13-3] TEMPERATURE 685 



ship between the stagnation temperature, speed, and altitude is 



r.= r„ + i^^ (13-1) 



where Ts = stagnation temperature, degrees Rankine 



Ta = ambient air temperature at altitude, degrees Rankine 



V = aircraft velocity, ft /sec 



J = Joule's constant, 778 ft Ib/Btu 



g = acceleration of gravity, 32.2 ft/sec^ 



Cp = specific heat of air at constant pressure, Btu/lb-°F. The 

 specific heat is a variable, dependent upon the temperature of 

 the air. A value of Cp = 0.24 Btu/lb-°F can be used for air 

 temperature below 1000°F without introducing a noticeable 

 error. 



To give an example of the serious nature of aerodynamic heating, the 

 stagnation temperature will be calculated for a Mach number of 2.5 

 (velocity = 2905 ft /sec) at sea-level conditions of the standard hot atmos- 

 phere. Ambient temperature, from Reference 1, is 103°F. 



Then, Ta = 460 + 103 = 563°Rankine 



2905^ 

 ^' = ^^^ + ^" (778) (32.2) (0.24) ^ ^^^^°^ 



/. = 1266 - 460 = 806° F. 



Fig. 13-2 shows the variation in stagnation temperature with Mach 

 number for the standard hot atmosphere. 



Another source of heat is solar radiation. For ground operations, solar 

 radiation can subject aircraft compartments to temperatures in the vicinity 

 of 160°F. These high temperatures can occur at times when the equipment 

 cooling system is inoperative and for periods of time long enough to render 

 useless the natural thermal lag capabilities of the electronic equipment in 

 its installed position. For flight conditions, the eff^ects of solar radiation are 

 of greatest relative magnitude for those portions of the flight profile 

 encompassing low-speed, high-altitude combinations, since aerodynamic 

 heating is small under these conditions. Therefore, time, as well as exposed 

 body surface area, is a determining factor in obtaining an appreciable heat 

 load from solar radiation. The higher the altitude of the body within the 

 earth's atmosphere, the greater the solar radiation intensity, because less 

 scattering and absorption of the sun's rays take place farther away from the 



