It seems taken as read universally that 2 engines are better than 4, but why? It seems to me that it is probably more nuanced than the general consensus:
for 2 engines:
1) Boundary layer effects - the bigger the engine, the more efficient the core
2) the larger the engine, the smaller the nacelle is in proportion to the fan area = less drag
3) 100% excess thrust requirement on take-off means faster climb to altitude
4) 2 sets of ancillaries vs 4 sets - less weight and cost (both manufacturing and probably maintenance)

for 4 engines:
1) better weight distribution across the wing
2) 33% excess thrust requirement (for engine out at take-off) - less weight - if a plane needs 100k lbs thrust to take off, then a 4 engined aircraft can have 4 engines of 33.3k lbs thrust rather than 2 of 100k lbs thrust. However, the boundary layer effects (point 1 above) counter this
3) less clearance required = smaller landing gear
4) Engines can be correctly sized for cruise, rather than single engine take off, so punch smaller hole in the air - less drag

Interesting that (according to Wiki) the CFM56-5C is 2.6t dry vs 7.8t for a 1st gen GE90 or a 6.1t for a Trent 800, so four are considerably lighter than 2 large ones.

I wonder how much of the "2 engines good, 4 engines bad" is :
1) simply technology differences - the A340-300 had the CFM56-5C (was this developed from the -5B or the -5A (it flew before the -5B)?) while the early GE90 was a bit of a dog of an engine and it really only came into its own with the GE90-110/115B, which was at least 10 years more advanced than the CFM56-5C, while the Trent 500 was a lower bypass, so less efficient (and based of older tech too).
2) the A340-600 was overweight - about a third due to engines, but the rest because it was a flying pencil, so the slenderness ratio wasn't great and the wings were stretches of stretches, so were probably compromises too. The A380-800 was also overweight as sized for the -900, while the engines were also less efficient for QC2 (although this presumably applies to twins too?)

My feeling is that the law of diminishing returns comes into effect and two 100k lbs engines are sufficiently more efficient than four 33.3k lbs engines, but may be two 133.3k lbs engines might not be more efficient than four 50k lbs engines?
any thoughts?

Looks like the industry has answered this question.

lhrnue wrote:

Looks like the industry has answered this question.

More than 20 years ago, really:

777ER vs. A342/343

Then 77W vs. A345/346

The later-gen A330ceos vs. 342/343

Add, perhaps finally for this generation, 77W vs. A380

You could add 763 vs. MD-11 to the 'more than 2 engine' list if you wanted.

This isn't a new question.
1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum. That us tough to justify.
2. Maintenance overhauls on the engines is $4.5 million each. Airlines try to drag it out, but about another $1 million per year for the quad
3. You are right on fuel burn. The big effect is tip losses which go with engine core diameter while thrust goes with diameter squared. The same with surface area. This means larger engines have free pressure ratio for the same combustor inlet temperature.
4. About another 8 tons more weight for the quad. That us 8 tons lost payload for... an increase in fuel burn, manufacturing cost, and maintenance costs. Some stuff like rotors, shafts, thrust bearing regulators (called sump back pressure regulator, but their job is to maintain the air bearing pressure to counter the thrust) weight pretty much the same per engine. Oh a little increase in weight with thrust, but not as much as you expect.

Quads worked before back when a 62,000 lbf thrust engine was the best that could be done. Now that 125,000 lbf wouldn't be risky, there won't be a quad unless it is huge! or a BWB.

In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets.

Lightsaber

Late edit:
There is one case where quads make sense. If an existing engine is used and the business case predicts less than 400 aircraft are to be sold, but forget anything competing with the 779, A350, or smaller. That market space has efficient twins, no quad need apply. Since a quad needs size to be economically competitive, we are talking 90m wingspan, which with folding wingtips is quite possible for enough airports. At least enough airports that would fill a new quad.

I believe the first BWB will have to be large (their efficiency gain is best over 500 seats). That would also be a 90m wingspan... Recall a BWB needs more wingspan for the same passenger load. Also recall underside laminar flow, the technology on wings still just entering the fleet (the Airbus "wing twist" had a little, but there is another 5%+ fuel burn reduction available). Underside laminar flow needs high aspect ratio wings.

That's partially my point, really:

777ER (1990 technology engine) vs. A342/343 (1980 technology engine)

Then 77W vs. A345/346 (too heavy - 6-7 tonnes heavier, of which about 2 tonnes was engine - fundamentally compromised by too thin a fuselage and double wing inserts - an attempt at countering the 77W with an update of what they had rather than spending huge sums on all new plane)

The later-gen A330ceos (1990 technology engines) vs. 342/343 (1980 technology engine)

Add, perhaps finally for this generation, 77W vs. A380 - perhaps, but massive size difference - in similar configuration, with similar passenger comfort, I'm not sure the CASM was that different, but the risk created by having to fill the A380 was much bigger, therefore the 77W won out.

You could add 763 vs. MD-11 to the 'more than 2 engine' list if you wanted - thought about that, but tail engine is a pretty huge compromise

Clearly the industry has gone to twins, but my question is whether it is causal or coincident and, if causal, what are the balances?

"In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets."

OK, that's fascinating - that would appear to suggest that the optimum size of turbofan is in the 60-100k lbs range, so it isn't "2 engines good, 4 engines bad" per se, but "at the current sizes of current wide bodies, 2 engines are better than 4", particularly with the downgauging trend we are seeing.

thank you

Has there ever been a case where four engines were chosen when two engines could have done the job?

Carmitage wrote:

"In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets."

OK, that's fascinating - that would appear to suggest that the optimum size of turbofan is in the 60-100k lbs range, so it isn't "2 engines good, 4 engines bad" per se, but "at the current sizes of current wide bodies, 2 engines are better than 4", particularly with the downgauging trend we are seeing.

thank you

We can agree, except, optimum size of a widebody turbofan is 45k to 125k. I was a fan of the A380. But I was a fan as the A388F enabled the A380R (high MTOW) which enabled the A389.

Studies are in work for 150k lbf engines. There is no natural limit, just an adversion to going into the unknown. We have 115k engines flying every day. The 779 only has a thrust reduction because the optimum CFRP wing has more wing area (lift) than a beer can wing.

It is difficult to explain the engine and airframe optimization process. Basically the airframer proposes an airframe with certain assumed engine dimensions, weights, efficiency, which gives the needed thrust. Then the airframer and engine vendor start optimizing.

The airframer finds out, for example, Alcoa has an aluminum at a higher TRL (technology readiness level) due to testing investment than the airframer realized. So they put in that aluminum which changes the tail shape just a little. A new carbon fiber epoxy might lighten the wing, but every stage if aircraft design requires a minimum TRL level and id that material isn't up to the TRL, do not consider it. If it is, the material and processes group (M&P) does a risk analysis. Depending on the program guidelines, more risk might or might not be allowed (lower TRL levels mean more risk and points are assigned as well as budgets. e.g., GE gas repeatedly spent hundreds if millions in testing quickly to bring up TRL levels, so they get more benefit if the doubt than Pratt):

NASA took Northrop's unpublished TRL levels and made them industry standard (no link where they came from):
https://www.nasa.gov/directorates/heo/s ... dion1.html

The best way us talk TRL levels and discuss how to get there:

My wording of TRL levels.
TRL9, been there, done that, and we are not extrapolating (e.g., to a higher thrust or much smaller thrust. e.g., now for GTF from 17k to 35k). Best example is CFM-56 for A340-300. Some companies won't consider an engine at TRL9 until it can prove it can serve the promised cycle or hour life. For example, the CF34-10 by most airlines is not yet at TRL9 by their standards, nor the T1000, T7000, PW GTFs, nor Passport.


TRL8 We've done it before, but maybe not this thrust level e.g. a GTF for a widebody. At EIS, new technology is at TRL8 at entry into passenger service. e.g., GE9x when 779 is certified. Certainly the PW1100G for the first few years in service.


TRL7 Billions were spent to prove the concept. e.g., Pratt on GTF before C-series launch. Multiple prototype engines, hundreds of hours of test time, and full failure modes testing. Unfortunately, I have failed concepts going from TRL levels to this level and below. I have personally discovered two failure modes the industry didn't think about and derived how to calculate margins to those failure modes using good 1st principal equations. That ended one concept and resulted in a changed design criteria for another.

Now Bombardier and Airbus agreed the GTF was at TRL7, Boeing considered it one TRL level lower and that made all the difference in who put it on their aircraft.

A350 had little that wasn't TRL7 at launch. Only the RR turbine type change was at TRL6 comes to mind (two stage intermediate turbine and single stage high turbine just wasn't the practice before).

The GTF took 30 years of development to hit TRL7 and Boeing wanted a lot more testing (about $200 million worth).

late edit:
An engine cannot be, by my opinion, at TRL7 until sand, ice, small bird and large bird ingestion tests are complete.

TRL6, we've got a great concept, look at our prototype. Today's ultrafan today or the GTF in 2000. For Pratt, the variable fan nozzle. This is the minimum most programs want at launch. This has been flown and went through over a third of failure modes testing.

Folks, the propfan has a future, but while GE and Douglas considered it to pass this level, airlines with their passenger comfort (noise) and maintenance (vibration) called it a failure. So we can debate if the propfan is at TRL5 (airline's opinion) or TRL6 (GE/Douglas opinion). Err... golden rule applies.

I haven't seen a major concept on an engine brought up to TRL6 in under 6 years. I've read the history of WW2 where they hit TRL6 fast in combat... not for today's commercial engines.

Gulfstream rejected the Passport, in my opinion, as it did not meet their TRL6 standards which is, in my opinion, the strictest in the industry for engines and went with Pearl. Pratt had to do an uneconomical amount of testing on the PW815 (which is internally tested to PW816 standards) to buy their way into the business jet market and meet Gulfstream's TRL6 standards which is below some airframers TRL7 standard for an engine!

TRL5 We've spent ten millions+ proving thus great concept is viable. This is the minimum for a high risk program at program launch. It spent at least a hundred hours on prototype engines being tested. If not in flight, in the temperature controlled vacuum chambers and has proven takeoff, climb, cruise, step climb, flight idle, decent, approach, reverse thrust, aborted takeoff, and over 10% if failure modes testing. The 787 was launched with an amazing amount of TRL5 technology, in my opinion we will never see that again (due to cost, delays, missed weight, and EIS reliability/issues). TRL4 to TRL5 I've never heard of being done in less than a year, and that was on Apollo program spend what it takes budgets.

TRL4 We've spent a million or two and it didn't fail. This did a few flight like environments, but us not a mass producible design. I have as a member of a team of two taken Whittle concepts (TRL1) up to TRL4 personally. It takes a few years and a good sized budget with a few person years if expense.

TRL3: Simulation proved it! Most if what a.net discussion is at this level or below. A good team can bring a TRL1 to TRL3 in less than 6 months. It just takes paying for 5+ people years if salary, benefits, and computer time. I was promoted early in my career for hitting TRL3 in a tear with 2 person years of labor (much better than typical) in my first year in industry.

TRL2: Look at our great rendering of this theoretical concept that has never been in a flight like environment. Some simulation by at most a small team. TRL2 I've personally hit in 2 weeks.

TRL1: The bright people want money to prove this great idea! TRL1 can be done in a day.

So only by understanding each airframer and engine company's take on TRL levels can you understand what can or cannot be done. 125k of thrust is at TRL7 in my opinion. 150k is at a good TRL 5. (again, my opinion).

Two decks was at TRL5 at A380 launch. It is now debatable if TRL9 as so many refinements are needed (lessons learned).

When debating topics on a.net, TRL level and applicability us always on my mind.

Lightsaber

lightsaber wrote:

Carmitage wrote:

"In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets."

OK, that's fascinating - that would appear to suggest that the optimum size of turbofan is in the 60-100k lbs range, so it isn't "2 engines good, 4 engines bad" per se, but "at the current sizes of current wide bodies, 2 engines are better than 4", particularly with the downgauging trend we are seeing.

thank you

We can agree, except, optimum size of a widebody turbofan is 45k to 125k. I was a fan of the A380. But I was a fan as the A388F enabled the A380R (high MTOW) which enabled the A389.

Studies are in work for 150k lbf engines. There is no natural limit, just an adversion to going into the unknown. We have 115k engines flying every day. The 779 only has a thrust reduction because the optimum CFRP wing has more wing area (lift) than a beer can wing.

It is difficult to explain the engine and airframe optimization process. Basically the airframer proposes an airframe with certain assumed engine dimensions, weights, efficiency, which gives the needed thrust. Then the airframer and engine vendor start optimizing.

The airframer finds out, for example, Alcoa has an aluminum at a higher TRL (technology readiness level) due to testing investment than the airframer realized. So they put in that aluminum which changes the tail shape just a little. A new carbon fiber epoxy might lighten the wing, but every stage if aircraft design requires a minimum TRL level and id that material isn't up to the TRL, do not consider it. If it is, the material and processes group (M&P) does a risk analysis. Depending on the program guidelines, more risk might or might not be allowed (lower TRL levels mean more risk and points are assigned as well as budgets. e.g., GE gas repeatedly spent hundreds if millions in testing quickly to bring up TRL levels, so they get more benefit if the doubt than Pratt):

NASA took Northrop's unpublished TRL levels and made them industry standard

First off, once again you deliver, thank you.

So, looking at this list, would the unducted turbofan be at TRL 4?

lightsaber wrote:

Carmitage wrote:

"In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets."

OK, that's fascinating - that would appear to suggest that the optimum size of turbofan is in the 60-100k lbs range, so it isn't "2 engines good, 4 engines bad" per se, but "at the current sizes of current wide bodies, 2 engines are better than 4", particularly with the downgauging trend we are seeing.

thank you

We can agree, except, optimum size of a widebody turbofan is 45k to 125k. I was a fan of the A380. But I was a fan as the A388F enabled the A380R (high MTOW) which enabled the A389.

Studies are in work for 150k lbf engines. There is no natural limit, just an adversion to going into the unknown. We have 115k engines flying every day. The 779 only has a thrust reduction because the optimum CFRP wing has more wing area (lift) than a beer can wing.

It is difficult to explain the engine and airframe optimization process. Basically the airframer proposes an airframe with certain assumed engine dimensions, weights, efficiency, which gives the needed thrust. Then the airframer and engine vendor start optimizing.

The airframer finds out, for example, Alcoa has an aluminum at a higher TRL (technology readiness level) due to testing investment than the airframer realized. So they put in that aluminum which changes the tail shape just a little. A new carbon fiber epoxy might lighten the wing, but every stage if aircraft design requires a minimum TRL level and id that material isn't up to the TRL, do not consider it. If it is, the material and processes group (M&P) does a risk analysis. Depending on the program guidelines, more risk might or might not be allowed (lower TRL levels mean more risk and points are assigned as well as budgets. e.g., GE gas repeatedly spent hundreds if millions in testing quickly to bring up TRL levels, so they get more benefit if the doubt than Pratt):

NASA took Northrop's unpublished TRL levels and made them industry standard (no link where they came from):
https://www.nasa.gov/directorates/heo/s ... dion1.html

The best way us talk TRL levels and discuss how to get there:

My wording of TRL levels.
TRL9, been there, done that, and we are not extrapolating (e.g., to a higher thrust or much smaller thrust. e.g., now for GTF from 17k to 35k). Best example is CFM-56 for A340-300. Some companies won't consider an engine at TRL9 until it can prove it can serve the promised cycle or hour life. For example, the CF34-10 by most airlines is not yet at TRL9 by their standards, nor the T1000, T7000, PW GTFs, nor Passport.


TRL8 We've done it before, but maybe not this thrust level e.g. a GTF for a widebody. At EIS, new technology is at TRL8 at entry into passenger service. e.g., GE9x when 779 is certified. Certainly the PW1100G for the first few years in service.


TRL7 Billions were spent to prove the concept. e.g., Pratt on GTF before C-series launch. Multiple prototype engines, hundreds of hours of test time, and full failure modes testing. Unfortunately, I have failed concepts going from TRL levels to this level and below. I have personally discovered two failure modes the industry didn't think about and derived how to calculate margins to those failure modes using good 1st principal equations. That ended one concept and resulted in a changed design criteria for another.

Now Bombardier and Airbus agreed the GTF was at TRL7, Boeing considered it one TRL level lower and that made all the difference in who put it on their aircraft.

A350 had little that wasn't TRL7 at launch. Only the RR turbine type change was at TRL6 comes to mind (two stage intermediate turbine and single stage high turbine just wasn't the practice before).

The GTF took 30 years of development to hit TRL7 and Boeing wanted a lot more testing (about $200 million worth).

late edit:
An engine cannot be, by my opinion, at TRL7 until sand, ice, small bird and large bird ingestion tests are complete.

TRL6, we've got a great concept, look at our prototype. Today's ultrafan today or the GTF in 2000. For Pratt, the variable fan nozzle. This is the minimum most programs want at launch. This has been flown and went through over a third of failure modes testing.

Folks, the propfan has a future, but while GE and Douglas considered it to pass this level, airlines with their passenger comfort (noise) and maintenance (vibration) called it a failure. So we can debate if the propfan is at TRL5 (airline's opinion) or TRL6 (GE/Douglas opinion). Err... golden rule applies.

I haven't seen a major concept on an engine brought up to TRL6 in under 6 years. I've read the history of WW2 where they hit TRL6 fast in combat... not for today's commercial engines.

Gulfstream rejected the Passport, in my opinion, as it did not meet their TRL6 standards which is, in my opinion, the strictest in the industry for engines and went with Pearl. Pratt had to do an uneconomical amount of testing on the PW815 (which is internally tested to PW816 standards) to buy their way into the business jet market and meet Gulfstream's TRL6 standards which is below some airframers TRL7 standard for an engine!

TRL5 We've spent ten millions+ proving thus great concept is viable. This is the minimum for a high risk program at program launch. It spent at least a hundred hours on prototype engines being tested. If not in flight, in the temperature controlled vacuum chambers and has proven takeoff, climb, cruise, step climb, flight idle, decent, approach, reverse thrust, aborted takeoff, and over 10% if failure modes testing. The 787 was launched with an amazing amount of TRL5 technology, in my opinion we will never see that again (due to cost, delays, missed weight, and EIS reliability/issues). TRL4 to TRL5 I've never heard of being done in less than a year, and that was on Apollo program spend what it takes budgets.

TRL4 We've spent a million or two and it didn't fail. This did a few flight like environments, but us not a mass producible design. I have as a member of a team of two taken Whittle concepts (TRL1) up to TRL4 personally. It takes a few years and a good sized budget with a few person years if expense.

TRL3: Simulation proved it! Most if what a.net discussion is at this level or below. A good team can bring a TRL1 to TRL3 in less than 6 months. It just takes paying for 5+ people years if salary, benefits, and computer time. I was promoted early in my career for hitting TRL3 in a tear with 2 person years of labor (much better than typical) in my first year in industry.

TRL2: Look at our great rendering of this theoretical concept that has never been in a flight like environment. Some simulation by at most a small team. TRL2 I've personally hit in 2 weeks.

TRL1: The bright people want money to prove this great idea! TRL1 can be done in a day.

So only by understanding each airframer and engine company's take on TRL levels can you understand what can or cannot be done. 125k of thrust is at TRL7 in my opinion. 150k is at a good TRL 5. (again, my opinion).

Two decks was at TRL5 at A380 launch. It is now debatable if TRL9 as so many refinements are needed (lessons learned).

When debating topics on a.net, TRL level and applicability us always on my mind.

Lightsaber

I enjoy your technical posts. Thanks.

lightsaber wrote:

Carmitage wrote:

"In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets."

OK, that's fascinating - that would appear to suggest that the optimum size of turbofan is in the 60-100k lbs range, so it isn't "2 engines good, 4 engines bad" per se, but "at the current sizes of current wide bodies, 2 engines are better than 4", particularly with the downgauging trend we are seeing.

thank you

We can agree, except, optimum size of a widebody turbofan is 45k to 125k. I was a fan of the A380. But I was a fan as the A388F enabled the A380R (high MTOW) which enabled the A389.

Studies are in work for 150k lbf engines. There is no natural limit, just an adversion to going into the unknown. We have 115k engines flying every day. The 779 only has a thrust reduction because the optimum CFRP wing has more wing area (lift) than a beer can wing.

It is difficult to explain the engine and airframe optimization process. Basically the airframer proposes an airframe with certain assumed engine dimensions, weights, efficiency, which gives the needed thrust. Then the airframer and engine vendor start optimizing.

The airframer finds out, for example, Alcoa has an aluminum at a higher TRL (technology readiness level) due to testing investment than the airframer realized. So they put in that aluminum which changes the tail shape just a little. A new carbon fiber epoxy might lighten the wing, but every stage if aircraft design requires a minimum TRL level and id that material isn't up to the TRL, do not consider it. If it is, the material and processes group (M&P) does a risk analysis. Depending on the program guidelines, more risk might or might not be allowed (lower TRL levels mean more risk and points are assigned as well as budgets. e.g., GE gas repeatedly spent hundreds if millions in testing quickly to bring up TRL levels, so they get more benefit if the doubt than Pratt):

NASA took Northrop's unpublished TRL levels and made them industry standard (no link where they came from):
https://www.nasa.gov/directorates/heo/s ... dion1.html

The best way us talk TRL levels and discuss how to get there:

My wording of TRL levels.
TRL9, been there, done that, and we are not extrapolating (e.g., to a higher thrust or much smaller thrust. e.g., now for GTF from 17k to 35k). Best example is CFM-56 for A340-300. Some companies won't consider an engine at TRL9 until it can prove it can serve the promised cycle or hour life. For example, the CF34-10 by most airlines is not yet at TRL9 by their standards, nor the T1000, T7000, PW GTFs, nor Passport.


TRL8 We've done it before, but maybe not this thrust level e.g. a GTF for a widebody. At EIS, new technology is at TRL8 at entry into passenger service. e.g., GE9x when 779 is certified. Certainly the PW1100G for the first few years in service.


TRL7 Billions were spent to prove the concept. e.g., Pratt on GTF before C-series launch. Multiple prototype engines, hundreds of hours of test time, and full failure modes testing. Unfortunately, I have failed concepts going from TRL levels to this level and below. I have personally discovered two failure modes the industry didn't think about and derived how to calculate margins to those failure modes using good 1st principal equations. That ended one concept and resulted in a changed design criteria for another.

Now Bombardier and Airbus agreed the GTF was at TRL7, Boeing considered it one TRL level lower and that made all the difference in who put it on their aircraft.

A350 had little that wasn't TRL7 at launch. Only the RR turbine type change was at TRL6 comes to mind (two stage intermediate turbine and single stage high turbine just wasn't the practice before).

The GTF took 30 years of development to hit TRL7 and Boeing wanted a lot more testing (about $200 million worth).

late edit:
An engine cannot be, by my opinion, at TRL7 until sand, ice, small bird and large bird ingestion tests are complete.

TRL6, we've got a great concept, look at our prototype. Today's ultrafan today or the GTF in 2000. For Pratt, the variable fan nozzle. This is the minimum most programs want at launch. This has been flown and went through over a third of failure modes testing.

Folks, the propfan has a future, but while GE and Douglas considered it to pass this level, airlines with their passenger comfort (noise) and maintenance (vibration) called it a failure. So we can debate if the propfan is at TRL5 (airline's opinion) or TRL6 (GE/Douglas opinion). Err... golden rule applies.

I haven't seen a major concept on an engine brought up to TRL6 in under 6 years. I've read the history of WW2 where they hit TRL6 fast in combat... not for today's commercial engines.

Gulfstream rejected the Passport, in my opinion, as it did not meet their TRL6 standards which is, in my opinion, the strictest in the industry for engines and went with Pearl. Pratt had to do an uneconomical amount of testing on the PW815 (which is internally tested to PW816 standards) to buy their way into the business jet market and meet Gulfstream's TRL6 standards which is below some airframers TRL7 standard for an engine!

TRL5 We've spent ten millions+ proving thus great concept is viable. This is the minimum for a high risk program at program launch. It spent at least a hundred hours on prototype engines being tested. If not in flight, in the temperature controlled vacuum chambers and has proven takeoff, climb, cruise, step climb, flight idle, decent, approach, reverse thrust, aborted takeoff, and over 10% if failure modes testing. The 787 was launched with an amazing amount of TRL5 technology, in my opinion we will never see that again (due to cost, delays, missed weight, and EIS reliability/issues). TRL4 to TRL5 I've never heard of being done in less than a year, and that was on Apollo program spend what it takes budgets.

TRL4 We've spent a million or two and it didn't fail. This did a few flight like environments, but us not a mass producible design. I have as a member of a team of two taken Whittle concepts (TRL1) up to TRL4 personally. It takes a few years and a good sized budget with a few person years if expense.

TRL3: Simulation proved it! Most if what a.net discussion is at this level or below. A good team can bring a TRL1 to TRL3 in less than 6 months. It just takes paying for 5+ people years if salary, benefits, and computer time. I was promoted early in my career for hitting TRL3 in a tear with 2 person years of labor (much better than typical) in my first year in industry.

TRL2: Look at our great rendering of this theoretical concept that has never been in a flight like environment. Some simulation by at most a small team. TRL2 I've personally hit in 2 weeks.

TRL1: The bright people want money to prove this great idea! TRL1 can be done in a day.

So only by understanding each airframer and engine company's take on TRL levels can you understand what can or cannot be done. 125k of thrust is at TRL7 in my opinion. 150k is at a good TRL 5. (again, my opinion).

Two decks was at TRL5 at A380 launch. It is now debatable if TRL9 as so many refinements are needed (lessons learned).

When debating topics on a.net, TRL level and applicability us always on my mind.

Lightsaber

I enjoy your technical posts. Thanks.

Sokes wrote:

Has there ever been a case where four engines were chosen when two engines could have done the job?

BAe146? I believe there was no suitable engine if they wanted two so they went with four.

lightsaber wrote:

This isn't a new question.
1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum. That us tough to justify.
2. Maintenance overhauls on the engines is $4.5 million each. Airlines try to drag it out, but about another $1 million per year for the quad
3. You are right on fuel burn. The big effect is tip losses which go with engine core diameter while thrust goes with diameter squared. The same with surface area. This means larger engines have free pressure ratio for the same combustor inlet temperature.
4. About another 8 tons more weight for the quad. That us 8 tons lost payload for... an increase in fuel burn, manufacturing cost, and maintenance costs. Some stuff like rotors, shafts, thrust bearing regulators (called sump back pressure regulator, but their job is to maintain the air bearing pressure to counter the thrust) weight pretty much the same per engine. Oh a little increase in weight with thrust, but not as much as you expect.

Quads worked before back when a 62,000 lbf thrust engine was the best that could be done. Now that 125,000 lbf wouldn't be risky, there won't be a quad unless it is huge! or a BWB.

In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets.

Lightsaber

Late edit:
There is one case where quads make sense. If an existing engine is used and the business case predicts less than 400 aircraft are to be sold, but forget anything competing with the 779, A350, or smaller. That market space has efficient twins, no quad need apply. Since a quad needs size to be economically competitive, we are talking 90m wingspan, which with folding wingtips is quite possible for enough airports. At least enough airports that would fill a new quad.

I believe the first BWB will have to be large (their efficiency gain is best over 500 seats). That would also be a 90m wingspan... Recall a BWB needs more wingspan for the same passenger load. Also recall underside laminar flow, the technology on wings still just entering the fleet (the Airbus "wing twist" had a little, but there is another 5%+ fuel burn reduction available). Underside laminar flow needs high aspect ratio wings.

How will possible electric or fuel cell powered BWB aircraft using electric engine instead of jet engine affect the number of optimal engine count?

lightsaber wrote:

...
Pratt had to do an uneconomical amount of testing on the PW815 (which is internally tested to PW816 standards) to buy their way into the business jet market and meet Gulfstream's TRL6 standards which is below some airframers TRL7 standard for an engine!

Interesting post.
I believe you mean to say Gulfstream's TRL6 standards is above some airframers' TRL7 standard?

The 787 was launched with an amazing amount of TRL5 technology, in my opinion we will never see that again (due to cost, delays, missed weight, and EIS reliability/issues).

That's something to think about.

NameOmitted wrote:

lightsaber wrote:

Carmitage wrote:

"In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets."

OK, that's fascinating - that would appear to suggest that the optimum size of turbofan is in the 60-100k lbs range, so it isn't "2 engines good, 4 engines bad" per se, but "at the current sizes of current wide bodies, 2 engines are better than 4", particularly with the downgauging trend we are seeing.

thank you

We can agree, except, optimum size of a widebody turbofan is 45k to 125k. I was a fan of the A380. But I was a fan as the A388F enabled the A380R (high MTOW) which enabled the A389.

Studies are in work for 150k lbf engines. There is no natural limit, just an adversion to going into the unknown. We have 115k engines flying every day. The 779 only has a thrust reduction because the optimum CFRP wing has more wing area (lift) than a beer can wing.

It is difficult to explain the engine and airframe optimization process. Basically the airframer proposes an airframe with certain assumed engine dimensions, weights, efficiency, which gives the needed thrust. Then the airframer and engine vendor start optimizing.

The airframer finds out, for example, Alcoa has an aluminum at a higher TRL (technology readiness level) due to testing investment than the airframer realized. So they put in that aluminum which changes the tail shape just a little. A new carbon fiber epoxy might lighten the wing, but every stage if aircraft design requires a minimum TRL level and id that material isn't up to the TRL, do not consider it. If it is, the material and processes group (M&P) does a risk analysis. Depending on the program guidelines, more risk might or might not be allowed (lower TRL levels mean more risk and points are assigned as well as budgets. e.g., GE gas repeatedly spent hundreds if millions in testing quickly to bring up TRL levels, so they get more benefit if the doubt than Pratt):

NASA took Northrop's unpublished TRL levels and made them industry standard

First off, once again you deliver, thank you.

So, looking at this list, would the unducted turbofan be at TRL 4?

In my opinion, TRL4. In GE's opinion TRL5.

I tried to be clear, as best I could, how different companies have different standards. Both can be correct by their interpretation.

TRL levels are used for risk management. They are a gut check on how many millions away from success the program is. The answer can be multiple kilo millions.

Lightsaber

c933103 wrote:

lightsaber wrote:

This isn't a new question.
1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum. That us tough to justify.
2. Maintenance overhauls on the engines is $4.5 million each. Airlines try to drag it out, but about another $1 million per year for the quad
3. You are right on fuel burn. The big effect is tip losses which go with engine core diameter while thrust goes with diameter squared. The same with surface area. This means larger engines have free pressure ratio for the same combustor inlet temperature.
4. About another 8 tons more weight for the quad. That us 8 tons lost payload for... an increase in fuel burn, manufacturing cost, and maintenance costs. Some stuff like rotors, shafts, thrust bearing regulators (called sump back pressure regulator, but their job is to maintain the air bearing pressure to counter the thrust) weight pretty much the same per engine. Oh a little increase in weight with thrust, but not as much as you expect.

Quads worked before back when a 62,000 lbf thrust engine was the best that could be done. Now that 125,000 lbf wouldn't be risky, there won't be a quad unless it is huge! or a BWB.

In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets.

Lightsaber

Late edit:
There is one case where quads make sense. If an existing engine is used and the business case predicts less than 400 aircraft are to be sold, but forget anything competing with the 779, A350, or smaller. That market space has efficient twins, no quad need apply. Since a quad needs size to be economically competitive, we are talking 90m wingspan, which with folding wingtips is quite possible for enough airports. At least enough airports that would fill a new quad.

I believe the first BWB will have to be large (their efficiency gain is best over 500 seats). That would also be a 90m wingspan... Recall a BWB needs more wingspan for the same passenger load. Also recall underside laminar flow, the technology on wings still just entering the fleet (the Airbus "wing twist" had a little, but there is another 5%+ fuel burn reduction available). Underside laminar flow needs high aspect ratio wings.

How will possible electric or fuel cell powered BWB aircraft using electric engine instead of jet engine affect the number of optimal engine count?

Several BWB designs do not have a large rudder. In the event of engine loss, there isn't a counter for asymmetric thrust.

Sokes wrote:

lightsaber wrote:

...
Pratt had to do an uneconomical amount of testing on the PW815 (which is internally tested to PW816 standards) to buy their way into the business jet market and meet Gulfstream's TRL6 standards which is below some airframers TRL7 standard for an engine!

Interesting post.
I believe you mean to say Gulfstream's TRL6 standards is above some airframers' TRL7 standard?

The 787 was launched with an amazing amount of TRL5 technology, in my opinion we will never see that again (due to cost, delays, missed weight, and EIS reliability/issues).

That's something to think about.

Yes typo, Gulfstream demands more on engine TRL6 than... I'll leave unnamed some consider good enough for TRL7.

The 787 was a moon shot. The Pratt engine was rejected. Back then I ranted about Boeing not accepting an excellent design. Back then I just couldn't realize Pratt had 4 major TRL5 level items and going forward with more than one is silly. So GE and RR had two each. In my opinion for that whole paragraph.

Lightsaber

PM wrote:

Sokes wrote:

Has there ever been a case where four engines were chosen when two engines could have done the job?

BAe146? I believe there was no suitable engine if they wanted two so they went with four.

BAe146 was indeed aimed market for shorter runways (like LCY) - if longer runways were available, there were more efficient options like the F-28.

Carmitage wrote:

It seems taken as read universally that 2 engines are better than 4, but why?

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

hivue wrote:

Carmitage wrote:

It seems taken as read universally that 2 engines are better than 4, but why?

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

Engine failure doesn’t result in a diversion to the nearest suitable airport—crew has more options.

Antarius wrote:

c933103 wrote:

lightsaber wrote:

This isn't a new question.
1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum. That us tough to justify.
2. Maintenance overhauls on the engines is $4.5 million each. Airlines try to drag it out, but about another $1 million per year for the quad
3. You are right on fuel burn. The big effect is tip losses which go with engine core diameter while thrust goes with diameter squared. The same with surface area. This means larger engines have free pressure ratio for the same combustor inlet temperature.
4. About another 8 tons more weight for the quad. That us 8 tons lost payload for... an increase in fuel burn, manufacturing cost, and maintenance costs. Some stuff like rotors, shafts, thrust bearing regulators (called sump back pressure regulator, but their job is to maintain the air bearing pressure to counter the thrust) weight pretty much the same per engine. Oh a little increase in weight with thrust, but not as much as you expect.

Quads worked before back when a 62,000 lbf thrust engine was the best that could be done. Now that 125,000 lbf wouldn't be risky, there won't be a quad unless it is huge! or a BWB.

In serious (industry) studies I have participated in, quads only work for 800+ passenger (which has wingspan issues) and BWBs which, in my opinion, should never be a twin (no rudder except a drag rudder for engine out). For BWBs, quads are usually the best answer, in my opinion. Except for smaller (200 to 300 seat) BWBs, those often optimize as trijets.

Lightsaber

Late edit:
There is one case where quads make sense. If an existing engine is used and the business case predicts less than 400 aircraft are to be sold, but forget anything competing with the 779, A350, or smaller. That market space has efficient twins, no quad need apply. Since a quad needs size to be economically competitive, we are talking 90m wingspan, which with folding wingtips is quite possible for enough airports. At least enough airports that would fill a new quad.

I believe the first BWB will have to be large (their efficiency gain is best over 500 seats). That would also be a 90m wingspan... Recall a BWB needs more wingspan for the same passenger load. Also recall underside laminar flow, the technology on wings still just entering the fleet (the Airbus "wing twist" had a little, but there is another 5%+ fuel burn reduction available). Underside laminar flow needs high aspect ratio wings.

How will possible electric or fuel cell powered BWB aircraft using electric engine instead of jet engine affect the number of optimal engine count?

Several BWB designs do not have a large rudder. In the event of engine loss, there isn't a counter for asymmetric thrust.

All Aircraft are designed to handle asymmetric thrust in an engine loss. All BWBs have drag rudders. The MAX issue highlighted no single point failures allowed.

The fuel cell/hybrid powering fans must include a loss of thust. The difference is, two just fans could be podded together on the centerline (think pair of B-52 engines). This cannot be done with jet engines due to the potential of rotor failure. The engines must be placed far enough apart such that there is a less than one in a million chance if the 3 (there are almost always three equal sized pieces) buts of rotor flung out, each weighing 25 to 30 kg, have less than a 1 in a million chance of taking out the other engine.

I'm so old that the engine rotors that failed in the early 1990s, I can't even find the articles! (Big events at the start of my career.) So scary that it is a major concern in all turbo machinery.

But electric motors driving fans aren't as scary and putting another 40kg of kevlar should do the job, so I see two near centerline fans or two fans with different flow paths, but coaxial would work (I'm being lazy, patented by Pratt on the sonic cruiser powered by a gas turbine, trivially simple when powered by electric motors).

Did I mention I was paid a day per week for 3 years to dig through engine patents? In particular expired ones? Of course I perused all the new ones... I was the only one of my starting team to stick around as I just thought it natural to package a business case for development... and I found good stuff every few weeks.

Lightsaber

lightsaber wrote:

The engines must be placed far enough apart such that there is a less than one in a million chance if the 3 (there are almost always three equal sized pieces) buts of rotor flung out, each weighing 25 to 30 kg, have less than a 1 in a million chance of taking out the other engine.

Almost impossible to imagine that the other engine will only taken out once in a million rotor failures.

Or is this combined i.e. every 1000000 engine cycle/hours 1 catastrophic rotor failure that takes the other engine out.

GalaxyFlyer wrote:

hivue wrote:

Carmitage wrote:

It seems taken as read universally that 2 engines are better than 4, but why?

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

Engine failure doesn’t result in a diversion to the nearest suitable airport—crew has more options.

I take your point. My contention is, to take a strictly hypothetical example, if you had 1000 instances of twins with engine failure xxx minutes single engine from the nearest suitable and 1000 instances of four engine airliners with engine failure xxx minutes three engine from the nearest suitable the outcomes -- whatever they might be -- would overall be the same for both groups. As I say, I have no data to back up this opinion.

Sokes wrote:

Has there ever been a case where four engines were chosen when two engines could have done the job?

British Aerospace 146 ? ( Avro RJ )

The outcomes will be the same, they land 99.999999% of the time. The 0.000001% of the time they crash (probabilities invented). The question is which has greater risk. The fact twins must divert to the nearest suitable airport, which could be 240 minutes away, still implies some minute greater risk, they cannot continue.

I’d say the likelihood of either type crashing due to engine failure is close to the same. As opposed to Part 23 planes, where twins have several times the fatal accident than singles

lightsaber wrote:

1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum.

This isn’t true, for example a GE90 list price is in excess of 40 million, airlines would purchase them for around 25.

lightsaber wrote:

[...]

TRL3: Simulation proved it! Most if what a.net discussion is at this level or below. A good team can bring a TRL1 to TRL3 in less than 6 months. It just takes paying for 5+ people years if salary, benefits, and computer time. I was promoted early in my career for hitting TRL3 in a tear with 2 person years of labor (much better than typical) in my first year in industry.

TRL2: Look at our great rendering of this theoretical concept that has never been in a flight like environment. Some simulation by at most a small team. TRL2 I've personally hit in 2 weeks.

TRL1: The bright people want money to prove this great idea! TRL1 can be done in a day.


[...]

Lightsaber

As someone who has made his living between TRLs 0 and 3, I very much enjoyed your post. IMHO especially the low TRLs are in the eye of the beholder. Still, comparing your timings for TRL 3 and below with my personal experience, I can only say "I wish!". However, I got very good evaluations and money on one or two TRL 0.

Going back to the topic, if life were a CAD project, one could conceive a BWB configuration with modular connections so you could use, say, up to 4-5 engines, not necessarily of the same type, with the combination of type and size tailored to the specific route and aircraft load. This would drive the FAA (or equivalent) people insane trying to come up with (combinatorial) certification criteria, and it would be fun to watch!

This is just a late evening thought experiment, no need to bring me back to reality.

Sokes wrote:

Has there ever been a case where four engines were chosen when two engines could have done the job?

QF regrets choosing the A380 instead of B77W?

Btw, the A380 vs the B77W is not a (near) 1:1 opponents just like the B772ER vs A343, or B77W vs A346.

zeke wrote:

lightsaber wrote:

1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum.

This isn’t true, for example a GE90 list price is in excess of 40 million, airlines would purchase them for around 25.

Engines deals have always been a big and important part of every other.
Look again at those days when RR sold 772 engines to SQ almost for free. Or the way that GE/CFM (feat. Boeing) stabbed Airbus when UA was swinging between the B737 and A320.

zeke wrote:

lightsaber wrote:

1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum.

This isn’t true, for example a GE90 list price is in excess of 40 million, airlines would purchase them for around 25.

Glad someone pointed that out - I saw that and thought great, he started with wrong information, what else of his is completely wrong? - and stopped reading.

Antaras wrote:

Sokes wrote:

Has there ever been a case where four engines were chosen when two engines could have done the job?

QF regrets choosing the A380 instead of B77W?

If you've got a quote saying this from someone at Qantas, now's the time as I'd like to see that.

This hasn’t been mentioned so far, but years ago I did a study of all crashes that I could find that were caused by engine issues (including engines falling off). What I found was that the same number of engine caused crashes had occurred in twins as in planes with more than two engines. I found this very interesting, as there are far more twins flying than planes with more engines. It indicates, contrary to popular opinion, that twins are actually safer than tris or quads. On closer examination, it actually makes sense. What are the engine issues likely to cause a crash? Two come to mind and are supported by history; the first is an uncontained engine failure doing structural damage. This is actually more likely the more engines you have. The second is an engine falling off doing structural damage and/or striking another one and causing it to fall off. Again, the first is more likely the more engines you have, and the second can only happen on a quad. Note that I can not find a single example of two unrelated engine failures on a single flight for jet engines, not even for B-52s. But since military records are less available I would not say there has never been one; I just couldn’t find it.

In theory, quads might have an advantage by incorporating cheaper and lower thrust geared turbo fan engines, as apposed to powerful, but very expensive and non-geared GE-90 or similar. The maintenance costs also really depend on overall engine thrust.
The answer to the original question, in my opinion, lies mainly in the modern economical paradigm. There are plenty of the good old A343 still flying around.

zeke wrote:

lightsaber wrote:

1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum.

This isn’t true, for example a GE90 list price is in excess of 40 million, airlines would purchase them for around 25.

Now I am confused, which quad uses the GE90, I was always under the impression that the GE90 was designed for a twin?
Is this a hypothetical discussion or does someone actually know what it cost for the A340 engines versus the A330 engines?
Since no Boeing quad uses the GE90 the only place these number can come from is airlines who use the A340 and or the A330.
What am I missing in the discussion?

par13del wrote:

zeke wrote:

lightsaber wrote:

1. Two engines cost $10 million each, 4 $8.5 million each. When you argue for a quad widebody, that means a $14 million higher sales price, minimum.

This isn’t true, for example a GE90 list price is in excess of 40 million, airlines would purchase them for around 25.

Now I am confused, which quad uses the GE90, I was always under the impression that the GE90 was designed for a twin?
Is this a hypothetical discussion or does someone actually know what it cost for the A340 engines versus the A330 engines?
Since no Boeing quad uses the GE90 the only place these number can come from is airlines who use the A340 and or the A330.
What am I missing in the discussion?

2 GE90s (777) vs 4 CFM56-5Cs (A340); that was the premise.
So, talking list price, engines on a 777 are listed at $80 million per shipset, vs $40 million for an A340 shipset.

hivue wrote:

Carmitage wrote:

It seems taken as read universally that 2 engines are better than 4, but why?

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

The quietest airliners have 4 engines.
Is that just coincidence?

Sent from my SM-G930F using Tapatalk

Junglejames wrote:

hivue wrote:

Carmitage wrote:

It seems taken as read universally that 2 engines are better than 4, but why?

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

The quietest airliners have 4 engines.
Is that just coincidence?

Sent from my SM-G930F using Tapatalk

Of course not, since each engine in a 4-engine plane runs at 1/2 the thrust than what would be needed in the same 2-engine plane (think A340 vs A330).

ClassicLover wrote:

If you've got a quote saying this from someone at Qantas, now's the time as I'd like to see that.

“It is great to be able to say I wish I could get in a time machine and go back to 2000 and change the fleet order made by not the last CEO, the CEO before that,” said Alan Joyce.

“But the reality is we have the aircraft we have.”

ClassicLover wrote:

Antaras wrote:

Sokes wrote:

Has there ever been a case where four engines were chosen when two engines could have done the job?

QF regrets choosing the A380 instead of B77W?

If you've got a quote saying this from someone at Qantas, now's the time as I'd like to see that.

He asked it as a question.

Haven't sales of 777, 787 and A350's answered these questions. Even when Airbus had both the A330 and A340, the A330 sold at triple the rate. The only reason the A380 has four engines is as a twin it would have been impractical. The A380 will be the last four engine plane built. Twins clearly have shown the way.

jfk777 wrote:

Haven't sales of 777, 787 and A350's answered these questions. Even when Airbus had both the A330 and A340, the A330 sold at triple the rate. The only reason the A380 has four engines is as a twin it would have been impractical. The A380 will be the last four engine plane built. Twins clearly have shown the way.

I’ll argue the 748F will be the last one built.

A concept that appeals to me combines a very wide fuselage and a not too long length. With two GTF engines from the A321 (30,000 lb) and two external from the A220 / 190 (20,000 lb), these without reversers. Landing gear relatively short and light.

johns624 wrote:

ClassicLover wrote:

Antaras wrote:

QF regrets choosing the A380 instead of B77W?

If you've got a quote saying this from someone at Qantas, now's the time as I'd like to see that.

He asked it as a question.

Yes, I said that it more than like a doubt or a question, not really a statement.

Nean1 wrote:

A concept that appeals to me combines a very wide fuselage and a not too long length. With two GTF engines from the A321 (30,000 lb) and two external from the A220 / 190 (20,000 lb), these without reversers. Landing gear relatively short and light.

Short, fat aircraft aren't usually very aerodynamic.

Carmitage wrote:

4) Engines can be correctly sized for cruise, rather than single engine take off, so punch smaller hole in the air - less drag

IMHO this one is not really valid, as engines on twins are correctly sized for cruise. Thrust is down to maybe 20% of take-off thrust, so they run at 80-85% N1 in the cruise. That's pretty much where they are most efficient.

The total engine frontal area might be larger with a twin, so you get more form drag, but four engines give more skin friction and interference drag. No idea which one wins here.

Junglejames wrote:

hivue wrote:

Carmitage wrote:

It seems taken as read universally that 2 engines are better than 4, but why?

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

The quietest airliners have 4 engines.
Is that just coincidence?

Sent from my SM-G930F using Tapatalk

The A350 and 787 would like a word.

bigb wrote:

jfk777 wrote:

Haven't sales of 777, 787 and A350's answered these questions. Even when Airbus had both the A330 and A340, the A330 sold at triple the rate. The only reason the A380 has four engines is as a twin it would have been impractical. The A380 will be the last four engine plane built. Twins clearly have shown the way.

I’ll argue the 748F will be the last one built.

The A380 was designed in an era when twins were crossing oceans no problems. The 747 was designed when such things were dreams in Juan Trippe's head. The only way to get enough power in 1970 for the 747 was with four engines. The A380 will be the last four engines because it came 30 years after the 747, even if a 748F is built physically after the last A380.

The new 748F is the 777-200LRF.

WayexTDI wrote:

Junglejames wrote:

hivue wrote:

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

The quietest airliners have 4 engines.
Is that just coincidence?

Sent from my SM-G930F using Tapatalk

Of course not, since each engine in a 4-engine plane runs at 1/2 the thrust than what would be needed in the same 2-engine plane (think A340 vs A330).

No I didn't for a minute think it was coincidence. But it was asked what benefits there were nowadays. Passenger comfort. That beats them all hands down.

Sent from my SM-G930F using Tapatalk

Starlionblue wrote:

Carmitage wrote:

4) Engines can be correctly sized for cruise, rather than single engine take off, so punch smaller hole in the air - less drag

IMHO this one is not really valid, as engines on twins are correctly sized for cruise. Thrust is down to maybe 20% of take-off thrust, so they run at 80-85% N1 in the cruise. That's pretty much where they are most efficient.

The total engine frontal area might be larger with a twin, so you get more form drag, but four engines give more skin friction and interference drag. No idea which one wins here.

Junglejames wrote:

hivue wrote:

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

The quietest airliners have 4 engines.
Is that just coincidence?

Sent from my SM-G930F using Tapatalk

The A350 and 787 would like a word.

787 doesn't come close. I'm not the only one that can't work out the name given to them. Unless of course the dream was a nightmare.
A350 also noisier.

They can have words all they like. I presume to agree with me.

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Nean1 wrote:

A concept that appeals to me combines a very wide fuselage and a not too long length. With two GTF engines from the A321 (30,000 lb) and two external from the A220 / 190 (20,000 lb), these without reversers. Landing gear relatively short and light.

I’d guess that would be very hard to certify on performance basis. On take-off, loss of the most critical engine would result in 30% loss of thrust while Part 25 performance is based on loss of 25%. Then, enroute performance for quads has to account for loss of two engines including landing and missed approach—pretty difficult with a loss of 60% of available thrust. I stand to be corrected by a Part 25 expert.

WayexTDI wrote:

Junglejames wrote:

hivue wrote:

Modern jet engines are extremely reliable. I don't have data to back this up, but I suspect that the ability of a two-engine airliner to stay flying with whatever engine issue is essentially the same as a four-engine with whatever engine issue. If two is as good as four, why would you want four?

The quietest airliners have 4 engines.
Is that just coincidence?

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Of course not, since each engine in a 4-engine plane runs at 1/2 the thrust than what would be needed in the same 2-engine plane (think A340 vs A330).

That makes no sense. No manufacturer would mount engines that ran at half thrust at takeoff, or in the cruise. If nothing else, the engines would very inefficient at half thrust.

The A340 engines run at or near full thrust at takeoff, and 80-85% in the cruise. Same as the A330 engines.

Quads and twins in the same weight range do not use engines in the same thrust range. Compare A340 and 777.

Side note: Noise on approach is mostly due to high lift devices.

Junglejames wrote:

Starlionblue wrote:

Carmitage wrote:

4) Engines can be correctly sized for cruise, rather than single engine take off, so punch smaller hole in the air - less drag

IMHO this one is not really valid, as engines on twins are correctly sized for cruise. Thrust is down to maybe 20% of take-off thrust, so they run at 80-85% N1 in the cruise. That's pretty much where they are most efficient.

The total engine frontal area might be larger with a twin, so you get more form drag, but four engines give more skin friction and interference drag. No idea which one wins here.

Junglejames wrote:

The quietest airliners have 4 engines.
Is that just coincidence?

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The A350 and 787 would like a word.

787 doesn't come close. I'm not the only one that can't work out the name given to them. Unless of course the dream was a nightmare.
A350 also noisier.

They can have words all they like. I presume to agree with me.

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C-Series, 787 and A350 are all quieter than the A380, which is by far the quietest quad.

Source: EASA Type Certificate Data Sheets for Noise. https://www.easa.europa.eu/document-lib ... =&year_to=