Product Tip of the Month

Product Tip of the Month

July 2001

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Sensor Orientation When Balancing Rotors

The majority of sensors used in the aviation world for balancing rotating components are of the piezoelectric type. These sensors are single-axis sensors. They only measure vibration in one direction. The connector end of the sensor is always the reference point. With this in mind, especially when using paper polar charts to manually plot solutions, it is imperative to adhere to the correct orientation of the sensor during the installation process.

After an OEM (Original Equipment Manufacturer) such as Bell Helicopter builds a paper polar chart for balancing a rotor, they put installation instructions for where to place the sensor and the direction that it must point are contained in the maintenance manual. The paper polar was designed with this installation as the standard. Since the sensors are single axis, any deviation from the installation or orientation of the balancing equipment will effect the balancing procedure and cause a deviation from the know response of the rotor. For instance, the maintenance manual may state to mount a sensor at the 9:00 position with the connector facing up. The equipment is installed on the aircraft correctly and on the first run an amplitude of .5 at 6:45 is observed. If the sensor was rotated so that the connector is facing down and the readings are taken again, the result would be a vibration reading of .5 at 12:45.

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Understand that once polar charts are designed, the installation locations and orientation of sensors also will aid in troubleshooting an aircraft. For example, let’s say we ran the aircraft and had a phase and amplitude reading of .5 at 6:45. The amplitude and phase are manually plotted on the polar chart and the solution is to add 10 grams to the target blade. The solution is implemented and the resulting phase and amplitude is now .9 at 6:30. The vibration increased instead of decreased. Only two errors could have occurred. The sensor was installed with the connector facing the wrong direction or the 10 grams was added to the wrong blade.

Understanding the importance of sensor installation and orientation as well as manually plotting solution results, can prevent excessive engine cycles and save man hours in the troubleshooting of problematic rotor systems.

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