Abstract :
A brief review is given of the methods of cooling electrical equipment and machines at present used in aircraft. Natural cooling, comprising radiation and natural convection, augmented where necessary by the use of fins, is much used, but where a greater amount of cooling is required forced convective air cooling must be employed, the air being circulated either by fan or by the use of blast air obtained from an external air intake. The theoretical aspects of natural and forced convective air cooling and the flow of air through machines are examined, and the limitations of forced cooling on high-speed and high-altitude aircraft are determined. Assuming a hot-spot temperature of 200°C, it is shown that cooling by blast air becomes inadequate at speeds in excess of about 1 000 m.h.p. owing to kinetic heating, and at altitudes in excess of about 60000ft owing to reduction of mass flow. The introduction of an efficient local cooling circuit and a blast-cooled radiator is shown to improve the altitude performance. Alternative methods of cooling to be used in the local circuit such as air at constant pressure, liquid cooling and evaporative cooling are discussed. Finally, methods of surmounting the thermal barrier so that effective cooling at very high speeds may be achieved are reviewed.
