Helicopters are characterized by a high number of rotating parts. These include the main rotor, the tail rotor, the engines, the transmission shafts, etc. Other rotating and non-rotating parts such as pumps, bearings, dampers, and the landing gear system can also induce vibrations. These are not always easy to identify and correct. Historically there have even been cases of cells that after being put into service had to be discarded because of a tendency to give rise to high frequency vibrations that could not be corrected. Regardless of origins, a high level of vibration is a main obstacle to comfort and therefore also to the commercial placement of a helicopter. Since vibrations are in most cases related to the action of the rotors, the dynamic balancing of rotors and shaft is a very critical helicopter maintenance operation.
“Nowadays, the technology allows the production of main parts such as blades with an equivalent degree of elasticity and of response to external stresses, thus guaranteeing a good starting point at the production level with regard to the overall comfort of the helicopter,” says Alidaunia Vice President and Flight Operations Manager Vincenzo Pucillo. It should also be noted that taking care of vibrations is necessary to avoid the occurrence of structural cracks. Vibrations also have negative effects on the electronic components causing frequent electronic failures.
Also, it is important to keep vibration levels as low as possible to reduce the rate of wear. “The wear rate will affect not only the lifespan of the rotors and shaft, but also the fatigue life of stationary components and airframe that vibrate in sympathy with the rotating components. Wear factors do not follow a proportional wear rate, but work in multiples. For example, if the wear factor on a certain bearing was one thousandth of an inch in 500 hours, it may take no more than 50 hours to accomplish the next one thousandth of an inch wear,” says Norman Serrano of International Vibration Technology (IVT). “If excessive vibration is allowed to continue, each extra one thousandth of an inch movement that is allowed will occur at a shorter interval. Since each component is attached to another, on the same platform, the wear factor of the adjoining component also increases. Such problems can lead to high maintenance cost in replacement parts and labor, which is often the difference of operating at a profit or a loss.”
For more than half a century, experts have struggled to find the most efficient and effective method for providing optimized, yet simple, helicopter track-and-balance solutions. This is firstly because each helicopter is different, from their manufactured elastomeric or composite rotor parts to their rotating mechanical tolerances, meaning that abnormal vibration is custom to a particular helicopter. The best practice or balancing method must meet two main criteria: It must adapt to each individual helicopter by learning the individual characteristics of each specific tail number and it must adapt to changes to each individual helicopter over time. “As rotor parts wear their behavior gradually changes, affecting the characteristics of vibration. Any balancing method must account for these changes in its solution or it will not be effective,” says Serrano.
The methods available today on the market to identify, analyze and correct the level of vibrations originate from companies, which developed specific software adaptable to any type of helicopter and/or rotating part. All such solutions allow good results both on new production helicopters and on helicopters already in service. “The systems can be broadly categorized as manual, semi-automatic or automatic, depending on their ability to provide indications, and their ability to process data to eventually indicate the corrective actions to be taken,” says Pucillo, who explains that basic equipment, which is still current at the introductory level, is the Chadwick 177/Strobex. In the past maintenance technicians have used it for the analysis and correction of vibrations. This instrument has the merit of having forced operators to deepen technical concepts and to learn how to face and correct almost any type of vibration. The successors of this basic system, regardless of the manufacturer, allow faster analyses that are often put into practice by operators according to the indications of the vibration identification systems. Applicable corrections include the addition or removal of weights or the variation of the length of the step control rods, which result in the variation of the aerodynamic load of the blades. Automatic systems are also used by manufacturers since with a few flights you get good results, thus saving on production times and costs.
“Imbalances in certain parts of the main rotor system have greater vibration impact than other out-of-balance parts. For example, a pitch link adjustment dynamically affects the blade differently compared to the trim tab adjustment. Even though you may experience a nonlinear balance response from an adjustment, you still have a linear relationship between adjustments and balance”, says Serrano. “Empirical evidence from the prioritized adaptive direct adjustment method that our Vibration Intelligent Balance Solution (VIBS™) technology verifies this to be true. This is because one of the main features of VIBS™ is the prioritized adaptive adjustment algorithm. In other words, we balance things in a particular sequence because that is effective. The VIBS™ learning algorithm is also simple, direct, and deterministic. In short, VIBS™ knows what to balance first. Moreover, its being deterministic means that it does not have to rely on learning from guesses. Empirical data shows that the first adjustment suggested by the VIBS™ method usually results in a dramatic vibration reduction. In addition, subsequent adjustments converge quickly and result in superior balancing results in just a few iterations.”
Some helicopter types currently install permanent helicopter usage monitoring systems (HUMS), which allow the vibration of the rotors and of major components such as engines and transmission boxes and their subcomponents to be read at any time of the flight. “HUMS allows to monitor every potentially critical component and provides the advantage of preventing potential mechanical failures and to anticipate the corrective actions with the great gain in terms of flight safety,” says Pucillo.
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 at 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 with car rims.,” says Pucillo. “The 1 Rev horizontal vibrations are much more evident on the ground than during the flight. Another type of horizontal vibration is that 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.”
The vertical vibrations develop vertically in relation to the rotating plane of the rotor. “These are generally low frequency vibrations induced by poor levelling (tracking) of the blades on the ground and in the air. While on the ground and in hovering the levelling is obtained with the adjustment of the step control rods, in translated flight the levelling is obtained by acting on the compensating flaps, which allow a targeted correction leaving unchanged the condition of the hovering. Sometimes forcing a blade to rotate in a determined plan can produce more negative than positive results. It is therefore better to have poor levelling with less vibrations rather than good levelling that causes higher vibrations”, says Pucillo. “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 the general improvement of 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.
Good maintenance practices are complemented by training for technicians so that 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 correct skill to operate the equipment in use 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, of the methods for their correction and good dexterity in interpreting the graphics generated by the vibration identification equipment used. The rest is only the result of direct experience gained on the helicopter,” says Pucillo.
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 to their specific training needs. The class will then track and balance an actual helicopter, if available, on the final day of the training course.