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The fuselage of the McDonnell Douglas DC-9-80 (marketed as the "MD-80") has a pronounced cusp at floor level, as can be seen in this diagram (courtesy of Boeing, eater of McDonnell Douglas - at least, according to @JWalters):

alleged DC-9-80 fuselage cross-section

Although the DC-9-80's fuselage cusp is nowhere near as extreme as the Boeing 377 Stratocruiser's, it still seems like an unnecessary stress concentrator with no apparent reason for being there; why did McDonnell Douglas put in the cusp, despite its potential for accelerating fatigue damage to the fuselage?

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    $\begingroup$ Not a unique feature of the DC-9. It's easier to name mainline jet the aircraft that aren't double lobe so they don't have a cusp: A300/310/330/340 (same circular fuselage), A380 (elliptical), 777 (circular). $\endgroup$ – user71659 Mar 3 at 7:13
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    $\begingroup$ It was inherited from the DC-8. The DC-8 was initially planned with a smaller fuselage width but the upper half was enlarged when the Boeing 707 was made wider before it. The DC-9 cross section was a smaller version of the DC-8 cross section. $\endgroup$ – Peter Kämpf Mar 3 at 13:03
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It's not a stress concentrator; it's just the opposite. What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle.

If I had a rubber balloon filled with air and was able to run a string internally from one side to the other, attached to the walls of the balloon, and then drew the string in to pinch the sides of the balloon into a figure eightish profile, I'd have the same thing. All the loads on the skins are still in tension, as well as the bridging floor beams (the string).

enter image description here

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To add to John's answer, why did they do it? To make more room in the passenger area.

It is important to remember that, opposite a submarine, pressure is higher on the inside of the aircraft in flight, rather than lower. So structural design favors a cross brace to hold it together. A submarine would be strongest if it were perfectly spherical.

Also, notice the "pinched" fuselage has lower frontal area, resulting in better fuel economy from less drag.

The MD-80 design, with its clean wings (engines mounted on rear fuselage), lived on as the Boeing 717.

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    $\begingroup$ The passenger area would be larger if they removed the concave area, so I don't understand the point of your first paragraph. $\endgroup$ – David Richerby Mar 3 at 13:56
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    $\begingroup$ Could be statisticly passenger + seat has more volume on average than luggage.🤔 But, apparently, enlarging the upper half was the best engineering choice. Also seen on the Super Guppy. $\endgroup$ – Robert DiGiovanni Mar 3 at 16:51
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    $\begingroup$ I mean using an egg-shaped cross-section. You see the point where it pinches in? Push that out to give a smooth curve: the passenger area gets larger and the floor still acts as a tension beam. $\endgroup$ – David Richerby Mar 3 at 17:20
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    $\begingroup$ @David Richerby: Shoulders are generally wider than feet, and that's where the increased room is. $\endgroup$ – jamesqf Mar 3 at 17:43
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    $\begingroup$ @DavidRicherby An oval shape like that disrupts the circular arc for each part circle and introduces bending forces in the frames above and below the floor beams as pressurization forces push out. You can do it that way, but you will add a bit of weight stiffening the frames. The double bubble profile is the lightest way to achieve an oval-ish cross section. $\endgroup$ – John K Mar 3 at 21:30
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This is called the "double bubble". The 737 has a similar but less pronounced design.

Both aircraft had a primary requirement to seat a specific number of passengers in each row: 5 in the DC-9 and its successors, 6 in the 737. Seat-bottoms are the widest parts of the seat, so they made the fuselage the widest there.

But continuing arc downward from there would result in insufficiently shallow cargo area, so they designed a lower bubble that provides more width and depth in the hold. And they net out to less materials (weight) and cross-section area (drag) than if you tried to fit that many seats per row in a purely circular cross-section.

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The answer is soap bubbles:

Soap bubbles

They are filled with slightly higher pressure air, and when they're attached to each other, they have a planar "reinforcement-like" part between them. (see the picture) This is similar to your "attached circles with reinforcement between them"-style structure. (DC fuselage)

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