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White Paper: Transient Balance

smooth flight

Revolutionary Trim Balance Process Saves Time, Yields Better Results


The technique of Transient Balance saves money by reducing the overall duration of the fan balance process.  Just recently available to the flight line incorporated in a battery-operated analyzer, the method reduces technician time, engine run time, and customer returns and complaints for recurring noise problems.

The industry has long recognized the benefits of reducing engine vibration on aircraft such as reduced cabin noise, increased passenger comfort, and smoother ride.  This has led to fan trim balance being a common maintenance practice in the test cell and on-wing.  Over time, automation of the process has made the job simpler for the A&P mechanic, but it can still be a time-intensive task, requiring dwells at specific speed points.  A technique called Transient Balance is an improved method to automate fan balancing by monitoring vibration at all engine speeds during a balance run.  This automation removes the need to manually select and dwell at specific speed points.   The maintenance technician is no longer required to refer back to a vibration survey to locate changing vibration levels since the entire range of speeds is monitored during the balance job.  Transient Balance also prevents the embarrassing occurrence of releasing an engine which passes vibration limits at one speed but exceeds them at others.


Early fan balance jobs were typically done at a single steady state engine speed.  Fans had more narrow chords relative to the fan diameter than they do today, and the weight was placed on the fan disk.  Over the years, the fan balancing process was simplified moving the weight placement forward and adding washers to the spinner or using different length spinner
attachment bolts.   This simplified the process of fan balancing, but also moved the balance weight forward of the fan. Additionally, to improve efficiencies the engine manufacturers have adopted wide chord fan blades.   That is, the blades have more depth when looking into the engine and are more three dimensional than before.   This complicates the process of fan balance as the dynamics of the fan and the engine have become much more complicated.   This manifests itself on some fans when the fan is balanced at selected speeds and then fails at other speeds.   Multi speed balancing can address this IF you know where the new imbalance will pop up.   The challenge is you may not know this until you start the balance job.  These balances reduce the vibration at a particular speed but leave other speeds uncorrected.

transient balance

As balancing evolved, manufacturers began to recommend balancing at several different speeds. This complicated the process by requiring the user to perform a vibration survey, review the results of that survey and select the engine speeds where the vibrations looked highest. This was a largely manual process that took time and didn’t address the shifting vibration response of the typical engine. On subsequent runs, it was possible that the original speed selections no longer contained the largest vibration peaks – those peaks may have moved to new engine speeds. This caused the technician to either run a new vibration survey to locate the new peaks and continue to try to manually reduce the vibration across shifting peaks throughout a wide engine speed range, or release an engine which still had annoying noise signatures within the operational range of the engine.


Today, using the Transient Balance technique incorporated in the ACES Systems VIPER II, the technician only has to accelerate the engine across its entire operating range and follow the analyzer’s recommended solutions. These solutions will automatically monitor the changing conditions in the engine and eliminate the highest vibration peaks found on every run. The analyzer will search and eliminate multiple peaks in up to 300 “bins” during each balance run.

With the Transient Balance technique, the balance data is collected during a slow sweep over the speed range of interest, taking between one and three minutes.
The actual data collection time is reduced relative to the multi-speed balance module.   When you elect to collect data at multiple discrete speed points, it is critical that each balance run return to the same speed point.   This requires the technician to slowly adjust speeds to match the desired speed points taking time and effort.   Once the technician is at the speed point, data is typically collected for about 30 seconds before moving onto the next speed point.   This translates to approximately one minute of fuel consumption per speed point measured.   Some larger engines require a multi-speed balance job that uses nine speeds translating to approximately nine minutes of fuel consumption per balance run.   Assuming this complicated job is completed with three runs, 27 minutes of fuel have been burned collecting data.   With the Transient Balance technique, the balance data is collected during a slow sweep over the speed range of interest, taking between one and three minutes.   For the same three-run balance job, the data collection time is only nine minutes, worst case.   This is a 67% reduction in data acquisition time.  For simple balance jobs with a single speed, there is no time savings, but because the balance is addressed over all speeds, there is no fear that another vibration peak showed up during the trim adjustments. .


Transient Balance saves time by reducing technician time in searching and identifying vibration peaks, reducing the amount of run time by eliminating long duration stabilization at individual speed settings, reducing unnecessary reduction of non-problem engine speeds (focusing only on the problem areas), and reducing the need for a separate vibration survey to display the final fan vibration peaks.  With the ACES Viper II, a survey is produced with each run and displayed in a final report generated by the analyzer that can be exported directly to PDF.

Written By: John Wilson
Executive Vice President of Engineering
Technology for Energy Corp.

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