It is a common misconception that if a plane has propellers, then it cannot be a "jet." However, this is not so. The turboprops used on planes like the C-130, P-3 Orion, and many commuter planes such as the ATR-42 do indeed use propellers to generate thrust, but the basic technology that turns those propellers is based on the turbojet. In more general terms, a turbojet is a gas turbine engine, and that same gas turbine cycle is used to power a turboprop as well as a turbofan. A turbofan is just like a turboprop except that some of the power generated by the gas turbine core is used to turn a series of ducted fan blades that generate additional thrust.
A similar class of engines that is just starting to be used on production aircraft is the propfan. Although the propfan also utilizes a gas turbine core, the power generated by the core is used to turn a series of unducted fan blades rather than propellers. These fan blades convert the power generated by the engine into thrust to push the aircraft forward.
The key point to remember is that both the turboprop and the propfan are classified as "jets" since a jet engine is
at the heart of the system. This is not true in the case of other propeller-driven aircraft, such as those of WWII
or most modern-day light general aviation planes. These craft are powered by piston engines, a much different
technology that is more akin to how an automobile engine works.
- answer by Joe Yoon, 3 August 2003
To further illustrate the point about turboprop engines being jet-powered, I recently came across the following quote in a copy of Skywest Airlines Magazine. The article was about the EMBRAER EMB-120 Brasilia commuter plane operated by the airline, and included the remark
"...Don't be fooled by the propellers you see: the same technology that powers jet aircraft actually powers the EMB-120 as well. Like jet engines, the EMB-120 is powered by a gas turbine design, allowing for the superior reliability and power that jet engines enjoy..."While the article does not mention the turboprop by name, it does a good job of summarizing the key points of our answer about the gas turbine core used in turbojet and turboprop engines.
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