A Modified Method of Deriving Fatigue Load Spectra from Historical Flight Test Data

Normalized design load spectra permit a recently proposed framework for fair, early, and meaningful assessments of rotorcraft dynamic-component fatigue durability. In prior work, a mission-rainflow method defines fatigue load spectra from historical flight test data. This paper proposes maximum-oscillatory and regime-rainflow alternatives with a goal of decreasing the effort required to generate each load spectrum. A maneuver-to-maneuver technique ensures representation of ground-air-ground cycles in the maximum-oscillatory and regime-rainflow alternatives. Discussion includes comparison to prior methods and an explanation of challenges and benefits associated with each method. To enable comparison to prior methods, tables provide the newly derived mission spectrum load occurrences for main rotor pitch link load and tail rotor flap-wise bending, as well as characterizing parameters using piecewise exponential distributions. Figures demonstrate the similarity of piecewise exponential distribution results between regime-rainflow and mission-rainflow methods, while also suggesting conservatism for the maximum-oscillatory method. Fatigue analysis supports similar conclusions.