Which device propels an aircraft by producing thrust approximately perpendicular to its plane of rotation?

• A. Jet engine
• B. Turbojet
• C. Propeller
• D. Rotor

Answer: (C)

Thrust is generated when blades push air backward, and the reaction on the blades pushes the aircraft forward along the axis of rotation. A propeller does exactly this: its blades rotate in a plane, and the resulting air flow is accelerated rearward, producing forward thrust that points along the propeller’s axis (perpendicular to the plane of rotation). This is how most airplanes gain forward motion.

Jet engines and turbojets produce thrust by accelerating air or combustion gases through the engine and expelling them rearward; the thrust comes from the jet exhaust, not from rotating blades in a plane. A rotor, like in helicopters, also has blades rotating to produce thrust along the rotor axis, but its primary role is to provide lift to keep the aircraft aloft, whereas a propeller is designed specifically to propel the aircraft forward.