NAVWEPS oo-EOT-80  OPERATING STRENGTH LIMITATIONS  GENERAL  DEFINITIONS  AND  STRUC-  TURAL REQUIREMENTS  STATIC STRENGTH  permanent deformation when subjected to the  limit load.  In fact, the components must with-  stand this load with a positive margin.  This  requirement implies that the aircraft should  withstand successfully  the limit load and then  return to the original unstressed shape when  the load is removed. Obviously, if the air-  craft is subjected to some load which is in  excess of the limit load, the overstress may  incur an objectionable permanent deformation  of the primary structure and require replace-  ment of the damaged parts.  There are strength requirements which ate  common to all aircraft. In general, these re-  quirements can be separated  into three particu-  lar areas. These are detailed in the following  discussion.  The static strength requirement is the con-  sideration given to the effect of simple static  loads with none of the ramifications of the  repetition or cyclic variation of loads.  An  important reference  point in the static strength  requirement is the “limit  load”  condition.  When the aircraft is at the design conligura-  tion, there will  be some maximum of load  which would be anticipated from the mission  requirement of the airplane. For example, a  fighter or attack type aircraft, at the design  configuration, may encounter a very peak load  factor of 7.5 in the accomplishment of its mis-  sion. Of course, such an aircraft may be sub-  ject to load factors of 3, 4, 5, 6, 1, etc., but no  more than 7.5 should be required to accom-  plish the mission.  Thus, the limit load condi-  tion is the maximum of loads anticipated in  normal operation of the aircraft,  Various  types of aircraft will have different limit load  factors according to the primary mission of  the aircraft.  Typical values are tabulated  below:  Many different flight and ground load condi-  tions must be considered to define the most  critical conditions for the structural com-  ponents. In addition to positive lift flight,  negative lift flight must be considered. Also,  the effect of flap and landing gear configura-  tion, gross weight, flight Mach,number, sym-  metry of loading, c.g. positions, etc., must be  studied to account for all possible sources of  critical loads.  To verify the capability of the  structure, ground static tests are conducted  and flight demonstrations ate required.  To provide for the rare instances of flight  when a load greater than the limit is required  to prevent a disaster, an “ultimate  factor of  safety”  is provided.  Experience has shown  that an ultimate factor of safety of 1.5 is suf-  ficient for piloted aircraft. Thus, the aircraft  must be capable of withstanding a load which  is 1.3 times the design limit load.  The primary  structure of the aircraft must withstand the  “ultimate  load”  (1.5 times limit)  without  failure.  Of course, permanent deformation  may be expected with this “overstress” but  no actual failure of the major load-carrying  components should take place at ultimate load  Ground static tests are necessary  to verify this  capability of the structure.  Of course, these examples are quite general and  it is important to note that there may be varia-  tions according to specific mission require-  ments.  Type  of aircraft:  limi,  hi,orror  Fighter  or attack.  7.5  Trainer.  7.5  T ransport,  patrol, antisubmarine.  3.0 or 2.5  An appreciation of the static strength re-  quirements may be obtained by inspection of  the basic properties of a typical aircraft metal.  Figure 3.1 illustrates the typical static strength  properties of a metal sample by a plot of applied  stress versus resulting strain.  At low values  Since the limit load is the maximum of the  normally anticipated loads, the aircraft struc-  ture must withstand this load with no ill  effects.  Specilicallv, the primary structure of  the aircraft should experience  no objectionable  3,26 