13-3] TEMPERATURE 687 



The principal thermal design problem is to keep the temperature of each 

 component low enough to allow satisfactory operation of the component 

 during the required period of time. The principal approaches to this 

 problem are 



id) Control of internal heat sources 



{U) Cooling (provision of heat sinks) 



{c) Increase in allowable component temperature. 



Control of internal heat sources consists of the use of components 

 generating a minimum of heat and the separation of heat sources from 

 temperature sensitive equipment. This approach is fundamental but 

 limited in applicability by the availability of components with the desired 

 characteristics. In the design of an airborne radar, every effort must be 

 made to use components tolerant of the highest temperatures, insofar as 

 this can be done without compromising other qualities. In most cases, 

 however, these two techniques alone will not produce equipment which can 

 operate reliably in the thermal environment. Resort to some types of 

 cooling will therefore be required. 



The equipment as a whole may normally lose heat in several of the 

 following ways: 



{a) By radiation 



{b) By conduction to structural members of the airframe 

 (c) By convection, i.e. forced cooling using a fluid 



{d) By the conversion of sensible heat to latent heat, i.e. by the evapora- 

 tion of a liquid of sublimation of a solid. 



As discussed above, the structure surrounding the radar in a supersonic 

 aircraft may well be hotter than the allowable temperature of some of the 

 components, or at least too hot for direct radiation and conduction to 

 provide significantly useful cooling. Thermal insulation techniques may 

 indeed be desirable to check the flow of heat into the equipment. 



Many systems, both simple and complex, have been developed for the 

 removal of heat from aircraft electronic equipment, using the principles ic) 

 and {d) above, alone or in combination. A few typical systems are depicted 

 in Fig. 13-3 in schematic form. In Fig. 13-3a the critical components are 

 cooled by a direct supply of ram air (forced air). In Fig. 13-3b, the elec- 

 tronic unit is sealed and pressurized. The sealed-in air is cooled by being 

 circulated through a heat exchanger cooled, in turn, by the ram air. In 

 Fig. 13-3c (another pressurized system), the design provides a conductive 

 path adequate to bring the heat from the critical elements to the stream of 

 ram air. This system is commonly called cold plate cooling. In any of these 

 three systems, if the ram air is not substantially cooler than the lowest 



