Abstract :
In blast-cooled electrical machines in aircraft, the minimum permissible mass flow of cooling air, the electrical output, the air outlet pressure (a function of aircraft altitude), the air inlet temperature (a function of aircraft speed and altitude) and the pressure drop across the machine are all related to each other. Experimental data concerning these quantities are obtained from rating tests on a prototype machine, and are generally presented graphically by means of a `rating-data chart¿. This paper describes a new form of rating-data chart, consisting of four component graphs. It has been derived from the heat-transfer and pressure-drop correlation equations of the physical system, and it expresses mass flow as a function of electrical output, pressure and temperature, and pressure drop as a function of mass flow, pressure and temperature. This form of presentation enables it to cover accurately the wide ranges of pressure and temperature which obtain in modern high-performance aircraft. The accuracy of the new form of chart has been assessed from charts for five machines, and is considerably greater than that of the simpler form of chart generally used at the present time.
