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A technique is described whereby helicopter rotor blade service life can be substantiated by combining laboratory fatigue tests with cyclic loading applied during conventional ground endurance tests of rotor control and power-transmission systems. The method for applying cyclic loading involves the use of excitation panels, which consist of an array of stationary panels properly spaced and located close beneath the rotor to excite a natural flapwise bending mode. During the investigation, blade cyclic moments were excited that were ten per cent higher than the maximum cyclic bending moment measured in flight on the same rotor blade. The level of fatigue loading could be controlled by adjusting the vertical distance between the panels and rotor. The more rapid application of load (third-mode excitation at nine per revolution instead of the conventional one-per-revolution occurrence of peak inflight loads), coupled with sustained application of damage (100 per cent of the time for panels instead of approximately 16 per cent of the time for a normal flight spectrum), permits the excitation panels to impose damage approximately 56 times faster than experienced in flight. Tests are now being conducted to determine generalized design rules for application of excitation panels to rotors of any number of blades of any size.
helicopters, rotor blades (helicopter), fatigue (materials), life testing, aircraft
Amer, K. B.
Manager, Hughes Tool Co., Culver City, Calif.
Sullivan, R. J.
Administrative assistant, Hughes Tool Co., Culver City, Calif.
Eakin, J. D.
Research project engineer, Hughes Tool Co., Culver City, Calif.