In a turboprop engine, what happens to most of the energy in the exhaust gases?

• A. A piston engine with a propeller.
• B. An electric motor-driven propeller.
• C. A turbine engine which drives a propeller through a reduction gearing arrangement. Most of the energy in the exhaust gases is converted into torque, rather than using its acceleration to drive the aircraft.
• D. A turbojet engine that uses afterburner.

Answer: (C)

In a turboprop, the hot exhaust gases feed a turbine that is connected to the propeller through a reduction gear. The turbine extracts most of the energy from those gases to turn the propeller shaft, producing torque that drives the aircraft forward. Because so much energy is used to spin the propeller, only a small portion is left to accelerate the exhaust, so jet thrust from the exhaust is minimal compared with the power delivered to the propeller. The other options don’t fit because a turboprop uses a gas turbine to drive a propeller, not a piston engine or an electric motor, and it’s not operated as a turbojet with afterburner.