13-8] ACCELERATION 707 



When it is not possible to substitute rugged components for more 

 sensitive components for either electrical or mechanical reasons, it is 

 desirable to group the sensitive components together. The area housing 

 these components can then be partially protected from the acoustic noise 

 by various sound absorption and sound insulation techniques. Sound- 

 absorbing materials are light and usually porous. Sound insulation 

 materials are massive and therefore impervious. Either or both techniques 

 can be efficiently used depending upon restrictions of weight, volume, form 

 factor, etc. for a particular design problem. 



13-8 ACCELERATION 



When an aircraft deviates from constant velocity flight, the equipment 

 on board is necessarily subjected to complex dynamic forces. When the 

 principal frequencies of these forces are low enough to be considered steady, 

 the environment is discussed in terms of acceleration. Such forces are 

 experienced notably in controlled maneuvers. Launching, landing, and air 

 turbulence will also induce dynamic forces, some of which may have 

 components of frequency low enough to be treated as acceleration, although 

 shock and vibration frequencies are more to be expected from these flight 

 conditions. 



Since the load induced by an acceleration is proportional to its magnitude 

 it is customary to measure loads in terms of acceleration. The ratio of the 

 magnitude of acceleration at a point to the magnitude of the acceleration of 

 gravity is called the load factor. The weights of all components and this 

 resultant combined with the weights are multiplied by this load factor to 

 obtain the net load. Load factors are often given for the center of gravity 

 of an aircraft. Since an aircraft is of finite size and is effectively a rigid body 

 at the frequencies under consideration here, the location of the equipment 

 must be considered before applying a load factor. It will be recalled from 

 the mechanics of rigid bodies that the acceleration of a point on such a body 

 is a function of the velocity and acceleration of the body, in all six of its 

 degrees of freedom, and of the position of the point on the body. 



Acceleration loads, when considered alone, do not present, for most 

 components of a radar, so severe a requirement as shock and vibration 

 loads, although acceleration is a difficult environment for certain instru- 

 ments, such as gyroscopes, which are auxiliary to radars. Because of the 

 comparatively long duration of an acceleration load, it may in some cases 

 affect the design of a major structural member. 



Steady acceleration probably has the greatest effect on packaging design 

 when considered in combination with vibration or shock. Accelerations of 

 an aircraft tend to increase the static deflection of installed vibration 

 isolators and may even cause full deflection, or snubbing. If this occurs, the 



