4 ms·

There are several reasons. A big one is having four jet engines rather than two. If you had two equivalent aircraft but one a twin jet the other a quad, the twi

by agurk 6y ago

There are several reasons. A big one is having four jet engines rather than two. If you had two equivalent aircraft but one a twin jet the other a quad, the twin would have higher efficiency. With each extra engine you pay for it with more drag and reduced overall efficiency. The engines on the A380 aren't actually that modern with the two options being 15-20 year old designs. One of the reasons it was discontinued is that no engine manufacturer was willing to make new engines for it, so they weren't able to produce a modern fuel efficient variant. And it also makes limited use of composites in its design, so it's heavier than it would be if designed today.

jabl 6y ago
> A big one is having four jet engines rather than two. If you had two equivalent aircraft but one a twin jet the other a quad, the twin would have higher efficiency. One can make a good case for turbine engines having better efficiency the larger they are (less losses due to boundary layer friction etc.), so yes, if you have two otherwise equivalent aircraft I'd assume fewer bigger engines would be better. But here we don't have equivalent aircraft, as the A380 is bigger than, well, pretty much any other passenger airliner out there. The A380 engines produce around 350 kN, which is slightly bigger than, say, the B787 engines. Admittedly the A350 engines are a bit bigger than that, but not by a huge amount. But the data seems to suggest both the 787 and 350 are much more fuel efficient than the A380? So to make an equivalent comparison, one would take an engine, make a twin engine plane and a quad engine plane with that exact same engine. Naively one could expect the quad version to take twice the number of passengers or cargo, and have roughly the same fuel economy/seat. If not, why not? Which scaling laws are non-linear here? > The engines on the A380 aren't actually that modern with the two options being 15-20 year old designs. One of the reasons it was discontinued is that no engine manufacturer was willing to make new engines for it, so they weren't able to produce a modern fuel efficient variant. > And it also makes limited use of composites in its design, so it's heavier than it would be if designed today. This explanation I can certainly buy. Also that the biggest turbofans are nowadays so large that a twin engine plane is already so big there's a very small market for quads.
- gsnedders 6y ago
  Some of the problems with the A380 come from the wing design: it has an unusually narrow wing for its weight (out of a desire to remain within the 80m box), along with the shaping of the wing being optimised for a heavier, long A380-900 that never came to market. An unusually large proportion of the drag comes from the wing on the A380. It's worthwhile looking at where Airbus proposed to improve the A380: * Densifying the main deck (going to 11-across in economy, still with 18" wide seats). This _alone_ in a typical configuration gives a 9% fuel-per-passenger gain. * They looked at new engines (derived from the A350, for example) which would provide a 5% fuel burn gain. * Minor wing modifications (new winglets and slightly reduced wing twist) which would provide a 4% fuel burn gain. * Longer maintenance intervals (so the aircraft spends more time in the air earning money rather). Other differences include that it holds much less freight in its belly than a 777 (a much larger proportion of its hold is consumed with passenger luggage), so you lose freight revenue. It's also worthwhile pointing out that engines are some of the most expensive parts to maintain on an aircraft, so by doubling the number of engines on an aircraft you make a very significant increase on maintenance costs. At the same time, you have the 777X lengthening the 777 (and increasing capacity) while reducing fuel-burn per seat, a much more versatile aircraft (an A380 will never be profitable on the major of 777X routes!), and more fleet commonality.

by agurk 6y ago

jabl 6y ago

> A big one is having four jet engines rather than two. If you had two equivalent aircraft but one a twin jet the other a quad, the twin would have higher efficiency. One can make a good case for turbine engines having better efficiency the larger they are (less losses due to boundary layer friction etc.), so yes, if you have two otherwise equivalent aircraft I'd assume fewer bigger engines would be better. But here we don't have equivalent aircraft, as the A380 is bigger than, well, pretty much any other passenger airliner out there. The A380 engines produce around 350 kN, which is slightly bigger than, say, the B787 engines. Admittedly the A350 engines are a bit bigger than that, but not by a huge amount. But the data seems to suggest both the 787 and 350 are much more fuel efficient than the A380? So to make an equivalent comparison, one would take an engine, make a twin engine plane and a quad engine plane with that exact same engine. Naively one could expect the quad version to take twice the number of passengers or cargo, and have roughly the same fuel economy/seat. If not, why not? Which scaling laws are non-linear here? > The engines on the A380 aren't actually that modern with the two options being 15-20 year old designs. One of the reasons it was discontinued is that no engine manufacturer was willing to make new engines for it, so they weren't able to produce a modern fuel efficient variant. > And it also makes limited use of composites in its design, so it's heavier than it would be if designed today. This explanation I can certainly buy. Also that the biggest turbofans are nowadays so large that a twin engine plane is already so big there's a very small market for quads.

gsnedders 6y ago
Some of the problems with the A380 come from the wing design: it has an unusually narrow wing for its weight (out of a desire to remain within the 80m box), along with the shaping of the wing being optimised for a heavier, long A380-900 that never came to market. An unusually large proportion of the drag comes from the wing on the A380. It's worthwhile looking at where Airbus proposed to improve the A380: * Densifying the main deck (going to 11-across in economy, still with 18" wide seats). This _alone_ in a typical configuration gives a 9% fuel-per-passenger gain. * They looked at new engines (derived from the A350, for example) which would provide a 5% fuel burn gain. * Minor wing modifications (new winglets and slightly reduced wing twist) which would provide a 4% fuel burn gain. * Longer maintenance intervals (so the aircraft spends more time in the air earning money rather). Other differences include that it holds much less freight in its belly than a 777 (a much larger proportion of its hold is consumed with passenger luggage), so you lose freight revenue. It's also worthwhile pointing out that engines are some of the most expensive parts to maintain on an aircraft, so by doubling the number of engines on an aircraft you make a very significant increase on maintenance costs. At the same time, you have the 777X lengthening the 777 (and increasing capacity) while reducing fuel-burn per seat, a much more versatile aircraft (an A380 will never be profitable on the major of 777X routes!), and more fleet commonality.