Framework for the Conceptual Decomposition of Unmanned Aircraft Propulsion Systems

Propulsion for the aircraft segment in unmanned aircraft systems (UAS) is provided not only by widely used propulsion technologies such as reciprocating piston engines and jet and prop turbine engines, but also by electric motors running off some combination of batteries, solar cells, and fuel cells. These disparate propulsion technologies pose a challenge to researchers and regulators in comparing systems, determining equivalent levels of safety, developing performance and safety requirements, etc. In the systems view of UAS propulsion presented here, the propulsion system is decomposed into five subsystems: (1) An energy source; (2) An energy transformer; (3) A powerplant; (4) A propulsion effecter; and (5) A control effecter. For example, a UAS employing an avgas fueled reciprocating piston engine would have petroleum distillates for its energy source, combustion as its energy transformer, the piston/crankshaft as its powerplant, a propeller as its propulsion effecter, and control of the flow of fuel, air, and electricity as its control effecter; a proton exchange membrane fuel cell driven UAS would have the hydrogen as its energy source, the fuel cell unit as its energy transformer, an electric motor as its powerplant, a propeller as its propulsion effecter, and control of applied voltage and current as its control effecter. The system framework is exploited in a technology survey of existing UAS propulsion systems that presents representative examples of the various propulsion technologies and compares the way in which the different generic subsystems are instantiated as the propulsion systems of various aircraft. The advantages and disadvantages of the propulsion technologies are discussed, as is the manner in which the systems framework facilitates comparison among different propulsion technologies and the way it may facilitate development of regulations appropriate for UAS. The ability of the framework to describe not only extant technologies, but also developing propulsion technologies such as fuel cells, micro jets, ultracapacitors, etc., is explained, and supported with key examples.