Tracking devices vary in technology and limitations from one manufacture to the next. If utilizing a strobe light, it is 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. The last and most practical option is the portable, hand-held optical tracker. This equipment requires no installation. You merely place the tracker into the cabin with the vibration analysis equipment. The tracker is hand held and operated, requires no tip targets, and operates completely from the analyzer's integrated power source.
Prior to starting the job, do a detailed inspection of the main rotor system and associated controls to ensure proper condition and operation. Few things are more frustrating than attempting to compensate for poor mechanical condition when performing a balance job.
If performing a periodic balance check, you need do nothing to the rotor system adjustments prior to starting. However, after major repair or overhaul work is performed on one or more of the main rotor system components, it may be beneficial to return main rotor system adjustments to nominal prior to starting the job.
After your equipment is set up, it is 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. Acquiring as many data as possible will enable you to better trend and troubleshoot the vibrations at hand. As you prepare to acquire data, one important consideration is the wind conditions. No wind is the preferred condition, but if there are winds present, you should ensure that the nose of the aircraft is pointed into the wind during data acquisition. Failure to do so may result in erroneous measurements being acquired, which complicates the balance job.
The typical first step in performing a balance job is to verify the idle ground track. This is accomplished with the aircraft at idle RPM with the blades at flat pitch. The goal here is to verify that the blade paths are relatively close to one another prior to bringing the engine to full throttle. Do not attempt to eliminate small blade splits at ground idle, as this may lead to readjusting the same blade after RPM and load are increased. If the idle track is satisfactory, move to the next condition.
Once the rotor RPM is advanced, check the lateral vibration reading and the blade track again. Don't place much emphasis on the lateral balance of the aircraft at this time. If the lateral vibration is excessive, make a correction; otherwise leave this until the vertical balancing (tracking) is completed. As noted earlier in this article, changes in blade track will affect the lateral balance. Again, verify the blade track to ensure the blade paths are relatively close. If track and balance are satisfactory, move to the next condition.
Bring the aircraft to a hover into the wind. Measure both the vertical and lateral vibration levels, as well as the blade track. At this point you may place more emphasis on the vertical vibration. With the aircraft in a hover, contact with the ground will no longer influence the vertical vibration reading. Record both the lateral and vertical measurements. If the vibrations levels allow, transition to forward flight. Measure the vertical and lateral vibration levels and the tracking. When all measurements for the run have been completed, land the aircraft and determine what adjustments are needed.
Helicopter Track and Balance Theory
By Mike Robinson
When finished acquiring data, what do you do with all of the information? We have already established the two planes of main rotor vibrations found in the airframe—vertical and lateral. Next, we will discuss how these vibrations are corrected and how to interpret the data collected.
As described earlier in this article, a lateral vibration is the unequal distribution of mass in the rotor "disk". Manufactures have provided several methods to correct for this. The most common corrections are to place weight on the main rotor hub at specific locations, and to "sweep" the main rotor blades. Sweeping the blades can be defined as moving the blade forward or aft of its angular position. When sweeping the blades, it is safer to sweep a blade aft, as sweeping a blade forward may cause the blade to be unstable. There are airframe types that utilize only main rotor weights, only blade sweeps, or a combination of both to correct for a lateral imbalance. This correction is based on the lateral vibration measurement, either on the ground or at hover.
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