Increasing the Fatigue Life of a Structural Web with a Large Rectangular Cutout Under Complex Loading

This paper details an analysis methodology for a primary structure component on a tandem rotor helicopter that has been shown to experience fatigue damage in operation. The primary structure component is a web in the aft pylon of the helicopter. The web carries a complex set of loads in flight with various forces and moments applied along its boundaries and rotor torque reacted around a large rectangular cutout in the web. Due to the complex loading applied to the web, there is no clear location that can be considered to carry a "gross" stress, rendering traditional hand calculation methods (such as the use of stress concentration factors applied to a gross stress) impractical. The analysis detailed in this paper considers the application of flight loads to Finite Element Models to determine stresses in the web, which are used to evaluate fatigue life based on various flight conditions.