Fan Trim Balance
The first indication of an out-of-balance fan in a business jet is, more often than not, a complaint from the guy sitting in the plush, white leather passenger seat. Although an engine meets or exceeds the standards of vibration and noise levels off the production line, the owner expects it to be as quiet as the one in his high-rise office. Don't be surprised by noise complaints even though the aircraft is brand spanking new.
During the balancing procedure, you may find that it still meets the manufacturer's vibration standards, but the boss wants it even quieter. With a little tender loving care, it can be quieted. Other indications may be in the aircraft's Engine Vibration Monitoring (EVM) system, or vibrations felt or seen in the flight controls, floor, instrument panel, etc. Engine manufacturers often dictate specific intervals for fan trim balancing, while others may tie it to an event such as overhaul or inspection cycles. As a rule of thumb, I would recommend at least every 400 hours — more frequently if you have a CEO with sensitive ears.
When an engine manufacturer assembles an engine, each disk of the compressor and turbine section is individually balanced. First, the disk is assembled using an optimizing technique to place each blade in the best possible position. A static balance is either part of the optimizing process or may be accomplished after the disk is assembled. The disk is then placed in a dynamic balancer and spun up to a balancing speed. After each disk is dynamically balanced, it is assembled with each of the other stages to form the compressor and turbine sections of an engine.
The fan is actually the first stage of compression as well as the major thrust producer. Because the fan assembly constitutes the largest rotating mass in the engine, an out of balance condition will most likely result in noise and vibration. Out of balance conditions can be caused by a variety of situations. Remember that the original assembly optimized the placement of each blade. If foreign object damage necessitates the change of a blade, the balance condition may change drastically. Blades are normally, but not always, changed in matched pairs. The damaged blade is removed along with its symmetrically opposing blade and the two are replaced with the matched pair. Blade erosion, FOD damage and repair, spinner rotation, and normal wear can also change the condition of the balance. In any of these cases, the balance of the fan will change. To top off the problem, the fan rotates at speeds that resonate in the cabin. I often compare it to attaching an engine to a tin can, then crawling inside the can for a listen.
Ease of use, size, durability, and price vary from one extreme to another when choosing the analyzer or balancer. New digital designs are more efficient and less expensive than older analog technology based instruments. Ease of use for the technician should be paramount. Make a list of what you want in a balancer. Shop around, ask for demonstrations, try them out for yourself, compare features, and make a decision based on what you need to fill your requirements.
For most fan trim balancing, a velocity sensor is the best choice. Its response range is well within the frequency bandwidth of most fans. Your application may call for a specific sensor capable of collecting higher frequencies for vibration surveys as well as fan trim balancing. In this case, the recommended sensor type for higher frequency requirements is normally an accelerometer. Some accelerometers, especially those designed for use in high temperature areas, may require an external charge unit.
A displacement sensor is designed for the lower frequency range where few jet engines operate. This type of sensor may be used in some rare cases to balance the fan, but almost never for vibration analysis. Refer to the maintenance manual if in doubt as to your specific needs. Manufacturers go to great lengths to match a specific sensor to their engine vibration analysis and balancing requirements. Take their advice first.
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