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With periodic, depending on the position of the helicopter blades, blade angle of attack change helicopter from above, with the rotor blades rotating breast actives are attacking the right of the longitudinal axis of the fuselage of the helicopter. Then the right part of the rotor produces more thrust than the left part, which is the source of the helicopter tilt to the left with t slope of the rotor helicopters can move freely in the directions of forward-backward and right-left progressive movement, ie, without executing the marketing of the main rotor axis The corresponding stress variables are called stress or stress fatigue. The course of time-varying loads is known as phantom load. May occur irregularly, randomly or in a manner determined. When the load segments are repeated, which is characteristic of periodically varying load, which is called a cyclic load. Sinusoidal stress cycle variables is described by the parameters: maximum stress σmax cycle, stress σmin minimum cycle, a period of change T or its inverse: the frequency of f. Pilot performs this maneuver through the appropriate joystick deflection.
Deflection of the joystick to the left to increase the angle of attack on the advancing blades and reduce the return on the blades. Looking at the he advent of the resultant force facing horizontally to the left, corresponding to the horizontal movement of the helicopter in that direction. Similarly, the helicopter behaves when wychylimy stick in any other direction left-right and front-back and all the intermediate directions. During one period there is a complete cycle of changes in load, and analogously to the full cycle of changes in stress. A special case of periodically varying load is sinusoidally variable load. This load was adopted as a basis for determining the fatigue properties of materials and structural elements.