A helicopter is a type of aircraft that operates with rotor blades revolving around a mast to generate lift. Helicopters are able to fly backwards, forwards and laterally as well as hover and land vertically. As with most machines, helicopters will vibrate, a problem typically caused by a rotor malfunction. There are three main types of vibrations known to affect helicopters.
About Helicopter Vibrations
Typically, an unusual amount of vibrations in a helicopter is due to a malfunction in the aircraft. These malfunctions may include loose hardware, out of track or out-of-balance conditions or worn bearings. Due to the various moving parts and rotor system stress during operation, helicopters have a high level of vibrations, which left unchecked will cause machine failure or other serious damage to the aircraft in a short amount of time.
Low to High Frequency
One type of helicopter vibration is a frequency vibration. This type of vibration may occur as a low, medium or high frequency. A low frequency vibration typically occurs when the revolution of the rotor is disturbed. A medium frequency vibration is a common rotor system vibration that occurs due to loose components of the aircraft. A high frequency vibration typically occurs when the tail rotor gears, tail drive wire and shaft or the tail rotor engine, fan or shaft assembly vibrates or rotates at an equal or greater speed than the tail rotor.
Ground resonance is a type of vibration that is the most destructive and dangerous of the vibrations and can destroy a helicopter within seconds. Ground resonance never occurs during flight and only affects grounded helicopters with turning rotors. Grand resonance is often the result of unbalanced forces in a rotor system that causes an aircraft to rock on the landing gear when the helicopter is at or near its natural frequency. Other causes of ground resonance are incorrect tire pressure, defective rotor blade lag dampeners and incorrect adjustments to landing gear shock struts.
Lateral and Vertical
Lateral and vertical vibrations are also a type of vibration that can affect a helicopter. Lateral vibrations are often the result of worn, loose or cracked parts or a lateral imbalance such as a span-wise imbalance, a chord-wise imbalance or a combination of both. Vertical vibrations typically occur when a rotor blade is out of track.