Moderators: richierich, ua900, PanAm_DC10, hOMSaR
lhrnue wrote:Looks like the industry has answered this question.
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
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
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
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
Sokes wrote:Has there ever been a case where four engines were chosen when two engines could have done the job?
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.
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!
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).
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?
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?
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.
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.
Carmitage wrote:It seems taken as read universally that 2 engines are better than 4, but why?
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?
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.
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.
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.
Sokes wrote:Has there ever been a case where four engines were chosen when two engines could have done the job?
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.
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
Sokes wrote:Has there ever been a case where four engines were chosen when two engines could have done the job?
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.
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.
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?
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.
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?
hivue wrote:The quietest airliners have 4 engines.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?
Junglejames wrote:hivue wrote:The quietest airliners have 4 engines.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?
Is that just coincidence?
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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.
ClassicLover wrote:He asked it as a question.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.
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.
johns624 wrote:ClassicLover wrote:He asked it as a question.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.
Nean1 wrote:Short, fat aircraft aren't usually very aerodynamic.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.
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
Junglejames wrote:hivue wrote:The quietest airliners have 4 engines.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?
Is that just coincidence?
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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.
WayexTDI wrote: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.Junglejames wrote:hivue wrote:The quietest airliners have 4 engines.
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?
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).
Starlionblue wrote: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.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:The quietest airliners have 4 engines.
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?
Is that just coincidence?
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The A350 and 787 would like a word.
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.
WayexTDI wrote:Junglejames wrote:hivue wrote:The quietest airliners have 4 engines.
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?
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).
Junglejames wrote:Starlionblue wrote: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.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.
A350 also noisier.
They can have words all they like. I presume to agree with me.
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