# Helicopter track and balance theory

Vertical Corrections

If using the polar chart method of making corrections, you must first plot the amplitude and phase angle of the measured vibrations on the chart. Be sure you have the correct chart, as there are different charts for the lateral and vertical planes. To plot information, find the phase angle (clock) line on the chart that represents the angle of the vibration recorded, then move outward to the amplitude "ring" of the vibration and make a plot. Next, move from this plot to the correction scale at the side of the chart. This scale will determine the amount of correction required to reduce the vibration. This same plotting method applies to both vertical and lateral polar charts.

Let's look at an example of these charts. The chart above represents the vertical vibration levels measured during a run for hover (plot #1), forward flight at 60 knots (plot #2), 80 knots (plot #3), 100 knots (plot #4), and 120 knots (plot #5). As you can see, the vibration levels for hover and flight at 60 knots are relatively close together. But forward flight at 80, 100, and 120 knots are quite a bit higher. The first correction to make in this case is a target blade pitch change link adjustment of approximately one flat upward. The second correction is to make a trim tab adjustment for the difference between hover and our fastest forward flight speed of 120 knots. This correction turns out to be approximately six degrees, target blade up. A note to remember when implementing tab corrections is that as the airspeed increases, the influence a trim tab has on the blade increases also. A blade showing an increase in track split and vibration as airspeed increases will require a trim tab movement. If the vibration plots had all been in relatively the same location, only the pitch change link would have been necessary.

An important point to remember when making multiple adjustments within the same run is the fact that the influence for each of the adjustments made will be affected by the other. This is true of both vertical and lateral adjustments and can make it difficult to accurately predict the aircraft's response to a specific correction. It is easier to track the aircraft's response one correction at a time.

The last step to accomplish when finished balancing is to verify, and correct if necessary, the autorotation RPM of the helicopter.

When Should a Rotor Track and Balance be Performed?
Most aircraft manufactures have specified intervals for rotor balance checks. However, it is generally recommended that the rotor system be checked and, if necessary, balanced any time a component of the system—such as pitch links, cyclic or collective control rod ends, or swashplate—is changed or adjusted. A track and balance should also be conducted any time the pilot reports a marked change in the vibration condition of the aircraft.

Tips and Hints About Track and Balance
Rotor smoothness should be achievable in all flight regimes. Manufacturers attempt to design and certify aircraft that are smooth in all conditions. If this is not 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 is not possible to smooth the aircraft through all flight conditions, you may be forced to compromise and sacrifice the smoothness of one flight condition in favor of another. In this case, the flight condition where your aircraft spends the most time, or the most critical condition, should take precedence.

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