Thermal consideration of generators in high-speed aircraft

THE PROVISION of electric power in aircraft becomes increasingly difficult as the speed and altitude of flight increases. The relatively large values of electric power generated per pound of equipment in subsonic aircraft are achieved by employing the low-temperature environmental air, directed as a blast through the generator, to remove the generator heat losses and limit the internal temperature. As actual and projected flight speeds increase, the air blast becomes less effective as a cooling medium, because the stagnation temperature of the air approaches and exceeds the allowable temperatures of the insulating materials, bearings, and brushes of the generator. To handle the cooling problem, the present trend in generator design is to introduce new materials which are able to operate at higher temperatures, to employ a circulating coolant such as oil, and an external heat exchanger, and to depend upon the evaporation of liquids for cooling.