Turboprop
What is a turboprop engine
The unsuspecting passenger will sometimes look across the airport tarmac and see aircraft that appears to be straight out of a Humphry Bogart movie - with all 4 propellers warming up. How oftern have I overheard those people saying, 'I hope we're not flying in one of those'.
In fact, most 2 or 4 engine aircraft with propellers are probably Turboprops - a combination of the inside of a jet engine with external propellers for added fuel efficiency.
Because propellers become less efficient as the speed of the aircraft increases, turboprops are used for low speed aircraft like cargo planes or small executive aircraft.
Turboprops therefore tend to fly at lower altitudes and at slower speeds than their Turbofan rivals. They are also cheaper to run and to hire - which is why they're used.
The noise level from turboprops tends to be higher than a turbofan, which has as much to do with altitude as it has to do with the engine itself. The main cause of this external noise are the large propellers to the front of the engine.
Additionally, the higher any aircraft goes, the quitier it will appear to become, the lower you fly, in any type of aircraft, the noisier it will be. This is to do with the density of the air around you.
A Turboprop ( Turbo-propeller ) or turboshaft engine therefore, differs from a Turbofan in that the design is optimized to produce rotating shaft power through a propeller, instead of thrust from the exhaust gas as in a true jet engine.
