Editing Airship (section)

===Envelope===
The envelope is the structure which contains the buoyant gas. Envelopes in the early 19th century were made from [[goldbeater's skin]], selected for its low weight, relatively high strength, and impermeability compared to paper or linen. By the 1920s, [[natural rubber]] treated with cotton became the predominant [[elastomer]] used in envelope construction. The natural rubber was succeeded by [[neoprene]] in the 1930s and [[Nylon]] and [[Polyethylene terephthalate|PET]] in the 1950s. A few [[metal-clad airship|airships have been metal-clad]].<ref>{{Cite book |last=Brooks |first=Peter |url=https://archive.org/details/zeppelin-rigid-airships-1893-1940_compress/page/93/mode/2up |title=Zeppelin: Rigid Airships, 1893-1940 |date=1992}}</ref> The most successful of which is the [[ZMC-2|Detroit ZMC-2]], which logged 2265 hours of flight time from 1929 to 1941 before being scrapped, as it was considered too small for operational use on anti-submarine patrols.<ref>[http://nasgi.org/zmc2.htm NAS Grosse Ile] {{Webarchive|url=https://web.archive.org/web/20110709085217/http://nasgi.org/zmc2.htm|date=2011-07-09}}, NASGIVM. 2006.</ref>

The problem of the exact determination of the pressure on an airship envelope is still problematic and has fascinated major scientists such as [[Theodor Von Karman]].<ref>{{Cite web|url=http://authors.library.caltech.edu/47901/1/19930094841.pdf|title=Von Karman, Theodor. "Calculation of pressure distribution on airship hulls." (1930).}}</ref>

The envelope may contain [[ballonets]] (see below), allowing adjustment of the density of the buoyant gas by adding or subtracting envelope volume.