A turboprop is an aircraft engine that combines features of both jet and reciprocating engines. A turboprop is a type of gas turbine (technically, this makes it a jet engine). But instead of getting its thrust from expelling hot gas as the jet aircraft we are familiar with do, the turboprop uses the power of the engine to turn a propeller.
The history of the turboprop engine dates to the late 1930s when engineers were trying to develop more powerful aircraft engines, particularly for military aircraft. In 1940, Gyorgy Jendrassik of Hungary was the first to test a turboprop engine, the J-CS1. Although the experiment was success, it was never put into production. Later in World War II, both the Germans and British developed turboprop engines and put them into production, beginning in 1942.
The turboprop engine operates in a manner similar to a conventional jet engine. Air enters through an intake and is fed to a compressor. The compressor forces the air through a series of stages to very high pressure and sends it to the combustor, where it combustion with the fuel takes pace (usually the fuel is aviation grade kerosene). Following combustion, the hot gases drive the turbine. Here is where the difference between a conventional jet engine and a turboprop enters in. A jet engine expels the hot gases produced by combustion to provide thrust. In a turboprop these gases power a turbine connected to a drive shaft that turns a propeller which provides most of the thrust. The vented exhaust provides only a small portion of the thrust.
This approach has advantages and disadvantages. A turboprop aircraft cannot achieve the extremely high speeds possible with jet engines. Propellers become inefficient at speeds over 450 miles per hour, when the combined speed of the aircraft and the propeller spin causes the tips of the propellers to move nearly at the speed of sound. When this happens, shock waves are generated that disrupt the airflow and reduce efficiency. In fact, because of the speeds turboprops do attain, they use specially designed propellers with a smaller diameter than normal propellers, in order to reduce the speed of the propeller tips.
For some purposes, turboprop-driven aircraft have distinct advantages. With cruising speeds of around 400 miles per hour, they are faster than conventional propeller-driven planes, and are more fuel efficient at those speeds than jet aircraft. They also are extremely powerful, and can accelerate quickly to take off on very short runways. The shaft-driven propellers can be reversed, enabling a turboprop aircraft to stop in a short distance as well. This STOL (short takeoff and landing) capability is very useful because the turboprop powered aircraft can use very short runways. A variation, the “turbo shaft” engine, is used to for high-speed helicopters, particularly for military use.
Turboprop aircraft are generally small. “Small” is relative, however. Though no turboprop airplane approaches today’s giant jumbo jets in size, some are fairly large. An important turboprop aircraft is the Russian TU-95 “Bear” strategic bomber. First flown in 1952, the TU-95 has a top speed of 575 miles per hour (the fastest of any production propeller-driven aircraft) and cruises at just over 400 miles per hour. It proved so reliable that it was manufactured into the 1990s, and is still active as part of Russia’s air force. The United States Air Force uses the turboprop-powered C-130 Hercules as a STOL cargo plane, capable of delivering supplies and personnel directly to a battlefield even when landing space is limited.
Commuter airlines rely on small to medium-sized turboprop aircraft to fly short-haul routes, where the STOL capability and fuel efficiency make them an ideal choice. They are also popular choices for air cargo duties, and for flying payloads to remote locations like McMurdo Station in Antarctica.