"A jet aircraft (or simply jet) is an aircraft (nearly always a fixed-wing aircraft) propelled by jet engines.
Whereas the engines in propeller-powered aircraft generally achieve their ...
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maximum efficiency at much lower speeds and altitudes, jet engines achieve maximum efficiency at speeds close to or even well above the speed of sound. Jet aircraft generally cruise most efficiently at about Mach 0.8 (981 km/h (610 mph)) and at altitudes around 10,000–15,000 m (33,000–49,000 ft) or more.
The idea of the jet engine was not new, but the technical problems involved could not begin to be solved until the 1930s." - (en.wikipedia.org 13.05.2021)