4 ms·

787-8 and 787-9 ETOPS Lowered (RR Engine Flaws)

2 points by hodgesrm 9y ago

ocdtrekkie 9y ago
Out of curiosity, is the benefits (I assume cost/efficiency) of having only two engines significant? Or with the significant conversation about engine failure of late, it make sense to lean back towards the four engine design of a 747, which presumably, doesn't tax the remaining engines as hard during a failure? (ETOPS, apparently, stands for "Extended-range Twin-engine Operational Performance Standards", and refers to how far you allow a plane with only two engines to fly from an airport it can land at, in case one fails and a single engine has to do all of the work.)
- hodgesrm 9y ago
  2 engine aircraft are cheaper to operate per seat due among other things to greater fuel efficiency which can be up to 50% over older 4-engine models. [0] The current four-engine models are at the bottom of the heap these days; the 787 is at the top. Also smaller cabins are easier to fill which means you can fly point-to-point from regional airports like Oakland CA (OAK) and still operate at max capacity. [0] https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft
  - AnimalMuppet 9y ago
    > 2 engine aircraft are cheaper to operate per seat due among other things to greater fuel efficiency which can be up to 50% over older 4-engine models. Why is this true? Is it because of the "older" part - because we've gotten better and designing fuel efficiency? Is it because the engines also increase drag? Is it because the four-engine planes typically don't have to run the engines as close to full throttle, and the engines are more efficient if run at higher power? Or something else?
    - hodgesrm 9y ago
      I'm not an aeronautical engineer but my understand is that a lot of the increase in efficiency is due to introduction of turbofan engines with higher bypass ratios. These economize on fuel since the fuel increasingly runs the turbofan to compress air that enters the bypass duct rather than generating thrust directly. Thrust instead comes from decompression of the air as it flows out behind the engine. B787, B777, AB380, and AB350 all have GE or RR turbofans with 8:1 or greater bypass according to numbers in Wikipedia. [0] This is not the only design improvement but it's a big one. [0] https://en.wikipedia.org/wiki/Bypass_ratio https://en.wikipedia.org/wiki/Bypass_ratio
      - ocdtrekkie 9y ago
        Wouldn't these design improvements equally apply to a new hypothetical four engine plane?
        hodgesrm 9y ago
        That's a good question. Other big factors in operating expense are load factor (i.e., percent of seats filled) and weight. The A380 is in trouble because it measures poorly on the preceding and in addition can only fly out of hubs with specially modified gates. The 747-8 seems to be largely a freighter with production down to about 1 a month. At this point it seems unlikely we'll see another big aircraft design program any time soon unless a country like China decides to do one for status reasons.
    - ocdtrekkie 9y ago
      From a high availability standpoint, amount of thrust used is actually opposite what you suggest. If I have two identical VMware servers in my datacenter, I can only run them each at 50% of their total capacity: I need to be sure that if one fails, I can run everything on only one server. If I have four servers, I can run each server at 75% capacity: If one fails, I can divide it's workload across the other three. When it comes to high availability, more assets actually permits you to push utilization higher. I know next to nothing about planes, but if you ran two engines at full throttle, you wouldn't have enough thrust if one failed.

2 points by hodgesrm 9y ago

Out of curiosity, is the benefits (I assume cost/efficiency) of having only two engines significant? Or with the significant conversation about engine failure of late, it make sense to lean back towards the four engine design of a 747, which presumably, doesn't tax the remaining engines as hard during a failure? (ETOPS, apparently, stands for "Extended-range Twin-engine Operational Performance Standards", and refers to how far you allow a plane with only two engines to fly from an airport it can land at, in case one fails and a single engine has to do all of the work.)

hodgesrm 9y ago
2 engine aircraft are cheaper to operate per seat due among other things to greater fuel efficiency which can be up to 50% over older 4-engine models. [0] The current four-engine models are at the bottom of the heap these days; the 787 is at the top. Also smaller cabins are easier to fill which means you can fly point-to-point from regional airports like Oakland CA (OAK) and still operate at max capacity. [0] https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft
- AnimalMuppet 9y ago
  > 2 engine aircraft are cheaper to operate per seat due among other things to greater fuel efficiency which can be up to 50% over older 4-engine models. Why is this true? Is it because of the "older" part - because we've gotten better and designing fuel efficiency? Is it because the engines also increase drag? Is it because the four-engine planes typically don't have to run the engines as close to full throttle, and the engines are more efficient if run at higher power? Or something else?
  - hodgesrm 9y ago
    I'm not an aeronautical engineer but my understand is that a lot of the increase in efficiency is due to introduction of turbofan engines with higher bypass ratios. These economize on fuel since the fuel increasingly runs the turbofan to compress air that enters the bypass duct rather than generating thrust directly. Thrust instead comes from decompression of the air as it flows out behind the engine. B787, B777, AB380, and AB350 all have GE or RR turbofans with 8:1 or greater bypass according to numbers in Wikipedia. [0] This is not the only design improvement but it's a big one. [0] https://en.wikipedia.org/wiki/Bypass_ratio https://en.wikipedia.org/wiki/Bypass_ratio
    - ocdtrekkie 9y ago
      Wouldn't these design improvements equally apply to a new hypothetical four engine plane?
      - hodgesrm 9y ago
        That's a good question. Other big factors in operating expense are load factor (i.e., percent of seats filled) and weight. The A380 is in trouble because it measures poorly on the preceding and in addition can only fly out of hubs with specially modified gates. The 747-8 seems to be largely a freighter with production down to about 1 a month. At this point it seems unlikely we'll see another big aircraft design program any time soon unless a country like China decides to do one for status reasons.
  - ocdtrekkie 9y ago
    From a high availability standpoint, amount of thrust used is actually opposite what you suggest. If I have two identical VMware servers in my datacenter, I can only run them each at 50% of their total capacity: I need to be sure that if one fails, I can run everything on only one server. If I have four servers, I can run each server at 75% capacity: If one fails, I can divide it's workload across the other three. When it comes to high availability, more assets actually permits you to push utilization higher. I know next to nothing about planes, but if you ran two engines at full throttle, you wouldn't have enough thrust if one failed.