Why do widebody aircraft generally cruise faster than narrowbody aircraft?

Thx

Speculation: Widebodies fly further on average. A shorter flight time means lower crew and maintenance costs for example. On shorter routes, a slightly higher Mach number won't make enough of a difference to matter. So airframers optimise for higher speed, which presumably means adding a little inefficiency in other areas like fuel burn.

It's a 3-way trade between ground-speed/time-of-flight, weather/altitude, and aircraft load/efficiency. The goal is to optimize the overall cost index for the flight.

Cost index is the ratio of time cost to fuel cost. The ideal cost index is that which minimizes the sum of the two costs for the flight.

As Starlionblue said, long-distance airliners spend a larger percentage of their flights in cruise, so are optimized differently for their design cost index. In reality, the cost index varies somewhat for actual flight conditions, and thus is calculated for each flight. But the design cost index is based on the typical or average flight of the airliner over its lifetime.

https://blog.openairlines.com/top-10-fa ... cost-index

Widebodies tend to operate within a more limited set of parameters on the ground, which also influences their design. When all your departure runways are 10000' or so, gates wide enough to handle more wing, etc., the aircraft can be built to take more space on the ground and use more runway, but then optimize for fast flight in climb and cruise. Certainly, there is significant value to being able to fly faster over long distances vs. shorter ones. Where a Q400 might well be competitive with a mainline or regional narrowbody on a flight of 500 NM or so, a 747/787/A350 is going to give an appreciable flight time advantage over a 767 or A330 on a 10 hour flight.

Avatar2go wrote:

It's a 3-way trade between ground-speed/time-of-flight, weather/altitude, and aircraft load/efficiency. The goal is to optimize the overall cost index for the flight.

Cost index is the ratio of time cost to fuel cost. The ideal cost index is that which minimizes the sum of the two costs for the flight.

As Starlionblue said, long-distance airliners spend a larger percentage of their flights in cruise, so are optimized differently for their design cost index. In reality, the cost index varies somewhat for actual flight conditions, and thus is calculated for each flight. But the design cost index is based on the typical or average flight of the airliner over its lifetime.

https://blog.openairlines.com/top-10-fa ... cost-index

Thanks for that link. Very nicely explained.

As you say, CI is calculated for every flight, though in practice during a given period the CI on the flight plans will not typically vary very much. We can also tactically change cost index in flight, for example decreasing it to slow down if we want to delay arrival until the end of a curfew.

N1120A wrote:

Widebodies tend to operate within a more limited set of parameters on the ground, which also influences their design. When all your departure runways are 10000' or so, gates wide enough to handle more wing, etc., the aircraft can be built to take more space on the ground and use more runway, but then optimize for fast flight in climb and cruise. Certainly, there is significant value to being able to fly faster over long distances vs. shorter ones. Where a Q400 might well be competitive with a mainline or regional narrowbody on a flight of 500 NM or so, a 747/787/A350 is going to give an appreciable flight time advantage over a 767 or A330 on a 10 hour flight.

I never really thought about it much, but its interesting the difference in widebody cruise speeds, all for different reasons. 747 was always designed to go fast, fuel burn wasn't as much of an issue. Then you have the 787/350 which also are relatively fast, but fuel burn was a huge factor in their designs. They have the advantage of new materials, advances in aerodynamics, and a mission profile right from design of long haul optimization.

The 767 and 330 are funny, most people loop them right in with "widebody" but their initial design put them in shorter to medium range markets, as ETOPS wasn't a thing and ULH hadn't really been developed beyond the 747. Airbus had envisioned the 340 being the long haul market leader, until ETOPS changed the landscape, and carriers wanted twins to fly near their maximum ranges and beyond. So you end up with the 330 and 767 both being extended beyond their initial design range goals, but still having a cruise speed around .82 if my research is correct.

Then over the Atlantic you end up with an A321 doing .78 which must make things interesting for ATC.

shamrock137 wrote:

N1120A wrote:

Widebodies tend to operate within a more limited set of parameters on the ground, which also influences their design. When all your departure runways are 10000' or so, gates wide enough to handle more wing, etc., the aircraft can be built to take more space on the ground and use more runway, but then optimize for fast flight in climb and cruise. Certainly, there is significant value to being able to fly faster over long distances vs. shorter ones. Where a Q400 might well be competitive with a mainline or regional narrowbody on a flight of 500 NM or so, a 747/787/A350 is going to give an appreciable flight time advantage over a 767 or A330 on a 10 hour flight.

I never really thought about it much, but its interesting the difference in widebody cruise speeds, all for different reasons. 747 was always designed to go fast, fuel burn wasn't as much of an issue. Then you have the 787/350 which also are relatively fast, but fuel burn was a huge factor in their designs. They have the advantage of new materials, advances in aerodynamics, and a mission profile right from design of long haul optimization.

The 767 and 330 are funny, most people loop them right in with "widebody" but their initial design put them in shorter to medium range markets, as ETOPS wasn't a thing and ULH hadn't really been developed beyond the 747. Airbus had envisioned the 340 being the long haul market leader, until ETOPS changed the landscape, and carriers wanted twins to fly near their maximum ranges and beyond. So you end up with the 330 and 767 both being extended beyond their initial design range goals, but still having a cruise speed around .82 if my research is correct.

Then over the Atlantic you end up with an A321 doing .78 which must make things interesting for ATC.

ETOPS started being "a thing" many years prior to the A330. The A300 started using 90 minute ETOPS in 1976. 120 minute ETOPS came along in 1985, and 180 minutes in 1988, well before A330 EIS.

The A340 was the long haul version with significantly higher MTOW than the A330, and four engines to carry that.

Starlionblue wrote:

shamrock137 wrote:

N1120A wrote:

Widebodies tend to operate within a more limited set of parameters on the ground, which also influences their design. When all your departure runways are 10000' or so, gates wide enough to handle more wing, etc., the aircraft can be built to take more space on the ground and use more runway, but then optimize for fast flight in climb and cruise. Certainly, there is significant value to being able to fly faster over long distances vs. shorter ones. Where a Q400 might well be competitive with a mainline or regional narrowbody on a flight of 500 NM or so, a 747/787/A350 is going to give an appreciable flight time advantage over a 767 or A330 on a 10 hour flight.

I never really thought about it much, but its interesting the difference in widebody cruise speeds, all for different reasons. 747 was always designed to go fast, fuel burn wasn't as much of an issue. Then you have the 787/350 which also are relatively fast, but fuel burn was a huge factor in their designs. They have the advantage of new materials, advances in aerodynamics, and a mission profile right from design of long haul optimization.

The 767 and 330 are funny, most people loop them right in with "widebody" but their initial design put them in shorter to medium range markets, as ETOPS wasn't a thing and ULH hadn't really been developed beyond the 747. Airbus had envisioned the 340 being the long haul market leader, until ETOPS changed the landscape, and carriers wanted twins to fly near their maximum ranges and beyond. So you end up with the 330 and 767 both being extended beyond their initial design range goals, but still having a cruise speed around .82 if my research is correct.

Then over the Atlantic you end up with an A321 doing .78 which must make things interesting for ATC.

ETOPS started being "a thing" many years prior to the A330. The A300 started using 90 minute ETOPS in 1976. 120 minute ETOPS came along in 1985, and 180 minutes in 1988, well before A330 EIS.

The A340 was the long haul version with significantly higher MTOW than the A330, and four engines to carry that.

Although ETOPS had been around for a while, previous aircraft had to prove their reliability over the course of the years before they were granted more than 60 minutes. It wasn't until the 777 and the A330 came along that they were trying to certify it for 180-minute ETOPS out of the box.

Starlionblue wrote:

shamrock137 wrote:

N1120A wrote:

Widebodies tend to operate within a more limited set of parameters on the ground, which also influences their design. When all your departure runways are 10000' or so, gates wide enough to handle more wing, etc., the aircraft can be built to take more space on the ground and use more runway, but then optimize for fast flight in climb and cruise. Certainly, there is significant value to being able to fly faster over long distances vs. shorter ones. Where a Q400 might well be competitive with a mainline or regional narrowbody on a flight of 500 NM or so, a 747/787/A350 is going to give an appreciable flight time advantage over a 767 or A330 on a 10 hour flight.

I never really thought about it much, but its interesting the difference in widebody cruise speeds, all for different reasons. 747 was always designed to go fast, fuel burn wasn't as much of an issue. Then you have the 787/350 which also are relatively fast, but fuel burn was a huge factor in their designs. They have the advantage of new materials, advances in aerodynamics, and a mission profile right from design of long haul optimization.

The 767 and 330 are funny, most people loop them right in with "widebody" but their initial design put them in shorter to medium range markets, as ETOPS wasn't a thing and ULH hadn't really been developed beyond the 747. Airbus had envisioned the 340 being the long haul market leader, until ETOPS changed the landscape, and carriers wanted twins to fly near their maximum ranges and beyond. So you end up with the 330 and 767 both being extended beyond their initial design range goals, but still having a cruise speed around .82 if my research is correct.

Then over the Atlantic you end up with an A321 doing .78 which must make things interesting for ATC.

ETOPS started being "a thing" many years prior to the A330. The A300 started using 90 minute ETOPS in 1976. 120 minute ETOPS came along in 1985, and 180 minutes in 1988, well before A330 EIS.

The A340 was the long haul version with significantly higher MTOW than the A330, and four engines to carry that.

Yes, should have worded it better. ETOPS existed, but development of the A330/340 started in the late 70's, early 80's, before it was accepted that twin engine aircraft would operate ULH flights. My point was simply that the A330 and 767 weren't developed as a the long haul aircraft they have turned into, especially the 330. The 340-200 was delivered with a range of 6,700nm, while the last 242t versions of the 332 are at 7260nm. Impressive how far the airframe has come.

Question for you though Starlionblue, since the thread is about cruise speed. Would you notice any difference between the 330 and 350 on the same route?

It's an optimization problem. Today's narrowbodies don't usually go as far, so speed is less important. Related to this, they usually have less sweep, around 25 degrees, versus 30-33 for most modern widebodies. That saves structural weight. Since the 737's 25-degree wings, the A320 and 737NG were able to move up the speed from .74 to .78 with better aerodynamics on the same sweep.

The narrowbody 707 was designed for .82, and often flown faster if fuel economy was unimportant. The 727 was pretty quick as well, arguably they could have brought the sweep angle down to save weight on the shorter sectors, but fuel was cheap then.

The 757/767 were designed when fuel was expensive, so they were optimized for efficiency over speed. Boeing deliberately made the wings on the larger end of optimal, which paid off in the 767 when better engines enabled the 767-300ER. Whether they anticipated that a decade earlier is anybody's guess, but it's done ok on 10-hour trips.

The A340 was also designed very deliberately to sacrifice speed for efficiency. If you look at the planform, it's got a thick cross section and high aspect ratio. Airlines voted for this trade, which helped them get sales away from the MD-11 (e.g. SQ). At the time, the A330 was not expected to play as big of a role, but subsequently became the better seller and replaced a lot of 777-200ERs. A lighter airframe and better CASM was a worhtwhile price for a bit of speed.

The 777 was initially designed for .83, and the aero came back a bit better than expected, so they were able to do .84. I think that .02 against the A340-300 made less of a difference than the better payload/range of the -200ER and the cheaper engines due to more competition.

The 340-500/600 needed more fuel capacity for the weight, so they added chord and gained a bit of speed. That was a cheap solution (same flaps, winglets, and mostly the same slats), but I wonder how that stacked up against a root extension to preserve more aspect ratio.

By the time the 380, 787, and 350 were on the drawing board, they'd figured out how to make better transonic wings, so those were all able to go .85. Some of that aero went into the 747-8, which can do .86, but a cleansheet would have had a less-swept and higher-span wing planform.

shamrock137 wrote:

Starlionblue wrote:

shamrock137 wrote:

I never really thought about it much, but its interesting the difference in widebody cruise speeds, all for different reasons. 747 was always designed to go fast, fuel burn wasn't as much of an issue. Then you have the 787/350 which also are relatively fast, but fuel burn was a huge factor in their designs. They have the advantage of new materials, advances in aerodynamics, and a mission profile right from design of long haul optimization.

The 767 and 330 are funny, most people loop them right in with "widebody" but their initial design put them in shorter to medium range markets, as ETOPS wasn't a thing and ULH hadn't really been developed beyond the 747. Airbus had envisioned the 340 being the long haul market leader, until ETOPS changed the landscape, and carriers wanted twins to fly near their maximum ranges and beyond. So you end up with the 330 and 767 both being extended beyond their initial design range goals, but still having a cruise speed around .82 if my research is correct.

Then over the Atlantic you end up with an A321 doing .78 which must make things interesting for ATC.

ETOPS started being "a thing" many years prior to the A330. The A300 started using 90 minute ETOPS in 1976. 120 minute ETOPS came along in 1985, and 180 minutes in 1988, well before A330 EIS.

The A340 was the long haul version with significantly higher MTOW than the A330, and four engines to carry that.

Yes, should have worded it better. ETOPS existed, but development of the A330/340 started in the late 70's, early 80's, before it was accepted that twin engine aircraft would operate ULH flights. My point was simply that the A330 and 767 weren't developed as a the long haul aircraft they have turned into, especially the 330. The 340-200 was delivered with a range of 6,700nm, while the last 242t versions of the 332 are at 7260nm. Impressive how far the airframe has come.

Question for you though Starlionblue, since the thread is about cruise speed. Would you notice any difference between the 330 and 350 on the same route?

Just looking at the PFD; yes sure. But to realy notice a difference in sector time, it would have to be more than a couple of hours sector length. Beyond 4-6 hours you definitely would.

The A350 sits at 0.85 all day long. The A330 likes 0.81-0.82. The fuel burn is about the same, but the A350 carries more, further, faster.

Where you really notice the difference is initial flight levels on sectors beyond a few hours. You can go significantly higher in the A350, which means you're more likely to get your optimal level, giving you better trip burn.

Starlionblue wrote:

shamrock137 wrote:

Starlionblue wrote:

ETOPS started being "a thing" many years prior to the A330. The A300 started using 90 minute ETOPS in 1976. 120 minute ETOPS came along in 1985, and 180 minutes in 1988, well before A330 EIS.

The A340 was the long haul version with significantly higher MTOW than the A330, and four engines to carry that.

Yes, should have worded it better. ETOPS existed, but development of the A330/340 started in the late 70's, early 80's, before it was accepted that twin engine aircraft would operate ULH flights. My point was simply that the A330 and 767 weren't developed as a the long haul aircraft they have turned into, especially the 330. The 340-200 was delivered with a range of 6,700nm, while the last 242t versions of the 332 are at 7260nm. Impressive how far the airframe has come.

Question for you though Starlionblue, since the thread is about cruise speed. Would you notice any difference between the 330 and 350 on the same route?

Just looking at the PFD; yes sure. But to realy notice a difference in sector time, it would have to be more than a couple of hours sector length. Beyond 4-6 hours you definitely would.

The A350 sits at 0.85 all day long. The A330 likes 0.81-0.82. The fuel burn is about the same, but the A350 carries more, further, faster.

Where you really notice the difference is initial flight levels on sectors beyond a few hours. You can go significantly higher in the A350, which means you're more likely to get your optimal level, giving you better trip burn.

Very interesting! Thank you for the first person expertise as always. Always fun to see a new aircraft type on the same sector and compare specs. I've been comparing the A220 to the 320/321 on some routes, 2 westbound flights at about a 6.5 hour flight, the difference in actual fuel burned for the 220 and 321 is about 11,000 lbs. 2,000lbs difference on a 2.0 hour flight between the 320 and 220.

Starlionblue wrote:

shamrock137 wrote:

Starlionblue wrote:

ETOPS started being "a thing" many years prior to the A330. The A300 started using 90 minute ETOPS in 1976. 120 minute ETOPS came along in 1985, and 180 minutes in 1988, well before A330 EIS.

The A340 was the long haul version with significantly higher MTOW than the A330, and four engines to carry that.

Yes, should have worded it better. ETOPS existed, but development of the A330/340 started in the late 70's, early 80's, before it was accepted that twin engine aircraft would operate ULH flights. My point was simply that the A330 and 767 weren't developed as a the long haul aircraft they have turned into, especially the 330. The 340-200 was delivered with a range of 6,700nm, while the last 242t versions of the 332 are at 7260nm. Impressive how far the airframe has come.

Question for you though Starlionblue, since the thread is about cruise speed. Would you notice any difference between the 330 and 350 on the same route?

Just looking at the PFD; yes sure. But to realy notice a difference in sector time, it would have to be more than a couple of hours sector length. Beyond 4-6 hours you definitely would.

The A350 sits at 0.85 all day long. The A330 likes 0.81-0.82. The fuel burn is about the same, but the A350 carries more, further, faster.

Where you really notice the difference is initial flight levels on sectors beyond a few hours. You can go significantly higher in the A350, which means you're more likely to get your optimal level, giving you better trip burn.

This makes total sense, and perfectly illustrates the advantage speed has no longer flights. It is similar to how an E-Jet can do near transcons like YYZ-PDX, but a 737/A320 is going to do it better and faster.

How would an L-1011 compare to a 767? For some reason I'm thinking the 767 would outperform the L-1011 by being able to fly higher and faster but no idea why I have that impression.

With NB now regularly flying 5hr + sectors, it will be interesting to see if the next generation of NB will indeed fly similar speeds to WB.

T54A wrote:

With NB now regularly flying 5hr + sectors, it will be interesting to see if the next generation of NB will indeed fly similar speeds to WB.

Well, they are faster than before. The 737 used to cruise at .74. Now they cruise more like .80.

N1120A wrote:

T54A wrote:

With NB now regularly flying 5hr + sectors, it will be interesting to see if the next generation of NB will indeed fly similar speeds to WB.

Well, they are faster than before. The 737 used to cruise at .74. Now they cruise more like .80.

I think that faster NB speeds will only happen if they're optimized primarily for the longer sectors, which I don't see happening. The vast majority of 737s and A321s do 1-3 hour flights, and then pinch-hit on 5-hour flights a small fraction of the time. The engines and layout are optimized for those shorter sectors, especially the MAX. If you can get the extra speed for free it would be nice, but if there's any measurable cost on the short sectors (weight, payload, efficiency) to shave 15 minutes on a 5-hour flight, I doubt you'd have many takers.

LH707330 wrote:

N1120A wrote:

T54A wrote:

With NB now regularly flying 5hr + sectors, it will be interesting to see if the next generation of NB will indeed fly similar speeds to WB.

Well, they are faster than before. The 737 used to cruise at .74. Now they cruise more like .80.

I think that faster NB speeds will only happen if they're optimized primarily for the longer sectors, which I don't see happening. The vast majority of 737s and A321s do 1-3 hour flights, and then pinch-hit on 5-hour flights a small fraction of the time. The engines and layout are optimized for those shorter sectors, especially the MAX. If you can get the extra speed for free it would be nice, but if there's any measurable cost on the short sectors (weight, payload, efficiency) to shave 15 minutes on a 5-hour flight, I doubt you'd have many takers.

The MAX and NEO engines, it can be argued, are absolutely not optimized for shorter sectors.

N1120A wrote:

LH707330 wrote:

N1120A wrote:

Well, they are faster than before. The 737 used to cruise at .74. Now they cruise more like .80.

I think that faster NB speeds will only happen if they're optimized primarily for the longer sectors, which I don't see happening. The vast majority of 737s and A321s do 1-3 hour flights, and then pinch-hit on 5-hour flights a small fraction of the time. The engines and layout are optimized for those shorter sectors, especially the MAX. If you can get the extra speed for free it would be nice, but if there's any measurable cost on the short sectors (weight, payload, efficiency) to shave 15 minutes on a 5-hour flight, I doubt you'd have many takers.

The MAX and NEO engines, it can be argued, are absolutely not optimized for shorter sectors.

Are you referring to the startup and shutdown cooling period? From what I know, I believe those issues were resolved, but stand to be corrected. Looking at some 321 vs 321NEO flights, the fuel burn difference for a 3 hour sector is about 3500-4000 lbs. I'd imagine operators are happy what that.

shamrock137 wrote:

N1120A wrote:

LH707330 wrote:

I think that faster NB speeds will only happen if they're optimized primarily for the longer sectors, which I don't see happening. The vast majority of 737s and A321s do 1-3 hour flights, and then pinch-hit on 5-hour flights a small fraction of the time. The engines and layout are optimized for those shorter sectors, especially the MAX. If you can get the extra speed for free it would be nice, but if there's any measurable cost on the short sectors (weight, payload, efficiency) to shave 15 minutes on a 5-hour flight, I doubt you'd have many takers.

The MAX and NEO engines, it can be argued, are absolutely not optimized for shorter sectors.

Are you referring to the startup and shutdown cooling period? From what I know, I believe those issues were resolved, but stand to be corrected. Looking at some 321 vs 321NEO flights, the fuel burn difference for a 3 hour sector is about 3500-4000 lbs. I'd imagine operators are happy what that.

The MAX and NEO definitely burn less fuel, but the cooling periods and the long start up both create issues that are less conducive for shorter legs.

N1120A wrote:

shamrock137 wrote:

N1120A wrote:

The MAX and NEO engines, it can be argued, are absolutely not optimized for shorter sectors.

Are you referring to the startup and shutdown cooling period? From what I know, I believe those issues were resolved, but stand to be corrected. Looking at some 321 vs 321NEO flights, the fuel burn difference for a 3 hour sector is about 3500-4000 lbs. I'd imagine operators are happy what that.

The MAX and NEO definitely burn less fuel, but the cooling periods and the long start up both create issues that are less conducive for shorter legs.

Today there is still a longer startup time for the LEAP than the earlier CFM-56. But it's been mitigated to generally under 2 minutes difference. Two engines can be cranked at once in the rotor motoring phase to save time. That takes the engine to 25% N1. Then the reliable start speed is also higher for the LEAP than the CFM-56 (63% rather 57%).

2 minutes feels like a long time while sitting there waiting, especially when the ramp is congested and pilots are ready to go. And it can add up when the offset is accumulated by many waiting aircraft. But doesn't significantly affect flight times or schedules.

It's just a matter of physics, more efficient engines run at higher temperatures and suffer more from heat soak and rotor bow on shutdown.

It may be time for manufacturers to find outside-the-box ground cooling solutions if startup (and cooldown) times start to have a negative impact on sector time. We're indeed getting to the point where extended starting sequences are costing as much time as a few % of Mach on a short sector.

Memories of tediously hand-spinning Garretts between flights have me wondering if a 'ground cooling' mode could be enabled by slowly spinning the inner shaft during turnarounds. Low pressure bleed through the starter would do it. It would have to be slow enough to not be a hazard to ground crew but fast enough to create a slow but steady flow of air through the core.

Francoflier wrote:

It may be time for manufacturers to find outside-the-box ground cooling solutions if startup (and cooldown) times start to have a negative impact on sector time. We're indeed getting to the point where extended starting sequences are costing as much time as a few % of Mach on a short sector.

Memories of tediously hand-spinning Garretts between flights have me wondering if a 'ground cooling' mode could be enabled by slowly spinning the inner shaft during turnarounds. Low pressure bleed through the starter would do it. It would have to be slow enough to not be a hazard to ground crew but fast enough to create a slow but steady flow of air through the core.

I asked that question of an aerospace engineer who works for an engine manufacturer. He said they had looked into it, but as you mentioned, the ground hazard is the issue. For a slow rotation of the fan, the air temperature coming out the back would burn any exposed skin. If you increase the speed to lower the exhaust temperature, the area of influence expands behind the engine. So Catch-22 situation.

Too much stored energy in too small a space and time interval. One instance where the power-to-size density ratio, which is otherwise a benefit, works against the turbofan engine.

I've wondered if a 90 degree elbow duct could be rolled up behind the engines, to point the exhaust vertically upward. But I'm sure there are still issues with having the fan spinning at the gate.

The total start time of an A320NEO with PW1100 engines with dual cooling function is only marginally longer than the total start time of CEO aircraft with IAE engines. I don't think that this marginally longer start time is a major factor, except in extraordinary circumstances (i.e. out of the dual cooling envelope).
An other thing to note is that the cruise speed of the A320NEO is slightly lower than that of an A320CEO at the same weight and cost index. The PW1100 has a better SFC at lower speeds (compared to the IAE/CFM engine). Furthermore the parasite drag of the aircraft is slightly higher due to the bigger engine nacelle size which also reduces optimum speed slightly (as the induced drag is the same on both variants).

Avatar2go wrote:

Francoflier wrote:

It may be time for manufacturers to find outside-the-box ground cooling solutions if startup (and cooldown) times start to have a negative impact on sector time. We're indeed getting to the point where extended starting sequences are costing as much time as a few % of Mach on a short sector.

Memories of tediously hand-spinning Garretts between flights have me wondering if a 'ground cooling' mode could be enabled by slowly spinning the inner shaft during turnarounds. Low pressure bleed through the starter would do it. It would have to be slow enough to not be a hazard to ground crew but fast enough to create a slow but steady flow of air through the core.

I asked that question of an aerospace engineer who works for an engine manufacturer. He said they had looked into it, but as you mentioned, the ground hazard is the issue. For a slow rotation of the fan, the air temperature coming out the back would burn any exposed skin. If you increase the speed to lower the exhaust temperature, the area of influence expands behind the engine. So Catch-22 situation.

Too much stored energy in too small a space and time interval. One instance where the power-to-size density ratio, which is otherwise a benefit, works against the turbofan engine.

I've wondered if a 90 degree elbow duct could be rolled up behind the engines, to point the exhaust vertically upward. But I'm sure there are still issues with having the fan spinning at the gate.

Great stuff, thanks.
I hadn't realised the exhaust temperature would be such a threat, at least not anymore than hot brakes or pack exhaust would be. I guess it really gets toasty in there...
It'll be interesting to see how such extreme heat cycling will affect engine service lives going forward.

shamrock137 wrote:

N1120A wrote:

LH707330 wrote:

I think that faster NB speeds will only happen if they're optimized primarily for the longer sectors, which I don't see happening. The vast majority of 737s and A321s do 1-3 hour flights, and then pinch-hit on 5-hour flights a small fraction of the time. The engines and layout are optimized for those shorter sectors, especially the MAX. If you can get the extra speed for free it would be nice, but if there's any measurable cost on the short sectors (weight, payload, efficiency) to shave 15 minutes on a 5-hour flight, I doubt you'd have many takers.

The MAX and NEO engines, it can be argued, are absolutely not optimized for shorter sectors.

Are you referring to the startup and shutdown cooling period? From what I know, I believe those issues were resolved, but stand to be corrected. Looking at some 321 vs 321NEO flights, the fuel burn difference for a 3 hour sector is about 3500-4000 lbs. I'd imagine operators are happy what that.

I was referring to the sector lengths. There's a thread somewhere where Lightsaber talks about the fan diameters, BPR, OPR, and variable nozzles on the 1100G optimized for 3-hour sectors, and the CFM LEAP 2B optimized for either 90 min or 2 hours, IIRC. If they wanted to optimize them for 5-hour sectors, they'd have higher BPRs and less cruise burn in exchange for worse mx costs and weight penalties in the climb.