NAVWEPS 00-801-80 
OPERATING STRENGTH LIMITATIONS 
many times the stall speed will require due 
consideration of the operating strength limits. 
The structural design of the aircraft must 
consider the possibility of negative load factors 
from maneuvers. Since the pilot cannot com- 
fortably tolerate large prolonged negative “g”, 
the aircraft need not be designed for negative 
load factors as great as the positive load factors. 
The effect of airplane gross weight during 
maneuvers must be appreciated because of the 
particular relation to flight operating strength 
limitations. During flight, the pilot appre- 
ciates the degree of a maneuver from the 
inertia forces produced by various load factors; 
the airplane structure senses the degree of a 
maneuver principally by the airloads involved. 
Thus, the pilot recognizes loadfactor while the 
structure recognizes only load. To better 
understand this relationship, consider an ex- 
ample airplane whose basic configuration gross 
weight is 20,000 lbs. At this basic configura- 
tion assume a limit load factor for symmetrical 
flight of 5.6 and an ultimate load factor of 8.4. 
If the airplane is operated at any other con- 
figuration, the load factor limits will be al- 
tered. The following data illustrate this fact 
by tabulating the load factors required to 
produce identical airloads at various gross 
weights. 
Grass weight, Ibs. Limit load Ultimate 
factor load factor 
20,wO (basic). 5.60 8.40 
30,003 (max. rakcoff). 3.73 5.60 
13,333 (min. f”cl):. 8.40 12.60 
As illustrated, at high gross weights above the 
basic configuration weight, the limit and ulti- 
mate load factors may be seriously reduced. 
For the airplane shown, a 5-g maneuver im- 
mediately after a high gross weight takeoff 
could be very near the “disaster regime,” 
especially if turbulence is associated with the 
maneuver. In the same sense, this airplane 
at very low operating weights below that of 
the basic configuration would experience great- 
ly increased limit and ultimate load factors. 
Operation in this region of high load factors 
at .low gross weight may create the impression 
that the airplane has great excess strength 
capability. This effect must be understood and 
intelligently appreciated since it is not uncom- 
mon to have a modern airplane configuration 
with more than SO percent of its gross weight 
as fuel. 
GUST LOAD FACTORS. Gusts are asso- 
ciated with the vertical and horizontal velocity 
gradients in the atmosphere. A horizontal 
gust produces a change in dynamic pressure on 
the airplane but causes relatively small and 
unimportant changes in flight load factor. 
The more important gusts are the vertical gusts 
which cause changes in angle of attack. This 
process is illustrated in figure 5.2. The vec- 
torial addition of the gust velocity to the air- 
plane velocity causes the change in angle of 
attack and change in lift. The change in angle 
of attack at some flight condition causes a 
change in the flight load factor. The incre- 
ment change in load factor due to the vertical 
gust can be determined from the following 
equation: 
where 
An=change in load factor due to gust 
m=lift curve slope, unit of C, per degree 
of 01 
o=altitude density ratio 
W/S= wing loading, psf 
V. = equivalent airspeed, knots 
KU=equivalent sharp edged gust velocity 
ft. per sec. 
As an example, consider the case of an air- 
plane with a lift curve slope m=O.OB and wing 
loading, (W/S)=60 psf. If this airplane were 
flying at sea level at 350 knots and encountered 
an effective gust of 30 ft. per sec., the gust 
would produce a load factor increment of 1.61. 
This increment would be added to the flight 
load factor of the airplane prior to the gust, 
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