In aviation, the term "propulsion" is used regularly in a variety of situations, and so has a wide range of meanings. In various instances, it includes the powered devices that provide thrust for flight, the systems that transmit, convert and condition power and energy for flight, and the energy storage. It typically does not include passive aerodynamic devices for lift, stability and control. It also does not include secondary and utility power systems (electrical, hydraulic, pneumatic, etc.) that do not directly maintain flight. For a conventional, commercial fixed-wing airplane, the definition of "propulsion" is relatively straightforward and well bounded, where the turbofan engines are externally mounted and procured from suppliers, and lift and control are provided by structural lifting surfaces (wings, tails, flaps, ailerons). The turbofan powerplants are the major component of a fixed-wing propulsion system. Propulsion on a commercial jet can conveniently be thought of, though not strictly correct, as everything in and associated with the nacelles typically hanging below the wings.
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