A turboprop is a jet engine that spends almost all its power turning a propeller
Inside a turboprop, air is squeezed, mixed with jet fuel and burned just as in a jet. But instead of blasting the hot gas out the back for thrust, the turbine squeezes nearly every last bit of energy from it to spin a propeller. The exhaust ends up supplying only around a tenth of the push.
The logic is efficiency. A propeller's wide blades move a huge volume of air a little, rather than a small volume a lot, and at modest speeds that uses less fuel for the same thrust. Because the turbine spins far too fast for a propeller, a reduction gearbox trades speed for torque. Many modern designs, such as the Pratt and Whitney Canada PT6, use a separate free turbine to drive the propeller, so the core can keep spinning independently and a damaged propeller need not wreck the gas generator.
The approach has a ceiling. Once the blade tips approach the speed of sound, a propeller loses its grip on the air and efficiency collapses, so turboprops rarely power aircraft faster than about 0.6 to 0.7 of the speed of sound. The Soviet Tupolev Tu-95 bomber is a famous exception, pairing four enormous engines with contra-rotating propellers. Pilots also handle them differently: a special ground mode lets the blades give zero or even reverse thrust for taxiing and braking.
The idea dates to the 1920s. Alan Arnold Griffith published influential work on compressors in 1926, while Frank Whittle chose to put all the gas energy into pure jet thrust. The Hungarian engineer György Jendrassik patented a turboprop in 1929 and tested a large prototype at the Ganz Works in Budapest from 1937.
The public first glimpsed the concept in a 1944 cutaway drawing in the British magazine Flight, and Rolls-Royce soon converted a Derwent jet into the first British turboprop. In America, General Electric's XT31 flew in 1945, and Allison's later T56 went on to power the C-130 Hercules transport and P-3 Orion patrol aircraft.
Source: Turboprop
