text: to distinguish between 
  strength and stiffness. Strength is simply the 
  resistance to load while stiffness is the resist- 
  ance to deflection or deformation. While 
  strength and stiffness are related, it is necessary 
  to appreciate that adequate structural strength 
  does not automatically provide adequate stiff- 
  ness. Thus, special consideration is necessary 
  the structural components with 
  to provide 
  specific stiffness characteristics to prevent un- 
  desirable aeroelastic effects during normal 
  operation. 
  
  It  is  important 
  
  An obvious solution to the apparent prob- 
  fatigue strength, 
  lems of static strength, 
  stiffness and rigidity would be to build 
  the 
  airplane like a product of an anvil works, 
  capable of withstanding all conceivable loads. 
  However, high performance airplane con- 
  inefi- 
  figurations cannot be developed with 
  cient, lowly stressed structures. The effect of 
  additional weight 
  is best illustrated by pre- 
  liminary design studies of a very long range, 
  the preliminary 
  high altitude bomber. 
  phases of design, each additional pound of 
  any weight would necessitate a 25-pound 
  increase in gross weight to maintain the same 
  performance. An increase in the weight of 
  any item produced a chain reaction-more 
  fuel, larger tanks, bigger engines, more fuel, 
  In the 
  heavier landing gear, more fuel, etc. 
  competitive sense of design, no additional 
  structural weight can be tolerated to provide 
  more strength than is specified as necessary 
  for the design mission requirement. 