Chapter 7. Turboprop engine

The turbojet and turbofan have relative low performance at the low speed. In this case the turboprop engines can be used (Fig. 7.1). In comparison to the turbofan engine, the turboprop engine uses a propeller instead of fan. Sometimes, the propeller’s turbine is called a free turbine. As the rotational speed of rotor is high the propeller is connected to the rotor by gearbox that reduces the rotational speed. The jet trust of turboprop usually is below 15% of total power. Most of the aircrafts with turboprop engines fly at speed not exceeding 800 km/h.

Fig. 7.1. Scheme of turboprop engine.

The calculation of turboprop engine is similar to the turbojet engine till section after compressor’s turbine.

Several specific equations are introduced below. The free isentropic work is

The optimal coefficient of free energy distribution between propeller and nozzle for can be calculated from

If .

The isentropic nozzle work is

and the isentropic work of free turbine is

The work of free turbine is

4 (frt). The pressure ratio of free turbine is

The total pressure after free turbine is

and the total temperature is

5. If the pressure is fully discharged in the nozzle the normalized gas speed from the nozzle can be calculated as

The normalized isentropic gas speed from nozzle  can be find from

and the total pressure at the nozzle exit is

The gas speed from the nozzle is

6. The specific propeller power is

The specific equivalent power for   can be calculated from

if :

The specific fuel consumption is

The equivalent engine power is