After the equipment is installed, it's time to start acquiring measurements. As a general rule, the maintenance manual will outline the flight conditions for making measurements and which measurements are required at each of these conditions. The typical first step is to verify the ground track at 100 percent ground. This is normally accomplished with the aircraft at 100 percent rpm and the blades at flat pitch. The next condition typically will be to hover the aircraft, record the lateral vibration, and check the track split. Next, vertical and track data is acquired at in-flight airspeeds as indicated by the maintenance manual.
The first situation to be corrected once data is acquired is out-of-track conditions at ground and hover. Although out-of-track conditions manifest themselves as a vertical vibration, they have a large effect on the lateral vibration readings as well. Track corrections are made by adjusting the pitch change links on some aircraft and outboard trim tabs on others. Regardless of the method, the end result is to achieve a flat track prior to making adjustments to correct for mass imbalances. It's always best to get the ground track as close as possible. This increases the degree of success achieved during forward flight. Digital analyzers with the aid of optical tracking devices increase the ability to measure track splits down to millimeters. When the ground and hover track is satisfactory, the lateral imbalance can be addressed.
Manufacturers have several methods to correct for lateral vibration. They include placing weight on the main rotor hub, adding tip weights to the blades, adjusting the chordwise balance of the blade, and sweeping the main rotor blades. Following the solutions from the analyzer, implement the lateral corrections until the vibration levels are below the maximum acceptable level. In most cases this is 0.2 IPS. Ideally, lower the levels as close to 0.1 IPS as possible. This makes the in-flight vertical limits easier to achieve, especially on two bladed helicopters.
There are two primary adjustments utilized to correct for a vertical vibration. The first is the main rotor pitch change links (PCL). To move a blade up or down, lengthen or shorten the PCL for that blade. This adjustment is typically used to correct for out-of-track conditions on the ground and in a hover. The second adjustment available is the main rotor trim tab. To make a blade fly higher or lower, bend the trim tab up or down. The adjustment of trim tabs is generally used to correct for out-of-track conditions that increase with airspeed. Trim tabs are very sensitive, so use care and caution when making adjustments.
Typical adjustment type results
Below are some general rules to help determine what to expect from an adjustment type.
Pitch links change the tip path plane of the blades through all speeds. Changes will affect vertical and lateral vibration levels.
Tabs change the tip path plane of the blades at higher speeds. Changes primarily affect vertical vibration levels. Sweep changes the mass of the rotor at all air speeds. Changes primarily affect lateral vibration levels. Tip weight changes the mass of the rotor at all air speeds. Changes primarily affect lateral vibration levels. Hub weight changes the mass of the rotor at all air speeds. Changes primarily affect lateral vibration levels
Blade chordwise weight changes the tip path plane of the blades. Changes have large effects on vertical vibration levels and on lateral vibration levels at ground and hover. Changes have an effect on the blade track from ground to hover.
There are exceptions to the rules listed above based on the design features of the helicopter's rotor system, but the rules are a basic guideline for the rotor behavior based on the adjustment that is implemented. If you're unsure of the best method for your aircraft, always refer to the maintenance manual or contact the OEM Product Support Division. Once the rotor is smoothed below all maximum acceptable limits, the last step is to adjust for autorotation rpm. The pitch change links are adjusted equally in the same direction. Autorotation adjustments have no effect on the rotor systems one-per-rev vibrations.
When to track and balance
Most aircraft manufacturers have specified intervals for rotor balance checks. However, it's generally recommended that the rotor system is checked and 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.
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