Turbo-props Versus Jet Engines
Props versus Jets
While both types of engine are propelled by a turbine, the way in which the engines deliver their output to propel the aircraft are totally different. Turbine propellers or turboprops are engines that drive an aircraft propeller using a series of reduction gears. Turbine Jets or turbojets are engines that propel the aircraft by heating air and using the expansion as a power propulsion system.
What is the major difference between turboprops and jet engines, and what are the pros and cons? The major difference is that turboprop engines perform better at lower speeds and jet engines perform better at higher speeds. The first major clue as to why the engines perform differently at different speeds is the way the way their output is measured. Turboprops output are measured in horsepower and jet engines are measured in thrust. Power being the rate at which total energy is transferred, and thrust being the amount of propulsion force applied to the aircraft by the engine.
While all this may seem technically complicated it can be understood in simpler terms and examples. Both engines use the same stages: intake, compression, combustion, and exhaust. The major difference being the turboprop uses the turbine engine to rotate the propeller which propels the aircraft, while the jet engine uses the expansion of the hot exhaust gases to push the aircraft forward.
Revisiting the earlier major difference, propellers have higher power at lower speeds. This however changes as the aircraft increases speed. The aircraft then starts running into propeller limitations. At higher speeds the lift created by the blade turns into rotational resistance forming drag and reducing thrust.
On the other hand, a jet engine pushes a smaller amount of air at low speeds. The jet’s thrust comes from the hot air. That is, the air leaving the exhaust is at a higher atmospheric pressure. The jet’s thrust does not fall as the jet goes faster. The engine actually takes in more air from the free flow velocity without the compressor doing any extra work. This results in better performance at the faster speeds.
Turboprop aircrafts are now being used by low cost carriers on short-leg flights because they are more economical on these types of flights. They also perform well in bush operations as they have Short Takeoff and Landing (STOL) capabilities which are important for shorter runways and landing strips. These can be seen in the use of the DHC-8 and the Cessna Caravan C-208.
Jet engine aircrafts are still the favourite for long-haul airline and larger cargo operations. They are efficient and perform better at higher speeds. They consume less fuel at these speeds. They also fly higher to more comfortable atmospheric conditions providing for smoother flights.
The noise levels are considered high on both the turbojet and the turboprop engines. The older engines were both noted to be very noisy. However new technology is providing for lower noise levels. Both types of engine manufacturers are working hard to develop engines that operate below noise pollution levels.