One of the main problem faces by designers and engineers of Helicopter is Vibration. Design of Helicopter is such that the vibration results from Engine, Rotors, Transmission and other moving parts. Since Vibrations can not be completely eliminated, focus of Helicopter design is to make them as low as possible.
The cause of vibration is the unbalanced condition of several parts. Unbalance condition can occur if a part is damage. Other than damage during manufacturing of part it cannot be perfectly balanced. Part can be manufactured near to balanced condition but not perfectly balance.
This problem intensifies in Helicopter due to the fact that Helicopter has many rotary components, especially Main Rotors and Tail Rotors.
Vibrations in Helicopter identified on the basis of there frequency. They are designated as extremely low, low, medium, and high.
Extremely Low Frequency Vibrations: – Frequency of these vibrations is less than 1/rev. Commonly occured in main rotor.
Low Frequency Vibrations: – Frequency of these vibrations is 1/ or 2/rev. These vibrations occur at between 0-500 RPM range. Main rotor usually rotates in this RPM range. It is most common and easiest to detect.
Medium Frequency Vibrations: -Frequency of these vibrations is 4/ or 6/rev. These vibrations occur at between 500- 2000 RPM range. Due to this wide range, it is very difficult to distinguish. Many rotors on Helicopter fall in this category of vibration’
High Frequency Vibration: – This type of vibration occurs in parts which rotates above 2000 RPM. Tail rotor rotates in this range therefore this vibration in Tail Rotor.
Vibration can also be classified as vertical and lateral. Lateral vibration occurs due to unbalance of main rotor. Unbalance of rotors can also be of two types: Spanwise And Chordwise. Spanwise unbalance is caused because of one blade or hub is being heavier than other. A chordwise unbalance occurs when there is more weight toward the trailing edge of one blade than the other.
Vibrations in Helicopter if not controlled can be catastrophic.
Some aircraft use Bifilar Vibration Absorber which are located at the head of main rotor. It reduces vibration directly from it’s source, i.e. Main rotor.