NAVWEPS 00-801-80  OPERATING STRENGTH LIMITATIONS  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.  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.  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 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.  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,  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  332 