You may have had the same sort of experience with your car. Suppose you drive to work every day at 60 mph. You begin to notice an increasing vibration in the steering wheel and you suspect that one of your front wheels is out of balance. You have the wheels rotated and balanced at your local garage. The next morning, you notice a slight vibration while accelerating through 37 mph that wasn't there before but everything seems fine at 60 mph. When you speed up to 74 mph to pass a truck, there's the out of balance vibration again. As you slow back to 60, the vibration goes away. What's going on here?
The influence of a mass on the balance of a disk changes with speed. You can't always get rid of a vibration at one speed without moving it to another speed — a process my boss calls "pushing down on a balloon." You push it down at one point and it pops up and grows at another.
Because of this phenomenon, it is best to balance at several speeds that encompass the normal range of in-flight power settings. Balancing at several speeds in the flight range doesn't lower the vibration amplitude and noise to its minimum achievable level at each speed, but rather brings each to an acceptable level without pushing the amplitude of any one too high.
What about speeds outside the normal flight range? The noise may be a little higher at takeoff power — but how much time will the power remain at this setting in a five-hour cross-country flight? On the other side of the coin, how much time will the power be at a cruise setting? Are you getting the picture here? If you must sacrifice "quiet" at one power setting to achieve it at another, consider which scenario will be more advantageous in the operation of your particular aircraft. Don't forget that you can also include maximum power as one of the multiple balance speeds if you wish.
Collecting the data
Now that we have a tach input, a sensor input, and a target speed, let's collect the data we need to arrive at a solution. Make sure the aircraft is positioned into the wind and that the wind is below 10 knots if possible. Crosswinds and wind gusts are not acceptable for balancing because they cause the fan speed to fluctuate. This in turn causes conditions to change. Reference the previous section on balancing speeds.
The analyzer is setup with the program for balancing the specific engine. The sensor type and speed are selected and the analyzer instructs you to start the engine. Before you begin collecting data, make sure the engine is at normal operating temperature. The analyzer will instruct you to accelerate to the first speed. When the speed is stabilized, begin the collection of data as instructed by the analyzer.
When data collection begins, a sequence of events occurs. As shown in the illustration on page 52, the vibration sensor is mounted at twelve o'clock and the laser tachometer is positioned to trigger at the six o'clock position as the reflective tape passes through its beam. In Figure A, on the next page, the mass causing the out of balance condition is located at the three o'clock position.
For the purpose of this example, let's assume the fan is rotating at one (1) RPM in a clockwise direction as viewed from the front of the engine looking aft (FLA). In Figure B, as the rotation progresses, approximately 15 seconds (90 degrees of rotation) have passed and the mass is now located at the six o'clock position. Accordingly, the reflective tape has rotated to the three o'clock position.
In Figure C, the mass has reached the nine o'clock position and the reflective tape has entered the laser beam. The laser beam is reflected back to the receiver of the laser tachometer and sends a pulse to the analyzer. At this point, 30 seconds (180 degrees of rotation) have passed.
In Figure D, the mass has reached the sensor position. The upward movement compresses the piezoelectric element inside the sensor and results in a voltage being produced, sent to the analyzer and measured as 10 mV. This value is then held in memory. The time is now 45 seconds or 270 degrees into the rotation. The rotation continues back to the position in Figure A and the process is repeated.
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