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Consider the fuselage of an A300. I have noticed this not only with airliners, but also for private jets and turboprops: why is the fuselage circular-shaped? Why don't aircraft have square-shaped fuselage? Does it have to do with too much drag?

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If you pressurize any hollow structure, it will try to assume a round shape. If you want to create a lightweight pressure vessel, again a sphere will be the most efficient result, because there the stresses in the skin will be equal at every point. Blow up a party balloon if you are in doubt.

A sphere is not the most efficient shape for aerodynamics, so fuselages are elongated spheres with a nice fairing at the end. If you make them cylindrical in the middle, you can build most bulkheads on the same jig and can interchange sections of the internal fairings. Plus, if you need a longer or shorter fuselage for the next version of the plane, you can easily add or remove sections - the parts will still fit after the modification.

Note that the fuselages of unpressurized aircraft don't follow this logic A Short Skyvan or a Dornier 228 have fuselages with a rectangular cross section, so large cargo can fit in.

Also, some fuselages are combinations of cylinders. The Boeing 377 Stratocruiser used the lower fuselage of the B-29 and had an upper, larger cylinder on top to give the passengers more room.

Boeing 377 cross section

Boeing 377 cross section (picture source)

For the same reason, rocket stages are cylindrical, too. They also need to be aerodynamic, have to tolerate high internal pressure and need to be lightweight.

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    $\begingroup$ +1 for answering the inverse question why some fuselages doesn't have circular cross section ? Should deserve a "true" +1 if compromise between aircraft purpose (cargo/compliant with containers specification..) and weight added (why are fuselage like those of ATRs, A380, CN235 and 737 doesn't take advantage of the extra space of a circular fuselage...) $\endgroup$
    – Karl Stephen
    Oct 2, 2015 at 13:40
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    $\begingroup$ The "will try to assume a round shape" part is due to gases' natural behavior of assuming the container's shape, which causes pressure to distribute equally in the perimeter area. If the container is flexible (think that metal is flexible given enough force) it will warp to distribute tension. A sphere distributes tension equally, whereas a cube does not. Accumulated tension requires structural reinforcement, thus weight, thus it's less economical for a plane. $\endgroup$
    – Ronan Paixão
    Oct 4, 2015 at 22:06
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The fuselages are circular (or nearly circular) in shape for two main reasons:

  • The main reason is that for a circular cross section, the pressure loads are resisted by tension, rather than by bending loads in non-circular sections. Also, the non-circular sections have stress concentrations when pressurized, which may lead to failure.

  • In case of a circular design, the flow will not separate under small (to moderate) angles of attack and in sideslip.

In case of non-pressurized aircraft, the fuselages are dictated by volume constraints and are usually rectangular in shape as it is more efficient in space utilization.

Commuter cabin cross sections

Source: adg.stanford.edu

In the case of pressurized aircraft, the best option structurally is to have a circular fuselage, but in order to have a useful internal space, an elliptical or 'double bubble' design is used, with an outer circular section and divided internal sections, like the A380.

A380 fuselage cross section
Source: EADS

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Why don't [airliners] have square shaped fuselage?

enter image description here Shorts Skyvan photo from Wings over Europe

enter image description here Shorts Skyvan diagram from A Tall Guy

Most airliners are pressurized. If you inflate a rubber balloon you'll notice that the most economical and strongest shape for a pressurized container is one with a circular cross-section.

You'll also notice, when you inflate an Origami water bomb (you should stop reading this and make an origami water bomb now), that the flat sides buckle and bulge - flat sheets are not good at resisting pressure.

Spherical airliners would have too many disadvantages but cylindrical ones have a good balance between strength, weight, drag and space-efficiency.

As you can see above, in some circumstances, aircraft manufacturers do beleieve that the advantages of a rectangular cross-section are worthwhile.

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    $\begingroup$ I personally believe in rectangle design. $\endgroup$
    – Ethan
    Oct 2, 2015 at 11:50
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    $\begingroup$ 1. Wow, that's an odd looking plane! I'm tempted to say the real reason is that rectangular fuselage planes are ugly ;-) 2. Seriously though, even taking aerodynamics out of the equation, spherical pressure vessels are quite rare. Flick through any industrial compressor catalog and you'll see all the tanks are cylindrical, because of the ease of fabricating cylindrical sides and the difficulty of fabricating hemispherical ends. The only difference is the aspect ratio is 2-3 times shorter than a typical plane. $\endgroup$
    – Level River St
    Oct 2, 2015 at 14:13
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    $\begingroup$ @steveverrill I wouldn't say ugly. I do have an aircraft design that has a rectangular fuselage and looks really cool looking when I picture it in my head. $\endgroup$
    – Ethan
    Oct 2, 2015 at 18:40
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    $\begingroup$ @steveverrill: I don't know about spherical pressure vessels being that rare. Consider natural gas tankers and natural gas tanks for examples. Granted, those are big tanks rather than something you'd see in a compressor catalogue but there's still quite common. $\endgroup$
    – mu is too short
    Oct 2, 2015 at 18:47
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    $\begingroup$ @muistooshort Liquefied natural gas tanks are spherical to reduce surface area and therefore insulation. Strictly speaking they aren't pressure vessels, as they operate at ambient pressure (and -162C temperature.) Methane has a critcal point of -83C and therefore cannot be liquefied at ambient temperature. A spherical pressure vessel offers 25% theoretical material saving over an infinitely long, thin cylinder. In practice it's rarely worth it if pressure is the only consideration. $\endgroup$
    – Level River St
    Oct 2, 2015 at 19:41
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Drag has little to nothing to do with it.

The primary reason why the fuselage is circular (or elliptical) shaped is that the cabin is pressurised.

This means that, mostly during cruise, the interior of the fuselage has an higher pressure than the outside atmosphere. The circular (or roughly circular) shape allow the fuselage to avoid blowing up like a baloon with minimal amount of material, minimizing the overall weight of the aircraft. A square fuselage would break apart at the corners due to stress concentration. A circular shape do not have corners were the tensile stress can concentrate.

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    $\begingroup$ Drag has insofar to do with it that otherwise aircraft fuselages would be spheres. The cylinder of the same volume causes much less drag when moved through air along its longitudinal axis. Plus, there are more window seats possible if you take a cylinder ;-) $\endgroup$
    – Peter Kämpf
    Oct 2, 2015 at 8:36
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    $\begingroup$ @PeterKämpf I agree, but the question is "circle vs square section", not about the 3D shape $\endgroup$
    – Federico
    Oct 2, 2015 at 8:37
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    $\begingroup$ @PeterKämpf More space to put your bags in a square shaped. And also more cabin width. $\endgroup$
    – Ethan
    Oct 2, 2015 at 8:38

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