What is the primary propulsion system for an airplane?

The primary propulsion system for an airplane is the engine. The engine generates thrust, which propels the aircraft forward and enables it to overcome drag and ascend into the air. In most aircraft, the engines can be either jet engines or piston engines, depending on the design and intended use of the aircraft.

Other components mentioned, such as the propeller, landing gear, and wings, play crucial roles in the operation of the aircraft but do not function as propulsion systems. The propeller is a mechanism driven by the engine to generate thrust, while the wings provide lift to support the aircraft in flight. The landing gear is essential for takeoff and landing but does not contribute to the propulsion of the aircraft. Therefore, the engine is the components that directly delivers the necessary power for flight.