Flow past a turboprop engine in operation
Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (up to that necessary to drive the compressor) from turbine expansion. Owing to the additional expansion in the turbine system, the residual energy in the exhaust jet is low.
 Consequently, the exhaust jet produces (typically) less than 10% of the total thrust,
 and turboprops can have
bypass ratios up to 50-100
 although the propulsion airflow is less clearly defined for propellers than for fans.
The propeller is coupled to the turbine through a
reduction gear that converts the high
torque output to low RPM, high torque. The propeller itself is normally a
constant speed (variable pitch) type similar to that used with larger
reciprocating aircraft engines.
Unlike the small diameter fans used in
turbofan jet engines, the propeller has a large diameter that lets it accelerate a large volume of air. This permits a lower airstream velocity for a given amount of thrust. As it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree,
 a low
disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use.
Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft
propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching
Mach 0.75. To increase propeller efficiency, a mechanism can be used to alter their pitch relative to the airspeed. A variable-pitch propeller, also called a
controllable-pitch propeller, can also be used to generate negative thrust while decelerating on the runway. Additionally, in the event of an engine outage, the pitch can be adjusted to a vaning pitch (called
feathering), thus minimizing the drag of the non-functioning propeller.
While most modern
turbofan engines use
axial-flow compressors, turboprop engines usually contain at least one stage of
centrifugal compression. Centrifugal compressors have the advantage of being simple and lightweight, at the expense of a streamlined shape.
While the power turbine may be integral with the gas generator section, many turboprops today feature a free power turbine on a separate coaxial shaft. This enables the propeller to rotate freely, independent of compressor speed.
Residual thrust on a turboshaft is avoided by further expansion in the turbine system and/or truncating and turning the exhaust 180 degrees, to produce two opposing jets. Apart from the above, there is very little difference between a turboprop and a turboshaft.