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 
NAVWEPS 00-EOT-80 
OPERATING STRENGTH LIMITATIONS 
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 
n=L/W 
n= load factor 
L=lift, Ibs. 
W= weight, Ibs. 
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. 
v.2 
=(-) V* 
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 
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