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I understand from this discussion about the Boeing 757 and 767 that drag is directly related to the front area of the airframe, hence flight efficiency of the dual aisle aircraft can only be significantly inferior to the single aisle design.

So how should we consider current Boeing reflection in the design of the so called MOM or NMA that they seek a twin aisle aircraft with single aisle economics? They are reported to aiming at offering "twin-aisle capabilities—range, comfort, capacity, and faster turnaround time—with single-aisle economics". Is it pure marketing rhetoric or are there parameters other than front area in the equation of such an airplane that would make the assertion sound?

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  • $\begingroup$ Improved streamlining? It reduces penalty from front area being larger and the increased mass/surface area would hurt a smaller plane more due to the scaling. Agree with ymb1 about it probably being marketing talk with some aspirational "we must improve our economics" thrown in. $\endgroup$ – Ville Niemi Feb 19 '18 at 15:32
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So it's definitely not ALL marketing spin, but it's also definitely not ALL engineering, either.

The frontal area is absolutely a major component of the drag on an airplane, but wingtip devices (sharklets, winglets, etc.) can significantly reduce drag, as can better nose shapes, smoother surfaces, and a number of other design considerations. If you kept all these other things the same, then increasing the frontal area would increase drag and reduce efficiency, but these other things aren't the same in a new airplane. The MoM proposal would certainly take advantage of all sorts of wing, engine, airframe design improvements that would reduce the drag components, and then you'd gobble most if not all of that up by making the airplane wider. ;-)

If I'm putting on my "engineer but worked in marketing for a while" hat, I'd say that the challenge put to me is that the marketing people want an airplane that can be operated at what used to be called single-aisle efficiency, but can now be done with widebody seat capacities.

I'd also contend that of the parameters mentioned: "range, comfort, capacity, and faster turnaround time" the only one of those things that really requires you to think seriously about a twin-aisle airplane is capacity. I can build a single-aisle aircraft that has every bit as much range, way more comfort, and a much faster turn around time as a twin-aisle aircraft, it just doesn't hold as many people. ;-)

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  • $\begingroup$ Using “wingtip devices” and “significantly” in the same sentence is wrong. Wingtip devices are aerodynamically never better than extending the wing, which is why they are still rare on new designs (main use is if they can make the plane fit in a smaller stand). They have some advantages over extending the wing for redesigns. In line with that, it is the A321-NEO that got fancy winglets while the early 797 concept pictures have none. $\endgroup$ – Jan Hudec Mar 28 '18 at 18:32
  • $\begingroup$ We can argue what qualifies as "significant" then... the question was in the context of economics, so if something provides a "significant" benefit relative to the cost, it gets implemented. Given how many of these wingtip devics are installed or were retrofitted, the airlines certainly seem to think that the investment is worth it. $\endgroup$ – ljwobker Mar 29 '18 at 18:56
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According to the marketing vice-president for Boeing:

[The NMA] would be "a little innovative" in terms of its configuration.

But he added: "We [would] wrap around this aircraft technologies that are proven and understood today – so no big technology push as we saw on [the] 787."

(flightglobal.com, March 2018)

In the 20 March Flight International issue, under the headline of "NMA timeline hinges on powerplants", the talk is that the missing piece is the engine. ETOPS had stalled the innovation in the 200 kN thrust range, and this is the area the engine manufacturers are working on. CFM, P&W, and R-R are "tight-lipped about the status of their NMA engines."

There were rumors of a 757 reboot/re-engine, but that was dismissed because the 757's wings are too big for the mission. So basically the NMA it will be "a little innovative," and the missing piece is the engine.

2019 update:

Rolls-Royce has pulled out of the NMA development. (flightglobal.com, Feb 2019)

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  1. Drag is first and foremost proportional to weight. If you can make the plane lighter, it will need less energy to fly. And the new carbon fibre composite structures promise just that. So a new design is likely to be lighter than the all-aluminium A321, which should more than make up for the increase in drag due to bigger cross-section.

  2. Efficiency means cost per seat-(or tonne-)mile and there are many more factors to that than just fuel cost and most of them are fixed costs, paid by day whether the plane is moving or not. If the plane can turn around faster due to its two aisles, it can fly further in a day and thus it will more efficient as the fixed costs will be spread over more miles. It would also help if they could make maintenance easier.

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  • $\begingroup$ A reference/link to support/quantify the statement about drag being proportional to weight would greatly help this answer. I would contend that it it not at all obvious to most readers whether a 5% increase in frontal area results in {more, less, or about the same} additional drag compared to a 5% increase in weight.... $\endgroup$ – ljwobker Mar 29 '18 at 19:00

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