696 



MECHANICAL DESIGN AND PACKAGING 



environment forcing the vibration, {b) the response characteristics of the 

 equipment and its parts, and {c) the fatigue characteristics of the materials 

 used. Design for reliability in a vibrating environment accordingly requires 

 constant awareness and consideration of these three factors. Vibration 

 testing machines are of use in simulating a known or specified environment 

 in order to obtain response characteristics or otherwise evaluate tentative 

 or finished designs. 



The character of the vibration environment of an installed airborne radar 

 can be measured in flight by means of vibration pickups and suitable 

 recording equipment. The prediction of this environment for a radar under 

 design is difficult, however, because the aircraft for which it is destined will 

 probably not be available at this stage. Even if a similar aircraft were 

 available, the results of vibration measurements must be used with caution, 

 for they will be completely valid only when the aircraft is loaded with a 

 dynamically similar model mounted in the exact manner planned for the 

 radar. The mechanical design and the early phases of vibration testing, 

 therefore, must usually depend upon an environment estimated from 

 past experience, possibly described in a military specification. Periodic 

 vibrations in aircraft have been observed at frequencies ranging from 

 about 5 cps to high acoustic frequencies; however, many authorities now 

 agree that the structure of a high-speed airframe usually vibrates in a 

 random manner forced by the cumulative effect of a very large number of 

 independent actions. 



Randotn jnotion means nonperiodic motion which is the result of a very 

 large number of impulses occurring by chance and which may be described 

 by a normal probability law. The instantaneous acceleration of a typical 

 random environment is plotted against time in Fig. 13-5. It can be seen 



2 + 



Fig. 13-5 Instantaneous Acceleration of Typical Random Motion. 



that some acceleration peaks have a greater magnitude than others and that 

 the period of time between values of zero acceleration varies irregularly. 

 Although the precise acceleration which will occur at any future instant 

 cannot be calculated, the probability of that acceleration occurring can be 

 stated. 



The characteristics of a random acceleration can be described in terms of 

 the mean square acceleration spectral density, or as it is often called, 



