The next solution to the tracking dilemma was to attach tip targets to the main rotor blades and visually "freeze" their flight path by use of a strobe light. This measurement could be performed for all flight speeds of interest, and is still in use today. Along with this new technology, the use of vibration sensors mounted to the airframe at specific locations was introduced. This facilitated the measurement and recording of the various vibration amplitudes in both the vertical and lateral planes. This amplitude (expressed in inches per second - IPS), combined with the phase angle (or clock angle) of the vibration, allowed the technician to manually plot corrections on a paper polar chart. The polar chart was for specific airframe use. When maintaining multiple airframe models or sub-models, each required the use of a chart relative to the specific model.
The next wave to arrive on the market saw the introduction of microprocessor-based analyzers that were capable of performing all of the balance calculations for the mechanic. Software programs developed for a specific airframe application drive these products. Along with these advances came various optical methods of acquiring track data. This allowed the user to collect track data without having to attach tip targets to the blade tips or visually interpret the position of the main rotor blades at a distance. The main drawback to the later systems is the fact that influence coefficients used in the software programs are not the same from one aircraft to the next of the same make and model.
There are many tools critical to the success of a main rotor track and balance job, the most important of which is reliable vibration analysis equipment. In today's high-technology maintenance facility, digital technology has largely replaced older analog equipment. Repair costs for aging analog analyzers is at an all-time high. A repair and recalibration can often reach half the original purchase price of the equipment. Lead-time for repairs and recalibrations can be weeks rather than days. To put it simply, new digital data acquisition systems are more economical, faster, and much more accurate. The leading digital equipment eliminates the need for strobe lights and charts, and even stores the complete job measurements for later review and printing.
Prior to acquiring data, you must first install the various 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 types of vibration sensors available for use today. Those most commonly found in aviation applications are the accelerometer and velometer. The manufacture of the vibration equipment you are using will dictate the type of sensor required. Some systems are designed to use only one type of sensor, which limits their range of application. Modern, state-of-the-art vibration and balancing equipment allows you to configure the analysis equipment to provide power to the sensor, convert the engineering units of measure if necessary, and automatically set the amplitude scale necessary for best viewing of the sensor's output on the vibration equipment.
Generally, a vibration sensor used to perform balancing needs to be located on the supporting structure as close to the rotating component as possible. The most common location used to mount a lateral vibration sensor is the upper portion of the main transmission, where the swashplate guide base is mounted. 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 cabin as possible, with the connector pointed up or down, depending on the airframe. This position allows the highest sensitivity for measuring the symmetry of lift forces developed by the main rotor as blades pass over the nose of the aircraft.
The once-per-revolution (one-per-rev) source is typically a magnetic pickup mounted on the non-rotating ring of the swashplate. A ferrous metal interrupter passing in close proximity to the magnetic core produces an electrical pulse, which triggers the pickup. A photo-optical device referred to as a phototach can also produce the one-per-rev signal. The Phototach is normally located in a position that allows a beam of light to be radiated onto a small piece of reflective tape on 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.
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