684 



MECHANICAL DESIGN AND PACKAGING 



100 



1^2 



60 



S o 



P o 

 =^°= 40 



20 



100 200 300 400 500 600 700 800 900 1000 



TEMPERATURE, °F 



Fig. 13-1 Decrease in Strength of Common Structural Materials at Elevated 

 Temperatures. 



nonstructural components. The life expectancy of vacuum tubes decreases 

 rapidly with increasing envelope temperatures. The transistors now- 

 available are even more sensitive to temperature than the vacuum tubes. 

 Wattage ratings of resistors must be reduced as ambient temperatures 

 increase. Magnetic properties of materials change with temperature. Other 

 examples could also be cited. 



The radar equipment will normally gain heat (a) from internal sources, 

 (^) from the environment, or (c) from the entering "cooling" fluid, if used. 



Although internally produced heat is, technically, not an environmental 

 factor, it is convenient and logical to consider it at this point. In craft 

 moving at subsonic speeds, this heat, produced by vacuum tubes and 

 resistive components, is the principal factor contributing to temperature 

 rise. It will still be important wherever use is made of heat-dissipating 

 components. 



As long as the equipment is cooler than its environment, heat may flow 

 into it by conduction through the supporting structure, by direct radiation 

 from the hot surfaces which it can "see," and by natural convection of air. 

 To the radar packaged inside the airframe, the principal environmental 

 sources of heat are the aircraft skin and the other electrical and electronic 

 components. In the forward part of the aircraft, the skin temperature will 

 approach the aerodynamic stagnation temperature. This means that 

 aerodynamic heating becomes serious at supersonic speeds. The relation- 



