This paper sets out the results from work instigated to determine the significance of the phase relationship occurring between the vibration responses arising from helicopter rotor blade passing frequencies. The work was initiated by an inquiry from the UK Ordnance Board as to whether the phase relationship between rotor blade passing harmonics should be included within the test requirements for ordance carried in helicopters. The results of the study indicated that the effects of phase relationship are significant over certain ranges of response frequencies and are sufficient to require consideration when setting test vibration amplitudes. However, the study also indicated that specifying a particular phase relationship for testing purposes is not usually necessary, especially for equipment that may be used at different locations within the helicopter.

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