Figure 5-51.  Asymmetrical loading of propeller (P-factor).  In aerodynamics, the maximum load factor (at given bank  angle) is a proportion between lift and weight and has a  trigonometric relationship. The load factor is measured in  Gs (acceleration of gravity), a unit of force equal to the force  exerted by gravity on a body at rest and indicates the force to  which a body is subjected when it is accelerated. Any force  applied to an aircraft to deflect its flight from a straight line  produces a stress on its structure. The amount of this force  is the load factor. While a course in aerodynamics is not a  prerequisite for obtaining a pilot’s license, the competent  pilot should have a solid understanding of the forces that act  on the aircraft, the advantageous use of these forces, and the  operating limitations of the aircraft being flown.  For example, a load factor of 3 means the total load on an  aircraft’s structure is three times its weight. Since load factors  are expressed in terms of Gs, a load factor of 3 may be spoken  of as 3 Gs, or a load factor of 4 as 4 Gs.  If an aircraft is pulled up from a dive, subjecting the pilot to  3 Gs, he or she would be pressed down into the seat with a  force equal to three times his or her weight. Since modern  aircraft operate at significantly higher speeds than older  aircraft, increasing the potential for large load factors, this  effect has become a primary consideration in the design of  the structure of all aircraft.  With the structural design of aircraft planned to withstand  only a certain amount of overload, a knowledge of load  factors has become essential for all pilots. Load factors are  important for two reasons:  Aircraft designed under the category system are readily  identified by a placard in the flight deck, which states the  operational category (or categories) in which the aircraft  An entirely different situation exists in aircraft design with  maneuvering load factors. It is necessary to discuss this matter  separately with respect to: (1) aircraft designed in accordance  with the category system (i.e., normal, utility, acrobatic); and  (2) older designs built according to requirements that did not  provide for operational categories.  The above considerations apply to all loading conditions,  whether they be due to gusts, maneuvers, or landings. The  gust load factor requirements now in effect are substantially  the same as those that have been in existence for years.  Hundreds of thousands of operational hours have proven  them adequate for safety. Since the pilot has little control over  gust load factors (except to reduce the aircraft’s speed when  rough air is encountered), the gust loading requirements are  substantially the same for most general aviation type aircraft  regardless of their operational use. Generally, the gust load  factors control the design of aircraft which are intended for  strictly nonacrobatic usage.  This 1.5 load limit factor is called the “factor of safety” and  provides, to some extent, for loads higher than those expected  under normal and reasonable operation. This strength reserve  is not something that pilots should willfully abuse; rather, it is  there for protection when encountering unexpected conditions.  The problem of load factors in aircraft design becomes how  to determine the highest load factors that can be expected in  normal operation under various operational situations. These  load factors are called “limit load factors.” For reasons of  safety, it is required that the aircraft be designed to withstand  these load factors without any structural damage. Although  the Code of Federal Regulations (CFR) requires the aircraft  structure be capable of supporting one and one-half times  these limit load factors without failure, it is accepted that  parts of the aircraft may bend or twist under these loads and  that some structural damage may occur.  The answer to the question “How strong should an aircraft  be?” is determined largely by the use to which the aircraft is  subjected. This is a difficult problem because the maximum  possible loads are much too high for use in efficient design.  It is true that any pilot can make a very hard landing or an  extremely sharp pull up from a dive, which would result in  abnormal loads. However, such extremely abnormal loads  must be dismissed somewhat if aircraft are built that take off  quickly, land slowly, and carry worthwhile payloads.  Load Factors in Aircraft Design  Load Factors  Low angle of attack  High angle of attack  Load on  upward moving propeller blade  Load on  downward moving propeller blade  Load on  downward moving propeller blade  Load on  upward moving propeller blade  1. It is possible for a pilot to impose a dangerous overload  on the aircraft structures.  2. An increased load factor increases the stalling speed and  makes stalls possible at seemingly safe flight speeds.  5-33 