measures. Put bluntly, a 5% hike in fan diameter doesn't provide such a large CASM change that it can't be absorbed by other things. That doesn't mean it will be, but it also doesn't mean it won't be.

 Quoting B777LRF (Reply 18):Now I don't have the numbers at hand, and suspect the same holds true for every other poster on this thread, but just looking at the engines offered for sale today makes it pretty damn obvious that a bigger fan = better efficiency.

Bigger fan = better *propulsive efficiency, all other things being equal. But all other things aren't equal, and propulsive efficiency is only one ingredient in CASM, which is what airlines actually care about. There's nothing wrong with arguing that the A320NEO has a leg up because of a larger fan, but it's really myopic to insist that a larger fan is the end of the argument.

 Quoting B777LRF (Reply 18):And it should be equally obvious that if you take an engine optimised for a large fan, and then bugger around with it to make it work with a smaller one, the result will always be a compromise, and thus yield worse performance figures.

That is *extremely* mission dependent. Given CFM's relationship with Boeing, it's just as likely that the LEAP-X core is optimized for the 737 and they buggered around with it to make it work with a larger fan for the A320.

 Quoting flipdewaf (Reply 20): Engine 1: Diameter:50" Core flow:50kg/s Bypassflow:450kg/s BPR:9:1 Engine 2: Diameter:50" Core flow:50kg/s Bypass flow: 600kg/s BPR:12:1 but from a propulsive efficiency stand point engine 1 will be more efficient.

Not really following you on this one...given equal turbine & fan efficiencies, propulsive efficiency is a fairly simple function of bypass ratio, so I don't see how you'd get that engine 1 has better propulsive efficiency.

 Quoting astuteman (Reply 22):Quoting tdscanuck (Reply 16): I'm quite sure that Boeing would strap an A320NEO-sized fan to the 737 if they thought it made sense I'd be interested to know why CFM, Pratt AND Airbus would all undertake something that didn't make sense....

Not following you on this one either...neither CFM, nor Pratt, nor Airbus are trying to strap an A320NEO-sized fan to the 737.

Tom.

BrouAviation
Posts: 280
Joined: Fri Jun 05, 2009 6:31 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting tdscanuck (Reply 23): Not really following you on this one...given equal turbine & fan efficiencies, propulsive efficiency is a fairly simple function of bypass ratio, so I don't see how you'd get that engine 1 has better propulsive efficiency.

Forgive me if I am saying something stupid, but hasn't propulsive energy everything to do with actual cruise speed and the airframe it is fitted to, and the ratio of that two?
An engine switched on, running idle on the platform has a Np of exactly zero, whereas the most propulsive efficient engine is actually one that is switched off.

EDIT: Are there significant differences in cross section areas of both A320/B738? With the same amount of payload, which model requires a higher cruise power setting to maintain M.82 for example, and how much more exactly?

[Edited 2011-07-24 06:56:39]
Never ask somebody if he's a pilot. If he is, he will let you know soon enough!

flipdewaf
Posts: 1814
Joined: Thu Jul 20, 2006 6:28 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting tdscanuck (Reply 23):given equal turbine & fan efficiencies
 Quoting tdscanuck (Reply 16): But all other things aren't equal.

Which is it?

 Quoting tdscanuck (Reply 23):so I don't see how you'd get that engine 1 has better propulsive efficiency.

Well work out the energy required for each pound of thrust and you will see.

The 737 and the A320 are as equal as it is possible to find in the aviation world and it is very difficult to see how the 737-leap-X at 70" is a more otimal overall solution then A320-leap-x at 75". The issue at stake is not that the 737 + 70" fan is the best solution available to boeing and that it is worse for them to to put a bigger fan on that airframe but that the limit of 70" is infact an efficiency limiting contraint.

I think we need to get back to the original topic of how the fan diameter can effect overall efficiency/CASM of the aircraft instead of hiding behind aircraft being a complete system that needs optimising as a system, it is easy to do that in any thread in the Tech/ops forum and can take any topic to a death if you dont have info to the Nth degree on an aircraft which is not conducive of a good thread .

Back on topic,

thanks

Fred

Stitch
Posts: 24022
Joined: Wed Jul 06, 2005 4:26 am

### RE: Big Fan, Little Fan...NEO V. RE...

I don't doubt the A320neo will be better than the 737RE, but if it was going to be so much better that the 737RE would not be competitive, then Boeing would not have launched it and would have continued work on the NSA, even if that means losing customers like AA, UA and DL in order to be sure they had a plane that would keep WN and FR and win them the bulk of RFPs for deliveries in the 2020s.

BrouAviation
Posts: 280
Joined: Fri Jun 05, 2009 6:31 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting flipdewaf (Reply 25):Well work out the energy required for each pound of thrust and you will see.

But propulsive energy has nothing to do with that. Propulsive efficiency is about how much energy is needed for travelling at a certain speed. And that is different per engine/airplane combination. Hence my question about the differences between Airbus and Boeing's narrowbodies. So if we really want to compare Big Fan, Little Fan, as in the thread title, we should not forget about the last part ...NEO V. RE...
Never ask somebody if he's a pilot. If he is, he will let you know soon enough!

flipdewaf
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### RE: Big Fan, Little Fan...NEO V. RE...

That might have to do with the different wordings used, we were tought at uni that propulsive efficiency was how the power taken from the low pressure turbine was used to create the bulk of the thrust (from the fan). This did involve losses from the nacelle and shaft losses etc but i think these can be ignored here.

Fred

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting BrouAviation (Reply 24):Forgive me if I am saying something stupid, but hasn't propulsive energy everything to do with actual cruise speed and the airframe it is fitted to, and the ratio of that two?

Propulsive *energy* is related to actual cruise speed, but not to the airframe the engine is fitted to. Propulsive efficiency, when comparing two engines at this level, isn't related to speed or airframe (it does change with speed, but it changes in the same way for similar engines so it's probably not meaningful in the 737RE vs. A320NEO discussion).

 Quoting BrouAviation (Reply 24):An engine switched on, running idle on the platform has a Np of exactly zero, whereas the most propulsive efficient engine is actually one that is switched off.

You're mixing work done with propulsive efficiency. An engine at idle on the thrust stand is doing no work...it still has a non-zero propulsive efficiency.

 Quoting BrouAviation (Reply 24):EDIT: Are there significant differences in cross section areas of both A320/B738? With the same amount of payload, which model requires a higher cruise power setting to maintain M.82 for example, and how much more exactly?

The cross-sections are a little different but that's not the main driver. The 737 is designed lighter and designed around a higher speed...the A320 requires slightly more thrust at the same speed but counters that with a slightly more efficient bypass. The net result is they're very very close.

 Quoting flipdewaf (Reply 25):Quoting tdscanuck (Reply 23): given equal turbine & fan efficiencies Quoting tdscanuck (Reply 16): But all other things aren't equal. Which is it?

All other things aren't equal. Sorry if that wasn't clear. *If* they were, then you could extrapolate airplane efficiency directly from fan size. But, since they're not actually equal, you can't go straight from fan size to airplane efficiency in any straightforward way.

 Quoting flipdewaf (Reply 25): Quoting tdscanuck (Reply 23): so I don't see how you'd get that engine 1 has better propulsive efficiency. Well work out the energy required for each pound of thrust and you will see.

Energy per pound of thrust is SFC, which is a useful engine measure but not at all the same as propulsive efficiency.

 Quoting flipdewaf (Reply 25):The 737 and the A320 are as equal as it is possible to find in the aviation world and it is very difficult to see how the 737-leap-X at 70" is a more otimal overall solution then A320-leap-x at 75".

It's rather unlikely that the 737RE at 70" is *more* optimal than the A320 at 75". But the fact that both airplanes, at the airplane level, are so close together despite being fairly different in some systems details is a key point. They're both exploiting different sides of several trades (weight, speed, drag, maintenance, production cost, etc.) to end up with two aircraft with about as identical overall economics as it's possible for two airplanes to get. But, because at the design detail level, they have some significant differences, you can't apply a change in one system to both airplanes in the same way.

 Quoting flipdewaf (Reply 25):I think we need to get back to the original topic of how the fan diameter can effect overall efficiency/CASM of the aircraft instead of hiding behind aircraft being a complete system that needs optimising as a system,

You can't invoke overall efficiency/CASM without invoking the whole system...it makes no sense to talk about CASM of a fan size.

 Quoting flipdewaf (Reply 28):That might have to do with the different wordings used, we were tought at uni that propulsive efficiency was how the power taken from the low pressure turbine was used to create the bulk of the thrust (from the fan).

That's sort of correct...propulsive efficiency is basically about how well the engine converts power extracted in the turbines (all of them, not just the LP turbine) into thrust. There is also thermal efficiency (how well the engine converts fuel energy to useful work in the turbines) and several other metrics related to integration of the powerplant onto the airplane, drag from the installation, etc.

Tom.

flipdewaf
Posts: 1814
Joined: Thu Jul 20, 2006 6:28 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting tdscanuck (Reply 29):That's sort of correct...propulsive efficiency is basically about how well the engine converts power extracted in the turbines (all of them, not just the LP turbine) into thrust. There is also thermal efficiency (how well the engine converts fuel energy to useful work in the turbines) and several other metrics related to integration of the powerplant onto the airplane, drag from the installation, etc.

Glad I wasn't the only one who learned it like that.

 Quoting tdscanuck (Reply 29):But the fact that both airplanes, at the airplane level, are so close together despite being fairly different in some systems details is a key point.

I think that we would have to say in terms of this topic that it is the key point

 Quoting tdscanuck (Reply 29):They're both exploiting different sides of several trades (weight, speed, drag, maintenance, production cost, etc.) to end up with two aircraft with about as identical overall economics as it's possible for two airplanes to get. But, because at the design detail level, they have some significant differences, you can't apply a change in one system to both airplanes in the same way.

The overall economics part is very important, both stating from the same level (on the only metric that really counts).

Both aircraft will have an engine of the same tech level and age and the rest of the airframe will remain much unchanged apart from the integration of the new engine so the only difference is in the detail.

It is the design detail differences that we are looking at here as this is a driver of what can be achieved on the given airframe with the new engine.

 Quoting tdscanuck (Reply 29):You can't invoke overall efficiency/CASM without invoking the whole system...it makes no sense to talk about CASM of a fan size.

CASM of a fan size is a rediculous measure   but here is a situation that could be "relatively" easlily compared and so we should compare to see the relative benefits of either aircraft, initially with top level numbers and drilling down if there is the relevant knowledge and inclination.

I shall use an annoying business mantra and say lets not ffind problems lets find solutions

Fred

sirtoby
Posts: 353
Joined: Sat Nov 10, 2007 11:56 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting flipdewaf (Reply 3):Here's how it stacks up for the different NEO/RE aircraft.

But you cannot take the whole front area - the hub-to-tip ratio plays a big role here, too. you can only consider the area in between the inner (hub) and outer (tip) fan diameter.

flipdewaf
Posts: 1814
Joined: Thu Jul 20, 2006 6:28 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting sirtoby (Reply 31):But you cannot take the whole front area - the hub-to-tip ratio plays a big role here, too. you can only consider the area in between the inner (hub) and outer (tip) fan diameter.

Yes, indeed, maybe you could find the hub to tip ratios?

Fred

sirtoby
Posts: 353
Joined: Sat Nov 10, 2007 11:56 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting flipdewaf (Reply 32):Yes, indeed, maybe you could find the hub to tip ratios?

Should be 0.28-0.29.

JoeCanuck
Topic Author
Posts: 4210
Joined: Mon Dec 19, 2005 3:30 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting flipdewaf (Reply 30):Quoting tdscanuck (Reply 29): But the fact that both airplanes, at the airplane level, are so close together despite being fairly different in some systems details is a key point. I think that we would have to say in terms of this topic that it is the key point

With the time Boeing and GE have, I have no doubt, (as do most), that the RE will end up close enough to the NEO efficiency wise to at least come close to maintaining the current narrow body sales ratio for the rest of this decade.

Given there is so little real data to draw from, we are dealing with general concepts instead of specifics, and with that in mind, I guess the point of all the questions I've been asking is to get a better idea the flight regions where a smaller fan has the greatest handicaps, (and why), what GE can do to maximize the efficiency of the RE version, given it's smaller fan handicap.

I have certainly found the information interesting and I will continue to probe you folks, (in a non invasive, non ET way), for more information and clarification from time to time.

Now...it's off to Oshkosh.
What the...?

Stitch
Posts: 24022
Joined: Wed Jul 06, 2005 4:26 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting JoeCanuck (Reply 34):With the time Boeing and GE have, I have no doubt, (as do most), that the RE will end up close enough to the NEO efficiency wise to at least come close to maintaining the current narrow body sales ratio for the rest of this decade.

Honestly, while the partisans only seem to care about who "wins", if Boeing can sell enough 737 Evolutions to keep the line full at 42 production units per month (or even higher), then even if they sell less every year to the A320neo, they're still going to be fine.

keesje
Posts: 9872
Joined: Thu Apr 12, 2001 2:08 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting Stitch (Reply 35):Honestly, while the partisans only seem to care about who "wins", if Boeing can sell enough 737 Evolutions to keep the line full at 42 production units per month (or even higher), then even if they sell less every year to the A320neo, they're still going to be fine.

I think that's why 737RE is the right track.

Continuity this decade & a realistic time frame for a competitive NSA.
"Never mistake motion for action." Ernest Hemingway

redflyer
Posts: 3886
Joined: Thu Feb 24, 2005 3:30 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting astuteman (Reply 22):To me, I'd have thought that if a 66" (say) to 70" fan optimised the system on the A320NEO, then that is what the engine makers and Airbus would have done. But they haven't. They've gone for 81"

So why doesn't Cessna hang a pair of 81 inchers on the back of their Citation X?
My other home is in the sky inside my Piper Cherokee 180.

flipdewaf
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting redflyer (Reply 37):So why doesn't Cessna hang a pair of 81 inchers on the back of their Citation X?

perhaps they would if it was an 80-90t MTOW aircraft that was designed to fly up to 3500nm with ~170 pax but seeing as it isnt it probably doesnt have an 81" fan for the same reason my PA38 doesn't.

Fred

redflyer
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Joined: Thu Feb 24, 2005 3:30 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting flipdewaf (Reply 38):perhaps they would if it was an 80-90t MTOW aircraft that was designed to fly up to 3500nm with ~170 pax

Fair enough. So can you say with the same certainty that any jet airplane that had an 80-90t MTOW, was designed to fly up to 3500nm, and with ~170 pax would, if it could, use absolutely nothing less than 81 inchers?
My other home is in the sky inside my Piper Cherokee 180.

flipdewaf
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting redflyer (Reply 39):Fair enough. So can you say with the same certainty that any jet airplane that had an 80-90t MTOW, was designed to fly up to 3500nm, and with ~170 pax would, if it could, use absolutely nothing less than 81 inchers?

No, not any aircraft but you distinctly pointed one jet out with a very different mission profile to those being discussed here.

No of course 81" isn't the be all and end all. 81" is simply the optimum found by airbus for an aircraft with as near as possible identical mission profile tothat of the 737 and that isn't (as far as I know) constrained in this aspect by the airframe. The 737 however has the fan diameter as a constraint that is very limited by the airframe and as such the optimum will likely be a smaller diameter.

This thread is discussing the effects that this constraint will have on both the 737 and A320 using reasonable assumptions.

Fred

redflyer
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Joined: Thu Feb 24, 2005 3:30 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting flipdewaf (Reply 40):No of course 81" isn't the be all and end all. 81" is simply the optimum found by airbus for an aircraft with as near as possible identical mission profile tothat of the 737 and that isn't (as far as I know) constrained in this aspect by the airframe. The 737 however has the fan diameter as a constraint that is very limited by the airframe and as such the optimum will likely be a smaller diameter.

I know very well the reasons why the 737RE is going with a smaller diameter fan. And I know if she wasn't restrained by landing gear height then Boeing would in all likelihood have selected the same or similar sized fan diameter engine. But what seems to be getting lost in this discussion, and what the OP asked originally, was if the most obvious shortcomings of a smaller diameter fan could be overcome in other ways. And after 40 replies, I'm not quite sure we know the answer. There are some here (as well as Boeing, which employs some top-notch engineers) who think it can be overcome, if not through the engine design itself then through other additional means. But there are others who disagree, and who haven't provided any irrefutable proof other than broad-brush generalizations, which are then used to bash Boeing.
My other home is in the sky inside my Piper Cherokee 180.

flipdewaf
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### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting redflyer (Reply 41):what the OP asked originally, was if the most obvious shortcomings of a smaller diameter fan could be overcome in other ways.

Really?

 Quoting JoeCanuck (Thread starter):What would the approximate fuel efficiency differences be of an 80 inch fan be over a 70 inch fan, pushing the same amount of air,
 Quoting redflyer (Reply 41):There are some here (as well as Boeing, which employs some top-notch engineers) who think it can be overcome, if not through the engine design itself then through other additional means.

There are also many here and some top notch engineers at airbus who say it won't. Personally I think it will compete on overall costs at the end of the day (±3%)

 Quoting redflyer (Reply 41):I know very well the reasons why the 737RE is going with a smaller diameter fan.

So why the comment about the CitationX if you knew all along?

 Quoting redflyer (Reply 41):And I know if she wasn't restrained by landing gear height then Boeing would in all likelihood have selected the same or similar sized fan diameter engine.

I agree and I'm pretty sure almost everyone here does aswel.

 Quoting redflyer (Reply 41):and who haven't provided any irrefutable proof other than broad-brush generalizations, which are then used to bash Boeing.

I haven't seen any Boeing bashing in this thread? All we are discussing is relative benefits of different aspects of aircraft, stop playing the victim.

Fred

redflyer
Posts: 3886
Joined: Thu Feb 24, 2005 3:30 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting flipdewaf (Reply 42):Quoting redflyer (Reply 41): I know very well the reasons why the 737RE is going with a smaller diameter fan. So why the comment about the CitationX if you knew all along?

To reveal the absurdity of certain absolute statements made by others.

 Quoting flipdewaf (Reply 42):Quoting redflyer (Reply 41): and who haven't provided any irrefutable proof other than broad-brush generalizations, which are then used to bash Boeing. I haven't seen any Boeing bashing in this thread?

Really?
My other home is in the sky inside my Piper Cherokee 180.

flipdewaf
Posts: 1814
Joined: Thu Jul 20, 2006 6:28 am

### RE: Big Fan, Little Fan...NEO V. RE...

 Quoting redflyer (Reply 43):To reveal the absurdity of certain absolute statements made by others.

Haven't seen them.

Sorry, again, you're going to have to find them for me, I can't see it.

Fred

JoeCanuck
Topic Author
Posts: 4210
Joined: Mon Dec 19, 2005 3:30 am

### RE: Big Fan, Little Fan...NEO V. RE...

It's been mostly civil in here folks, and I appreciate that. I realise I steered a fine line with the topic and it's heartening to see that it's possible to have a well reasoned discussion based on merits.

 Quoting flipdewaf (Reply 42): There are also many here and some top notch engineers at airbus who say it won't. Personally I think it will compete on overall costs at the end of the day (±3%)

...which will probably end up being about the same difference as achieved by the two CFM models. So while A and B maintain their relative sales, they do close the gap enough on the CSeries and others to make selling them a bit more problematic.

On the other hand, the sales success of the NEO, (soon to be followed by RE sales, I reckon), may leave the only available open slots to BBD. Still...thousands of sales into the second half of the decade has a way of healing many wounds.
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