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OF ALL rotating electric aircraft equipment, the inverter is probably one of the most difficult and most interesting items. A semi-independent power conversion unit, it presents a combination of design problems inherent to its components, together with additional ones, such as special vibration, ventilation, starting, and regulation problems. Some of the inherent problems are partially eased by its limited output such as, for instance, commutation; others, such as symmetry requirements, are aggravated. Thus, the inverter is a highly compacted motor generator set, regulated to very close limits, self-ventilated or, lately, mixed ventilated throughout a considerable range of altitudes and temperatures, vibrated with as high as 500 cycles per second (cps) equivalent to accelerations of 5 to 10 G's, and exposed to shocks, fungus, humidity, frequent starting, etc. In the following, an attempt will be made to outline the design problems, discuss possible cooling and regulation methods, and anticipate future lines of development.