APPLYING CORRECTIONS


of horizontal vibration is due to a possible malfunction of one or more dampers acting on the swing (lead lag) of the blades during their rotation. The correction of this vibration involves a maintenance action on the damper identified as faulty.”


Actual application results of the VIBS method at work on HH-60 vibration analysis runs.


The corrections that can be made to the systems that induce vibrations depend on the type of vibration (whether it is vertical or horizontal) and the frequency at which the vibration manifests itself. Basically the vibrations are minimized with the addition or removal of weights and/or by acting on the step-change rods. “The ‘horizontal’ ones, which develop in the rotating plane of the rotor, are generally low frequency vibrations (1 rev). They develop the peak of the vibration at each complete revolution of the rotor and are easy to identify. These vibrations are corrected by the addition or removal of weights, pretty much like car rims.,” Pucillo says. “The 1 rev horizontal vibrations are much more evident on the ground than during the flight. Another type


TRAINING


Good maintenance practices are complemented by training for technicians, so they become more proficient


in identifying and rectifying


vibrations. “The training courses are often delivered by the companies that develop the software systems. Often these trainings are useful to acquire the skill to operate equipment, but do not fully clarify the concepts of how to minimize vibrations. In order to tackle this sensitive task, one must have a good theoretical knowledge of the causes of vibrations and the methods for their correction, as well as skill at interpreting the graphics generated by the vibration identification equipment. The rest is only the result of direct experience gained on the helicopter,” Pucillo says.


80 July/Aug 2018


Serrano says IVT offers different levels of courses in basic principles of dynamic balancing theory and vibration troubleshooting. The training course can be conducted at the technicians’ maintenance facility and tailored to accommodate their specific training needs. The class will then track and balance an actual helicopter, if available, on the final day of the training course.


The vertical vibrations develop vertically in relation to the rotating plane of the rotor. “These are generally low frequency vibrations induced by poor leveling (tracking) of the blades on the ground and in the air. While on the ground and hovering, the leveling is obtained with the adjustment of the step control rods. In flight, the leveling is obtained by acting on the compensating flaps, which allow a targeted correction and leave the condition of the hovering unchanged. Sometimes forcing a blade to rotate in a determined plan can produce more negative than positive results. It is therefore better to have poor leveling with less vibrations, rather than good leveling that causes higher vibrations,” Pucillo explains. “Higher frequency vibrations – e.g. type 4 rev, 5 rev, etc. — are corrected by adding masses, which essentially develop vibrations in contrast to those of the helicopter. These masses are designed by the manufacturers, and operators are left only with the task of performing small adjustments resulting from the operation of the helicopter.” The addition of masses for improved comfort does not exempt operators from those preventive checks on mechanical connections, flight controls, landing gear, emergency floats, and external loads that affect aerodynamics and inevitably induce vibrations.


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