Aerodynamics of Flight

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