Helicopter Track and Balance Theory
've heard it said on many occasions that a helicopter is several thousand rotating components trying desperately to shake themselves free of one another. In my experience, this is often a true statement, but a correctable one. A helicopter is a complex collection of rotating assemblies that allow flight characteristics unavailable to fixed wing aircraft.
Premature wear and failures in rotating helicopter components can be attributed to excess vibration levels. Reducing these vibration levels in the airframe to a minimum is absolutely essential in order to ensure the safety and longevity of the helicopter.
Rotor track and balance is the process of smoothing vibrations in the airframe, which are caused by the main rotor. The main rotor is not the only rotating assembly of concern in a helicopter; there are others such as the tail rotor assembly, drive shaft assemblies, and oil cooler fans. However, this article will address only the main rotor.
First, it is important to realize exactly what type of vibrations you are trying to reduce or eliminate. A helicopter main rotor is capable of producing vibrations in both the vertical and lateral planes.
A vertical vibration is a result of unequal lift produced by the main rotor blades. This unequal lift can be a result of blade chord profile variances from one blade to the next or improper adjustment of pitch change links and trim tabs. These constitute the most common causes of vertical vibration.
A lateral vibration is the result of an unequal distribution of mass in the main rotor "disk." This unequal distribution can be a result of the manufacturing process, which allows blade or component weight differences. Poor assembly techniques, improper alignment of a main rotor trunion, erosion, and a host of other possibilities also contribute to the situation. A lateral vibration may also be felt as a result of an aircraft that is out of "track," or vertical balance. This vibration is a result of the airframe rolling with the mass effect caused by the unequal vertical lift component.
The term "rotor track and balance" is somewhat misleading, in that "track" or "tracking" refers to adjusting the blade tip paths to make them fly in the same rotational plane. This does not always result in the smoothest ride. Some airframe and blade combinations will ride smoother with a "track split." The desired end result of the track and balance job should be the smoothest possible ride.
It is important to note that balancing is performed in the primary rotational frequency of the main rotor. There are other main rotor vibrations present, such as the blade pass frequency of the main rotor. This is referred to as the n-per-rev (n = number of blades) frequency of the main rotor. If the mechanical condition of the helicopter is suspect, these vibrations can be quite noticeable once the main rotor 1-per-rev vibrations are reduced.
As I will note throughout this article, all airframe types are not the same, and this discussion is not intended to educate the reader on the intricacies of a specific airframe. The applications discussed will be general in scope and will focus more on the general principles of helicopter rotor balancing.
A Little History
The first method employed by helicopter manufacturers and maintenance personnel to accomplish a rotor track and balance was limited to the use of static balance equipment and tracking flags. A static balance device utilizes a "bubble" type level and balance arbor assembly suspended from a fixture to adjust the main rotor span- and chord-wise mass distribution. A tracking flag is a long lightweight pole held vertically, with two horizontal arms extending from it. Multiple strands of tape were attached between the horizontal arms, making a ribbon-like connection from one to the other. The individual main rotor blade tips were coated with different colored grease pencil or chalk. With the helicopter running on the ground, the flag was moved in toward the rotor disk. As the individually colored blade tips made contact with the tape, each left a mark corresponding to its assigned color. If the marks were vertically separated, a pitch change adjustment was needed to move the blades tips closer together. If the marks overlapped one another, no adjustment was needed. The drawbacks to this method are obvious. It was dangerous, restricted to ground only, and did not allow for track measurements in flight. The use of static rotor balancing devices is not applicable to some aircraft.
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