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Comment by Sn0wCoder | Hacker News Reader

Come on, we all know so they can go faster.

They could go faster with a single larger engine. Why do you think they went with two engines instead of a single larger engine? Redundancy.

Not clear to me that you'd design a commercial turbofan aircraft with one engine if you eliminated reliability requirements. You need to feed air to the engine and, to the degree you have wings, might as well have 2 engines.

The idea that the engines need to sit on the wings like they do is something seems important, because it's so often done, but is actually not necessary. You can put a single engine in the tail, or on top of the cabin, or on the top of the cockpit, or underneath. They go on the wings because when you need two (for redundancy) it's a good place to put them for balance, and because it makes them nicely accessible for maintenance.

Of course military aircraft have different requirements (and many have just one engine in the tail). Certainly I'm not enough of an aero/astro engineer to know if, positing a perfectly reliable engine, you'd mount just one somewhere on the fuselage.

> it's so often done

The MD-80 had two rear engines and was a fairly popular plane. A lot of smaller jets still have rear engines, too.

Not sure what you mean? I didn't say it was always done. In fact the point I was making was that it wasn't always done. I gave my own counter-examples. It's just when people look at modern planes they always see two engines and think that's the way it has to be. It clearly isn't.

The MD-80 (and 727) were old turbojets though. Probably a lot harder to come up with a practical tailmounted design with a turbofan although I'm open to it being possible to mount one turbofan above the fuselage.

Trijets are cool!

While redundancy makes it a moot issue, if I am not mistaken, 777s already use some of the most powerful engines available. As for how fast they fly, I believe it is mostly set by the speed of sound.

Yes they use two of the most powerful engines... and just one of those is more than enough to fly the plane just fine almost as fast as they are allowed to... so why else would they have two of them if not redundancy?

Fundamentally people can look it up themselves - 'ETOPS' - the redundancy requirement is a written regulation.

I think it should be obvious that I am responding to the claim "they could go faster with a single larger engine." no-one is arguing against redundancy (in which matter, ETOPS is just one aspect.)

If they needed to go faster, they could do that with a single larger engine if they wanted to (which they may have to develop.) But they don't, because they're limited as you've explained.

> no-one is arguing against redundancy

Sn0wCoder said they reason they had two engines was to go faster and not redundancy, and m00dy thinks they can't fly without two engines.

Point taken that some people had mistaken ideas about twin-engine airplanes, but those mistakes are most straightforwardly resolved by the redundancy consideration alone, which dominates other issues.

> If they needed to go faster, they could do that with a single larger engine if they wanted to (which they may have to develop.)

Plus a significantly different airframe and perhaps engine intakes to deal with the transonic issue that I also mentioned. It is an idea that bubbles up perennially [1], but so far, the economics have ensured that it doesn't go much beyond the concept stage.

This has become rather silly, but it seques into a related issue: It seems to me that fan-blade failure incidents (or at least uncontained fan-blade failure incidents) have increased in frequency in the last few years. If so, maybe this is a sign that we are already pushing a bit too hard on the limits of current technology?

[1] https://www.boeing.com/features/2019/01/spreading-our-wings-...

Citation needed. It's not at all obvious that a single huge engine is in any way better than two for a given speed.

This is basic aeronautical engineering. Barring the need for redundancy, turbofans are most efficient and quieter with larger intake sizes, the main limiting factor being fan tip speed.

This is the reason that the 737 had the MCAS stuff. They increased the size of the engine that caused ground clearance problems, which required shifting them.

Note sure this is reliable source but “ In general, twin engine aircraft allow for faster speeds, and faster pickup...”[1]