NAVWEPS 00-EOT-80  OPERATING STRENGTH LIMITATIONS  It  is important  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  to provide the structural components with  specific stiffness characteristics to prevent un-  desirable aeroelastic effects during normal  operation.  An obvious solution to the apparent prob-  lems of static strength, fatigue 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-  figurations cannot be developed with inefi-  cient, lowly stressed  structures. The effect of  additional weight is best illustrated by pre-  liminary design studies of a very long range,  high altitude bomber.  In the preliminary  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,  heavier landing gear, more fuel, etc. In the  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.  AIRCRAFT  LOADS  AND  OPERATING  LIMITATIONS  FLIGHT LOADS-MANEUVERS  AND  GUSTS  The loads imposed on an aircraft in flight  are the result of maneuvers and gusts. The  maneuver loads may predominate in  the  design of fighter airplanes while gust loads  may predominate in the design of the large  multiengine aircraft. The maneuver loads an  331  airplane may encounter depend in great part  on the mission type of the airplane. However,  the maximum maneuvering capability is of  interest because of the relationship with  strength limits.  The flight load factor is defined as the pro-  portion between airplane lift  and weight,  where  MANEUVERING  LOAD FACTORS. The  maximum lift attainable at any airspeed occurs  when the airplane is at CLmU. With the use  of the basic lift equation, this maximum lift  is expressed  as:  Since maximum lift must be equal to the  weight at the stall speed,  If the effects of compressibility and viscosity  on Ch  are neglected for simplification, the  maximum load factor attainable is determined  by the following relationship.  Thus, if the airplane is flying at twice the  stall speed and the angle of attack is increased  to obtain maximum lift, a maximum load  factor of four will result. At three times the  stall speed, nine “g ’ s”  would result; four  times the stall speed, sixteen g ’ s result; five  times the stall speed, twenty-five g ’ s result;  etc. Therefore, any airplane which has high  speed performance  may have the capability of  high maneuvering load factors. The airplane  which is capable of flight speeds that are  n=L/W  n= load factor  L=lift,  Ibs.  W= weight, Ibs. 