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Hi Bypass Engine - What's The Limit In Diameter?  
User currently offlineA380900 From France, joined Dec 2003, 1118 posts, RR: 1
Posted (3 years 11 months 1 week 6 days 18 hours ago) and read 6929 times:

It looks like when it comes to hi bypass engine, the larger the diameter, the better. Yet some airplanes do not seem to use the full diameter that would be possible once the ground clearance is taken into account. (A380, A330 do not max out diameter as much as the 737 for instance). Also, the 777 has engines of different diameter depending on their power. Why not use the larger fan size for all powers if bigger is always better?

I understand also that the tip of the blades cannot go faster than the speed of sound (or can they?). Is there an absolute limit of the hi-bypass engine diameter or could we see larger diameters than the GE115 or the Trent 900?

34 replies: All unread, showing first 25:
 
User currently offlineRevelation From United States of America, joined Feb 2005, 12930 posts, RR: 25
Reply 1, posted (3 years 11 months 1 week 6 days 15 hours ago) and read 6869 times:

Quoting A380900 (Thread starter):
Why not use the larger fan size for all powers if bigger is always better?

Bigger is not always better.

Some factors that come to mind:

Bigger diameter means larger and thus heavier case, heaver fan blades, more frontal drag, and their knock-on effects.

Bigger diameter means more fan blade flex, so tip clearance has to be increased, decreasing efficiency.



Inspiration, move me brightly!
User currently offlinenomadd22 From United States of America, joined Feb 2008, 1899 posts, RR: 0
Reply 2, posted (3 years 11 months 1 week 6 days 15 hours ago) and read 6865 times:

Mostly because the engines weren't available when they bought the plane. Adding a new engine type to an airliner is a huge undertaking.
And bigger is not always better. The bigger the fan is the slower it spins. And the fan speed is tied to the core speed, which generally is more efficient the faster it goes. Pratt is working on geared fans that can go slower and be bigger without compromising core efficiency, but they don't have any big ones on the market yet.

Also, bigger fans mean more weight and drag. There are a lot of considerations besides fan efficiency to consider.



Andy Goetsch
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17166 posts, RR: 66
Reply 3, posted (3 years 11 months 1 week 6 days 12 hours ago) and read 6780 times:

Quoting nomadd22 (Reply 2):
The bigger the fan is the slower it spins. And the fan speed is tied to the core speed, which generally is more efficient the faster it goes.

The core and the fan are on different spools so they can and do spin at very different speeds. Not saying there is no relation between the speeds but a slow fan need not mean a slow core.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineprebennorholm From Denmark, joined Mar 2000, 6534 posts, RR: 54
Reply 4, posted (3 years 11 months 1 week 6 days 10 hours ago) and read 6736 times:

Quoting Starlionblue (Reply 3):
Quoting nomadd22 (Reply 2):
The bigger the fan is the slower it spins. And the fan speed is tied to the core speed, which generally is more efficient the faster it goes.

The core and the fan are on different spools so they can and do spin at very different speeds. Not saying there is no relation between the speeds but a slow fan need not mean a slow core.

Dear Stallion, I think that nomadd22 means the LP turbine, when he mentions "the core". The LP turbine is considered part of the core since it never touches the bypass air.

On all non-geared tubofan engines the LP turbine and the fan are locked to the same rpm. And in practically all cases the LP turbine would gain efficiency if allowed to spin faster. Consequently fan diameter is a compromise which is involving the LP turbine.

Quoting A380900 (Thread starter):
I understand also that the tip of the blades cannot go faster than the speed of sound (or can they?).

Fan tips always operate at supersonic speed. With the plane going at 0.8 Mach the fan tips would also be at M=0.8 even if the engine wasn't spinning at all, so it doesn't take much spinning to make the tips supersonic at cruise speed. Fan tip speed on a cruising airliner is typically between 1.5 and 2.0 Mach. The new P&W geared turbofans will be a little slower, but still supersonic, typically between 1.3 and 1.5 Mach. Most of the efficiency gain will be due to a faster spinning LP turbine.

Turboprop planes, however, they keep the propeller tips just below Mach one.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineKlaus From Germany, joined Jul 2001, 21520 posts, RR: 53
Reply 5, posted (3 years 11 months 1 week 6 days 9 hours ago) and read 6720 times:

Quoting prebennorholm (Reply 4):
Fan tip speed on a cruising airliner is typically between 1.5 and 2.0 Mach.

Isn't that modified by the different pressures before and aft of the fan?


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17166 posts, RR: 66
Reply 6, posted (3 years 11 months 1 week 6 days 8 hours ago) and read 6691 times:

Quoting prebennorholm (Reply 4):

Dear Stallion, I think that nomadd22 means the LP turbine, when he mentions "the core". The LP turbine is considered part of the core since it never touches the bypass air.

Fair enough.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineprebennorholm From Denmark, joined Mar 2000, 6534 posts, RR: 54
Reply 7, posted (3 years 11 months 1 week 6 days 7 hours ago) and read 6671 times:

Quoting Klaus (Reply 5):
Quoting prebennorholm (Reply 4):
Fan tip speed on a cruising airliner is typically between 1.5 and 2.0 Mach.

Isn't that modified by the different pressures before and aft of the fan?

It is the Mach number with which the leading edge of the fan blade tip hits the still air at cruise conditions.

The front of the fan blades are like the upper surface on a lifting wing, where the air flow is in fact acellerated. That's at least how it works at subsonic speed. Supersonic aerodynamics is a lot more complicated, and I will leave it to others to explain that - or write books about it. Optimizing the profile of supersonic fan blades is an extremely complicated science.

In any case, for single stage fan tips we are talking "Concorde-speeds" rather than transonic speeds. It's not like a multi stage axial compressor, where pressure is built up to several times ambient pressure, and the local airspeed is reduced by the same factor. The air flow in the combustion chambre is in fact much slower than the bypass air in the fan.

The turbines - both HP and LP - take advantage of that, so they in fact operate in subsonic mode. Here it also helps that sonic speed is temperature dependent - in the hot exhaust gasses sonic speed is quite a bit faster in mph than what we are used to.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineKlaus From Germany, joined Jul 2001, 21520 posts, RR: 53
Reply 8, posted (3 years 11 months 1 week 6 days 7 hours ago) and read 6655 times:

Quoting prebennorholm (Reply 7):
It is the Mach number with which the leading edge of the fan blade tip hits the still air at cruise conditions.

But it's actually not really still air, isn't it? That was why I asked.

In a subsonic high-bypass engine the air should already be accelerating towards the fan within the inlet due to the lowered pressure in front of the fan ("above" the blades). And correspondingly the air should be slowing down again behind the fan, the differential effectively producing bypass thrust (minus the air going into the core compressor).

So unless I've missed something here, the fan blades should actually be operating in a mixed environment with partially below and above ambient pressures and correspondingly different mach speeds, wouldn't they?

Isn't that a factor here?


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 9, posted (3 years 11 months 1 week 6 days 6 hours ago) and read 6648 times:

Quoting A380900 (Thread starter):

It looks like when it comes to hi bypass engine, the larger the diameter, the better

The larger the *bypass ratio* the better...there is no inherent advantage (for fans) to large diameter other than tip clearance issues, which isn't nearly as big a deal for the fan as the core.

Quoting A380900 (Thread starter):
Why not use the larger fan size for all powers if bigger is always better?

Because bigger isn't always better. More bypass ratio always provides better propulsive efficiency, but there's no value in having a fan bigger than you need for whatever thrust you require. What you really want is the smallest fan you can get the thrust you need, and then the largest bypass ratio you can get with that fan (smallest possible core).

Quoting A380900 (Thread starter):
Is there an absolute limit of the hi-bypass engine diameter or could we see larger diameters than the GE115 or the Trent 900?

There's no technical reason you can't go to a larger diameter than those engines, but there may never be an economic case. Right now, there's no reason to build an engine with more thrust than that, and the efficiency gains will come by shrinking the core.

Quoting Klaus (Reply 8):
So unless I've missed something here, the fan blades should actually be operating in a mixed environment with partially below and above ambient pressures and correspondingly different mach speeds, wouldn't they?

No. Mach number has no dependence on pressure, only on temperature. The air starts out at freestream Mach (~0.85), and may or may not accelerate into the inlet depending on the particular design...done properly on most designs, it should actually slow down going through the inlet to lower the Mach number at the fan face. At that point, this high-subsonic air encounters a fan blade whizzing around tremendously fast...the local mach number on the fan blade is the vector product of the fan blade speed and the airspeed...that can be well above Mach 1. The air pressure goes up through the fan (not a tremendous amount of velocity change within the fan itself), causing a temperature rise. Higher temperature means higher sonic speed, so you get a Mach number drop through the fan. The rest of the fan duct converts that pressure rise to velocity, and you get the air coming out the back of the nozzle just below Mach 1.

Tom.


User currently offlineprebennorholm From Denmark, joined Mar 2000, 6534 posts, RR: 54
Reply 10, posted (3 years 11 months 1 week 5 days 10 hours ago) and read 6397 times:

Quoting Klaus (Reply 8):
the fan blades should actually be operating in a mixed environment with partially below and above ambient pressures and correspondingly different mach speeds, wouldn't they?

Let's look at an example: CFM56-3 (22k lbs engine on B737-300), fan diameter 60 inches, 100% N1 = 5175 rpm.

Simple math (diameter multiplied by pi multiplied by rpm) gives a fan tip rotational speed of 923 mph = 1486 km/h.

Then vector in the airplane cruise speed (0.8 Mach or whatever) and we are well on the way to Mach 2.

Then there are - as tdscanuck explains in reply #9 - some pressure and temperature variations which are complicating exact calculations of local Mach numbers.

The latter may give or take low double digit percentages here and there and everywhere, but no more than that. At the end of the day, the fan tips local airspeed on a typical high bypass turbofan engine is in the 1.5 to 2.0 Mach frame.

For curiosity, Volkswagen Golf 1.4 TSI, turbocharger compressor wheel diameter = 40 mm, rpm at max power (122 HP output) = 220,000 rpm. Compressor tip speed = 1,658 km/h or 1,031 mph. (Well, that's a centrifugal compressor, and aerodynamics is therefore an entirely different animal compared to a fan. But interesting data anyway. Just accelerate a little fast away from a green traffic light, and a tiny CFRP component on your car goes supersonic in a split second).



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineKlaus From Germany, joined Jul 2001, 21520 posts, RR: 53
Reply 11, posted (3 years 11 months 1 week 5 days 2 hours ago) and read 6320 times:

Quoting prebennorholm (Reply 10):
Then there are - as tdscanuck explains in reply #9 - some pressure and temperature variations which are complicating exact calculations of local Mach numbers.

The latter may give or take low double digit percentages here and there and everywhere, but no more than that.

Okay, I was just wondering whether the effect was in fact present there, and apparently it is. (Actual numbers would still be interesting, but are probably manufacturer-proprietary.)

Thanks.

Quoting prebennorholm (Reply 10):
Just accelerate a little fast away from a green traffic light, and a tiny CFRP component on your car goes supersonic in a split second).


Cool!
But CFRP? Are you sure?


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17166 posts, RR: 66
Reply 12, posted (3 years 11 months 1 week 5 days ago) and read 6295 times:

Quoting Klaus (Reply 11):
Quoting prebennorholm (Reply 10):
Just accelerate a little fast away from a green traffic light, and a tiny CFRP component on your car goes supersonic in a split second).


Cool!
But CFRP? Are you sure?

Modern turbos typically have rather exotic materials for the turbines. They need to be very light in order to spool up quickly, and they need to handle very high rotational speeds. Ceramics are not unusual due to their lightness.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineKlaus From Germany, joined Jul 2001, 21520 posts, RR: 53
Reply 13, posted (3 years 11 months 1 week 5 days ago) and read 6292 times:

Quoting Starlionblue (Reply 12):
Modern turbos typically have rather exotic materials for the turbines. They need to be very light in order to spool up quickly, and they need to handle very high rotational speeds. Ceramics are not unusual due to their lightness.

Yes, ceramics are pretty much the only way to go for high-performance turbines as far as I know.

The fan side could theoretically use less temperature-resistant materials, but CFRP? That sounds extremely hard to manufacture in the shapes required for a car turbo.

[Edited 2011-01-11 03:59:31]

User currently offlinejetlife2 From United States of America, joined Jul 2006, 221 posts, RR: 25
Reply 14, posted (3 years 11 months 1 week 4 days 9 hours ago) and read 6150 times:

Good questions, and a lot of good points raised already.

There are a couple of factors not mentioned so far that are of interest also.

The GE90-115B fan case external diameter was sized to fit in the cargo door of a 747F (and by extension others) for transportability. The propulsor architecture means that the fan stator does not need to be transported (or even purchased) all that often, so this is a good system. Go higher than that diameter, and you have to either accept the transportation restrictions (which might lead customers to want more fan stators on hand, and defeat that advantage somewhat); or split the fan case. The latter is possible but unfriendly to aero and containment. Very large fan cases are really not easy to make when you want to hold them round in operation....Such a system has to be studied very carefully and designed nearly completely before the real trades are apparent.

As you go up in bypass ratio and demand more from the fan, the LPT has to get bigger also...usually longer (more stages). So the system weight trade has to be worked through carefully.

Another factor is the length of the landing gear. Those are already some very large forgings, and have to be stowed in the aircraft, so going longer propagates some other design and manufacturing challenges.

The attractions of bypass ratio (sfc) and acoustics sure make it interesting and enticing, so large (really large) fans keep reappearing in the studies.....will they win - or is it not worth it in the end? We will see...


User currently offlinecobra27 From Slovenia, joined May 2001, 1033 posts, RR: 0
Reply 15, posted (3 years 11 months 1 week 3 days 22 hours ago) and read 6038 times:

Where is lightsaber?

Anyway, GE90-94 seems to have better SFC than GE90-115. I don't think this is the upper limit. I would be nice to see A380 with 2 engines.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 16, posted (3 years 11 months 1 week 3 days 21 hours ago) and read 6025 times:

Quoting cobra27 (Reply 15):
It would be nice to see A380 with 2 engines.

From a technical standpoint, absolutely. But, right now, there's zero business case to develop such an engine. You'd need them to be very nearly twice the size of the GE90-115B, and figure out some way to pay for such an awesome project based on lifetime sales that, right now, look like they'd be in the neighborhood of 2000-3000...that is an extremely tall order.

Tom.


User currently offlineBMI727 From United States of America, joined Feb 2009, 15828 posts, RR: 27
Reply 17, posted (3 years 11 months 1 week 3 days 20 hours ago) and read 6007 times:

Quoting prebennorholm (Reply 4):
Fan tips always operate at supersonic speed.

Then what causes the loud obnoxious buzzing we hear (notably from RB211s) at high thrust settings?



Why do Aerospace Engineering students have to turn things in on time?
User currently offlineDocLightning From United States of America, joined Nov 2005, 20334 posts, RR: 59
Reply 18, posted (3 years 11 months 1 week 3 days 6 hours ago) and read 5884 times:

Quoting tdscanuck (Reply 16):

From a technical standpoint, absolutely. But, right now, there's zero business case to develop such an engine. You'd need them to be very nearly twice the size of the GE90-115B, and figure out some way to pay for such an awesome project based on lifetime sales that, right now, look like they'd be in the neighborhood of 2000-3000...that is an extremely tall order.

Also, can you imagine the transport headaches if you have to replace one of those at an outstation? Either you'd have to deliver it by ship and then maybe by some rail-based contrivance, or you'd have to make the casing disassemblable/reassamblable which would present its own spectrum of heartburn.

On the upside, if such an engine were developed, you might have a business case to build more An-225s!  


User currently offlinespeedygonzales From Norway, joined Sep 2007, 745 posts, RR: 0
Reply 19, posted (3 years 11 months 1 week 2 days 23 hours ago) and read 5837 times:

Could a twin fan be feasible to achieve very high BPR? One fan driven directly, and the other through a drive shaft like the lift fan on F-35B.


Las Malvinas son Argentinas
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 20, posted (3 years 11 months 1 week 2 days 22 hours ago) and read 5829 times:

Quoting speedygonzales (Reply 19):
Could a twin fan be feasible to achieve very high BPR? One fan driven directly, and the other through a drive shaft like the lift fan on F-35B.

It's technically possible, but the certification challenges would be huge (something like what the LearFan went through). If you want to go to remote driven, I think you'd be better off just going to ducted turboprops (which is very nearly what the GTF is) rather than running driveshafts around.

Tom.


User currently onlinerwessel From United States of America, joined Jan 2007, 2410 posts, RR: 2
Reply 21, posted (3 years 11 months 1 week 2 days 11 hours ago) and read 5734 times:
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Quoting DocLightning (Reply 18):
Also, can you imagine the transport headaches if you have to replace one of those at an outstation? Either you'd have to deliver it by ship and then maybe by some rail-based contrivance, or you'd have to make the casing disassemblable/reassamblable which would present its own spectrum of heartburn.

Twice the thrust would require a 41% bigger diameter fan. You'd need a bigger cargo door, but it would fit inside a 747, or even a 777. And the fan assembly should fit handily through the standard nose door of an An-124. It might even fit in a stock C-17.


User currently offlineDocLightning From United States of America, joined Nov 2005, 20334 posts, RR: 59
Reply 22, posted (3 years 11 months 1 week 1 day 11 hours ago) and read 5586 times:

Quoting rwessel (Reply 21):

Twice the thrust would require a 41% bigger diameter fan. You'd need a bigger cargo door, but it would fit inside a 747, or even a 777. And the fan assembly should fit handily through the standard nose door of an An-124. It might even fit in a stock C-17.

And do you think any airline wants to deal with a logistical nightmare that requires an An-124 every time something goes wrong? I don't.

Could you even fit such a thing on a train?


User currently onlinerwessel From United States of America, joined Jan 2007, 2410 posts, RR: 2
Reply 23, posted (3 years 11 months 1 week 1 day 1 hour ago) and read 5519 times:
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Quoting DocLightning (Reply 22):
And do you think any airline wants to deal with a logistical nightmare that requires an An-124 every time something goes wrong? I don't.

Could you even fit such a thing on a train?

Errr.. You suggested that delivery would require a ship. Clearly a number of aircraft available could transport a fan assembly of the required size without modifications, and while they might not be as convenient to use as some other methods, chartering an An-124 is not that hard, and would delivery anywhere your down aircraft might be in a few days (unlike a seagoing vessel). And there are other aircraft that could take that role (C-17, probably C-5, 747-LCF, the various Belugas), although arranging to have some of those like that available would take some doing. Or a bigger side door mod to a handful of 747s. Point is, this would probably be a relatively minor concerns in building an engine of this class. If GE built such a thing, they could *buy* a couple of An-124s for well under 10% of the likely development costs of such an engine, just to have the transports available at a moments notice (although that sounds like the most expensive option).

As to trains, well cars regularly transport stacked (two high) shipping containers, although those are route restricted because of overhead clearance issues (in the US you'd need all Plate H compatible routes). But that 17ft total height (two 8.5 ft high containers) would accommodate the fan stood on its side, so long as the length of the fan assembly wasn't more than a bit wider than the rail car (about 10 ft - again with route restrictions wider loads are possible).

OTOH, overland transport could also be done with a heavy lift helicopter with an external sling (the fan assembly is probably only about half the ~35klbs such an engine would weigh). Sure the range isn't great, but it would probably be faster and easier than trying to do it by train (although certainly not cheaper!), even if you had to stop for gas every couple of hundred miles. Of course if you got a Mi-26*, you could transport the fully assembled engine, but I'm guessing you're going to object to that again.    Of course the Russians would be more than happy to sell you a couple (cheap at $15m/ea!). A CH-53K might be able to do that too, but you'd be right on the edge of its (projected) capabilities (although just the fan should be no problem).


*That is one heck of a big helicopter.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17166 posts, RR: 66
Reply 24, posted (3 years 11 months 1 week 1 day ago) and read 5518 times:

Quoting rwessel (Reply 23):
As to trains, well cars regularly transport stacked (two high) shipping containers,

AFAIK only in North America. In any case as you say I think train is impractical.



"There are no stupid questions, but there are a lot of inquisitive idiots."
25 rwessel : There's some of that in at least Australia, China and India.
26 DocLightning : I think it's only certified to fly with Dreamliner parts aboard, for some reason.
27 tdscanuck : Dreamlifter parts + related tooling. It's actually written into the Type Certificate Data Sheet for the -LCF. There's no particular reason that can't
28 nomadd22 : For some reason I was thinking that the LCF wasn't couldn't carry anything but Boeing owned cargo. That certifying it for general cargo would be a ma
29 Starlionblue : Well, Belugas carry non-Airbus stuff from time to time. As Tom says, paperwork.
30 tdscanuck : Technically, it's 787 cargo...some of those parts probably don't belong to Boeing at the time they're transported. But yes, the types of cargo the Dr
31 Pihero : That's an aspect that people don' really see : Three years ago, a 777 ETOPS-diverted back to Northern Brazil : an engine change was required. Then pr
32 speedygonzales : The LCF also has the problem that it can't open the crago door on its own. It needs a specialized vehicle (modified forklift, IIRC) to support the wei
33 BrouAviation : Ceramics are not used in aviation, because of their brittleness. All turbine parts and the combustion chamber of practically any jet engine fitted on
34 Klaus : I had been referring to car turbos there, but apparently the earlier development targets to that effect I had remembered have actually not been reali
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