The offset flapping hinge is offset from the center of the rotor  hub and can produce powerful moments useful for controlling  the helicopter. The distance of the hinge from the hub (the  offset) multiplied by the force produced at the hinge produces  a moment at the hub. Obviously, the larger the offset, the  greater the moment for the same force produced by the blade.  The flapping motion is the result of the constantly changing  balance between lift, centrifugal, and inertial forces. This  rising and falling of the blades is characteristic of most  helicopters and has often been compared to the beating of  a bird’s wing. The flapping hinge, together with the natural  flexibility found in most blades, permits the blade to droop  considerably when the helicopter is at rest and the rotor is not  turning over. During flight, the necessary rigidity is provided  by the powerful centrifugal force that results from the rotation  of the blades. This force pulls outward from the tip, stiffening  the blade, and is the only factor that keeps it from folding up. Some helicopters incorporate stability augmentation systems  (SAS) to help stabilize the helicopter in flight and in a hover.  The simplest of these systems is a force trim system, which  uses a magnetic clutch and springs to hold the cyclic control in  the position at which it was released. More advanced systems  use electric actuators that make inputs to the hydraulic servos.  These servos receive control commands from a computer  that senses helicopter attitude. Other inputs, such as heading,  speed, altitude, and navigation information may be supplied  to the computer to form a complete autopilot system. The  SAS may be overridden or disconnected by the pilot at any  time. SAS reduces pilot workload by improving basic aircraft  control harmony and decreasing disturbances. These systems  are very useful when the pilot is required to perform other  duties, such as sling loading and search and rescue operations. The following paragraphs describe the various types of  vibrations.  Figure 2-50  shows the general levels into which  frequencies are divided. The flag and pole method, as shown in  Figure 2-51,  shows  the relative positions of the rotor blades. The blade tips are  marked with chalk or a grease pencil. Each blade tip should  be marked with a different color so that it is easy to determine  the relationship of the other tips of the rotor blades to each  other. This method can be used on all types of helicopters  that do not have jet propulsion at the blade tips. Refer to  the applicable maintenance manual for specific procedures. Blade tracking is the process of determining the positions  of the tips of the rotor blade relative to each other while the  rotor head is turning, and of determining the corrections  necessary to hold these positions within certain tolerances.  The blades should all track one another as closely as possible.  The purpose of blade tracking is to bring the tips of all blades  into the same tip path throughout their entire cycle of rotation.  Various methods of blade tracking are explained below. High frequency vibrations can be caused by anything in the  helicopter that rotates or vibrates at extremely high speeds.  A high frequency vibration typically occurs when the tail  rotor gears, tail drive shaft or the tail rotor engine, fan or  shaft assembly vibrates or rotates at an equal or greater speed  than the tail rotor. Medium frequency vibration (4/rev and 6/rev) is another  vibration inherent in most rotors. An increase in the level of  these vibrations is caused by a change in the capability of the  fuselage to absorb vibration, or a loose airframe component,  such as the skids, vibrating at that frequency.  Low frequency vibrations (1/rev and 2/rev) are caused by the  rotor itself. 1/rev vibrations are of two basic types: vertical  or lateral. A 1/rev is caused simply by one blade developing  more lift at a given point than the other blade develops at  the same point.  Extreme low frequency vibration is pretty well limited to pylon  rock. Pylon rocking (two to three cycles per second) is inherent  with the rotor, mast, and transmission system. To keep the  vibration from reaching noticeable levels, transmission mount  dampening is incorporated to absorb the rocking.  Extreme Low Frequency Vibration Offset Flapping Hinge Low Frequency Vibration Medium Frequency Vibration Stability Augmentation Systems (SAS) High Frequency Vibration Helicopter Vibration Rotor Blade Tracking Flag and Pole  Helicopter Vibration Types  Frequency Level Extreme low frequency Low frequency Medium frequency High frequency Less than 1/rev PYLON ROCK 1/rev or 2/rev type vibration Generally 4, 5, or 6/rev Tail rotor speed or faster Vibration Figure 2-50.  Various helicopter vibration types. 2-31