Rotor track and balance
By John Sharski Making a tab adjustment to correct for a vertical imbalance
A helicopter is a complex assembly of rotating components that allows flight characteristics unavailable to fixed wing aircraft. In these aircraft, excessive vibration levels can lead to premature wear and failures in rotating components Reducing vibration levels in the airframe to a minimum is an essential component in extending the longevity and safety of the helicopter. Rotor track and balance is the process of smoothing vibrations in the airframe, which are caused by rotating components. 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.
Types of vibration
Before you can balance the rotor, you must determine what type of vibration you have. A helicopter main rotor is capable of producing vibrations in both the vertical and lateral planes.
Vertical vibration, sometimes referred to as an aerodynamic imbalance, is a result of unequal lift produced by the main rotor blades. This 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.
Lateral vibration is the result of an unequal distribution of mass, or mass imbalance in the main rotor. This imbalance is a heavy spot on the rotor system that can be felt during rotation. The greater the mass imbalance or the farther the mass imbalance is from the center of rotation, the greater the severity.
A lateral vibration may also be felt when an aircraft is out of "track," or has vertical imbalance. This lateral vibration is a result of the airframe rolling with the mass effect caused by unequal blade lift. "Tracking" of the main rotor blades 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 slight "track split." The desired end result of the track and balance job should be the smoothest possible ride.
Prior to acquiring data you must install vibration sensors, tachometer signal sources, tracking devices, and associated connecting cables and mounts. The sensor types, installation locations, and material for these components have been performance optimized by manufacturer testing and are specified in the helicopter's maintenance manual. There are numerous sensors available. Those most commonly found in aviation are the acceleration and velocity sensors. The manufacturer of the equipment you are using will dictate what type of sensor is required. Generally, a vibration sensor used to perform balancing needs to be located on the support structure as close to the rotating component as possible.
One of the most common locations used to mount a lateral sensor is the upper portion of the main transmission, on the swashplate support. The connector of the sensor is positioned perpendicular to the left or right of the ship's centerline. The vertical vibration sensor is normally mounted as far forward in the cockpit as possible, with the connector pointed up or down depending on the OEM's directions. This position allows the highest sensitivity for measuring the symmetry of lifting forces developed by the main rotor as the blades pass over the nose of the aircraft. The once-per-revolution (one-per-rev) source is typically a magnetic pickup mounted on the stationary swashplate. A ferrous metal interrupter passing close to the magnetic pickup causes an electrical pulse, triggering the tach event.
A photo optical device referred to as a Phototach can also produce the one-per-rev signal. The Phototach is normally located where a beam of light can be projected onto a small piece of reflective tape attached to the mast or other rotating component of the main rotor system. The light is then reflected back to an optical receiver in the Phototach lens, which triggers the electrical pulse to the analysis equipment.
If utilizing a strobe light, it's necessary to install targets at the tips of the blades. Additionally, the strobe requires a DC power supply for operation. This power is usually tapped from the ship's power source because it cannot be supplied by portable equipment. Airframe mounted optical trackers require the use of a mounting bracket for a solid, stable mounted position.
Lastly, there are hand-held optical trackers. This equipment requires no installation. The tracker is operated in the cabin with the vibration analysis equipment. It requires no tip targets, and operates from the analyzer's integrated power source. Chordwise adjustments can be hub weight or sweep as shown on this Bell 206B
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