When to normalize rotor settings
If a flight crew or customer complains of a slight vertical or lateral vibration, there's no need to strip the rotor of weight or zero tabs. It's likely that the rotor requires a little tweaking to bring it back below the maximum allowable vibration levels. Rotor systems can even be sensitive to the climate and environment. Aircraft that have elastomeric components in the rotor system, at times, require re-balancing due to changes in outside temperature or seasonal changes.
However, if a new set of blades is installed, all adjustments should be nominalized prior to tracking and balancing. Nominalizing the settings will increase the chances of successfully balancing the rotor in as few flights as possible. It also will help prevent reaching the maximum allowable adjustment for weight, tab, sweep, and pitch change links.
The "weight" for weight adjustments for span weight is usually lead shot A phototach can be used to obtain a once-per-rev tach pulse A magnetic pickup can also be used to obtain the once-per-rev tach pulse
Tips and hints about track and balance
Rotor smoothness should be achievable in all flight regimes. If this isn't achievable, an influencing force present in the airframe may be causing the problem. As aircraft get older, parts begin to wear resulting in higher vibration levels. When it isn't possible to smooth the aircraft through all flight conditions, you may be forced to compromise and sacrifice the smoothness of one over another. In this case, the fight condition where your aircraft spends the most time, or the most critical condition, should take precedence.
You may notice certain flags during the rotor smoothing process that will certainly save you the time and frustration of attempting to correct for a mechanical problem present in the rotor system. For every adjustment on the rotor system, there is an associated influence. For example, say we have a tail rotor head that should require approximately 30 grams to correct for an imbalance of 1.00 IPS. If in the course of a balance job, you make a weight addition of 15 grams and the vibration level changes by 0.9 IPS. The "flag" in this case would be the change in vibration of 0.9 IPS. This much change would normally require the addition of 28 grams of weight. In addition, the actual moveline may not be in the desired direction. This could indicate that other factors are involved which may not be correctable by normal procedures. An aircraft which is not mechanically sound, will be extremely difficult, if not impossible, to balance.
Finally, don't forget the human factor. If everything seems correct but you still can't achieve acceptable results, you may have made a misadjustment. First, verify that the equipment has been installed correctly. Sensors are single axis, if a sensor is pointed in the wrong direction, the result of an adjustment will be opposite the desired outcome. Secondly, it's easy to return the aircraft to its initial configuration and start over. When returned to the original configuration, if values are not similar to the original readings, a mechanical problem may exist.
Special thanks to the maintenance team at Vertiflite in Maryville, TN, for the use of their facilities and aircraft for the photo shoot for the article.
About the author
John Sharski has six years' commercial aviation experience and 10 years' experience with the U.S. Air Force. During his tenure with the USAF, John spent four years at the USAF Helicopter Training Facility as a Master Instructor, and served as a subject matter expert in the ground-up design and implementation of H-53 and UH-1N training courses. John has spent the last six years working in commercial aviation as an applications engineer. He currently fills this position for ACES Systems/TEC Aviation Division in Knoxville, TN.
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