Downloaded from https://www.everyspec.com ADS-79D-HDBK APPENDIX F ROTOR TRACK AND BALANCE F.1 SCOPE The purpose of this Appendix is to provide methodology and guidance for the use of on-board information from the DSC to aid in the application of rotor smoothing processes. F.2 REFERENCES AND APPLICABLE DOCUMENTS F.3 DEFINITIONS F.4 INTRODUCTION The primary purpose of rotor smoothing is to reduce crew fatigue and wear and tear on the airframe and subcomponents.
The vibration of interest is the rotor once per revolution (1P) vibration, which is caused by dissimilarities in the rotor blades such as subtle differences in airfoil contour, span moment, blade twist, stiffness distribution, and chord balance. Aircraft are equipped with pitch change links, trim tabs, blade wedges, balance weights, and blade sweep devices to reduce these 1P vibrations. F.5 GENERAL GUIDANCE In order to perform rotor smoothing, the aircraft is instrumented with devices to measure the blade height, vibration, and rotor position over multiple rotations of each shaft or rotor system of interest.
Blade height is typically measured by blade trackers placed to view the rotor blades in the advancing direction, measuring the distance of each passing blade from the tracker itself. Vibration is typically measured by accelerometers or velocimeters placed in the cabin or near each shaft of the rotor system of interest, measuring the amplitude of the vibration signal. Rotor position is typically measured by a tachometer placed on each shaft or rotor of interest, measuring the phase of the vibration signal relative to a known position. This known position is typically referred to as the master position.
F.6 TECHNICAL GUIDANCE The tachometer signal is critical in that it is used to process the time history of both the vibration and blade height measurements into an actionable set of information. Using the master position, individual rotations of the rotor or shaft of interest are identified in the tachometer signal. The timing of these individual rotations is then used to average together the multiple rotations of the vibration signal and blade track measurement. This averaging process reduces the contribution of non synchronous noise sources, such as turbulence, changes in control position or drive train vibration.
The result is an averaged blade track height for each individual blade and a vibration signal synchronous with the rotor or shaft of interest. The blade track height is typically expressed in terms of the total track split, which is simply the maximum separation between blades, and referenced to the rotor blade hub. The vibration is typically expressed in terms of the 1P vibration magnitude in units of inches per second and the vibration phase angle relative to the master position. Higher harmonics of the rotor can be calculated from the averaged signal but are not typically considered for rotor smoothing operations. The vibrations are typically measured in and out of plane of the rotor.
In-plane vibrations are primarily caused by a difference of rotor blade mass, but other factors include span moments, 108