Pratt and Whitney's Next-Gen Geared Turbofan and the “Double-Bubble” Airliner | Hacker News Reader

Pratt and Whitney's Next-Gen Geared Turbofan and the “Double-Bubble” Airliner

aviationweek.com

34 pointsaerocapture20 comments

The clustered beneath the pi-tail of the NASA/MIT D8 design is bad for maintainability, isn't it?

All we learnt from early commercial jetliner like the De Havilland Comet (http://en.wikipedia.org/wiki/De_Havilland_Comet), where two pairs of turbojet engines were buried into the wings, is that such configuration increased the structural weight and complexity of the wings. Armour had to be placed around the engine cells to contain debris from any serious engine failures; also, placing the engines inside the wing required a more complicated wing structure.

Trijets have been around for a very long time, and aside from the DC-10's various misfeatures, have been rather reliable, even when the number two engine goes out with or without detonating itself in various ways. Given that this plane will almost certainly be fly by wire, I imagine that it'll be a lot easier to design the hydraulic systems in ways to further avoid certain failure modes that did occur in those planes.

Regarding maintenance, the article discusses how the reverse design gives a benefit of having the parts of the engine most difficult to service in a position to make such service easier, so I imagine maintainability will be /different/ but not necessarily worse.

Trijet design will almost always consume more fuel than a comparable twin engine design. This results in higher operating costs and reduced range. Although trijets are more efficient than four-engine aircraft, the difficulty and complexity of mounting the center engine through the tail will somewhat negate this advantage.

quote from http://en.wikipedia.org/wiki/Trijet

http://en.wikipedia.org/wiki/Tupolev_Tu-154

Is also a popular trijet. Not only that but one of the fastest passenger civilian aircraft. It is on its way out and in limited operation.

Much of the downsides of burying engines in the wing versus in the fuselage (below the H-tail) are irrelevant because the engines are not in a cantilever loaded portion of the structure. A beam has zero bending moment at a free end, and we may treat the fuselage as a long beam with free ends, like a see-saw balanced on a fulcrum in the middle (the wing in this case; we are neglecting the downforce from the horizontal tail). Placing the engines at a far end of the minimizes the bending load they must withstand. Placing engines at a wing root means putting them at the location of maximum bending moment due to the lift acting on the wing.

The article addresses debris-related engine failure mitigation.

I would still think that intake at the top of the fuselage should be less likely to ingest debris, the military would be an ideal means to test it in transport configurations.

There's been a decent amount of research that the upper portions of the fuselage are more durable locations for engines. The A-10 for example, has its engines located up and to the rear for that reason, and the experimental YC-14 [1] also had the engines placed up high, albeit for different reasons, but avoidance of ingestion was definitely a nice side effect.

[1] http://en.wikipedia.org/wiki/Boeing_YC-14

Only in as far as a novel 'core canting' approach to protecting one engine from an unconfined failure of the adjacent unit. It didn't discuss protecting the tail or fuselage (unless I'm forgetting something; read it a couple hours ago so maybe I did.)

Yes the second engine may be saved (not mechanically linked).

But the debris could destroy parts of the tail/wing/fuselage.

This is a tradeoff due to mission priorities. Embedded nacelles (as in fighters) reduce drag (the point of this project is to increase efficiency), but complicate matters of structural support and increase engine vulnerability in case of fire. These have superior shielding to runway debris and birdstrikes, but such incidents are extremely rare.

More on the D8: https://www.nasa.gov/content/the-double-bubble-d8-0/

From the article:

"carrying 180 passengers 3,000 nautical miles in a coach cabin roomier than that of a Boeing 737-800"

I would LOVE it if a bigger cabin meant that people got more space. But considering how we've all accepted being crammed into ever smaller coach seats, I suspect that any breakthrough here will simply to go make airlines more profitable while ticket prices drop a miniscule amount.

Right? What are the chances that if the DoubleBubble is 20% more spacious than a 180-seat 737, that they'd give customers 20% more space rather than just flying a 216-passenger plane?

Depends on the load factor ( # of kilos/m2 ) that applies to the plane. You could increase the cabin size by 20% but only increase max load by 5% and then you'd have 15% more space into which to put passengers and luggage.

I myself would be happy with larger, lighter, slower and more fuel-efficient planes. But then I am also perennially hoping that dirigible passenger liners will make a comeback.

> Depends on the load factor ( # of kilos/m2 ) that applies to the plane. You could increase the cabin size by 20% but only increase max load by 5% and then you'd have 15% more space into which to put passengers and luggage.

Actually what you'd need to do is increase 20% past the point at which the load factor starts to become about weight rather than volume. What I mean is that right now planes might be able to carry more weight, but don't because there's not enough room. I'm not an aerospace guy so I can't tell you where stuff lies on the spectrum right now. But it seems possible to me, which is why the airlines keep cramming more seats into planes; they've got the lift capacity for the increased weight.

> I myself would be happy with larger, lighter, slower and more fuel-efficient planes. But then I am also perennially hoping that dirigible passenger liners will make a comeback.

I wouldn't mind that much either. I'm headed to Scotland from Texas for a wedding in about 6 weeks and I'd much prefer 36 hours each way on a sofa to 12-14 hours each way crammed into a chair barely big enough for me. At 4500 miles the airship would only need to manage 125mph which is fast, but not crazy fast.

I'd also be a fan of big, ocean going ground effect airplanes. But when I say big, I mean big. It's going to be able to have ground effect at 20-30 feet up so that you can cut across most of the chop. I suspect that the wings would need to be an awful shape to get that high, but that's what it would take for comfort.

http://en.wikipedia.org/wiki/File:Pelican-01.jpg

WIG vehicles are pretty damn cool. Unfortunately they are rather vulnerable to weather. I am disappointed that Russia hasn't made regular Ekranoplan service a thing, both across the polar areas and on the black sea.

IIRC airlines are very unprofitable

For those of us who aren't aeronautical engineers, why is it called "double-bubble"?

It's just a reference to the shape of it. Page 4:

http://web.mit.edu/drela/Public/papers/Hawaii_11/Drela_AIAA2...

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