A turboprop engine is a type of turbine engine used to drive an aircraft propeller. The basic components of a turboprop engine are the intake, compressor, combustor, turbine, and propelling nozzle. Air is funneled in through the intake and compressed by the compressor. From here, fuel is added to the compressed air in the combustor, where the fuel-air mixture ignites. The hot gases resulting from this combustion then expand through the turbine. Thrust is obtained by the combustion gases pushing towards a vectored surface in front of the expanding gas. The remaining gas is transmitted through the reduction gearing to the propeller. Here, in the propeller nozzle, further expansion of these gases occurs, where they exhaust to atmospheric pressure.
Turboprop engines sacrifice exhaust thrust in exchange for shaft power, which is obtained by extracting additional power from turbine expansion. Thanks to the additional expansion within the turbine system, the residual energy in the exhaust jet is relatively low. As such, the exhaust jet produces at most only 10% of the total thrust. A greater percentage of the thrust comes from the propeller when at lower speeds than when at higher speeds. Turboprops can have bypass ratios up to 50 or 100, although the propulsion airflow is not as accurately defined in propellers as it is in fans.
In turboprop engines, the propeller is coupled to the turbine through a reduction gear that converts the high RPM/low torque output into low RPM/high torque. The propeller itself is typically a constant-speed propeller similar to those used on large reciprocating engine aircraft. Unlike the fans found in turbofan jet engines, which have small diameters, the propeller has a large diameter that allows it to speed up a large quantity of air. This enables it to produce a lower airstream velocity for a given amount of thrust. Because at low speeds the engine is better at accelerating a large amount of air by a small degree than a small amount of air by a large degree, a low disc loading increases the aircraft's energy efficiency, and reduces its fuel use.
Propellers lose efficiency as aircraft speed increases, so turboprop engines are normally not used on aircraft whose speeds exceed 0.6-0.7 Mach. Despite this, propfan engines, which share many similarities with turboprop engines, can cruise at flight speeds up to 0.7 Mach. To increase propeller efficiency over a broader range of airspeeds, turboprops are typically equipped with constant-speed propellers. The blades of this propeller type increase pitch as aircraft speed increases, allowing for operation at more airspeeds than a fixed-pitch propeller. A second benefit of constant-speed propellers is that they can also be used to generate negative thrust while decelerating on the runway upon landing.
In comparison to turbofans, turboprops are far more efficient at flight speeds under 450 mph because the jet velocity of the propeller is comparatively low. Modern turboprop airliners operate at nearly the same speed as small regional jet airliners but do so using approximately 30% less fuel per passenger. That said, compared to turbojets, propeller aircraft have much lower ceilings. Compared to piston engines, the greater power-to-weight ratio allows for shorter takeoffs, and reliability can offset the higher initial cost, as well as maintenance and fuel consumption.
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