You’ve spent the time tracking and balancing the helicopter as smoothly as possible. You’ve balanced the tail rotor and all available driveshafts. You’ve replaced a few components. You’ve even tightened the loose nut behind the stick. However, a vibration still remains in the aircraft. You’re left scratching your head with no further course of action. How do you continue your troubleshooting and find a resolution?
Most modern track and balance equipment contains a very powerful tool that you can use to improve the quality of information at your disposal. The vibration spectrum survey can bring quite a bit of seemingly unrelated data into an organized format that you can use as a valuable troubleshooting tool.
There are several things to keep in mind concerning the limits of the equipment itself. The first limit is that vibration sensors see all vibrations at the same time; the engineers call this the time domain (Figure 1). What this means is that using the raw data, you cannot identify a specific component with a problem. The sensor simply sees all the vibrations occurring at its location. The closer the sensor is to the source of the vibration, the more powerful the signal will be.
For this reason, it is often necessary to locate several sensors throughout the aircraft. Each sensor will see the entire range of vibration readings, but the sensors closest to the actual problem will show the greatest vibration amplitude levels. Once the raw data is gathered, some mathematical magic called Fast Fourier Transform will be performed inside the analyzer to convert the raw time data into data displayed in the frequency domain (Figure 2). What this means to us as mechanics is that now we can identify a specific frequency at which the vibration is occurring.
The vibration survey, converted to frequency units, will be displayed on the analyzer. Some analyzers will simply list numeric values for the peak vibration levels and the associated frequency. Some analyzers will actually display a graphical representation of the entire range of defined frequencies. These surveys are typically taken with the aircraft set at a single operating condition. For this reason, these vibration surveys are called steady state surveys.
Displaying the Data
Several different methods of displaying the steady state vibration survey plot exist. The first method is an exponential type of display. This plot displays the spectral plot as the data is processed. This display is constantly changing to reflect the operation of the component. The limit to this plot is that it is very difficult to capture a random vibration event. The only information saved is the information displayed at the time the data capture occurs.
The second method is the peak hold survey. It will display the highest peak at each individual frequency until it is replaced by a higher peak. It shows the “worst case” vibration at each specific frequency. This does not mean that the maximum vibration amplitude is occurring at each frequency during the entire duration of the run, it is simply a method to display the highest peaks recorded during a period of time.
Another type of plot is the waterfall plot (Figure 3). This type of plot captures a sample of the vibration survey and places them back to back for display on a single screen. It is possible to watch for trends or peaks that appear or disappear. The waterfall plot is very useful for recording operating conditions that change.
There is even equipment that can record the vibration survey in “real time” allowing the technician the ability to find a vibration in a specific operation condition. This type of survey is called the transient vibration survey. The transient vibration survey can gather data as the aircraft changes flight conditions.
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