When jet engines were first introduced in the 1940s, they were not very efficient. In fact, the cost of operating a jet airplane was so great that only military uses could be justified. At the time, commercial airline companies decided to compromise between the well-tested piston engines they were then using and the more powerful, but more expensive, jet engines. The result was the turboprop engine. In a turboprop engine, a conventional propeller is attached to the turbine in a turbojet engine. As the turbine is turned by the series of reactions described above, it turns the airplane's propeller. Much greater propeller speeds can be attained by this combination that are possible with simple piston-driven propeller planes. The problem is that at high rotational speeds, propellers begin to develop such serious eddying problems that they actually begin to slow the plane down. Thus, the maximum efficient speed at which turboprop airplanes can operate is less than 450 MPH (724 km/h).
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David E. Newton