Vertical vibrations can be caused by several different factors, but result in the rotor blades producing unequal lift at a given point in their rotation. Vibrations can be separated into two different types, vertical and lateral. When the mechanical condition of the helicopter is suspect, these vibrations can be quite noticeable once the main rotor 1-per-rev vibration is reduced. Three-per-rev frequency, a four-bladed system an inherent four-per-rev frequency and a five-bladed system an inherent five-per-rev frequency. A three-bladed rotor system would have an inherent This is expressed as the n-per-rev, where n = the number of blades in the system. Excessive vibration levels can lead to premature wear and failure in components, and this leads to increased maintenance costs and aircraft downtime.īalancing of the main rotor system is accomplished in the helicopter’s main rotational frequency. This vibration will also be felt in other components on the helicopter. If the blades are out of balance, they will cause a vibration or beat frequency that is transmitted into the rotor head, transmission and airframe. Ideally, if all the blades are perfectly identical and have been installed exactly to specifications, then for a given cyclic and collective setting, the rotor blade tips will all “track” through the exact same point in space at a given point in the rotation around the hub. In flight, they also change their pitch angle to generate the helicopter’s direction of flight, speed and lift. I guess a good place to start is to ask the question, “Why do rotor blades need to be tracked and balanced?” Rotor blades travel around a hub at a given speed. Tail rotors also need to be tracked and balanced, but we are not going to address that in this article.
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This is probably not too far from the truth, but the whole concept of rotating parts is where the tracking and balancing of the main rotor blades comes into being. Many perceive a helicopter as a conglomeration of rotating parts that, in order to fly, work in unison to beat the air into submission.
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A helicopter’s flight dynamics are so different from a fixed-wing aircraft’s that we encounter maintenance issues that are totally unique to helicopters. Helicopters, on the other hand, are inherently unstable, and if you put one in straight and level flight and take your hands off the controls, it will undoubtedly try to roll over on its back. That is, if you place the aircraft in straight and level flight and take your hands off of the controls, the aircraft will pretty much stay where it is. Most fixed-wing aircraft are inherently stable. No answers are wrong - they just differ from each other. Asking seven helicopter maintenance professionals for their insight on rotor blade track and balance is like asking seven sportscasters which is the best college football team … you will get seven different answers.