Which of the following is NOT a type of aircraft engine?

The distinction that makes the selected answer correct lies in the nature of aircraft engines versus aircraft types. Piston, turboprop, and turbojet refer specifically to the types of engines used to power aircraft.

Piston engines, similar to those found in cars, are typically used in smaller, general aviation aircraft and operate on the principle of combustion in cylinders. Turboprop engines, which are a type of turbine engine, utilize a gas turbine to spin a propeller, combining jet engine characteristics with propeller-driven performance, commonly used in regional airliners. Turbojet engines are designed for high-speed flight, with all the airflow passing through the engine for propulsion, which is characteristic of many military and older aircraft.

On the other hand, a helicopter refers to a specific type of aircraft that utilizes rotating blades to achieve lift and does not classify as an engine type. Helicopters may use either piston or turbine engines to power their rotors, but the term "helicopter" itself denotes the aircraft rather than any specific engine type. Thus, the assertion that "helicopter" is not a type of aircraft engine is accurate, as it represents a fundamental category of aircraft design rather than an engine classification.