Refer to figures.
Generally in flight on a helicopter, the arranged

• Rotors;
• Engines and
• Drive shafts,

transmit their own vibrations to the structure which are stabilised because there is no real focal point for them to attach to.

On the other hand, Ground resonance is a problem which can occur whilst the aircraft is in contact with the ground.

Ground resonance is an oscillation of increasing amplitude caused by the interaction between vibrations in the rotor system and fuselage vibrations of similar frequency.

For ground resonance to occur 4 conditions must be met:

• Self-excited vibration must be present in the rotor system caused by
  • Faulty tracking
  • Blades of unequal weight or
  • Faulty drag dumpers
• Fuselage vibration must be present caused by
  • Mishandling the cyclic stick during landing (bounce side to side);
  • Taxiing, running take-off or landing on a rough surface (holes, ruts etc.);
  • Incorrect or unequal tyre and oleo pressures;
  • Passengers or stores contacting the undercarriage during the disembarkation.
• Rotor and fuselage vibrations must be similar in frequency so that they reinforce each other
• The helicopter must be in contact with the ground

Typically, ground resonance happens during:

• Startup;
• Lift off and
• Landing.

The phenomenon starts with a slow lateral rocking of the fuselage but can increase very rapidly in sympathy with rotor RPM.

During start-up as the rotor spins up, it passes through two regions where resonance can happen, about 30% and 60% NR.

Moreover, the condition is also more common with wheeled helicopters that have shock absorbers. Skid-equipped helicopters can absorb more vibration due to rigidity.

Finally, talking about recovery actions, the most effective methods of recovering from ground resonance are:

• Lift Off as soon as possible if Nr is sufficient
• Lower the collective lever, Close the throttle, Shut down the helicopter and Apply the rotor brake as quickly as possible if Nr is insufficient.