Airplane engines may be classified as driven by propeller, Jet, Turbojet, and Rocket. Most engines originally were of the internal-combustion, piston-operated type, which may be air-or liquidcooled.
During and after world war II, duct-type and gas-turbine engines became increasingly important, and since then jet
propulsion has became the main form of power in most commercial and military aircraft. The landing gear is the understructure that supports the weight of the craft when on