Rotor track and balance
By John Sharski Making a tab adjustment to correct for a vertical imbalance
A helicopter is a complex assembly of rotating components that allows flight characteristics unavailable to fixed wing aircraft. In these aircraft, excessive vibration levels can lead to premature wear and failures in rotating components Reducing vibration levels in the airframe to a minimum is an essential component in extending the longevity and safety of the helicopter. Rotor track and balance is the process of smoothing vibrations in the airframe, which are caused by rotating components. The main rotor is not the only rotating assembly of concern in a helicopter; there are others such as the tail rotor assembly, drive shaft assemblies, and oil cooler fans.
Types of vibration
Before you can balance the rotor, you must determine what type of vibration you have. A helicopter main rotor is capable of producing vibrations in both the vertical and lateral planes.
Vertical vibration, sometimes referred to as an aerodynamic imbalance, is a result of unequal lift produced by the main rotor blades. This can be a result of blade chord profile variances from one blade to the next or improper adjustment of pitch change links and trim tabs.
Lateral vibration is the result of an unequal distribution of mass, or mass imbalance in the main rotor. This imbalance is a heavy spot on the rotor system that can be felt during rotation. The greater the mass imbalance or the farther the mass imbalance is from the center of rotation, the greater the severity.
A lateral vibration may also be felt when an aircraft is out of "track," or has vertical imbalance. This lateral vibration is a result of the airframe rolling with the mass effect caused by unequal blade lift. "Tracking" of the main rotor blades refers to adjusting the blade tip paths to make them fly in the same rotational plane. This does not always result in the smoothest ride. Some airframe and blade combinations will ride smoother with a slight "track split." The desired end result of the track and balance job should be the smoothest possible ride.
Prior to acquiring data you must install vibration sensors, tachometer signal sources, tracking devices, and associated connecting cables and mounts. The sensor types, installation locations, and material for these components have been performance optimized by manufacturer testing and are specified in the helicopter's maintenance manual. There are numerous sensors available. Those most commonly found in aviation are the acceleration and velocity sensors. The manufacturer of the equipment you are using will dictate what type of sensor is required. Generally, a vibration sensor used to perform balancing needs to be located on the support structure as close to the rotating component as possible.
One of the most common locations used to mount a lateral sensor is the upper portion of the main transmission, on the swashplate support. The connector of the sensor is positioned perpendicular to the left or right of the ship's centerline. The vertical vibration sensor is normally mounted as far forward in the cockpit as possible, with the connector pointed up or down depending on the OEM's directions. This position allows the highest sensitivity for measuring the symmetry of lifting forces developed by the main rotor as the blades pass over the nose of the aircraft. The once-per-revolution (one-per-rev) source is typically a magnetic pickup mounted on the stationary swashplate. A ferrous metal interrupter passing close to the magnetic pickup causes an electrical pulse, triggering the tach event.
A photo optical device referred to as a Phototach can also produce the one-per-rev signal. The Phototach is normally located where a beam of light can be projected onto a small piece of reflective tape attached to the mast or other rotating component of the main rotor system. The light is then reflected back to an optical receiver in the Phototach lens, which triggers the electrical pulse to the analysis equipment.
If utilizing a strobe light, it's necessary to install targets at the tips of the blades. Additionally, the strobe requires a DC power supply for operation. This power is usually tapped from the ship's power source because it cannot be supplied by portable equipment. Airframe mounted optical trackers require the use of a mounting bracket for a solid, stable mounted position.
Lastly, there are hand-held optical trackers. This equipment requires no installation. The tracker is operated in the cabin with the vibration analysis equipment. It requires no tip targets, and operates from the analyzer's integrated power source. Chordwise adjustments can be hub weight or sweep as shown on this Bell 206B
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.
When to normalize rotor settings
If a flight crew or customer complains of a slight vertical or lateral vibration, there's no need to strip the rotor of weight or zero tabs. It's likely that the rotor requires a little tweaking to bring it back below the maximum allowable vibration levels. Rotor systems can even be sensitive to the climate and environment. Aircraft that have elastomeric components in the rotor system, at times, require re-balancing due to changes in outside temperature or seasonal changes.
However, if a new set of blades is installed, all adjustments should be nominalized prior to tracking and balancing. Nominalizing the settings will increase the chances of successfully balancing the rotor in as few flights as possible. It also will help prevent reaching the maximum allowable adjustment for weight, tab, sweep, and pitch change links.
The "weight" for weight adjustments for span weight is usually lead shot A phototach can be used to obtain a once-per-rev tach pulse A magnetic pickup can also be used to obtain the once-per-rev tach pulse
Tips and hints about track and balance
Rotor smoothness should be achievable in all flight regimes. If this isn't 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 isn't possible to smooth the aircraft through all flight conditions, you may be forced to compromise and sacrifice the smoothness of one over another. In this case, the fight condition where your aircraft spends the most time, or the most critical condition, should take precedence.
You may notice certain flags during the rotor smoothing process that will certainly save you the time and frustration of attempting to correct for a mechanical problem present in the rotor system. For every adjustment on the rotor system, there is an associated influence. For example, say we have a tail rotor head that should require approximately 30 grams to correct for an imbalance of 1.00 IPS. If in the course of a balance job, you make a weight addition of 15 grams and the vibration level changes by 0.9 IPS. The "flag" in this case would be the change in vibration of 0.9 IPS. This much change would normally require the addition of 28 grams of weight. In addition, the actual moveline may not be in the desired direction. This could indicate that other factors are involved which may not be correctable by normal procedures. An aircraft which is not mechanically sound, will be extremely difficult, if not impossible, to balance.
Finally, don't forget the human factor. If everything seems correct but you still can't achieve acceptable results, you may have made a misadjustment. First, verify that the equipment has been installed correctly. Sensors are single axis, if a sensor is pointed in the wrong direction, the result of an adjustment will be opposite the desired outcome. Secondly, it's easy to return the aircraft to its initial configuration and start over. When returned to the original configuration, if values are not similar to the original readings, a mechanical problem may exist.
Special thanks to the maintenance team at Vertiflite in Maryville, TN, for the use of their facilities and aircraft for the photo shoot for the article.
About the author
John Sharski has six years' commercial aviation experience and 10 years' experience with the U.S. Air Force. During his tenure with the USAF, John spent four years at the USAF Helicopter Training Facility as a Master Instructor, and served as a subject matter expert in the ground-up design and implementation of H-53 and UH-1N training courses. John has spent the last six years working in commercial aviation as an applications engineer. He currently fills this position for ACES Systems/TEC Aviation Division in Knoxville, TN.