5 ms·

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 Havi

by frik 12y ago

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 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.

usefulcat 12y ago
The article addresses debris-related engine failure mitigation.
- scrumper 12y ago
  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.)
- frik 12y ago
  Yes the second engine may be saved (not mechanically linked). But the debris could destroy parts of the tail/wing/fuselage.
  - nether 12y ago
    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.
- Shivetya 12y ago
  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.
  - Sanddancer 12y ago
    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 http://en.wikipedia.org/wiki/Boeing_YC-14
Sanddancer 12y ago
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.
- rdtsc 12y ago
  http://en.wikipedia.org/wiki/Tupolev_Tu-154 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.
- frik 12y ago
  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/Trijet
nether 12y ago
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.

by frik 12y ago

usefulcat 12y ago

The article addresses debris-related engine failure mitigation.

scrumper 12y ago
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.)
frik 12y ago
Yes the second engine may be saved (not mechanically linked). But the debris could destroy parts of the tail/wing/fuselage.
- nether 12y ago
  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.
Shivetya 12y ago
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.
- Sanddancer 12y ago
  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 http://en.wikipedia.org/wiki/Boeing_YC-14

Sanddancer 12y ago

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.

rdtsc 12y ago
http://en.wikipedia.org/wiki/Tupolev_Tu-154 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.
frik 12y ago
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/Trijet

nether 12y ago

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.