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Nearly all modern transport aircraft are designed with a plug-system fuselage, allowing longer and shorter variants to be built by adding or removing plugs.

In addition, the plug system simplifies construction, as large sections of the aircraft can be assembled from modular parts.

What was the first aircraft to employ this system?

And what was the reason for adopting it in the first place:

  • to make possible the construction of variants
  • to enjoy efficiencies and economies of design and manufacture
  • both of the above?
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This should be a Zeppelin. You said "aircraft", not "airplane", so lighter-than-air systems qualify also.

Already the model submitted with the patent application in 1895 (yes, back then the inventor had to demonstrate that the invention is actually implementable!) showed several cylindrical sections, akin to a train of cylindrical balloons. The final patent (Reichspatent No. 98580) was for a "Lenkbarer Luftzug" (dirigible aerial train). Adding more sections would increase length and payload.

Later, when the maximum diameter of Zeppelin airships was limited by the size of available hangars, their mid section was of constant size and allowed to add plugs to increase payload and range. This was, among others, the case with LZ 104 which turned out 30 m longer than initially planned.

The last big airliner not designed for stretching was the Lockheed Constellation. It had a continuously varying cross section from tip to tail.

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  • $\begingroup$ Interesting, though if it is the balloon that is made of sections, does that count as a fuselage? $\endgroup$ – Daniele Procida Nov 13 '18 at 11:31
  • $\begingroup$ @DanieleProcida: Good question! At least it was the part holding the payload, and the idea was to spread the payload over the length of the airship. Only it is not called a fuselage. $\endgroup$ – Peter Kämpf Nov 13 '18 at 19:47
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In the US, that was most likely the Douglas DC- series of aircraft. Douglas used the plug-stretch design principle throughout the series, as more powerful engines became available over time which permitted more payload.

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