NAVWEPS oo-EOT-80 
OPERATING STRENGTH LIMITATIONS 
GENERAL DEFINITIONS AND STRUC- 
TURAL REQUIREMENTS 
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. 
STATIC STRENGTH 
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: 
Type of aircraft: hbi”< limi, hi,orror 
Fighter or attack. 7.5 
Trainer. 7.5 
T ransport, patrol, antisubmarine. 3.0 or 2.5 
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. 
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 
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. 
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. 
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 
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