Lower-speed aircraft can use propellers for thrust generation. Therefore, instead of driving a smaller encased fan (i. e., turbofan), a large propeller (i. e., turboprop) can be driven by a gas turbine engine to improve efficiency because the exhaust energy can be further extracted to a very low exhaust velocity (i. e., nearly zero nozzle thrust). Some residual jet thrust is left at the nozzle exit plane when it needs to be added to the propeller thrust. The nozzle thrust is converted to HP and, together with the SHP generated, it becomes the equivalent SHP (ESHP).
However, a large propeller diameter limits rotational speed due to both aerodynamic (i. e., transonic blade tips) and structural (i. e., centrifugal force) considerations. Heavy reduction gears are required to reduce the propeller rpm to a desirable level. Propeller efficiency decreases when aircraft are operating at flight speeds above Mach 0.5. For shorter-range flights, a turboprop’s slower speed does not become time-critical to the users, yet it offers better fuel economy. Figure 10.7 is a schematic diagram of a typical turboprop engine. Modern turboprops have up to eight blades (see Figure 10.31), which allow a reduction of the diameter size and operate at a relatively higher rpm and aircraft speed.