WaspAir
Traditionally tracking in the helicopter world means getting the "blades flying the same path in flight" as you say. However modern electronics has allowed us to measure and demonstrate what many old time helicopter mechanics kept telling us that "the smoothest ride often results from a track split".
You don't have to believe me here are 3 quotes from Enstrom, Robinson and DSS micro who are pretty well established in the helicopter world and probably know what they're talking about.
Here is a quote from an
Enstrom document called “Tracking Main Rotor Blades “.
General: The purpose of tracking the main rotor blades is to obtain a smooth ride. This is accomplished by adjusting the blade track to reduce vibration. An out-of-track condition will produce a vibration, usually a one per rev which is felt as a vertical vibration.
However, for the rotor system to be smooth, it does not necessarily mean that the blades are flying in the same plane. With the advent of digital tracking equipment it has been found that the best ride is not necessarily a condition of the blades flying in plane, but in a track that gives the least magnitude of vertical vibration.
Here is a quote from a
Robinson document called “Helicopter track and balance theory
Helicopter Track and Balance Theory By Mike Robinson February 1999 've heard it said on many occasions that a helicopter is several thousand rotating components trying desperately...
www.aviationpros.com
The term "rotor track and balance" is somewhat misleading, in that "track" or "tracking" 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 "track split." The desired end result of the track and balance job should be the smoothest possible ride.”
Also from
DSS.......https://www.dssmicro.com/theory/th_hist_rotor_smoothing.htm
Tracking using Vibration Sensors Users found the track conditions of the rotor directly related to vibrations in the airframe. Experimentally it was found that the vibration information could be used to adjust pitch links and tabs to produce minimum vibrations at all forward speeds. After this process was complete, the blade track could be measured optically and surprisingly the blades were not in perfect track! This lead to a quandary... do we want perfect track or minimum vibrations?
Is Tracking of any value? In the process of using these tracking methods and measuring the vibrations that resulted, users found that "perfect track" rarely produced minimum vibrations. Various theories have been proposed to explain this effect. One theory is that each blade has a slightly different shape, twist, flexibility etc. and only by putting them slightly out of track can these variations in lift be compensated. Another theory is that each blade produces a "turbulent wake" that the trailing blade must fly through. If alternating blades are set to fly high and then low, each blade will have "calmer air" to fly through resulting in smoother flight. This effect is more pronounced on aircraft with four or more blades on the main rotor.
The RTB procedure that I developed for Smart Avionics PB3/4 users tracks the rotors using vibration during the balancing process.
Mike G
