[AlexandrB]

Yup, I've been seeing these futuristic airplane concepts for most of my life, but commercial airframes seem to be converging to be more and more similar. Unusual elements like a third engine in the tail or horizontal stabilizers mounted above the rudder have largely disappeared. Given the thin margins in air travel, I suspect this is because this design is the most successful. It might be a local minimum, but when it comes to these radically different designs: "I'll believe it when I see it".

[hotpotamus]

My understanding is that tri-engine jetliners were actually more to fulfill a regulation than for engineering reasons. There are regulatory limits to how far a plane is allowed to fly on one engine called ETOPS - I forget what it really stands for, but the colloquial expression is: Engines Turn Or Passengers Swim. It used to be limited to around 2 hours I believe, meaning you've got a bit of time to divert and make an emergency landing. But if you're crossing the ocean, it's not really possible, so for a long time if you were crossing an ocean, you needed a 3 or 4 engined plane to do it.

Jet engines are extremely reliable however, they fail on the order of several hundred times less often than piston engines, and are very well proven and have basically only improved, and so ETOPS rules have been relaxed quite a bit meaning that a lot more ocean crossing routes are available to twinjets.

[robocat]

Plus having more engines doesn’t help if failures are correlated, such as some fuel issues, or http://news.bbc.co.uk/2/hi/uk_news/magazine/8622099.stm

[jabl]

Yes, ETOPS ratings have been extended as engines have gotten more reliable.

Another factor is that we have learned to make bigger engines as well. So nowadays with two engines you can power a pretty big plane. And due to how turbine efficiency tends to scale with size, two big engines is more fuel efficient as well as saves on maintenance costs vs. having more but smaller engines.

For an extreme example, look at the B-52 with 8 engines. That was what was available back when the plane was designed, but nowadays the thrust from those 8 engines (about 600kN in total) can easily be exceeded by two modern large turbofans.

[coredog64]

Extended Twin Operations.

[jabl]

Horizontal stabilizers above the rudder went away with the engines mounted on the body in the back, the reason for the high tail was to keep the elevators away from the jet blast of the engines.

As to why create a design like that in the first place, I don't know. My understanding is that the 'engines under the wings' layout won because with the engines in the back the fuselage needs to be stronger (and thus heavier) to support the engines.

(Business jets tend to still have the engines in the back layout, because mounting the engines high allows shorter landing gear so that a stair that is part of the door is enough to board the plane, no need for an external stair. But that's not much of a consideration for a passenger plane operating out of airports with infrastructure available.)

[ramesh31]

A lot of the reasoning for business/regional jets maintaining the high tail mount engine layout is FOD avoidance. Operating out of shorter runways/smaller airports this becomes an issue.

[jabl]

Isn't much FOD ingestion due to the wheels kicking up something? If so, mounting the engines in the back seems like a bad idea, as even if they are higher up than an under wing mounting there's a risk that the wheels might kick up some debris?

See also SAS flight 751, where ice broke off from the wings and were ingested into the engines https://en.wikipedia.org/wiki/Scandinavian_Airlines_System_F...

From a FOD perspective, in my admittedly very non-expert opinion the best location would be to have either a high mounted wing with engines in the traditional under wing position, or then engines above the wings like the Honda business jet?

[lstodd]

Something like An-72.

But maintenance becomes a bitch, so they put'em back under the wings eventually.