A helicopter’s rotor system, engine(s), and other dynamic components generate vibrations in the airframe. These components will vibrate at a natural frequency which in turn causes other parts like the landing gear, tail boom, cabin etc… to vibrate as well. Each part’s frequency will vary according to its weight, stiffness, shape, etc…. As such, a helicopter contains a complex set of vibrations that add up to a resulting airframe vibration. Engineers attempt to reduce the overall vibration level by tuning the natural frequency of all the components.
When a helicopter is in flight, the airframe’s natural frequency (the sum of all the components’ frequencies) will vibrate without interference. However, when ground contact is made with the landing gear it can interfere with the airframe’s ability to vibrate at its natural frequency. Ground resonance happens when ground contact alters the natural frequency of the main rotor system. This unbalanced condition triggers vibrations that are augmented with every blade revolution causing a reflected impulse that increases in amplitude very quickly. The only rotor systems susceptible to ground resonance have three or more blades. This is due to each blade’s ability to lead and lag (speeding up and slowing down) independently. If something causes the blades to depart from their symmetry, the rotor system’s center of gravity shift causes it to become out of balance allowing divergent oscillations to rapidly become strong enough to cause serious damage to the helicopter. In some cases, complete destruction can occur with many componets coming loose and being thrown from the helicopter.
Engineers design damping systems for the main rotor and landing gear to absorb the energy and prevent the oscillations from accelerating. Still, a sudden shock to the airframe, like a hard landing, can unbalance the main rotor system beyond the damping system’s ability to absorb the energy and ground resonance can start. Improperly serviced or malfunctioning dampeners are usually the cause. Ground resonance happens very fast, however, if the pilot recognizes it in time and there is enough power and rotor RPM available to lift the helicopter off the ground the divergent oscillations will stop. This is the quickest way to stop it and hopefully will result in little or no damage. If the situation is such that there is not enough power to lift into a hover, then a full power reduction is the only option. However, this approach will take time for the vibrations to fade and significant damage can occur.