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Required Number Of Engines For Widebodies  
User currently offlineAlasdair1982 From UK - Scotland, joined Mar 2008, 468 posts, RR: 0
Posted (5 years 9 months 1 week 2 days 13 hours ago) and read 3159 times:

Something that got me wondering

The 777-300ER (with KLM) can hold 390 passengers in economy, along with all their luggage and meals, and 35 in business class. Yet only needs two engines

So, why will the 747-8I still require four engines? Will the increase in capacity really not be able to be powered by two engines?

Why does the A340 require four engines?

11 replies: All unread, jump to last
 
User currently offlineAlessandro From , joined Dec 1969, posts, RR:
Reply 1, posted (5 years 9 months 1 week 2 days 12 hours ago) and read 3155 times:

Since it´s an older aircraft the A340 uses 4 engines (2 engined version of same family is called A330), the B777 got lone user rights for the GE90-115 (world most powerful engine) and other engines in the GE90. Also A340 shouldn´t compete with the inhouse A380.
B748i, cost is lower than a cleanpaper design due to it´s an upgrade not a new plane which makes it easier to pass the redtape. Could it fly with 2 GE90-115? Probably but why compete against your own B777 line and raise the cost for certification?


User currently offlineAA737-823 From United States of America, joined Mar 2000, 5726 posts, RR: 11
Reply 2, posted (5 years 9 months 1 week 2 days 12 hours ago) and read 3144 times:

There are several pros and cons on each side.
With a three or four-engined airplane, you don't expose yourself to the ETOPS system. Thus, you can fly a straight line from anywhere to anywhere (barring MOAs, no-fly-zones, and countless other real-life restrictions). But four engines adds complexity. That's double the number of engines to go wrong.

On the other side, there are great efficiencies gained by using just two engines. The ETOPS system is annoying and expensive to participate in, but has actually generated a mode of flying that is statistically SAFER than 3 or 4 engine flying. But, you have to stay within your time limits to a suitable diversion airport.
However, there is a lot less system redundancy. So, other ways to provide redundancy have to be devised, like the ADP hydraulic pumps on 767s and 777s, RATs on 757s and up, and the list goes on.

It's all calculated risks.
Personally, I don't hesitate to get on an A330 to go from Amsterdam to Dallas or Minneapolis, but that said, there's something psychologically comforting about looking out your window and seeing TWO engines on EACH wing, as on the 744.

Also, keep in mind that due to engine-out takeoff requirements, twin-jets have to have OVERPOWERED engines, to be able to keep the airplane flying in a single-engine climbout scenario. Multi-engines airplanes suffer less from this requirement, as losing one engine on a 744 isn't as critical as it is on a 773ER.

Pros and cons, pros and cons.


User currently offlineKeesje From , joined Dec 1969, posts, RR:
Reply 3, posted (5 years 9 months 1 week 2 days 11 hours ago) and read 3129 times:



Quoting AA737-823 (Reply 2):
But four engines adds complexity. That's double the number of engines to go wrong.

Strange statistics. For safety the risk of enough engines to break down to bring the aircraft down is the real important factor. If the engines on a quad are as reliable as on a twin extra engines reduce the risk of a cash resulting from engine falure.

I think a tripple engined design will eventually take over in the 400-550 seat segment.



User currently offlineKELPkid From United States of America, joined Nov 2005, 6343 posts, RR: 3
Reply 4, posted (5 years 9 months 1 week 2 days 11 hours ago) and read 3118 times:



Quoting Keesje (Reply 3):
I think a tripple engined design will eventually take over in the 400-550 seat segment.

Someone might want to tell genius in that photograph (preparing a wind tunnel mock-up) that hollowed-out engine nacelles won't adequately replicate the actual drag encountered in flight by a nacelle containing an actual engine...  Wink Or are they proposing MAD (Magnetic Aerodynamic Drive) on this model? Big grin



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (5 years 9 months 1 week 2 days 7 hours ago) and read 3046 times:



Quoting Alasdair1982 (Thread starter):
The 777-300ER (with KLM) can hold 390 passengers in economy, along with all their luggage and meals, and 35 in business class. Yet only needs two engines

So, why will the 747-8I still require four engines? Will the increase in capacity really not be able to be powered by two engines?

Mostly, because there are engine big enough for the 777-300ER to operate with two but not for the 747-8 to operate with two, and making the 747-8 a trijet isn't an option.

Quoting Alasdair1982 (Thread starter):
Why does the A340 require four engines?

It sort of doesn't...a twin-engine A340 is called an A330. Having four engines allows it to pack less thrust and greater range, at the expense of complexity and cost.

Quoting Alessandro (Reply 1):
B748i, cost is lower than a cleanpaper design due to it´s an upgrade not a new plane which makes it easier to pass the redtape. Could it fly with 2 GE90-115?

Nope. Installed thrust is almost always sized around engine-out climb requirements. A single GE90-115 is good to heft ~775000 pounds. The 747-8 comes in at about 975000...you'd need twin 145k engines, which nobody currently builds (or is planning to build).

Quoting Keesje (Reply 3):
For safety the risk of enough engines to break down to bring the aircraft down is the real important factor. If the engines on a quad are as reliable as on a twin extra engines reduce the risk of a cash resulting from engine falure.

That's not exactly correct. They reduce the risk of a crash due to inadequate thrust. However, there are several engine failure modes that can damage the airplane, like a rotor burst. These are twice as likely on a quad as on a twin, given equally reliable engines. Far more common than engine induced crashes are engine-induced diversions and maintenance delays, and those are also twice as likely on a quad.

Quoting Keesje (Reply 3):
I think a tripple engined design will eventually take over in the 400-550 seat segment.

I agree. Three is almost always the best balance between cost and reliability (that's why triple-redundancy is so common but dual and quad much less so), but there are severe integration challenges with the third engine on a conventional airframe. BWB's avoid most of those by their nature.

Quoting KELPkid (Reply 4):
Someone might want to tell genius in that photograph (preparing a wind tunnel mock-up) that hollowed-out engine nacelles won't adequately replicate the actual drag encountered in flight by a nacelle containing an actual engine...

They're not supposed to...in flight, the engine is providing thrust so there's negative drag on the nacelle (from the airplane's point of view). This type of design is very common on wind tunnel models. I suspect they're built that way to make sure that the pressure field on the outside is accurate so that the flow over the wings and strut is realistic.

Tom.


User currently offlineVikkyvik From United States of America, joined Jul 2003, 9777 posts, RR: 26
Reply 6, posted (5 years 9 months 1 week 2 days 1 hour ago) and read 2979 times:
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Quoting KELPkid (Reply 4):
Someone might want to tell genius in that photograph (preparing a wind tunnel mock-up) that hollowed-out engine nacelles won't adequately replicate the actual drag encountered in flight by a nacelle containing an actual engine... Wink Or are they proposing MAD (Magnetic Aerodynamic Drive) on this model?

Nah, nothing so complicated. I think they're just going to run it on a treadmill............. duck 

Quoting Tdscanuck (Reply 5):

They're not supposed to...in flight, the engine is providing thrust so there's negative drag on the nacelle (from the airplane's point of view). This type of design is very common on wind tunnel models. I suspect they're built that way to make sure that the pressure field on the outside is accurate so that the flow over the wings and strut is realistic.

True, the sum of the forces on the engine would be a net thrust. But the nacelles will still add aerodynamic drag. I'd assume that this drag isn't factored into the quoted thrust of the engine.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineAlessandro From , joined Dec 1969, posts, RR:
Reply 7, posted (5 years 9 months 1 week 2 days ago) and read 2960 times:



Quoting Keesje (Reply 3):
Strange statistics. For safety the risk of enough engines to break down to bring the aircraft down is the real important factor. If the engines on a quad are as reliable as on a twin extra engines reduce the risk of a cash resulting from engine falure.

I think a tripple engined design will eventually take over in the 400-550 seat segment.

So how are you supposed to evacute 400 passengers in 90 seconds from a BWB?

Quoting Tdscanuck (Reply 5):
Nope. Installed thrust is almost always sized around engine-out climb requirements. A single GE90-115 is good to heft ~775000 pounds. The 747-8 comes in at about 975000...you'd need twin 145k engines, which nobody currently builds (or is planning to build).

Well, that´s 20,5% difference, how much weight will be saved from building a 2-engined B748i
instead of a 4-engined?


User currently offlineAA737-823 From United States of America, joined Mar 2000, 5726 posts, RR: 11
Reply 8, posted (5 years 9 months 1 week 1 day 23 hours ago) and read 2944 times:



Quoting Alessandro (Reply 7):
So how are you supposed to evacute 400 passengers in 90 seconds from a BWB?

Holes in the floor!


User currently offlineKeesje From , joined Dec 1969, posts, RR:
Reply 9, posted (5 years 9 months 1 week 1 day 21 hours ago) and read 2918 times:



Quoting Alessandro (Reply 7):
So how are you supposed to evacute 400 passengers in 90 seconds from a BWB?

I doubt it will be a BWB. A flat pressure body requires a lot of structure..

Quoting Alasdair1982 (Thread starter):
The 777-300ER (with KLM) can hold 390 passengers in economy, along with all their luggage and meals, and 35 in business class. Yet only needs two engines

So, why will the 747-8I still require four engines? Will the increase in capacity really not be able to be powered by two engines?



Quoting Alessandro (Reply 7):
Well, that´s 20,5% difference, how much weight will be saved from building a 2-engined B748i
instead of a 4-engined?

Ground clearance would be an issue.. very long heavy landing gears x 5..

http://www.cardatabase.net/modifiedairlinerphotos/photos/big/00005184.jpg


User currently offlineAlessandro From , joined Dec 1969, posts, RR:
Reply 10, posted (5 years 9 months 1 week 1 day 21 hours ago) and read 2913 times:



Quoting AA737-823 (Reply 8):
Quoting Alessandro (Reply 7):
So how are you supposed to evacute 400 passengers in 90 seconds from a BWB?

Holes in the floor!

So you are trapped in flames if a BWB makes a belly landing and catch fire?


User currently offlineAlessandro From , joined Dec 1969, posts, RR:
Reply 11, posted (5 years 9 months 1 week 1 day 21 hours ago) and read 2911 times:



Quoting Keesje (Reply 9):


Ground clearance would be an issue.. very long heavy landing gears x 5..

Sure it would, Tu-114 landing gear style would be handy.


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