As described earlier in this article, a vertical vibration is a result of unequal lift produced by the main rotor blades. There are two primary adjustments utilized to correct for a vertical vibration. The first is the main rotor pitch change links (PCL). The PCL controls the angle of each main rotor blade individually. To move a blade up or down, lengthen or shorten the PCL for that blade. This adjustment is typically used to correct for the vertical blade separation on the ground and the vertical vibration at hover. The second adjustment available is the main rotor trim tab. To make a blade fly higher or lower, move the trim tab up or down. The adjustment of trim tabs is primarily used for in-flight corrections. Trim tabs are very sensitive, and it is important to use care and caution when making adjustments. One point to consider when making track adjustments is that as you change a blade path, you will either increase or decrease the amount of that blade's drag. This change in drag has the tendency to move the blade forward (lead) or aft (lag) of its original position. As the blade pitch is increased, drag is increased, causing the blade to lag. If the blade pitch is decreased, drag is decreased, causing the blade to lead. This lead/lag effect results in a change in the lateral balance of the rotor disk. Thus the adage, "track affects balance" is realized.
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.
Are You On The Right Track? Rotor track and balance By John Sharski Making a tab adjustment to correct for a vertical imbalance A helicopter is a complex assembly of...
No Bad Vibes Propeller Balancing and Vibration Analysis in Reciprocating Engines By John Sharski November 2000 So, your aircraft has a brand-new propeller fitted on a fresh...
NOTAR® Maintenance — No Sweat By Greg Napert September 1999 NOTAR (No Tail Rotor) helicopters were introduced in 1991 as a revolutionary way to provide anti-torque to helicopters...