Under what conditions is ground resonance most likely to develop in a helicopter?

Ground resonance occurs primarily due to the interaction of the helicopter's rotor dynamics with the ground, particularly when the rotor system becomes unbalanced. This phenomenon typically arises in circumstances where shocks or vibrations affect the structure of the helicopter, such as when it lands or conducts rapid maneuvers. If a series of shocks disrupts the balance of the rotor system, it can lead to oscillations that amplify as a result of the interaction between the rotor system and the ground, increasing the risk of damaging vibrations.

In contrast, conditions such as high altitude takeoffs, steady flight at low speeds, or hovering at altitude do not inherently introduce the same kind of unbalancing forces. These scenarios are characterized by relatively stable flight dynamics, where the rotor system does not have the same interactions that can trigger the resonance phenomenon seen during transitions from ground to flight or during certain disturbances while on the ground. Therefore, the best scenario for the development of ground resonance is indeed when an unbalanced state is introduced through a series of shocks to the rotor system.