Method and apparatus to determine need for rotor blade pitch adjustment and/or blade substitution

In the method of testing a bladed rotor having multiple blades defining an axis of rotation, said rotor comprising a helicopter lift rotor and wherein helicopter structure associated with the rotor is subject to vibratory motion due to an out-of-track condition of the rotor blades, the method employing vibration pick-up means, the steps that include shafts

• water mounting the pick-up means on the helicopter structure with longitudinal spacing from the lift rotor,
• b. rotating the rotor and operating the pick-up means to produce first vibratory signals corresponding to vibratory motion of the structure generally parallel to said axis, said first signals having an associated amplitude and clock angle about said axis,
• c. adjusting the effective pitch of at least one of said blades in an effort to reduce the amplitude of said vibratory signals and maintaining the adjustment constant as the rotor rotates, and
• d. repeating at least said (b) step to produce second vibratory signals corresponding to vibratory motion of the structure generally parallel to said axis, said second signals having an associated amplitude and clock angle about said axis, and
• e. determining from said first and second signals amplitudes and from said clock angles a minimum or near minimum value for said vibratory signal amplitude, and which is less than each of said first and second signal amplitudes, and
• f. substituting a new blade or blades for at least one of said rotor blades if said minimum or near minimum value is unacceptably large, thereby to reduce said out-of-track condition of the rotor blades.