We discovered that our Scout had a bit of a “bounce” on the ground and decided to do a basic blade track to find out what was wrong. Tracking is the process of smoothing vibrations in the airframe, which are caused by the main rotor blades. Premature wear and failure of components can be attributed to excess vibration levels. Reducing these vibration levels in the airframe to a minimum is absolutely essential in order to ensure the safety and longevity of the helicopter. It is also very tiring and irritating for the crew to fly in a vibrating helicopter.
A vertical vibration is a result of unequal lift produced by the main rotor blades. This unequal lift is usually the result of improper adjustment of the pitch change links and/or the trim tabs. These are the most common causes of vertical vibration. Tracking refers to adjusting the main rotor 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.
A tracking flag is a long lightweight pole held vertically, with two horizontal arms extending from it. A wide band of cloth is attached between the horizontal arms, making a ribbon-like connection from one to the other. A wide piece of masking tape is further added to avoid tearing the cloth.
The individual main rotor blade tips are coated with different coloured grease pencils. With the helicopter running on the ground, the flag is moved in toward the rotor disk. As the individually coloured blade tips make contact with the tape, each left a mark corresponding to its assigned colour.
If the marks were vertically separated, a pitch change adjustment was needed to move the blades tips closer together. If the marks overlapped one another, no adjustment was needed. As you can see, the marks are spread so a slight adjustment was required.
After that adjustment was made, a handling flight proved that we had improved the vibration level and the overall ride of our Scout. Happy days!